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The New Idria Serpentinite A thesis presented by Marcus Robinson Van Baaien, II to The Department of Earth and Planetary Sciences in partial fulfillment of the requirements for the degree of Doctor of Philospohy in the subject of Geology Harvard University Cambridge, Massachusetts May, 1995 DMI Number: 9539051 Copyright 1995 by Van Baalen, Marcus Robinson, II All rights reserved. UMI MicroEotn 9539051 Copyright 1995, by OMI Company. All rights reserved. This aicroform edition is protected against unauthorized copying under Title 17, United States Code. 300 North Zecb Road Ann Arbor, HI 48103 HARVARD UNIVERSITY rut (iftADLati. school oi aiits ami surscis THESIS ACCEPTANCE CERTIFICATE The undersigned, appointed In the Division Department Earth and Planetary Sciences Committee have examined a thesis entitled "The New Idria Serpentinite" presented by Marcus van Baalen candidate for the degree of Doctor of Philosophy and hereby certify that it is worthy of acceptance. SiRruturr fhinrsd^ ^ Typed tutiie Pmf. Heinrich D. Holland Si/future Typed njme Prof. Charles W. Burnham Signature Typed name  1995 by Marcus Robinson Van Baalcn II All rights reserved. Abstract This thesis describes the mineralogy and petrology of the serpentinite massif at New Idria, California, USA. This serpendnite is an oval, 23 x 8 km body, interpreted as a pan of the Mesozoic Coast Range Ophiolite. Diapiric emplacement within the Coast Ranges has resulted in a piercement structure or tectonic window, lying just cast of the San Andreas fault system ncmheast of Parkfield. The mineralogy of the serpentinite is dominated by chrysorile; New Idria is one of the largest deposits of chrysorile asbestos in the world. A model for the generation of short fiber chrysotile asbestos is proposed. Locally within the serpentinite, tectonic blocks or "knockers" of several distinct rock types are preserved. Some blocks contain exotic Ti-rich minerals that previously were regarded as metasomatic; these arc reinterpreted as the products of essentially isochemical metamorphism. Blocks of all types have experienced Ml and M2 metamorphism, of blucschist/grcenschist and sub* grecnschist facies respectively. It is proposed that M2 metamorphism is related to passage of the Mendocino Triple Junction in the Miocene. Blocks consisting of chlorite + diopside + Ti-gamet are interpreted as metapyroxenites. Blocks containing the rare Ti-minerals benitoite and_neptunite are interpreted as re-metamorphosed Franciscan mafic schists and greenstones. Massive antigorite blocks have a thermal history separate in part from the rest of the serpentinite. Approximately 300 chemical analyses of Ti-andradite and Ti-grossular garnets and their coexisting minerals are presented. Public policy issues, related to the largely unfounded concerns about health hazards of chrysotile asbestos, have brought recent attention to New Idria. Fluvial transport of chrysotile asbestos from New Idria eastward into the San Joaquin Valley has been taking place for as much as 12 million years. Since the completion of the California Aqueduct, some of the chrysotile asbestos has gone into the canal, mainly under flood conditions. Therefore the downstream users of this water have found chrysotile asbestos fibers in the water. Concern about the possible health hazards of this condition has been greatly overstated; the hazard is negligible. It will be a matter for science and public policy together to find a safe and economical solution to these issues. Principal Advisor Prof. Heinrich D. Holland 0-3 Table of Contents Chapter I - Introduction and Regional Geology T. Outline and Statement of Purpose.........................................................1 II. The New Idria District of California............................................................. 3 A. Location.....................................................................................................3 B. Description................................................................................................ 6 C. Index Maps.......................................................................................... 13 D. Flora .......................................................................................................... 14 E. Fauna....................................................................................................... 16 III. Parts of the Serpentinite.............. 1.................................................................17 A. The Serpentinite Massif............................................... 17 B. The Tectonic Blocks....................... 17 C. The Mercury Mines..................................................................................17 D. The New Idria Syenite............................................................................. 18 IV. Laboratory Analytical and Computational Techniques...............................20 A. Petrography.............................................................................................. 20 B. Mineral Analyses..................................................................................... 21 C. Whole Rock Analyses....... .................................................................... 24 D. X-ray Diffraction..................................................................................... 24 E. Fluid Inclusion Microthermometry........................................................ 24 F. Thermodynamic Modeling..................................................................... 25 G. Norm Calculations........................................................................ 25 H. Bond Distance Calculations....................................................................26 I. Mineral Abbreviations............................................................................ 27 V. Conclusions................................................................................................... 28 Appendix A - Glossary of Place Names.................................................................30 Tables ...................................................................................................................34 04 Chapter 2 - History of Scientific Investigations of the New Idria District Prologue: .................................................................................................................... I I. Geological studies prior to 1950...................................................................... 1 II. Studies of Coleman and others, 1950 onward............................ ............... 8 III. MineraJogicai studies.......................................................................................9 IV. More Mining Studies....................................................................................... 10 V. Geophysical Studies, 1943 onward...... ........................................................10 VI. Studies by the EPA, 1982 onward.......................................................13 Epilogue: .................................................................................................................... 14 0-5 Chapter 3 - Mineralogy & Petrology of the New Idria Serpentinite I. Introduction.................................................................................................... 1 II. Geological Setting of the New Idria District..................................................1 III. Serpentine and Serpentinites..................................................................2 IV. Some Serpentine Mineralogy.......................................................................... 3 A. Lizardite......................................................................................................7 B. Antigorite....................................................................... 11 C. Chrysotile................................................................................................... 14 V. Phase Relations among the Serpentine Minerals................... ..................... 16 VI. Phase Relations in Serpentinites...................................................................17 VII. Distinguishing Among the Serpentine Minerals........................................... 21 VIII. Progressive Alteration of Ultramafic Rocks................................................ 24 A. Progressive Serpendnization of Dunite.............................................. ..26 B. Progressive Serpendnizationof Harzburgite...........................................27 IX. Petrology of the New Idria Serpentinite............................................. 36 A. Compositional Analysis........................................................................ 36 B. Geothermometry from Relict Igneous Minerals............................ 46 Accessory Minerals of the Serpentinite...............................................53 A. Brucite........................................................................................................53 B. Magnetite...... ............................................................................................ 55 C. Sulfides............................................................ 57 D. Chromite..................................................................................................57 E. Chromian Andradite Garnets.......................................................... 61 F. A Blue Amphibole............................................................................69 G. Secondary Alteration Minerals.................................................................73 XI. Discussion and Conclusions.................................................................. 73 Tables ....................................................................................................................84 0-6 Chapter 4 - Mineralogy and Petrology of the Tectonic Inclusions I. Introduction........................................................... 1 II. Geological Setting of the New Idria District.................................................6 III. Chlorite-Diopsidc-Gamet Rocks....................................................................7 A. General Statement.....................................................................................7 B. The 34-50 locality.....................................................................................8 C. Perovskite Knob................................................................................. 22 D. The Melanitc Mine........................................................................... 38 E. Discussion of Chlorite Compositions.....................................................46 F. Discussion of Bulk Compositions................................................. 47 G. Models for the Origin of Chlorite-Diopsidc-Gamet Rocks..................53 I. Serpentinization of Fertile Rocks............................................................ 66 I. Temperatures of M1 and M2 Metamorphism.......................................74 J. About Rodingites...................................................................................... 75 K. Chlorite-Diopside-Gamet Rocks from Other Localities.................. 76 IV. Mafic Schists and Greenstones............................................................ 80 A. General Statement................................................................................. ..80 . B. Jadeite-bcaring Blocks................. ............................................................81 C. Blocks Containing Benitoite, Neptunite, and Joaquinite................ ...84 V. Antigorite Knockers................................................................. ...................... 98 A. General Statement.................................................................................... 98 B. Santa Rita Peak........................................................................................ 98 C. San Carlos Peak................................................. 104 D. Perovskite Knob............................................................. 104 VI. Fluid Inclusion Constraints on M2 metamorphism.....................................105 VII. Conclusions.................................................................................................... 110 Tables ....................................................................................................................116 0-7 Chapter 5 - Compositions and Mineral Associations of Garnets from the New Idria Serpentinite I. Introduction....................................................................................................1 II. Geological Setting of the New Idria District................................................. 3 III. Titaniferous Garnets.........................................................................................3 IV. Titaniferous Garnets at New Idria:......................................................5 V. The SAG Projection............ ........................................................................... 6 VI. Compositions of New Idria Garnets................................................... 7 VII. Compositions of Ti-gamets from Other Localities....................................... 13 VIII. Some Crystal Chemical Considerations.................................... ..24 IX. Exchange Vectors............................................................................................ 25 X. A Three-Garnet Rock............................................................................. 26 XI. Compositions of Minerals Coexisting with Garnet..................................... 27 XII. The Colors of New Idria Garnets.................................................................. 32 XIII. Conclusions.....................................................................................................34 Tables .................................................................................................................... 37 0-8 Chapter 6 - Titanium Mobility in Metamorphic Systems Abstract ........................................................................................................... 1 1. Introduction.............................................................................................................. I 1.1 Element mobility...................................................................................... 3 2. Experimental data and Ti concentrations in natural waters.......................... 7 2.1 Measurements of Ti in natural waters.......................................... 7 2.2 Low Temperature Experimental Data.................................................... 7 2.3 High Temperature Experimental Data........................................... 9 2.4 Comparing the effects of Temperature and Pressure......................16 2.5 Speciadon of Titanium in Aqueous Fluids.............................................16 3. Titanium mobility observed in low grade metamorphic rocks..................... 17 3.1 Alteration of silicate minerals............. .................................................... 17 3.2 New Idria, California............................................................................... 19 3.3 Siena Blanca, Texas................................................................................24 3.4 Adamello, Italy..................................................................... 25 3.5 Magnet Cove, Arkansas.......................................................................... 26 3.6 The Alpine Clefts.......... ................................................................ 28 4. Titanium mobility observed in high grade metamorphic rocks...........................28 4.1 Monviso, France............................. 29 4.2 Santa Catalina Island, California............................................................ 30 4.3 Tauem Window, Austria........................................................................ 31 5. Conclusions............................................................................................................ 32 Tables ................................................................................................................... 34 0-9 Chapter 7 - Discussion and Conclusions Opening Statement...................................................................................................... ..1 I. Minerals and Mineral Associations..................................................................1 A. Serpentine.................................................................................................. 3 B. Olivine, Pyroxene and Chromite............................................................ 5 C. Chlorite and Diopsidc............................................................................... 5 D. Garnets........................................................... 6 E. Sodic Amphibole...................................................................................... 8 II. Tectonic Blocks............................................................................................... 8 III. Intrusive Rocks............................................................................................... 12 IV. Element Mobility During Metamorphism..................................................... 13 V. Progressive Serpentinization........................... 14 VI. Formation of Short Fiber Chrysotile Asbestos.....................................15 VII. Regional Implications...................................... 21 A. Tectonic History.............................................. 21 B. Jurassic Tectonics.................................................................................... 21 C. Cretaceous and Neogene Tectonics........................................................22 D. Miocene Plate Rearrangements in the Eastern Pacific........................... 22 E. Miocene Events........................................................................................ 22 F. Multiple Effects of Triple Junction Passage at New Idria....................23 VIII. New Idria and the San Andreas Fault Today................. 28 A. New Idria as a Tectonic Window................................................. 28 B. Recent Seismicity..............................................................................28 Closing Statement................................................................................................29 A. Comparative Studies of Ophiolites.........................................................29 B. Areas for Future Research........................................... 30 Tables .................................................................................................................. 31 0-10 List of Plates Frontispiece LANDSAT photograph of the New Idria District 0-20 Plate 1.1 Plate 1.2 Geology of the Santa Rita Peak 7.5 Minute Quadrangle (in pocket) The New Idria Thrust Fault, Corbett-Byles Mine 1-11 Plate 2.1 Plate 2.2 The New Idria Serpentinite from San Benito Mt. Guillemin-Tarayre 1867 Geologic Map of California 2-5 2-7 Plate 3.1 Plate 3.2 Plate 3.3 Plate 3.4 Plate 3.5 Plate 3.6 Plate 3.7 Plate 3.8 . The New Idria Serpentinite from Condon Peak Bastite Grain in Serpentinized Harsburgite Serpentinized Dunite at the Butler Estate Mine Serpentinized Dunite at the J.M. Christie Mine Photomicrograph of Pscudomorphic Texture Serpentinized Harzburgite, Hand Sample Chalcopyrite Overgrown by Ti-gamet Hydromagnesite Veins in Serpentinite Outcrop Plate 4.1 Plate 4.2 Plate 4.3 Plate 4.4 Plate 4.5 Plate 4.6 Plate 4.7 Plate 4.8 Photo of 34-50 locality Photomicrograph of Sample 990-22 Photomicrograph of Sample 990-15 Photo of Pcrovskite Knob Basal Contact at West End of Perovskite Knob Basal Contact at East End of Perovskite Knob Photomicrograph of Sample 688-6 Photo of Melanite Mine 3-5 3-23 3-29 3-31 3-33 3-35 3-59 3-77 4-13 4-17 4-19 4-25 4-31 4-33 4-35 4-41 Plate 4.9 Plate 4.10 Plate 4.11 Plate 4.12 Cat's-eye Garnets from Melanite Mine Photomicrograph of Benitoite with Neptunite Photo of Santa Rita Peak Antigoritc Mylonite Zone at Santa Rita Peak 4-45 4-87 4-101 4-103 Plate 5.1 Plate 5.2 Photo of M2 garnet vein Three Coexisting Garnets in Sample 990-1 5-23 5-31 Plate 6.1 Plate 6.2 Exsolution of Rutile in Chlorite Hydrothermal Rutile Vein List of Figures 6-21 6-23 Fig 1.1 Fig 1.2 Fig 1.3 Map of Central California Simplified Geologic Map of the New Idria District Sample Locations in the Santa Rita Peak Quadrangle 1-5 1-9 1-23 Fig 2.1 . Recent Seismicity in Northern California 2-12 Ftg 3.1a Fig 3.1b Fig 3.2 Fig 3.3 Fig 3.4 Fig 3.5 Fig 3.6 Fig 3.7 The Iizardite IT structure Simplified antigorite structure Univariant Reactions in the MSH System IUGS Classification of Ultramafic Rocks Serpentinite Bulk Compositions Compositional Variation in Serpentinites Pyroxene Compositions A Subsolidus Phase Diagram for Lherzolitc 0-12 3-8 3-12 3-19 3-39 3-41 3-43 3-49 3-51 Fig 3.8 Fig 3.9 Fig 3.10 Fig 3.11 Compositions of Chromian Garnets Cr-Fe Exchange in Chromian Garnets Si Variation in Chromian Garnets Condensed Space for Sodic-Calcic Amphiboles 3-63 3-65 3-67 3-71 Fig 4.1 Fig 4.2 Fig 4.3 Fig 4.4 Fig 4.5 Fig 4.6 Fig 4.7 Fig 4.8 Fig 4.9 Fig 4.10 Fig 4.11 Fig.4.12a Fig 4.12b' Fig.4.12c Fig 4.13 Fig 4.14 Fig 4.15 Fig 4.16 Fig 4.17 Orientations of Structural Features at New Idria Geological Sketch Map of 34-50 Locality Schematic Diagram of 34-50 Locality Geological Sketch Map of Perovskite Knob Schematic Diagram of Perovskite Knob Schematic Diagram of Melanite Mine Compositional Variation in Chlorite Tschermak Variation in Chlorite Chlorite-Diopside-Gamet Bulk Compositions Arrangements of lnMgOj-2Ca0+Ali0J+9Si02+6HI0 Univariant Reacu s in le C> 1ASH system Phase Relations it te S :tem : rc-Tlc-Chl-Di Major Element Pai ..tiotu ; During Serpentinization Prograde Metamorphism in a Portion of CMASH Chlorite-Diopside-Gamet Rocks versus Rodingites Tectonic Inclusions in the Serpentinite Geological Sketch Map of the Gem Mine Fluid Inclusion Homogenization Temperatures Fluid Inclusion Final Melting Temperatures 4-5 4-11 4-15 4-27 4-29 4-43 4-49 4-51 4-57 4-59 4-73 4-69 4-71 4-73 4-79 4-83 4-89 4-107 4-109 Fig 5.1 Fig 5.2 Compositions of New Idria Garnets Additional Garnet Compositions 0-13 5-11 5-15 Fig 5.3 Fig 5.4 Fig 5.5 Fig 5.6 M2 Garnet Vein Microprobe Traverse Fe-Ti Exchange in Garnet (Fe+Al) - Ti Exchange in Garnet Garnet Compositions in a Three Garnet Rock 5-17 5-19 5-21 5-29 Fig 6.1 Fig 6.2 Fig 6.3 Advection of Ti Across a Boundary Layer Solubility of Ti Oxides Tt Concentrations in Natural Waters 6-11 6-11 6-13 Fig 7.1a Fig 7.1b Fig 7.2 Fig 7.3 Kinematic Model for Dunite Kernel Rotation Model for Development of Shear Planes in Harzburgite Late Miocene Plate Configurations Tectonic Windows in the Coast Ranges 7-17 7-19 7-25 7-27 List of Tables Table 1.1 Table 1.2 ' Whole-Rock Analyses of the New Idria Syenite Amphibole in the New Idria Syenite 1-34 1-35 Table 3.1 Table 3.2 Table 3.3 Table 3.4 Table 3.5 Table 3.6 Table 3.7 Table 3.8 XRD Data for Serpentine Sample 989-45 Whole Rock Analyses of the New Idria Serpentinite Petrographic Identification of Serpentine Microtextures Spinel-Norm Calculation of Serpentinite Analyses Representative Pyroxene and Olivine Compositions Chromite Compositions Kammererite (Cr-chlorite) Analyses Chromian Garnet Analyses 3-84 3-85 3-86 3-87 3-94 3-96 3-97 3-98 0-14 Table 3.9 Table 3.10 Blue Amphibole Chemical Analysis and Site Assignments 3-99 Blue Amphibole XRD Data 3-100 Table 4.1 Table 4.2 Table 4.3 Table 4.4 Table 4.5 Table 4.6 Table 4.7 Mctamorphic Diopsidc Compositions Chlorite Compositions at 34-50 Locality Chlorite Compositions at Perovskite Knob Miscellaneous Mineral Compositions Bulk Compositions of Chlorite-Diopside Rocks Spinel-Norm Calculations for Chlorite-Diopside Rocks Bulk Compositions for Mafic Schists and Greenstones 4-116 4-117 4-118 4-119 4-120 4-121 4-134 Table 5.1 Preferred New Idria Garnet Analyses Table 5.2 Additional New Idria Garnet Analyses Table 5.3 " Compositions and Colors for 51 New Idria garnets 5-37 5-51 5-63 Table 6.1 Table 6.2 Tx Concentrations in Natural Waters Experimental Ti Oxide Solubility Data 6-34 6-35 Table 7.1 Correlation Chart for Mineral Generations 7-31 0-15 To Louisa and Emily 0-16 Acknowledgements The successful completion of this thesis is due in large pan to the good advice and counsel that I received from a large number of people. In this section I will try to acknowledge these contributions as best I can, but inevitably some people will be left off the list. To these unnamed people, thank you nonetheless. First I want to thank Prof. Heinrich D. Holland, my principal advisor, for his unflagging suppon and assistance, and for the battles on the back of envelopes, as described in song by Tony Lasaga. I was very fortunate to be able to show Dick something of New Idria, albeit under ferocious weather conditions, during a post-GSA field trip in 1991. Next, I am indebted to Prof. James B. Thompson, Jr., who provided early encouragement and wisdom in the thesis project, as well as theoretical and field-oriented guidance during the course of the investigations. When I began this study, Jim was my principal advisor, at the time of his retirement and change to emeritus status, he was succeeded in this role by Dick Holland. This change was a bonanza to me as a graduate student, because thenceforth I enjoyed, in practice, the benefits of two principal advisors. Their encouragement, seemingly limitless patience, and keen insights from complementary points of view, were critical success factors in the study. Next, I wish to acknowledge the long-time friendship and support of Prof. Charles W. Burnham. As of this writing I have known Charlie for 29 years; it was he who first encouraged me to take up the study of geology in a rigorous fashion. During these years he has been a mentor and unfailing friend. During the thesis project he was a continual source of mineralogical wisdom, and of reality checks on some of my more outlandish hypotheses. This thesis would never have been written without the impetus provided by Elizabeth A. Zbinden, of Sranford University. Elizabeth introduced me to the New Idria District, and equally importantly, to many fine colleagues at Stanford. She served as my field assistant during the 1989 field season, and as a good friend during some of the discouraging times that seem to be an inescapable part of graduate study. 0-17 Dr. David S. O'Hanley of the University of Saskatchewan played a key role in this study. Dave introduced me to the complexities of serpentine textures and phase relations. I was pleased to visit New Idria on two occasions with him. At one time or another I consulted about my project with nearly every faculty member in the Dept, of Earth and Planetary Sciences at Harvard; in particular I want to thank Visiting Prof. Theresa S. Bowers and Profs. Goran Ekstrom, the late John Haller, Stein B. Jacobsen, Ulrich Petersen, Roberta Rudnick, Jane Selverstone, Raymond Sicver, and Joann Stock. Many members of the professional staff of the EPS Dept at Harvard, also contributed to my project. These include Dr. Antje Danielson, Joan Donahue, Dr. Carl Francis, Dr. Joan Fryxell, Dave Lange, Bill Metropolis, Dr. Ron Pflaum, Debbie Rich, Dr. Ed Roedder, Pauline Solomon, Harold Thompson and Connie Wick. Special thanks go to Larry Pitman and the corps of volunteers at the Harvard Mineralogical Museum. Additional thanks go to Pete Dawson of U. Mass. Amherst. Much of the field work for this study was supported by grants from the Reginald Daly Fund at Harvard, as well as the Graduate Student Council. I am indebted to the Geology Dept at Stanford for providing an intellectual "base camp" and the use of their field vehicles for my investigations at New Idria, which is about a four hour drive from the Stanford campus. I particularly want to thank Profs. Gordon E. Brown, Jr., Robert G. Coleman, George A. Parks, and Jonathan F. Stebbins for their encouragement and advice. I also want to thank Dr. Barbara M. Bekken and Prof. Michael F. Hochella for intellectual and logistical support. Many faculty members at other universities and government institutions also helped significantly. In particular I wish to acknowledge John Ayers (Vanderbilt), Rob Berman (GSC), Wally Bothner (UNH), Bob Christiansen (USGS), Barb Dutrow (LSU), Marta Flohr (USGS), Reto Gier (ETH), Tim Grove (MIT), Chuck Guidotti (Maine), George Guthrie (Los Alamos), Bob Jachens (USGS), Jo Laird (UNH), Marvin Lanphere (USGS), Mac Ross (USGS), George Rossman (Cal Tech), Cathy Skinner (Yale), Joe Smyth (Colorado), and Walt Trzcienski (Montreal). Many ranchers, miners, and government officials in the New Idria area also helped my cause. Some of these include Buzz Gray (Gem Mine), Dave and Sharon Teague (Los 0-18 Gatos Creek Ranch), Roger Hopper and John Myers (KCAC), Dan Meer and Dick Procunicr (EPA, San Francisco), and Tim Moore and Ron Smith (BLM, Hollister). No graduate student can really survive without the friendship and support of fellow-traveler students. I particularly wish to acknowledge the friendship and useful scientific conversations with John Amason (Stanford), Gary Axen, Ximena Barrientos, Bill Carey, Lou Derry, Chip Green, Hany Hummel, Raymond Jannas, Carlos Jove (Stanford), Ro Kinzlcr (MIT), Claudia Lewis, Mark Linker, Suzanne MacEnroe (U. Mass.), Andy MacFarlane, Othmar Mllntener (ETH), English Pearcy, Kirsten Peters, Rob Rye, Giovanni Ruggieri (Firenze), Zeke Snow, Naylor Stone and Marc Willis (Washington). My field work at New Idria was made more pleasant and productive through the assistance of Mr. Stephen L. Hansen and his wife Deborah, and their three daughters Anna, Sarah, and Naomi. The Hansens provided food and lodging at their remote cabin, and tireless field assistance throughout the area. I wish also to acknowledge the support and encouragement of present and former members of the Harvard administration in this project, particularly Dale P. Flecker, Robert H. Scott, Stephen C. Hall, Howard L. Resnikoff and Ronald Orcutt In the end of course it was the patience and understanding of my family that made the entire effort both possible and enjoyable. My wife Louisa, my daughter Emily, my father Joseph and my sister Frieda were a limitless source of inspiration and unstinting love to me; without their help, all my efforts would have been for nought. Cambridge, Massachusetts May, 1995 Frontispiece - Infrared LANDSAT photograph of the central California coast from Monterey to the San Joaquin Valley, September 6,1992. The San Andreas Fault traverses the image from lower right to upper left The oval New Idria Serpentinite and the surrounding anticlinal structures protrude, thumb-like, eastward from the Diablo Range in the right central portion of the image. Photograph courtesy of EROS Data Center, Sioux Falls. S.D. _______ _____________________________ _____ 0-19 Chapter 1 - Introduction and Regional Geology I. Outline and Statement of Purpose................................................................. l II. The New Idria District of California...................................................3 A. Location.............................................................................................. ....3 B. Description................................................................................................6 C. Index Maps.............................................................................................13 D. Flora...........................................................................................................14 E. Fauna......................................................................................................... 16 III. Parts of the Serpentinite......................................................................17 A. The Serpentinite Massif...........................................................................17 B. The Tectonic Blocks................................................................................ 17 C. The Mercury Mines..................................................................................17 D. The New Idria Syenite....................................................................18 IV. Laboratory Analytical and Computational Techniques............................... 20 A. Petrography..............................................................................................20 B. Mineral Analyses..................................................................................... 21 C. Whole Rock Analyses...................... 24 D. X-ray Diffraction.....................................................................................24 E. Fluid Inclusion Microthermometry................................................24 F. Thermodynamic Modeling..................................................................... 25 G. Nomi Calculations.................................................................................. 25 H. Bond Distance Calculations....................................................................26 I. Mineral Abbreviations............................................................................ 27 V. Conclusions................................................................................................... 28 Appendix A - Glossary of Place Names..................................................................30 Tables .................................................-........... -................................................. 34 Chapter 1 Introduction and Regional Geology Chapter 1 Introduction and Regional Geology I. Outline and Statement of Purpose The major purpose of this thesis is to describe the mineralogy and petrology of the serpentinite massif at New Idria, California. Of particular interest is the evolution of exotic mineral suites contained in tectonic blocks or "knockers" within the setpentinite body. These blocks contain minerals enriched in titanium, an element with extremely low abundance (< 1 ppm) in the bulk serpentinite outside the blocks. Some tectonic blocks are characterized by the assemblage chlorite + diopside + Ti-gamet; these blocks are interpreted as metapyroxenites. Other blocks contain the rare minerals benitoite and neptunite; these blocks are interpreted as altered mafic schists and greenstones; still others contain near end-memberjadeite; following Coleman (1961) jadeite-bcaring blocks arc also interpreted as altered mafic schists. Chapter 1 introduces the New Idria District and describes the methods of study employed. Field and laboratory study of rocks in the southern portion of the serpentinite, along with reconnaissance study of rocks in the northern portion, were undertaken in the years 1988 1993. Chapter 2 describes in the history of scientific work at New Idria; the District has been the focus of investigations with a variety of purposes for nearly 150 years. These include mining studies (including oil exploration), mincralogical studies of rare minerals, geophysical studies (especially since the destructive Coalinga earthquake of 1983), and most recently asbestos-related studies by the Environmental Protection Agency. 1-1 Chapter 3 discusses the mineralogy and petrology of the serpenrinite as a whole, exclusive of the tectonic blocks. The protolith of the serpenrinite is seen to be dunitc with lesser amounts of harzburgite and other ultramafic rock types. The entire massif is interpreted as an ophiotitic flake of oceanic crust or underlying mantle, of Jurassic to Cretaceous age. Chapter 4 describes the mineralogy and petrology of the tectonic inclusions in the serpenrinite. These inclusions are subdivided into three groups: metapyroxenites, metamorphosed Franciscan mafic schists and greenstones, and anrigorite knockers. A model is proposed for the formation of exotic minerals in the blocks as products of essentially isochemical metamorphism. Chapter 5 discusses the most prominent of the titaniferous minerals of the District, garnet. Approximately 300 chemical analyses of garnets and coexisting minerals are presented, along with a discussion of chemical variation using exchange vectors and crystal chemical constraints. Chapter 6 discusses the possibilities for mobility of titanium in aqueous fluids as a factor in the paragenesis of titaniferous minerals. Ti mobility on a length scale of only about one meter can be shown at New Idria. Comparative studies of Tt mobility are also discussed. Chapter 7 offers a regional interpretation for the evolution of the New Idria Serpenrinite. The chapter also summarizes the conclusions of the thesis and lays out a framework for further investigations. Each of these chapters contains a chapter table of contents and a brief summary of the regional geology, so that it is possible to read the chapters in any order. 1-2 II. The New Idria District of California A. Location The New Idria District includes portions of San Benito and Fresno Counties: it is roughly bounded on the north and south by the towns of Panoche and Coalinga respectively (Figure 1.1). The District lies between the western margin of the San Joaquin Valley and the San Benito River in the southern portion of the Diablo Range, a constituent member of the Coast Ranges Province of California. (Page, 1966). The Diablo Range, as defined by Anderson & Pack (1915), forms the eastern margin of the Coast Ranges between Carquinez Straits at San Francisco Bay and Polonio Pass in San Luis Obispo County, a distance of some 300 km. The highest summits of this range lie within the District: wooded San Benito Mt. (1579 m.) and barren Santa Rita Peak, (1574 m.). The eastern side of the Diablo Range forms a precipitous escarpment; the relief from the summit of Santa Rita Peak to the junedon of Cantua Creek and Arroyo Leona, 7 km to the cast, is over 1000 m. The total relief from the crest of the range to the San Joaquin Valley 25 km. east is 1500 m. The New Idria District is sparsely inhabited, mainly by isolated ranchers living in the valleys. Only one settlement lies fully within the District: New Idria, also known as Idria, now nearly a ghost town, at the site of the abandoned New Idria merrily mines. Drainage of the region is to the west, via the San Benito River, and to the east, via Cantua Creek and a network of intermittent streams descending to the San Joaquin Valley and the California Aqueduct. To the southeast, drainage is also into the San Joaquin Valley, via the perennial Los Gatos Creek and the Arroyo Pasajero. 1-3 Figure 1.1 - Map of Central California, showing the region from Monterey Bay to the San Joaquin Valley. This map covers approximately the same area as the satellite photo in the frontispiece. The New Idria District lies to the east of the San Andreas Fault. 1-4 Physical access to the edge of the District is via paved road, about a four hour drive from San Francisco. The Coalinga-Bitterwater Road, diverging from SR 25 at Bittcrwater, and following the San Benito River and Los Gatos Creek valleys, provides access from the west and south. County Road J1, that runs from Paicines south of Hollister through the Panoche Valley to Idria, provides access from the north. Within the District a bewildering maze of unpaved and generally unmaintained roads makes a four wheel drive vehicle essential in moving about. The Bureau of Land Management (BLM) office in Hollister has current information about access to the District. As of 1993, BLM has proposed to limit vehicular access to what is termed the Hazardous Asbestos Area, that generally coincides with the zone of serpentinite rock. The frontispiece of this study shows a satellite photograph of the central Coast Ranges using infrared film, at a scale of approximately 1:1,000,000. The oval outline of the anticlinal structure containing the New Idria Serpentinite can be seen to the east of the San Andreas Fault. B. Description The term New Idria District, referred to in this study simply as the District, was first used by Becker to describe the general region encompassing the historic mercury mines at New Idria. California (Becker, 1888). The most remarkable feature of the District is an oval, 23 by 8 km, fault bounded, serpentinite massif flanked by steeply dipping, locally overturned, sedimentary and mctamorphic rocks of the Jurassic-Cretaceous Franciscan and the Cretaceous to Pliocene Great Valley Sequence of California (Figure 1.2). The serpentinite is older than the sandstones and shales of the Great Valley Sequence. The outcrop area of serpentinite rock exceeds 150 km2. The New Idria Serpentinite is a 1-6 serpentinite diapir (Coleman, 1957), a pan of the Coast Range Ophiolite in California. The age of the ophiolite is 153 to 165 Ma. (Hopson et al., 1981); its emplacement within the Franciscan Formation is related to subduction of the Farallon plate in the Jurassic. The tectonic setting of the ophiolite was a forearc basin lying between the Siena Nevada volcanic arc to the east and a trench-subduction complex to the west, represented by the Franciscan Formation (Bartow, 1990). A piece of oceanic crust or underlying mantle was obducted onto the North American continent, later to rise through the Franciscan Formation as a serpentinitc diapir. The structural trend of the serpentinite body and related folds and faults is 300, and so is inclined 20 degrees to the San Andreas fault system, that trends 320 in this part of California. Emplacement of the New Idria Serpentinite as a diapir within the Franciscan has resulted in a piercement structure or tectonic window, lying just east of the San Andreas fault system northeast of Parkfield. The serpentinite body was shown by Coleman (1957) to be fault bounded on all sides (Plate 1*2), with the sense of shear where it could be determined showing upward movement of the serpentinite against the country rock. No evidence of contact metamorphism was found. The diapir breached the surface in the Miocene, as recorded by the sudden appearance of serpentine boulder conglomerates in the Big Blue member of the Trembler Formation. Th; New Idria Serpentinite lies in the core of one of a series of anticlines that characterize the Coast Ranges in this pan of California (Arnold & Anderson, 1908; Taliaferro, 1943; Eckel & Myers, 1946; Zigleret aL, 1986). Numerous other partially to completely serpentinized peridotites lie in a similar structural position along the east side of the San Andreas fault in central and northern California. During the Miocene, the New Idria Serpentinite experienced an interval of rapid uplift, sub-greenschist fades metamorphism, and intrusion of small syenite stocks. 1-7 Figure 1.2 - Simplified geologic map of the New Idria District. See the Santa Cruz 1:250.000 geologic map, (Jennings & Strand, 1958) for more detail. 1-8 5 km Plate 1.2 - Photograph of the fault contact between the New Idria Seipentinite and the Cretaceous Great Valley Sequence, at the Corbctt-Byles Mine. 1-10 The mineralogy of the serpentinite is dominated by chrysotile. Brucite, accessory magnetite, and minor lizanlite and antigoritc arc also present The protolith for the serpentinite was a depleted peridotite consisting of dunite with minor harzburgite. Locally within the serpentinite, tectonic blocks of metapyroxenite, greenstone, mafic schist, and massive antigoritc are preserved. These blocks are of limited extent, with sheared contacts against the surrounding serpentinite. They are recognizable in the field by distinct soil color and erosion properties, contrasting vegetation above, and anomalous mineral assemblages within. The bulk composition of the blocks is significantly different from that of the bulk serpentinite. New Idria is one of the largest known deposits of chrysotile asbestos in the world. Indeed, Klein (1993) has proposed that eolian erosion from New Idria is the main contributor to background levels of airborne chrysotile asbestos in the northern hemisphere. At this time, the Environmental Protection Agency (EPA) has designated two sites in the District as Superfund sites under the Comprehensive Environmental Response Compensation and Liability Act of 1980 (CERCLA). At the same time, the only remaining asbestos mine (KCAC) in the United States continues in operation in the District. The District is rich in mineral resources, including mercury, magnesite, chromite, petroleum, asbestos, and gemstone minerals. The District has a rich human history, chiefly on account of these mineral deposits. The most valuable economic commodities have been mercury and petroleum, that occur at the margin or entirely outside the serpentinite body. In California the occurrences of mercury and petroleum are generally linked spatially, and perhaps genetically (Peabody, 1989; Peabody & Einaudi, 1992). 1-12 The New Idria District is renowned as a mineral collecting area, with over 100 mineral species reported to date. The District lies partly in San Benito County and partly in Fresno County, so that New Idria minerals are also known by these geographic names, e.g. San Benito garnets. The minerals for which the District is known are, for the most pan, associated with the boundary faults of the serpentinite or with tectonic blocks of various rock types included in the serpentinite. C. Index Maps 1. Geographical Maps The USGS Santa Cruz 1:250,000 map, the Coalinga 1:100,000 metric topographic map, the 15 minute New Idria quadrangle, as well as the 7.5 minute Santa Rita Peak. San Benito Ml, Idria and Hernandez Reservoir quadrangles, cover the area described in this study. 2. Geological Maps There is a deanh of published geologic maps at suitable scales for work in the New Idria District. A State geologic map or especially the 1:750,000 Isostatic Residual Gravity Map (Roberts et al., 1990) are needed to place the New Idria District in its regional context. The 1:250,000 Geologic Map of California, Santa Cruz sheet (Jennings & Strand, 1958) provides further details of the southern Diablo Range. The general map of the New Idria Serpentinite produced by Eckel & Myers (1946) is thus far the only complete geologic map of the serpentinite. and as such has been used as a base map for other geologic compilations for the past 50 years. Unpublished maps by T.W. Dibblee (Dibblee, 1971, 1979) are useful in the study of the rocks immediately adjacent to the serpentinite. 1-13 Several of the engineering and soil maps prepared for the EPA by Levine-Fricke Consultants (Levine-Fricke, 1989) are also useful. This study presents a geologic map of the Santa Rita Peak quadrangle, at a scale of 1:24,000 (Plate 1.1, in pocket). This map is compiled from field work during this study as well as from the above sources. 3. A Note on Place Names Description of a remote area is made more difficult by the lack of commonly accepted place names. The geological and geophysical literature may use different names for the same features, confounding the reader. For example, in the asbestos industry the New Idria District is more generally known as the Coalinga asbestos deposit. Some of the EPA studies have used the name Sew idria Formation in reference to the New Idria Serpentinite exclusive of the Franciscan rocks. This term is inappropriate, and its use is discouraged. In this study, official names are used where available, but informal names are used as well. A glossary of place names is included as an appendix to this chapter. D. Flora Approaching New Idria from a distance, one is struck by the overall white color of the ridges and open slopes of the serpentine barren, in contrast with the red-brown color of the vegetation and soil of surrounding areas that have different bedrock types. Likewise, there is a strong contrast in the vegetation that grows on the serpentine soil: a unique serpentine chapparal replaces open oak woodland (Kruckeberg, 1982). Brewer (1861) commented on the desolate nature of the region but did not connect the vegetation with the unusual soil type. At the present time, the vegetative cover varies considerably within the region, perhaps due to the pattern of recent forest fires. 1-14 To the non-specialist, the most prominent plants of the arid serpentine barren are the scrubby chaparrals that grow on the mountain slopes and frequently make access to outcrops a difficult struggle. Characteristic plants are the orange-barked manzanita with red berries, Arctstaphylosfranciscam; the scrubby oak Quercus durata; and the spiny buckthorn Ceanothusjepsonii. Where trees arc found on wooded slopes, they are a mixture of pines bearing pineapple-sized cones, mainly the digger Pinus sabirtiana, but locally are the jeffrey P.jeffreyi, the coulter P. coulteri, and hybrids of these two (Dann, 1988). Occasionally cypresses are found; Cupressus sargentii is the characteristic species. A variety of small bushes and grasses cling to life on otherwise barren slopes. The small Parishes buckwheat Erigonum parishii has hardly any foliage, while the aromatic rabbit bush Chrysothamnus graveoleus is able to colonize the wastelands of abandoned mines, e.g. at the J.M. Christie mine. This characteristic should be noted by those attempting reclamation of mining land. Recently the San Benito evening primrose Camissonia benitensis has been designated a threatened species under the Endangered Species Act, which means that special management concents exist for its habitat, which is concentrated in the Gear Creek watesshed at the northern end of the serpentinite. Some of the larger tectonic inclusions within the serpentinite support a "normal" vegetation for California, e.g. the live oak Quercus agrifolia.. The occurrence of these prominent trees allows the geologist to locate soil, and hence bedrock, of a contrasting type within the serpentinite. From any of the higher summits, the margin of the serpentinite can be easily recognized once this vegetation contrast is understood. 1-15 Soils developed on serpentine are shallow and stony, typically enriched strongly in Mg and the heavy metals Cr and Ni, but seriously depleted in Ca, K, N and P, making normal plant growth difficult. The soil pH at New Idria is ca. 7.2 (Kruckeberg, 1982). Locally, however, acid mine drainage from the mercury mines at Idria produces a stream pH as low as 1.7. E. Fauna Little work has been done on the endemic fauna of serpentinites, but the visitor immediately notices the general absence of mammals, particularly small rodents. During four field seasons the author never had any food snitched by furry creatures at a campsite. The lack of an important food source for snakes contributes to their low population in the serpentinite. While rattlesnakes axe abundant in the surrounding hills, they are generally absent within the serpentinite, and where found tend to be living on one of the tectonic blocks, e.g. at the Gem Mine. An exception is the two-striped garter snake, that has a small habitat around Spanish Lake. Small lizards are ubiquitous. During the hot season (June - October) flies are occasionally annoying. The characteristic birds of the District arc the frequently heard but seldom seen wientit, along with the scrub jay, plain titmouse, townsend's warbler, olive-sided flycatcher, red tailed hawk, and turkey vulture. In the surrounding valleys one also sees the yellow billed magpie. California quail, mourning dove, roadrunner, rough winged swallow, redshafted flicker, and western bluebird. 1-16 III. Parts of the Serpentinite In this study the New Idria Serpentinite is subdivided into its constituent pans for convenience. A. The Serpentinite Massif The mineralogy and petrology of the serpentinite and its ultramafic protolith are discussed in Chapter 3. B . The Tectonic Blocks The mineralogy and petrology of the tectonic blocks entrained by the serpentinite are the subject of Chapter 4. C. The Mercury Mines As described in Chapter 2, it was the 1852 discovery of the cinnabar deposits at New Idria that brought this region to the attention of the outside world. The mines became in time the largest operating mercury mines in the United States, prior to closing for environmental reasons in 1972. The mercury mineralization is chiefly cinnabar with minor metacinnabar, and is fault controlled. The deposits are hydrothermal in nature. Peabody (1989) and Peabody & Einaudi (1992) proposed that the mercury has been remobilized from sedimentary rocks of the Great Valley Sequence. The geology of the mercury mineralization at the New Idria Mines has been described by Henderson (1965). The technology of mercury mining 1-17 is ihe subject of numerous reports in the Bulletin of the California Bureau of Mines and some of the older references that are given in Chapter 2. This study does not deal per se with the mercury mines, but it is important to note their physical placement around the margins of the serpentinite, astride the boundary faults. A few mines are located within the serpentinite body. These may be related to throughgoing faults that are parallel or subparallel to the eastern boundary fault This fault is generally known as the New Idria Thrust Fault (Plate 1.1). Further indication of the existence of throughgoing faults is suggested by the 2 km.-Iong row of silicified serpentine outcrops near Spanish Lake, also shown in Plate 1.1. In these outcrops, chalcedony or locally quartz has apparently precipitated from fault-related fluids. D. The New Idria Syenite The New Idria Serpentinite is intruded by small syenite stocks of Miocene age, here collectively named the New Idria Syenite. Stream boulders of this syenite were noted by Anderson & Pack (1915), and their source was traced to the headwaters of White Creek. The original description of the syenite was of this stock in the White Creek valley, hence the body was named the White Creek Stock (Robertson, 1940). Since there are additional outcrops not in the White Creek drainage, it is here proposed that that name be abandoned in favor of the name New Idria Syenite. Significantly, these stocks form a linear array on the map (Plate 1.1); their emplacement may be partially fault controlled. It is significant that the White Creek Stock intrudes the boundary between the serpentinite and the adjacent Panoche Shale. Coleman (1957) also described the New Idria Syenite. The New Idria Syenite is a soda syenite, composed almost entirely of sodic feldspar and kaersutite (formerly barkeviktite) amphibole. The rock is very coarse grained in places. 1-18 with amphibole crystals up to 30 cm. long. Elsewhere the rock is fine-grained with only millimeter-sized amphibole crystals. Enclaves of coarse grained rock are found in fine grained rock, and vice-versa. In places, olivine and pyroxene are found; Coleman (1957) described that facies as camptonite. Accessory minerals include biodte, apatite, dtanitc (sphene), magnetite, ilmenitc, pyralspite garnet and zircon. Deuteric and later alteration products include zoisite, prchnite, zeolites, white mica, chlorite and calcite. Table 1.1 presents some whole-rock analyses of the syenite. A CIPW norm calculation shows that in all cases the rock is nepheline normative. Table 1.2 presents some kaersutite amphibole analyses. The abundance of tectonic blocks of several types in the New Idria Seipentinite raises the question of whether the syenite outcrops arc intrusive or not At a large syenite outcrop named The Nose, located to the west of Perovskite Knob, the seipentinite surrounding the syenite has been contact metamorphosed: both antigorite and prograde olivine are present. The aureole grades outward to the "normal" seipentinite rock consisting of chrysotile and lizardite. This observation shows that the contact is indeed intrusive; the margins of other tectonic blocks do not have contact aureoles. The age of the New Idria Syenite has been determined by Marvin Lanphere of the U.S. Geological Survey using the ^Ar/^Ar total fusion technique on an amphibole mineral separate. This unpublished age is 12.4 0.8 Ma (M. Lanphere, written communication, 1993). Such an age is consistent with the intrusive nature of the contact, assuming that the serpentinite as a whole has a late Jurassic age. The occurrence of the prehnite-zoisite pair in deutericaily altered syenite provides a pressure estimate at the time of alteration. The stability fields of prehnite and zoisite 1-19 overlap only between 1 and 3 kb (Frey ct al., 1991). Therefore the outcrop presently at the surface lay at a depth of 3.5 to 10 km. at the time of alteration, assuming an average density of 2.8 g/cm3 for the rock formerly overlying the outcrop (the average density of several syenite samples was determined to be 2.8 g/cm3 using a Jolly balance; the density of serpentinite was found to be 2.4 to 2.8 g/cm3). This is the only quantitative pressure estimate obtained during the course of this study, although mineral parageneses described in Chapters 3 and 4 provide general estimates. If the 12.4 Ma age of the syenite measured by Lanphcre is close to the time of deuteric alteration, which is reasonable for a small intrusion, then the unroofing rate since the late Miocene may be calculated. The average unroofing rate is calculated to be 0.3 to 0.8 mm/year for the past 12 million years. This calculation agrees well with the estimated rate of 1 mm/year for the Coast Ranges, obtained using other data (Ring & Brandon 1994). IV. Laboratory Analytical and Computational Techniques Field and laboratory study of rocks in the southern portion of the serpentinite, along with reconnaissance study of rocks in the northern portion, were undertaken in the years 1988 1993. Sample locations are shown in Figure L3. In this study several analytical techniques have been used, which arc described below. A. Petrography Approximately 100 thin sections were studied using an Olympus BH2 polarizing microscope. With few exceptions the thin sections were prepared by Mr. Harold Thompson of Harvard University. 1-20 B. Mineral Analyses Harvard's Camcca MBX electron microprobe with three crystal spectrometers and Tracor automation and Bence-Albee matrix correction was used extensively. The microprobe laboratory runs under the direction of Mr. David Lange. Normal operating conditions for anhydrous minerals were 15 kV accelerating voltage, 15 nA absorbed current, and a point beam. Hydrous minerals were analyzed using a 2K raster beam, with 800X and special calibration used for some serpentine samples. WDS standards used were the natural or synthetic silicate standards at Harvard. REE standards were specially obtained for this project from the Smithsonian Institution (Jarosewich & Boatner, 1991). EDS spectrometry was used frequently for phase identification and qualitative analyses. All specimens were examined using BSE prior to chemical analysis. In an effort to understand the uncertainty of element concentrations measured in this study, a project working standard was chosen early in the study. This standard was a point in a relatively homogeneous portion of a Ti-rich garnet; the point was repeatedly analyzed for during the course of the study to determine precision and accuracy. Error analysis shows that nearly all of the variability in analyses of the working standard can be explained by the counting statistics in the detectors on the crystal spectrometers. The compositions reported in this study are considered to be accurate to approximately 1 2% of the amount reported. Chapter 5 discusses compositional errors further. 1-21 Figure 1.3 - Sample locations in the Santa Rita Peak 7.5 minute quadrangle. A: 34-50 locality; B: Perovskite Knob; C: Mclanite Mine; D: The Nose; E: Gem Mine; F: 990-1 locality; G: The Teeth; H: KCAC Mine; I: Atlas Mine site. 1-22 Figure 1.3 - Sample Locations in the Santa Rita Peak Quadrangle I km C. Whole Rock Analyses The Siemens MRS 400MP and Siemens SRS gold tube XRF spectrometers at the University of Massachusetts, Amherst, were used for whole rock analyses, major and trace respectively. This laboratory runs under the direction of Dr. Michael Rhodes and Mr. Peter Dawson. Sample preparation procedures were according to University of Massachusetts standards, following the techniques of Gardner (1990). Duplicate analyses using dual standards were performed. D. X-ray Diffraction The Scintag XDS 2000 diffractometer at Harvard was used in this study for large samples. Cu- Ka radiation was used; operating conditions were 45 KV, 35 Ma, 1 per minute scan rate for most continuous scans. This laboratory operates under the direction of Prof. Charles W. Burnham. Small crystals were studied with a Gandolfi camera mounted on a Philips X-ray generator, also using Cu-Ka radiation at 45 KV. Mr. Lawrence C. Pitman performed Gandolfi X-ray experiments for the author. E. Fluid Inclusion Microthermometry Fluid inclusion measurements were done using the Fluid, Inc. modified USGS heating/freezing stage at Harvard. Calibration using synthetic fluid inclusions was completed prior to the measurements reported in this study. The precision and accuracy of measurements arc considered to be 0.1 C and 1.0C respectively. 1-24 F. Thermodynamic Modeling Some modeling of mineral equilibria was done using the SUPCRT (Helgeson et aJ., 1978) and SUPCRT 92 (Johnson et al,, 1991) computer codes, running on a Sun 386i workstation or a Macintosh computer respectively. Thermochemical data for a few phases were added to the SUPCRT database from various external sources including Robie etal., (1978) and Berman (1990). TheTWQ2S (Berman, 1991) computer code, also known as GEOCALC, was used for calculations of mineral equilibria in Chapters 3 and 4. This code was run on a Sun 386i workstation. G. Norm Calculations CIPW norm calculations were performed using an Excel spreadsheet on a Macintosh computer, and using the program NORMCALC obtained from the Geology Dept, at Stanford University. NORMCALC ran on a VAX computer at the Harvard Psychology Dept. The CEPW norm calculation (Cross et al,, 1902) was originally designed for feldspar-bearing rocks that crystallized at low pressures, and is not appropriate for high pressure rocks that crystallized above the stability field of anorthite. A modified Excel calculation procedure was developed for these rocks by the author, in which low pressure phases were replaced by their high pressure equivalents. Albite was replaced by jadeite, anorthite by spinel + clinopyroxene. A few extremely calcic rocks also required a Active wollastonite component to accommodate Ca; in actuality such rocks would accommodate excess Ca in garnet. A computer code widely used in Switzerland (MANNOR, written by P. Ulmer) has adopted a similar strategy. 1-25 H. Bond Distance Calculations The BDTEA computer code of L.W. Finger was used for bond distance calculations in Chapter 3. 1-26 I. Mineral Abbreviations In this study the abbreviations for minerals recommended by Kretz (1983) have been used. Note that abbreviations for mineral names are capitalized but abbreviations for chemical components are not capitalized, to distinguish them from mineral names. The following table lists the principal abbreviations employed: Name andradite anorthite antigorite brucitc chlorite chromite chrysotile diopside enstatite forsterite hedenbergite garnet grossular magnesite magnetite olivine spinel talc tremolite Abbreviation Adr An Atg Bit: Chi duCtl Di En Fo Hd Grt Grs Mgs Mag 01 Spl Tic Tr 1-27 V. Conclusions The main points of this chapter that will be used in other chapters are: 1. The New Idria Serpendnite is a serpentinite diapir, a pan of the Coast Range Ophiolite in California. The age of the ophiolite is 153 to 165 Ma. (Hopson et al., 1981); its emplacement within the Franciscan Formation is related to subduction of the Farallon plate in the Jurassic. The serpentinite is a tectonic window in the Coast Ranges. 2. The mineralogy of the serpentinite is dominated by chrysotile; New Idria is one of the largest deposits of chrysotile asbestos in the world and the site of the only operating asbestos mine in the United States at the present time. 3. The New Idria Serpentinite contains small syenite stocks of Miocene age, here collectively named the New Idria Syenite. The syenite is shown to have an intrusive relationship with the surrounding serpentinite. In the contact aureole around the intrusive syenite, prograde olivine replaces antigorite. The aureole grades outwards to a mixture of chrysotile and lizardite. 4. The age of the New Idria Syenite has been determined by Marvin Lanphcre to be 12.4 0.8 Ma, using the <Ar/"Ar total fusion technique on an amphibole mineral separate from the syenite. 5. Coexisting prehnite and zoisite in deuterically altered syenite provide a pressure estimate at the time of alteration. The stability fields of prehnite and zoisite overlap only between 1 and 3 kb (Frey et al., 1991). Therefore the outcrop presently at the surface lay 1-28 at a depth of 3.5 to 10 km. at the time of alteration, assuming an average density of 2.8 g/cm^ for the rock formerly overlying the outcrop. 6. If the 12.4 Ma age of the syenite measured by Lanphcre is close to the time of deuteric alteration, which is reasonable for a small intrusion, then the unroofing rate since the late Miocene may be calculated. The average unroofing rate is calculated to be 0.3 to 0.8 mm/year for the past 12 million years. This calculation agrees well with the estimated rate of 1 mm/year for the Coast Ranges, obtained using other data (Ring & Brandon 1994). 1-29 Appendix A - Glossary of Place Names All localities, unless otherwise noted, lie in the USGS Santa Rita Peak 7.5 minute quadrangle map. Atlas Mine: The large open pit mine and abandoned mill of the Atlas Asbestos Co. cover several acres on the south-facing escarpment of the serpentinite in the White Creek drainage. A gated road from Spanish Lake that descends to the White Creek valley transects the mine site. The mine and mill are now an EPA Superfund site. Buder Estate Pit: This moderately large, nonh-facing, U-shaped excavation was formerly a chromite mine. The mine, also known as the Mistake Mine, lies in the extreme southeast comer of the serpentinite in the Pine Creek drainage. The New Idria thrust fault crops out on the headwall of the pit. J.M. Christie Pit: This large open pit asbestos mine, formerly operated by Johns ManviUe, lies high on the western slope of Wright Mt. in the drainage of the Arroyo Leona. Corbett-Byles Pit: This moderately large open pit was formerly a chromite mine. The mine lies in the southeast comer of the serpentinite in the White Creek drainage. The New Idria thrust fault crops out on the headwall of the pit. The Archer Mine, a mercury mine, lies in the fault zone directly underneath the Corbett-Byles pit. 1-30 Gem Mine: This small open pit benitoite mine lies at the toe of a ridge extending southwest from the summit of Santa Rita Peak, in the valley of the San Benito River. The mine is a patented claim and is marked by a sign warning of private property. KCAC Mine: This large open pit asbestos mine is operated by the KCAC Co. of King City, California. The mine was formerly operated by the Union Carbide Co. The mine lies on the western slope of Santa Rita Peak and can be reached via a gated, private paved road from the Coalinga-Bitterwatcr Rd. The private road is known as the KCAC Road. Hill 4857: This prominent unnamed hill is marked on the USGS Santa Rita Peak 7.5 minute quadrangle map with its elevadon. The hill is south of Santa Rita Peak, east of Condon Peak, and west of the Atlas Mine site. Perovskite Knob lies on the western slope of Hill 4857, the Melanite Mine on the north. Melanite Mine: This small excavation on the north slope of hill 4857 lies in a gully above the south branch of the Santa Benito River, which splits into a north and a south branch just upstream of the Gem Mine. New Idria Thrust Fa lit: This high angle thrust or reverse fault forms the northeastern boundary of the New Idria Serpentinite. The fault represents a breached anticline. The continuation of the anticlinal axis to the southeast is known as the Coalinga Anticline, and still further to the south the Kettleman Hills Anticline. Nose: This name is given to the prominent syenite dike that crops out on the north side of Snakebite Hill directly uphill from a 90 degree bend in the San Benito 1-31 River where its course alters from south to west. Large syenite boulders are found at the stream bend. This is the dike that was mapped by Coleman (1957). Perovskite Knob: This prominent, 15 m. high knob lies on the western slope of Hill 4857 and looks westward to Snakebite Hill. The Santa Rita Peak. Condon Peak, the Nose and the Teeth are visible from Perovskite Knob. Snakebite Hill: This name is given to a low, twin-summited hill that lies between Hill 4857 and Condon Peak. The name comes from the map pattern on the quadrangle map, on which the contour lines about the twin summits resemble a snake bite. The hill may be reached via a very rough 4WD road from the west. Teeth: This name is given to a pair of 3 m. high resistant serpentine knobs that lie on a ridge that lies to the west of and above the KCAC Mine, facing southwards towards the San Benito River valley. The Teeth can be reached by a rough 4WD road leading directly uphill to the west from the KCAC Mine road at the point where it enters the mine pit. About 100 m. to the north of the Teeth lies the syenite/camptonite outcrop mentioned by Coleman (1957). As of this writing the syenite outcrop has completely disintegrated and the only sample fragments for this study were obtained by uprooting a bush and digging a small pit Victor Claim: This small staked claim consists of set of outcrops lies on the north slope of San Benito Mt., in a side valley off the Clear Creek Valley, approximately one km. from the Clear Creek Road and 0.5 km upstream from a small mercury mine. The Victor Claim is marked by a monument post, about 10 m. above the bed of a small stream. The locality is found on the USGS San Benito Mt 7.5 minute quadrangle map. 1-32 The 34-50 locality: This set of four rocky knobs, 2-3 m. tall, lies at an elevation of about 1300 m. on a southward projecting ridge above the bed of the San Benito River just to the south of the KCAC Mine and just to the west of the Gem Mine. The locality is reached via a short spur road. The name is the one given by R.G. Coleman in 1950. 1-33 Table 1.1 Whole-rock analyses of the New Idrla Syenite Sample -> location -> 1 989-8 White Crk. 2 345 989-39 A&A, 1910 Coleman Coleman The Nose White Crk. White Crk. White Crk. Si02 Ti02 AI203 Fo203 FeO MO MO CeO Na20 K20 P205 H20> H20Tojal wt. % 51.46 1.53 18.56 8.42 n.d. 0.14 2.92 6.11 9.89 0.42 0.50 n.d. n.d. 99.94 47.85 2.44 15.38 10.61 n.d. 0.16 9.45 9.03 3.27 1.47 0.14 n.d. n.d. 99.79 60.00 0.42 16.88 1.83 3.02 0.12 1.40 3.16 9.31 0.94 0.14 1.53 0.43 99.18 48.96 2.01 15.72 0.36 9.65 0.17 6.52 7.76 5.22 0.75 n.d. 2.29 0.42 99.83 51.42 2.20 16.58 1.48 7.62 0.23 3.68 6.77 6.04 1.06 0.49 1.41 0.34 99.32 Trace (ppm) l\b Zr Sr Zn Ni Cr V Ce Ba La 39.87 260.83 836.15 90.41 9.28 0.00 64.68 58.98 562.37 23.00 25.30 123.50 410.35 88.53 24.03 0.60 314.23 30.08 441.39 11.61 n.d. n.d. n.d. 300 n.d. trace 200 n.d. 800 n.d. n.d. n.d. n.d. n.d. n.d. 0 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 600 500 trace n.d. n.d. n.d. Analyses 1&2 anhydrous; all Fe expressed as Fe203 In analysis 3. CO2-0.59 wt. % In analysis 5. CO2n0.45 wt. % Analyses 1 & 2 (this study) Analysis 3 from Arnold & Anderson (1910) Analyses 4&5 from Coleman (1957) 1 . I Table 1.1 Tabla 1.2 Amphlbole In tha Naw Idrla Syenite Sample > Si02 TI02 AI203 Cr203 Fe203 FaO* MO MO CaO ZnO BaO Na20 K20 F Cl H20 (dill) Totals 990-39.3 39.81 3.2S 13.40 0.00 n.d. 9.67 0.10 14.45 11.20 0.02 0.02 2.66 0.85 0.01 0.02 4.54 100.00 990-39.4 40.65 3.31 13.65 0.00 n.d. 9.99 0.10 14.60 11.16 0.00 0.01 2.62 0.86 0.01 0.02 3.02 100.00 note: all Fe as FeO Tabla 1.2 Chapter 2 History of Scientific Investigations of the New Idria District Chapter 2 - History of Scientific Investigations of the New Idria District Prologue: ....................................................................................................... 1 I. Geological studies prior to 1950.............................................................. 1 II. S tudies of Coleman and others, 1950 onward.........................................8 III. Mincralogical studies.................................................. ...........................9 IV. More Mining Studies.................. 10 V. Geophysical Studies, 1943 onward........................... 10 VI. Studies by the EPA, 1982 onward .......................................................... 13 Epilogue: ........................................................................................................14 Chapter 2 - History of Scientific Investigations of the New Idria District Prologue: Two sixteenth century events separated by only four years set the backdrop for studies of the New Idria District. In 1542, Juan Cabrillo became the first European to sight California (Morison, 1974). He sailed as far north as Bodega Bay, and anchored in the shadow of the serpentine-rich Franciscan hills. In 1546, the first modem text on mineralogy, De Fossilium Naturum, was published in Basel by Georg Bauer, writing under the pseudonym of Georgius Agricola. In this book. Agricola first used the term serpentenaria to refer to serpentine rock. The histories of California and serpentenaria have been intertwined ever since. One of the mysteries of the early exploration of California is that the Golden Gate remained undiscovered by coastal sailors, including the redoubtable Sir Francis Drake, for over two hundred years: this narrow pass through the Franciscan Formation of the Coast Ranges, and the bay of San Friincisco behind it, were discovered from land by the Portola expedition in 1769. The Fages expedition of 1772 was the first group to traverse the lands to the south and east of San Francisco Bay, and so discovered the San Benito River. This river was forded and named by Fray Juan Crespi, a member of the Fages expedition (Bolton, 1927). The New Idria District lies at the headwaters of the San Benito River, and benitoite from the District has become the California state gemstone. I. Geological studies prior to 1950 Almost eighty years later in 1851, a silver prospector named Jesse Smith made the inadvertent discovery of cinnabar in the New Idria District, which opened the rich mining history of this region (Gilbert, 1984). Smith established the Aurora Silver Mine, which 2-1 promptly went under, owing to the absence of silver. Further exploration in 1854, however, led to the opening of the New Idria Quicksilver Mine in 1855. Intrigue and bloodshed accompanying these events are described in a Bret Harte novel. The Story ofa Mine (1877). This mine became one of the largest producers of mercury in the U.S., and together with the nearby San Carlos mine produced nearly 500,000 76 lb. flasks of mercury prior to closing in 1972 for environmental reasons. Gilbert (1984), has written a history of the legal disputes (which were litigated before the U.S. Supreme Court) and mining operations at the New Idria mines. The New Idria District and the mines were visited in 1861 by a party from the Geological Survey of California under J.D. Whitney1 (Whitney, 1865). The region was described and metamorphic rocks including serpentine were mentioned, but the 1873 map produced by the Whitney Survey appears to have been lost to history. W.H. Brewer, a member of the 1861 party, described a forbidding scene in in his journal (Brewer, 1861,1966): "... chain after chain of mountains, most barren and desolate. No words can describe one chain, at the foot of which we had passed on our way - gray and dry rocks or soil, furrowed by ancient streams into innumerable canyons, now perfectly dry, without a tree, scarcely a shrub or other vegetation - none, absolutely, could be seen. It was a scene of unmixed desolation, more terrible for a stranger to be lost in than even the snows and glaciers of the Alps.... How still it was! - no sound of a bird in the evening twilight, no chirrup of insect, but silence, deathly stillness reigned." 1 J.D. Whitney later joined the faculty at Harvard, where he served as Sturgis Hooper Professor of Geology from 1875 until his death in 1896. 2-2 Plate 2.1, a modem view to the northeast from the summit of San Benito Mt., includes a pan of the barren vista described by Brewer. Assuming that the serpentine vegetation in 1861 was roughly comparable to that of the present day, Brewer's description may apply to the arid region that lies to the nonheast in the wind shadow of the New Idria massif. New Idria appeared on the first state geologic map of California, (Plate 2.2, GuilleminTarayre, 1867), but the accompanying text is incomplete, and the extant portion does not mention the mining operations at New Idria. Perhaps in the future both the missing Whitney maps and the GuiUemin-Tarayre text will emerge from some forgotten archive. Early references which deal with New Idria concentrated on mining methods and production statistics for the mercury mines, (e.g. Becker, 1888; Forstner, 1903; Bradley, 1918). Other references described efforts to produce magnesite, chromite, and gemquality benitoite (e.g. Gale, 1912; Louderback, 1907,1909). Becker (1888) described the geology of the New Idria district, but the accompanying map included only the immediate vicinity of the mercury mines at New Idria. Arnold and Anderson (1910) were the first to describe the huge extent of the serpentinite, which extends far beyond the limits of their map. Significantly, their report also mentioned the occurrence of asbestos, and the intimate relations between asbestos-bearing rocks and the Franciscan formation. They also mentioned the White Creek syenite, which crops out on the southern margin of the serpentinite. The first published map showing the New Idria district as an extensive serpentinite mass was that of Anderson & Pack (1915), who were mainly interested in the oil potential of the area. Later, R.C. Miclenz described the stratigraphy and structure of part of the district in an unpublished doctoral thesis (Mielenz, 1939). The White Creek syenite at the southern end of the New Idria district was the subject of an unpublished M.A. thesis by M.S. Robertson (Robertson, 1940). Taliaferro (1943), in describing the structure of the Diablo Range, concluded that the steeply dipping New Idria Thrust Fault 2-3 Plate 2.1 - View of the New Idria Serpcntinitc and adjacent regions from the summit of San Benito Mt., looking northeast. Prominent peaks, from L-R, are Samson Peak, Idria Peak, and San Carlos Peak. Boundary of serpentinite can be distinguished as a change in vegetation and soil color on open slopes: serpentinite soil is greenish grey, sandstone is light brown. 2-4 Plate 2.2 - The first geologic map of California and adjoining regions, prepared by Edmond Guillemin-Tarayre in 1867. New Idria is shown on the eastern side of the Coast Range near latitude 37. 2-6 iiim u e was responsible for an appreciable pan cf the uplift of the Diablo Range in this section. He also showed how serpentine formed the cores of anticlines within the Franciscan formation, and suggested a subsurface connection between the New Idria serpentinite and the Laguna Mountain body to the west. To the extent that early studies mentioned the origin of the serpentinite body, they generally attributed it to an igneous intrusion of an ultrabasic magma (e.g. Taliaferro, 1943). In 1922, Laizure noted the establishment of a small asbestos mining operation on Clear Creek, but used the adjective "desultory" to describe the asbestos industry at that time. The study by Eckel & Myers (1946) focussed mainly on the quicksilver mines at New Idria, but brief mention of the serpentinite was made, and a geologic sketch map of the serpentinite was included. This map, modified and enhanced by others, has been the "base map" for the majority of recent studies of the District. II. Studies of Coleman and others, 1950 onward Starting in 1950, R.G. Coleman of Stanford University began a study of the serpentinite and its remarkable suite of associated minerals (Coleman, 1957). He was assisted by a well-timed forest fire which burned over a portion of the District and facilitated access to remote outcrops. His study was the first to attempt to explain the New Idria District in an overall petrologic and structural context. He showed that the serpentinite body was fault bounded on all sides, with the sense of shear where it could be determined showing upward movement of the serpentinite against the country rock. Coleman showed that the intrusion of the serpentinite took place at low temperatures, given the lack of contact metamorphism of the country rock, and suggested that the entire massif was a diapir, that rose because of the density contrast between the serpentinite and the surrounding country rocks, aided by tectonic overpressure related to development of the fold systems. 2-8 Coleman concluded that some of the exotic mineral suites were associated with tectonic blocks entrained by the serpentinite, while others were due to metasomatic alteration of the serpentinite, probably due to the intrusion of small igneous bodies. Subsequent generations of students have visited the District, frequently for topical studies, but no general re-evaluation of the rocks studied by Coleman has been undertaken. Several field trip guides have been prepared in recent years, and these serve as useful introductions to the District (e.g. Brown et al,, 1985; Coleman, 1986). The guidebook prepared by E.J. Fowkcs (1982) of West Hills College in Coalinga is an excellent sourcebook for the stratigraphy, structure, and paleontology of the region. The guidebook has many useful photographs and local maps. III. Mineralogical studies The unique suite of minerals found at the Gem Mine, particularly benitoite, neptunite, and joaquinite, have received much attention (e.g. Louderback, 1907, 1909; Bradley, 1909; Laird & Albce, 1972; Wise & Gill, 1977). Millage (1981) completed a mineralogical study-of the Victor Claim, another benitoite and neptunite locality in the District. Other mineralogical studies have included specimens from the District, e.g. that of Lager et.al. (1989) on Ti-bearing garnets, and that of Murdock & Ingram (1966) on REE-enriched vesuvianites. These mineralogical studies have generally investigated isolated mineral specimens and have not delved into the petrology and geochemistry of the District 2-9 IV. More Mining Studies The mercury mines of the New Idria District arc fault controlled. Henderson (1965) studied die cinnabar deposits at the New Idria Mines and proposed a low temperature hydrothermal model for their origin. Beginning in I960, the New Idria District became widely known for the industrial properties of the short fiber chrysotile asbestos that occurs here in abundance, and an "asbestos rush" occurred with several companies establishing large open pit mines. Some of the literature for this time is proprietary and unpublished, with only brief mention in trade journals of the period. The modem asbestos mining history is summarized by Levine-Fricke (1989). As of 1995, only one mine, the KCAC mine, remains in operation; indeed KCAC is now the only operating asbestos mine in the United States. Mumpton and Thompson (1975) described the mineralogy of the serpentinite and suggested that the short fiber chrysotile resulted from milling, or repeated rcciystallization under shearing stress. V. Geophysical Studies, 1943 onward There is a large literature on the structure and tectonics of the California Coast Ranges; it is beyond the scope of this pa,x:r to review that literature A useful starting point however is the monograph Geotectonic Development ofCalifornia (Ernst, 1981). There has been renewed interest in geophysical studies of the region in the wake of the destructive May 2, 1983 Coalinga earthquake. The Coast Ranges, including the Diablo Range, have been the subject of regional studies by a variety of geophysical methods, including gravity and aeromagnetic measurements, as well as scismographic techniques. Starring in the 1920s, refraction and reflection seismology were used mainly in support of oil company 2-10 exploration efforts, but helped to unravel the deep structure underlying the Coast Ranges and San Joaquin Valley (Vaughan, 1943). Byerly (1954) studied the regional gravity profile of the Coast Ranges in Central California. He reported that a 60 mgal negative anomaly underlies the New Idria Serpendnitc, with the minimum value reached at the southern end. Byerly concluded that the serpentinite body is thickest at the southern end. and that no large body of unserpendnized peridodte underlies the serpendnite at the surface (or alternatively, if such a body exists it must be deeply buried). Later studies by Jachens (1991), with a higher station density, supported this conclusion. Aeromagnetic measurements in this region have generally been inconclusive, or have merely served to reinforce conclusions obtained by other methods (Vaughan. 1943; Coleman 1986). The region around and immediately to the south of the New Idria District is seismically active today, as shown in Figure 2.1. There are swarms of magnitude 1-3 earthquakes and an occasional larger event (e.g. the 1926 Idria, Mw = 5.5,1982 M^S.4 New Idria, i9S3 -6.5 Coalinga, and 1985 M^pti.l Kettleman Hills events, ref. Stein & Ekstrdm, (1992)). The 1983 earthquake of magnitude 6.5 effectively levelled the town of Coalinga, but stimulated research interest in this pan of California (e.g. Rymer & Ellsworth, 1990). The 1983 event occurred near the crest of the Coalinga Anticline, a southerly continuation of the New Idria Thrust Fault. The predominant mode of recent seismicity has been reverse or thrust faulting, due to crustal shonening from compressional stress normal to the strike of the San Andreas fault, (e.g. Montgomery, 1993; Bartow, 1990; Wentworth & Zoback, 1989). Another part of the seismic activity may be due to continued diapiric rise of the serpentinite massif, expressed as small movements on the boundary faults. 2-11 Figure 2.1 - Recent Seismicity in Northern California NORTHERN CALIFORNIA 1980 - 1985 Figure 2.1 data from Hill et al. (1991). New Idria lies just to the north of the town of Coalinga. Recent seismicity has been concentrated in the region immediately to the south and east of New Idria. VI. Studies by the EPA, 1982 onward The California Aqueduct, completed in 1967, transports municipal and agricultural water from northern California to southern California. The aqueduct forms a barrier to stream drainage eastward from the New Idria District, especially at the Arroyo Pasajero east of Coalinga. Starting in 1982, public concern was expressed about the amount of chrysotile asbestos in the drinking water of the communities served by the California Aqueduct, chiefly Los Angeles. This concern resulted in a series of studies by the Environmental Protection Agency (EPA) to locate the source of this asbestos and to evaluate the perceived public health hazard. (e.g. Levine-Fricke, 1989; Woodward-Clyde, 1990). Stream runoff from the New Idria District was shown to be the major chrysotile asbestos source, particularly drainage through Los Gatos Creek and the Arroyo Pasajero. Notwithstanding the lack of evidence for any health hazard associated with ingestion of chrysotile asbestos in drinking water, the EPA moved to establish two Superfund sites in the District, one at the abandoned Atlas Mine, the other at the abandoned Coalinga Asbestos Mill site in Pine Canyon. Both of these locations are listed by EPA as uncontrolled hazardous waste sites under the provisions of the Comprehensive Environmental Response, Compensation, and Liability Act, or CERCLA (Superfund). Continued public pressure has resulted in a number of planned remedial actions, the estimated cost of which is now approaching SI billion. The Bureau of Land Management has implemented a series of access control procedures, in order to reduce the disturbance of unstable hillsides by trail bikes and off-road vehicles in the Gear Creek Management Area. The Levine-Fricke (1989) study, done under contract to the EPA, includes a good summary of the geomorphology and stratigraphy of the District As noted above, the EPA was also concerned about the environmental impact of mercury mining at the New Idria Mines, which led to the closing of these mines in 1972. Arid 2-13 mine drainage from the sulfide-rich mine dumps results today in a pH as low as 1.7 in the stream that drains the area. The stream drains into a sparsely inhabited region, and no active remediation is taking place at the present time. The remote town of Idria is presently occupied by a drug rehabilitation program operated by the Futures Foundation of San Jose, California. Most recently, concern has been expressed about the habitat of the San Benito evening primrose, which has been designated a threatened species under the Endangered Species Act. Botanical studies supervised by the Bureau of Land Management in Hollister provide an additional source of information on the District. Epilogue: Both the historical and scientific literature of the New Idria District are extremely scattered, making research time-consuming. This chapter is offered in the hope of saving a great deal of time for future investigators. The greatest body of scientific literature concerning the New Idria District in the next few years is likely to be related to geophysical and tectonic studies of the Coast Ranges, and to environmental and engineering studies of the asbestos deposits. 2-14 Chapter 3 Mineralogy and Petrology of the New Idria Serpentinite Chapter 3 - Mineralogy & Petrology of the New Idria Serpentinite I. Introduction.............................................................................................1 II. Geological Setting of theNew Idria District........................................I III. Serpentine and Serpentinites................................................................... 2 IV. Some Serpentine Mineralogy............................................................3 A. Lizardite............................................................................................ 7 B. Antigorite....................................................................................... 11 C. Chrysotilc........................................................................................ 14 V. Phase Relations among the Serpentine Minerals........................................16 VI. Phase Relations in Serpentinites.............................................................. 17 VII. Distinguishing Among the Serpentine Minerals............................... 21 VIII. Progressive Alteration of Ultramafic Rocks............................................ 24 A. Progressive Serpentinization of Dunite.............................................26 B. Progressive Scrpentinization of Harzburgite.............................. 27 IX. Petrology of the New Idria Serpentinite...................................................36 A. Compositional Analysis................................................................... 36 B. Geothermometry from Relict Igneous Minerals............................... 46 X. Accessory Minerals of the Serpentinite....................................................53 A. Brucite...................................................................................... .....53 B. Magnetite.......................... ............................ ................................. 55 C. Sulfides..........................................................................................57 D. Chromite..................................................................... 1..................57 E. Chromian Andradite Garnets............................................................61 F. A Blue Amphibole........................................................................... 69 G. Secondary AlterationMinerals.................................................... 73 XI. Discussion and Conclusions.................................................................. 78 Tables ........................................................................................................84 Chapter 3 - Mineralogy & Petrology of the New Idria Serpentinite I. Introduction This chapter describes the mineralogy and petrology of the New Idria Serpentinite and its accessory minerals exclusive of the tectonic blocks and igneous intrusives, which are described in Chapter 4 and Chapter 1 respectively. After a brief description of the geological setting of the New Idria District, a brief summary of serpentine mineralogy as it applies to the rocks of the District is offered. The process of progressive alteration of ultramafic rocks is shown to be responsible for the present mineralogy and rock textures observed at New Idria. The likely protolith for the serpentinite is discussed. Finally, accessory minerals of the serpentinite are described. II. Geological Setting of the New Idria District The term New Idria District, referred to in this study simply as the District, was first used by Becker to describe the general region encompassing the historic mercury mines at New Idria, California (Becker, 1888). The most remarkable feature of the District is an oval. 23 by 8 km, fault bounded, serpentinite massif flanked by steeply dipping, locally overturned, sedimentary and metamorphic rocks of the Jurassic-Cretaceous Franciscan and the Cretaceous to Pliocene Great Valley Sequence of California. The New Idria Serpentinite is a part of the Coast Range Ophiolite in California (Hopson ct al., 1981). Its emplacement within the Franciscan Formation is related to subduction of the Famllon plate in the Jurassic. The tectonic setting of the ophiolite was a forearc basin 3-1 lying between the Sierra Nevada volcanic arc to the cast and a trcnch-subduction complex to the west, represented by the Franciscan Formation (Bartow, 1990). A piece of oceanic crust or underlying mantle was obducted onto the North American continent, later to rise through the Franciscan Formation as a serpentinite diapir. Emplacement of the New Idria Serpcntinite as a diapir within the Franciscan has resulted in a piercement structure or tectonic window, lying just east of the San Andreas fault system northeast of Parkfield. Further details about the geological environment of the District can be found in Chapter 1. III. Serpentine and Serpentinites Serpentine is the name of a mineral family; serpentinite is a rock type consisting of one or more of the serpentine minerals, along with accessory phases, typically brucite, magnetite and chromite. The mineralogy and petrology of the New Idria Serpentinite have received much less attention than those of the related mineral deposits within and along its margins. Bradley (1918) noted that "The most striking feature of the geology of this district is the large area of serpentine...", but limited his description of this serpentine to a single paragraph in a lengthy report. His accompanying map simply showed undifferentiated serpentine. The 1946 Eckel & Myers map, which has been used as a base map for most geologic work for the past 50 years, showed some of the heterogeneity within the serpentinite. While Coleman ( 957) studied the physical emplacement of the serpentinite, he discussed the serpentine minerals themselves only briefly; he suggested that the dominant mineral was antigorite. Coleman also noted the local occurrence of tabular bastite-rich rock, resulting from serpentinization of harzburgite. (Bastite is the term used to describe serpentine that has formed after a chain silicate mineral, commonly orthopyroxene.) More recently, it has become apparent that the serpentinite is chrysotilcdominated (e.g. Mumpton & Thompson. 1975; Coleman, 1980; this study). Mumpton & 3-2 Plate 3.1 General view of the New Idria serpentinite looking northeast from Condon Peak. Rolling hills are partially covered with scrubby vegetation on nearly white soil. Numerous rough roads, abandoned mines and exploration pits dot the landscape. The rocky knob to the left is Santa Rita Peak (1574 m). an antigorite knob. Beyond the peak is a steep escarpment which descends to the San Joaquin Valley. Summits of the Sierra Nevada are visible in the distance. 3-4 among these three minerals. The reader is referred to Wicks & O'HanJcy (1988) fora discussion of serpentine polymorphism and polytypism. The serpentine minerals have many structural features in common. Divalent cations, chiefly Mg, in six coordination with oxygen and hydroxyl groups, form an octahedral sheet with a b dimension of 9.4A. Tetravalent cations, chiefly Si, form a tetrahedral sheet with a b dimension of 9.1 A, i.e. slightly smaller than the octahedral sheet It is this misfit that is presumed to lead to the observed structural variations in the serpentine minerals. The apices of the tetrahedra are bonded to one side of the octahedral sheet Hydroxyl groups lie at the center of the six-coordinated rings of the tetrahedral sheet, and at the same level as apical oxygens. Hydroxyl groups also lie in the basal plane of the octahedral sheet on the side opposite the tetrahedral sheeL Weak bonds involving hydrogen connect the layers of the serpentine structure; geometric variations in the positioning of adjacent layers allow for extensive polytypism. Figure 3.1a shows the ideal serpentine structure in perspective view, seen perpendicular to c*. The unit cell dimension along c is approximately ik. Fe and A1 are the principal compositional variants in serpentine. Compositional variation occurs along the three exchange vectors AljMg.jSi.j (Tk); FeMg_, (Fm); and Fe^Al^ (Fa). The dioctahedral vacancy substitution QAljMg.j is not known to occur in serpentine. Trace amounts of Ca, Na, and K reported in serpentine analyses may represent mixtures of other silicates with serpentine in the analyzed samples. Serpentine gTtv'p minerals most often form by hydration of minerals in ultramafic rocks, chiefly olivine and pyroxene. Other parageneses of serpentine are rare but do occur at Globe, Arizona, at Newbury. Massachusetts, and at Bolton, Massachusetts, chrysotile 3-6 has formed as an alteration product after tremolite or forstcrite, which have in turn formed by contact metamorphism of siliceous dolomite. If the hydration of peridodte occurs at temperatures above die stability field of serpentine minerals, then other phases may form instead of serpentine minerals: e.g. anthophyllite, talc, and members of the humite group, especially if the activity of water (aH2q) in the fluid is low. At New Idria no evidence for the formation of these minerals has been found, and it is shown below that serpentinization at relatively low temperatures took place. For this reason the complex phase relations of these minerals are omitted from the following discussions. A. Lizardite The planar smicture of lizardite is conceptually the simplest of the serpentine minerals, and approaches the ideal structure shown in Figure 3. la. Lizardite accommodates the misfit between the octahedral and tetrahedral sheets by two mechanisms: cation substitution, and minor geometric adjustments that include tetrahedral rotation and changes in thickness of the sheets. Substitution of Fe2*, Fe3* and A1 for Mg2* occur in the octahedral sites, and Fe3* and A1 for Si4* in the tetrahedral sites. Introduction of Fe3* and A! in the octahedral sheet reduces the dimensions of this sheet, and therefore improves the fit with a pure Si tetrahedral sheet Fe3* and A1 in the tetrahedral sheet increase the dimensions of this sheet, also improving the fit Fe2* in the octahedral sheet, on the other hand, worsens the fit O'Hanley & Dyar (1993) suggest that A1 and Fe2* arc favored in the octahedral sites; Fe3* appears to be favored in the tetrahedral sites. Tetrahedral rotation, on the other hand, has the effect of reducing the size of the tetrahedral sheet which would appear to worsen the fit in near-end-member 3-7 O Figure 3.1a - The lizardite IT structure viewed normal to c* Mg3Si205(0 H ) 4 compositions. The ionic substitutions couid in principle increase the dimensions of the tetrahedral sheet so that it becomes larger than the octahedral sheet, as in micas. In these cases, tetrahedral rotation could reduce the size of the tetrahedral sheet to achieve proper fit. However, Mellini & Zanazzi (1987) found small tetrahedral rotations with led < 6 in near-end-member compositions. In their study the magnitude and sign of the rotation varied according to the Iizaidite polytype. The tetrahedral rotation observed in lizardite is not what one would expect from observation of the structures of similar minerals. New calculations using the BDTEA computer code of L.W. Finger and the data of Mellini & Zanazzi (1987), Pendikatsis & Burzlaff (1981), Zigan & Rothbaucr (1967) and Hazcn (1976) show that the unshared edges of the octahedral sheet in Iizaidite are 3.07A compared with 3.22A in brucite, and 2.97A in talc. The octahedral edge dimension in a slab from the periclase structure is 2.98A. In Iizaidite and talc the unshared edge dimensions must be the same as the apical oxygen distances in the tetrahedral sheet. The lateral dimensions of the octahedral sheet in lizardite are less than in brucite but greater than in periclase; this must be due to the presence of the single tetrahedral sheet in the 1:1 lizardite structure. In talc, a mineral with many structural similarities to Iizaidite (except that two tetrahedral sheets are attached in the 2:1 structure), tetrahedral rotation in the end member composition is extremely small (a = 3.6, Evans & Guggenheim. 1988). The unshared edge length in talc is smaller than in lizardite: I would therefore expect that tetrahedral rotation in talc would be greater, not less, than in lizardite. The opposite appears to be true. The unshared edges in lizardite might be expected to be shorter on the tetrahedral side than on the non-tetrahedral side, but the difference is less than O.OOlA. The shared octahedral edges in lizardite are 2.78A compared with 2.79A in brucite, 2.98A in periclase and 2.50A in talc. As is common in sheet silicates, the octahedral sheet in Iizaidite is compressed along the c axis compared with the dimensions of a sheet of regular MgOfi or Mg(OH)6 octahedra. This slight 3-9 compression also reduces the lateral dimensions of the octahedral sheet by a small amount, improving the fit. The formation of lizardite is promoted by the availability of A1 (Caruso & Chemosky, 1979), Fc (Wicks & Plant, 1979), and relatively high oxygen fugacity (O'Hanley & Dyar, 1993). Lizardite formation is favored over chrysotile by a relatively low activity of water (O'Hanley et al.. 1989, and references therein). The lizardite IT polytype is the most common product of the retrograde alteration of olivine (Wicks & Whittaker, 1977) and is the most abundant of the serpentine minerals (Wicks & O'Hanley, 1988). In parageneses involving both lizardite and chrysotile, the presence of unserpentinized olivine in the rock tends to depress aH2o> resulting in early lizardite. Once olivine has been consumed and the activity of water has risen, late chrysotile tends to form (Prichard, 1979). Prichard suggested that the composition of serpentinizing fluids may change once olivinehas been consumed, and that this is the explanation for the late stage appearance of chrysotile. Another possibility is that the water content of lizardite is lower than that of chrysotile, as discussed in the section on chrysotile. Alternatively, it is possible that kinetic factors control which "polymorph" appears in the formation of serpentine after olivine. At New Idria, lizardite is much less abundant than chrysotile, possibly due to the recrystallization of earlier lizardite under shearing stress (Mumpton & Thompson, 1975). An alternative explanation for the abundance of chrysotile is the extensive growth of cataclastic fabric due to the predominance of dunite in the protolith (D. O'Hanley, personal communication, 1993). This idea is also discussed below in the section on progressive serpentinization, and in Chapter 7. 3-10 B. Antigorite The next most abundant serpentine mineral is antigorite, which employs a different strategy for resolving misfit in the structure. Antigorite has a modulated structure, in which the misfit between the tetrahedral and octahedral sheets is accommodated by corrugations in the two sheets (Kunze, 1956). The continuous, undulatory octahedral sheet is always concave towards the tetrahedral sheet The tetrahedral sheet is also continuous, but inverts at the corrugation boundaries or points of inflection, so that the tetrahedral apices point alternately towards +c and -c. This modulated structure causes octahedral sites to be "lost" at the corrugation boundaries or points of inflection, with the result that antigorite is Mg and (OH) - deficient, or Si-enriched, compared to the ideal serpentine formula. The proper end member formula for antigorite is necessarily arbitrary since it depends on the supercell periodicity: Thermochemical data for the following approximation to the antigorite formula are given by Berman (1988) and Helgeson et al. (1978): ^48^*34^85^^62 Figure 3.1b shows in schematic form the antigorite structure with the corrugations flattened out in order to see the bonding between layers. It is apparent that the Si-O-Si bonds at the points where the tetrahedral sheets invert are responsible for the tough, resistant nature of antigorite compared with lizardite or chrysotile, in which the adjacent layers are held together by weak bonds involving hydrogen. This toughness is well displayed in the resistant antigorite knobs at New Idria. Consideration of the antigorite chemistry and structure also suggest that antigorite should be a member of a polysomatic scries between lizardite and talc (e.g. Ottcn, 1993; Sanford, 1978). The above antigorite formula is equivalent to 15 Liz + 1 Tic. If the wavelength of the modulation were to 3-11 H O H O f- O decrease the structure would become increasingly taJc-Iike, while if the wavelength were to increase the structure would become increasingly lizardite-like. In the 1:1 lizardite structure, each octahedron is bonded to two apical oxygens and four hydroxyl groups; the apical oxygens lie on the same side of the octahedral sheet In the 2:1 talc structure each octahedron is bonded to four apical oxygens and two hydroxyl groups; the apical oxygens lie on both sides of the octahedral sheet In the andgorite structure, at the point in each layer where the tetrahedral sheet inverts, there is a distinct type of octahedron bonded to four apical oxygens and two hydroxyls; the apical oxygens lie on both sides of the octahedral sheet. The position of one of these special talc-like octahcdra is shown by the heavy black lines in Figure 3.1b. Sanford (1978) considered polysomatic models of the antigorite structure. Polysomatic models for the antigorite structure based on HRTEM studies were also proposed by Spinnler (1985) and Ottcn (1993), who considered more complex structural building blocks than talc and lizardite. The stability field of antigorite extends to higher temperatures than that of the other serpentine minerals - in this sense it is the high T "polymoTph". Antigorite does not often form as a retrograde alteration product of olivine; it generally forms as a prograde metamorphic product, together with brucite, after pre-existing lizardite and chrysotile, according to the endothermic reaction: HMgjSijCtyOHXt -Mg^Si^O^OH)^ + 3Mg(OH)2 chrysotile antigorite brucite (1) Using the data of Berman (1988), I find this reaction consumes 2 kJ/mol chrysotile at 1 bar and 25C. At New Idria, antigorite is found principally in tough, resistant knobs which arc interpreted as tectonic inclusions or knockers in the chrysotile-dominated serpentinite (Plate 3.1). Antigorite knockers are discussed further in Chapter 4. 3-13 One of the unexpected phenomena associated with serpendnites is that during hydration of peridodtc at low temperatures, andgorite rarely forms; instead one sees lizardire or chrysodle. On the other hand, dehydration of a serpentinite by progressive metamorphism produces andgorite, which eventually dehydrates with rising temperature to form olivine. Why does hydration of olivine not produce antigorite? One possible explanation has to do with the relationship between PH2q and P^ui. Sanford (1981) proposed that the lower limit of the antigorite stability field may be bypassed during serpentinization when PH2o < P^, typical of infiltration of water along cracks at shallow depths, while antigorite is stabilized during prograde metamorphism of serpentinites when PH2q = P^^. Sanford's model did not, however, consider the formation of iizardite versus chrysotile. C. Chrysotile Another structural variant in the serpentine family is chrysotile. Although the least abundant of the serpentine minerals, chrysotile dominates at New Idria. The structure of chrysotile accommodates the misfit between the octahedral and tetrahedral sheets by rolling up the sheets into cylinders; the tetrahedral sheet faces the concave side of the octahedral sheet. The result is that crystallites of chrysotile are actually hollow tubules with diameters of up to several microns, easily visible with a scanning election microscope (Yada, 1967). The tubules may aggregate into fibers which form chrysotile asbestos, although massive chrysotile also occurs at New Idria. Chrysotile asbestos, representing 95% of commercial production, is the subject of much current medical research because of concerns about its pathogenicity in lung tissue (e.g. Guthrie & Mossman, 1993). 3-14 Chrysorile 2M is the most abundant chrysotile polytypc; it commonly forms at low temperatures as a late stage alteration product of olivine. As noted in the discussion of lizardite stability, low temperature progressive serpentinization of olivine usually produces lizardite until olivine has been consumed, followed by late stage formation of chrysotile. At the time olivine is completely consumed, rises and the composition of the serpentinizing fluids may possibly change (Prichard, 1979). If aH2Q by itself affects the formation of lizardite versus chrysotile, then the assumption that the water content of lizardite and chrysotile is equal must be challenged. The dioctahedral analogue of lizardite is kaolinite (A^SijOjfOH^). A related hydrated form of kaolinite known as halloysite (Al2Si203(0H)4*2H20) has interlayer molecular water its structure tends to roll up into tubules, somewhat like tubules of chrysotile (except that in halloysite the octahedral layer is convex towards the tetrahedral layer, not concave as in chrysotile). The molecular water in the halloysite structure could possibly correspond to (thus far undetected) water in the chrysotile structure. This would lead to a hypothetical chrysotile formula of MgjSijO^OH^nH^O. If this could be established by experiment, the interaction between a^and the chrysotile might be explained by the reaction pain olivine + water = lizardite + brucite lizardite + n*water = chrysotile The equilibrium constant for both of these reactions obviously depends on aH20. For the second reaction, that produces chrysotile, we have the relation K = l/[aH20Jn. The value of n is uncertain, but is probably small, perhaps less than 1. It is noteworthy that Faust & Fahey (1962) found that some, but not all, chrysotile dehydration curves using the DTA 3-15 technique had a double peak while antigorite always had a single peak. They believed that excess water in chrysotile was a surface absorbtion phenomenon. V. Phase Relations among the Serpentine Minerals The phase relations among the serpentine minerals arc difficult to show schematically, and are not completely understood. In general, lizardite and chrysotile are the low temperature serpentine minerals; antigorite is the high temperature phase. The stability field of lizardite probably extends to higher temperature than that of chrysotile, due to the effect of ionic substitutions of A1 and Fe (O'Hanley et al., 1989). The presence of lizardite and antigorite in blueschist facies serpentinites suggests these two phases are the stable high pressure phases. Reversed experiments showing the above relationships remain to be done; no reliable thermochemical data for lizardite have been published. Anempts to synthesize lizardite in the laboratory have almost invariably produced chrysotile, making reversal experiments difficult (but see Chemosky, 1975). Substitution of Fe and Al probably expands the thermal stability of lizardite relative to the other serpentine minerals. Antigorite is the stable serpentine mineral at high temperature conditions; the reaction boundaries separating antigorite from lizardite and chrysotile arc not simple polymorphic transitions, but probably correspond to reaction (1) above. Figure 3.2 shows the calculated univariant curve for reaction (1) using the TWQ2S program of Berman (1991 1994) and thermodynamic data of Berman (1988). The upper stability limit of chrysotile is less than 300C at 1 bar; the univariant line has a negative slope. This is consistent with the observations of Sanford (1981) and of Evans et al., (1976). All of these observations are summarized in the following schematic diagram: 3-16 Antigorite Lizardite P Chrysotilc T Due to the importance of Fe and AI, discussion of serpentine phase relations in MSH alone is probably unproductive; MASH is the simplest realistic system (O'Hanley, D.S., personal communication 1993). The reader is referred to O'Hanley et al. (1989) for complex phase diagrams in the MgO-SiOj-^O (MSH) and Mg0-Al203*Si02*H20 (MASH) systems, constructed using the method of dual networks (Kujawa, 1965). The MSH system, however, can be used as a first-order approximation for formulating phase relations between serpentine minerals and other minerals found in serpentinites. These relations are explored in the next section. VI. Phase Relations in Serpentinites The thermal stabilities of common phase assemblages in serpentinites are limited by the univariant reactions shown in Figure 3.2. This figure is calculated using the TWQ2S program for the MSH system and includes only the stable reactions for the portion of MSH in which chrysotilc + brucitc arc the low temperature assemblage. The calculated positions of the univariant equilibria agree well with experimental evidence (Bowen & Tuttle, 1949), and field evidence (Evans et al., 1976). 3-17 Figure 3.2 - Univariant reactions in the magnesian portion of the MSH system, for bulk compositions corresponding to mixtures of forsterite and enstadte. Compositions more magnesian than forsterite or more siliceous than enstadte not shown. Calculations using the TWQ2S code (Berman, 1991). Numbered reactions given in text. Abbreviations arc Cthchrysotile, Brc:brucite, Atg:antigorite, Fo:forsterite, Tlc:talc, En:enstatite, Wrwater. 3-18 I(XX) (snq)d The univariant reactions in MSH with a fluid phase of pure HjO are, in addition to reaction (1): Mg4gSi34Og5(OH)62 *20Mg(OH)2 = 34Mg2Si04 + 51H,0 antigorite brucite forstcrite Mg^Si^OgjCOH)^ = 4Mg3Si4O,0(OH)2 + 18Mg2Si04 + 27H20 antigorite talc forstcrite 2Mg2Si04 + 2Mg3Si4O10(OH)2 = 10MgSiO3 + 2Hp forsterite talc enstatite (2) (3) (4) The following discussion of progressive metamorphic reactions assumes that the lowtemperature serpentine mineral is chrysotile, since there are no thermochemical data available for Iizaidite. Accessory oxide minerals are also ignored; they do not significantly affect the stabilities of coexisting silicate minerals. At New Idria, the dominant phase assemblage of chrysotile + brucite indicates an Mg/Si ratio lying between forsterite and enstatite, and a low temperature equilibrium (<250C), in the divariant field below reaction (1). Antigorite knockers described in Chapter 4 have a higher temperature assemblage; they lie in the divariant field between reactions (1) and (2). In the contact aureole around the intrusive New Idria syenite, progradc olivine replacing antigorite shows that reaction boundary (2) was encountered. No evidence was found at New Idria for higher grade metamorphism, i.e. reaction (3) or higher. I therefore conclude that at New Idria, based on evidence from the serpentinite itself, the rocks equilibrated at temperatures at or below that of the greenschist facies. Note that in the bulk compositions modelled here, with fluids consisting of pure HjO, talc docs not appear as a low temperature mineral, but rather as a metamorphic mineral in the amphibolite facies. Bulk compositions more siliceous than enstatite, in which chrysotile + talc are stable at low temperatures, were not found in this study. Bulk compositions more magnesian than 3-20 forsterite were also not found in this study; ultramafic rocks with such compositions are extremely rare in nature. If these unusual compositions were explicitly considered. Figure 3.2 would require two more univariant reactions for the dehydration of talc and brucite respectively, at upper amphibolite grade. Likewise, if the aqueous phase were not pure HjO, the above univariant reactions would become divariant in P-T-aH2o space, and talc might appear as a low-T phase. VII. Distinguishing Among the Serpentine Minerals In many cases it is difficult to distinguish among the serpentine group minerals with a single test, particularly since more than one serpentine mineral may exist in a single sample. X-ray diffraction analysis (XRD) is often ambiguous for two reasons: the peaks which distinguish the serpentine minerals are weak, and their intensities may be affected by preferred orientation of the sample. Large single crystals of serpentine are rare, making single crystal analysis difficult Observation of optical properties, using the criteria of Wicks & Whittaker (1977), arc another useful, but not definitive method. Elevated A1 contents in a serpentine chemical analysis suggest the presence of lizarditc, as noted above. In this study, determinations of serpentine mineralogy were done by a combination of XRD and chemical analyses, as well as the optical and textural criteria of Wicks & Whit aker (1977), as simplified by Wicks & O'Hanley, (1988). (sec Table 3J). In some cases the specific serpentine minerals present could not be determined unambiguously. These cases are noted in the text. To illustrate these points, sample 989-45 from New Idria was studied using all of the above techniques. 989-45 is a dark green, sheared rock from Perovskite Knob. Table 3.1 gives a measured XRD pattern for this sample. The indexed pattern suggests that 3-21 Plate 3.2 - Photomicrograph of serpentinized harzburgite, sample 989-45. Bastite grain in center shows development of both lizardite and chrysodle. Crossed nicols with gypsum plate. Crossed nicols; Field of view 1 mm. 3-22 this sample contains a mixture of chrysotiie and aluminous lizardite. High A1 contents (1.45%) in whole rock analytical data in Table 3.2 (column 1), electron microprobe analysis of individual grains, and petrographic observations using the Wicks & Whittaker criteria, confirm that the sample contains a mixture of chrysotiie and lizardite. Plate 3.2 shows a portion of this sample in which serpentine of both positive and negative sign of elongation occurs within a single bastitc grain, because of the presence of chrysotiie and lizardite respectively. The replacement appears to be controlled by exsolution lamellae or cleavage planes in the orthopyroxene. Bastite, the pseudomorphic replacement of chain silicates by setpentine, is widely distributed at New Idria. VIII. Progressive Alteration of Ultramafic Rocks It is important to view the formation of serpentinites as a multi-stage process, in which earlier minerals may recrystallize or be replaced more than once along their path from protolith to geologist's sample bag. The term progressive serpertrinizarion is used here to describe this multi-stage process. The following discussion assumes a depleted ultramafic protolith of dunite and/or harzburgite, and that the model reactions are in the MS H system (i.e. the fluid phase is pure HjO). Chapter 4 extends this analysis to the CMASH system. Olivine and orthopyroxene are not stable at low temperatures in the presence of water, as shown in Figure 3.2. If hydration takes place at temperatures below reaction boundary (1), one of the following component reactions may occur 2Mg2Si04 + SHjO = Mg3Si205(0H)4 + Mg(OH)2 forsterite serpentine brucite (5) 3-24 Mg2Si04 + MgSi03 + 2HjO = Mg3Si203(0H)4 (6) forstcritc enstarite serpentine 6MgSi03 + 3H20 = Mg3Si4O10(OH)2 + Mg3Si205(0H)4 (7) enstarite talc serpentine In reaction (5) pure forsterite reacts with water to produce serpentine, and, since magnesia is in excess, brucite. The resulting serpentine-brucite assemblage may persist, may undergo recrystallization by prograde metamorphism in some later thermal event, or may undergo further replacement reactions, particularly if C02-rich meteoric waters are present. Evidence for all three of these processes is abundant at New Idria. When orthopyioxene is present as a reactant, modal brucite is reduced - to zero in the case of reaction (6), from which only serpentine is produced. As the ratio of orthopyioxene to olivine increases beyond 1:1 molar proportions, talc may appear as a product together with serpentine. Reaction (7), with olivine absent, produces the assemblage talc + serpentine in equal proportions. Therefore, at low temperatures in this model system, in which the hydrous phases have essentially fixed composidons, the amount of modal brucite depends on the orthopyroxene/olivine ratio. If iron in solid solution is added as an additional component, reactions (8a), (8b) and (9) described below in Section X may produce magnetite; the coexisting hydrous minerals will be low in Fe (but see Section X concerning the formation of Fe-rich brucite). The fractionation of oxygen isotopes between serpentine and magnetite has been used to record the temperature of serpentinization (Wenner & Taylor, 1971). Application of this technique to New Idria samples would provide useful information on the evolution of the serpentinite. 3D-3180 studies of the serpentine minerals at New Idria also hold promise for constraining the environment of serpentinization, potentially distinguishing ocean- 3-25 floor serpendnizadon from scrpentinization that look place after emplacement of the ophiolite. In principle, serpendnized ultramaftc rocks could contain talc as pan of the low temperature assemblage, when the serpenrinizing fluid is pure HjO. In fact, this is rare, because the required silica-rich bulk composition would place the parent rock in the "olivine orthopyroxenitc" field of the IUGS classification, which is an extremely rare rock type. Therefore, talc does not normally appear as a low temperature mineral when fluids are pure H2O, but instead as a progiade metamorphic replacement of antigorite at roughly staurolite grade (TrommsdorfT & Evans, 1974). If the serpenrinizing fluids are rich in C02, however, then magnesite and talc or tremolite may indeed appear as low temperature products. This has not happened at New Idria but has happened in the Appalachian serpentinites of Vermont and Quebec. Sec Sanford (1978) for a discussion of serpendnizadon with C02-rich fluids. A. Progressive Serpentinization of Dunite Serpentinite formed after dunite often has a knobby outcrop texture formed by intersecting serpendne/brurite veins which isolate rounded kernels of unsetpendnized dunite. This macroscopic texture is well displayed in resistant bouldrrs at New Idria (Plate 3.3, 3.4). O'Hanley (1992) has shown that this kernel pattern results from a volume increase during serpendnizadon. Development of the intersecting veins is aided by the relatively isotropic nature of dunite - there is no preferred direction for vein development, and strain is not partitioned along well-developed shear planes. At New Idria, the nearly complete pulverization of the original bedrock in the southern half of the District may be explained by positive feedback between progressive serpendnizadon and decreasing rock competency. This feedback, in an environment of shear stress due to tectonic 3-26 overpressure, may lead to the development of additional vein systems and continued serpentinization, which further weaken the rock. At the KCAC Mine, large rounded boulders, which populate the waste pile, are mainly kernels of dunite. Chapter 7 uses these concepts to propose a model for development of short-fiber chrysotile asbestos at New Idria. Microtextures of serpentine also show the kernel pattern, as illustrated in Plate 3.5. In this photomicrograph, the intersecting fractures of the parent rock isolate olivine and pyroxene cores into polygonal kernels. The serpentine minerals here are a mixture of lizardite and chrysotile; the vein filling mineral is mostly chrysotile. This texture is pseudomorphic and is common. Opaque minerals are magnetite and chromite. B. Progressive Serpentinization of Harzburgite Serpentinite formed after harzburgite often has a slabby texture, and the serpentinized rock fractures leaving sharp edges, rather than the rounded kernels of serpentinized dunite. Two aspects of the harzburgite protolith are responsible for this difference mineralogy and layering. Harzburgite differs from dunite in having significant amounts of orthopyroxene, which is frequently found in parallel layers that give the rock a banded appearance. These layers may be a primary igneous feature or may have been produced by deformation of the harzburgite prior to serpentinization. At New Idria harzburgite outcrops are not numerous; the banding is displayed only faintly due to the advanced degree of serpentinization. Fractures along which serpentinizing fluids flow tend to form preferentially parallel to the layers, rather than at random angles as in dunite. The resulting serpentinization tends to produce angular blocks as shown in Plate 3.6. The presence of orthopyroxene also inhibits the production of brucite, according to reaction (6) above. Brucite is unstable in the presence of C02 - rich meteoric fluids, altering 3-27 Plate 3.3 - Serpentinized dunite at Butler Estate Mine showing early development of kernel texture by intersecting veins. 3-28 Plate 3.4 - Serpentinized dunite at J.M. Christie Pit showing progressive development of kernel texture as isolated, rounded masses. 3-30 Plate 3.5 - Photomicrograph of kernel texture with relict cores of olivine (center) and pyroxene (left). Intersecting veinlets isolate kernels as serpentinization proceeds. Opaques include magnetite and chromite. Crossed nicols; field of view 2 mm. 3-32 Plate 3.6 - Photo of hand sample of partially serpentinized harzburgite, showing slabby texture, bastite grains and weathering rind. 3-34 quickly to one of the Mg carbonates. Reactive brucite therefore tends to destabilize the rock mechanically, while in its absence the rock retains much of its original strength until serpentinization is quite advanced (see Chidester et al., 1978). Serpentine formed after orthopyroxene tends to preserve the original rock texture as bastite pscudomorphs. Basdte is found in samples at New Idria, where orthopyroxene has been more resistant to serpentinization than has olivine. The bastite grains, visible in hand sample as dark spots, also contribute to maintaining the physical integrity of the rock and enhance its resistance to further degradation. The mineralogy and textures of serpentinites depend on several factors, particularly on the protolith but also on the temperature at which serpentinization occurs. Table 3J summarizes processes and resulting microtextures inferred by Wicks and co-workers. In this study the simplified classification scheme of Wicks and O'Hanley (1988) is used unless otherwise stated. A factor in the production of antigorite, not shown directly in Table 3-3 is the importance of prograde metamorphism of low-temperature serpentine minerals, following reaction (1) above. Most antigorite forms by this process. IX. Petrology of the New Idria Serpentinite A. Compositional Analysis Serpentine group minerals most often form as alteration products of minerals in ultramafic rock, chiefly olivine and pyroxene. Ultramafic rocks are conveniently studied in the ternary projection forsterite-enstatite-diopside (OI-Opx-Cpx), projected from a specified aluminous phase, as shown in Figure 3-3. Serpentinized ultramafic rocks may also be 3-36 studied in this projection, so long as one keeps in mind that mobile components such as Ca may be lost during serpentinization, resulting for example in an underestimate of the modal clinopyroxcne of the protolith. Table 3.2 gives representative whole rock analyses of the New Idria serpentinite from this study and from other sources. Examination of this table shows that Mg, Si, and Fe account for almost all of the cations in the anhydrous analyses. The two drill hole samples (columns 2 and 4) are more similar to each other than either is to the "hard rock" samples. The variations in major element chemistry are quite significant - a large number of whole rock analyses would be required to estimate adequately the mean bulk composition of the serpentinite. Table 3.4 displays the anhydrous bulk compositions of Table 3.2 recalculated into the normative Ol-Opx-Cpx-spinel ulcramafic system, using a method called "spinel norm". The CIPW norm calculation (Cross et al., 1902), was originally designed for feldspar bearing rocks which crystallized at low pressures, and is not appropriate for high pressure rocks which crystallized above the stability field of anorthite. Modal mineralogy produced by the CDPW norm does not correspond to that present in high pressure ultramafic rocks. I therefore utilized a modified calculation procedure called "spinel norm" for these rocks, in which low pressure phases are replaced by their high pressure equivalents, and the resulting ternary plots are projected through spinel. I replaced albite by jadeite and anorthite by spinel + clinopyioxene; the Tschermak (Tk) component of the pyroxenes was set to zero for these samples. A few extremely calcic rocks also required a fictive wollastonite component to accommodate Ca: in actuality such rocks would accommodate excess Ca in garnet. A computer code widely used in Switzerland (MANNOR, written by P. Ulmer) has adopted a similar strategy. 3-37 Figure 3.3 - IUGS (1973) classification scheme for uitramafic rocks. The units in the ultramafic ternary are modal (volume) percent, very nearly equal to oxygen equivalent percent. 3-38 Figure 3.3 - IUGS Classification o f Ultramafic Rocks o orlhopyroxtnilc wehstcrite clinopyroxenitr Figure 3.4 - Projection of serpentinite bulk compositions into the ultramafic ternary of Figure 3.3. See Figure 3.3 for field boundaries. Projection through spinel. Note analyses lie in two distinct groups (see text). 3-40 Figure 3.4 - Projection o f Serpentinite Bulk Compositions Figure 3.5 - Compositional variation in serpentinites expressed as mixing line between end members of pure serpentine, and a mixture of brucite + magnetite appropriate to the Mg-Fc ratio in host rock. 3-42 o ca a t------ ------- 1------ ------- r 3K R o2w+eorj gure 3.5 - Compositional Variation in Serpentinites S102+ A 1203 Figure 3.4 projects the calculated compositions of Table 3.3 into the ultramafic ternary system Ol-Opx-Cpx, where they plot along the Ol-Opx join. The analyses fall into two groups, a "dunite group" low in Opx and a "harzburgite group" rich in Opx. Although both groups lie within the harzburgite field according to the IUGS classification scheme, in this study it is convenient to refer to the rocks as dunite and harzburgite. In the field, these two groups are readily distinguishable by their mineralogy and alteration properties. Figure 3.5 shows the inverse correlation, observed in the whole rock serpentinite analyses, between Si02 + A1203 (which are contained in modal serpentine minerals), and "excess" MgO and Fe^, which are contained in modal brucite and magnetite respectively. The compositional trend is a mixing line between two end members defined in the following way: a pure serpentine composition containing 50 wt. % MgO, and a composition consisting of a mixture of brucite + magnetite, in proportions appropriate to the Mg/Fe ratio of the rock. MgO and FejOj were added together and are defined as "excess" when their sum exceeds 50 wt %. This excess amount then represents variation toward an end member component of brucite + magnetite. Nearly all of the bulk analyses of the serpentinite plot along or close to this mixing line. The New Idria serpentinite is far from homogeneous, with mineralogical and petrological variations on several scales. The rock consists principally of fractured and sheared serpentine rock of several types, weathered to a depth of several meters in places. This weathered zone is best seen in the southern portion of the District - a powdery matrix of crumbled chips and pulverized sheets consisting principally of chrysotile. The fractured serpentine rock underlies the weathered zone and stands out in positive relief as knobs and towers in a variety of shapes. These knobs and towers are frequently green in color when freshly exposed, weathering to a brick red after a short time. In general at New 3-44 Idria, knobs and towers of all rock types weather to about the same brick red color. The abundance of resistant knobs and towers increases north of San Benito Mountain. Within the serpentinite there are numerous tectonic inclusions of contrasting rock type which may also form resistant knobs: these are discussed in Chapter 4. Finding representative rock samples in this environment is difficult because of the uncertain mixture of resistant and crumbled rock. This may explain the differences in bulk mineralogy reported by previous workers. In an environment dominated by crumbly rock there is a tendency to ovcrsample the resistant portions. This may be the reason why Coleman (1957) originally suggested that the serpentinite is dominated by andgorite, while later studies (e.g. Mumpton & Thompson, 1975; Coleman, 1980; this study) conclude that chrysotile is the dominant phase, and that andgorite and lizardite are less abundant. Based on bulk composidon and relict textures in the seipendnite, I conclude that the ultiamafic protolith was a peridodte consisting of dunite and harzburgite, together with minor Iherzolitc and pyroxenite. The serpentinization process has not affected these rock types equally. The ultramafic protolith at New idria was apparently richer in harzburgite in the northern half of the district; serpendnization and weathering are less complete there. While the first asbestos mine was in the northern end of the District, (Laizure, 1922) all of the large open pit asbestos mines opened since 1960 have been located in the southern half of the district where serpentinizarion after dunite is more complete. Magnedte and chromite arc both found as accessory minerals in serpenunized harzburgite, as in dunite. 3-45 B . Geothermometry from Relict Igneous Minerals As noted earlier, the ultramafic protolith at New Idria was a peridotite consisting mainly of dunite and harzburgite. However, a low outcrop near PerovsJdte Knob consists of harzburgite crosscut by a late stage Iherzolite vein. The vein at this locality is the only sample found in this study containing unserpentinized cores of relict olivine, orthopyroxene, and clinopyroxene in the same rock. If prior to serpentinization this Iherzolite was an equilibrium phase assemblage, and if serpentinization has not affected the composition of the relict cores, then it is possible to calculate the crystallization temperature of this Iherzolite. If successful, this would be an important clue to the origin of the New Idria District. Table 3.5 shows representative igneous pyroxene and olivine compositions from New Idria (under the above assumptions) compared with similar pyroxenes in other alpine serpentinites. Coexisting olivine compositions from New Idria are also given. The pyroxene analyses are plotted in the pyroxene quadrilateral. Figure 3.6. New Idria pyroxene compositions are very similar to those of the other igneous pyroxenes; they differ" from the others in the table mainly by having lower A1 contents. The ratio Mg/(Mg+Fe) or XMg in Cpx is higher than in coexisting Opx for all samples in the sample set, reflecting apparent equilibrium partitioning of Mg and Fe between these phases. XMg in olivine is essentially equal to that in coexisting Opx, for the magnesian compositions found in this study. Bowen & Schairer (1935), however, showed that for more iron-rich compositions, XMg in olivine is less than that in coexisting Opx. Combining these facts we have: 3-46 It is interesting to speculate on the crystal chemical reasons for unequal Mg-Fe partitioning between Cpx and Opx. Apparently it is energetically less favorable to accommodate Fc in the Cpx structure. The order of preference for the larger M2 site in pyroxene is Ca > Fe > Mg. In Cpx, most of the 8 coordinated M2 site is occupied by Ca. forcing Fe into the smaller Ml site. In Opx, however, where Ca is minor. Fe can more readily occupy the distorted 6-coordinated M2 site. Ohashi & Burnham (1975) discussed the relationship between ionic substitution and structural changes in clinopyroxenes. We may apply the pyroxene geothermometer of Wells (1977) to the New Idria samples. This solvus model is based on the solubility of Opx in coexisting Cpx, for which the equation of equilibrium is: (MgjS^Ofilop* = (Mg2Si206)cpx An activity model, based on that of Wood & Banno (1973), represents the distribution of cations between the Ml and M2 sites in pyroxene: Ca2*, Na* and Mn2* are assigned uniquely to M2; Al3*, Cr3*, Ti4*, and Fe3* to Ml. Mg2* and Fe2* are initially assumed to be randomly distributed between Ml and M2, with a small empirical correction applied later. The parametric equation for the Wells model is as follows: T (K) = mi This model yields 877C for the crystallization temperature, which is almost certainly below the solidus temperature for this bulk composition. Therefore these pyroxenes have probably re-equilibrated in sub-solidus conditions and the 877C temperature estimate applies to pre-serpentinization but post-igneous crystallization conditions in the mantle. 341 Figure 3.6 - Igneous pyroxene compositions in the pyroxene quadrilateral. Data from Table 3.5. Units are mole fraction. Note Cpx is more magnesian than coexisting Opx. 3-48 Ue1 Figure 3.7 - P-T subsolidus phase diagram for Iherzolite (CMAS), showing the nature of the aluminous phase as a function of pressure (Hall, 19S7). 3-50 Figure 3.7 - A Phase Diagram for Lherzolite Pressure IKbarsi The Boyd (1973) gcothermometer is based on Ca partitioning between coexisting orthopyroxenc and clinopyroxene. Boyd provided a table with experimentally determined points on the diopside-enstatite solvus; applying his geothermometer is simply a matter of calculating the Ca/(Ca + Mg) ratio (atom fraction) for the Cpx phase, and interpolating temperature in his tabic. The Boyd model yields a temperature of about 900C, which is in good agreement with the estimate provided by the Wells model. How reliable are the temperature estimates? All of these models suffer from the possibility of errors in formulation of the activity - composition relationships, and all require an assumption of phase equilibrium. Neither of these conditions can in general be proved beyond reasonable doubt Therefore, the temperature estimates should be considered a useful, but not definitive, guide. None of the existing Al-in-Opx geobarometers can be used in the present case owing to lack of plagioclase or garnet The A1 content of Opx coexisting with either plagioclase or garnet depends strongly on pressure, a circumstance favorable to the development of geobarometers. The corresponding relationship of A1 content in Opx in assemblages containing spinel depends only weakly on pressure. This gap in workable geobarometers is a stumbling block for petrologists today. Evans (1977) reviewed potential geothermometers and geobarometers for ultramafic rocks; he presented many more thermometers than barometers. More recently, Carswell & Gibb (1987) reviewed geothermometers and geobarometers for garnet lherzolites. Another approach to obtaining a rough pressure estimate is the examination of the phases present in the New Idria serpentinite itself. The lherzolite phase diagram (Figure 3.7), shows that if the pressure of equilibration was below 9 kb, I would expect to see relict plagioclase or its alteration products, e.g. amphibole or epidote. If the equilibrium 3-52 pressure was between 9 and 20 kb, I would expect to see spinel. If the equilibrium pressure was 30 kb or higher I would expect to see pyrope garnet or its alteration products, e.g. chlorite pseudomorphs, and I would expect to see a significant chrome diopside component in the clinopyroxene. At New Idria, since igneous (pyrope) garnet, plagioclase, and their alteration products are absent from the least altered samples, and the Cr content of primary Cpx is very low (less than 0.50 wt. % Cr203 in microprobe analyses - see Table 3.5), the best esdmate of pressure of original equilibnadon lies within the spinel stability field of 9-20 kb. This corresponds to a depth estimate of 30-60 km. X. Accessory Minerals of the Serpentinite As noted earlier, the remarkable minerals for which the District is known are, for the most pan, associated with the boundary faults of the serpentinite or with tectonic blocks within the serpentinite. There are also accessory minerals found within the serpentine mass per se. In this section I focus on some of these minerals. Some of the occurrences are quite typical of alpine serpentinites, while others are anomalous and merit further study. In this study no attempt was made to survey accessory minerals of the seipentinite; those mentioned here were studied only is encountered. A. Brucite Brucite is a common product of the serpentinization of dunite, as in reaction (2). At New Idria, brucite is widely distributed but is inconspicuous and easily overlooked. It was suggested by Mumpton & Thompson (1975) that extreme weathering and alteration of the serpentinite has resulted in a brucite depletion zone in the uppermost 10 m. of the 3-53 deposit. X-ray diffraction analysis of samples taken from the surface of the deposit generally show no identifiable brucite peaks, as in Table 3.1. However, X-ray analysis of sample 29653 did show the presence of brucite; this sample was retrieved from a drill hole 100 ft. below the surface. As discussed below in the section on carbonates, the near-surface absence of brucite may be due to reactions with C02-rich meteoric waters, and the presence of brucite at deeper levels may show that meteoric waters have not yet reacted with brucite there. Many thin sections of massive serpentinite examined in this study contain no observable brucite; in the Mumpton & Thompson (1975) study about half of their massive samples contained brucite. Where observed in thin section, brucite is frequently found in sheared chrysodle/lizardite rock as bundles of slip fibers showing first order interference colors. The fibrous form of brucite is called nemalite (Whittaker & Middleton, 1979). The modal abundance of brucite does not exceed 5% in most samples. Brucite was occasionally found as dispersed grains in antigorite rock, with a mode of < 1 %. In"this study no brucite compositions were determined; Mumpton & Thompson (1975) however, commented on the anomalously high Fe content of New Idria brucite; they suggested that the elevated Fe content contributed to the rapid brucite breakdown in the surface weathering zone, resulting in the production of the basic carbonates pyroaurite (Mg6Fc2C03(0H)j64H20) and coalingite (Mgj0Fe2CO3(OH)242H2O), which give open slopes at New Idria their characteristic light brown color. Coleman (1977) noted that Fe-Mg partitioning among brucite, olivine and serpentine, follows the relationship: 3-54 B. Magnetite Magnetite is ubiquitous at New Idria and is the most easiJy recognized accessory mineral in the serpendnite. In this regard New Idria is typical of alpine scrpentinites. At least two generations of magnetite were found in this study. Magnetite occurs as disseminated grains, in foliation-parallel stringers, and as drusy coatings in open fractures. Magnetite was formed from the fayalite component of olivine under relatively reducing conditions during serpentinization, probably by a reaction similar to that proposed by Betechtin (1961): 6(Mg,JFe0J)SiO4 + 71^0 = 3Mg3Si203(0H)4+ Fe^ + ft, olivine serpentine magnetite (8a) " or alternatively, since 21^0 = 2ft, + 02, 6(Mg,jFe0_j)SiO4 + 6HjO + 0.5O2 = 3Mg3Si205(0H)4+ Fe304 (8a) olivine serpentine magnetite Ramdohr (1967) suggested that serpentinization of olivine under strongly reducing conditions would produce serpentine, magnetite, and native iron or iron alloys. One possible reaction with these products is: 8(MguFe0_5)SiO4 + 8*^0 = 4Mg3Si203(0H)4+ Fc304 + Fe olivine serpentine magnetite (9) Some alpine setpentinites have preserved such extremely reduced assemblages including native metals and the nickel-iron alloy awamite, N^ 3Fe (e.g. Perctti et al., 1992), but only limited evidence for native iron or iron alloys at New Idria has been found so far. 3-55 Coleman (1986) included josephinite in his list of New Idria minerals, but did not comment on its occurrence. Josephinite is actually a rock type, consisting of taenite, awamite, and andradite at its type locality (Botto & Morrison, 1976). However, many workers now considerjosephinite to be a synonym of awaruite. On the basis of this limited evidence, I conclude in this study that the environment during serpentinization was relatively reducing. The occurrence of magnetite at New Idria includes randomly dispersed opaques with varying modal abundance (0*2%) within serpentine rock, but also includes filamentous and undulatory bands parallel to schistosity in schistose serpentine. In massive antigorite serpentinite, magnetite may occur as randomly dispersed, irregular grains up to a few hundred microns across, or may be finely comminuted where the texture is mylonitic. Finely dispersed magnetite in serpentine gives the rock an overall black color. Foliationparallefmagnetite bands in some rocks suggest pressure solution of the surrounding rock; this texture deserves further investigation. Finally, magnetite appears as a second generation vein-filling mineral, or as drusy coatings on walls of veins and vugs. In many of these cases, perfectly formed octahedra of magnetite up to 1 mm in diameter arc found coating rock surfaces. Sometimes these black coatings resemble black garnets which also form drusy coatings, as described in Chapter 4. A pocket magnet is an essential field tool for identifying the mineralogy of these fine-grained coatings. Most New Idria magnetites appear to have compositions near the ideal end member formula. Magnetite compositions were not studied intensively, but using EDS analysis, in which the detection limit for minor elements is about 1%, no discemable ulvospinel or hercynite components were found in the samples examined. 3-56 c. Sulfides Copper sulfides are not abundant but are widely distributed in all types of New Tdria rocks. Chalcopyrite is found within serpentinite rock, while djurleite (Cu3,S6) is found within altered mafic schists. At the Gem Mine locality, chrysocolla forms alteration haloes around sulfide grains, typically the low temperature phase djurleite. In chloritediopside rocks, chalcopyrite is sometimes overgrown by garnet (Plate 3.7). D. Chromite The peridotite from which the serpentinite formed contained chromite. The occurrence of chromite within the serpentinite is widespread but irregular. There are few concentrations of economic grade. Exceptions are those of the Butler Estate, Corbctt-Byles and Big Ridge mines in the southern portion of the District. At these mines boulders arc found containing 75% or more chromite with interstitial serpentine minerals. Mining of chromite as a strategic commodity took place until the late 1950s, when cheaper sources became available in politically stable areas. Matthews (1961) discussed the mining operations and historical background of the Butler Estate chromite mine. Table 3.6 shows representative analyses of New Idria chromites. There arc two groups in the table, a primary group and an altered group. The primary group includes chromites with appreciable contents of Cr, Fe, Mg. and Al, but only trace amounts of Ti, Mn, Ni, and Zn. The compositions are typical of chromites from alpine-type ultramafic bodies worldwide, and are properly namedferroan magnesiochromites. The key ratios Mg/(Mg+Fe), Cr/(Cr+Al), and Fe/(Fe+Cr+Al), given in Table 3.6; all plot within the alpine peridotite field of Irvine (1967). These chromites are interpreted as primary magmatic products. They are unzoned, with no discemable compositional variation 3-57 Plate 3.7 - Photomicrograph of chalcopyrite grain overgrown by filigree of Ti-gamet, in a matrix of chlorite. Perimeter of garnet overgrowth is decorated with octahedra of magnetite. Sample 989-46b. Backscatter electron imaging, 400X; field of view 25 pro. 3-58 between core and rim. In thin section they range from deep red to opaque. Unaltered chromite grains occur in fully serpentinized ultramafic rock. A few chromites contain ovoid inclusions of serpentine, which may be serpentinized primary olivine inclusions. The compositions of the chromites may be adequately expressed in terms of the quaternary system chromite (FeCr204), magnesiochromite (MgCr204), magnetite (FeFe^), and spinel (MgAl204). Only limited solid solution towards magnetite is observed. Igneous clinopyioxenc associated with chromite has a small chromc-diopside component (less than 0.50 wt. % Cr203 in microprobe analyses - see Table 3.5). Ramdohr (1967) noted that chromite is the only opaque mineral commonly found in serpentinites which is likely to have persisted from the ultramafic pnotolith. At New Idria this interpretation is consistent with field evidence, which shows that chromite alteration haloes overprint the serpentine fabric and therefore postdate serpentinization. The altered group of chromites shown in Table 3.6 occurs in rocks which have clearly undergone post-serpentinization metamorphism, tentatively assigned to the M2 event (sec Chapter 4). Altered chromites are frequently found rimmed by the chromian chlorite kammererite; this chlorite overprints the surrounding serpentine fabric in these rocks. Chromian garnet is also associated with chromite alteration (see below). These altered chromites arc cryptically zoned, with Cr-rich cores and Cr-poor rims, as shown in Table 3.6. The rim compositions are slightly chromian magnetites: Mg and A1 occur at the trace level, while Ni and sometimes Mn are enriched compared with the primary chromite compositions. This enrichment may be due to Fe, Ni, and Mn all behaving similarly during M2 metamorphism. Table 3.7 shows representative compositions of kammererite which rims altered chromite. Under the metamorphic conditions in which the chromite and kammererite 3-60 formed, Cr, Al and Mg partitioned strongly in to the chlorite phase, while Fc was concentrated in the oxide phase, an impure magnetite. The question may be raised whether any P-T information can be garnered from the compositions of chromite and coexisting minerals. Sack & Ghiorso (1991) reviewed the role of chromite as a pctrogcnetic indicator, and in particular discussed the olivine-spinel geothermometer. This avenue is worth pursuing in the future, provided that suitable samples from New Idria can be gathered. E. Chromian Andradite Garnets Andradite garnets containing up to 38 mole percent uvarovite component are found in slabby serpentinized harzburgite blocks. These garnets are anhedral, emerald green, and often associated with relict chromite grains. They form irregular masses up to l mm. in size. Table 3.8 gives garnet analyses from several grains in a sample from New Idria, along with comparative analyses from other localities. The principal variation in the New Idria garnet analyses is the ratio of Cr/Fe; Ca is relatively constant, AI, Ti, Mg, and Mn are all minor. All of the analysis totals are under 100%, suggesting that these garnets may be hydrous. Figure 3.8 shows the positions of these ugrandite garnets in the ugrandite ternary space (uvarovite-grossular-andrati.te). Additional published garnet analyses from other localities are also plotted. The New Idria specimens are essentially chromian andradites; compositional variation in the ternary system may be almost completely accounted for by the exchange vector CrFe.,. Figure 3.9 shows the near-perfect inverse correlation between Cr and Fe in New Idria uvarovites, consistent with the CrFe.j exchange vector. 3-61 Figure 3.8 - Projection of Cr garnet compositions into the Ugrandite garnet ternary space using mole fractions. New Idria garnets are chromian andradites. Symbols as follows: filled squares: New Idria samples (this study); open squares, Val Malenco samples, (Milntener, 1991); filled triangles, Ontario samples (Duke & Bonardi, 1982); open circles. South Africa and Finland samples, undifferentiated: see Table 3.8 for details. 3-62 mole proportions, projected through pyrulspite components, Ti, and water. Figure 3.9 - Octahedral Fe and octahedral Cr in New Idria garnets, showing near-perfect inverse correlation, demonstrating progress along the CrFe.i exchange vector. 3-64 aj |Bjpaqnao Figure 3.9 - Cr-Fe Exchange in New Idria Garnets Octahedral Cr Figure 3.10 - Si variation in chromian andradites, expressed as Si02 weight % against (100% - oxide totals). Good correlation is evidence for hydrogamet H4Si j exchange. 3-66 Figure 3.10 - Si variation in Chromian Garnets from New Idria <0 CO # cino CO Ibjoj sislteav - %00t m* rojs With few exceptions, chromian garnets from other localities are aluminous (Isaacs, 1965; Kalamarides & Berg, 1988); they vary along the join uvarovite-grossular, according to the exchange vector CrAl.j. Figure 3,8 shows that the New Idria samples arc the least aluminous of the garnets plotted. The chromian garnets from the Val Malcnco serpentinite arc most similar to the New Idria garnets, but have less Cr and more Al. The Ontario samples (Duke & Bonardi, 1982) arc garnets from a serpentinized wchrlite in Ontario. Two other occurrences are not shown in Figure 3.8: Jan et al. (1984) reported chromian andradites from a chromitiie layer in Pakistan. Rost et al. (1979) also reported chromian andradite from the Urals, but their paragenesis is unknown. The New Idria specimens are believed to be the tint chromian andradites reported from the United States. Table 3.8 shows also that the New Idria garnets have low totals and may be hydrous, as arc many other New Idria garnets. Therefore the relation between Si and the analysis total was studied. Figure 3.10 shows a good correlation between Si02 wt % and the deficiency in the analysis total. This trend is to be expected if the deficiency is due to presence of tetrahedral OH groups substituting for Si. However, the amount of apparent water thus calculated exceeds the amount predicted by simple progress along the hydrogamet exchange vector HjSi^. It is also possible that some other divalent cations have been overlooked. EDS analysis, however, did not show any additional elements other than those presented in Table 3.8. X-ray refinement of the cell edge in one sample (by L.C. Pitman) gives a value of 12.071A 0.017, compared with tabulated values for uvarovite (11.990A) and andradite (12.059A), as reported by Meagher (1982). Shoji (1974) reported a steep positive relationship between the cell edge and progress along the hydrogamet exchange vector in grossular, the same is probably true for andradite, and one would expect the cell edge for a hydrous andradite to exceed 12.059A. The refined cell edge of 12.071 A therefore 3-68 suggests and is consistent with compositional variation along the two exchange vectors CrFe.j and H4Si.j. Future study, particularly including IR measurements, is required to resolve this question quantitatively. F. A Blue Amphibole An unidentified blue sodic-calcic amphibole was found in veins within antigorite serpentinite along the KCAC road near the western margin of the New Idria seipcntinite. The locality was brought to the author's attention by D.S. O'Hanley and R.G. Coleman. The amphibole composition lies roughly one third of the way between winchite and eckermannite (Figure 3.11); the paragenesis is a new one. Winchite is an A-site-empty sodic-calcic amphibole with a general formula of: NaCa(Mg,Fe,Mn,Al)5Si8022(0H)2 Winchite is related to tremolite by the two exchange vectors NaSiCa^Al.j (plag) and Al2Mg.iSi.i (tk). Winchite is found in mafic schists of the medium pressure series. Winchite has not previously been reported from a serpentinite. Eckermannite is an A-site-occupied sodic amphibole, with a general formula of: NaNa2Mg4Al5Si8022(OH)2 Eckermannite is related to tremolite by the exchange vectors NaAlfJ^Si.! (edenite), NaSiCa^Al.j (plagioclase), and AUMg.jSi.j (tk). Extensive substitution of both ferrous 3-69 Figure 3.11 - Condensed Space for Sodic-Calcic Amphiboles, redrawn after Thompson (1981). Lower left comer of cube is the tremolite composition Ca2Mg5Si8022(0H)2, other amphibole compositions obtained via the exchange vectors shown. Shaded plane is limit of accessible compositions for clinoamphibole, in which all tetrahedral sites are occupied by Si. Symbols are Tr. tremolite; Et: edenite; Wn: winchite; Rt: richterite; Kt: katophorite; Bn banoisite; Tm: taramite; Gl: glaucophanc; Ek: eckcrmannite. Mi: miyashiroite; At: Ca2Mg4Al2Si7022(0H)2; Pg: pargasitc; Ap: NaCa2Mg3AlJSi5022(0H)2. Black striped ball is the blue amphibole from this study, lying in the shaded plane about 1/3 of the distance from Wn to Ek. 3-70 Figure 3.11 - Condensed Space for Sodic-Calcic Amphiboles a u t/J CM and ferric iron for Mg and Ai occurs. The ferroan members of this family are blue. Eckermannitc is a rare mineral, normally associated with peralkaline igneous rocks. To the best of the author's knowledge, no occurrence of eckermannitc in serpentinites has been reported (but see Matsyuk et al., 1991, fora report of K-richterite in dunite). The New Idria amphibole specimens were examined petrographically, by EDS analysis, and by X-ray analysis. Petrographic examination showed birefringem fibrous mats with fibers too small to obtain interference figures. Many fibers are curved, especially those near the center of the veins. Associated minerals in the veins are andradite garnet and chromite. Some of the chromite has altered to kammererite. Table 3.9 gives a representative microprobe analysis and possible structural formula for this amphibole. Measurable F and G were present in all analyses. No significant compositional variation was observed between amphibole in the vein center and that along the vein walls. The M4-site of the structure is inferred by stoichiometry to be shared between Na and Ca with a small amount of Fe2*. K and excess Na must partially fill the A-sitc; the A site occupancy could be as low as 0.16, depending on the ferrous/ferric ratio chosen. A gravimetric analysis would be needed in order to determine this ratio. In an end-member eckermannitc the A-site would be completely filled, and in winchite empty. An approximate formula for this amphibole, under the above assumptions, is: (Na,K)oj,(Na.a,Fe2^)2(Mg,Fe3*.Fe2^.Al,Cr.Mn.Ti)JSi8.oo022(OH,a.F)2 A small fragment was studied by X-ray powder diffraction; Table 3.10 displays the indexed pattern. The pattern is similar to those of richterite and winchite. 3-72 A definite statement about the formation of these amphibole veins in anrigorite would be premature, but the physical proximity to the boundary fault, at which the serpentine is faulted against Franciscan grcywackes and metavolcanics, suggests that the occurrence may be metasomatic: the required Na may have been advected in aqueous fluids from the Franciscan rocks. The environment is A1 deficient, resulting in a strongly peralkaline composition when Na is added. The conditions of metamorphism are unknown, but the appearance of winchite is consistent with the medium to high pressure, low temperature assemblages found in some tectonic blocks at New Idria. G. Secondary Alteration Minerals Many episodes of metamorphism and mineralization have occurred during the evolution of the New Idria District One process, ongoing today, is the formation of a variety of carbonate minerals due to the percolation of meteoric water through the serpentinite. Calcite, the basic Mg carbonates hydromagnesitc ^MgCOj^MgfOH^HjO) and aninite (MgC03*Mg(0H)2*3H20). and complex Fe-Mg carbonates such as coalingite and pyroaurite (see Section A) are products of this process. Calcite is found frequently as an alteration product of all the rock types in the District; textures indicate that calcite precipitation was mainly along veins and fractures. As noted by Coleman (1977), Ca-rich ground waters are ubiquitous in serpentinites; the original source of Ca at New Idria may have been Cpx in the protolith. The formation of calcite must postdate serpentinization, because serpentine itself is unstable relative to a mixture of talc and magnesite in the presence of C02-bearing fluids, (e.g. fluids with XC02 0.01 at 350C, 1000 bars) according to the component reaction: 3-73 2Mg3Si205(0H)4 + 3C02 = Mg3Si4OI0(OH)2+ 3MgC03 + 31^0 (10) serpentine talc magnesite The assemblage talc + magnesite has not been found at New Idria, but is common in metamorphosed ultramafics from Vermont where C02-rich fluids infiltrated during Acadian metamorphism (Sanford, 1981; Chidester et ah, 1978). The basic carbonates hydromagnesite, artinite and dypingitc (MgjCOj'MgfOHIjOHjO) are common at New Idria, where they have formed through reactions between C02bearing meteoric waters and the minerals of the serpendnite. Brucite, a product of primary serpendnizadon through reaction (2) or of prograde metamorphism of serpendnite through reacdon (1), is nonetheless scarce in outcrop. Brucite is reaedve and has largely disappeared from the weathering zone (upper 10 m.) because of component reactions similar to: 4Mg(OH), + 3C02 = 3MgC03Mg(0H)23H20 brucite hydromagnesite (11) Analogous reactions are probably responsible for the appearance of artinite and dypingitc. Artinite forms in fractures and cavities where open space allows spheroids of radial artinite fibers to grow to centimeter size. Dypingite forms millimeter-sized non-fibrous pisolites which coat vein walls. However, hydromagnesite also locally has a pisoliric habit, making field identifications somewhat uncertain. Ordinarily, however, hydromagnesite forms in intersecting veins which quickly destroy the integrity of the host rock (Plate 3.8). 3-74 H. An Unidentified Blue Coating A bluish grey coating found on fracture walls and slickensided serpentine surfaces was sampled in this study. X-ray analysis by Malcolm and Daphne Ross (personal communication. 1993) showed the material to be at least partially crystalline. The material is apparently a polyphase material, with detectable peaks for montmorillonite, quartz, and feldspar. Chemical tests showed the blue coating to be insoluble in HQ and moderately soluble in concentrated NaOH. This blue coating has been observed in Appalachian serpentinites by the author, but its exact origin is unknown. 3-75 Plate 3.8 - Photo of hydromagnesite veins in a serpentinite outcrop on San Carlos Peak. Hydromagnesite forms by reaction between brucite and meteoric waters. 3-76 XI. Discussion and Conclusions The mineralogy and petrology of the New Idria Serpentinite is seen to be complex, yet not without parallels in other alpine serpendnites and ultramafic bodies. The very high degree of alteration and weathering has obscured much of the evidence at New Idria, but there remain telltale signs of the evolution of this body. The protolith for the New Idria Serpentinite was a depleted dunite with lesser amounts of harzburgite, Iherzolite and pyroxenite. The dunites and harzburgites are probably residues of one or more partial melting events. The Iherzolites described in this chapter occur in crosscutting veins and have relatively depleted compositions. These ultramafic rocks probably represent pieces of oceanic crust or underlying mantle from the Farallon plate, that were obducted onto the western margin of North America in the late Jurassic. The serpentinized ultramafic rocks rose as a diapir through the overlying Franciscan Formation and Great Valley Sequence. Many of the mineral associations and compositions at New Idria arc similar to those at Burro Ml, California, which lies on the west side of the San Andreas Fault (Loney et al., 1971). Serpentinization at Burro Ml is much less complete that at New Idria. Loney ct al. interpreted Burro Ml as a dismembered portion of the Coast Range Ophiolite. It is likely that Burro Ml, which was not visited in the present study, contains important clues to the origin of the New Idria Serpentinite. The main points of this chapter that will be used in other chapters are: 3-78 1. The phase relations among the serpentine minerals are difficult to show schematically, and are not completely understood. In general, lizardite and chrysotile are the low temperature serpentine minerals; antigorite is the high temperature phase. 2. Due to the importance of Fe and Al, discussion of serpentine phase relations in MSH alone is probably unproductive; MASH is the simplest realistic system. The MSH system, however, can be used as a first-order approximation for formulating phase relations between serpentine minerals and other minerals found in serpentinites. 3. A necessary condition for the serpentine minerals to be true polymorphs is that they be chemically identical: it is now clear that antigorite, chrysotile and lizardite are not chemically identical. Each occupies a compositional range, and these ranges partially overlap. 4. Compositional variation in serpentine occurs along the three exchange vectors Al2Mg4Si., (Tk); FeMg4 (Fm); and Fe?*Al., (Fa). The dioctahedral vacancy substitution [)Al2Mg 3 is not known to occur in serpentine. Trace amounts of Ca, Na, and K reported in serpentine analyses may represent mixtures of other silicates with serpentine in the analyzed samples. 5. The unshared edges of the octahedral sheet in lizardite are 3.07A compared with 3.22A in brucite, and 2.97A in talc. The octahedral edge dimension in a slab from the pcriclase structure is 2.98A. The unshared edges in lizardite might be expected to be shorter on the tetrahedral side than on the non-tetrahedral side, but the difference is less than 0.001 A. 3-79 6. The unshared edge length in talc is smaller than in lizardite: I would therefore expect that tetrahedral rotation in talc would be greater, not less, than in lizardite. The opposite appears to be true. 7. In antigorite, the Si-O-Si bonds at the points where the tetrahedral sheets invert are responsible for the tough, resistant nature of andgorite compared with lizardite or chrysotile, in which the adjacent layers are held together by weak bonds involving hydrogen. Antigorite is a member of a polysomatic series between lizardite and talc (e.g. Otten, 1993; Sanford, 1978; Thompson, 1978). The conventional antigorite formula is equivalent to 15 Liz + 1 Tic. If the wavelength of the modulation were to decrease the structure would become increasingly talc-like, while if the wavelength were to increase the structure would become increasingly lizardite-like. 8. If aH20 by itself affects the formation of lizardite versus chrysotile, then the assumption that the water content of lizardite and chrysotile is equal must be challenged. The molecular water in the halloysite structure could possibly correspond to (thus far undetected) water in the chrysotile structure. This would lead to a hypothetical chrysotile formula of MgjSijOjfOHj^nHjO. The value of n is unknown but is probably small. 9. At New Idria. the dominant phase assemblage of chrysotile + brucite indicates an Mg/Si ratio lying between forsterite and enstatite, and a low temperature equilibrium (<250C). Antigorite knockers described in Chapter 4 record a higher temperature. 10. At New Idria, lizardite is much less abundant than chrysotile, possibly due to the recrystallization of earlier lizardite under shearing stress. An alternative explanation for the abundance of chrysotile is the extensive growth of cataclastic fabric due to the predominance of dunite in the protolith. 3-80 11. In principle, serpentinized ultramafic rocks could contain talc as part of the low temperature assemblage, when the serpentinizing fluid is pure HjO. In fact, this is rare, because the required silica-rich bulk composition would place the parent rock in the "olivine orthopyroxenite" field of the IUGS classification, which is an extremely rare rock type. Therefore, talc does not normally appear as a low temperature mineral when fluids are pure HjO, but instead as a prograde metamorphic replacement of andgorite at roughly staurolite grade (Trommsdorff & Evans, 1974). If the seipendnizing fluids are rich in C02. however, then magnesite and talc or tremolite may indeed appear as low temperature products. This has not happened at New Idria but has happened in the Appalachian serpentinites of Vermont and Quebec. 12. Serpentine formed after dunite often has a knobby outcrop texture formed by intersecring serpenrine/brucite veins which isolate rounded kernels of unserpentinized dunite. 13. Serpentine formed after harzburgite often has a slabby texture, and the serpentinized rock fractures, leaving sharp edges rather than the rounded kernels of serpentinized dunite. 14. Based on bulk composition and relict textures in the serpenrinite, I conclude that the ultramafic protolith was a peridotite consisting of dunite and harzburgite, together with minor Iherzolite and pyroxenite. The ultramafic protolith at New Idria was apparently richer in harzburgite in the northern half of the district; serpenrinizarion and weathering are less complete there. 3-81 15. Relict olivine, orthopyroxene and clinopyroxcne are locally preserved in Iherzolitc veins. Their compositions are related by the inequality: reflecting apparent equilibrium partitioning of Mg and Fe among these phases. The Wells (1977) geothermometer based on solubility of Opx-in-Cpx yields a crystallization temperature of 877C for an Opx-Cpx mineral pair from a Iherzolitc vein. The Boyd (1973) geothermometer based on Ca-partitioning yields a temperature of 900C, which is in good agreement. These pyroxenes have probably re-equilibrated in sub-solidus conditions and the 877-900C temperature estimate applies to pre-serpentinization but post-igneous crystallization conditions in the mantle.. 16. At New Idria, since igneous (pyrope) garnet, plagioclasc, and their alteration products are absent from the least altered samples, and the Cr content of primary Cpx is very low, the best estimate of pressure of original equilibration lies within the spinel stability field of 9-20 kb. This corresponds to a depth estimate of 30-60 km. 17. On the basis of trace occurrences of awaruite (josephinite), I conclude that the environment during serpentinization was relatively reducing. 18. Accessory chromite includes two groups, a primary group and an altered group. The primary group includes chromites with appreciable contents of Cr, Fe, Mg, and Al, but only trace amounts ofTi, Mn, Ni, and Zn. The compositions are typical of chromites from alpine-type ultramafic bodies worldwide, and are properly namedferroan magnesiochromitcs. 3-82 19. The altered group of chromites occurs in rocks which have dearly undergone post-serpentinization metamorphism. Altered chromites are frequently found rimmed by the chromian chlorite kammererite; this chlorite overprints the surrounding serpentine fabric in these rocks. These altered chromites are cryptically zoned, with Cr-rich cores and Cr-poor rims. 20. Andradite garnets containing up to 38 mole percent uvarovite component are found in slabby serpentinized harzburgite blocks. These garnets arc anhedral, emerald green, and often associated with relict chromite grains. Chromian andradites are very rare: these arc believed to be the first chromian andradites reported from the United States. 21. A blue amphibole was found in veins within antigorite serpentinite along the KCAC road near the western margin of the New Idria serpentinite. The amphibole has a partially filled A-site and appears to lie approximately one-third of the distance between a winchite and eckermannite; the paragenesis is a new one. An approximate formula for this amphibole is: " (Na,K)031(Na,Ca,Fe2*)2(Mg,Fe3+,Fe2<',Al,Cr,Mn,Ti)5Si8 00O22(OH,Cl,F)2 3-83 Table 3.1 XRD Data for Serpentine Sampla 989-45 2 Theta intensity degrees I/I* d-spacing A hkl 12.15 19.40 19.91 24.45 26.00 26.28 35.52 35.72 35.90 36.38 36.68 37.00 38.58 41.91 42.05 46.22 59.68 60.02 60.27 100 10 4 59 4 5 27 26 31 4 4 4 5 7 8 4 4 6 10 7.279 4.572 4.456 3.638 3.424 3.389 2.525' 2.512 2.499 2.468 2.448 2.428 2.332 2.154 2.147 1.963 1.548 1.540 1.534 002 020 004 121 -202 chrysotila -202 lizardite 202 023 132 330 029 Table 3.1 Low aclivlly ol water. In primary serpenlinization of olivino, favors production of lizardite In most cases lizardile Is longlh-fast while antigorile and chrysolite arc length-slow Table 3.4 folio EOO'O eoo'o [pr | 2 uw. *3 O 73 c 0 000 000 0 000 0 000 000 0 000 iO C _| -^21ol|oiIS<tj| 8ml|~eil ^i1 pj1oj| o Oi S; . t-. <i O' u. ii Si 2Tio 1 500 0 1 000 0 Weighl % 29653 43.85 060 09 79.700 101.940 152.02o| MPfop. 0.730 ISO I O od 000 oo ldO L sOiIsO: 1 Tolals COM Oo o do 080'9S 9 9 0 0.12 46.10 71.050 70.940 40.320 0.20 61.960 94.200 141.850 18.016 e 2 0.120 1.143 0.012 n c (0 IU<71 >X.1 Oi ooo Xa, M Q. o o! | EsI Mode Mol proportion (A O 75 O MB EOO'O ooo ooo do oOO o 000 0 d o Oo `Hi 00 6E0 0 EOO'O 000 0 000 0 000 0 000 0 000 0 0000 500 0 000 0 o Oo o " if CM i Q Q. (0 i ooo d llm l 0.001 0.013 i 0.024 | 0.0021 Ooo o 01 Totals 90 ____Si balance 0.559 0.729 0.729 i <c/j; i!i i excess 0.000 1 vf il 0.000 CM O d CooM d OI mols 0.132 1.118 1.263 1.263 0.559 [%06 " 000 0 0.012 1 j 0.012 K in Cdx i EOO'O 000 0 1 i i iM 1i ooo oO oo OoO d d i S6E6 0 001 96E0 00 800 0 ! i o do to to CM CM d Ooo d 'ii iii ii iiii ii 0.072 2.236 1 14.41 81.2 2.7 82.7 ( 0.0121 0.132 2.236 1 0.51 1 5.54 2.753 2.380 100 i 1 II 1 11 j i Table 3.4 (olio 000*0 | |000*0 m* o "!s is l 21 1 600 0 Si excess! 01 m olsl 0.373 Ka u .s CO 03 uo*ss hJC- ii|!jIj ii !i M 1 I i i 1 < ! i 1i 1i !II! 1 2.849 _____Ol T o ta ls 0.373 000*0 000*0 000*0 [000 0 I 600 0 ooo CM o O oO oo o CCOO r-* oo ooa ooo d d o d d d d o d O o d o CCMM q Oo 0.423 22.00 i i 5 CCVMJ o t 5 o ooo oo roo* Oo do 0.423| 0.746 OCMS cm q cn COMS q m m r-.' CCGOMO in V'T q in *T* o d tn oo d US q d Table 3.4 folio 3 Wo Spl Co*M* d -oo o CM o o ooo d oGVO d, o oo o o d 060*09 pr oo ooo dd oOo o 6 o oo ooo dd Ej ` 1 QJ ; <i ! ooo d ooo d ooo ooo 11 d d ado. CL ooCD ooo CM o oo q ooq OuqsS ooddoo CCwMD roo- ooq OOo ooo OOO doddo 2c 000*0 000*0 000*0 o oo o o o ooo o d d q d o d 79.700 101.940 152.0201 159.700 70.940 40.320 61.9B0 94.200 141.050 18.016 i oo uqs GqO {1 CMDS si CO 1 CO 212 o q d CCMO oo d oo d CCMD V oo d 03 us q d o o d oo d oo d oo d 5.o5 ii : slslslallsis;o X 1n O1l cm CM 1 CM o i^l < Ol l in| i 00*001 US c _(>0 'o i13! t > * Ol Est Mode (Ol-Opx-Cpx coordinates |Mol proportion o o E Sc *eoO^3 o 2 sample Si02 iT o la ls O x id e t W e ig h t % llm pr 0.724 0.000 Ol mols 0.5B0 1 0.000 --1 JO iw 1 (A/ t a CO 1 ^1 ! ,K <3 c JC CocMn 0) h- jj ! CM 1^ 0 ooo d ooo d ooo o ooo o ooo d OCoM) o oo o *r o o qOO d o oo o OM=1lil 111 ooO d j T o ta ls 0.000 OSS'O io 0.000 0.014 | T"1d ! CCoOM o a CO ooo d ooo o r-* ( o1 o d ooo d ooo d OoO ooo dd oo o 6 o 11 OO io ooo o ooo o --JOo |o oo d *1 1 | !!I ooq O O O oo CL O V CrM- ooo ooo ooo ooo <** S cn r- q o oo o oo ooo ooo 2 o o d d o o U. c dddod i 0o039 94.200 141.850 18.016 I 56.080 40.320 70.940 71.850 152.020 159.700 101.940 79.700 06009 CO CM 2 CcMn cn oo d O o o oq o o o o d oo o 03 CM d CO f-. d oo d oo o oo o o o oo do {T o ta ls |C r 203 1 F e 203 1 S i02 s a m p le 1 cn CM O o CM < L SIS CaO m 1 O O xy E q u iv a le n ts 000 0 0000 m o o o !1< 1 1i1' i i 2.318 94.66 CM rM*l CM O d o q od 0V3 CM* oo 03 CM CM cn CO 09r CM Ml cn d h* C4"M CM fN d IrN*. V q cn o cn o rU*l d o q d o o 0.084 o d O oo o d o ooo o d d oo o o o d U3 <c5 o o (J Xa q aK T3 O Ui o M o l p ro p o rtio n q o b o c 5c a o 2 Table 3.4 lolio Table 3.4 folio 5 M 9 lJ . .-_89 Si b a la n c e 2 .7 9 4 0 .4 5 0 24.52 75.48 0 .4 5 0 0.692 _____ 0 1 T o t a ls > i 0 .4 5 0 0.346 0.796 0000 |0 0 0 0 |0 0 0 0 I 000 0 0000 0000 S LO 'O > (Of ^0)1 ! O s' O 1O !S d : ;wl i!1 ' O d O l m o ls 0 .3 4 6 X 1Q 10 i -- i jc i? r* m CO fs 0 m *- q0 d 000 0 , i* i! !i 11 ! 1 00 00 00 dd i 1 i 5 I 1 1 ; 1 M !, ! ! ; ! 1 11 1 1 j INI i i 1 II 1{ l1 O q -4T 0 d 0 d 0 n CO 0 ISIS q d m |1 1 a1 ** n d d 1 qQO 000 0 0000 Table 3.4 folio s a m p le Si02_ Ti02 " Cr203 T o ta ls Esl Mode GFW 6 0 .0 9 0 0.795 7 9 .7 0 0 I 1 5 9 .7 0 0 I 7 1 .8 5 0 7 0 . 9 4 0 (in F e O A) 4 0 .3 2 0 56.O 0O | 9 4 .2 0 0 1 4 1 .8 5 0 1 8 .0 1 6 S100 0000 0 a0 0 d a CO 0 .0 1 5 Wo 0 .0 0 0 0000 000 0 pp ! o E 1 ! <i ! 1! CL O 2 0 .0 0 0 0 0 0 d 0 0 0 d ooo d oo m 0 0 00 ddd 000 0 0 0 q d O O O d 0 0q d fs CM 0COl 0 0q dd 0000 0 q0 d 00 CM o> q 0 iCnM o *1* o> is* g iCnO 0 q 0q Oq CinM 0 0 0 q 0 q OO O q 2! 9 rs! d dddd ddddd .9* o cc O *o X O gjg|g<n O CM < 8 lislsil CM u. 2 0000 000 0 00 001 0 9 6 19 0000 0000 0000 0000 q0 dd CM CO 0 CM d 0q 0 q d d 00 0 q d d 00 0 0 d d 00 0 q d d 0 d O O d 0000 to q 0 a .S 0 u3 C flj 'a 0 0 u Xa O c 0 t: 0a 0c c 0 0 0 LU aX a O ol 0 O22 pr q * 0 E 0c c 0 0 0 2 5 000 0 000 0 000 0 000 0 0100 000 0 000 3E d i* a LU 8 CM n O CM CM O Oxide sample Si02 ' IAI203 03 |000 [060 09 CO CM GFW MT7 42.79 0.00 79.700 101.940 152.020 o.oo|I 159.700 1 8.62 71.850 o.oool 0.120 si 111-! 3 47.74| 40.320 080 95 00 001 poo poo jooo 000 950 1 61.980 94.200! g (Totals 18.016 |Oxy Equivalents Est Mode [Ol-Opx-Cpx coordinates |Mot proportion 00q d 0 m09 |000 0 jc o o o |000'0 oOO o 00 9 U. #c o C5 ro** d 0 d 00q 0000 0000 loooo I Ooo Ooo 6 oo o a! i do CM d 2 0j d 000 0 000 0 000 0 |oooo 000 0 0 d llm Orl o.oool 2 0 d d 0 d 000 p 6 0 '0 |p tO 'O | looo o jle o o o OZO'O j MOd d d coon a CO 090 0 Q oO oO oo <JJ fuclj <0 X) : j *1 O ifi 0.093 Totals 0.712 0.712 000 0 I 600 0 lo o o o 669 0 I. o<*/> X c/i qoo d ooo ooo 1.198 o oo ooo 1.304 Ol molsl 1.304 91% 000 0 J 669 0 IG60 0 1 1 000 0 000 0 I 000 0 010 0 ! ac . 1 j oOO o 0 001 1 i 1 I1 1 11 O d0 CM 03 0.280 2.396 2.747 CM CM dd CM CM CM* CM 87.6 2.396 2.499 1____ 0.40 3.73 rq** d 100 !j!ji j Uii i!!! i !! i !! Table 3.4 lolio ; is- <i H i 1! (f X 3 X 8 CL o < 8 tn *" X .3 re--i ior>i o c\' utr)1 | c' Ol r5| d( co*>>1t1kdO"j> i Ma), Ooi|rf*11 ! i i>> ml 1 1 0*1 ^el ol k! Representative Pvroxene and Olivine Compositions A o cn Phase -> -O(JJ Sample ->| CM o w |Ti02 1 1 OCM O j < O ll. 1 ITolal 1 1 |1 1 1 <' oi UQ. 5 8 1 Total |80'0 | iO n COM o i(n9 o d oor r* 0crC5>>1f * CC in oCM CaOt i0n d Cpx 1 Cpx Cpx Idrla Idrla Idrla Idria Idria idrla 1 793-15a|1793-15a|I 793-15a|[ 793-15a|I 793-15a 54.18 54.79 i 54.67 56.981 57.14 57.18 IS'S SE'O 000 |90'0 I SS'S |69'0 | to o Oo o 1.0211 1.17| 0.45 0.44 1.61 1.35 1 0.071 iB.nl ____IL Z J j1 24.1611 24.70| 1 IIVO-O 1 0q d 99.6511 100.261 1 I[2 0 0 0.29 1 8 .4 1 11 24.6411 100.1511 IlOO'O o o 0.31 1 35.3411 0.4511 100.1311 1 0.25 5.49 1 0.07|1 0.08 35.2511 35.6311 0.48| ____ 1 000 ]|00'0 1 100.25 1 100.34 pu I 000 T> *d *(>39 c c Totalp 52.47! 6.83 1 2.73 1 16.03 20.761 1.66 [ 1 |S0 l | cm WCM *>(50 PV) Totalp 1 54.251 n.d. 3.40 1 n.d. 6.20j n.d. 1 1 n.d 1 99.15 1 S60 160*0 p re s jn ro*. >C7 d A lb e rt 3.171 1.88 17.191 24.091 1 0.22! 100.75 pu | A lb e rt 55.70 0.02 0.13 33.40 0.35 100.61 s 00 0C3M oq (>oQ o (D at 60 0 00 l CD cvi OooO) 1 ir Cpx __ Opx Trinilv Tr initv 0.25 3.25 2.67 17.39 22.65 cdv C0O1 1 o a\ fV. GCOD d0 |86'0 poo SO'O 000 0 d 0 d 1.97!1 0.04 1 l 0.941 0.00 1 OO'O [960 | S6'0 ]000 I 100 | 1 1.9811 0.00 0.05 0 .0 4I 3 .9 9 !1 OO'O | S6'0 166*0 I|00'0 I[ t o o |[eoo |000 1 1.98|1 1 1 4 .0 1 I1 OO'O |20'0 [1OO'O 1 110 0 |90'0 000 1.9611 1.961 0 OCO d I 0. I 6! 0.061 1.811 1 1 0 .0 2 Il OO'O M i l4.01I_____ 1 |p o o I91'0j 1 000 1189 [99'0 |000 10 0 0 000 | |p o o [9 ro II 10' 0 000 1 0.29 1 0.14 1 0.08 1 0.18 000 1.82! 0.02I L4 .01I 0O0 CM dO 1 I 4 .03!I 1.78 0.04| 4.04| OO'O |000 | |[0 0 0 000 1 | 000 90 0 [GOO 000 O d 1.92 0.92 0.93 0.02 4.011 000 to o 000 000 1.91 0.171 0.02! 0.17! 1.71 ( 0a cCVJI 0' GaC)} c' C0M>>' cc>! G' T* cr) CGC> O0C G a OC' { ;. o<0>;. 0011 mo|i c l <^1 o'i 0"; c ! 'i !!i _ U.u 1 ____ 0.16 ____p . p o ____ 1.82 r\ nn M3|lCCS3 31!C5 --| 91 cf 1 CO d aq CO d to o |10'0 to o i 1. 3C i1 c3 C i L1L/1.I i1. X' OS'O 0.49 0.49 ZO'O 0.02 1 0.95 cni ! : l ! 1! 1i ; i1 i i 1 i Nolos: total Fo oxprossod as FoC Totalp samplos from Polors M9 OS'O 1 11 II o'j 0.021 t_ . )m ! * v) 160 !. 11 -ux ! -5 i , E 1~ I !s ! !E \l ,c !- :s i2 i: 3t/1 aa 1< !| 0 11,da0 s5 !E P 1 !10 i it Cl 000 dd 1 0.92 1 0.91 o.osl 0.92|_____ 0,92 COM d GOO d 60 0 j 68 0 j 0.46 0.49 0.05] 0.91 0.91 E0 0 at d co 06 0 d| c` c 0.01 1 GOJ a 0.48 a) u.9 1 5! c 0.94 ___0.91 -- _qj?8 ____ p.92 ____0.92 1981) n co o a Eo o !:! ' 11 M i N 1 1 !iiii! i !ii: !' i I 1' 1 ! -' 1 i !I ! ! 1 ! j > j i O oo > nc o a cc in b >- 6 Oo o o 6 o .3 2 i i i bo cb 1!: ill Phase > (Location -] New Idria [New Idria z * |New Idria New Idria |New Idria 1 ISample ->| 793-15a|t 7 9 3 -15a|1 793-15a]1 793-15a 1 1091-75 1 1091-75 1 40.24 CM CM O* CM o 40.11 40.54 40.28j CM CM Oo CO i-- 000 000 1100 0 iEO'O [900 000 000 jooo 000 loro | | 000 000 ] 000 000 liro | [OO O I' [OO'O 81 oli. n O CM o IAI203 NO Total oo o 1 0.0211 0.011 1 0.08 |99'8 i 8.59]1 8.531 cb 1 8.23 I 9.30 1 0.12 L____ 0J2J1 0.27|1 0.34 50.03 49.74 49.92 49.91 49.89 49.11 I1000 | |8G*0 I llO'O |6e'o 1 00'0 j I 1 0.43|I 0.381 1 0.2911 0.1611 o b o o' 1 0.02I1 0.02I 110 0 1 o.ool 1 1 99.4311 99.03|! 99.2611 99.1911 9 9 .3 1 11 99.381 1 0 ' 0 |0 1 600 | pu Albert 1 Albert 1 817 1 826 1 38.23 38.38 n.d. 1 b c n.d. 1n.d. 9.18 ob o GO I bo cb b c 51.66 0.26 1 1 n.d. 99.4311 1 51.65 0.2311 n.d. I 1n.d. 99.151I pu | A lb e rt 835 39.44 __ ad. 0.11 50.24 0.29 n.d. 99.78 pu i pu [10 0 0 1 [S6'0 [ I00'0 000 1 I[00 0 000 I1000 l1OO'O 166"0 16'0 00'0 000 10-0 OO'O OO'O 66'0 cn H- < LL 5 0! Total li CO b 1 0.00 0.00 1.83 1 3 -01 16' 0 000 10-0 OO'O 000 I I 00 q bb b o' b 00'0 000 ] I|00'0 IlOO'O | cn q 1 o.ool 0.99! 0 0 qO b CO 1 I 1.83 1 1 0.1 el o.ool 1.83|1 0.01 1 ___ M il1 I 0.9111 GO 0 bb 1 i ||00'E [[OO O' 000 too 11 I1IO -O 000 |000 000 1 _____O J l i 1 1.831 1 1.83|1 3.011 0 .9 1 1^ 0.921 O q 060 |ioe [OO'O 000 |10 0 0 [10-0 000 000 000 | 0.99 I__ 0.95 I 0.97 bb cC "O GO c b b c |0 0 0 10 0 0 pu 1 0.1911 1.81 j1 0q b n.d.lI n.d. 0.191 1.91 1 n.d. 1 1.9li n.d. 0.20 b c iqn b 1 n.d. 11 n.d. | 1 0.911 0.911 |S0C 1 b c n.d. 2 <3 3 11i i 1 i t 1 1 l 1t 11 I1 11 i (Noios: total Fo expressed as FoO* 1 [Tolalp samples from Potors 11968). Alborl samples (rom MacGr . i i1 i 1 1 1 f| 11 too j00`0 1 1 iII iqn ! 1 1 1 [06 0 |S0'G ! 11! 1iI l !1, GQS) !1 1 != ' i2 i0 1* 1l M if </> > . fON). o 3t/> 1 \& '1 1 1l0 Tnblo 3.5 folio 2 06 0 90 0 |0 C 0 100 lia b le 3.6 - Primary Chromlteal *O i i i i (Jl b o b Q z UO9_ CO 004 U9. CM O K ISample -> IAI203 |Cr203 iTolal wl / 1091-70.1 1091-70.2 1091-70.3 1091-70.4 1091-70.5 1091-70.6 1091-70.7 *2 2b 9* z o *cnr CM fOs b in b New Id rial New Idria)1 New Idria) t o.ioi t l 0.12I CO C7> 9.62 9.75 59.33 59.84 59.81 0609 New Idria!1 New Idria!1 New Idria I Mont Alberti 1 0.12 9.73 9.90 10.03 29.22 60.20 57.45 58.48 ____ 38.13 CM 04 GOO 1 bb 04 bb 0tr CO C0O O tri b b 0 5.36 4.99 1 11.2311 11.3 2 I1 11.3 5 !1 10.95]! ! 0.16| 2 00 .1 ! 11.20 1 1 10.4311 0 .1s] 10.46 16.27 1 14.64|1 14.83|[ 14.80|I 14.93] I 147551| 15.00 G00 Ot'O C0O 0 b 0.07 0.12 SO'O ZOO | 0.02 0.04] 0.04 100.55 101.58 102.14 101.69 97.84 *-- tr. ci n 04 b Oi V O a C 00 O 04 b 0b3 s 0 0 0 |jo o o ] 00 0 I00 0 Ito o o |100'0 Ito o o 1 6E0 | |9G'0 | Gc1 cr O T- CO 0 03 *4* + 1- < 0 u0_ UCOM ccc} - c> cCc>> cc3) cC>3 CC3> a` a c` c* c> Ci r> 1 0.36{ I 0.36) 1 0.36| 1 0.37 i OS'l Gl'O OO'O OG'O in O Orbb OCO 0 bb 1.50 1.50 1.50 1 0.13 ! o .i3 |1 0.121 0J3 1 0.30 1 0.29 1 0.30 | 1 0.00 0 .7 0 '1 0 .7 0 ]1 0 .7 0 ! 169 0 | ||o o o [6Z0 | [OO'O C**M fs. bb 1.49 1 |00'G |00'0 ||00'0 !|00'G [100 0 ]000 ||00'G [10 0 0 [1OO'O |1 |G0 0 ' I[OO'O 1 1OO O' OO'O looo 00 00 OO bbb Cc..1 CO z rS |90'0 Total cations 1 1 CO O o' b j 00 o' b + tcJ0nL 5 CM L9O+L: 0+ 2 < 4 a 21 0 <+ O'W Un0L 90'0 900 |08'0 I 0.70 1 0.7011 1 ! 0 .7 0 I 0.0611 0 .7 0 I 0.061 0.70| r> Cr*D bb 0 COD COD O bbbb ' 0.79 0.70 1 0 u9. CM 9 1 !i 1 i i Nolos: Fo analyzod with microprobo as FeO; Fo203 calculated Irom stoichiomelrv. 3 ra iin n i rf, (Notes: Mont Albort sample from McGroqor & Basu. 1979 1 90 0 080 OO'O O o' O OO 0 0.12 0.28 0.72 3.001 0.721 0.70 <C153 rt*Hr cTr3 ctsi c) ci c) C 1 b !! 1( 1 1 1 11 l 1 ! Tablo 3.G (olio Ti02 AI203 IZnO ITotal wt % 1 11 1 I 0.631 99.17|! 98.81 [1 101.131 99.94 | 1 j 1 1 1 1 | 1 660 t1 oo | ooe [too 100 1 | |00'0 |00'0 [60 0 G l'0 [600 ||SC'0 I[OO'O 1ooe 1 I[OO'O too 61 1 0 OO'O | OO'O |C90C |SC'0 | [too | OO'O |OO'O j j too too 96'0 020 | OO'O |00 0 1 600 1 j j 1600 [960 |66'0 ooe 01 0 0 | ZO'O too OO'O |00 0 |00 0 |I 10' 0 190 0 [oeoe |09'S9 | |S0'0 CC'O 860 OO'O eoo OO'O 000 000 00 0 oeo [S00 11 !i E w iqn d CO 1 ro- rin* o* o* 67.801 30.021 [ 62.311 30.17] o.oal KCACRD.il KCAC-RD.2 KCACRD.3 KCAC-RD.4 core! intermediate! intermediate 1 E w o CJ O CM aq% d m f* CM CO COM CO O d o oo 63.281 0.07| *> CEow u GO m** d cc 2 6< % o sc z TJ O < u> A ci O oa 3a- cE(n3 0.16) COM o CM o CO r^ o CO OCM dodo in CM p** dd 15.29] 52.791 29.091 | 1 [ cn CO OCM O O CUQM. 9 Li. cm Q z 11 1 OCD) rc-n. d CM d 0.03 0.99 fo** CcMn d 1 CM q o 0.97 CD CoM oo dd 1 iCnO 0) COM o oo + < w o uCoO. CuOM. z 5 0.011 1 ITotal cations ! 3.00 0.02 0.94 III1! |!M ! | |3: i 1 i i 0.9S| o cn od qo o Oo q o rr*-* o 1 1 *+ + ucn. a UCOCMT. o < o Oj u, 2 < " ? 21 o ucn. CM O IL 0.38| co i R n itt o oE JuC o 3 oE T3 5<3 3O 3 n uOCOM. uOo. tfi a a O) oa u E || 1s 1 !! "-! ! <n'i 15! CM o o ta n .o3 Tabla 3.7 Kammererlta (Cr*chlorlta) Analyaaa Sample -> !L I KCAC-RD.5 1 ii KCAC-RD.6 Si02 Ti02 1 32.271 32.90 1 O.OOl 0.00 AI203 Cr203 1 7.31 1 5.76 i 7.43 5.30 FaO* 19.25 17.14 MO m______________ CO 0.07 22.14 0.26 0.05 23.97 0.27 Na20 H20 Totals 0.02 12.92 100.00 0.02 12.92 100.00 Si Tl Al Cr Fa* Mi MJ Ca Na H Totals 3.38 0.00 0.90 0.48 1.69 0.01 3.45 0.03 0.00 9.02 18.96 3.40 0.00 0.90 0.43 1.48 0.00 3.69 0.03 0.00 8.90 18.83 MqfAl 3.83 4.08 Cr/(AI+Cr) 0.35 0.32 Cr/Fe (total) 0.28 0.29 Mg(Mq-fFa) 0.67 0.71 1 Notes: Fe analyzed as FeO: H20 calculated by difference Table 3.7 31 CMJ O | | HS z -z rr* CCOO lt osiirSs I CaCS>|i CvOi* 1 lia b le 3.8 - Chromlan Garnet Analyses] ! 1 11 11 z z CO c O 1 h- 0 11 ca2*nii|1 3CO (8 O CM 8 Os (0 location *> ) sample > |Si02 IAI203 i 1 [C9 96 | |600 CO d oCMs CM CO cd CM z cm CO m 688-2.2 34.88 34.50 O .lll 1 20.0B 0.07| 33.18| 32.56| 1 j 97.0011 980 l u o |80 0 |60'0 d 00 d CD O co V CO z 99CO <M m Oq O cd CO z z z0 d CO csi 688-2.4 688-2.5 688-2.6 688-2.7 688-2.8 34.75] 34.93 34.90 34.23 35.03 35.53 1010 |60'0 O CO d cd O O O r0- Cr*O. d d dV O .lll 1 o.oal 0.051 9.57| 9.07|, 6.57 i 9.13 1900 CIO I ll'O lEO'O __ 19.85 i 20.38 22.62 18.32 20.48 23.41 O d I 0 .0 6 1I 0.04 1 OC'O 0.15 0.61 0.22 1 33.84|I 33.6211 33.65|1 33.651I 33.531 1 98.3511 98.551I 98.2611 97.711I 98.611I 99.6611 C0D3 CO <0 0 d in os Os cvi i 1 0.111 0.08|! B.63 | 21.31 I 0.12 33.571I 99.661 |Z0*00l 1 1 lo c i VM 2 .A .1 35.49 I 0.641 2.3B 0.04 0.12 33.82! iCnM cCMo cd CM " 00 |000 COO in < 0.231 ____ 1.J4 1.90 ___ 0.76 2.64 ___ 4.82 23.79 ___ 0.00 010 33.89 99.13 61 0 160*1 I jis-o I | 1 O cc ' r ml I 11 r iiii 8 1-- 2 Z) O < a 3 _c a Ol CO dd MS O 3 > s "> O 10 |UGA mols I 1 1 lAndradlte u O5 69 59 I lo ro I space: 1 0.18 13.5411 30.88!1 0.5711 0.48 1 I 68.64 100*99 | as d I 33.43]i 0.57] I CM d 0.19|1 31.7311 67.8611 BS'O 0.19|1 23.241 1 76.1811 CM GO <0 iqn CO o .io l1 1 0.3711 61.681 0.191 31.7611 0.421 1 1 23.16 1 78.3311 V d 0.19|1 29.721 69.87 1 0.19 8.39 3.37 88.24 8.98 9.64 81.39 II !i1 1!1 m O in CO vi 1 1100-total 1 1.47 I 1 3 4 .8 8 3.00 \ 1.65 1 1.7411 2.29| 1 34.7511 34.93 1 34.90 1 34.23!1 35.03 | t 35.53 0.14 35.97 INoles on Location: INI analyses from New Idria. this study i 1 I ; 1 1! 1 ! i i ,1 1 1 , ; , . ; 1 ' 1 ' |j !i ! i 1 ; i i 1 !! 1 !I i os CCS 0chT 0 0c 2 Js to 0 u c0 m 2 *5 > e0 ses > *a c (Q 2 > 1 ! 1 ; INoles on analysis totals (weight % ): INew Idria qarnets (Nl) froquentiv have low totals and Eire probably hydrous , 1 Tablo 3.8 folio IZC001 | leoo | liNO in * >. C < e m O c 5 E ow . U o ri 3 m H- CM O CO location -4 sample -> Ti02 AI203 4.A.3 33.97 0.431 2.04 VM 4.A.4 34.251 2.22 CO o> dd <D O COM o CM CO s 6 U- 24.801 23.511 o d CO a JC CQ 8 VO- 0.07| i 1 33.711 97.791 iMole fraction In UGA space: 0908 I 610 |UGA mots I 0.19 "> o 5> 53<OM0 Z3 a < I 9.O il 1 13.211[ 10.39 11.18 39 c5 1 75.61 6S18 I 89 8 ItO O O l I[02001 |eiee | |C00 |000 i co |JNO <c0 CM CM co 0.32] 1.831 26.34] n.d.l 0.23]1 0.20 9.53 1 1 CM co CO d *o c in 0) d CO in 34.88| 0.33| 2.061 0.111 1 0.21 17.11 1 9.84 73.05 1 co CO CD d f** o d c CD CD d 35.511 0.45| 1.69 ei ^ l l i I I 1 9 9 .9 1 1I 0.20 28.2311 9.39 62.38 1 'C 35.191 0.55| 1.76 10.541 n.d.| 0.47]1 33.78]1 99.70| 0.20 35.4411 8.83 55.73 1 in TO m 33.501 34.64| 2.241 TT G> r** (6 Oi 9.121 ____ LL49 1 9J7 1 CM d t d c 1.67|1 33.70]1 1 0 .7 8 1I 33.26]] 100.06 0.22 33.3311 40.62 26.06 1 0.22 34.421 1 42.17 23.42 OS'OOt 1108 66 | pu ji S0'6 | oCM o rr 1 1 (D TR i9 ;9 |89 l 36.861 0.40| 33.331 2.501 34.52 6.051 r co 1 ? ion o * i*ne d 5.471 COM o 0.03 0.62|l___ 1.281 _____ r e 35.931I 35.12] 32.81 100.39 1 11 CM o CO ii CnM\] Cc di 9 38.291 17.27 _____0 .2 2 44 7fi 35.51 27.351 !i 1 i1 !1 i1 ii l !1 i1 1 ! 1 1 Notes on Location: |ONT analyses from Reaume Township. ONtario; Duke & Bonardi. 1982 1TR analyses from Transvaal. South Africa: Frankel, 1959 |KAR analyses from Karelia. Finland: von Knorrinq. 1954 Tablo 3 0 folio 2 Table 3.9 Chemical Analyais and Site Assignments (or a Blue Amphlbole i `j j i j j Sample -> Si02 KCAC-RD.3I 56.321 1________ ii i1 Ti02 0.061 1 1 i 1 AI203 0.101 i11 Cr203 0.141 iii FeO' MID 16.861 0.09 j 1!........................................... lii : 14.53 ________ 11 CcO 3.621 ________ I ZrO 0.02 iii BaO 0.00 r1 Na20 K20 F Cl 5.75 0.23 0.07 0.02 ... .. . 1 ! 1 i H20 2.22 l Total 100.00 site assic nments 1 atom pfu A M4 Ml-M3 TI (OH.F.CI) Si 8.00 8.00l Ti 0.01 0.01 1 Al 0.02 0.02 1 Cr 0.02 0.02 f Fe3 0.94 0.94 1 Fe2 1.06 0.13 0.93 1 Mi - 0.01 0.01 1 M? 3.08 3.08 _... 1 Ca 0.55 0.55 1 Zn 0.00 0.00 i Ba 0.00 0.00 i Na 1.58 0.27 1.32 1! K 0.04 0.041 i1 F 0.061 ! i 0.06 Cl 0.011 1 1 0.01 OH 1.93 j ________ 1 1.93 1!I!!i Structural formula: 0.31 2.00 5.00 8.001 2.00 J________________ !_________ 1i1 Notes: All Fe as FeO' in probe analysis, Fe2/Fe3 by stoichiometry Table 3.9 Table 3.10 - XRO Data for Blua Amphlbola 793-19a 2 Theta degrees intensity l/f d, A 10.53 16.44 18.21 19.74 26.01 26.27 27.31 28.65 30.23 33.06 33.18 34.61 35.25 35.44 38.80 39.26 40.87 41.68 42.61 60.68 45 8.393 37 5.386 12 4.868 24 4.494 57 3.424 100 3.390 16 3.263 48 3.113 12 2.954 55 2.707 50 2.698 18 2.590 24 2.544 26 2.531 11 2.319 13 2.293 31 2.206 15 2.165 17 2.120 20 1.545 Table 3.10 Chapter 4 Tectonic Blocks of the New Idria Serpentinite Chapter 4 - Mineralogy and Petrology of the Tectonic Inclusions I. Introduction.......................................................................................... 1 II. Geological Setting of theNew Idria District..............................................6 III. Chlorite-Diopside-Gamet Rocks.............. 7 A. General Statement..............................................................................7 B. The 34-50 locality..............................................................................8 C. Perovskite Knob...................................... 22 D. The Melanitc Mine..................................................................... 38 E. Discussion of Chlorite Compositions................................................ 46 F. Discussion of Bulk Compositions........ ................... ................ 47 G. Models for the Origin of Chlorite-Diopside-Gamet Rocks................ 53 I. Serpentinization of Fertile Rocks.......................................................66 I. Temperatures of M1 and M2 Metamorphism....................................74 J. About Rodingites.............................................................................. 75 K. Chlorite-Diopside-Gamet Rocksfrom Other Localities.................. 76 IV. Mafic Schists and Greenstones.......................... ........................... 80 A. General Statement..................................................... ....................... 80 . B. Jadeite-bearing Blocks........................................................................81 C. Blocks Containing Benitoite, Neptunite, and Joaquinite...................84 V. Andgorite Knockers.................................................................................98 A. General Statement.............................................................................98 B. Santa Rita Peak................................................................................ 98 C. San Carlos Peak......................................................... 104 D. Perovskite Knob.........................................................................104 VI. Fluid Inclusion Constraints on M2 metamorphism..................................105 VII. Conclusions........................................................................................... 110 Tables .........................................................................................................116 Chapter 4 - Mineralogy and Petrology of (he Tectonic Inclusions I. Introduction The New Idria Serpentinite is locally heterogeneous: numerous rock bodies of limited extent and contrasting physical character are included within the mass of the serpentinite. These bodies arc isolated from each other, have sheared contacts with the surrounding serpentinite, and show evidence of brittle to ductile deformational processes; they are interpreted as tectonic blocks or knockers. A consequence of this interpretation is that the tectonic blocks have a history separate in pan from the rest of the serpentinite. Coleman (1957) noted the lithologic similarity between many of these blocks and the Franciscan rocks found at the margin of the serpentinite; following a suggestion made by Loudcrback (1909), he proposed that some tectonic blocks were derived from the Franciscan section during emplacement of the serpentinite. The pre-entrainment history of some of the blocks is fairly clear, for others the evidence is equivocal. The dimensions of the blocks range from 1-1000 meters; many of the larger blocks are elongate and have a southeast-northwest orientation. Most of the smaller blocks and some of the larger blocks have hidden margins, so that their shape and orientation cannot be determined unambiguously. The blocks can be distinguished in the field by distinct soil color and erosion properties, contrasting vegetation above, and anomalous mineral assemblages within. These exotic rock bodies have attracted much attention from mineralogists and mineral collectors for the past century: they are the host rocks for museum-quality specimens of titanium-rich, chromium-rich, and rare-earth-rich minerals, including perovskitc. 4-1 schorlomite garnet (containing up to 16 wt % Ti02), chromian garnet, REE-vesuvianite (idocrase), titanite (sphene), zinconolite, benitoite, neptunite, joaquinite, and a numerous other accessory minerals. The renowned San Benito County garnets are hosted in blocks of chlorite-diopside-gamet rock; benitoite, the state gemstone of California, is hosted in blocks of mafic schist. Coleman (1986), Millage (1981) and Wise & Gill (1977) have prepared lists of mineral species from the New Idria District. Figure 4.1 shows the orientation of some structural features related to tectonic blocks. Examination of Figure 4.1 shows that there are two preferred structural trends . northwest and northeast. Northwest trending features parallel to the regional structure and the New Idria Thrust Fault include the physical orientation of elongate tectonic blocks and foliation in the larger blocks. The northeast-trending features include tension cracks and shear and fracture planes within blocks. Crosscutting relationships show that the northwest trending features generally predate the northeast trending features. Coleman (1957) recognized and mapped numerous tectonic blocks; he demonstrated that the exposed contacts of the blocks are faulted against the surrounding serpentinite. Evidence of shearing and development of slickensides is common. However, in many cases the actual contacts are not exposed, and the relationship of the blocks to the host serpentinite is obscured. In this study, most of Coleman's larger blocks in the southern portion of the District were re-examined; some of the smaller blocks could not be located in the dense brush and may have disintegrated, or disappeared under landslides. A few new localities not described by Coleman were visited and studied. Coleman (1957) established five categories of tectonic blocks based upon their mineralogy; one category was reserved for jadeite-bearing rocks. In the present study these five categories have been collapsed into the single category of Mafic Schists and Greenstones. 4-2 Some of the rocks regarded by Coleman as metasomadc veins are reinterpreted as tectonic blocks. In the present study only three categories of tectonic blocks are defined, based upon mineralogy, bulk composition, and probable history. Descripuons of these categories of tectonic blocks follow; while numerous examples of each type of tectonic block exist in the New Idria District, in most cases the exposures arc poor and the geologic interpretation ambiguous. Therefore, this study concentrates on a few key outcrops at which contrasting rock types and in some cases geologic contacts arc exposed. Tectonic Block Cateeorv. this Studv Coleman Category A. Chlorite-Diopside-Gamet Rocks Metasomadc chlorite-rich rocks B. Mafic Schists and Greenstones Tectonic blocks I-IV, jadeite-bearing rocks, and metasomadc soda-rich rocks C. Antigorite Knockers Rock ofSanta Rita Peak All types of tectonic blocks show evidence of two metamorphic events, here defined as Ml and M2. Ml metamorphism is associated with the development of penetrative foliation in the chlorite-diopside-gamet rocks and the mafic schists, and the interpenetrating fabric of the antigorite blocks. Some of the Ml metamorphism must have been of the high-pressure blucschist type. Ml metamorphism occurred prior to entrainment of the blocks, because 4-3 Figure 4.1 - Orientations of structural features at New Idria. Northwest-trending features are sub parallel to regional structure and the New Idria Thrust Fault, oblique to the San Andreas Fault Northeast-trending features crosscut northwest set. 4-4 Figure 4.1 - Orientations of Structural Features Legend:1 1 - Foliation at Gem Mine 2 - Strike of New Idria Thrust Fault 3 - Foliation at Melanite Mine 4 - Foliation at 34-50 locality 5 - Pyroxenite layers near Perovskite Knob 6 Shear planes at Santa Rita Peak 7 - Fractures at Melanite Mine the penetrative foliation of the blocks is not matched by development of adjacent schistose serpentine. Ml metamorphism in blocks of different rock types may have occurred at different times prior to entrainment, and for some blocks the Ml event may have been a series of pre-entrainment events. M2 metamorphism is associated with the development of tension cracks and veins that crosscut the Ml foliation in chlorite-diopside-gamet blocks and mafic schists, and with crosscutting garnet veins in the antigorite knockers. M2 metamorphism took place after entrainment, and may have occurred for all blocks at the same time, probably in the Miocene. The only quantitative age controls on the timing of M2 metamorphism are contained in the mafic schists that host benitoite and neptunite at the Gem Mine, described below. M2 mctamorphic reactions in most blocks were essentially isochemical, with only water required as an open system component There may have been Ca metasomatism associated with M2 metamorphism of antigorite knockers. It is proposed here that M2 metamorphism is related to passage of the Mendocino Fracture Zone in the Miocene, with attendant regional uplift, sudden appearance of clastic serpentine in the sediments of the Great Valley Sequence, and development of the northeast-trending structural features described in this chapter.II. II. Geological Setting of the New Idria District The term New Idria District, referred to in this study simply as the District, was first used by Becker to describe the general region encompassing the historic mercury mines at New Idria, California (Becker. 1888). The most remarkable feature of the District is an oval. 23 by 8 km, fault bounded, serpentinite massif flanked by steeply dipping, locally overturned, sedimentary and metamorphic rocks of the Jurassic-Cretaceous Franciscan and the Cretaceous to Pliocene Great Valley Sequence of California. 4-6 The New Idria Serpendnite is a part of the Coast Range Ophioiite in California (Hopson et al.. 1981). Its emplacement within the Franciscan Formation is related tosubduction of the Farallon plate in the Jurassic. The tectonic setting of the ophioiite was a forcarc basin lying between the Siena Nevada volcanic arc to the east and a trench-subduction complex to the west, represented by the Franciscan Formation (Bartow, 1990). A piece of oceanic crust or underlying mantle was obducted onto the North American continent, later to rise through the Franciscan Formation as a serpentinite diapir. Emplacement of the New Idria Serpendnite as a diapir within the Franciscan has resulted in a piercemcnt structure or tectonic window, lying just east of the San Andreas fault system northeast of Parkfield. Further details about the geological environment of the District can be found in Chapter 1. III. Chlorite-Diopside-Garnet Rocks A. General Statement These green to yellow-weathering, blue-grey to white, foliated rocks consist mainly of chlorite and diopside, with mclanite garnet (1-5 wt % TiOj), schorlomite garnet (> 5 wt % Ti02), variable amounts of serpentine and magnetite, and several Ti-rich accessory minerals, notably vesuvianite (idocrase), perovskite, and titanite (sphenc), along with minor chromite, kammererite, chromian garnet, apatite, zirconolite, pyrolusite and copper sulfides. Other accessory minerals have also been reported (e.g. Coleman, 1986). Carbonate minerals including calcite and hydromagnesite sometimes fill voids in the rock. These surface alteration minerals were probably precipitated from meteoric waters and are not part of the mctamorphic phase assemblage. The rocks are poorly exposed in most places; exceptions occur where a slab of more resistant serpentine cap rock overlies the chlorite-diopside-gamet rock. The best exposures of these rocks occur at Coleman locality 4-7 34-50, at Pcrovskite Knob, and at the Melanite Mine, ail in the southern portion of the District (Figure 13). Chlorite-diopside-gamet rocks have not been reported from the nonhem half of the District. B . The 34-50 locality At this locality a set of four resistant rock towers surrounded by talus sits astride a low north-south ridge at an elevation of 4360 ft. (1329 m.) MSL, above the stream bed of the San Benito River, a short distance downstream from the Gem Mine. The coordinates are 3620'.07 N, 12036'.86 W. The KCAC Mine is clearly visible to the north, the Gem Mine to the east, and a syenite dike referred to in this study as The Nose on a prominent hill to the south. The locality name of 34-50 as used in this study is carried forward from Coleman (1957). Access to the outcrop is via a short spur from the (usually) graded road leading up from the KCAC mine to Santa Rita Peak. The four towers are 2-3 m. tall and arranged roughly in a 5 x 10 m. rectangle oriented with its long axis SE-NW (Figure 4.2). The towers and associated talus cover approximately 100 sq. m. and are surrounded by dense brush. Each of the towers is capped by dark green, sheared but relatively resistant serpentine, under which lies a slab of light green tc white, chlorite-diopside-gamet rock dipping gently to the east The four towers are apparently the eroded remnants of a once-conrinuous slab of chlorite-diopside-gamet rock capped by serpentine. The strike and dip of this slab vary from 25-27E and I4-32S respectively, subparallel to the New Idria Thrust Fault. The outer contacts of the slab are not exposed, so its original lateral dimensions are unknown. However, the upper contact of chlorite-diopside-gamet rock against the serpentinite is exposed in each tower, and in the NW tower the upper and lower contacts are both exposed, so that a complete cross section is visible. Plate 4.1 is a photograph of the west face of the NW tower, showing both 4-8 contacts. Figure 4.3 shows the essential features of this outcrop, in schematic form (see also Coleman, 1957). The chlorite-diopside-gamet rock of the 34-50 locality is divided into a central chlorite-rich zone and two diopside-rich outer zones, an upper and a lower. The central zone includes the prominent horizontal green band in Plate 4.1, while the outer zones include the white horizontal bands in that photograph. The mineralogy of the three zones is similar, but the modal proportions of the minerals vary. This locality is remarkable for the absence of magnetite, a nearly ubiquitous accessory mineral at New Idria. The contacts between the various zones are abrupt to gradational; locally there is considerable interfmgering, especially between diopside and serpentine in the outer margin. The rock of the outcrop is strongly sheared. The interfmgering has probably resulted from recrystallization of diopside and serpentine under shearing stress, and possibly from local mobility of Ca in metamorphic fluids (e.g. Labotka & Albee, 1979). 1. Central Zone Chlorite gives a green color and imparts a strong foliation to the chlorite-rich central zone. Petrographic study reveals foliated Ml chlorite in nearly monomineralic bands, fine grained (< 100 pm) Ml diopside in nearly monomineralic bands, and domains containing both foliated chlorite and fine grained diopside (Plate 4.2). The width of these bands is quite variable, on a scale of millimeters. At the boundaries between chlorite-rich and diopsiderich domains, the foliation is disrupted and chlorite occurs in a variety of orientations. This may be due to mineral growth under shearing stress at a point of contrast in rock competency. The chlorite in the central zone has nonnal birefringence and does not display anomalous blue interference colors. This chlorite has an intermediate Fe-Mg ratio as 4-9 Figure 4.2 - Geologic sketch map of the 34-50 locality; geology revised from Coleman, 1957. The four towers are the eroded remnants of a once-continuous, easterly dipping stab. 4-10 Figure 4.2 - Geologic Sketch Map o f 34-50 Locality Plate 4.1 - Photo of the NW tower at the 34-50 locality. Serpentinite cap rock protects a 1 m. high slab of chloritc-diopside-gamet rock that dips gently away from the viewer. Green, foliated rock near hammer head is an Ml chlorite-rich domain. White band near hammer handle is an Ml diopside-rich domain. A second white band is visible in the lower portion of the photo, just above the green serpentinite that underlies the slab. 4-12 Figure 4.3 - Schematic diagram of northwest tower of 34-50 locality (see Plate 4.1), showing mineralogical variation with mirror symmetry about a horizontal plane. 4-14 Figure 4.3 - Schematic of 34-50 Locality Measured Vertical Section nature of garnets major phases none Ti-free andradite Ti-rich andradite Ti-rich andradite Ti-rich andradite Ti-free andradite none serpentine serpentine+diopside+gamet diopside+garnet chiorite+diopside+gamet diopside+gamet serpenune+diopside-t-garnet serpentine distance above ref point (cm) 235 175 160 -- 130 -- 1*"AA TV _. /J " OU " 0 Plate 4.2 - Photomicrograph of sample 990-22 from 34-50 locality, showing foliated Ml chlorite at top and bottom of photo, fine grained Ml diopside lens in center, and stringers of black Ml Ti-gamet. Crossed nicols with gypsum plate. Field of view 4 mm. 4-16 Plate 4.3 - Photomicrograph of sample 990-15 from 34-50 locality, in a diopside-rich domain. Stubby crystals of Ml diopside show good cleavage. Isolated Ml garnets visible near left and right of photo, minor Ml chlorite in lower left. Crossed nicols with gypsum plate. Field of view 4 mm. 4-18 shown in Table 4.2. Small amounts of a serpentine mineral may also be intergrown with the chlorite; distinguishing these phases optically is difficult. Chromite and its alteration products kammcrcrite and chromian garnet are minor accessories. Recrystallized metamorphic diopside occurs in at least two generations; (Ml) massive, fine to occasionally coarse grained diopside and (M2) bladed diopside associated with vein filling garnet. No significant compositional differences were found between the first and second generations of diopside. Table 4.1 and Table 4.2 show compositions of metamorphic diopside and chlorite respectively from the 34-50 locality. The diopside here and at other localities is more Mg-rich (higher XMg) than coexisting chlorite. Comparison of diopside compositions with those of the relict igneous clinopyroxenes in Table 3.5 shows them to be very similar, although the metamorphic pyroxenes arc slightly richer in Fe than the igneous (XMg = 0.92 and XMg = 0.95 respectively). Ti-rich andradite garnet is present in all samples examined. Metamorphic Ti-gamets are anhedral to subhedral and occur in two generations. Ml garnets occur as isolated matrix crystals or blebs and stringers parallel to the foliation; M2 vein-filling garnets are associated with bladed M2 diopside in millimeter-wide veins that crosscut the foliation nearly at right angles. The veins appear to radiate outward from the central zone. Ml garnet blebs conform to local crcnulations in the foliation and therefore predate the crcnulation. At 34 50. most of the Ti-rich garnet of both generations is isotropic to slightly birefringent. Under straight nicols the garnet varies in color from light pink to reddish brown. Occasionally earlier, low Ti, slightly birefringent Ml garnet is overgrown by later, Ti-rich, isotropic M2 garnet in a crosscutting vein. Along the walls of veins, M2 diopside is seen to crystallize as thin whiskers or blades, some of which are in turn overgrown by garnet, occasionally resulting in a "bearded" appearance at the contact between vein filling garnet 4-20 and the vein wall. Open spaces in the veins arc frequently filled by late stage calcite precipitated from ground waters. 2. Outer Zone Examination of samples from the diopside-rich outer zone reveals moderately coarse grained Ml diopside (1 mm) with minor Ml chlorite, and ubiquitous chromite (and chromian alteration minerals). Some coarse diopside grains exhibit an equant texture, with good cleavage (Plate 4.3). Others have an elongate bladed shape. Some early diopside crystals arc seen to have been fractured and bent. The larger (1 mm) blades are parallel to the foliation while the smaller blades are randomly oriented. The compositions of the two types of Ml diopside are indistinguishable; they average XMg = 0.92, which is the same as diopside from the central zone. At the outermost edge of the diopside-rich zone serpentine interfingers with the diopside. The Ml chlorite is strongly foliated, and frequently shows anomalous blue interference colors, suggesting an Fe-rich composition. However, as shown in Table 4.2, the chlorite in the outer zone is generally more magnesian than in the central zone. Unaltered chromite has a deep red color, and is always surrounded by chlorite; chromite was never seen in contact with Cpx. This chromite has the same composition and optical properties as the unaltered chromite discussed in Chapter 3. However, some chromite grains have been partially replaced by a mixture of chlorite and a green Cr-rich garnet that also contains up to 3 wt. % Ti02. Magnetite is conspicuously absent at 34-50. The outer zone is also crosscut by tension cracks with vein-filling M2 minerals similar to those in the central zone. No significant compositional or textural differences were observed between M2 assemblages in the central and outer zones. 4-21 3. Comparison of Central and Outer Zones The textures and compositions described above suggest that the chlorite-diopside-gamet slab seen at the 34-50 locality is a strongly sheared and almost completely recrystallized rock embedded within the serpcntinite. The foliated rock texture is inconsistent with a simple metasomatic replacement of lizardite and chrysotile; nor do the M1 minerals appear to have precipitated in a hydrothermal vein. Shearing is manifest in the central zone by strong foliation of the dominant Fe-rich Ml chlorite, and in the outer zone by fracturing and recrystallizadon of Ml diopside. These observations show that the Ml metamorphism was synkincmatic. The anhedral to subhedral Ti-rich garnets of the central zone occur in two generations. The first garnet generation (Ml) is associated with the shearing and recrystallization event which produced the M1 chlorite, while the vein-filling M2 garnets which crosscut the foliation formed later. The veins that host the garnet are probably tension cracks; the minerals in the veins have beautifully preserved, delicate textures with no sign of post-mineralization deformation. Therefore the M2 mineralization is postkinematic. C. Perovskite Knob This prominent red knob. 25 m. long and 30 m. wide, rises 15 m. above its base elevation of 4520 ft. (1378 m.) MSL on the western slope of Hill 4857 (Plate 4.4 and Figure 4.4). The coordinates are 36 19'.65 N, 120 36'.31 W. The knob is best reached by descending a short spur road from a higher gravel road which traverses the western slope of Hill 4857. This gravel road was blocked in 1990 by a steel gate placed by the Bureau of Land Management (BLM) to prevent vehicle access to the EPA Superfund site at the abandoned Atlas Mine that lies on the eastern side of Hill 4857. As of this writing, it is 4-22 generally no longer possible to drive to Perovskite Knob without special arrangements with BLM. Another approach from the unnamed stream valley below is possible but involves ascending through dense brush. The origin of the name Perovskite Knob is unknown, but presumably was given by local mineral collectors who found perovskite there. The rock of the knob consists of three sub-horizontal zones (Figure 4.4, 4.5). The topmost zone, consisting mostly of red-weathering, dark green, tough serpentinite, is resistant to erosion and is the main reason for survival of the knob. This cap rock is about 3 meters thick; it is a dense rock consisting mainly of antigorite with a sugary texture; it is similar to the rock at the summit of Santa Rita Peak. The cap rock is underlain by strongly sheared serpentine rock consisting of a mixture of lizardite and massive chrysotile. that forms the middle zone, about 10 m. thick. This rock tends to split along the boundaries of lenticular blocks or phacoids. The overall appearance is reptilian and scaly; the field term "dragon skin" is used to describe this rock. The classic reptilian appearance of serpentine gave rise to the Latin term serpentenaria, first used by Agricola in 1546 to describe serpentine rock. This rock weathers to an orange-red color, but fresh surfaces show a variety of colors from brown to green to white. Dragon skin rock is slippery and treacherous in wet weather. In both the tough antigorite rock and the sheared serpentine rock, magnetite is abundant, sometimes as macroscopically visible grains but frequently as finely disseminated panicles which give the rock a dark color. The lowermost zone consists of dark, blue-grey massive chloritc-diopside-gamct rock containing a host of accessory minerals. The contact between the chlorite-diopside-gamet rock and the dragon skin rock is sharp in most places (Plate 4.5). Mapping of this contact around the base of the knob shows that the contact surface is roughly planar, dipping gently to the south: the dip varies from 10-20. The contact may be followed along the base of the north side of Perovskite Knob, generally about 1 m above present ground level. It is 4-23 Plate 4.4 - Photo of Pcrovskite Knob from the north. Knob rises 15 m. above its base and is capped with resistant. red-weathering antigorite. Bush in foreground is manzanita; trees are digger pines. 4-24 : t r" Figure 4.4 - Geological sketch map of Pcrovskite Knob, showing antigoritc cap rock, sheared serpentine "dragon skin" rock, and underlying slab of southerly dipping chloritediopside-gamet rock. Compare with Figure 4.2. 4-26 < o 3aC* tj C3 a a 5 >>> > CoO Oej o :v Z Figure 4.4 - Geological Sketch Map of Perovskite Knob ciiloritc-diopsidc-gamcl rock Figure 4.5 - Schematic diagram of Perovskite Knob, showing mineralogical variation. Compare with Figures 4.3, 4.4. 4-28 Figure 4.5 - Schematic of Perovskite Knob Measured Vertical Section nature of garnets none Ti-free andradite in lower portion Ti-rich andradite major phases andgorite sheared serpendne distance above ref point (m) 15 ~ 12 - chlorite+diopside+gamet 0 -- Plate 4.5 - Photo of base of Perovskite Knob on the north side. Hammer sits on subhorizontal fracture surface of yellow-weathering, chlorite-diopside-gamet rock that underlies Perovskite Knob. Contact with overlying sheared, green serpentinite is the prominent fracture, 1 m. above hammer, that dips 30 to right. 4-30 Plate 4.6 - Photo of block of red-to-bluc weathering chlorite-diopside-gamet rock, showing sharp contacts, at east end of Perovskite Knob. Hammer handle is on chlorite-diopsidegamet rock, head on enclosing rcd-to-green weathering serpentinite rock, block dips 60 left. 4-32 Plate 4.7 - Photomicrograph of M2 garnet vein in sample 688-6 from Perovskite Knob. M2 Ti-garnet is birefringent. sector twinned, and composidonally zoned. Fans of yellow, M2 chlorite grow out from vein wall below birefringent garnet. Wallrock of vein occupies left half of photo; it consists mainly of Ml chlorite, some of which shows anomalous blue interference colors. Opaque minerals in upper left include deep red Ml garnet and magnetite. Crossed nicols. Field of view 2 nun. 4-34 nearly continuous with an offset below the east end of the knob which separates the east and west summit blocks higher up. At the NW comer of the knob the exposed contact ends abruptly at a large talus pile. On the south side of the knob only one small outcropping of the chlorite-diopside-gamet rock was found, near the base of a digger pine. At the present erosion level, the lower boundary of the chlorite-diopside-gamet rock is not exposed. The exposed portion, however, including its upper contact, bears strong resemblance in mineralogy and bulk composition to the slab of chlorite-diopside-gamet rock at the 34*50 locality. In contrast to the occurrence at 34-50, an outer zone of diopsiderich rock has not developed at Pcrovskite Knob. It is nevertheless proposed here that this contact between chlorite-diopside-gamet rock and overlying serpentinite is the outer boundary of a block consisting of chlorite-diopside-gamet rock. Shearing along this contact during emplacement of the serpentinite has resulted in the slickensided "dragon skin" rock on the less-competent serpentine side of the contact The exposed cap rock at 34-50 corresponds to the dragon skin rock at Perovskite Knob. The antigorite rock at Pcrovskite Knob has no counterpart at 34-50: if once present it has been eroded, an interpretation consistent with the poor, crumbly condition of the 34-50 towers compared with the High relief at Perovskite Knob. At the extreme eastern (uphill) end of Perovskite Knob the contact between chloritediopside-gamet rock and overlying dragon skin includes an angular block of chloritediopside-gamet rock surrounded by sheared serpentine rock. This 2 meter high angular block is about 30 cm. wide, lies in sub vertical orientation, and superficially resembles a dike (Plate 4.6). Close examination of the block shows that it is not a dike, but an intact block of massive, fine grained chlorite-diopside-gamet rock. The contacts between the chlorite-diopside-gamet block and the surrounding serpentine are sharp: there are no visible 4-36 textural gradients across the block. This block is interpreted as an irregular portion of the otherwise generally planar contact of underlying chlorite-diopside-gamet rock. The blue-grey chlorite-diopside-gamet rock at Perovskite Knob does not exhibit the strong foliation observed at the 34*50 locality. It is massive to weakly foliated, and is cut by parallel vertical veins and fractures that strike northeast. These fractures are pan of the late stage northeast trending features shown in Figure 4.1. The fractures contain museum quality mineral specimens of garnet, vesuvianite (idocrase), and perovskite (Table 4.4). Extensive blasting by mineral collectors has occurred at Perovskite Knob. This is a mixed blessing for the geologist. Some excellent mineral specimens were exposed by the blasting but geologic relations were obscured by talus. The only rattlesnake observed within the serpentinite during the course of this study was found in the talus pile at Perovskite Knob by the authofs daughter in the summer of 1990. The mineralogy of the chlorite-diopside-gamet rock at Perovskite Knob is rich and varied; this is a famous collecting locality. Phases encountered in the course of this study include: chlorite, dlopside, titaniferous garnet, andradite garnet, magnetite, perovskite, vesuvianite (idocrase), titanite (sphene), apatite, zirconolite, pyrolusite, chalcopvrite, and the secondary minerals calcite and hydromagnesite. Other minerals have been found at this locality, the reader is referred to Coleman (1986) for a more complete list The titaniferous garnets from Perovskite Knob and other nearby localities are of particular interest both mineralogically and petrologically; they are discussed further in Chapter 5. Chlorite at Perovskite Knob occurs in more than one generation, as at the 34-50 locality. The first generation of chlorite defines the weak foliation of the rock. A few grains of this MI chlorite show a blue anomalous interference color in thin section. The M2 chlorite is 4-37 associated with vein-filling M2 garnet, and is locally found in millimeter-sized hexagonal plates coating the walls of veins. In thin section the M2 chlorite is displayed as blades arranged in the shape of a fan, attached to the vein wall (Plate 4.7). The M2 chlorite is more magnesian than the first generation. Table 43 shows chlorite compositions at Perovskite Knob. Comparison with Table 4.2 shows that Perovskite Knob chlorites have a higher Fe content than 34-50 central zone chlorites. However, chlorite 688-8C.7 in Table 43 is second generation; it is magnesian, comparable to chlorites from the outer zone at 34-50. This observation suggests that some of the chlorite in the outer zone of 34 50 may be of the M2 generation. Ml diopside at Perovskite Knob occurs chiefly as isolated equant matrix grains, unlike at 34-50 where there are diopside-dominated domains. The overall abundance of diopside at Perovskite Knob is significandy lower than at 34-50, reflecting a different bulk composition. The diopside at Perovskite Knob is also somewhat more Fe-rich than at 34 50 (Table 4.1). D. The Melanite Mine This locality is a low and deeply weathered exposure of yellow-weathering chloritediopside-gamet rock, lying high on the north side of Hill 4857 above the streambed of the south fork of the San Benito River (which forks just upstream from the Gem Mine). The outcrop lies straight downhill via a pair of rough roads from the EPA gate at the northwest comer of the Atlas Mine site. The coordinates are 3619'.88N, 12036'.02W; the elevation is about 4700 ft. (1433 m.). The Melanite Mine may be also be approached on foot from below, by ascending a gully from a road which traverses the north slope of Hill 4857. This locality was unknown at the time of Coleman's work in the 1950s. 4-38 Unlike 34-50 and PerovsJdte Knob, the Melanite Mine is not protected by resistant cap rock, which accounts for its low and generally unimpressive appearance (Plate 4.8). The locality might be overlooked altogether were it not for abundant drusy crystals of black Tirich M2 garnets coating vein surfaces: mineral collectors have blasted the outcrop with dynamite, leaving a large talus pile with easily gathered grab samples. The outcrop is a low bench of yellow-weathering, chlorite-diopside-gamet rock about 1 m. high and 15 m. wide. The bench is flanked on both sides by green-weathering serpentine rock. Here, as at many other New Idria localities, contact relations ate best seen from a few meters away, as subtle color contrasts between adjacent rock types. At the south end of the bench a backhoe excavation leading up the hill a short distance has exposed the green-weathering serpentine rock at the south end of the outcrop. Evidently mineral collectors wished to see if a buried portion of the outcrop could be uncovered; as a result the contact at the edge of the chlorite-diopside-gamet rock is now exposed. Figure 4.6 shows in schematic form the essential rock types at the Melanite Mine: comparison with Figures 4.2 and 4J shows the same progression of rock types as those seen at the 34 50 locality and Perovskite Knob. The Melanite Mine is accordingly interpreted as a 15 m. wide slab of chlorite-diopside-gamet rock, bounded on both sides by sheared serpentinite rock. The thickness of this slab cannot be determined from the existing exposure. Like the 34-50 locality, the central zone of chlorite-diopside-gamet rock here contains both chlorite-rich and diopside-rich domains. The chlorite-rich domains are moderately well foliated with Ml chlorite defining the foliation. The diopside-rich domains are massive to weakly foliated. The abundance of Ml garnets is somewhat greater in the diopside-rich domains. The exposed face of chlorite-diopside-gamet rock in the central zone also contains irregular regions of serpentinite, making the field relations very confusing at the Melanite Mine. These serpentinite regions appear to be artifacts of blasting and may 4-39 Plate 4.8 - View of the Melanite Mine, consisting of a slab of yellow-weathering chloritcdiopside-gamet rock above talus slope, enclosed by green-weathering serpentinite. Total width of slab IS m. Contact of slab with serpentinite visible by color change, one third of distance from left side of photo. 4-40 Figure 4.6 - Schematic diagram of Melanite Mine tabular outcrop, showing mineralogical variation from left to right. The progression of rock types is similar to that of 34-50 (Figure 4.3) but rotated 90. 4-42 Figure 4.6 - Schematic of Melanite Mine Co9 cn C a3. tn nn so` serpentine + diopside + gamet serpentine f%iAt 3 g I 9O serpentine serpentine + diopside + garnet iu-> natureof garnets none Ti-free andradite Ti-rich andradite cats eye garnets (sec text) nonc t't-fr ajfuipg uta Kq oioqj *uim -o msia jo ppy -3urj\ sinrcppq sqi jc suoz uopisireu puaitj sip ui punoj ussq 3Acq sjsureS asaqj, `suiu .iood-tj. `MOipX pirc shoo qou-tx `jpejq qii*v sioun:3 .pfo s.ico,, jo qduSaioiuioioiy - 6'f Wld therefore represent pan of an irregular contact surface at the margin of the chloritc-diopsidcgamet slab. Weathering at this outcrop makes an exact interpretation of the contact relations problematical. The mineralogy at the Melanite Mine is quite simple compared with Perovskiie Knob; the great variety of accessory phases there is absent at the Melanite Mine. In the left hand or southern transitional zone between the central zone and the serpentinite, patches of M2 diopsidc whiskers about 5 mm. long are found coating serpentinite, giving the rock a fuzzy appearance. In the central zone Ti-rich M2 garnets coat vein walls. These garnets are jet black and of generally uniform size, under 1 mm. At the right hand or northern transitional zone, some remarkable "cat's eye" garnets were found, as indicated in Figure 4.6. These small (< 1 mm.) andradite garnets have black, Ti-rich cores, and yellow, Ti-free rims. The compositional zoning is abrupt and extreme, as seen in Plate 4.9. The diopside-rich substrate on which the garnets lie has a porous or spongy texture, suggesting high fluid flux. These cuhcdral M2 garnets formed in the altered rind of the relatively Tirich central zone. Aqueous fluids from which the garnets precipitated during the M2 event were able to mobilize 71 on a scale of about one meter, the width of the rind. Outside this rind only Ti-free garnets were found, due to the greater mobility of Ca under M2 conditions. The interpretation of these cat's eye garnets is also discussed in Chapter 6, as an example of Ti mobility in aqueous fluids. Miintcner & Hermann (1994) have reported similar garnets from Val Malenco, Italy. E. Discussion of Chlorite Compositions Table 4.2 and Table 43 give compositions of chlorite from the 34-50 and Perovskite Knob localities respectively. All compositions are ordinary mctamorphic chlorites; they fall into the categories of clinochlore, pycnochloritc, and penninite in the nomenclature of Hey 4-46 (1954). Figure 4.7 shows these compositions in the system Al203-Fc0-Mg0 projected through Si02 and H20. Examination of Table 4.2, 4.3 and Figure 4.7 shows a considerable compositional range for New Idria chlorites, particularly in Fe/Mg ratio and in A1 content. Much of this variation may be explained by the exchange vectors AljMg ,Si.| (Tk) and FcMg., (Fm). Additionally, substitution of ferric iron via Fe3+AI_, (Fa) probably takes place, but in this study ferrous/ferric ratios were not determined. Figure 4.8 plots the progress along the Tk exchange vector against the ratio Fe/(Fe+Mg), in order to show strong covariance of Fe and Al. Laird (1988) demonstrated that in mafic and ultramafic rocks, a strong positive correlation exists between progress along the Tk vector in chlorite and metamorphic grade. I therefore conclude from Figure 4.8 that the Ml chlorites record higher grade conditions than the M2, and that the MI chlorites generally did not recrystallizc during the M2 event In both Figure 4.7 and Figure 4.8 the data points for M2 chlorites are distinguished from Ml. The M2 chlorites are more magnesian and less aluminous than the MI. The FeMg ratio in chlorite reflects the local bulk composition of the rock (Maruyama et al.. 1986). As noted before, all New Idria chlorites are less magnesian than coexisting diopside, which is an exception to the general rule that chlorites tend to be more magnesian than coexisting mafic minerals. Using the thcrmochemical data of Helgeson et aL (1978) in the model system FMASH, I find that at sub-greenschist conditions, the assemblage Fe-chlorite (daphnite) + diopside is stable relative to Mg-chlorite (clinochlore) + hedenbergite, which is consistent with the field observations at New Idria. F. Discussion of Bulk Compositions Bulk compositions of the chlorite-diopside-gamet rocks are very similar to pyroxenites from other ultramafic rocks world wide. Table 4.5 shows representative bulk 4-47 Figure 4.7 - Chlorite compositions projected into the system MgO-(Fe+MnO)*Alj03 system. Ml and M2 chlorites form distinct groups as shown. 4-48 mole fractions Figure 4.8 - Tschcrtnak variation in chlorites plotted against Fe/(Fe+Mg), showing covariance. Ml chlorites record higher grade conditions than M2 (Laird, 1988). 4-50 compositions of chlorite-diopside-gamet rocks, along with pyroxcnitcs from other localities and with New Idria Serpentinite sample 989-45 for comparison. The bulk compositions of the chlorite-diopside-gamet rocks vary over a considerable range: Al, Ca, and the ratio Mg/Si show the largest excursions. The variability in the New Idria samples is partly explained by the nature of rock samples chosen for analysis: chlorite-diopside-gamet rock containing chlorite and diopside in all proportions is found at New Idria: ensuring that a given sample is "representative" is difficult. Some trace element data are shown in Table 4.5 for the samples in this study and for the comparison samples. The trace element concentrations for the chlorite-diopside-gamet rocks fall within the normal range for pyroxcnitcs; both are different from the depleted 989 45 sample, which is a serpentinized harzburgite. Table 4.6 recalculates the analyses from Table 4JS using the spinel norm method described in Chapter 3. Figure 4.9 shows the analyses from Table 4.6 projected into the ultramafic ternary system ol-opx-cpx (see also figure Figure 3-3). The projection is through spinel and minor components, c.g. Ti02. A similar projection through garnet or anorthitc would slightly affect the plotted positions by moving all points incrementally away from the Cpx vertex, but the effect would be small, and the equivalent rock types would not change. The shaded oval outlines the area of serpentinite bulk compositions from Figure 3.4: these lie in the dunite to harzburgite fields. All of the data points ploned in Figure 4.9 lie in the pyroxenite field: most fall within the olivine websterite category. The dotted line in Figure 4.9 represents the locus of compositions along a mixing line between two end members: pure chlorite and pure diopside, using actual mineral compositions from the 34-50 locality. All compositions with the exception of the pure chlorite sample lie in the pyroxenite portion of composition space; the pure chlorite end member lies along the ol-opx join. The open triangle symbol that lies nearly on the dotted 4-52 line represents a model composition used for thermodynamic calculations in the following section. G. Models for the Origin of Chlorite-Diopside-Garnet Rocks 1. Metasomatism Coleman (1957) referred to the chlorite-diopside-gamet rocks of New Idria as metasomatic chlorite-rich rocks, and proposed that they were formed by metasomatic alteration of ordinary serpentine rock. He proposed a process of alteration of serpentine rock-to one containing chlorite, diopside and garnet, by fluids importing Ca, Al, Ti, and Fe while exporting Mg and Si to the surrounding environment He proposed that the metasomatic fluids were hydrothermal fluids related to the intrusion of some small intrusive syenite bodies 1-2 lan. distant The main reason for this hypothesis was to explain the extreme enrichment of Ti in the "metasomatic" rocks (more than four orders of magnitude). He suggested that breakdown ofTi-rich amphibole in the intrusives could have contributed aqueous Ti complexes to the hydrothermal fluids. However, kilometer scale movement of Ti-rich hydrothermal fluids is problematical given current understanding of the very limited mobility of Ti in crustal rocks (see Chapter 7, also Van Baalcn, 1993). Ti (and Al) complexes in aqueous fluids at low temperatures have such low concentrations that the required advection of these elements could be accomplished only by massive fluxes of fluids infiltrating through a well developed subterranean plumbing system. At New Idria, however, there is no evidence of a large scale system of fluid conduits and vein systems. On the contrary, the chlorite-diopside-gamet blocks arc relatively small, isolated from one another, and show no obvious spatial relationships to each other. No evidence of veins connecting blocks has been found. Furthermore, close examination of serpentinite 4-53 rock at the contact with the syenite intrusive shows no sign of metasomatic alteration. The contact instead appears to be relatively dry and baked, with an aureole containing in tum olivine, antigorite. and lizarditc/chrysotile at increasing distances from the contact. This succession of mineral zones in the aureole represents successively lower peak temperatures within an essentially isochemical system. On the syenite side of the contact, however, there is in places a rind of albitite or chloritized amphibole with titanitc (sphene) that suggests deuteric alteration. Formation of titanite (sphene) as a breakdown product of Tiamphibole suggests that Ti may have been conserved rather than exported during this process. The penetratively foliated textures of the chlorite-diopside-gamet rocks manifest powerful shear during the crystallization of the chlorite. Serpentine rock in the area, while generally sheared and crudely foliated, never shows this penetrative foliation. This observation suggests that the development of penetrative foliation predates entrainment of the blocks. 2. Isochemical Metamorphism Ail of these problems with the metasomatism hypothesis are solved by a simpler proposal adopted here, that the chlorite-diopside-gamet rocks represent nearly isochemically metamorphosed blocks of a pyroxr.nite bulk composition that are included within the larger serpentinite body; these are not the lherzolite veins of Chapter 3. Only water is required as an open system component. This interpretation is suggested by the field relations and rock textures, by comparison of bulk compositions, and finally by analogy with similar rocks from other localities that do not have the same local geology as New Idria. If the blocks are indeed metapyroxenites, the question of the origin of the blocks must be addressed; there are several possibilities. It is possible that the metapyroxenite blocks are 4-54 tectonic inclusions from an unknown source entrained within the scrpendnite: tectonic inclusions of a variety of other rock types arc present. It is also possible that the metapyroxenite blocks might be primary igneous features or even have a cognate origin with the depleted dunites and harzburgites. They may represent cumulate layers from passing basaltic melts, or crosscutting veins and dikes; this possibility might be investigated with a trace element study if it could be shown that trace clement patterns persist through serpentinization. The mirror symmetry displayed at the 34-50 locality suggests but does not prove an origin as a multiple injection dike. Other, less-altered ophiolitic slices in the Coast Ranges, c.g. Elder Creek in Tehama County, have meterscale veins and dikes of clinopyroxenite (Rynearson, 1946; Shervais & Beaman,-1994). The present random orientation of the blocks may well result from boudinage or other tectonic dismemberment of dikes followed by block rotation during the diapiric ascent of the New Idria Serpentinite. 3. Whole Rock Reactions If the chlorite-diopside-gamct rocks arc derived from pyroxenites, then it should be possible to write whole-rock reactions relating the two rock types. These whole-rock reactions in turn must be linear combinations of the component reactions. Since the Tigarnets, magnetite and chromite arc only accessory minerals to the rock-forming minerals chlorite and diopside, we arc justified in ignoring them at first This simplification allows us to initially employ the model system CaO-MgO-A^Oj-SiC^-f^O (CMASH). A representative model composition in CMASH for a pyroxenite is lOMgO + 2CaO + A^Qj + 9SiO, + 6^0. Under upper mantle conditions this composition (ignoring the water) would exist as a mixture of olivine + orthopyroxene + clinopyroxene + spinel. 4-55 Figure 4.9 - Chlorite-diopside-gamet bulk compositions in the ultnunafic ternary (see Figure 3.3 for field boundaries). Explanation of symbols: filled squares, chlorite-diopsidegamet rocks from New Idria; filled triangle, chlorite-diopside-garnet rock from Val Malenco (Miintencr, 1994); filled circle, pyroxenitc sample 66SAL-1 from Hawaii; open triangle, model composition used for calculations (see text). Dotted line is locus of points containing mixtures of chlorite and diopside, from 0% to 100% diopside. Shaded oval is region of serpcntinite bulk compositions from dunite to harzburgite (see Figure 3.4). 4-56 projected from spinel and minor components Figure 4.10 - Equivalent mineral assemblages for composition shown, P-T conditions generally increasing from top of table to bottom. 4-58 o <N vO + oO'! co CN + m O _r^i < + O S3 u <N + o ea_ o c 0e <y cCO Si-3i b- < 1 o Ou, 3 CO E c~ o <--N *'ct3/ij. `&I s 13 S3 Urs aS si o'Ifvl U I/J C/3 rO CO 2 30 Zo <=> o 0*1 -iT1 jr CO o _rs <irj so 2 s, WM 3 + m 2x CC/3O oQ. 5 2 %-g CO 2o <r + vO . CeOo rUO* 2 0 + oCO CM r5? 60 a U , vO m i . 2 m 2g BO -- 2 cn + VO Ox -"7* CcOo U 2 u cn O - ^3 CO cto` o JN *** vO + 2* # o 2 + rr 2o co . cm > eo a 2o m + vO O rs co eo x 5 jo- u + CM 6" r*l CO tN <1 1co S 8> U o JN 3 + co co jtj 2JN P < + m 2x co a. eo O 2 co + tT 2g 00 *5 2 m + vO CO X CO Q. 2U rs u <N Recast in terms of New Idria minerals, this composition is equivalent to a mixture of chlorite + serpentine + diopside. The composition corresponds to the open triangle symbol in Figure 4.9. This point falls within the pyroxenite (olivine websterite) field, and lies near the Hawaiian reference sample. Figure 4.10 shows the equivalent mineralogical arrangements of this model composition expressed in terms of anhydrous mantle minerals. Whole rock reactions therefore relate the phase assemblages in successive lines. The equivalence of the anhydrous phase assemblages in Figure 4.10 and the metamorphic reactions connecting them were demonstrated by Thompson using the concepts of modal space and reaction space (Thompson, 1991,1982). At New Idria, there is no reason to prefer one of these upper mantle assemblages over another, for present purposes they will be considered equivalent. In other words, the conclusions drawn here do not depend on the starting mantle mineralogy. 4. Component Reactions Chapter 3 showed how the serpentinization of dunites and harzburgites results in a mixture of serpentine + brucite as the low-temperature assemblage. The modal proportions of serpentine and brucite are determined by the modal proportions of olivine and Opx in the parent rock, i.e. by the Mg/Si ratio. This traditional, simplistic model of serpentinization is limited by the MSH model system. a. Conservation of Al While a serpentine phase is produced during hydration of these rocks, chlorite appears as the characteristic aluminous phase; chlorite is apparently stable relative to aluminous serpentine. The reaction that relates lizardite to chlorite involves the Tschermak substitution: 4-60 2Mg3Si205(0H)4 + Al2Mg.,Si.j = Mg5Al2Si3O,0(OH)g lizardite Tk clinochlore O'Hanley et al. (1989) briefly considered the issue of A1 partitioning betv/ecn aluminous lizardite and clinochlore but did not resolve the relative roles of these minerals during serpentinization. The relative stabilities of the 7A phases such as aluminous serpentine and amesite versus the 14A chlorites are not well understood. Recently Bailey1 et al. (1995) described dozyiie, a 1:1 regular interstratification of serpentine and chlorite. The component reaction relating plagioclase and chlorite in CMASH is straightforward: CaAl2Si208 + 3Mg2Si04 + 4H20 = Mg5Al2Si3O,0(OH)8 + CaMgSi206 anorthite forsterite chlorite diopside This reaction shows that, in the presence of olivine and water, the aluminum in plagioclase is conserved as the aluminum in chlorite. Note also that this reaction has diopside as a product, resulting in an increase of modal pyroxene during hydration! However, serpentinization of Opx will decrease modal pyroxene, so that the abundance of total pyroxene in the hydrated rock will depend on the Cpx/Opx ratio in the anhydrous assemblage. Spinel or pyropc garnet may be related to chlorite with analogous reactions in which aluminum is conserved. 1 This paper was published posthumously; Sturges Bailey passed away November 30,1994. 4-61 Figure 4.11 - Univariant reactions in a portion of the CMASH system relevant to serpendnized peridotites and pyroxenites. Note that chlorite coexists with all of the lowtemperature assemblages. Tremolite appears and diopside disappears at low amphibolite grade. Complex reactions at upper amphibolite grade omined - see Evans (1977) for further details.. Calculations using TWQ2S code of Berman (1991). 4-62 Figure 4.11 - Univariant Curves in the CMASH System Pressure (bars) Figure 4.11 shows some of the key metamorphic reactions in CMASH involving serpentine, chlorite, and the accompanying calcic minerals. In this figure, chlorite is stable over nearly the entire diagram; its breakdown reactions at high temperatures have been omitted. According to Trommsdorff & Evans (1974), chlorite undergoes a continuous reaction at upper amphibolite grade during Alpine-style mctamotphism, producing green spinel. Breakdown curves for dissociation of chlorite at high temperatures have been also been investigated experimentally by Cho & Fawcett (1986), Jenkins & Chemosky, (1986) and Staudigel & Schreyer (1977). Some experiments have produced cordierite as a product of chlorite breakdown, but cordierite is only found in metamorphosed ultramafic rocks under high-temperature low-pressure conditions. The New Idria rocks have not . experienced high grade metamotphism; the rocks have remained well within the stability field of chlorite. b. The Role of Ca During low temperature hydration of fertile ultramafic rocks, diopside appears as the characteristic calcic phase, because there are no calcic chlorites. It is at fust surprising that diopside, not tremolite is the stable low temperature product of the hydration of fertile ultramaficrrocks. This behavior distinguishes diopside in ultramafic compositions from diopside in siliceous dolomites. Figure 4.11 shows that tremolite forms by prograde metamorphism of diopside + antigorite at temperatures in excess of 400C Evans & Trommsdorff (1970) showed in chemographic form the mineral assemblages encountered by progressive metamorphism of rocks of this bulk composition. At high temperatures tremolite breaks down and diopside reappears. Thus in this bulk composition diopside has not one but two stability fields. Although a full discussion of the stabilities of these minerals is beyond the scope of this study, it is relevant that the stability of tremolite at low 4-64 temperatures is enhanced by a high activity of C02 in the metamorphic fluids - at New Idria the scrpentinizing fluids were probably low in C02, as was shown in Chapter 3. Another important observation is that Ca is frequently lost to the rock body during serpentinization, so that the bulk composition of the serpentinite is lower in Ca than the precursor ultramafic rocks. The Ca is contained in aqueous fluids; the fluids associated with active scrpcntinization have high pH and are generally saturated or supersaturated with respect to Ca(OH)2 (Coleman, 1977; Barnes & O'Neil, 1972). On the other hand, streamwaters draining completely serpendnized bodies such as New Idria are low in Ca; instead they are enriched in Mg2* and HCOj (ibid). - c. The Roles of Fe and Ti Thus far we have focused on the rock forming minerals and used CMASH as a model system. Consideradon of metamorphic Ti-gamet and magnedte requires us to include Fe and Ti in the discussion, and thus expand our model system to CFTMASH. Chapter 3 discussed the stability of magnedte during serpendnizadon. The formation of andradite garnet is probably due to component reactions involving the hedenbergite component of Cpx (Taylor & Liou, 1978): 9CaFeSi206 + 202 -- 3Ca3Fe2Si30j2 + Fe304 + 9Si02 or 9CaFeSi206 + 4H20 = SCa^S^O^ + Fe304 + 9Si02+ 4H2 Hd Adr Mag 4-65 In these reactions the Si02 component does not appear as free quartz; rather, its effect is equivalent to a decrease in the Ol/Opx ratio and a change in the amounts of modal chlorite, serpentine and brucite produced during serpentinization. As noted in Chapter 3, the presence of awaruite (FcNi) and other phases characteristic of relatively reducing conditions demonstrates that serpentinization may occur at very low oxygen fugacities. Peretti et al. (1992) demonstrated that andradite garnet occurs in serpentinites in conjunction with these reduced phases. The formation of Ti-gamet probably occurs in reactions similar to those above, in which a Ti component of the precursor Cpx, (expressed as the exchange vector TiFe.,), enters the garnet phase. See Chapter 5 for a discussion of Ti-gamets and the exchange vector TiFe.j. . 9CaFcSi206 + 3TiFe_j + 4H20 = 3Ca3TiFeSi30,2 + Fe^ + 9Si02+ 4H2 Hd Ti-Adr I. Serpentinization of Fertile Rocks Traditional discussions of serpentinization imply, without generally stating, that the starting material was a mixture of olivine and Opx, Le. a dunite or a harzburgite. The reactions usually written are expressed in MSH. This simplification is justified, because the majority of exposed serpentinites world-wide appear to be hydrated dunites and harzburgites. However, ultramafic rocks in nature frequently have more complex, fertile compositions, and so a more general view of serpentinization is needed. The hydration of fertile lherzolites and pyroxenites is more complex than that of their depleted cousins because of the presence of the elements Ca and Al. As before, Fe and 'll may initially be ignored. The addition of C02 as a component in the serpentinizing fluids would affect the phase relations discussed here, mainly by the addition of magnesite, and changes in the stability 4-66 fields of talc and tremolite. A full discussion of the effects of C02 in serpentinizing fluids is beyond the scope of this discussion; the reader is referred to Sanford (1981). For bulk compositions that can be expressed as points within the triangles of Figure 4.9 or Figure 3.3, i.e. a mixture of 01 + Opx + Cpx with minor Spl, the low-temperature hydrated equivalent can be represented by a point in the tetrahedron brucite-talc-diopsideclinochiore, (Brc-Tlc-Di-Chl) as shown in Figure 4.12a. Only compositions more silicic than a 1:1 ratio of 01:0px will plot to the right of the shaded plane Ctl-Di-Chl in Figure 4.12a. Such compositions will have talc and possibly tremolite as part of their lowtemperature assemblage; as noted in Chapter 3, such compositions are extremely rare: they include olivine onhopyroxenites, orthopyroxenites, and websterites. More typical serpentinized peridotites and pyioxenites, however, will lie in the reduced tetrahedron brucite-chrysotile-chlorite-diopside. At New Idria, the depleted rocks are a mixture of chrysotile arid brucite; they lie on the basal edge of the tetrahedron. The chlorite-diopsidegarnet rocks lie inside the reduced tetrahedron close to the Di-Chi join. Ca loss during serpentinization would cause the bulk composition to migrate away from the diopside vertex but remain inside the tetrahedron. The presence of C02 in the serpentinizing fluids will result in the low-temperature assemblage talc + magnesite instead of serpentine. Talc and diopside do not coexist at low temperatures, but are replaced by tremolite, as discussed in Chapter 3. The resulting phase assemblage of Chi + Tr + Tic + Mgs lies in the shaded plane Tlc-Tr-Chl of Figure 4.12a, provided that the figure is redefined so as to project from Mgs. As a general rule, Fe and Mg follow separate geochemical pathways during hydration of fertile mantle rocks; Ti partitions into andradite garnet. The minerals in the hydrated assemblage accommodate Fe and Mg to different degrees, resulting in a greater number of 4-67 Figure 4.12a - Phase relations in the system brucite-diopside-talc-chlorite at 200C, l kb, projected through water. Compositions with Ol*Opx ratio > l lie behind shaded plane passing through Ctl-Di-Chl. Compositions with Ol-Opx ratio < 1 may contain Tr as part of low -T assemblage, indicated by smaller shaded plane Cd-Tr-Chl. Abbreviations are Ctltchrysotile, Brc:brucite, Dkdiopside, Tlc:talc, Chlxhlorite, Tntremolite. 4-68 Figure 4.12b - Major element partitioning during serpendnizadon of fertile peridodtes and pyroxenites. Chlorite is the characteristic aluminous phase when Al*exchange capacity of lizardite is exceeded (see text). 4-70 oc Figure 4.12c - Prograde metamorphism in a portion of CMASH. Topological changes in phase relations correspond to univariant reactions in Figure 4.11. Abbreviations as in Figure 4.11. 4-72 Figure 4.12c - Prograde Metamorphism in a Portion o f CM ASH 450C, 1kb u minerals. This greater number in turn reduces the thermodynamic variance of the assemblage according to the phase rule, when compared with the anhydrous assemblage. The forsterite component of olivine, the enstadte component of Opx and the diopside component of Cpx reappear in the hydrated assemblage in the form of chlorite, serpentine brucite and diopside. The fayalite component of olivine, the ferrosilite component of Opx and the hedenbergite component of Cpx reappear in the hydrated assemblage as chlorite, andradite garnet, magnetite and possibly brucite. Because the distribudon coefficients for the elements in the hydrated assemblage differ from those in the anhycLous assemblage, element redistribudon takes place duringserpendnizadon. This partitioning is summarized in Figure 4.12b. This figure shows in schematic form the chemical pathways for major elements and Ti. The modal abundance of the hydrous product phases depends on the exchange capacity (e.g. for Tk) of the hydrous minerals, and on the starting composition. Prograde metamorphism of the low temperature phase assemblages of Figure 4.12a produces a series of reactions that change the topology within the tetrahedron. These reactions are shown in Figure 4.11 and in the topological changes in Figure 4.12c. A more extended discussion of prograde metamorphism of alpine serpentinites is contained in Evans (1977) and in earlier papers by Evans and Trommsdorff. One of the important conclusions of this study is that the work of Trommsdorff and Evans in the Alps is also applicable to the evolution of the New Idria Serpentinite. I. Temperatures of Ml and M2 Metamorphism The above discussion using model systems shows that the phase assemblage chlorite + diopside + Ti-gamet magnetite can be derived from the low-temperature hydration of an 4-74 ultramafic precursor of pyroxenite composition. This event is defined as the Ml event. The phase assemblages found at New Idria suggest that greenschist or blucschist facies condidons accompanied this event The blocks containing these phases are enclosed within chrysotile-lizardite host rock, which is also stable under low greenschist and blucschist conditions. Chlorite compositions discussed earlier (see Figure 4.8) show that the Ml event represents a higher metamorphic grade than the M2. Since the M2 mineralogy is the same as the Ml, the phase assemblage must have been stable under the condiuons of Ml and M2. In an effort to learn more about the condidons of M2 metamorphism, a fluid inclusion study was undertaken, using fluid inclusions in M2 garnets. The results of the study were that homogenizadon temperatures averaged 235C, suggesting that trapping temperatures during M2 metamorphism were below 400C, well within the greenschist facies. Details of this study are described below in Section VI. J. About Rodingites The chlorite-diopside-garpet rocks at New Idria bear a superficial resemblance to a coarse grained rock type known as rodingite. Rodingites are metasotnatic calc-silicate rock bodies associated with certain serpentinites, such as the one at Belvidere Ml, Vermont The characteristic minerals of rodingites are hydnogrossular, idocrase, diopside, and chlorite. Rodingites were first described at exposures along the Roding River in the Dun Ml ultramafic complex. New Zealand (Bell et al.,1911). Mafic dikes were intruded into the ultramafite prior to scrpentinization, so that when serpentinization occurred, chemical potential gradients and incompatible mineral assemblages provided the driving force for metasomatic exchange of Ca (out of the ultramafite) and Si (into the ultramafite), producing 4-75 the characteristic suite of calc-silicate minerals. The term rodingite has since been used more generally (e.g. Coleman, 1977) to describe metasomatic rinds that form during serpendnization, by reactions either with intrusive dikes or siliceous country nock. Figure 4.13 shows on an ACF diagram the bulk compositions of chlorite-diopside-gamet rock from Table 4.6 compared with the rodingite field from Coleman (1977). While the New Idria rocks plot near a tie line between chlorite and diopside, the rodingite field sits astride the grossular-diopside join. The chlorite-diopside-gamet rocks of New Idria did not form as rodingites because the bulk composition of both the metapyroxenite blocks and the enclosing serpentinite were both silica-poor. Chlorite, diopside and titaniferous andradite garnets arc all stable phases in association with serpentine minerals. There is no regional scale rodingite rind at the boundary ofthe New Idria Serpentinite because the emplacement into the country rock postdated serpendnization. One possible exception to this conclusion is the small rock body of locality 990-1 (see Figure 1.3), at which the rock contains both vesuvianite and grossular garnet, the characteristic rodingite minerals. The bulk composition of this outcrop is unknown, but could lie in the rodingite field. K. Chlorite-Diopside-Garnet Rocks from Other Localities Chloritc-diopside-Ti garnet rocks arc not abundant on land; pyroxenites are seldom exposed at the earth's surface and their metamorphic products, chlorite-diopside-gamet rock, weather and erode rapidly. In Val Malenco, Italy, however, chlorite-diopside-gamet rocks with mineral compositions similar to those of New Idria are found in altered 4-76 pyroxenite blocks contained in the Malenco scrpentinitc (Milntcner, & Hennann, 1994). This serpentinite, one of the alpine ophiolites, lies structurally below the Austroalpine Margna nappe and above the Penninic Suretta nappe, and is one of the largest exposed serpentinites in the Alps. The Ti-rich melanite and schorlomite garnets were found first at the Forcella Fellaria locality, associated with a metamorphosed clinopyroxenite layer. However, field work in the summer of 1994 uncovered several other similar localities in the Malenco serpentinite (Miintcner, 1994, pers. comm.). Unlike at New Idria, the metapyroxenites at Malenco may turn out to be continuous enough to extract structural information about the . serpentinite. Val Malenco is also widely known for the green demantoid variety of andradite garnet. These Ti-free garnets are widespread at Val Malenco, but are not associated with chlorite-diopside-gamet rocks. At New Idria, similar demantoid garnets are found at die Green Fire Mine and other localities. See Chapter 5 for a discussion of garnet compositions. Two other occurrences of Ti-gamets are noteworthy: Switzer et al. (1971) describe Tigamets from serpentinized peridotites in the Mid-Atlantic Ridge near 43N. The garnets are hosted by chlorite-diopside rock. The authors compared this discovery to the New Idria occurrence, which at the time was the only known example of this garnet paragenesis. The authors also speculated that, unless their dredge sample was extremely unusual, there may be much more chlorite-diopside-gamet rock in the submarine environment than previously realized. Still another interesting occurrence ofTi-gamet is in the Sangabawa District of Japan, described by Onuki et al., (1981). This raetamorphic occurrence is in rock described as "olivine clinopyroxenite" that is included within a serpentinite in the paired metamorphic belts of Honshu. 4-77 Figure 4.13* Bulk compositions of chloritc-diopside-gamet rocks compared with Rodingitc field of Coleman (1977). 4-78 Figure 4.13 - Chlorite-Diopside-Gamet Rocks versus Rodingitcs lewJdna samples u u.H IV. Mafic Schists and Greenstones A. General Statement These blocks of grey-green to blue, massive to penetratively foliated mafic schists and greenstone are irregularly distributed throughout the New Idria Serpenrinite. Figure 4.14 shows the distribution of the larger blocks, here labelled simply "tectonic inclusions". Loudcrback & Blasdale (1909) and Coleman (1957) proposed that these blocks were Franciscan rocks entrained within the serpentinitc during emplacement. The bulk compositions and mineralogies of the blocks are very similar to Franciscan rocks found throughout the Coast Ranges (e.g. Ernst, 1965). No blocks of Great Valley sandstones and shales have been found within the serpenrinite. All of the blocks have sheared contacts and are relatively resistant to weathering, and arc therefore ridge formers. The sizes of the blocks vary from kilometer scale to meter scale. The largest of these blocks forms the southwestern boundary of the oval serpenrinite massif itself; this large marginal flake of Franciscan rock lies in fault contact with the serpenrinite on one side and the younger Great Valley rocks on the other, as shown in Figure 1.2. Other marginal Franciscan blocks are found along the New Idria Thrust Fault which marks the northeastern margin of the serpenrinite (Plate 1.1). Some physical mechanisms for emplacement of tectonic blocks in serpenrinites were described by Barriga et al. (1992). Table 4.7 compares representative bulk compositions of some of the tectonic blocks of mafic schist and greenstone from New Idria (columns 1-3) with Franciscan rocks (columns 4-6). The samples in columns 1-2 arc from the Gem Mine locality described below; column 3 is from a jadcite-bearing locality in Gear Creek described by Coleman (1961). Examination of these analyses shows that all of these rocks arc relatively high in silica and 4-80 soda compared with the the other types of tectonic blocks and with the bulk serpentinite. All of the analyses are also relatively low in alumina and lime; two are pcralkaline with acmite in the norm All of the analyses therefore would plot near the F vertex in a classical ACF diagram (not shown). As with the other types of tectonic blocks, the mafic schists and greenstones have experienced at least one episode of pre-entrainment Ml metamorphism. M2 metamorphism is manifest by mineralized veins that crosscut the Ml foliation. B. Jadeite-bearing Blocks ' Tectonic blocks of albite-crossite schist that contain jadeite are found in the Clear Creek Valley at the northern end of the District The blocks are easily found at a distance by looking for the oak trees which they support in contrast to the digger pines of surrounding serpentine soils. Coleman (1961) described the mineralogy and petrology of these mafic schists. He interpreted these blocks as desilicated keratophyres (Na-rich silicic volcanics) of the Franciscan Formation. Some of the blocks contain white, near end-memberjadeite in pods and in crosscutting veins. The jadeite formed in veins that crosscut the Ml foliation, but it is difficult to say whether the jadeite formed pre- or post-entrainment of the blocks. Coleman developed a model for the origin ofjadeite at low temperature and moderate pressure. The effect of M2 metamorphism on these jadeite-bearing blocks is manifest in the development of late stage zeolites and pectolite in the veins and pods. The reader is referred to Coleman (1961) for further details on the development ofjadeite in the tectonic blocks of the Clear Creek Valley. 4-81 Figure 4.14 - Distribution of some tectonic blocks in the New Idria Serpentinite Modified from Eckel & Myers (1946), Coleman (1961). 4-82 Geology from Uekel & Myers. 1946, Coleman, 1961, This situly Table 4.7 and Table 8 in Coleman (1961) show that, as a group, the jadeite-bearing blocks have low Ti contents, averaging 0.5 wt. % Ti02. This geochemical characteristic is used to distinguish these blocks from those with more normal Franciscan values of 1-3 wt. % described next. C. Blocks Containing Benitoite, Neptunite, and Joaquinite Blocks of mafic schist and greenstone that contain up to 2 wt % TiOj are exposed in the vicinity of the Gem Mine in the southern part of the District, at the Victor Claim in the Clear Creek Valley, and on the east summit of Santa Rita Peak. The blocks of mafic schist contain blue amphiboles (crossite, glaucophane) and are composidonally similar to Franciscan blueschists; they are interpreted as altered blueschists. Blocks of greenstone contain albite, chlorite, and non-jadeitic pyroxene; they are interpreted as Franciscan metagreywackes and metavolcanics. Mineralized M2 veins crosscut the Ml foliadon of the mafic schists. 1. The Gem Mine The Gem Mine is located near the headwaters of the San Benito River on the western flank of Santa Rita Peak. The coordinates are 3619.90' N, 12036.71' W. The open cut mine is reached by one of several poorly maintained roads that may require four wheel drive vehicles, especially in wet weather. The Gem Mine is a patented claim located on private property and permission from the owners is required in order to visit the mine. Mr. Elvis "Buzz" Gray of Missoula, Montana, and Mr. William Forrest of Fresno, California, control access to the property at this time. Wise & Gill (1977) reviewed the history and development of the Gem Mine, which has operated under several names, including 4-84 Bcnitoitc Gem mine, Benitoite mine. Gem Mine, Dallas Benitoite mine, Dallas Gem mine and Sapphire mine. This famous locality is best known as the site of the 1906 discovery of benitoite (BaTiS^C^), a blue mineral that is now the state gemstone of California (Louderback & Blasdale, 1907). Ncptunite (Na2KLi(Fe,Mg,Mn)2Ti202Si8022. the second reported occurrence) and joaquinite (Ba2NaCe2Fe(Ti.Nb)2Sig026(0H,F>H20, a new mineral) from the mine were described along with benitoite by Louderback & Blasdale (1909). All of these minerals are strongly peralkaline. Laird & Albee (1972) discussed the composition and physical properties of benitoite, neptunite and joaquinite. Wise et al., (1977) described jonesite, another new mineral from this locality. Wise & Gill (1977) described the minerals of the Gem Mine and concurred with Coleman on their probable metasomatic origin. Plate 4.10 shows in thin section the assemblage of benitoite and neptunite in a gangue of zeolite, primarily natrolite with minor thomsonite. Figure 4.15 shows a sketch map and cross section of the Gera Mine, redrawn after Coleman (1957) and Wise & Gill (1977). The main feature of the map is an oval, northwest-trending body of mafic schist and greenstone that is embedded in the surrounding serpentinite. This oval body contains two parts: greenstone to the north and mafic schist to the south. The mineralized zone lies in a brecciated zone along the cont; ct between the two pans. The excavated working area of the Gem Mine lies in the mafic schist of the footwall, where veins and fractures contain the ore minerals. The greenstone of the hanging wall forms a 10 m. high cliff on the north side of the mine. Barren mafic schist forms a low cliff to the south. At the eastern end of the marie schist body are some small, spheroidal bodies of altered gabbro that contain labradorite feldspar. 4-85 Plate 4.10 Photomicrograph of sample 688-H from the Gem Mine, containing benitoite (high relief trigonal mineral) and neptunite (orange bladed mineral) in a gangue of polycrystalline natrolite. Crossed nicols. Field of view 2 mm. 4-86 Figure 4.15 * Geological map and cross section of Gem Mine, redrawn after Coleman (1957), Wise & Gill (1977). 4-88 Figure 4.15 - Geologic Map and Cross Section of the Gem Mine The original geologic relations at the Gem Mine have been largely obscured by decades of mining. For this reason, the original field descriptions by Louderback are of paramount importance. Louderback and Blasdale (1909) reported that the brecciated and mineralized zone was 520 ft long and about 400 ft. wide in its widest part Within that zone, the benitoite-bearing rocks were only 64 ft. wide and 400 ft. long, with the abundance of benitoite decreasing to the west Bcnitoitc was only found at the surface for 230 ft. of the 400 ft. Louderback also noted that the greenstone diminished in abundance to the west. The general nature of the mineralized zone was. from the center outward, dmsy cavities containing benitoite, neptunite and natrolite deposited concurrently, next white, relatively pure natrolite, then bluish natrolite containing abundant amphibole needles, and finally the altered wall rock in which cavities filled with amphiboles in a felty texture suggested the leaching out of the feldspathic components of the wall rock. Outside the mineralized zone, albite rather than natrolite was the characteristic sodic aluminosilicate mineral. Louderback also suggested that the mineralized zone was the locus of deformation and faulting, together with development of northwest striking foliation in the footwall blueschist. Post-mineralization transverse (i.e. northeast-trending) veins were also noted. Finally, he noted that brecciation of the serpentinite rock extended to the east of the mineralized zone, in the same direction. He gave the orientation of the outcrop as 300, with a dip averaging 70N. As noted above, the greenstone and mafic schist at the Gem Mine are interpreted as Franciscan rock entrained within the serpentinite. A large block of Franciscan rock containing a contact between greenstone and mafic schist may have been caught up with the serpentinite during emplacement Alternatively, a block of Franciscan greenstone and a block of Franciscan mafic schist may have been separately entrained but moved to their present adjacent positions by tectonic movements during ascent of the diapir. Relative 4-90 movement between the two blocks would help to explain the brccriated zone along the contact. The greenstone of the hanging wall consists of two distinct types: the relationship between these types is obscure. The first type is a fine grained, massive, light brown-weathering, green granular rock composed of albite, chlorite, green pumpellyite, a non-sodic pyroxene, stiipnomelane, titanite (sphene), barite, and zircon. This greenstone is crosscut by veins containing albite and chlorite. Table 4.7 (column 1) contains a bulk analysis of this rock. The second greenstone is a massive, grey-weathering, dark green rock composed of albite, chlorite, a pyroxene near aegirine (acmite), stiipnomelane, and strongly corroded opaques. Limonite stains some of the albite orange. The bulk composition of this second greenstone was not determined in this study, but is probably higher in soda than the first greenstone, as suggested by sodic pyroxene. The second greenstone may also have a higher pressure Ml metamoiphic history. Both greenstones are relatively resistant to weathering so that the northern portion of the Gem Mine claim is a positive weathering ridge on the western slope of Santa Rita Peak. The altered mafic schist of the footwall, along the contact with greenstone, is a light to medium blue, brecciated rock containing actinolite, crossite, natrolite, albite, that is crosscut by zeolite veins (natrolite thomsonite) containing benitoite, neptunite. joaquinite, jonesite and several accessory minerals, including apatite, gypsum, a member of the zoisite group and several copper sulfides: djurleite (this study; Wise & Gill, 1977) digenite, covellitc (Wise & Gill. 1977), and possibly chalcocite (Louderback & Blasdale, 1909). The copper sulfides are in turn surrounded with alteration haloes of chrysocolla. The productive zone of the mine is quite narrow, being restricted to the brecciated zone immediately adjacent to the contact. A 1914 photo at the Gem Mine (Bradley, 1938) 4-91 showed that there were underground workings starting from an adit, but presently all work takes place in the open cut. The unaltered, barren mafic schist of the footwall is grey-blue, moderately resistant, massive to weakly foliated, actinolite-crossite-albite schist. In the more massive samples the actinolite occurs in randomly oriented blades and bundles, and is more abundant than crossite. In more foliated samples the amphiboles, especially crossite, take on a preferred orientation. Accessory stilpnomelane is present, together with unidentified opaques. Unaltered mafic schist (but not greenstone) crops out along the road that curves around the west end of the Gem Mine. - 2. The Victor Claim The Victor Claim consists of some weathered outcrops of mafic schist in and above an unnamed stream that forms a tributary to Gear Geek in the northern portion of the District. The coordinates are 36 22.17' N, 12042.22' W. The mineralogy of the Victor Gaim was described by Millage (1982): small colorless grains of benitoite have been formed here, probably by reactions similar to the one below. Neptunite also occurs, but at much lower abundance than at the Gem Mine. As pointed out by Millage, the benitoite at Victor Gaim is hosted in albite rather than natrolite. This may have been due to relatively lower activity of silica. Slight modifications to the benitoite-generating reaction proposed below would be required if albite were to be a product rather than a reactant. During this study the Victor Gaim was visited only once, and its mineralogy was not studied in detail Whether Franciscan greenstone is present at the Victor Gaim is unknown; Millage (1981) did not mention this rock type. The extent of hydrothermal wallrock alteration appeared to be much less than that at the Gem Mine. One notable 4-92 feature of the locality not mentioned by Millagc was the occurrence of a nearby mercury mine. In general, the mercury mines at New Idria are found along the periphery of the serpentinite, not deep within the serpentinite as in this case. It is possible that the mercury mine and the Victor Gaim both lie along a throughgoing fault that is suggested by the Jennings & Strand (1958) map The southerly continuation of this fault, that lies parallel to the New Idria Thrust Fault, also coincides with the linear array of syenite intrusives discussed in Chapter 1 (Figure 1.2, Plate 1.1). It is suggested here that this fault provided a conduit for circulating fluids during the M2 metamorphic event, and may have been responsible for the mineralization at the Victor Gaim and and the adjacent mercury mine. - 3. Other Benitoite Localities There is another benitoite occurrence at an undisclosed locality within the New Idria Serpentinite. according to Wise & Gill (1977). Outside of the New Idria District, benitoite is extremely rare. Grains of benitoite have been found in drill cores elsewhere in California, in Eocene sands of Texas, and in Belgian sands (Roberts et al., 1990). If the mechanism proposed below for the formation of benitoite is correct, then there may well be additional undiscovered benitoite localities. A promising place to look would be in the vicinity of tectonic blocks in serpentinites, especially at a contact between two different tectonic blocks. 4. The Formation of Benitoite One of the enduring mysteries of the New Idria District has been the origin of the rare mineral benitoite, BaTiS^O^ Coleman (1957) hypothesized that benitoite formed by metasomatic alteration of the Franciscan rocks at the Gem Mine, as a result of fluids 4-93 associated with syenite intrusions 1 km or more distant. This hypothesis was analogous to that for the formation of chlorite-diopsidc-gamet rocks from ordinary scrpentinite, as described in the previous secdon. An ahemative hypothesis is proposed here, that benitoite at the Gem Mine formed by metamorphism of the contact between the hanging wall greenstone and footwall mafic schist, in the presence of Na-rich, low silica fluids. Mineralized drusy cavities and extensive development of vein systems certainly suggest hydrothermal mineralization at the Gem Mine, but the chemical components needed to produce benitoite and probably the other exotic minerals are available in the adjacent wall rocks. Therefore there is no need to advect Ti, Al, and other components from a great distance. Ordinary brines associated with M2 metamorphism probably produced local mobility of the all the required chemical components of benitoite and other minerals. The pockets of felty amphiboles that are found throughout the brecciated zone and altered wall rock suggest that sodic feldspar was leached out of this rock and redeposited in the veins as natrolite thomsonite, with attendant loss of silica to the environment Natrolite must have precipitated from a fluid with low silica activity. A hypothesis for origin of the hydrothermal fluids is described in Chapter 7. By assuming the end member formulas for required minerals, we have the following simplified component reaction: barite + titanite + albite + chlorite + H20 = benitoite + actinolite + clinozoisite + natrolite + S02(aq). The actual whole rock reaction that took place in the presence of hydrothermal fluids is likely to be different that this reaction, particularly in the specific aqueous species present. 4-94 There is also no guarantee that the system was closed except for water. Extensive wall rock alteration in the mineralized zone suggests large volumes of fluid have passed through. However, all of the reactant phases in this equation are hanging wall phases; all of the product phases occur in the footwall. The above reaction is not unique: other similar reactions could be constructed, e.g. by substituting thomsonite for clinozoisite as a product In all of these reactions, however, the Ba required to produce benitoite can be found in barite, a rather common mineral in Franciscan rocks. Likewise, the required Ti can be found in (titanite (sphene). Louderback & Blasdale (1909) commented on the absence of barite or any other apparent source of barium in their original report. Only with the advent of the electron microprobe did it become possible to recognize the relatively abundant, micron-sized grains of this mineral. A mass balance of the Ba budget for these rocks shows that the abundance of benitoite is consistent with the abundance of barite in the surrounding rocks. Table 4.7 shows the amount of Ba to be 100-200 ppm, which compares with ISO ppm in the Catalina greenstone. Assuming that all of the Ba is incorporated in benitoite after the above reacdon, the amount of benitoite should be approximately 0.01 wt. % in the mineralized zone, or 1 part in 10,000. The actual amount of benitoite present is well under this amount. A similar mass balance of the Ti budget shows adequate dtanium to account for abundance of the Ti-bearing minerals such as benitoite and neptunitc. The temperature and pressure of formation of benitoite are not well constrained. Millage (1982) suggested that the upper stability limit of benitoite is about 600C. If benitoite and natrolite formed concurrently, then the conditions must have been within the stability field of natrolite, which is limited to about 250C at 1 bar (Peacor, 1973). Wise & Gill (1977) argued that the presence of djurleite as a companion phase placed a constraint on the temperature of formation of benitoite. The upper limit of stability of djurleite is about 95C. 4-95 However, as pointed out by Ulrich Petersen (pers. comm., 1992), the copper sulfide minerals readily recrystallize and invert to other forms with falling temperatures, unlike most silicates. This means that the crystals that are now djurleite could have originally formed as digenite or another high-temperature sulfide. Therefore, the implied constraint on the temperature of formation of benitoite at or below 100C should not be considered a reliable limit. Fluid inclusion evidence from garnets in nearby tectonic blocks suggests that M2 metamorphism may have occurred at about 300C; similar temperatures may have occurred at the Gem Mine at the same time. 5. The Formation of Neptunite _ The veins that contain benitoite also contain neptunite, a rare peralkaline mineral associated with alkalic rocks. A simplified formula for neptunite is Na2KLi(Fe,Mg,Mn)2Ti2Si8024. Neptunite ha$ both igneous and metamorphic parageneses. The New Idria occurrence is considered to be metamorphic, and is similar to an Australian occurrence of neptunite in a tectonic block included in the Woodsreef Serpcntinite (Slansky & Glen, 1982). Neptunite is hosted by albite, rather than natrolite, in the Australian occurrence. At New Idria, textural relations suggest that benitoite, neptunite and natrolite crystallized at the same time. The abundance of neptunite is everywhere greater than that of benitoite. Neptunite probably is produced by a reaction similar to that for benitoite formation, that involves hanging wall and footwall phases. Neptunite contains potash as an essential constituent and Mn as a significant impurity. Bulk analyses of the wall rock (Table 4.7) show sufficient K and Mn to account for the amount of neptunite present 4-96 6. The Ages of Benitoite and Neptunite In an effort to understand the timing of M2 metamorphism at New Idria, a reconnaissance study of the age of crystallization of the benitoite-neptunite crystals was undertaken. The minerals are assumed to have crystallized simultaneously. Using Rb/Sr dating techniques on samples from this study, Marvin Lanpherc of the USGS constructed a two-point isochron of a mineral pair, yielding an age of 11.96 Ma. With a two point isochron no reliable error estimate is possible, so the benitoite-neptunite age should be considered preliminary. A further attempt to date the neptunite crystals using 40Ar/39Ar technique is currently underway. However, this preliminary age agrees well with the 12.4 0.8 Ma age of the New Idria Syenite, mentioned in Chapter 1. The interpretation of this Miocene age of mineralization is discussed in Chapter 7. 7. The'Formation of Natroiite The abundance of natroiite as a gangue mineral at the Gem Mine is much greater than any of the exotic Ti-bearing minerals. Natroiite can form at relatively low temperatures by hydration of albite in a low silica environment, i.e. 2albite + 8H20 = natroiite + BSiO^aq) The excess silica is readily absorbed by the surrounding serpentinite and does not appear as free quartz. This reaction probably accounts for the natroiite veins found on the east summit of Santa Rita Peak, which is another tectonic block of mafic schist. Benitoite has not been found at this locality, although Coleman (1957) reported joaquinite. 4-97 V. Antigorite Knockers A. General Statement Resistant knobs or knockers of red-weathering, dark green rock stand in high relief against the crumbly rock which otherwise characterizes the District. These knobs consist almost entirely of dense, tough antigorite with a sugary texture. Prominent examples occur on the main (west) summit of Santa Rita Peak, on the west side of San Carlos Peak, and on the top of Perovskite Knob. The antigorite knobs may be sheared or mylonitized as at Santa Rita Peak, with recrystallization of antigorite within the mylonite zone. Where exposed, the contacts of these blocks are strongly sheared. Some knobs are crosscut by veins of yellow andradite garnet, as at Santa Rita Peak and at San Carlos Peak. The knobs probably represent prograde metamorphism of pre-existing chrysotile and lizardite to form antigorite. Alternatively^ they may record primary serpendnization of peridotite at a high temperature (see Chapter 3). In either case, these knobs have a thermal history separate in part from that of the bulk serpentinite, and must be older than the immediately surrounding serpentinite. B . Santa Rita Peak The summit of Santa Rita Peak (1574 m) is the most prominent topographic landmark in the New Idria District (Plate 4.11). The peak, unnamed at the time, was used as a geographic landmark by Arnold & Anderson (1908). Along with wooded San Benito Mt. 5 km. north, the summit of Santa Rita Peak forms the highest portion of the Diablo Range. Santa Rita Peak has two summits separated by a saddle; the craggy western summit is the higher of the two. A microwave communications tower maintained by Chevron Oil stands on the saddle; the gravel road to the tower is generally well graded and provides ready 4-98 access to the summit region. The upper slopes of Santa Rita Peak are treeless, due to recurring forest fires. Access to outcrops on the mountain is hindered only by occasionally dense chapparai. One of the few sources of potable water within the District, Agua Buena Spring, lies on the south slope of the peak. The western summit of Santa Rita Peak is a large, resistant knob of red-weathering antigoritc. The antigorite has a non-pseudomorphic, interpenetrating texture. This texture is typical of prograde serpentine - antigoritc - replacing earlier lizardite and chrysotile (Wicks & Whittaker, 1977; Wicks & O'Hanley, 1988). The immediately adjacent rock, separated by sheared contacts, consists of the normal lizardite and chrysotile that characterize the District Portions of the summit knob are sheared and mylonitized; shear planes strike 060 and dip 3SSE, roughly perpendicular to the regional structure. Plate 4.12 shows this brecciated zone. Presence of antigorite in this block, together with its absence in adjacent rock, shows that the thermal history of Santa Rita Peak differs from the bulk serpentinite. Santa Rita Peak must be older than the adjacent serpentinite, and probably represents a remnant of an earlier serpentinization event. A stable isotope study of the antigorite block might help to clarify its relationship to the bulk serpentinite. The antigorite is crosscut locally by veins of yellow M2 andradite garnet, and has a dark green, almost black color due to disseminated and locally comminuted particles of magnetite and chromite. The veins of yellow andradite garnet are known by their varietal name, topazolite; many garnets contain fluid inclusions. At Santa Rita Peak these garnets are not uniformly distributed through the rock, but are concentrated in a band a few meters above the mylonite zone. The relationship and timing of the formation of the garnet veins to the development of the mylonite zone deserves study; both events postdate crystallization of the antigorite block 4-99 Plate 4.11 * Santa Rita Peak (1574 m.), with its blocky summit, dominates the southern portion of the New Idria landscape. The summit is a red-weathering antigorite knocker. Gem Mine visible as brown scar on lower slope of peak, KCAC Mine as large open pit area near left side of photo. Photo from Condon Peak. 4-100 Plate 4.12 - Andgorite mylonite zone at summit of Santa Rita Peak. 4-102 and arc assigned to the M2 event The author is indebted to David O'Hanley for pointing out the significance of this mylonite zone on a 1991 field trip. Some of the chromite grains in the antigorite at Santa Rita Peak are surrounded by alteration haloes of kammererite that crosscut the antigorite fabric; this alteradon is also assigned to the M2 event and is discussed in Chapter 3. The eastern summit of Santa Rita Peak is held up by mafic schist crosscut by natrolite veins (sec preceding section); this summit is interpreted as a separate tectonic block unrelated to the antigorite of the west summit " C. San Carlos Peak A prominent"knob on the west side of San Carlos Peak in the northern part of the District has rock very similar to that of Santa Rita Peak: andgorite crosscut by veins of yellow garnet It may be significant that both this knob and Santa Rita Peak lie close to the eastern margin of the serpentinite and the New Idria Thrust Fault D. Perovskite Knob Perovskite Knob is described in the section on chlorite-diopside-gamet rocks (see Figure 4.4, 4.5). The summit of Perovskiie Knob consists of massive antigorite, but without the veins of yellow gameL There are also faint northeast-trending shear planes crosscutting the antigorite, that may represent a poorly developed mylonite zone. Perovskite Knob is here interpreted as an antigorite knob welded to a tectonic block of chlorite-diopside-gamet rock. This physical juxtaposition may be a clue to the origin of both types of tectonic block. 4-104 VI. Fluid Inclusion Constraints on M2 metamorphism The garnets of Santa Rita Peak contain fluid inclusions of a type similar to those in garnets of the chlorite-diopside-gamct rocks described earlier. In an effort to learn more about the conditions of M2 metamorphism, a fluid inclusion study was undertaken. Primary, pseudcsecondary, and secondary fluid inclusions, as defined by Roedder (1984) are present. The inclusions are irregular in shape and vary from 5 to 30 pm. in size. The majority of the inclusions are in the 5- 10pm range. The euhedral garnet crystals hosting these inclusions range in size from 1-10 mm; the anhedral vein-filling garnets occupy veins typically 5-10 mm in width. The size and abundance of fluid inclusions in the garnets correlate negatively with the Ti content of the garnets and positively with their Fe-content. Except for the smallest, inclusions are of the two-phase aqueous type; no daughter crystals were observed. No evidence for C02 in the inclusions was found, as expected from the experimental stability studies of low temperature andradites by Taylor and Liou (1978). Microthermomctric measurements on 31 two-phase inclusions showed homogenization temperatures averaging 255C. Chapter 1 derived an upper limit of 3 kb for Miocene deuteric alteration of the New Idria syenite at the present erosion level. If this alteration occurred at the same time and at the same depth as the formation of M2 garnets, as proposed in Chapter 7, then the 3 kb upper limit also applies to the garnets. If the maximum pressure of M2 metamorphism was 3 kb, then the maximum inclusion trapping temperature was 400C. Internal reflections and the dark color of Ti-bearing garnets in thin section made melting temperature measurements especially difficult, but measurements on large inclusions showed Tm (ice) = -3.5 to -2.0C, indicating that the salinity was equivalent to 3-5 wt % NaCI (Roedder, 1984). 4-105 Figure 4.16 - Fluid inclusion homogenization temperatures from M2 garnets. Mean is 235C. See text for details. 4-106 Figure 4.17 - Fluid inclusion temperatures of final melting from M2 garnets. Salinity equivalent to 3-5 wt.% NaG. 4-108 Except for one sample, no clathrate formation was observed. In this sample, from Santa Rita Peak, a reconnaissance study using Raman spectroscopy in the laboratory of Prof. Jill D. Pasteris, Washington University, showed the presence of a small amount of methane (approx 0.5 bars). Re-examination of this sample on the fluid inclusion stage showed the possibility of formation of methane clathrates. The significance of the presence of methane is unknown, but suggests that low oxygen fugacity prevailed during M2 metamorphism. Figure 4.16 and Figure 4.17 show histograms for homogenization and final melting temperatures respectively. The only phases in this study found to contain fluid inclusions were the M2 garnets, which occur as anhedral and euhedral vein filling crystals. At Santa Rita Peak the compositions of the garnets are near end-member andradite; in chloritediopside-gamet rocks the garnets are titaniferous andradites. There were no systematic differences in properties between the Santa Rita Peak garnets and those of the chloritediopside-gamet rocks. This similarity supports the proposal that M2 metamorphism affected the region with similar temperatures over a broad area. VII. Conclusions 1. Three types of tectonic blocks are recognized in the New Idria Scrpentinite: chloritcdiopside-gamet rocks, mafic schists, and antigorite knockers. 2. All types of tectonic blocks show evidence of two metamorphic events, here defined as Ml and M2. Ml metamorphism is associated with the development of penetrative foliation in the chlorite-diopside-gamet rocks and the mafic schists, and the interpenetrating fabric of the antigorite blocks. Some of the Ml metamorphism must have been of the high-pressure blueschist type. Ml metamorphism occurred prior to entrainment 4-110 of the blocks, and may have been a muld-stagc event. M2 metamorphism is postcntrainmcnt of the blocks and is associated with the development of tension cracks and veins that crosscut the Ml foliation in chlorite-diopside-gamet blocks and mafic schists, and with crosscutting garnet veins in the antigorite knockers. 3. The 34-50 locality consists of four rock towers each 2 m. high. Each tower is capped by dark green, sheared but relatively resistant serpentine, under which lies a slab of light green to white, chlorite-diopside-gamet rock dipping gently to the east The towers are apparently the eroded remnants of a once-continuous slab of chlorite-diopside-gamet rock preserved under resistant serpentine cap rock. 4. Ml chlorite defines the foliation in the chlorite-rich central zone of 34-50. M1 chlorite in the outer, diopside-rich zones is more magnesian than chlorite from the central zone. M1 diopside is more magnesian than coexisting chlorite, in both central and outer zones. Ml garnet occurs in foliation-parallel veins and blebs. 5. At 34-50 shearing is manifest in the central zone by strong foliation of the dominant Fe-rich Ml chlorite, and in the outer zone by fracturing and recrystallization of Ml diopside. These observations show that the Ml metamorphism was synkinematic. 6. The Ml fabric at 34-50 is crosscut by veins interpreted as tension cracks. These veins are filled with M2 garnet, diopside, and minor chlorite. The M2 minerals in the veins have beautifully preserved delicate textures with no sign of post-mineralization deformation. Therefore the M2 mineralization was postkinematic. 7. The Perovskite Knob locality consists of resistant serpentine cap rock overlying a slab of chlorite-diopside-gamet rock that dips gendy to the south. The summit of 4-111 Pcrovskitc Knob consists of the same tough antigorite rock that is found at the summit of Santa Rita Peak. 8. In contrast to 34-50, the chlorite-diopside-gamet rock at Pcrovskitc knob is only weakly foliated, determined by M1 chlorite. M2 chlorite is found in millimeter-sized cuhedraJ crystals coating vein walls in places. M2 chlorite is more magnesian than Ml chlorite. Ml diopside is more magnesian than coexisting Ml chlorite. 9. The Melanite Mine locality consists of a 15 m. wide slab of chlorite-diopside-gamet rock bounded laterally by serpentine rock. There is no resistant cap rock and the exposure is poor. 10. M2 "cats eye" garnets in the lateral transition zone at the Melanite Mine show evidence for meter-scale Ti mobility. Cores of Ti-rich garnet overgrown by Ti-poor garnet are found only in the lateral transition zone. These are interpreted as the result of Tibearing fluids moving out of the Ti-rich chlorite-diopside-gamet rock into Ti-free serpentinite rock. 11. Bulk compositions of the chlorite-diopside-gamet rocks are different from the bulk serpentinite but are very similar to pyroxenites from other ultramafic rocks world-wide. The trace element concentrations for the chlorite-diopside-gamet rocks fall within the normal range for pyroxenites; both are different from a serpentinized harzburgite sample. 12. Using the "spinel norm" calculation the chlorite-diopside-gamet rocks are shown to be the metamorphosed equivalents of olivine websterites. 4-112 13. Low solubilities of Ti oxide minerals, lack of a well-developed vein system between chlorite-diopside-gamet blocks, lack of penetrative foliation in serpentine rock adjacent to foliated chlorite-rich rocks, and lack of apparent Ti-metasomatism at the margin of the igneous intrusives all argue against formation of chlorite-diopside-gamet rock by metasomatic alteration of ordinary serpentine rock by fluids from igneous intrusives. 14. Bulk compositional similarities, the observation of M1 and M2 metamorphism, rock textures, and existence of chlorite-diopside-gamet rocks from other localities argue for essentially isochemical metamorphism of blocks of pyroxenite to produce the chloritediopside-gamet rocks at New Idria. 15. It is possible that the metapyroxenite blocks are tectonic inclusions from an unknown source entrained within the serpentinite: tectonic inclusions of a variety of other rock types are present. It is also possible that the metapyroxenite blocks might be primary igneous features or even have a cognate origin with the depleted dunites and harzburgites. They may represent cumulate layers or crosscutting veins and dikes. The present random orientation of the blocks may well result from boudinage or other tectonic dismemberment of dikes followed by block rotation during the diapiric ascent of the New Idria Serpentinite. 16. An analysis using model systems shows that the phase assemblage chlorite + diopside + Ti-gamet magnetite can be derived from the low-temperature hydration of an ultramafic precursor of pyroxenite composition. The phase assemblages found at New Idria suggest that greenschist or blueschist facies conditions accompanied this hydration. The stability of this phase assemblage is at present the only control on the temperature of Ml metamorphism. However, comparison of Ml and M2 chlorite compositions shows that the Ml event was of higher metamorphic grade than the M2. 4-113 17. A study of fluid inclusions in M2 minerals showed that homogenization temperatures averaged 235C, suggesting that trapping temperatures during M2 metamorphism were below 400C. No quantitative pressure estimate was obtained from this study. 18. Models for serpentinization are frequently expressed in the MSH system. A more general model for serpentinization of fertile ultramafic rocks in the system CMASH and CFTMASH is proposed here. The resulting phases all lie inside the brucite-talc-diopsidcchlorite tetrahedron. Ca is considered a mobile component during serpentinization; Al, Fe and Ti are conserved. 19. Chlorite-diopside-gamet rocks at New Idria are shown not to be Rodingites. 20. Chlorite + diopside + Ti-gamct rocks are now known from three localities outside the New Idria District: Val Malenco, Italy, Sangabawa District, Japan; Mid-Atlantic Ridge at 43N. All parageneses are in serpentinized ultramafic rocks. 21. Greenstone and mafic schist at the Gem Mine are interpreted as Franciscan rock entrained within the serpentinite. The mineralized zone that contains benitoite lies along or near the contact between these two rock types. 22. It is here proposed that benitoite has formed by an essentially isochemical reaction involving barite, titanite, albite and chlorite as reactants and benitoite, actinolite, clinozoisite thomsonite and natrolitc as products. Neptunite probably has formed by a similar reaction. 23. A preliminary determination of the age of benitoite and neptunite has been made using the Rb-Sr method and a two-point isochron. The age is estimated to be 11.96 Ma. 4-114 No error bar is available, however. This age should be compared with the age of the New Idria Syenite, 12.4 0.8 Ma reported in Chapter 1. A further attempt to date neptunite crystals using `,0Ar/39Ar technique is currendy underway. 24. Knockers of dense, tough antigorite with a sugary texture form resistant, positive weathering knobs in the District These knockers may be sheared or mylonitized as at Santa Rita Peak, with recrystallization of antigorite within the mylonite zone. Locally, the knockers are crosscut by veins of M2 garnet that contain fluid inclusions. Some of the fluid inclusions contain methane. The knockers have a thermal history separate in pan from that of the bulk serpentinite, and must be older than the immediately surrounding serpentinite. 4-115 I > oiqci 990-16.91 outerl 32.71 oo o* 14.681 T5 C CoO b o b qo CO too o* oo o' q o' 13.56 oo oo 32.571 14.39| 1 990-16.611 32.66 i 3O oo o' *d c v(/n> b o o CoO oo o o b o* 13.72 oo oo*" 00001 13.90 090-22.41 1 central! 27.91 1 1 16.671 18.52| 1 > Ou q o eg b oeg o meg eegg GoD o eog b "cd s 4c* 1 ! 22.201 1 central! 28.12 e o 7api n eegg 6ao> eog C<Oo o eg obo b eog o' o b x c Eoo co oeg eg <eog beg o ed o ca b IN. eegg eog o eog o' b c 13.66 oo oo 990-22.l| central 16.581 1 o o N AA 8*r aHs 3a ce(o0 > "Uo5 eg O CO eg O h- n O eg < n 8 ow 9 u. i H20 diff Total AFM space: 1 13.49 100.00 * Oo fN O CO o' b o b g oo ro* oo obob o e*gr ftNo oK ooob o> teog ctno oq b O o' o o eg CinO < ooob i s< i i notes: [total Fe expressed as FeO l' |H20 dotermlned by difference 1i i ` C .2 u0 30 E u01. 1 ? 2 -2 c XE Table 4.2 688-4A.11I 688-8C.3I 688-8C.5I 688-8C.7 1 14.921 10.83 _____ 0.42 4.09 2 2 .7 b I 35.79 29.961 34.69 CoM d CD 212 d dl 1 in o d d c i0n) 00 CCOD d CM Oq Oq d d d CO 15.24 19.621 22.061 1 OO'OOl I[OO'OOl j * in CCMM COM) d d c ine d CqO C0M r0 in d d d CM 12.651 18.03| 1 18.881I 21.I ll 1 28.721 eoo o d c CO d 00) q CM 0 ddd 0 0 d0* _______ TO 13.8B 100.60 0.10 1 o(Ojl C^OT Tj 01 d| 0 0 m1 5 CM d 0in d qCO d 1!: . 111 !i! 1j i! in 0CM> iCnO qco dddO CO CCOM m qCO dddd 1 7 .7 1 11 ! 23.26|1 Tablo 4.3 11.13 O o coo < TCC<OOn oco o q o d c O CD d d c imn d 0O) CM 0 0 ddd COM 5 ddd 13.23 1L25] 1 26.90 1 17.551I 27.69| 28.25| 29.14) 29.18) 30.341 688-4A.2I 68B-4A.7I 688-4A.8I I OO'OOl I OcO 17.92) 21.7211 ion d 16.96| d c m0 d 0* COM aq ddd 13.55 0q d0 >o aS 16.05| 23.8711 coo d 16.20| d c in<e d CCMO <s 0 0 ddd 13.93 0 0 Vd0" m co m* mo ** < a u0 CC(0OD 5 q d o o 17.85) 17.39] d c d c 21.481i 19.78)| 20.261 20.92]1 CmO d <0 d CM 0 CaO 0 0 ddd CO 0 0 0 ddd 12.50 12.59 0q d0 0 q d0 688-4.31 , O I | 1 AA O . at- i a CEaO *oo5 04 O CO CM o h- AI2Q3 co 89 0 U. *o 8% ! 8 OCM X ITolal |AFM space: [OO'OOl m CM m in r0* dd0d CO CCOO 0CO dddd rCM- mCO CCOO dddd CO CCOM 0CO Cr-M. dddd 0CO mCO CO dddd CO CO m CM CO dddd < 5i Inolas: Ilolal Fe expressed as FeQ* |H 20 determined by difference C JO 0<Q w $ 0 E u. 0 2 1 3 O E X Table 4.4 Miscellaneous Mineral Compositions Sample > phase > Si02 !!! | 688-8C I 688-5 I I Titanite 1 Perovskite 1 1 30.001 0.02 990-3 Vesuvianite 36.10 Ti02 Zr02 AI203 Cr203 Fe203* FeO* MO MO CO Na20 Nb205 1 38.27 n.d. 1.29 0.07 n.d. 0.63 0.04 0.04 27.97 n.d. n.d. 58.23 n.d. n.d. i n.d. n.d. 0.23 0.00 0.01 40.89 n.d. 0.19 1.34 n.d. 15.78 n.d. 2.44 n.d. 0.11 2.05 35.89 0.02 0.00 Ce203 La203 Y203 H20(dif() Total n.d. n.d. n.d. n.d. 98.31 0.10 0.00 0.02 n.d. 99.68 0.13 0.02 0.00 6.11 100.00 Si 1.00 0.00 8.87 Ti 0.9S 1.00 0.25 Zr 0.00 0.00 0.00 At Cr Fe* Ml M3 Ca Na Ns Ce La Y Total 0.05 0.00 0.02 0.00 0.00 0.99 0.00 0.00 0.00 o.ool o.ool 3.01 j 0.00 0.00 0.00 0.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 2.0oi 4.57 0.00 0.45 0.02 0.75 9.44 0.02 0.00 0.02 0.00 0.00 24.39 ______:_______ !______________ !_____________ Total Fe expressed as either FeO* or Fe203*, as shown Table 4.4 1i 18 1 *5 1 "D ! ai jt u o CC 2 * a O 0 1 X O 2 u O 9 c 0 *9 O a, E 0 a 3 a 0 > c % w a e a p 9 n 9 m r a Ok CD a 3 n M 9 9 0c a 9 M a. O > CL 9 Jt 0 < 9 * <0 </> (3 9 J> X <0 9 5 0 m a 0 m w ? 6 O n O CC CM a X O at O CO r<*3~ 5 0 m at O co 0 CM CM a 0 m O2 at 0 n a AA A >s 9 a| I1 O 0 a a .e 0 CO 0 C h l-O ll V a l M a le n c o ) N e w Id rla l N e w td ria l 4 5 .8 6 |0 0 0 ] l TM ___________ 3 0 .7 7 1 2 .2 3 o.ool 0 .0 3 CM O O OO dd O O d 0 1 1 at 0 in o CO (0 to 0 O 0 CO 0 p p CO 0 pO0 0 cd d cd d d d d d 'T 0 CO 0 m <3 CM in m m 0 0 CM 0 in p-. p p O 0 in 0 w d d dddd d d co 0 CM 00 CM m CM CM 01 CO in CM v" 0 03 CO p s CO p V0 edddddd CM d Ok CO CM CM r* at rs r at n 0 a Ok 0 9r CO CM to 00 0 p CO CM 0 d d ai CO d d d d 0 CM to p* <0 0 r- CM to CO a CM 0 in 0 CM pO0 CO d co ** d d pi d d d d 0 Cb in a w CM <0 r** to 0 Ok N. co CO 0 CM p CM CM p O in V d d b d d d CM Ok a> m CM to 0 0 CD 0 CM O m CM CM p CM p 0 w a e d ai CM d d d d co a CO CO 8CM O CM O 0 CM O CM 9 s 2 CO h- < U. ! s2 g CM 0 Q. V- 1 1 1 Ok CO d 6 CM X 0 S' CM d to O 4- OX CM X d Cm 0 m d 9 CO 0 CM in b b 0 CO 0 02 CM 9 PT LL 0 to r- CM CO d 0 CM O9 0 Li. u. sCO E 0 0d CM 9 U. 1 O 9 *1 9 U_ 9 3E j* 0 1 9 31 9 & K CO 9 O 9 U. O CM 9 U. 3 2 9 O 9 2d in CO a * 0 w CO b 0 O CM 9 LL X in m JJ 3 O >1 -3 *3 >3 <C 5= w >. a c 9 (T race (ppm )! | R G C -3 4 -5 0 n .a .1 ____________n .a . n .a . ffl T3 c O E 1d c ft C d> d| d> a c! cl c| c 1 !ii !1 1 ! in O CD in CD CM in Olfllfl O CM r> co m Ok 01 p 1 -V bdV CO 0 in *T b; d 1 ci m p** 0 CM I I CO CO CM Cm 11 !! 8 C 2 *9 V 0m 0 0 O O 0 0 O O *d V V b fC CO d c CM m U> V1 c i 2 CO CD CO 1 1 -3 >1 9 * d c 9 c d c d c O0 O0 O0 d c d c d| d cl c 9 CM CM X \ | *6 d d d d d T3 *d "d T3 ccccccC ccC 0 9 'Z Z "o F lv CM Ok 10 03 r* r* 0 in 10 2 CM w p O 04 p CO CO * b d V CO cd CO CM CO d Ok 03 m CO CD O CO f* CM m in CD o> CO <0 p CM in m CO p 0 at CM b in V CM d CD d at 0 Ok CM 0 CM 0 CO 'C r*. px. CM Ok p* CO CO in CM p n r-* p Ok to Cki p at rl 9 O Ok Ok d CD Ok V w CM d m CO cd CM O is.* <0 to m mi r-* 1w ! <35 f5 CO -5 z 0 > 9a O - Tablo 4.5 Tabto 4.6 (olio Table 4.6 lolio T o la ls j ____ Y - 0.617 Si excess 0.001 01 trial 0.200 s1! II i l i aH i ; i j ! O c JO I I ! i i * C**O * H* 1 8 oi rCO ooCO C*Or o oo ooo ooo CO o> oOo iConO COM O o oo oo o ooo doddoddddoood O ooCNJ o o oO' d S oCCMO o o00 CM o Q oC*M> ooq do mcoo iConO do CM CO CM CO o 2 o oo d d o oo d -oo 03 Oo CooM do CooM d 5 oo ooo dd oo q o e CO oo 00 o q do a < ooq oo q do 9do (0 CO oCoO CO oo o ooo IinA rCvO in CoO oqCM oo o o oq oo o qL d o d d o d U. o d o o d d 2c i oa> o o IV. o q o COM roov IoA q o q oCM CO oCO q oCO q oCoM mo q CO o d (0 dfv S CM in din rv rdv d idn co d Oxide 1 Weight %| C\J COM fCvO rv o q o o CO CO CO CM rCvO q CM o o CM o o o o o 3 Pco d CO d d o o dCO CO o o o d oo CO aoA a E m to CM O c/5 CM p CO OCM < CO OCM o u. I8 l s tA O i : I; ! : j 1 11 ! 1 1 i! ) 110 R o od o oo i<n9* ! ij O qo d co Oi CM d CO o o03 d CD q d co o q o CO oCO o03 CM o Pv VCM 0aco>9 CM Mr d iConO iqn in dd Oqri q CcMo d o o o o d d cCooM CO d o oo oo d d in Oi CoM o d o oo o d 1 ! 10 o crt o UJ Co a 3O' ui *oou2 Ka U Xa co r so a c5 co *5 2 o>X Utoi O O o 2 o 2 Table 4.6 (olio 3 ] | ZO!l| It 1 11 iS COoM d a 2 K o u S iU. o d dd dd d * 00 d 6d E ns 40 * in COD 2: n CM 5 --y a GFW M-Prop.l a < flm RGC-34-5C Si02 |S29 0 [060 09 0.003| 0.0061 79.700 0.0261 0.026 Oo dd AI203 11.61 101.940 000 |00'0 n O ewg 152.020 joooo 8 CM o 159.700 | 14.9S] r^. iOpn 0.213 1 0.29] 70.940I c 2 u. 0.026 C0O d 19.99 40.320 00 d 8 I 56.080 ! 0.238 0.000 61.980 1000 94.200 000 d 141.850 1000 |1000 Issoo SSO'O 0.112 o.oool 01 .112|I 0.183| 0.183 I99C0 OCoM d 0000 OO OO 980 0 000 0 ooo o ; ffl O h* O Totals 0.1931 0.6251 0.0261 3 ____ Y 0.625 Si excess! 0.000 Ooo >-- 22 Ol trial 0.708 0.193 CoO 000 0.238 k in Cpx 000 0 0 0 0 0 ___ |91001 [000 0 1000 |00 0 1 000 ________m 600 50 01 1 ii 1 1 O OO O O 0d [Totals SOO'O | 90 | 600 0 j 00 1 000 0 1 0001 11i tOo JaC *o 0 0 0 M a O aK O ID O |Oxy Equivalents [Mot proportion I Mol Fraction norm to 1.0 10.077 1 3.0 | 0.017 1 0.165 CM CD n d d 1.099 0.772 | 0.2 29.8 CM d 58.6 41.2 0.183 0.002 [ 0.772 d 0.16 80.69 2.592 100.0 0.957 100 11ii i1! 11 -- Tablo 4.6 folio 4 | j & O 5 OOO d a rC0-O* d $ ooo TJ 2O X O Q. <(&A9 aX U .c 1 h5to? m 8118 [000 0 1 000 |ooo I coo SIO'O 086 19 699 0 |000 000 |060'09 .S5 5 * M-Prop. J/ C<DO A CM O CD CM O AI203 [ nm d in in 70.700| 8.34 101.940 152.020 OOO CM C0O 0.7581 0.007 ddd CD 6 OOO 0 0h* 0in CD d **- d d CD r*- 8 CM UO- in CM d 2 |H20 [Totals O d 0 0.19I 26.96 8.25 56.080 0.90 94.200 1 141.850 18.0161 O CqM 0 CD 9 U. c ddd 0.131 0.147 JO c 9_ 9 _> I3OD >X O Ui m 9 ac *0 05u X 0a aX O O [Mol proportion |Mo! Fraction norm to 1.0 i O OO 000 000 CM d d 0 d <CL 000 000 dd o 000 d llm 0.002| 0.058 0.0071 0.000 0.015| fOO^. d 0.015 SOO'O 0.020 0.1751 d 000 0 00001 0 d Totals 0.294| 0.227 0.176 0.758| 0.0071 ! ____ Y - 0.758 Si excess! 000 0 |008'0 | [000 0 | 000 0 i 000 0 280 0 0.067 dd rs. | 0.227] 0.352 0.147 GOOD 000 Od Ol trial 0.176 84% 0.000 000 0 j 0000 1 stoo 0Oa d 980 ! 1 1 ' 1 1 001 j 01c 0001 I 0.268 | 0.682 CD d I 32.3 38.9 1 30.1 0.147 0.227 | 13.64 I 21.08 rO* d Ol V CM 0.704 25.7 65.28 2.737 100.0 1.079 1 :i11 1 ! 1:i111 ! 11 1j i 1 j! l__ III1:! j Tablo 4.6 folio Totals Si excess] Ol trial 0.258 >R|| (tOo ib s! 000 0 ' --. . - |0000 M !1 u^ c * CO O' p fC*sD >J--C d i 1 ii p(tOo oCoM CM o a oo oop ooo CD OoO CoO oOO ooo 0Ol 01 ooo o d d o d o o o d o d o| d o tPCoO d to to o i i i >5 CM d C*-M d 5 rCtoO ooo od CO COO) do a CM o CM O do $ ooo o ooo o o ooo ooo dd ooo d o oop aoo dd ooo o e oCoM d COM O o a < opo ooo oo Ido ttpoo CoM o CM O oo o ooo o *> to C0O5 ooo ooo ooo Oa O o dodoo ddddod 2 GFW o o o fos. o*r p o OCM o o too p o p o CpM o OCD o pCD opo o tpo to O d to rd* o tCro--M dto rd* o d to CO V< * d 2 03 woo COM d Ol d CM <e OO o o o o pto d CPM o CO p d oo d oo o oo d oo d oo d o *5 a. 5 * o 8;Xo s 8 1 O CO CO Cl E <9 to CM O c/5 CM O h- O CM < O CM o LCOLM ii to iO S! to a --o ii j;| i1 ii ii* cn rC^M. CM o oo o i to d o p O] of 1 i CM p v CM OCO fs! CO d OCO p d K t5oT CO ?J CO p V CM oo d do oto * U) CM t'To CO d tpo - to CO d d ooo o o o oQ o d o ooo o d d r^. oo CO d o oop p d o 3 ca d ce o uo ac (9 U.3c2Jr aX O oa Xa o a X>. O U*5J O 5 o 2 |Mol Fraction norm to 1.0 Table 4.6 folio Tablo 4.6 Iolio Tablo 4.6 folio 11m l 000 0 |000 0 Totals Si excess! Ol trial] 0.052 i aX a i i c HJO 3 g fms. U--9.1 d CO GO poo CM O OOO ooO ooo m poo C0VO3 OOM ooo ooo oOo ddooodddooodd i 1 i O CionM o iS CCoMO d a 0CCO3D ooo oo 9S GCoOO d $ ooo d o o CCOMD o cs 03 CO p dd 0CoO0 d ooo d -*ao CO o ooe od Oo d w O ooo o oo dd ooo o ooo ooo dd Q. < ooo ooo od Ido CTOT CD ooo CM O ooo ooQ mmo oddooo 00 in e> CO *oo*r ooo ooo ooo ddd od o 2c i ooCO poo o p oOCM opo oipn o p oCCMO oCD oop ooCM o mp CD O dCO d o CmTM" m*d* r* dr*- d CO in <o 0d3 d * oCO in p CM op oCD o CpO CO op CinD rCMt oo o o oo oo JO .5 Q d m o V o d CO d d CM d o d o d d o 9 5 1 s 1 i;oo X o a <E(90 oCM cn oCM 3-- CO O CM < CO O CM U uo. JO m O s o lO IO pxC px coordinates fMol proportion |MoI Fraction norm to 1.0 iji i| 1 j i O) p o od o oo CcCrOOs o oV-- CM d aops d p p CO po o min CCMO CD p o o as p as CGoMO o mp CXM* 0OC3O CM d p CGMO* 03 CO d iOnS mV CmO p in o ooo o d d r*. p o d o o d o oo o d d ooo p d d to fiS 9 Jr>o 3cr U4 >XN O at Tablo 4.6 folio O ifi Q ooo o 8 ho* o 2 o d *o C*M* o o 8 s6 ll i* i:ii I.E ! in!111 29 X O [000 [0000 |060'09 Weight % GFW T n. Eato Di 60 CM o CO 47.56 0.792 ItI02 I 0.02 79.700 AI2Q3 1 7.57 101.940 0.074 o q d r* CM CD O d o oo D 152.020 159.700 4.95 71.850 000 0 000 0 COO'O 00 001 000 000 090 9S 181i (in FeO A) 24.88 40.320| 0.617 1 14.67 0.262] 61.980 94.200 IP205 i 141.850 18.0161 (Totals ca c O 9> Xa 3a UOJ' Xa >X. O o6 O (Q s"Owoou |Est Mode |MoI proportion |Mol Fraction norm to 1.0 OOO d ooo d d o q d ooo oo d 0.001 ao. OOO ooo O odo 2 a. < 00 SCO'O 200 0 [000 0 | 000 000 0 00 0 000 0 1000 E OOo oo o o C) T" o ooo d 00 000 0 000 0 000 000 0 9610 000 0 ] 000 0 000 0 000 0 960 0 EZS0 0.158 | 0.0711 0.262 0.158 0.262 Totals 0.792 0.074 1 sI u9. 1 CO ____Y - 0.792 i excess! ooo o 0.687 0.262 01 trial 0.687 90% 000 0 | 860 0 2 > hJC- i 1 OOo o 0 001 ji Mi 0.285 10.3 1.570j 0.475 I 0.392 56.9] 17.2 14.2 64.4 19.5 16.1 0.262 0.158 0.392 32.22 19.501 48.28 2.759 100.0 0.812 100 : ! 1 l i 1 Table 4.6 folio 10 llm 000 0 000 0 0.735 j s XUWU)) Ol trial! 0.145 i jX 1 oa JC 1 UCD- ii i s* r* oCD CD h- d 1 pInA*- o oo CoO oOo ooo oOo CoO ooo irA** peog oao oao oo p oooddddo dddod Totals O (A o eocg d & eoegg d emegg o 0.349 a ooo IA rt-A* d od 1 i 1 i ao d * ooo o -> a) oo eoog do w O oo oo dd ooo d <a od IOrA-* aoo Co*O odo i o p o 000 0 o d ooo d ooeg d ooo o 000 0 ooo o ooo o CCOO O o 2 U. in p* d CO o eopg d C [000 0 ooo d ooo ooo dd 086 19 OOeg oIpA ad 18.016 60.090 79.700 152.020 159.700 71.850 1 70.940 40.320 56.080 * o o <gz>- n o o *** O pOi eg o Oo o oo .S' Q d o o d d o deg d d d o o do 5 A ;o X O a Eau> CVJ O in eg g >-- IAI203 |Cf203 s 8 18 lCiM. i IA O s Totals Oxy Equivalents |! ! `i 1i 1!! 11! rrC--O*. eg O oo O oo OCD o | it ICoAD o o eg o IeAg coCnO d prv. IaAd 0.175 1 0.232 N0CO3. d eg ieng o dCO I*AT ergi p * p T- rC o IA p03 P* in o ooo o o o neg o CD d d eoog o o ooo o d o ooo o o o C *o oou QX O Xa O UJ 6 |Mol proportion |Mol Fraction norm to 1.0 Table 4.6 lolio I I Totals Si excess! 0.000 Ol trial 0.915 0.191 II CD i 1 !6 a!1 s| ; I 1.1 0aX-j q00 ! : i ; i 1c d i 1 ; 31 hJC Ml'! 1i . r" 0 1 : as 10 ooq 0CO OOO OOO OOO in os 0q0 00q 0q 1' d o d d d d d d d 0000 OOO 000 dd O Is 1' 00CM 1 d1 ifi i i , i i 1 : : ' a irn* oqo do 00CO d 000 000 dd a CO r0-% o i oao o d 000 d -o oo o o od 6 oo ooo do JE ooo d <a PAC*O. ooq 0CT--O 00q odod 00 d 000 d 000 000 dd rO d 2 0 d 000 d 0q d 00q d 000 d 000 d c . 1 1* 11 i1 1 i i11 i !i!1 ! 1!!1 1 IS 1 r-. O d0 0 0d 00 ! 1" j Is 1= 0 rv CM 0 CO r0v* d 001I 00I CqM d q ttoo p*. 00CO d i0n C0M 00CM d a 0 0 tCoM 0q0 d 00 N d 0.549 0 00 0 d d CM 0d d d OOO 0 d d 000 q d d Tabio 4.6 (olio 12 000 0000 0000 060 09 GFW M-Prop. |Mol proportion (Mol Fraction norm to 1.0 79.700 101.940I 152.020 159.700 71.850 70.94o| 40.320I 56.080 61.980 94.200 141.850 18.016 * oCM 0 o 0 O O O 0 os q 00 q tqo CM O O q OO 00 jCas a o o dd d C0M V d d d d 0d '5 2 AI203 I |Cf203 nir a Ere V) CM .O c/5 CM o 8 0 jo a CM s i 1 9 i i H0- |Oxy Equivalents re rce 000 X a 0 Xa O UToJ O lOxido 1 [Oxide 1 IAI203 |Cf203 [Totals ____ Y - 0.623 Ol trial 0.238 i______ k in Cpx 0.000 *> f4Ifr/lti 1> ooo 1 1 12 d 1_ C/5 jf CoCMD 2 CD A 3 tcCooM ooo o ooo ooo oOO oCCMD oOO oo Ooo!j ooo o h- ddo ddd d d o| o 6 qCcoO o CD o 000 0 i i oOo o & CCCOOM d Q CooM ooo od :> : I tc0om3 i !* oo d oo d * ho ro*> o 1 ooo d o ooo d Hml o.oool "O ooo o ooo d w o *o* OOo oo ooo o ooo ooo dd <a ooo OOo do CCOM CD oa or* ooo ooO r* I CM q oo ooo ooo odoooo dddo 000 0 ooo d c i o q (qoA ooCM CO o d d** MProp. 79.700 159.7001 70.040 1 i 40.320 | 56.080 61.080 141.650 020ZSI |060'09 *o oo (CAO OO oO oco CD q qCO (oA oo CoM Oo o 5 5 JB CO d fv d d d d dCO d d d d o hr a Ea(A CM o CO CM o h- s 8 1 L8CLM. i IA iO 00 001 1 iji i 1! 1iiM 1i J lCAD P oo o d o CqO oo 0.952 71.35 S00 0 I q l CO CdO (A IqCAM d (A * T" q (dA C0M9 q d q CdM CM o q d oo o co q o OCO o CTdM ooo O o d ooo o d d oo d o ooo o d o ooo o d o o _<c5 u c oou a> *Ua5J gXa Xa >* O U(0J O O 1Mol proportion 1Mol Fraction norm lo 1.0 Table 4.6 lolio 13 Tablo 4 7 (olio (Table 4.7 - CIPW Normative Minerals for Mafic Schists and Greenstones CaM d o <r>o. cCqO CO in n CM - CO >CoZ o a Ert llocatlon > A A 990-89 __ 1 1190-15 I Coleman I Ernst I Ernst I Sorensen 1 greenstone | malic schist] mafic schist) blueschist I greenstone I greenstone 1 ] New Idria j New Idria | New Idria I Franciscan ] Franciscan 1 Catalina ] c 0 <occ a 2 < Quartz Corundum lOrthoctase lA c m lte INepheline iDiopside |Hyper6thene iMaqnelile lllmenite IApatite ITotal 1 1 | | 1 I I I 1 6C66 | I |0S*9 | cq fisn- rc^q CM d OCM O) CO O) CO d oT" iOn GO CM 0.411 1 8.63] 19.47 1 1 I\LL'Z O<Di cq CO d 43.90| 15.08 14.551 1 1 99.26| d 0 !S6`0 668 0 79.14| 1 0.31 1 1 0.17! IC fE o d CM Ge--O COD 0 d0 CCOO d cj CO CGOO CM in 10.28 I 1 16.81 1 11.48 1.98 1 1 917. CVO CO L 26.151 1 20.331 1 16.35 17.12! 1 0.B5 100.02! 1. io 0C0M CM r>." 5.08 15.231 32.22 20.781 2.28 1 i1 [References: Coleman sample from Coleman (1961), Ernst samples from Ernst 11965) [Sorensen Sample from Sorensen (1986) I | Tablo 4.7 (olio Chapter 5 Compositions and Mineral Associations of Garnets from the New Idria Serpentinite Chapter 5 - Compositions and Mineral Associations of Garnets from the New Idria Serpcntinite I. Introduction......................................................................................... 1 II. Geological Setting of the New Idria District........................................... 3 III. Titaniferous Garnets............................................................................... 3 IV. Titaniferous Garnets at New Idria:.................................................5 V. The SAG Projection............................................................................... 6 VI. Compositions of New Idria Garnets...............................................7 VII. Compositions of Ti-gamets from Other Localities................................... 13 VIII. Some Crystal Chemical Considerations...................................................24 IX. Exchange Vectors...................................................................................25 X. A Three-Garnet Rock..................................................................... 26 XI. Compositions of Minerals Coexisting with Garnet..................... 27 XII. " The Colors of New Idria Garnets...........................................................32 XIII. Conclusions...........................................................................................34 Tables ........................................................................................................37 Chapter 5 - Compositions and Mineral Associations of Garnets from the New Idria Serpentinite I. Introduction The purpose of this chapter is to present and discuss approximately 300 electron microprobe analyses of Ti-gamets from the New Idria District. Analyses of coexisting minerals, principally chlorite and diopsidc, are given in Chapter 4. The garnet compositions are shown in the Schorlomite-Andradite-firossular (SAG) projection; compositional variations of the garnets are explored using exchange vectors. Garnets from the New Idria serpentinite in San Benito and Fresno Counties, California are widely known for their unusual colors and titanium-rich compositions. Most of the New Idria garnets are calcic and belong to the andradite clan with end-member formula (Ca3Fe^Si30I2). Titanium substitution occurs principally along the exchange vector TiFe.j, making these garnets Ti-andradites. Some of the garnets are also hydrous and vary along, the hydrogamet exchange vector fySi^. Some of the garnets are Ti-grossulars, a portion of composition space not previously explored. Compositional variation in these garnets occurs along the vector TiFe^Al.j. This study presents approximately 300 electron microprobe analyses of garnets from the New Idria District, together with analyses of coexisting minerals, principally chlorite and diopsidc. The mean 1102 content of the garnets in this study is 5 wl %. One of the garnets contains 15.62 wt % Ti02, which corresponds to approximately one Ti atom per formula unit, i.e. the schorlomite end member. 5-1 Most New Idria garnets are hosted in tectonic blocks of chlorite-diopside-garnet rocks that are interpreted as mctapyroxenites entrained by the serpcntinite. These garnets occur in two generations assigned to metamorphic events Ml and M2 (see Chapter 4). At one locality, near end-member grossular, end-member andradite, and Ti-andradite coexist in a single rock. Some yellow, Ti-free andradites occur in tectonic blocks of massive antigorite. Outside of the tectonic blocks, widespread but minor uvarovitic garnet is found as an alteration product of the ubiquitous chromite grains in the serpcntinite. These garnets are discussed separately, in Chapter 3. Uvarovitic garnet inside tectonic blocks may also be litaniferous, however. The colors and varietal names of andradite garnets found at New Idria range from light green (demantoid) to yellow (topazolite), through brown (unnamed) to black (melanite and schorlomite). Uvarovitic garnets are emerald green. There is a general correlation between garnet color and composition that is a useful field guide at New Idria, although not necessarily applicable elsewhere. Many, but not all garnets of the New Idria district are strongly anisotropic, sector twinned, and zoned. Cation ordering on the octahedral sites has been cited as a principal cause of anisotropy in garnets that reduces the cubic symmetry to tetragonal or even triclinic (e.g. Akizuki, 1984; Griffin eLal., 1990; Kingma & Downs, 1989). Recent spectroscopic work by Lager ct al. (1989) has demonstrated the presence of (OH) groups in New Idria garnets. If these (OH) groups enter the garnet structure through the hydrogamet substitution, they may also contribute to anisotropy. Sector twinning and zoning are most frequently observed in vein-filling garnets at New Idria; these garnets generally have Ti-rich cores and Ti-poor rims, but sometimes the zoning pattern is reversed. The appearance of these garnets is suggestive of skam deposits, e.g. Harris & Einaudi (1982), although the New Idria occurrences arc not associated with skams. 5-2 II. Geological Setting of the New Idria District The term New Idria District, referred to in this study simply as the District, was first used by Becker to describe the general region encompassing the historic mercury mines at New Idria, California (Becker, 1888). The most remarkable feature of the District is an oval, 23 by 8 km, fault bounded, serpentinite massif flanked by steeply dipping, locally overturned, sedimentary and metamorphic rocks of the Jurassic-Cretaceous Franciscan and the Cretaceous to Pliocene Great Valley Sequence of California. The New Idria Serpendnite is a pan of the Coast Range Ophiolite in California (Hopson et ah, 1981). Its emplacement within the Franciscan Formation is related to subduction of the Farallon plate in the Jurassic. The tectonic setting of the ophiolite was a forearc basin lying between the Sierra Nevada volcanic arc to the east and a tiench-subduction complex to the west, represented by the Franciscan Formation (Bartow, 1990). A piece of oceanic crust or underlying mantle was obducted onto the North American continent, later to rise through the Franciscan Formation as a serpentinite diapir. Emplacement of the New Idria Serpentinite as a diapir within the Franciscan has resulted in a piercement structure or tectonic window, lying just east of the San Andreas fault system northeast of Parkfield. Further details about the geological environment of the District can be found in Chapter 1. III. Titaniferous Garnets Titanifcrous andradites have been recognized as a unique variety of garnet for over a century. Tschermak (1885) mentioned Ti-rich garnets from serpentinites and alkaline igneous rocks in the Alps, and recognized them as members of the andradite clan. The nature of the exchange vectors that incorporate Ti in garnet, and the valence state of Ti in 5-3 garnet, have led to an extensive literature on this subject (e.g. Kilhbcrgeret al., 1989; Huggins ct al., 1977a,b; Dowty, 1969; Huckenholz, 1969; Howie & Wooley, 1968; Isaacs, T., 1968; Ito & Frondel, 1967; Piners, 1894). Garnets with more than about 1 wt % Ti02 in their analyses are strongly colored: black in hand sample and deep red in thin section. The nature of the electronic absorption that leads to this coloration has led to much speculation. Current theories include the possibility of electron hopping between Tt and Fe-occupied sites, making the actual valence state of Ti and Fe indeterminate. The possible presence of Ti3*- cations in garnet is still hotly debated. Studies by Huggins et al. (1977a) and Dowty (1969) used Mossbauer spectroscopy in an attempt to determine site occupancy of cations in the garnet structure. Dowty (1969) doubted the occurrence of Ti3+ in garnet; KUhberger ct al. (1989) reached the opposite conclusion. The limitations of these studies are that, due to the techniques employed, only indirect evidence is available regarding the valence state of Ti; model-dependent assumptions must be made. The presence or absence of Ti3* can be deduced from assumptions about crystal chemical preferences and charge balance combined with measurements of the site occupancy and valence state of Fe. Waychunas (1987) attempted a direct measurement of the valence state of Ti in silicate minerals using synchrotron radiation and XANES spectroscopy. He concluded that while Ti3* in terrestrial minerals cannot be completely ruled out, its concentration must be very low. Further studies using a brighter synchrotron source and EXAFS spectroscopic techniques might resolve this question in a direct manner. Therefore, I conclude at the present time that presence of Ti3* in garnets has not been completely ruled out, but is only supported by indirect and equivocal evidence. The stability field of andradite garnets and the location in P-T space of univariant reactions involving Ti-gamet have been investigated by Taylor & Liou (1972), Huckenholz (1969), Huckenholz et al. (1976) and others in synthesis experiments. The evidence from these studies and from natural occurrences in metamorphic and igneous rocks is that both 5-4 andradites and Ti-andraditcs are stable from about 300C up to, and including, igneous temperatures. None of the studies has delimited the stability field of the Ti-rich varieties explicitly, but there is a positive dependency on oxygen fugacity (Huckenholz et al,, 1976). Virgo ct al. (1976) suggested that Ti and A1 stabilize Fe3+ in andradite, allowing these andradites to persist as stable phases at lower oxygen fugacitics than otherwise possible. The paragencsis of Ti-rich garnets is largely restricted to alkaline igneous rocks and serpentinites. Apparently, low silica activity and peralkaline bulk compositions favor Tigamet, provided that Ti is available. Saturating Ti minerals, e.g. rutile, perovskite, titanite and ilmenite, appear in extremely Ti-rich bulk compositions. While all of these minerals occur at New Idria, they are very rare. Other localities that have Ti-gamets associated with serpentinites include the Sangabawa District of Japan (Onuki et al., 1981), and some of the Alpine serpentinites. Recently, Miintener & Hermann (1994) have described an occurrence of Ti-gamets in the Malenco serpentinite in the Italian Alps. Their paragenesis in chlorite-diopside host rocks appears to be very similar to the New Idria occurrences; comparative studies among these localities are underway. The Malenco Ti-gamets are not associated with igneous intrusives. Switzer et al., (1971) reported an oreanic occurrence of Ti-gamet hosted by chlorite-diopside rock in a dredge sample of scrpciitinized peridotite from the Mid-Atlantic Ridge near 43 N. IV. Titaniferous Garnets at New Idria: The occurrence of Ti-gamets at New Idria is restricted to host rocks found in isolated tectonic blocks consisting of chlorite, diopsidc, garnet, and accessory minerals. These blocks are interpreted as metapyroxenites. The nature and origin of these blocks is described in Chapter 4. 5-5 Garnet in the blocks occurs in two generations: an early (Ml) generation of foliationparallel blebs, stringers, and isolated grains, and a late (M2) generation of subhedral to euhcdral vein-filling crystals. The second generation M2 garnets, which fill crosscutting veins and form drusy coatings on the walls of cavities, have received the most attention by mineral collectors because of their relatively large size (up to 1 cm) and the relative abundance of gem and museum-quality specimens. The first generation Ml garnets are anhedral to subhedral; the individual grains are usually too small to be seen with a hand lens: using SEM, isolated garnets of micron size can be found. Typically the garnets have a light to deep red color in thin section and are isotropic; they tend to form elongate, polycrystalline aggregates up to 5 mm. long. The Ml garnets are principally found in chlorite-rich domains of the host rock, so that the garnets coexist with abundant chlorite and minor diopside, along with accessory minerals. Second generation M2 garnets are much larger (up to cm. size), are generally euhcdral to subhedral, and are commonly, though not universally, anisotropic. They are generally deep red in thin section. These garnets coexist with diopside whiskers that are affixed to vein walls, and with chlorite that sometimes forms fan-shaped bundles. V. The SAG Projection Titaniferous garnets may best be considered as "improved" andradite and grossular garnets, in which Ti substitutes for Fe and A1 respectively. A convenient projection for this portion of composition space is the SAG projection, using the ternary components schorlomite (CajTiFe^S^O^), andradite (Ca3Fe^+Si3C>i2) and grossular (Ca3Al2Si3012), projected 5-6 through pyralspitc and all other components. The transformation equations for this projection, using atom fractions of Ti, Fe and A1 respectively, arc as follows: c = 2Ti Ti+Fe+Al a - CFe-Tt) Ti+Fe+Al G_ _A1_.... Ti+Fe+Al Using the wt. % of Ti02, FejOj, and A1203 provides a more convenient form of the transformation equations: 2HO2 79.90_________ T1O2 , 2Fe2Q3 . 2Al;Oj 79.90 159.70 101.94 2Fe2Q3 , TiQ? A=___ 159JO .75L9.Q____ TiOz 2FejCh - 2Ah(h 79.90 159.70 101.94 2AhCh 101.94________ TiQ? . 2Fc?Ch , 2AhC>3 79.90 159.70 101.94VI. VI. Compositions of New Idria Garnets Figure 5.1 shows 300 electron microprobe analyses of garnets from New Idria in the SAG projection. All but one of the garnet analyses in this study plot within the triangle defined by these components. One Ti-rich analysis plots just outside the triangle beyond the S vertex. Compositional variation along the three binaries S-A, A-G and S-G can be expressed by the exchange vectors TiFe.j, AlFe.j, and TiFeAl.2 respectively. 5-7 Due to the strongly zoned nature of the majority of New Idria garnets, each analysis in Figure 5.1 represents the composition at a specific point in a garnet crystal; compositions are not averaged. In the majority of cases New Idria garnets were found to be zoned, with Ti-rich cores and Ti-poor rims. This was particularly true in vein-filling subhedral garnets, suggesting that the composition of fluids in the veins evolved from Ti-rich to Ti-poor, as reported in Van Baalen & Zbindcn, (1989). Some exceptions to the normal zoning patterns were observed: some garnets varied from Ti-poor to Ti-rich and back to Ti-poor in a traverse from core to rim. In Figure 5.1, most of the analyses arc relatively low in A1 and plot near the S-A join. These garnets arc interpreted as modified andradites. The exchange vector TiFe.j relates compositions along the S-A join to the andradite composition. TiFe.j substitution may be achieved by more than one crystal chemical process: the vector is equivalent to either or both of the vectors TiFe2+Fe?2 and T^^Fe3*. As noted previously, the Ti3+Fe3,* vector is considered the less likely of the two, but until the proper spectroscopic studies are completed, the debate will rage on. In this study using microprobc analytical techniques it was not possible to distinguish between these two possibilities; for present purposes the vector TiFe.j is used. Table 5.1 and 5.2 present the garnet analyses plotted in Figure 5.1. The data are divided into two groups: those analyses with totals between 99.00 - 100.99 wt. % arc considered preferred analyses; they are given in Table 5.1. Those analyses with low totals of 97.00-98.99 wt. % are given in Table 5.2. It is likely that most of the analyses in Table S2 represent garnets with a significant hydrogamet component, making them hydroandradites and titanohydroandradites. In this study analytic determination of the 5-8 hydrogamet component was not possible. However, recent spectroscopic work by Lager et al. (1989) has demonstrated the presence of (OH) groups in New Idria garnets. In routine garnet analyses in this study, the major elements measured are those shown in Table 5.1. However, each garnet sample was examined for major impurities using EDS analysis, with a detection limit of approximately 1 wt. %. A representative garnet sample was also analyzed using the WDS technique for the presence of V, Zr, Sn, P, Na, and REE. All of these elements were below the detection limits of approximately 0.01 wt %. Cr, however, was found to occur in significant amounts in some samples. In an effort to understand the uncertainty of garnet compositions measured in this study, a working standard was chosen early in the study. This standard was a point in a relatively homogeneous portion of a Ti-rich hydrogamet; this point was repeatedly analyzed for consistency during the course of the study. The composition of the working standard in wt. 9c, with 1 a errors, was thereby determined to be: Oxide Wt. % I 0 Si02 33.55 0,29 T1O2 5.56 0.13 ai2o3 0.73 0.02 Cr203 0.01 0.01 FC203* 23.29 0.20 MnO 0.16 0.03 MgO 0.48 0.02 CaO 33.65 0.18 Totals 97.44 0.29 5-9 Figure 5.1 - Compositions of 300 garnets from New Idria in ternary system schorlomite* andradite-grossular (SAG), projected from pyralspitc and all other components. See text for details of projection. Dotted line is along apparent compositional trend shown by a significant number of analyses. 5-10 Figure 5.1 - Compositions o f New Idria Garnets - schorlomile - andraditc - grossular O a "rt C < S 00 < o Using the site preference rules of Huggins et al. (1977), who found that tetrahedral site preference was in the sequence Al > Fe > Ti, and with normalization to 12 oxygens, this well-determined composition is equivalent to an anhydrous structural formula of: Ca3.0O.02(Ti0.36*0lFc1.45.0lMn0.0tMg0.06Ca0.09XSt2.88i.02Al0.07Fc0.05) In this formulation the tetrahedral sites and the 8-coordinatcd sites sum to the ideal number of 3.0; the octahedral cations sum to 1.98. Error estimates arc shown for major elements; errors for Mn.Mg and Al are approximately 0.002. Note that the analysis appears to be Si-deficient and to have excess Ca. As been noted by most other workers, Ti-gamcts world-wide are commonly subsilicic, i.e. they have < 3 Si pfu, and supercalcic, i.c.they have > 3 Ca pfu. This compositional feature is apparently real and not an artifact of the analytical techniques used. The chief source of random variability in microprobe analyses is the counting statistics in the detectors on the spectrometers. The working standards used for the microprobe at Harvard have also been analyzed using wet chemistry in order to constrain systematic errors. In this study no other significant sources of error were found, i.e. no error terms other than counting statistics were required to explain the variability in the analyses. The mean compositions of the garnets in Tables 5.1 (totals 99%) and Table 5.2 (< 99%) are given in the final column of each table and are listed below for convenience. A comparison of the two groups shows that the garnets in the second group are slightly lower in Si and Ca; noticeably higher in Ti and Al; noticeably lower in Fe, and higher in the trace constituents Cr and Mg. Mn as a trace constituent is the same in both groups. I therefore conclude that the garnets of the first group are essentially Ti-andradites, with only minor 5-12 amounts of other components, while the garnets of the second group vary more broadly in composition space. Oxide Table 5.1 Table 5.2 Si02 TiOz AI2O3 Cr203 MnO MgO CaO Totals 34.47 4.40 1.85 0.06 24.08 0.15 0.27 34.21 99.50 34.01 5.73 2.85 0.15 20.75 0.15 0.70 34.06 98.40 The lower Si contents of the garnets in the second group correlate in a general way with lower analysis totals, but a plot (not shown) of Si wt. % versus analysis totals shows considerable scatter. Therefore, the simple hypothesis that low analysis totals are mainly due to the hydrogamet substitution does not fully explain the compositional complexity in these garnets. VII. Compositions of Ti-garnets from Other Localities Figure 5.2 presents analyses of Ti-gamets from other localities for comparison with the New Idria samples. The Onuki ct al. (1981) data are from the Sangabawa district ofJapan; the Hermann & Miintcner (1991) data arc from Val Malenco, Italy. In both cases the garnets are found in serpendnites. The Switzer et al. (1971) samples arc from 5-13 Figure 5.2 - Compositions of garnets from other localities for comparison with Figure 5.1. Symbols as noted in figure. 5-14 63 Analyses Figure 5.3 - Microprobe traverse of an M2 gamer vein in sample 989*65a. Early Ti-rich garnet grew along vein walls; late Ti-poor garnet filled vein center. 5-16 (njd) sooty millimeters Figure 5.4 - Fe-Ti exchange in same garnet traverse as Figure 5.3. Good inverse con-elation argues for progress along TiFe., vector (see text for discussion of TiFe.,). 5-18 Figure 5.5 - (Fe+Al)*Ti exchange, compare with Figure 5.4. Improved fit shows that nearly all compositional variation can be explained by progress along AlFc.( exchange vector combined with Felt!. Minor tetrahedral Fc and A1 is also likely. 5-20 Plate 5.1 - Photo of M2 garnet vein crosscutting Ml chlorite-diopside-gamet rock, sample 989*65a. Garnet in veins is black and Ti-rich at edge, but yellow and Ti-poor in center of veins, showing that fluids precipitating garnet evolved from Ti-rich to Ti-poor compositions. Width of sample 3 cm. ' 5-22 scrpcndnized peridotite dredged from the Mid-Atlantic Ridge near 43 N. Ti-andradites also have an igneous paragenesis, in association with alkalic rocks such as nepheline syenites. The Flohr & Ross (1989) data are from Magnet Cove, Arkansas, USA, where the garnets have an igneous origin. Figure 5.2 shows that the trends of the Onuki et al. (1981) and the Flohr & Ross (1989) analyses arc roughly parallel to the S-A join at relatively constant G. One of the Onuki samples is somewhat enriched in Al (4.75 wt.% A12Oj). The Hermann & Milntener (1991) samples fall into two groups: a low-Tt group that lies along the S-G join and a Tienriched group that lies near the S-A join at low and constant Al. I therefore conclude that the dominant exchange vector for the garnets in Figure 5.2 is along FeTi.i, with variation along AlFe.i for some of the Hermann & Milntener samples. Comparison of Figures 5.1 and 5.2 shows that the New Idria garnets very over a .much wider compositional range than the garnets from other localities. This is particularly noticeable in the Al contents: some New Idria garnets are quite aluminous. Figure 5.1 also shows that some compositions lie along a trend that projects to the G-vertex. Less obvious is the observation that some garnets in Figure 5.2 also lie along this same trend. This trend is shown as a dotted line in Figure 5.1; the intersection along the S-A join is at about S6gA32. The exchange vector that defines this trend is a weighted sum of the two vectors TiFe.j and TiFcAl.j. VIII. Some Crystal Chemical Considerations Examination of Table 5.1 and 5.2 shows that most analyses have less than 3 Si cations per formula unit (pfu), and more than 3 Ca cations pfu. The New Idria garnets are 5-24 therefore similar to many other reported dtaniferous garnets in that they are subsilicic and supercalcic. From a crystal chemical standpoint the subsilicic compositions can be explained by proposing that some tetrahedral sites arc occupied by Al. Fc3''', and even Ti4*. Huggins et al. (1977a) discussed tetrahedral occupancy in titaniferous garnets and concluded that the order of preference for these cations is Al > Fe > Ti. The occurrence of more than 3 Ca cations pfu apparently requires some Ca in the octahedral sites. MUntcncr & Hermann (1994) suggested that the apparent Ca excess in Ti-andradites may be an artifact of the water free cation normalization, for those garnets containing a significant hydrogamet component. However, in this study even garnets with analysis totals close to 100% (Table 5.1) and which therefore should have a low hydrogamet component, appear to be supercalcic. I therefore conclude that the excess Ca is real, and may be explained by the larger octahedral site in andradite as compared to the other common garnets. According to Smyth & Bish (1988), the mean bond distance and polyhedral volume respectively in the relatively large octahedral site of andradite are 2.024A and 11.046A2, compared with 1.924A and 9.49lA2 in grossular and 1.887A and 8.937A2 in pyrope. However, it should be noted that the 8-coordinated site in andradite measures 2.433A and 24.55A2 respectively, which is very much larger than the octahedral site. IX. Exchange Vectors In an attempt to understand the nature ofTi-Al-Fe exchange in these garnets, detailed studies of individual crystals were undertaken. Figure S3 presents the results of a microprobe traverse across a garnet vein in sample 989-65A (Plate 5.1). The garnet vein crosscuts the foliation in a chlorite-diopside-gamet rock, and the garnets are considered to be M2. The garnets with the greatest Ti contents occur along the walls of the vein and grew earlier than the low-Ti andradites that fill the center of the vein. Whiskers of diopside attached to the vein walls arc enveloped by garnet that continued to precipitate in the 5-25 absence of diopsidc. The inverse correlation of Ti and Fe in this traverse is striking. Figure 5.4 plots Fe-Ti exchange in the vein garnet; the inverse correlation is good and supports the the suggestion of a simple substitution of Ti for Fe, according to the exchange vector TiFe.j. However, particularly for the low Ti-gamets the scattci in Figure 5.4 is noticeable and the Fe content at zero Ti is not two Fe atoms pfu. In these garnets an Fe-Al exchange also appears to be significant. Figure 5.5 plots Ti versus Fe+Al; the anticorrelation is further improved and the intercept is nearly 2. This figure also argues for minor, but noticeable, amounts of tetrahedral Fe and Al, possibly via vectors similar to Ti ,VA1 v,Al.}Si.i and Ti ,vFe v`Fc.1Si.1,which are both equivalent to TiSi.t. X . A Three-Garnet Rock In the course of this study a minor outcrop designated 990-1, in a stream gully south of the Gem Mine, was found to consist of chlorite-diopside-gamet rock with very aluminous garnets. This locality may be the same as Coleman's outcrop 92-52, although the description given by Coleman (1957) described a much larger outcrop than presendy exists at 990-1. Garnets from the 990-1 locality have unusual parageneses and compositions. Figure 5.6 shows garnet analyses from this locality in the SAG projection. The compositions fall into two groups, a "normal" group and an aluminous group. Plate 5.2 shows a backscatter electron micrograph of a sample from 990-1. In the upper portion of this photo, left of center, is a composite garnet grain containing at least three generations of garnet The central portion in medium grey is an aluminous garnet with the composition given in Table 52 column 97. This earliest generation was apparendy fractured, and the pieces were overgrown by a Ti-rich garnet that appears light grey in the photo (Table 5.2 column 101). Later the entire grain was overgrown by near endmember andradite that shows as white in the photo (Table 5.1 column 152). These three 5-26 compositions arc the vertices of the dashed triangle in Figure 5.6. The three generations of garnet meet at two points as shown in Plate 5.2. The host rock at the 990-1 locality is strongly foliated. The garnet textures suggest brecciation or mylonitization, and the garnets appear to have grown in a succession of equilibrium states (each garnet generadon appears homogeneous). Therefore we may not conclude on the strength of this evidence that the three garnet compositions are in equilibrium with each other. The assignment of the garnets to an Ml or an M2 event is ambiguous in this sample. Apparently, the latest generadon of garnet precipitated from fluids percolating through the brecciated rock,and acted as a crack-filling material that welded the rock into a dense, tough mass. The 990-1 samples also contain REE-enriched dtanite (sphene) and zircon as accessory minerals. ' " XI. Compositions of Minerals Coexisting with Garnet Chapter.4 deals with chloritc-diopside-gamet rocks which are the host rock for the garnets described in this chapter. It was suggested in Chapter 4 that the ultimate source of Ti for the Ti-bearing garnets was a /ertile ultramafic rock of pyroxenite composition. In such a rock Ti would have been present in Cpx and spinel. It was also suggested in Chapter 4 that the gamet-forming reactions involved the breakdown of the hedenbergite component of clinopyroxene, resulting in production of garnet, magnetite and free silica, probably SiO^aq). It was shown further that these reactions may have taken place under conditions of very low oxygen fugacity. The presence of small amounts of methane in fluid inclusions from the garnets of Santa Rita Peak also implies low oxygen fugacity during M2 metamorphism. Understanding the formation of the minerals in these rocks requires 5-27 Figure 5.6 Garnet compositions in chlorite-diopside-gamet sample 990-1, projection as in Figure 5.1. Open squares are all garnet compositions. Dashed triangle with filled squares at vertices outlines compositions of three coexisting garnets shown in Plate 5.2. 5-28 Plate 5.2 - Backseatter electron photomicrograph of sample from 990-1 locality. Three generations of garnet comprise the large grain in center. Earliest garnet (dark grey) is grossular, second generation (medium grey) is Ti-andradite; third generation (white) is Ti- frcc andradite. Field of view 25 pro. ' 5-30 knowledge of the compositions of coexisting minerals. The reader is referred to Tables 4.1, 4.2, 4.3, 5.1 and 5.2 for some of these compositions. One New Idria sample used in this study was 92453, from the collection of the Harvard Mineralogical Museum. This sample, from an unknown location at New Idria, contains some M2 veins of homogeneous garnet coexisting with homogeneous diopside. The table below gives the mean compositions of 22 garnet and 8 diopside analyses respectively. Oxide Si02 TiC>2 A12C>3 Cr203 FeO* MnO MgO CaO Na20 Totals garnet 33.74 8.31 3.71 0.03 17.85 n.d. 0.18 0.27 34.68 n.d. 98.76 diopside 53.09 0.03 0.36 0.05 n.d. 8.18 0.40 13.33 24.63 0.12 100.20 XII. The Colors of New Idria Garnets New Idria andradite garnets occur in a variety of hues, including yellows, reds, greens, browns, and black. The colors of garnets and other minerals relate broadly to their chemical composition, both major and trace elements. The exact mechanisms are a controversial subject, and it is not the intention of this study to discuss detailed mechanisms. It is instead shown here that the colors of andradites relate in a logical manner to major element chemistry. The relationships are a useful field guide at New 5-32 Idria, but may not be universally applicable. Table 5.3 shows averaged chemical analyses for groups of garnets with a specified color. One color scries relates to the relative proportions of Ti and Fe. Groups 1-5 in Table 5J show increasing Ti content and decreasing Fe content, in accordance with progress along the TiFe., exchange vector. Garnets with Ti02 < 0.05 wt % (Group 1) are light gTecn; they take the the varietal name of demantoid. Garnets with Ti02 between 0.05 and 1 0 wt % (Group 2) are honey yellow, with the varietal name of topazoliie. This group is the most widespread at New Idria. Increasing Ti02 contents cause the color to darken: yellow-brown (Group 3) garnets have Ti02 from about 1.0 to 2.0 wt. %, Garnets with 2.0 to 3.5 wt. % Ti02 (Group 4) have a characteristic dull brown color, which may also be related to the greater Mn content of this group: MnO is up to ten times higher in Group 4 than in other garnets in Groups 1-5. There are two archaic varietal names for brown, manganoan andradites: allochroite and polyadelphire. Garnets with Ti02 greater than 3.5 wt. % and low A12Oj (Group 5) are a brilliant jet black: there are two varietal names used for this group, melanite and schorlomite. The distinction between melanite (under 5 wt % TiO^, and schorlomite (> 5 wt % TiC^.is arbitrary and based on Ti02 content, not color. At New Idria this study shows that some garnets that chemically are in the melanite group are not in fact black, suggesting that the term melanite needs revising or abandoning. Some New Idria garnets do not fit into the above simple color progression. Chromiferous garnets. Group 6 in Table 5.3, are emerald green and almost always anhedral. These garnets have a significant uvarovite component, and should be called chromian andradites. Aluminous garnets, Group 7 in Table S3, are a distinctive cinnamon brown and are frequently euhedral. These garnets have a significant 5-33 grossular component, although many are also high-Ti: these should be called titaniferous grossulars. The detennination of the exact electronic configurations responsible for the observed colors is not a goal of this study. The major color progression correlates well with . progress along the TiFe.i exchange vector. Ti4* should have no direct effect on mineral color, since it has no 3d electrons with absorption in the visible spectrum. However, if the vector TiFe.! is equivalent to TiFe2*Fe32 then the coloring may be due to the presence of Fe2* or to electron hopping between Fe and Ti ions in adjacent sites. If the vector TiFe.t is equivalent to Ti3*Fe3j* then the presence ofTi3*, with one 3d electron, might have an impact on color. As discussed earlier, this possibility is considered doubtful. Elevated Mn contents arc known to be related empirically to brown coloration in garnets, as evidenced by the varietal names for these types. The emerald green color of the chromian garnets is well established, as is the cinnamon color of grossular. It is interesting and perhaps relevant to note that the elevated Ti content of Group 7 garnets has no apparent effect on their color, giving further weight to the suggestion that Ti per se has no great coloration effect New Idria garnets form an excellent data set for future spectroscopic studies of mineral colors. XIII. Conclusions 1. 300 electron microprobe analyses of garnets from the New Idria District are presented. Analyses ofcoexisting minerals, principally chlorite and diopside, are given in Chapter 4. 5-34 2. Most of the New Idria garnets are calcic and belong to the andradite clan. Many, but not all garnets of the New Idria district are strongly anisotropic, sector twinned, and zoned. Titanium substitution occurs principally along the exchange vector TiFe.j, making these garnets Ti-andraditcs. Some of the garnets arc also hydrous and vary along the hydrogamet exchange vector H4Si.j. Some of the garnets are Ti-grossulars, a portion of composition space not previously explored. Compositional variation in these garnets occurs along the vector TiFe2+A1.2. The mean TIC^ content of the garnets in this study is 5 wt. %. One of the garnets contains 15.62 wt. % Ti02, which corresponds to approximately one Ti atom per formula unit, i.e. the schorlomite end member. . 3. At the present time that presence of Ti3* in garnets has not been completely ruled out, but is only supported by indirect and equivocal evidence. TiFe.j substitution may be achieved by more than one crystal chemical process: the vector is equivalent toeither or both of the vectors TiFe2*Fe32 and Ti3+Fe3\ The Ti3*Fe3* vector is considered the less likely of the two, but until the proper EXAFS studies are completed, the debate will rage on. In this study using microprobe analytical techniques it was not possible to distinguish between these two possibilities and TiFe.j is used to describe compositional variations in garnets. 4. A representative garnet sample v/as also analyzed using the WDS technique for the presence of V, Zr, Sn, P, Na, and REE. All of these elements were below the detection limits of approximately 0.01 wt %. 5. Most New Idria garnets are hosted in tectonic blocks of chlorite-diopside-gamet rocks that are interpreted as metapyroxenites entrained by the serpentinite. The ultimate 5-35 source of the Ti in the Ti-gamets is from these pyroxcnites. The garnets occur in two generations assigned to metamorphic events Ml and M2. 6. Most analyses have less than 3 Si cations per formula unit (pfu), and more than 3 Ca cations pfu. The New Idria garnets are therefore similar to many other reported titaniferous garnets in that they arc subsilicic and supercalcic. 7. One sample contains three coexisting garnets. The garnet textures suggest brccciation or mylonitization and the garnets appear to have grown in a succession of equilibrium states (each garnet generation appears homogeneous). 8. New Idria andradite garnets occur in a variety of hues, including yellows, reds, greens, browns, and black. The colors of garnets and other minerals relate broadly to chemical composition, both major and trace elements. The principal color series is from yellow to black, correlating with progress along the TiFe.t exchange vector. Chromian garnets are emerald green, grossular garnets are cinnamon. 9. Ti4* should have no direct effect on mineral color since it has no 3d electrons with absorption in the visible spectrum. However, if the vector TiFe., is equivalent to TiFe2*Fe?2 t*lcn t*ic coloring may be due to the presence of Fc2* or to electron hopping between Fe and H ions in adjacent sites. 5-36 i Cr"\s<<rvr)>1 - * tf> i o in o 0 01 m *5 O H t 9 c < *mc CM o "O Q. to jj 2o <Cl>> r> r r-. ^cc:> rcrj>) rc)> c> C-) cc ' rIT*"- ci rc>> c) <<>> rH) cOJ) aai CO CO CM cv O CO d CO 03 d |0Z 66 900 _____ 0 0 0m o. ir o V- o GO d oo fs. CO Number > Sample --> 92453 1 1! 92453 I1 92453 1[ 92453 I1 92453 1 92453 I1 92453 1 92453 i1 92453 92453 92453 1 8 S Si02 TI02 AI2Q3 Cr203 Totals n 1 34.861I 3.83 34.0311 34.041 1 8.78 33.9111 7.41 34.10!1 6.74 3 5 .0 1 I1 3.02 34.0311 35.7311 35.121 6.79 2.71 2.69 | |j0 0 0 IZO'O 09SZ | 1 |000 600 !IZO'O I CM o CM CM in CO GO CO CM CD CO 03 O CM ** 1 0.03 25.46 3.971 1 17.18 |1E0'0 2.941 0fs3 1 1 20.91 1.3211 0.0111 22.06 1.56|1 0.021 1.581 1 1.93!I ]000 CoO 25.24 1.65| 25.52 I1800 CoO fs o CD o 1 0.261 ! 0.141 0.26 0.27 0.44 0.29 0.19 0.34 0.25 33.88 1 99.521 34.78 99.051 34.64 99.33|1 99.161 |00`66 | 34.39 34.16 1 99.6311 99.141 P) tn CM o q 34.05 I 34.00 99.22| 33.791 8.751 18.27 0.14' 34.77 99.31 j 35.07 3.13 25.62 0.08 0.21 34.06 O uCM. |Sl 0 |80E |00 too [000 190 'E | |00'0 1 1 lEO'O B S 'l j[00 0 j 000 | [000 1 IZO'O 000 |10 0 '0 sro |60' 1 1000 1 1C IO lO l'E | too 00 0 1 810 1 1100 0 ISS'O EO'O j000 1 zoo HO ac c\ o o o n o o d fos o d oo d to y- < o u. 5 g <3 Cations 2.93 ' 24 ! 2.81 0.541 0.39 2.82 1 0.29 2.83 0.47|1 2.86 0.431! 2.93 0.19|1 CM in CD 0.431 1 0.16 2.97 0.1711 0.19 2.95 0. .171 0.16 2.80 0.54 0.32 0) 0.20 1 rc on < G Totals 1.61 M-------- il 1 3.051I 7.97 1 3.0811 7.93 1.15 0.0311 3.07] 7.92 1.32 0.05 11 7.95 Nolos: all Fe oxpressod as Fe203l INolos: cations calculated on basis of 12 oxvoons 1.39 1.61 0.041 1 7.95 .3.0711 7.99 d o 1.39 1 3.071| 7.95 CD O) is.* 0.01 3.031 dd 1.61 7.99 CM q Oo 1.14 7.93 q 7.9B ra>). iirf)> rr>)1l cc> rV) c*- C*3r> caa3 Cr>) f cvi c> c'1c) T-- c C) ri N 1 1 1 Tablo 5.1 Folio Table 5.1 n CM I ; COOf. rj; o03-, vOi cevqj,j --CY| toP.5|eo, n1,>I n1 V: loiiioi oii <ri roMi di ; o'1 ;| [ f*frl':oe!o.oO: O'ol iinn 1. O--l Orri mOl Otoi.l 11 ;, I 1 1 1 | 1 ! !1i m; CO CM < CO n v CO CD CO CO n o' d d i Number -> I I Sample --> | 92453 I 92453 CO CD in CO 19 688-6 1 688-6 I 688-6 I CO | 9-889 20 I 688-6 I CO CCOO CM CO 22 1 __g 688-6 I 688-6 1 688-6 CM O |Si02 IA1203 1 CO OCM O CO 0UCM- s t e e _______ ! 8 ITotals 1 | 1 |Z00 33.57| 8.62| 0.33] 34.671 1 99.031| (GOO 33.66| 8.58] 16.02 0.15] 0 .3 11i 34.9111 99.13]| CO C IO d cn6 I 3.08] 14.97] 0.21 0.32|1 35.1211 99.97| 34.39] 5.4B 34.55) 5.29 coo 080 CO COM dd 0.20] 0.47|1 34.33!! | 25.061 0.16 0.45]1 34.341! 100.74]! V CM |CC'0 | OZ'O soo coo zoo coo 680 ess C'TM 34.09] 1 1 0.20 0.41 1 34.27!1 99.85 j1 in d O4T Oi * 34.1411 1.05 24.40 1 0.15 34.2611 99.9311 33.701 1 6.54 1.33 22.7811 34.31 1 99.2211 33.971i 6.28 1.23 22.94 1 0.22 ! 34.44 1 99.42 1 in CO nri in O CM 34.22I 5.91 0.94 0.05 23.70|I___ 24.471 coo 010 ozo 0.291 34.32! 1 99.9811 CCOM d 34.36] 99.67! CC>) 0 c- d i1 i oil oii 11 ! l ! 1 00 COD d0 d 0000 d Cl CM ciji 00 0 1 000 10 0 0 10 0 0 100 0 ]000 10 0 0 11 0 0 ]000 100 0 1 dil t o o |[to o 1 SS't 1 060 | - , 00 1 O dd CO 00 dd CO < 0 UO_l 5 Icalions 4T m d 2.79 1 0.33 1 0.34 CS'O 2.94 0.19 2.87 0.34 1 2.87 0.33 0.09 1 2.86 0.09 SCO SCO 0 d 2.86 cro 1 2.83 0.41 1 2.85 0.12 1 2.86 0. 12! 2.82 0.37! o re d: d o'i r-ii 1 i iiilii in <00 d o re | *0 iTolols I 1 1.12 1 1.54 1 1.57 to o 1 1t o o 1t o o [to o 90'C 1 O d o> 1 0.04 1 0.04 1 0.06 1 3.09 1 3.01 1 3.07 7.94 7.98 1 7.98 7.97 ] 1 1.54 1s o o '3.08 1 7.98 1 3.07 7.98 to o |to o |to o ] SOO 60C ^Or Oi d 1 1.45 1 1.49 I 0.02 0 d 1 0.04 3.10 3.07I 7.97 7.96 7.99 1 1 1 J 1 1 i 1 iNoles: all Fe expressed as Fe203 |Notes: cations calculated on basis ol 12 oxvaens Table 5.1 Folio 2 Table 5.1 5 3 z . c->. c? in rri ant' I <X)| ODI CM, CD, O ri|si isi "I i 1 i ii ! h 00Cr)M)r1j1/ (i 96 ee * 04 CM 0) CD CM od o O h* O A 25 1 26 CO I Sample - > | 6 8 8 - 6 1 1 CO GO CD 27 28 29 30 32 33 | 688-6 688-6 11 I! 6 8 8 -6 I1 6 8 8 -6 11 6 8 8 -6 11 6 8 8 -6 1E 6 8 8 -6 1 GO C(0O n <0 SC 686-6 36 CM 04 Oo CO V CD IAI203 I CD O 04 O CD O uCM. m\M O _______ 1 8 Totals 1 soo |900 CD CD o in o fC*M* CO in 34.081 1 E 24.53 1 0.341 34.24 99.92 1 6.03 0.971| 0.05 24.22 0.14] 1 34.36 100.23 33.72 0.651! 6.32 0.94| 23.41 23.26 ! . .0.181 0.40 j 0.20 0.37) 34.43 34.36 100.12 99.46 CD in -** O) o do n 01 CM |SC0 I 33.81 I 7.04 33.82 \ 6.34 1 34.00 6.181 1.75|I 6.57 1-90 j ! 6.57 1.831 33.54 in o soo too coo o CM o 21.81 1 1 34.5B 99.37 22.40 0.171 0.32! 34.67 99.46 22.69 1 I 34.57 99.70 0.15! 0.29!1 34.52 99.85 22.40 0.28 1 34.30 99.59 22.26 34.23 99.43 03 CM CM d o* 10 d G0O) 0003 o CO o n coo 33.08 7.63 1.83 21.65 0.20 0.29 ____ 0.2? 34.2B 34 5P 99.00 C00 to o |C0'0 |to o 1 |1i o o | 100 11I TM _____ t 1 o o 111 0 0 too 1 ] too I [COO 1 |S0'0 SOO 1 , o'; d| o'; r^i i! !i ii M; a Q 1 o O CD N ii' CO i CM a O CoQ C0 d o d pv VCD n r0-> CM d CO K O o od oo dd < 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 OO'O 0 0 0 OO'O OO'O Cations 1 2.85 I 2.851| ! 0.361 E o.ssi I o.oel 0.00 |S i 0 I|60'0 ||90'0 | (0 ^r d 2.82 1 1 2.8511 0.40|1 2.83|1 0.4411 ! 2.831 0.401 0.17 1E |eeo | 1 ! 0.171 L2.83! o .4 i! C*D dd 2.84 1 1 1.551 _____ L 5 ? 1 1.47j1 1.4711 1.37 1 .1 -41 j 1 1.411 0.44 1.40 6f0 CC O 2.7a! ___ 2,83 0 48 ___ 0.45 ___ 0.18 n nn 1.37 5 U_: s 3 01 Q 0CC i fs. O d Tolals 1 O l'C 60 C O d | 0.04 !80'C 3.07 3.07 1 7.97 1 7.97] 1 7.96j1 7.9711 7.96|1 7.981 6O'C 3.07 7.95 ____ 7.96 i 1i i1 ! Notos: all Fe expressed as Fe203 iNotos: cations calculated on basis of 12 oxvaens Table 5.1 Folio 3 )' -cc-5. --o, -oO-, Cl - LD oici, ;. , ' |! m Tablo 5.1 i i I1 eo 1 Ii ! ' <{ 1 j aoj 1 1 i l ;i CCOO COO' cCvOi O' CCOO of oCM od O' 1 Number -> 1 38 3 91; | CM O' ___ I 44___ 1E 45 IL 46 1 47 ' iSampte > | 6 8 8 -6 1 6 8 8 -6 I1 6 8 8 -6 1 6 8 8 -6 1! 6 8 8 -6 11 6 8 8 -6 I1 6 8 8 -6 11 6 8 8 -6 1I 6 8 8 -6 11 6 8 8 -6 | 6 8 8 -6 <3 e u. w 0 < b- 35 u. 8 s s O CeM CM O K |Si02 |At203 Cr203 Totals [Cations m Totals CO coo 200 i S001 200 S00 900 |0C6 CoM eo 0) 33.63 32.95 32.801I 32.96 1 33.07 32.86 32.32 34.17 1 8.25 9.15 6.95 10.61 11.19 11.52 5.41 o CO co d 11 1.751 1 1.68| 1 1.651 1 1.871 1 1.761L _ . 1.551 L. 1-0 5 1 oCO O o' d 0.04 0.05! CM .1 20.1811 20.05|I 2 0 1 2 || 19.41 ]| 18.051| 18.89] 24.15 0 CM 1 0.221i 0.221 d o CM 1 0.20 1 0.21 1 0.22 |EE'0 | 90 66 [ECO I 1 0.291 CnM d 1 0.33!1 0.3711 0.381 I 0.47| 34.38 34.49 34.74 34.55 34.78 34.89 34.99 34.76 34.70 9966 99.66 99.20 99.01 99.25 99.24 99.29 99.15 99.26 ISO j 00 0 000 [S8'0 ] Orr) ci n O CD CD 03 6 CD OCO 1 __ 33.79 5.38 _____5.99 0.84 0.78 ___ 0.00 23.98 ___ 23.9G 0.27 ____ 0.26 0.52 _____ 0.52 34.04 ___ 33.82 99.21 ____ 99.12 lo re ]190*0 ][1 0 0 |000 1 |[C IE [1S 00 111 0 0 1 1 1SOO [100 000 |C9` 0 lire 1 I|1 0 0 [10 0 0 | |lo r e 100 1 10 0 0 19S*0 | 1Z 0 ` 0 (10 0 0 910 ||6S 0 1ZOO 100 0 1 1lo o I000 r0*. r* d fv 00 CM d CD CD 40T dd 0 d a> O CM CD CO ci ! 1 2.761 1 2.75|1 2.7611 2.771l. 2.761 1 2.73| 2.71 1 0.52!1 0.581 | 0.58! 1 0.67| 1 0.731 0.17! 0.17 0.19 0.17 0.15 0.14 1.27 1.271 1.27 1.22 1.19 1.19 1 0.04 1 0.04 : 1 3.07 1 3.10 1 3.12I 1 7.93 1 7.95 1 0.041 1 ' 3.12! 1 7.93]| 7 .9 3 |1 7 .9 1 11 3.12 7.921 ? 19 0 '0 [10 0 |000 010 90'0 000 800 OCM d l CD CD n CM d 80 0 G G c: cv c CCCl fs c cc\1 fcs TJ r cCcG ____2.85 ____ 0.38 0.08 ____ 0.00 ____ 0.00 1.53 1.52 ____ 1.52 ____ 0.02 6.07 3.06 ____ 3.0G 7.97 ~7.97 1 i j1 i 1 11 !1 1I `i 11 j 1 n O uC*M. % *0 g <10d 0 QXO u! O </) 0 0 Z in c0 a OK CM O J*<uO35 c 0 75 O3 3 %n 3 iOn O Z Tablo 5.1 Folio 4 Table 5.1 1 . I ,1(01 ! ol coi ' 2 i' ! 50 hj SI i1 i .1 A u. Cf) m 1Number > | 49 I 50 I 51 688-6 I 688-6 1 J5 21! S3 1 II 54 5 5IE 1 CO IA ! 57 1 58 688-6 1 688-6 1 II 6 8 8 -6 |1 6 8 8 -6 I1 6 8 8 -6 11 6 8 8 -6 1 688-6 CO CO CO 96 EE S9CE <c |OI CO CO q qCO 03 CM CO a oCea/3 S <3 Uoi 5 w o < W a U. 8 s r> O C9M n CM OCM |Si02 IAI203 GO Totals (Cations Totals ES0 lOO S I' |90'E [|90'E ] |90'0 ||90 0 [ [20 0 ||Z 0 0 | 00 0 000 0 0 0 0 0 0 00 0 000 00 0 [0 0 0 |0 0 0 0 0 0 34.321 34.02 34.24 ! 33.70 33.91 33.83 34.06 ! 34.03 lots 0.761 5J8I 1 0.77 S8'0 1 6.031! 0.78 6.0911 5.96| 1 0.77 5.871 1 0.77 S8 0 5.721t C"TD 5.711 |10 0 0 |000 1 000 1 |1Z0O |00 1 0 |0 0 0 1 j |0S'66 | OS'O ES0 ES0 |9Z0 | O o CO GCMO d CO CM oo oo 1 o.ool 1 24.781[ 24.541 1 24.3811 24.361 1 24.5911 24.0211 24.091i 23.741 CO CM o rr o Oi o o ad* 1 0.271 1 0.271 1 0.2711 0.27 11 0.24 ] 0.53 0.54 0.54 0.47 1 33.9211 34.0411 33.961 1 33.9311 33.7711 33.8711 33.8711 34.04| 1 99.311 1 99.3411 99.6111 99.40]1 99.6511 99.121 I 99.04| q GqO in CM d |0 0 0 |80 0 |[9 0 0 | 8 0 0 2.88 1 0.3211 0.33|1 0.32! fs. GO qo CM* o [8'0 | 2.83 1 2.85 0.39| I |8E0 j 2.83 1 0.08 | 2.85 0.37!I 0.00 2.86 0.36| oo o dd 1 1.57 1 1.5611 1.54 [ l 1 . 5 2 |1.54 1 1.5211 1.56)1 1.53! [90E | ZOO | COM d oCM O oo COM ro* oo CM oO od 1 0.02 1 0.07 1 0.07 o d [ 0.06 1 3.05 1 3.07 1 3.06 1 3.0411 3.05! 7.98 7.99 7.98 7.96 7.98 7.97 7.97 CoO d 3.07 7.97 EE 0 |9 E 0 | [too 1 [66EE | Cl CD C*M d [000 S8'0 5.26 0.19 99.19 5066 ISO 34.05 5.63 _____ 5.49 0.78 _____ 0.89 0.00 ____ 0.00 23.79 ___24.29 0.24 0 ?3 _____ 0,50 34.05 ___ 33.93 ___ 99.29 (inA c o 2.86 2.87 ____ 2.86 80'0 CCO3 OCQ aa _____ 0.35 ____ 0.09 ____0.00 1.51 _____ 1.54 11 ^ 1o ____ 0.02 0.06 ____ 0.06 ____0.06 3.07 3 OR _____3.06 CB Cl N 7.98 ____ ' 7.98 1j n O CM Q U. </) m *T3 Wl V) S a. So ' I75 1 i 21 \} 11 1 i j iNotes: cations calculated on basis ot 12 oxyqens Tablo 5.1 Folio 5 Table 5.1 1 . <0 1 1 I'*' CO i; iI 11 <o ! 1^ ii ii o iA i i i . n*- co-n--OoNr*O.. cm'-*cr.Qco1n. ., cr-o'tmo *- co oCM 0O4l e cn CO co C<EO0 "S e 1 Number > 1 | 9-889 62 688-6 688-6 | 69 CD CCOO TCO CO 65 1 6 6 I___ 67___ 11 68 I 69 | 688-6 1 1 688-6 I1 6 8 8 -6 |1 6 8 6 -6 I( 6 8 8 -6 | 688-6 | 688-6 ZOIS ' nVti uv, *-',! o- rr cji of. 6,, i i , i . ; i ; ; j : [ |! 1 i 1 : i > ! i 1 : 1 ; 1 1l 68*66 | ncm; ooil> CCMO i o O o o hCO* o IoD CoM o o 4-- o 8 s CO OCM uO. CO o(M 6 <0 OCM < CM g ITotals --- 1 |S0*00l ||CS*66 | |0 0 0 i|0 0 0 ]|ooo ; |96`0 BSS 0.931L . 0.941 24.371 1 0.281 M i li_____ 1 1 1 33.981 1 | 24.32| 0.241 0.5211 33.941 1 | UCM) CVO o> ro* VCM 34.50 5.21 0.251 0 .4 4 11 33.89|1 99.94|| 34.101 5.34 1 o.ool 24.59| 0.301 20.4 11 33.80| 1 99.6311 CD o CCOM d CD CM oo 34.04 1.421l 1 1 0.3211 33.9611 99.5711 CO |G8*C6 | |6C*0 | SCO 1 [0 0 0 ||00*0 |100 0 |100 0 ||0 0 0 j 33.82 6.12 1.14|1 23.89|1 0.2 II 0.37| 33.9411 99.551I 34.24 5.37 1.031 2 4 .0 l|1 0.26|1 33.9911 99.25|1 |06`S 34.06 23.73|1 0 .2 4 11 1 33.961 99.2811 34.29 5.54 24.221 1 0.26| 1 0.36 1 I 99.5211 OCOi CM ** o 33.92 IS s 5.45 1 1.15 _____ 0.00 23.60 24.01 0.27 j 0.45' 0.46 34.05 99.34 i 99.17 I 1I iCations 1 98*Z 00 0 no SCO SCO I |*co |6C0 | 00*0 00*0 00*0 00 0 00*0 ZOO I |SSl | 00*0 00 0 00*0 0 0 0 60*0 6 0 0 60*0 [ceo ||SC0 i1cco | *} cM. |d |d-o co 2.87 2.8B 2.88 2.86 2.85 2.84 2.88 2.88 2.89 2.86 oq dd *CO C0M dd H-l 1 0.39| 1 0.341 < 0.11 0.14 0.11 0.11 o 8 5 U. ITotals 1 1.54!1 1.53| 1 0.021 d CM IIDD ZOO | 0.00 1 1.4711 1.51 1 1.52| r 0.021 l _ . 1-53 COM 0 dd S00 SOO | SOO SOO 90*0 0.06 soe 1 3.0611 3.03i1 3.03 1 3.04| IqD pi I 3.06|1 3.06|1____ 3.041 7.98 7.96' 7.97 7.97 7.96 7.96 7.97 7.96 7.96 120*0 ] | |S0*0 |90*6 O) OCM o 90 0 CM O o 1.52 _____ 1.1 _____ 0.01 0.06 _____ 0.05 3.06 _____ 3.05 7.97 ____ 7.9B ____ 7.96 i 1 i iNolos: all Fe expres&od as Fe203| |Nolos: cations calculated on basis of 12 oxvaens Tablo 5.1 Folio 6 Table 5.1 1 j if ! jsis ! 1 i* ! IOrtj a-~iMr OV3 CM Cl 8s 00 d 03 C0M d I80SE I ' in'i oi oi d> ~ n! i r r*** 0 CD d d CCDD Oq nCM Oq rC*O. CM d VCO ihn. 0 oi d ocwi d CO CM nd 1 Number -> I 77 I 9 79 A a E (9 CD 688-6 | 608-6 1 688-6 1 6B8-6 I 688-6 I 688-6 I 80 1 82 i 83 688-7 1 688-7 1 688-7 I 688-7 O OCM 9 LL a OCM Ow rt OCM < CM O CM O in ITotals |96*GG 1 |00*S iCnO 0q0 CqD CCOO C5 cCMi d 34.64 5.481 0.02 23.711 0.271 1 o.ool 0.251 34.011 1 99.63 99.91 CCMO or>- IqO CqM 00 6.34 0.231 99.60 80*66 000 000 33.951 6.861 21.691 0.35|1 1 99.081 ^qr CM 34.16 6.971 1.9611 1 0.2611 0.32]! 34.3711 I | 1 000 33.1BI 10.731 1.64 18.4611 0.291 0.461 34.3411 99.09 ||0C0 |180*0 35.211 0 .73] > 28.7211 33.5711 99.05 SC*66 | ||G0*0 1100 1 35.20| 0.22 1 0.32 30.1711 33.381 0SEC |600 |C0 'O S90 0.171 29.3811 I 1 99.07 |900 1 090 6s o qO i<nn o 34.75 CM CM) rr** O ____ 0.25 0.05 coo ISO-O [SS0C 29.741 0.04 1 ____6.03 ____ o'05 33.31 1 33.421 ___ 33.45 99.25 99.791 _ 1 0 0 1B1 (A OC 15 0 00*0 00*0 00*0 20*0 00*0 ooc 06*2 | W H- I 1 0 .3 4 ! [ 0.11 81*0 | ICO 1 2.80| i 1 l 0.201 2.84|1 0.431 1 00 0 1 |100'0 100 0 10 0 0 I OS'l | OCM q dd < w 0 * qCM *Or dd 9 LL 1 1.49 1 1.37 1 CqM q d ri qCM -q* d ci <3 1 0.05 1 0.04 SO G 90G ITotals 1 7.96 1 7.97 1 7.96 1 7.95 I 1Notes: all Fe expressed as Fe203 INotos: cations calculated on basis ol 12 oxvoens q0 CM d 20 0 |20 0 | 100 0 q dd 2.86 1____ 2.77 1 1 0.671 0.19|1 0.16'1 100*0 80G 1.32 1 1 1 7.95 1 1.16 0.02 0.06 1 3.07 7.90 1 000 iqn CD q dd 2.9911 0.04 1 1 0.04 1 3.06 8 .0 2 I1 00*0 1 0 0 0 10 0 0 1 20 0 | 000 1 COM x0r dn 2.99 1 1 1.93] 8 .0 2 I 90'C 10*0 10*0 1 j | | 1 1 11 11 1 1 1 CqJ Cl qe 0q dd O 0q dd 2.99 1 | 0 .0 2 !1 ! 2.96 0.04 1 0.02 1.91 1 3.04 ____ 8.02 j ___ 2.98 ____ a q i 0.02 ____ 9.60 1.95 ____1.98 ____ 0.00 ____0.01 3.03 ____3.01 8.01 8.01 Tablo 5.1 Folio Table S.1 1 I98 . <tJ i ocoioior-iInViI , , r* .a . cc)P * 2c0o!l i r*. o Z9'66 900 ][G00 9G0 | 8GSG S00 | 600 9GSG i irt I 0 to d d a 1 Number > 1 85 ] A a E reaCO 688-7 1 1i 688-7 1 r^ CO 0 CO to to 0 CD 1 B9 I1 1 688-7 1 to CO 90 0 I1___ 9J___ 1 1 688-7 I to 0 CO <CJM> 1 93 94I! 1 [ 680-7 11 608-7 1 95 r*. 0 0 to 96 V CO CM CM O oin O CM O 80 8s u. IAI203 Totals nn d d d 1 0.071I i1 0.17] 0.11 ! ! 1 33.351\ 89.141 rp** 0 in to CO to co n OO d0 34.76 0 .0 2 I! ! 30.221| o.osl 1 0.11 33.541 900 1 | O CM d 34.49 0.051 1 0.23 30.60|| o.osl | 99.151 V CO 900 |000 | |900 1 0.15|1 0.17 30.37| I 33.43|| 99.18 | |690C 34.54 0.031 0.18]1 0.19 1 0.021 0.11 33.42|1 99.18 | 35.23 35.52 110'P____ |69'0 ! O0.1411 ^ ll 1 1 0.52 28.8311 30.15 1 600 000 900 00 1 33.7011 33.32!| 99.20 99.38 O d 0O d 0.0311 0.1911 30.10 1 33.55 1 99.41 0.251 0.16!I 29.97!1 33.42j 35.41 0.20 0.12 30.19| 33.15 99.64 too 1 I[00 0 161 1zoo |[zoo 00G SO'G I 10 0 0 I {0 0 0 00C GOG [10 0 0 100 [ZOO |000 too [000 1 too 1 | 00G tO I [10 0 0 |1Z 0 0 CO'Q |10 0 0 too 1 [000 | [too |00'0 1 1 t-- I110 0' 11000 100 SO'G 1 I000 [zoo COM OCM d0 0 d < Cations 2.96 2.971 1 o.ool 1 o.ool 1 0 .0 2 I 1.97 1.95 00 OCM dd 2.95 1 0.02 1.97 2.98 0 .02I 1.95 2.95 1.98 0p 0 db iOt CD d sr* 0.03 3.01 1.93 0.01 1.92 0.02 1.92 O0 dd w 0 O u <3 Totals 1 B.03 3.07 l 3.07 8.04] 0 O0 d ci *O 0 0O d0 3.06 | 3.07 8 .0 1 1! 1 8 .0 1 1 0.01 1 I 3.03 8.011 3.01 8.02 iNotos: all Fe expressed as Fe203 INotos: cal ions calculated on basis ol 12 oxvoens 1 |90'0 |000 100 |Z00 [too 00 C (000 ||6 0 0 1 i 11 1 1 11 I rr nO 0 ** d 00 0 00 0 000 100 900 |000 0 CO -- a w G ii i!111i 35.49 _____0.05 0.25 ____ 0.20 0.12 0.04 33.43 99.70 3.01 0.02 1.94 ____ 0.00 _____ PJJ1 3.03 _____ 3.04 _____ 8.03 1 Gn bi1 bl:bliaoijl| ;! ic\' c' Cit oj - Table 5.1 Folio 8 CCvJ>1I O--; COC Table 5.1 801 /O t l J CO (CG0OD 1 1 !CC c\ <B CCMM *-- 01 0 ex} cvi,ni*-;'qi di 01 V cii 1 in. esf m c> r-- on -- s 10 eoini^ 0. v 0|0O'cn cvi d; ol d.' --1 d 1 d| dj r-.\ ,I. j , I LOO I( |S0SC | ; , J 60X soo to o 000 SO'O (10*0 (000 |60'0 10 8 sox Ijooo |000 1 | 80 0 |U '0 |10 0 0 |1Z 0 0 ||00 0 |S0X cox 10 0 1 |10 0 0 ]11 0 0 j toe [1too [10 0 0 11 0 0 |Z00 000 Z0'8 1 10 0 0 1too 1 000 1 [10 0 0 |100 1 |Z00 |10 0 0 000 10 0 0 1too zoo 1 11000 00X coo I 00 c 1 1 i 1 ;! 1 1 i i : ! 1 1 1 1 ! i <q d CtCoOD CM od Number > 97 ISample > 688-7 98 99 1 100 102 103 104 105 688-7 688-7 688-7 688-7 688-7 688-8 106 668-8 6BB-B CO q dCO CM CCCOOO 0 q d u0COM. s 1 ISi02 T102 AI203 |Cr203 l&O Totals CO COO OCX 100 I 680 jooo 1 |0E*O |000 1 | I|0 0 0 I|Z 0 0 j cox CO CO <r d n 11 35.60l 1 35.431I 35.19| 1 35.421| 35.7311 36.871I 35.13 33.46 ! 0.231 jOZ'O I 0.021 ! 0.1911 0.231 1 1.221I 6.33| 7.071 1 0.14| 1 0.4111 0.22!1 0.9211 1.24| O CCOD d CCMM |Z000l ISE'CE SOO |S00 [SOOOl 1 | 900 1 1 ||0 0 0 zoo 11000 I CM CO 0 O d CdO d d | I 0.151 1 0.1211 0.1711 0.13 29.64 30.44 31.19 30.84 29.79 28.95 23.11 1 0.031 0.20 0 0.77 0.14 32.95 1 33.571 1 33.3911 33.301 0.37 1 33.561 1 32.7811 33.6011 34.011 0.36 34.27 q0 d9 CM CO dO) 1 99.761 COM d0 T+ q O I 99.50 I 99.01 !1 99.031 IE 0 33.95 6.38 _____ 6JI6 1.85 I f if l 0.05 22.13 0.19 34.18 99.04 1 '1 5O' 1W d Od Gc c G c G Ctl a f- a G n r* CO b- < 0 u0 5 <3 (Cations 1 o.ool 1 0 .0 2 I 2.98 2.97 2.99 3.01 2.97 2.87 CqM CM 2.85 O d OO 0 dd 0.45 0.40 COM 1 0.04I COM d 0.12 0.1 B q OCM dd CqD (Totals 0.00 1 0.10 2.98 8.02 1.94 1 1.971 1 1 0.02 3.03 8.02 4" 0 1.99 1 3.02 1.961l 3.02 q d 1.9311 1.861 2.91 7.99 1.84 0.04 1.46 3.07 7.95 1.42 7.96 .! i| j! Notes: all Fe expressed as Fe203 iNolos: cations calculated on basis ol 12 oxvaens 1 1 Table 5.1 Folio 9 __ .120 9S9-6SA ___ 34.28 ___ 4 .7 0 ____ 333 0.02 ____ 0.01 16.29 ___ \ \ . J 4 ____0JD8 O Ifi 35.30 99.01 Cl oT ccoo' CD IT i i)ii ro'*. noi j co; ooil 1i , j I I , to. c ieg|lrc nco* oo nr.< OI 6,or o'i C\J.-- Cl O;-- 0| o'1oi (O 1 | ; ii i iii,,, 1 i! ! l ! CG fv rC^D ueg: am oa a*- C cc\ O*J* C0O) 1 | OQ eg o a o V" o Q n N VS9-686 eg CCOO O o CO M CO CO CO d d o O) eg qeg d tqn o oo d CO 1 19 o CqO d oi q co' r^* 1 34.81 99.01 18.57 34.98 99.01 34.31 33.48 33.53 688-8 I1 688-8 11989-65AI 989-65A eegg in o og>; GO h* CD co d d d o fo*> tn q oq CO o eg o eg tqn oi o d o -* d o CO r^' 34.361 99.621 34.2811 99.5411 22.671! 24.01 ! 23.40 eg oq CO o Cl CoO CO in d d d rq-* qco co qo rq* o O4T CO o q eg d d o o o co (Totals i (Notes: cations calculated on basis of 12 oxvqens 1 Tablo 5.1 Folio 10 Table 5.1 w ** CO r-- CCOD CD co 33.79 q oq o Cl tn o o o CoO> mo Oo qCO qo o* o CD o fs. coo od ceog rc*o. od n Oeg) dd 34.431| 99.4711 cCoD ceog C*O o o eqg o q CO o qGO eg d d d o d co cqo e OI o oq cqo eg o in o <31 o qto oi d o d o d co r^* q CD CO co o o q o o qCD q oi o d d d d co 34.371! 99.4411 E 22.781! 33.96 1 I Number -> 1 688-8 1 688-8 I 688-8 I 688-6 I 688-8 | O) Uo) o> mo C1O^ eegg tcno cno o eegg o d co d 23.801 33.62 GhO* CO CO o in o ecog oeg od CO 34.221 22.511 ton *r ehg* coo g- ecog o Vco <> d d d 34.371 1 33.771 CeOg Oql ** o oeg CO o CO d o o dO) 22.53! 1 i A a a e C9O O04 eg O CO OCe9Og s s U. 34.211 1 99.301 1 99.171 qoi tn * o eg d oo d q eg o d o d GoO co cqo CqO 0CO* co o o q o o OoI q oi d d d o o co (qA o q r* o eg q o *o<r rq* cqo eg d o d o d co* q 5oT oo eg q o o OqI 01 oi o d d d o co c/5 t-- < Ofcv 9 LL. 5 <3 CO U0e9g. 3 1 V9I &K9i U9. ffl /) 1 zo Totals (Cations 1AI203 ]Cr203 Table 5.1 ! I i i 1 i Is ); c \ io\ . . qCO;' ^-r. cnT*v* 1 Number * > 1 121 1 122 ]1 123 11 124 11 125 1! 126 I1 127 11 128 12911 I 130 131 Sample > 989-65A 9B9-65A 989-65A 989-65A 989-65A 989-65A 9B9-65A 989-65A 989-65A 989-65A 989-65A .< O*?l CCOD |C9'GG j Si02 ITI02 AI203 1 34.141 1 3.671 to 0 r*. ZOO CO o O o LL. cn d CO OCM s m! _____ 1 0.14I1 0.171 Oto d CO 3 ITolals ! 99.081 |C i'0 1 GOO 0 0 0 CM dd 0 I 7.771 4.75] 1 17.36 0 .1 6 1 1 35.32 1 34.191 3.70 3.66] 22.53 0.12Il 34.81 99.17| CO d <d0 dCO CO q | 000 CM 00 1 3.46 1 22.04 0.1211 34.77 I 90'SG [600 |LZL CM d 35.2111 1.94 3.96] 23.36 0 . 1 1 11 | 34.53 9 9 . 2 1 1| CO o OCM a cm 33.6711 1 19.60 0.12Ii 0.23 1 99.26]| | 100 35.2111 1.05 4.2611 23.91 0.12I 34.68 99.32|1 |C10 ____0 1 CoO o cVo qV CO cn 35.1511 4.171 0.02 23.23 l 99.43]1 34.911 1 3.10 22.53 0.10] 34.77 99.44I 34.641 2.68 3.75 23.52 0.13 34.59 99.47 00 0 coo 1~ a d ro* o (*fi * 35.34 1.78 ____ 2.27 _____3.99 ____ 0,01 23.0B __ 23.29 0.10 _____0.13 ___ (M l 34.93 id an !* CO : Of). 1 CMi c 0| oi - o'jo o'coj j | '1 1 ! i in 1 C^l T * ^5 C 1 GCOli CM o G a a CO 0 0O d dd CO < j|1!|'' ' i 1 1 1 1M ! w 0 O LL. Cations 2.83 2.78 2.86 2.90 2.93 2.B0 2.94 2.92 2.90 |60'C 100 000 | ] [ s i 000 [e tc GOO 000 I 100 000 ]6 C 0 CM dd in 0 dd CD d CM d d ddd OO'O 1 0.23 ^I 0.36| _____0 6] 0.00 |9G'0 | OO'O OO'O 0CO0 0.22 | 0.12 I 0.32]1 0.07 0.42] 5 Totals 1 1.2211 1.08 I1.4211 1.381 1.47 1 23| 1 1.45] 1100 too too 0.01 0.01 0.00 0.02 0.02 0.01 COM 0.02 1 3.0911 3.1311 3.121 7.95 7.96 CM O d 0 q O CqM d 1 3.081 1 7.97 3.10] 8.04 1 3.10I 8.01 too 16 CO 0q dd 2.89 0.17 0.37 I60C zoo 10 0 000 0.00 8 03 1 l CO qCM 1 cb 1 <3s 1 qCM d 11 1 |Notos: all Fo expressed as Fo203 INotos: cations calculalod on basis ot 12 oxvaens 1 1 ' Tablo 5.1 Folio 11 nCDi oOjl JvOlI -- 01 O--i tpoi, O 01 Table 5.1 i.. 1 i<r 1 !2 a 1 i |8 , |S* .. P4*j >1 . CM' Or-' I<XX5?< |O-J-ji || iCvli!<r"-|T|OIVIO|-O-lOto O--, j : too to o |io-o to o j j! t o o _________ \1 2 2 _____ |1 TM t o o _____ |Z0'0 1 t 1 l o o |11TM_____ ito 'o _ |Ito o I|to o lo oliTM | j oil o'l d) dj -'i o'i b, n: 'i , ; 11 1IIj 1| i; 1i !i l1 1 !: 1| i` ;i ro* C0O0 CoM d d CD o CO o o* d co 100 | to o 1to o 11t o |600 1 ooe |1Gt*0 1 to o | |C0'0 l1OO'O IfO ]|e o o |ISO'O ||00'0 [SSO C9SC <m r03r to CO O) oC>D o o 04 I Number > | 133 1 I1 134 13511 11 136 11 137 11 138 13911 1 Sample 989-65A 989-65A 989-65A 989-65A 989-65A 989-65A COD) O) <mcp 9B9-65A VS9-606 142 I oro I I Si02 Ti02 IA1203 34.89 1 2.371 3.691 35.42 1_____ L27l1 4.44 35.15 1.9811 4.37 35.25 2.1911 3.90 34.84 3.7211 3.74 34.81 2.651 35.12 1 3.91 35.81 1.19] 1 4.25' 2.341 O o CD V CM O Oi d CO CM CO O CO o CM CM s uo. w io 86 66 0 1 0 18 0 0 | CoD fCsO d CVO o CO 1-- < o U. |CaO Totals Cations (A 6 _________ 23.56 23.32 1 0.121 o o' 23.40 22.38 1 0.10 i24.63 23.86 23.68 1 0.09! 1 o.i ! 1 0.111 1_____ 0,13| 0rn0 VCO 1 34.8811 34.5511 34.8111 34.6911 34.7311 34.8711 34.46!1 34.751 99.57 1 99.671 1 99.7311 99.7411 99.7411 99.7911 99.821| 99.86{ 99.92 2.91 1 1.471 OO'O 8C0 9C0 coo s io 2.96 1 000 2.94 0.08|1 1.471 000 2.91 2.92 0.12 0.14 0.43 L . 1-46|1 000 |00'0 2.89 0.23 j 0.37 1.40| [SSI 1 OO'O 2.90 0.17 0.29 1 OO'O 2.91 1.49] 2.96 0.41 1.47 000 00 0 23.28 2.94 1.45 ts to zoo CIO 09'S6 VS9*686 C) GC!> c<r>|i ! G^w*!* Ca\1. e\' VG rO>' acr a CO cCnT co 1i r1 in * 35.27 2.22 3.68 00 0 ____ 0.02 23.79 ' 2.1 2fl 0.12 C C o :ror> j i f^a-); c*-) c> 1 r cc**-!i 11 fCsO 1 o osr in o oo q d o CO O o COM o o ci O & CoM o co to <3 |80'C ||60'C | raci>i COJJ cc) Totals 1 8.03 1 3.061 8.031 |C0'8 ] 1 3.081 1 8.02] Notos: all Fe expressed as Fo203l INotos: calions calculated on basis ol 12 oxvaens | | 8.0 o] ! 3.08] B.0l| Z0'8 | 3.07 8.00 1 Tablo 5.1 Folio 12 Table 5.1 l < :i '' 11 IT & :r '1 a> irf* cn- 1 vni c1 =c=>'>>!|lC^tcCtc>)> CM O 3 IcT)>! c> C>< cv c> Vnn r r OCMrI 0)1 ' ' i ' nO. 1 o'l 1i 1 U 1 1 i1 S6SG CfNM. V rv xn CM d ro> : ui G 00 O C<Oe d Number > 145 146 I Sample > 989-65A 9B9-65A 989-65A 9B9-65A 990-1 150 ! 1 152 153 990-1 990-1 990-1 1 990-1 154 GT-A 1 155 1 GT-A CcvJ 0~1 O CM s s uO. TI02 AI203 |Cr203 Totals Cations CO |90`66 | 900 600 zoo 800 |IC 0 I1600 |1C10 1 | COO CM V CM CM O Vo 35.55 1 2.041 1 1 23.45 _____ I 35.38 1.6211 o .o il 24.04 0.121 I I000 35.67 0.711 _____ 4.06 4.02 24.76 23.89 CD 0) CO d CO 34.69 1 34.6411 34.821! 100.15 1 100.191 1 100.32)1 100.3B |89*S | 35.471 1 10.49 35.26 1.88 2.24 COO 180 0 11.17 1 25.11 L____ 0.14 1 0.21 35.801 1 34.251I 1 99.13)1 35.01 1 1 1 25.53 0.12 0.20 34.39'I 99.18)1 oo CM od 35.52 2.521 2.51 0.02 24.30 34.46) 99.64) CO CD CM O ooV CO O GO oin CO cm CM d 35.37 24.78 100.04 910 U 0 000 2.93 1 0.13 0.41 1 1-46 |00 0 [10 0 0 I 2.921____ 1,97 1 [6C'0 1 0.41! 1 1.49 1 1.54| 2.9411 2.851| 0.34 [660 | 0.391 1 M1_____ i l 1 0.6711 0 0 0 [10 0 0 1 2.96)1 0.12 0.22)1 1.591 1 2.95] 0.11 0.21) 1.62) 1.52 CD CM O CD cn o O) CM O CM o d |000 2.94 0.16) 1.55 000 95 0 | 34.90 1.74 1.83 25.62! 0.32 34.07 1 99.04 CO5i CO cn 580 oO 35.35 2.47 26.261 0.22' IGT 2.941 0.11 0.04 1.63 NC M` C) C> c C\l c cc c OJ O G o Q a cn C 03 Oo Oo _____ to o |[to o [to o 1 1 [to o I[to o O o CoM o d od o oo O ddO Oo o co CO h- < a 5 c3 CM O |20'0 | [G00 j|C00 1 100 oo o r1 IT> cvf c c c Totals 3.07 001 3.07 1 8.02 3.06 3.07 8.01 3.Q8 ! 7.971 q CD 3.08 01C con CD I 0.02 1 0.04 qCD oO d CD 1 3.06 3.08 B.00 1 8.02 1 INotos: all Fe expressed os Fe203l INoles: calions calculated on basis of 12 oxyqens Table 5.1 Folio 13 nO' tpj-,i oO' tQr?| o'l ! O' 1 o'l i i ci. c3 C>' cV C5 c) r C3 r3 c3, C a! , j- i 1i 2 .B 9 I l_______0 . 2 7 | 2 4 .0 8 1 I 9 9 -1 0 0 % 1 3 4 .4 7 1 |C 0 '0 |0 5 '6 6 <r- ui mc c 35 o H o in co in *-- V CO o r- cm <r* d o CO 0 0 o CM CM r-- o in o d d d r" d 0 op co r* 1 L Tablo 5.1 Folio 14 cm o CO o m CO K o oo CD H o CinJ < o CO < IA h o rin-* < h o A A s 2E3 0 a E C6O3 r*. p m CO CO *1 co o ai CM CO p CM a> in d d d OV o o CO d CM CO 03 CO CO o n* co CO o co CM rv CO CO ro-- 0 p CO cvi CM d in CM d d d co 01 o> CO CD in co oo IO * r*. cn r- V co' CO d in CM o d CO CO cdn CO o*r C'MM- CoM T" *o* coo CM CD 03 in CO o d d oco d d CO CO d 03 VO) ion o CO coo fo*. CM CM CO O) CCOO in d d d CO d CO CM CO 03 Oo) CO 03 o CM oo p ion co CM op n*r in o o d d d d CO d CO CM CO O) 8 cm O iti CM o H CO o CM < co O CwM o CO O uC0M. CO a ho- o> <0 co o in o O' poo opoppO CM o o o o d co 0 CM O CM oo i0n3 P o*T o0 O CM d d d d d CO 0 CpM o CM co oo CM p o 0o ro^ O CM o d d d d CO 0 < O oo p oo p oo oo o * O CO d d o d d CO 0 0 p ro^ 0 oo pCO oo oco ion O CM d d d d d co 0 0 p 0o coo oo in 0 oo o ro** CM O CM o d d d d co 0 a c <0 O "5 o K < o U0. 5s3 a o to ca> oX CrM *3 0 o C0 CU0M- oc 8 T9 IOA a X o 0u CO u- co *5 8 to CO Zo zo 1 * 5 00> C0D1 CD 6 m m o fm m m >m *<em c 0W o 0 2 * 04 z 0c *so < e* IO 5 * r> o* O5cv!) 1 C> C**"* ^a> r*' j 00 0 G^OT\ tC>"> 1 -'!<=' 1 j c1^\-;I ic->11|1 Cr^-> i Cxv1 , rOt rc*> acc>) 1 000 Oa4> ^0*r 0*-c> cc>> r^r> c*> rc)) c) aMrt c\J c> C> c> *": c> c> r> r- 1 1 11 C IUT1 ICTO 6 1 8ll a in *cc Number -> Sample - > 1 92453 I1 92453 11 92453 I1 92453 |1 92453 |! 92453 I1 92453 |1 92453 i1 92453 o ion 004) o> CD - 92453 CO 0CM CO 0CM |Si02 JTI02 IAI203 Totals eoo 600 600 |9B'6E | n. OCO dCO CO 000 600 |68'0 | 160 200 ]65 8 |' m O 0p0 CO o' CO C^Or m iOf CD* 1 0 , 1 5 ] 1 8 . 11 33.711 1 1 8.59] 1 33.65! 1 33.7811 33.6111 33.90 1 33.97]I 0| | 6.02]I 33.49] 9.08] L1 4.42]1 3.80] 4.7 9 I 1 4.31 j 1 4.77| I 3.44] C0O d CCDD 0C0M COO) 0.01 1 16.7911 16.971 1 16.111| 1 0.21 0.23 16.48]I 0.20 16.46]I 0.20 16.59| 0.23 1 23.22] 0.22 CO in d CO | 0.25 0.27 0.221 0.21 i 0.29 34.66 I 34.8411 34.77] 1 34.69]1 34.7 9 I 1 98.011 1 9B.19I 1 98.231 I 98.33]I 98.36|I 0p CoO* 0.24 1 98.48| |S5'86 | 0.21 34.14] 1 98.57] 166*0 iinn CO rs, fs! d GO C^Mr 0* d 34.05 8.27! 4.741 0.02 16.32 34.82 98.63 n 0a1s at CO Nr- 33.49 0.18 0.38 0p 0 d in GVO dd TT d p d cn 1- < 0 L0L <3 Cations 1 2.791 1 2.811 1 2 .8 l|1 2.801 0.52 0.541 0.511 0.44 0.38 o.ool o.ool 1 1.06 1.06 1.01 ]1LOO 60'1 [10 0 0 I ZO'O 1 80`G 000 Totals 001 j 0.03 1 3.10 7.95 0.031! 7.93 0 .0 3 I1 3.10 7.94 0.0 3]I 3.10l 7.94 ] |100 0 6S0 2.811 0.54 0.42 2.79 ! 0.42 606 |to o |000 1 1.03] 1 0.04| 7.93 1.04 0.0 2 I I ,3.10 7.94 1 606 1 |Z0'0 ]|00'0 ES0 2 . 8 1 11 0.47 0.03] ] 7.93 |G00 ]jo o o 600 d d N. CD P -r~ 0 CvD; CCOO 2 .8 8 1f 3.10 1 1 1 |000 rin- p dd 2.791 0.34 1.09 0.05 3.08 7.93 1 2.81 0.46 7.93 80 6 600 t0'0 000 ISO a a r cv Cr>l 0 56 0.35 1.10 C0M ( Notes: all Fe expressed as Fe203l INotos: cations calculated on basis of 12 oxvaens G iGn rGv 01 1 1 | 1 1 Table 5 2 Folio Table 5.2 at r* r eo -- - *t- cm, o toi o. u'nvif'- Oj 0| *- mi toi *-| bi ml o> . bi bi o'i r> im O i.. 1____ CW| i | to o |Z9`0 . o m.f- o 01 - o o a 1 i co nl0:O!'ri0;O'r' ci | ` c\i| o'bio'--b, o-b. rvl I : ; I .: i I , 18 CO 0 CM uO. eo 0 CM 0 CO OCM < CM o h- CM o cn E cE<ns iTotals a i AA < V CCMi CCDD ID 100 000 |Z.S'CC o o* CM in 00 Ci <6 r 92453 1 92453 i 92453 at mto CM t*o 20 1 22 1 92453 1 92453 |1 92453 | 92453 | 688-4 1 686-4 I m CO CO 33.791 0 .0 2 I 16.681 .1 0 19I 0.24| 1 34.871 I1 98.761 aeos at CM |O8'06 | i4nT ttVo CO* d VCO 33.58 8.80| 3.221 17.791 0.32| 1 18.20| 0.15| 0.2B| 34.671! 98.82 |SB'86 | (0 OS 33.40| 3.48] 0.02 17.941 0.19|I 0.311 34.86]I | 33.571 9.021 8.85| iant o CD to o tv 1 0.13]! 0.30l E 34.76|| 98.87|| 17.9611 0.1411 0.34|1 34.65]1 98.87'1 C806 | 000 too zoo to rj 00 CO r* CP aost 1 17.7011 0.14|1 0.3211 34.79!1 98.92 33.521 9.031 3.5311 17.7511 0.1611 0.3711 34.55 1 1 32.511 32.321 11.371 11.891 0.491_____ 051 1 |SC0 1 C00 18.31 1 0.281 0.711 1 33.8411 97.59 1 CD* I 1 0 .7 7 I 33.71 i1 97.69| 1 OM O d fv O) o CCMM CaM> CD ci <0 CD o' to 15.99 0.22 34.77] 0c5 JO 75 0w Op CMD; CM d ZS0 1 1 2.79] | 0.55] I |SS`0 | 1 1 0.32 1 [SCO 1 <0 [10 0 0 10 0 0 I0 0 0 1 p -d 1 1.04 1 0.01 1 0.01 0 d 60C ] coo ] COO 1 5 s <3 ITotals 1 7 . 9 31 3.09 I 1 7.94 3.09 1 I 7.93 INotos: all Fe oxpressod as Fo203 iNolos: cations calculated on basis of 12 oxv at CM (c0aA ] 1 000 1 dd 0 ** 0 2.78 1 0.54 1 0.34 1 I 3.11 7.95 i CM ] 60C 100 000 1 ] p d 2.79!| 0.561 0.35 1 1.09 1 ] | 7.93 1 60C { 10 0 0 1 SS`0 2.78 1 1 0.34 1.12 0.01 0.04 1 7.93 1 1 11 0 0 10 0 0 |9 C 0 0 d 2.77!1 0.56 1 3.09 1 7.94 ] 1S0 0 too 10 0 0 seo to at * ti 2.78]1 0.56 1 1 1 3.07 I0 6 -L GOO 1 100 0 | 1 1 1 1 0 d CD p ci 2.7611 0.73 1 0.05 1 0.02 | 1 COO [000 ||S 0 0 2.74| 0.76 1.16] 3.06 7 .90! 1i !i nr;| 1 091!1 9 2.82 0.39 1.01 0.02 Ol CD Tablo 5.2 Folio 2 :, r 1 u)|. . tCoDlI o--il 0*-| OCMj- -- -Wy Prd-j CM| tMI ,1 1 O-JT; -T Cp3J Oo. PJ| --pi mOloc001 oUilli ; . 1 ii: i < i!i cecc,i .; moil dii Pliii o';ic--v'ii o', di10-w|'iOao) 'l I i i;;| 1 1i 1 i l1 i 11 i1 !ciiiiolc1 J o'l' m' o`1| Ol1 PJ r-'j i i ii !i !i 1i 1' 1 11 1 11 ! ' 1 .1 l i i1 < in co ffi <G0O 1 J rC>Vi Irs. r cfi a GCCC iGr v- C rcs> CM cN***' cr^>;* srO>*i CC\>1 aa* G1C/)I CNi cM> c OyaJ; c* cCc>!* rec)): G*-*j Ci cr12>) c> Ci au9J ^ 1 1 11 33.75 23.39 33.94 80 86 iCnM m oT CO O"Jj C0O0 inn GoO a a ain -- o cc a O G0C to b o o o co CM o o a G 0 CO rs 608-6 0.40 0.19 688-6 1 33.72 22.97 98.01 CM CCMD oCM CoM CGOO cCoO CO o o o d 99CC rs CD GoO oo rs G m0 rqs m0) cvi d d d d CO h- 1 CM co CO G09O oo is.' r* d 20.43 fT" GCMO COO) vr Cl o o CCOO OJ O) CD 'C CM d oo o COO 5 o 0 d rs 9co cqo rs 32.7211 33.631 11 688-6 !1 668-6 CCMO CcoO CoO o> iCnO b d dd CO 19.41 34.26| 97.83| Nrs. mOJ rs oo <e CM o 0 r* to Cl CM o d o o d CO is 1 33.611I 97.55!| 33.661I 11 688-6 in rC*O oo O GO co o in o d co' d d CO CM G0D icno rs o oo in o cqo 0q0 q CM d d d o d co 3.091 7.98 23.311 0.161 0.491 33.6711 97.5411 33.35!1 |1 688-6 CD ros. o Cl in o d CM 2.861 ceoo rs o do [000 oin o C0D od 1 0.021 2.90|1 23.4111 1 0.4911 33.3611 97.5311 1 5.381 0.76 !1 688-6 I|00`0 800 |88'CC | oCO CM CD d d CM icno tn CqD CqO o co* 900 i 1 0.361 o.ool 23.4511 1 33.7411 1 33.281 1 1 11 688-6 I0S`0 [000 iCnO <0 fs. co CM CD CD ros CinM 0 COD OJ CD CM in o d a> CM d d d 1 1 20.6411 33.901 1 97.381 33.091I | 1 II 688-6 CM CqM OO oO CO CO o d d CM COM CD C*--l o O CM CO 0 0 qOJ iqn CM o o d d d co* is.' 23.151! 33.48 1 97.22 1 33.761 1 09 V CO fs. oo tn GO V in in o d d o CM 2.90 0.35 0.00 roo OJ q d CD O d qCO eo* r* q Tablo 5 2 fo lio 3 Table 5.2 3.10 ITotals Notes: all Fe expressed as Fe203l INoles: cations calculated on basis of 12 oxvaens 688-6 1 1 Sample > I Number > Cations Ti02 IAI203 Cr203 ITotals CO 8 si sCM O i CO O CM s (JL to < o UL <3 Tablo 5.2 ISO 1 S00 [10 0 0 1100 1 OSCE ! __ IV 1 i11 1 .l i' l j !to i ! to i a 1 cCpO| CcoOfl OOi' lO/Tjf cuf Cr>>1 cVoj' rG*C> cdi *-i oj oi o' iH Mill CM oa o> o c`.i cVn) ccc 1 1 ; r~,i CmO , f*--*- O0'. 1 !1 CM.1 OI, O O.1 1-- ci 0 r>j f^i : 1 ! ;1 ! j III!:!! 00*0 6 9 ' CC o o o Vcn [Number *> I___ 37___ ___ 1 3B 3911 1 * 1 42 43 44I1 . 11 4 5j1 1 iSample > | 1 688-6 11 688-6 11 688-6 !1 688-6 11 688-6 11 688-6 11 688-6 |1 688-6 ! 688-6 688-6 CM CO CD b CCOO |Si02 TI02 AI203 1 cn o o uCOM. cn O m1 _______ 1! Totals ! 6.36 0.931 1 22.941 1 0.261 1 33.621 1 98.43 CO pCO b 32.33 11.23 0.021 1 18.371 1 0.271i 34.351 1 98.49 |89'0 | [860 | 32.35 33.79 33.46 12.19 5.23 9.12 1_____ L?0|1_____ L99J |00*0 1 1 o4T 0.131 CO CM b o Vcn 17.6311 23.631 1 CO 0.221 1 1 34.36| CM CM 0.361 ccnn 1 98.52 9B.63 98.68 |08EB | |9*0 | |00*0 CGOO o 33.79 5.47 1 1 24.04!1 0.25|1 0.4911 I 98.72 32.61 12.33 1.1511 0.021 17.3511 0.2711 0.711 34.31 j 98.75 33.67 33.91 6.29 5.34 0.7711_____1.25| 23.24!1___ 23.891 0.27!1 0.26 1 0.41 j 33.76! 98.78 98.82 6.70 22.12 98.85 CoMp 0CM0 CM o b cVn 33.25 1.84 19.43 0.241 0.36 34.33 98.86 1 rC^C 1 Ifff G--C cc CM O 0 G w' O O CJ OV cn vCnM COM bb COM 0 C0M Oto cWn b b b 03 COM OpJ bb 800 800 600 (Cations u h- OO OCM CM b 2.86 ____ 2^21 1 2.7211 2.8 6 ! | 1 0.411 1 0.711 1 0.7711 0.33!1 0.571 [ [000 00 bb 0.14 0.12 0.00 |SE'0 | 2.86 1 2 .7 3 I1 0.11 2.85 < jooo 1 I0 0 0 |[0 0 0 10 0 0 w 0 |ES'l | uO. 1 1.4611 1.161 1 1.5111 _ 1.211 1 1.091I 1.48 [00 0 |000 2.86 0.12 000 2.82 0.42 0.18 to o z o o z o o 10*0 z o o z o o zoo CCNNIjI GCM 5 |90'0 | 90*0 G [60'E <3 cGc> Tolals 0.05 1 3.07! 1 3.10| 60E | COD b 0 0 1 0.051 3 ji; 1 1 1 3.0811 .3.071 |B0* | 0.091 I 7.96 1 7.9111 7.90 1 7.99]1 7 . 9 2 11 7.98 1 1 3.08| 7-.97| 0 .0 5 I! 3.06 7.97 0.04 7.96 CN C INolos: all Fe expressed as Fe2Q3l INolos: calions calculated on basis ol 12 oxvnens 1 i 1 i 1 Tablo 5.2 Folio 4 ^ cmi 0c4) i O. O-j 1!i Crl | J` *i !! i1 !! Table 5.2 1 1 11 lc ) Ps | < 1' ca C0O3 . ; ocm-| cr.j.cJ)n;cOmfmcoi jtoo,irc**nj, ccav., gudi,i i ini o| oid| cm o, o'i pi odi H . .H ; 1 1 i !! 1 1 1 I ' j ; qoin0n1i v,jOoii vooi o0| no* ^oj ro- i I . p>| o 6i o`i --': oi O' <o'i ajl !i 1 i ji! 1! 1 1 1 1 ! i ! 1 i |S6'S | |S8*CC | 0V CCOO 0 CCOM CO d o> d CD tn r*> Ocn O Oo d d d GO O r- CO CO ca GO CO CD d m CO tCoO CD deo CD CO INumber > Sample 49 50 688-6 ___ 52___ 53 1 1 54 688-6 688-6 { 688-6 I 55 688-7 688-7 1 57 58 688-7 1 688-7 59 688-7 8 CO 0 00 OCM CM O IL |Si02 Ti02 AI203 1AO Totals 1 6.711 1.79 to CM CO to d 34.021 1 1 0.81 32.7211 11.2111 1.22 32.41 1 11.30 1.381 100 1 37.54 1 1 0.33 34.97 1 1.34] 0.341 37.68 0.41 900 900 000 OO'O OO'O 100 1 22.201 1 24.00]1 24.55 11 18.90] d 0.23| 0.271 |SS'8 l | | 0.21 COO 0.17 20.00]1 27.54]I 900 0.14 20.86| CD CM dd [ 0.2B I CO M* d _____o ^a !1 17.051 1 19.2 0 ] 06'G 34.16 1 33.6411 34.1511 34.6411 22.59 1 33.13] [06 06 |196'86 | 98.88 1 98.8811 98.931 I 97.89]1 97.971 10 0 |1000 1 CCOO* d od V n CVO* GOO CnO 29.70 98.05 |e io | UCO TT 0 eoo -1 , zoo CM CD d CCOO 34.75 0.17 0.14 29.441 98.28 U NCC r*s- OC CCB\ a nCO OOCI o to no 0.46 O.0 7 ! 0.12 60 c 00 0 00 0 OOJI tcr ICT GC CC c rc G ' Ci OJ G O a T- 0 C n CC' *0 0 CA < u uO. 5 5 <3 ICalions iTotals OO' O CO 0 0 d dd 100 1 1 ZLZ 2.82 0.42 1.41 3.08 1 7.96 I OO'O | SCO C0M 0CO dd CO CD oi GO GO OCO oi d 1.53 3.0711 7.97 |90`0 |00'0 |80'0 0.34 1.56 0.02 3.0511 7.98 |90`0 zoo CM 00 dd 2.74 0.71 1 1 1.19 1 3.06| 7.90 d ]00'0 [1GOO 0.71 0.141 o.ool 3.04 1.22 OO'O 0.0511 1 7.921 2.061 .1.9611 8.33 000 jCO'O 3.00 1.76 3-041 l1i TM _____ |00'0 3.02 1.26 2.301 1.69 2.97 0.02 1.93 8.05 zoo OO'O 0 0 0 _____ |10'0m OO'O 0 e' OO d 2.99 0.02 1.91 0.03 8.04 Notes: all Fe expressed as Fe203 INoles: cations calculated on basis of 12 oxvaens 1 l 1 j i Table 5.2 Folio 5 i :, , : !! Table 5 2 !' i ~U; go ! i ' 1" |i j * ir1' mi * ( t| Q>i ml c il ^ i?> CC -',:c>1 c\1 c> 1 ( c\ 11 r u>1 V) c>lc)i a> c r ai r> a . 1 ; i i i ir Cl cc PI Oi a E a Z 2 CO CO E00 zoo E00 |0 0 0 1 zoo 000 000 000 000 A IS a m p le > 6 8 8 -7 CO 62 6 8 8 -7 63 6 8 8 -7 8g O u. 6 CM CM CM CM O o35 IA I2 0 3 MO T o ta ls OO I 09O E 610 |S 0 0 j 03 CM GO GO CM oO <0 1 3 4 .2 7 1 1 3 5 .1 7 1 1 1 .3 2 1 1 j1 0 .1 8 1 1 I 0 .0 3 1 0 .1 7 ]' CO d 0 .1 9 110 0 600G 2 8 .2 1 I 0 .0 3 ] IS 0 0 1 0 .0 7 ! 0 .3 4 ! 3 3 .4 4 1 1 3 3 .3 1 1 1 3 3 .3 2 1 1 I 9 8 .8 5 1 1 9 8 .9 1 1 ! |0 6 `8 6 | C a tio n s 21 2 .9 4 1 1 .9 9 ] 1 o CD 0 CM c CO O 0 CD 0 dd O (O < 0 <3u. {T o ta ls 0 .0 2 1 0 .0 1 1 1 1 .9 8 | 1100 I 1 1-94] 1 jo o o 1 10 0 0 I0 0 0 1 0 .0 1 ] 1 0 .0 4 |1 3 .0 8 1 3 .0 3 1 3 .0 5 1 1 ] 8 .0 5 1 8 .OO!I 8 .0 3 ]1 CO CO |0 6 '8 6 | COO |S C '0 CO 64 65 6 8 8 -7 6 8 8 -7 A 3 6 .5 8 ] 3 4 .7 1 11 3 5 .4 0 | 1 | | j S00 CM r*. Oo dd o o o 0 .2 7 1 ! 2 6 .6 2 i 0 .1 6 1 1 0 .1 1 3 0 .3 6 0 .3 7 1 2 8 .8 8 CM O o 5 .3 8 2 9 .9 9 1 9 8 .9 2 1 0 .2 3 1 3 3 .5 7 1 I || 6 6 ` 8 6 |s s e g 11 0 0 | 3 .0 5 1 2 .9 7 L 3 .0 1 0 .0 2 M1____ i l 1 .6 7 1 1______ 1 - 9 6 ] 1 1 1| 0 0 0 0 d 1.851 1 11 0 *0 10 0 0 1[ 0 0 * 0 0 .6 7 1 2 .6 8 1 1 8 .1 0 ]1 O O 1 3 .0 8 1 1 8 .0 4 ]1 . 0 .0 3 |1 3 .0 6 1 8 .0 2 |1 |s o *e | 69 99 CD CO CM oo CD o d CO CO CD r> -j* Ps. CO 0 o> CM d isi 1 u -i CO CO CO cq 67 68 6 8 8 -8 6 8 8 -8 1 3 3 .7 2 1 1 3 3 .7 7 1 CO <d n CO to | B E G 900 1 2 2 .3 7 1 1 9 8 .6 6 1 1 0E0 1 1 .7 2 1 0 .0 5 2 2 .4 9 0 .1 5 1 3 4 .1 9 1 I 0 8 .7 5 1 1 CD d 0 .0 4 2 2 .0 9 0 .3 2 3 4 .3 6 | 9 8 .7 7 1 1| 0 0 0 0 d O GO * dd 2 .8 5 1 .4 2 0 .0 3 1 | 7 .9 5 ] 1 0 -0 1( 0 0 0 1 0 .3 9 1 0 .1 7 3 .0 9 1 |0 0 0 1 2 .8 4 | 0 .4 0 0 .0 1 3 .1 0 7 .9 7 1 1 O d r** O 0 dd o CO ei CO CO GO CO CM CO id i - 000 610 |S I'0 c o o COO f 8 EC 70 6 .7 5 1 .8 7 2 1 .7 7 0 .2 8 3 4 .5 2 9 8 .7 0 2 .8 1 0 .4 3 1 .3 8 3 .1 2 7 .9 7 71 6 8 8 -8 6 .2 1 1 .7 3 2 2 .4 6 0 .1 0 0 .2 9 9 8 .9 2 *CO a a a cn N ______ 2 . 6 2 0 .1 7 0________ . 4 4 0________ . 1 5 0________ . 0 0 O CM * a G G G cr _______ 1 . 4 1 ________0 . 0 1 ________ 0 . 0 4 CC CC 3________ . 0 7 _________ 7.96 n O CM 0 ll 8 1 <A O a X O O Li. 75 0 0 Z j J ] IN o le s : c a lio n s c a lc u la te d o n b a s is o f 12 o x v a e n s Table 5,2 Folio Table 5.2 ii jj s 1! 1VS9*686 (VS9-686 CoM d d o CM d GO orr Ok rj CO CO O a E CO C/3 A I Number > 1 688-8 1 68B-BC 1 <U) CD (O Ook Ok O in GO GO GO CD + [VS9-6B6 CO <itno at CO Ok fs. IV59-686 <m o <0 GO 0k GO Ok CD 83 989-65A 91 89-65AI s CO O CM uO. CO O u CO O CM < CM o OJ O CO \ m _______ ^ 8 [Totals I1 |seo ; 6S'0 6.88| 15.621 0.061 22.461 1 0.221 34.11 I 98.9711 15.3311 t I 32.24!| 97.181I CD o f*. <0 CM CM do in CO 34.7b ! 32.541 8.971 9.751 |SB'0 4.94 | Izo o 0.611 17.631 14.79| 0.27| 3.0811____ 0 J4 ]1 31.97|1 35.241 98.1611 97.60!1 [910 [coo C*M in CO CM in Ok in CM CD Ok 33.051 1 0.12|1 1 97.8911 Ok o d 32.431 4.221 15.691 0.151 0.1511 35.2511 68.021 80'86 | [coo OSC CM 01 GO CD Ok CD CM cm CD CO 17.991 0.1311 0.20|1 34.9811 1 | leoo 33.23 3.41 17.621 0.17| 0.2311 34.7|1 98.3111 |sose | [e t' | in CO* ** 33.38 i 1 4.75 o.ool 0.13 1 35.20 98.30 1 o CM 0) CM o 33.591I 7 . 0 1 11 1 0.15| 1 98.461 CO Sl'O [EO'O CD o 33.891___ 33,36 6.331 _____7,29 4.03 _____3.65 _____0J)0 19.081 ___ 18.98 ____ 0.13 _____0.J7 34.87! ___ 35.01 98.54 ___ 98.59 (c0 P3 O CO 0 d GOD 0 o' d OCM GO dd CM 0 T* d 2.81 2.66 2.89 I 0.44 1 0.99 I 0.56 | 0.15 860 ZOO 10 0 0 I 90 0 | < 0 uO. <3 ITotals 1.10 0.02 1 0.02 1 0.38 1 3.08 1 2.92 1 2.85 1 1 7.96 1 7.82 1 7.93 1 |Notes: all Fe expressed as Fe203l lNotos: cations calculated on basis of 12 ox* CCOCD to o 1z o o 2.72 0.61 0.49 0.93 3.16 1 7.95 to o 1to o 1 q r>.` mCD CD dd 2.76 1 1 3.14 1 1 CqM d 2.71 0.64 0.42 3.16 7.95 66'0 |9 6 0 | |0 0 0 |100'0 10 0 0 I0 0 0 10 0 0 10 0 0 10 0 0 10 0 0 1 9S'0 | ZS0 1 ci c o o coo seo CO Ok i*n 2.77 1.14 0.01 1 1 O -d 2.78 1 0.34 1 3.11 I 7.94 1 0 d 2.78 0.52 0.47 1.04 0.02 3.14 1 7.96 I too 2.81 0.41 1.15 0.02 3.14 7.97 1 rr d 11 q d 00 dd |i: ; ; Miii' !i 2.83 _____2.79 0.46 1.20 0.021 .3 12! 7.97! Tablo 5 2 Folio 7 1co0/l OqS; 01- j q| comj< rr p0si.r ,1 ,. oj o| *- i d{ n n!. ! Table 5.2 VS9-686 96 1 56 j 3 eeg z </> 8 s CO O UCM. CO o CM 6 CM O |Si02 IAI203 ITotals CL 1 A A 85 I 86 1___ 87___ 1 88 | 89 | 90 | ___ 1 92___ I1 93 1 94 9B9-65AI 989-65A 91 89-65A] 989-65AI 989-65AI1089-65AI d0>0 O) <iCnO 0O0) <i<n0 9B9-65AI 989-65A 00 IoN. fCNO r> 04 CO < CM \CTOt ri COC7O1 04 * roi CO PN CO V CO 04 V CO* 33.70 1 33.15 33.77 1 33.41 1 32.73 pN PCMN to <0 8.201 t 6.971 1 10.021 oCO 600 100 I |S0'0 ||E 0 0 j| t o o pN O in o d C0O) O CO o I 3.451| 3.60| 0,021 1 0.03| 22.21 21.70 1 18.18 18.57 ! 22.90 18.85 21.34 15.61 800 0.11 0.12 0.14 0.18 0.17 0.13 00 0.13 0.13 1 0.21 0.17 0.14 0.14 0.131 00SE 34.76 I 34.81 34.97 34.66 34.99 34.91 34.85 II 9B.60| 1 98.7111 98.711 CINO* d 1 98.8411 98.871! 98.88!! 10 |S00 I 33.60 35.04 _____2.271 1 CCOO CO V 16.90 22.84 0.13 35.27 98.681! 0.10 34.48 98.941 PON COM V6 33.82 5.92 0.02 21.13 0.20 34.92 96 86 010 u o iCnO CM cn i o 1 < 0 1- 34.08 4.78 0.02 21.87 0 .1BI 34.76 1 1 IN 0C1O i ; j IoN. CJ 00 d (CA o ffl o OT 1 ootl | rec *- 1 cc*-!I0C9M !|rc-\ CD PCNM. CM d h- 1 1 0.34 2.88!1 0.2611 2.79|1 0.51 j1 0.35 2.7711 0 . 5 2 11 0.34 <1 0 000 1 [1OO'O 10 0 0 11000 I u. 1.371 1.17 1to o COO 111.0*0 1110*0 1 0 COM dd 5 s <3 (Totals 1 3.14 1 1_____ L02 0.02 3.13 1 8.00,1 0.02 3.13 1 3.1311 7.96 1_____L 9 6 1 [Notes: all Fe expressed as Fe203 iNolos: cations calculated on basis ol 12 oxyqens 2.89!1 0.1711 0.37 2.81 11 0.44|1 2.80|1 0.361| 0.32| 2.7011 0.62 j 0.52 1 [100*0 6E0 [100*0 ][OO'O \|0 0 0 60 1.45 1.35 0.97 I 1 0 0 _____ |1 1 0 0 I(to o 0 n d pC 0.02 0.02 0.02 3.1211____ 3 1? 1_____3 J 4 ]| 8.03!1 7.9711 s.ooi 0.02 3.08| 1 1 11 0 0 [ |000 2.78 0.43 1.05 3.13| 7.95I [60*E | 100 zoo 1 000 0.14! 0.40 8.01 n Ip O CVJ d liTM _____ 2.83! 0.37 0.2B 2.85 n nn cot 61 000 i1 j 1.33 0.02 3.13 7.99 _____a.oo Table 5.2 Folio 8 Table 5.2 ' 1 1 1901 1 66 * t 1 r^, ar- cCoOil 0"! Ncojl ro-j.ccjmjj a*- 1 cav . 2* Al ;SlS I^ j"l cii <\i| b| rij bi oi | ;~l 1 l" 1 ; !"! 1 ! i I 1 I 1 i CQGl 1I ,10^1 i' cc\ iicovjiio-'|OiinOi 0i1n01s0100 ,: . 1 | W a 0] o'i -- j o'l01cjj cd'I 1 ' 1 1 1 iI ! i j | 1 1 1 1! 1 i 1 1 OS'SC o CM OCM b o' cr> 1Number -> | 98 1 1 100 1 I 101 11 102 103 11 I1 104 I1 105 | ISample > 990-1 990-1 990-1 990-1 990-1 990-1 o<o31 990-1 990-1 990-1 107 990-1 eg O in 3 5 LOL w 0 < h- C/5 23 h uOCeM. 8 |Ti02 IAI203 |Cr203 CaO Cations Totals CO JO soo in o ne CO 36.761 36.30 36.41 33.69 34.70 34.56 34.23 1 34.95 |C0'0 ] 69'SC 800 |ero | sex |90'0 | SS SC' 60SE |60'0 |S10 j t*o o CoO o o' ba> at CM ob ob nMl o V CM | 2.131 1 4.191 1 3.70 ! 5.921 7.28 1.19 I 6.111 15.43 13.141I 13.72 9.67 9.55 o iT 2.66 9.87 9.60 1 0.171 oO o [ 0.13| ! 0.23|1 0.12 iCnD m 5.46 6.83 oCD 25.57 12.71 11.97 2.34 2.79 1 1.49 0.1311 0.25 0.261I 0.2911 0.08 0.271 0.08 0.32 OS'SC 35.78 34.34 35.69 34.35 35.47 35.63 1 97.0611 97.2911 97.8011 97.8211 97.9311 98.2511 98.3611 98.40]1 98.75] 35.07 3.41 2.86 22.93 34.21 98.81 600 ie x 610 900 2.92 2.90 2.90 2.85 2.82 2.83 2.79 2.91 2.87 ] 1 |00'0 1 |100'0 800 to o ]|Z0'0 |09 0 to o 1 [660 ] I |ZO0 1 | 1110*0 | 9G0 |to o |Z00 1 1110 0 |C8'0 |10X 1 I COX |81'0 CO C0O r- b 0 b iCnM b in 0 O bb 0.13 0.22 0.29 0.37 0.44 0.27 ] | 1 1.241___ ] 1.29]1 0.331 I 0.92 000 I100X t1c c o [ >0" |CC'0 ]1t o o I SOX I .1 1 1 1 o.ool 1 0.26] 1 0.281 1 0.411l ____M il 1 1 _____ 1 1-36]| 1 0.03| 1_____M 2 ] 1 0.70|1 0.72| ____M il 1 3.12| '3.Ill 1 ^l 0.27|1 0.94| --------0 I 0.04] ____ 3-13| 1 1 8.06]1 8.03| 1 7.99]1 7.99|1 7.96] 1 7.99| ||8 0 X |to o 1 |C0'0 to o 1 |60'C |to o 100 COM o' 2.83 0.37 0.92 0.73 1 7.97 CCNOi GG IT O O r- 2.94 0.21 001 1 0.02 3.07 7.98 iNoles: all Fe expressed as Fe203l (Notes: cations calculated on basis of 12 oxvoens 1 111 1 Tablo 5.2 Folio 9 Table 5.2 i u< 1 CM|>_ rlo ii O o' O d O to CO dd eg CM O 8CO u s e sese zese GO d eg CO d CJ CM 0 CO 0 - dd [Number > | 109 I 110 1 ISample --> 1 GT-A 1 GT-A GT-A 112 1I 113 1 114 1 115 11 116 I___ 117 1 GT-A GT-A GT-A 1 GT-A GT-A GT-A 119 GT-A ! GT-A |90 B8 |` CNI CM O dd |Ti02 I 33.60| 0.03 CO O CM < CO 0 CwM 0 CO 0 CM u9. s \ m ______ iTotals 1 I1 27.09 34.061I 97.091 |0 0 0 j|00 0 j 000 000 O CO 0 CD 0 dd Op d * cm 33.50 ! 27.56 0.53 33.8611 1 33.79 1 0.03 1.851 1 27.54 I 34.2611 97.651 34.55 2.78 I.IBl1 24.80 0 .0 2 I 0.47 34.2211 1 |10 0 1 2.05 3.2111 5.41 17.60 1 0.43 34.2411 98.2611 100 SCO 6 1 0 | 000 100 000 100 35.29 0.5511 0.051 33.601 1 98.4611 34.90 1.93 1.541i 26.121 33.6511 98.521I 35.00 1.70l 0.3711_____ L73|1 29.70 25.59 |S0 0 |1e o o 33.461| 98.59 | 0.37 33.90| 98.691 |98B6 | cm CO 0 <0 CO 2.35 1.691 0.32 34.17 10 8 60C 000 I 10'S I [00*0 |000 000 10 0 0 10 0 0 000 1 ooe 10 0 0 |10 0 0 1 ;1o o o 10 0 0 |C00 to e |60B 10 0 0 000 1 ||0 0 0 | |10 0 0 I 100 0 I |0Z0 ] 100 0 100 0 I soo |000 900 10 0 0 d CO CM nr ** 0 dd CM O CO d dd CO h- < 0 t9L. 5 s <3 ICalions 2.91 1 0 .22I frs**. 0 0 1G l:_____ je r o i 2.90 [ 1.80]1 2.92 0.19] 1.7911 |000 2.95 1 1 1.59 2.96 0.3211 0.36| 1 1 2.96 0.06|l_____QJ5|1 1.82]1 1.6711 2.98 0.041 1.91 1 .. 1.63| r* 0.15 0.171 0.14 0 0 CM 0 h* p Tolats CM 0 d 0.07 0.02 .1 3.16 1 3.14 1 3.17 1 3.13 1 3.08 1 3.07|1 3.06|1 3 .0 7 ! 1 8.09 O CD 1 8.02 1 8.01 1 8.0l|I 8.03] 0.05 so 0 000 00*0 35.01 1.98 2.90 4.46 19.63 25.47 _____0.04 0.40 ____ 0.34 34.40 . 34.14 98.87 ___ 98.90 _ .2.94 2.95 0.12 ____0.12 0.29 _____0.19 0 30 ____ 0.00 ____1.62 ___ 0.00 _____0.04 3.09 _____3.09 ____ _8.02 100 s ifi S6'i C\ CM O 1 00 1 1 <T) CD 0 CM d CO 0 CM 0 u. !3 1 td 9 & X 9 9 U. *5 Vi 9 O 2' |Notes: cations calculated on basis of 12 oxvaens Tablo 5.2 Folio 10 Table 5.2 |60'S6 | I a-19 | IC l 0V0 rC*O. 0CM3 Vo ci 2a z3 . GcoOi, fq*>* CvOi COMit r05 1 *T\ --`i oi 0*1 h* ! 1 .CM ill! 1 CO 1 < i cnn 00 0 K to no b io 0 6 ` 86 1910 |0Z0 I 1 ZOIS CM n AA 121 I 122 1 CO 124 125 C0 CM ! 127 129 GT-A 1 GT-A 1 GT-A 1 GT-B 1 GT-B 1! GT-B 1! GT-B 1! GT-B 1 GT-B 130 i 132 1 GT-B 1 a> O nn oCM O 1AI203 | CM 6 1 ICoLM \m 1 _______ 1 1 |CaO Totals 11 35.411 I 0.581 1 o.ool! 28.751 1 0.091I 33.721 1 35.271 1.551 2.321I 0.411 25.071I 0.041 0.22 34.061 88.93 | 66 86 100 |coo | |E0'0 od o IN. 35.421! 0.3111 29.451 I GEO 9 8 0 |0'0 | inC CO VT* CD o 34.67) 1 7.16|1 I 1 1 34.58]| 97.23 I [96`E 33.171! 1 3.3811 0.11 1 22.581! 0.19| 33.52]1 97.23 11 0 0 dd CCOO CO o 32.B91 8 . 6 1 1! 2.00|! 18.56|! 1 34.501[ 97.51 o oO dd cj 3.48| 1.5411 I I 2.17|1 SG0 IlOO'O I 24.451[ 23.2611 0.17| _____ 1 0.34 33.841I 97.93 34.0511 98.13 34.24| 3.9811 1.9111 23.731 1 0.201I 0.34 33.94]1 98.35 o o 2.0B I 2.151 24.341 0.171 0.45 34.07 n V 34.16 3.27 1.81 24.57 9G86 9G'86 6E0 9 f0 |000 I 100 o 0.02 0.35 98.41 I 60 E S00 1100 000 |06'Z |60'C 10-0 LSJ L i 000 CIO | ]|80'G | 000 000 1 110-0 000 |00 0 1 000 |SS'0 1110-0 100 1 I|t o o 000 000 ] |10 0 0 |1000 1 0 0 0 G00 000 |00'0 101-0 _ 110-0 1 10'G O -e 0 d 01 0CO q CqM 0q5 d dd d CO < O U.I 5 s <3 Cations 12^71 1 0.23 d nq 3.0211 1 2 7 9 12.90]1 2.831 1 1____ 0.251 1 0.71 0.34 0.20 2 . 9 1 11 0 .2 2 I1 0.15 2.92]1 0 .2 3 ]1 0.22 2.90 | 0 .2 5 ] 0.19 2.97j 0.21 0.21 0.18 2.9B 1 1.84]1 1.59]i109] Totals 1 0 .0 3 ] L 0.12I 1 3.07 1 3.07)1 3.08] 1 8.01 | 8.02]| 8.08] i INolos: alt Fe expressed as Fe203 iNotus: cations calculated on basis ol 12 oxvaens in O q q d oi d co fi*n* q q d cd t 1 i 1 1 i 1 1 1 1 0.11] 1 .3.1311 1 7.971 1 3.0911 OO d 0O dd 1.48]1 1 3.091 1 q d 1.51 1 0.0 4 I1 7.99| 1.57 ____ 176 o.oel ____ 0.00 __ 0.04 ____ 3.08 8.02 ___ 8.01 8.01 Table 5.2 Folio 11 1 Callons] 34.061 20.75] 97-98% 1 34.0l] ITotals [Notes: all Fe expressed as Fe203 (Notes: cations calculated on basis of 12 oxyqens Table 5 2 Folio 12 1Sample > 1 GT-B 1 GT-B 11 GT-B 11 GT-B 1 GT-B 11 GT-B 1 34.67] 34.541 34.68 34.26 34.95 35.14 cm d C> xe> 2 H CcOo CO CCOO mCt*O* CO CCOO 23.701 1 98.6111 CO GinO m in CM o mr-* 0 dd O <dn C0O3 CCOO CO cm q CCMO q qCD rq* 0q0 CM d d d r" d d co* 1 ! 0C\J G0O m0 6 CO d 0<r dd 26.4011 qCO CM d q CCOO dd 23.9911 tCnO CCMO 0q CO CM d co cq dd 24.131 1 CCOO Oin cqo CM CM d cV"o dd 25.5311 cI*o* iCnO 0q CM d dd CCOD qco 0 CO d CO icno dd 25.811 33.8911 98.64| 34.0411 1 34.1311 98.7911 33.6711 98.9111 34.421 98.96| |S9`86 | CO Cl 0 CO *" 0 q4J- 00 C0M mq CM d d d d d co d in Cl CM in 00 GCDO 00 mq iqn q CM d d d d d co d 0q1 cCMo CCMM 00 qCM 0 *q co 0 q CM d d d d d CO d CqM h* iCnM 0q CqO q m0 qGO CqM CM d d d d d co d CqO 00 co 00 cqo 0 m0 GqO 0 CM d d d d d co* d qCM CM 0q q0 q O rq*. 0q CM d d d d d co |96`CC | I |! 1 iTotals [Cations IAI2Q3 ICr203 A w 5 Ea Z CM CM OO C/5 CO 0uCeM. s 1 S < 0 0 LL 5 5 3  Oo o o t c n O m tm 2 z* in o *o "o5 *cmo ome i S, oEo ce 2 n in ah(0 | Group m CO CM o 600 600 610 000 SI02 33.63 35.43 35.10 34.40 33.70i 34.97 i 35.91 TI02 AI203 CoM 1 1.69 3.27 7.36 0.45 1.66 2.04 1.55 4.37 i3 .ie l 00 a O o CCOD COM o ddod n OCM n UON. t 1 8.11 0.04 27.40 29.21 25.25 24.26 20.89 21.40 7 .03! eoo 0.17 0.07 0.19| |0 E I SOO EGO OE'O 1o o 0.37 0.49 0.31 CbO ! 34.06 1 33.60 | 33.97 33.96 34.43 33.49 35.7 1 ! OS'86 Total 97.39 99.02 98.92 98.49 98.50 9 7 .7 4 ! c i ccn o Color light green honey yellow yellow-brown brown black emerald greon 1 1 1 [ 1 I Table 5.3 Chapter 6 Titanium Mobility in Metamorphic Systems Note: This chapter appeared in its entirety as an article in Chemical Geology, V. 110, pp. 233-249; 1993. For this reason the section numbering in this chapter differs from the other chapters._______________________________________________________________________ Chapter 6 * Titanium Mobility in Metamorphic Systems Abstract ................................................................... .......................................1 1. Introduction..................................................................................................... 1 1.1 Element mobility.............................................................................. 3 2. Experimental data and Ti concentrations in natural waters....................... 7 2.1 Measurements of Ti in natural waters.......................................7 2.2 Low Temperature Experimental Data............................................... 7 2.3 High Temperature Experimental Data....................................... 9 2.4 Comparing the effects of Temperature and Pressure....... ............16 2.5 Speciadon of Titanium in Aqueous Fluids.........................................16 3. Titanium mobility observed in low grade metamorphic rocks................... 17 3.1 Alteration of silicate minerals.............................................................17 3.2 New Idria, California......................................................................... 19 3.3 Siena Blanca, Texas......................................................... 7..............24 3.4 Adamello, Italy....................................... 25 3.5 Magnet Cove, Arkansas..................................................... ...26 3.6 The Alpine Clefts......................................................................28 4: Titanium mobility observed in high grade metamorphic rocks........................ 28 4.1 Monviso, France............................................................................... 29 4.2 Santa Catalina Island, California....................................................... 30 4.3 Tauem Window, Austria................................................. 31 5. Conclusions...................................................................................................32 Tables ........................................................................................................... 34 Chapter 6 - Titanium Mobility in Metamorphic Systems Abstract Titanium is a dispersed lithophile element commonly regarded as chemically immobile at submagmatic temperatures. However, a growing number of field studies demonstrate that Ti is somewhat mobile in certain environments. This review examines the current state of knowledge concerning the mobility of titanium during alteration and metamorphism of crustal rocks. Also considered are relevant Ti-oxide solubility data from experiments and determinations of Ti concentrations in natural waters. Where Ti mobility is well documented, its length scale is 10 m. However, Ti may move greater distances in deep cmstal shear zones. The solubility of Ti-oxides in geologic fluids is strongly temperature dependent. Under low to medium grade metamorphic conditions Ti mobility is limited by the low solubility ofTl-oxides in the associated fluids. Under eclogite conditions Tt mobility is probably limited not by Ti-oxide solubility (which reaches 1% (Til) but by restricted fluid flow in low permeability rocks. 1. Introduction Titanium, a dispersed lithophile element, is the ninth most abundant element in the earth's crust: estimated Ti02 concentration in the earth's crust is 0.9 wt. % (Mason & Moore, 1982); in the average shale 0.80 wt. % (Ronov et al., 1992). Oxides of Ti have low solubilites in aqueous fluids under most geologic conditions, and Tt is commonly regarded as chemically immobile at sub-magmatic temperatures, Lc. in sedimentary and metamorphic environments. 6-1 Titanium is incompatible in most igneous processes, with excess Ti usually incorporated in oxides or dtanite (sphene). However, when the activity of water is high, biopyriboles may form, containing up to 10 wt. % TiOj. In silica-deficient or peralkaline magmas, other Ti phases may form, including schorlomite garnet, perovskite, neptunite, zirconolite, and other rare Ti minerals. The term high field strength clement, or HFSE. has appeared in the petrologic literature in recent years as a shorthand notation for a group of chemical elements with broadly similar properties. Titanium is the most abundant member of this group. Elements frequently cited as members of the HFSE group arc Ti, Zr, Hf, Nb, Ta, plus occasionally Th. Early mentions of the term HFSE include Reagan & Meijer, (1984) and Stebbins ct al., (1984). An older, and much preferable, term for clement field strength is ionic potential, originally defined by Cartledge (1928) as the numeric ratio of ionic charge to ionic radius (in A). This preferable term not only has historical precedence, but avoids awkward questions ofjust where in the electric field of an ion its field strength is to be measured. Barth (1952) noted empirically that the numeric value of ionic potential divides the chemical elements into three groups, based on the solubilities of many of their compounds in water (1) Elements whose most frequently observed ions have an ionic potential less than 3 (c.g. Na*) have highly soluble oxides and hydroxides. (2) Those with an ionic potential between 3 and 12 (e.g. Ti4*) hydrolize easily in solution, and their hydroxides and oxides arc only sparingly soluble. The HFSE fall into this group. 6-2 (3) Elements whose most frequently observed ions have an ionic potential greater than 12 (e.g. P5*) form soluble oxyacids in solution. 1.1 Element mobility Element mobility directly addresses the question of the "circulation of the elements in nature", in the sense intended by Goldschmidt (1954). Furthermore, for the pctrologist, mobility directly addresses the question of open-system versus closed-system behavior. The definition of mobility used in this paper is this: An element is considered mobile if measurable changes in its concentration relative to a specified referenceframe occur during a particular process. A chemist might object that all elements are mobile to a degree, but our definition says that if mobility is too small to be measured then we will call the element immobile. Ordinarily one relies upon the concentrations of elements relative to each other in mass balance,calculations, and makes arguments that at least one element is immobile - and thereby constitutes a reference frame. Since certain elements are believed to be immobile during most petrologic processes, the concentrations of these elements are often used, without specific proof, as the basis of a reference frame for determining the mobility of other elements. Al, Ti, Zr, Hf, Cr, Ce4*, are often used in this sense. The use of one or more immobile elements as a reference frame lies at the heart of a number of useful methods for analyzing mass balance in petrologic processes. Examples include Pearce element ratios (Pearce, 1968), isocon diagrams (Grant, 1986), and the rock cell (Barth, 1952). 1.1.1 Length scales The length scale of titanium mobility in metamorphic environments is poorly understood. Sorting out what is known about this subject is best done by considering the characteristic length scale of such mobility. The strongest case for significant Ti mobility is the appearance ofTi-bcaring phases in a rock otherwise devoid of 7102. when these phases are limited to a 6-3 Figure 6.1 - Idealized cross section showing advection of Ti across boundary layer between Ti-rich Rock Type 1 (e.g.1.0% Ti02) and Ti-free Rock Type 2 (e.g. 0.0% Ti02). Segment AB is Type 1, with boundary at B. CD is Type 2. BC is boundary layer, with phase assemblage typical of Type 2, but with introduced Ti-bearing phases. The length scale of Ti transport is the distance BC. Ti concentration profile shown in lower portion of figure. 6-4 <v >> C3 -3 >> V* C3 T3 C zos GQ ea CO 00 O 1-4 <o H o co u <U >o '< I VO oVx 3 Wm*) UU boundary layer and where textural evidence shows advection across this boundary layer. Figure 6.1 illustrates this concept in an idealized manner. 1.1.2 Mobility in fluids Experimental and field studies to date have concentrated on aqueous fluids as the principal vehicle for titanium mobility. The Ti concentration in most geologic fluids is controlled by the solubility of Ti-oxides. Titanium mobility resulting from fluid transport during metamorphic and metasomatic processes involves the dissolution of solids, fluid transport, and precipitation. Each of these three stages is an important topic in its own right, but here the emphasis is on the net effects of these processes, rather than the details of their mechanisms. Whether intergranular fluids differ from free fluids in their ability to tranport titanium is unknown. 1.1.3 Solid stale diffusion - Solid state diffusion may be important as a transport mechanism for certain elements, but there is no evidence supporting diffusive transport of titanium beyond the dimensions of single crystals in the metamorphic environment. No studies known to this author have reported monomineralic zones of titanium-saturated minerals created by the sort of diffusive processes modeled by Thompson (1959), in which chemical potential gradients are the driving forces. Diffusion kinetics arc well discussed by Mueller (1966). Mueller concludes that lattice diffusion is ineffective as a vehicle for element mobility except on a very short length scale, although he suggests that diffusion in a differential stress field may contribute to the formation of gneissic banding. He also makes the simple observation that components with low solubilities in fluids arc likely to have limited mobility. It will be shown below that the converse of this statement is not necessarily true. 6-6 2. Experimental data and Ti concentrations in natural waters 2.1 Measurements of Ti in natural waters Numerous studies confirm the essential immobility of titanium during weathering and diagenesis (e.g. Piotrowsld et al., 1989, Patterson, 1971, Hoppe, 1941). The concentration of Ti in natural waters is very low, except under extremes of pH or at high P T conditions. The few recent determinations ofTt concentrations in natural waters at low temperatures are summarized in Table 6.1. These data show appreciable Ti concentrations (a few ppm) at pH 12 (Kranov, 1973) and pH < 2 (White et al., 1963), but low concentrations (ppb or lower) in the near-neutral pH range most often found in natural waters. The absence of studies reporting significant aqueous Tt in the near-neutral pH range in natural waters is consistent with low solubility of Ti02 in these fluids. No direct measurements ofTi concentration have been made in natural waters at high P-T conditions. 2.2 Low Temperature Experimental Data Experimental data for Ti02 solubility are sparse, and a number of experiments have been made under geologically unrealistic conditions. Table 62 summarizes the best available measurements from several studies; the data arc organized in terms of increasing temperature. Nabivanets & Lukachina (1964), in a series of experiments apparently never repeated, showed constant Ti02 solubility of about 0.1 ppm Ti02 between pH 4-7, at 18C and 1 bar, in 0.1 M perchlorate solutions. If these results were applicable in natural systems, then we would expect to see much higher concentrations of Ti in natural waters in the near-neutral pH range than shown in Table 6.1. Other experiments have explored the effect of pH on Ti02 solubility: Babko et al. (1962) and Liberti et al. (1963) demonstrated that Ti02 solubility at pH < 2,5 has a steep inverse relationship with pH, rising to 72 ppm Ti02 at pH 1.3. These results support the existence of a dipositive mononuclear 'll 6-7 hydroxyl complex in this pH range, as discussed below. Other experiments (e.g. Bright & Readey, 1987, and Barsukova ct al. 1979) have explored the effect of possible fluoride complexing on the solubility of Ti02. Their results support the idea that fluoride complexes (probably TiF62`) significantly enhance Ti02 solubility at high fluoride concentrations (0.5-2.1M F` or 1-4 wt. % fluoride). Extrapolation of these results to the concentrations of fluoride normally found in natural waters (1-10 ppm) suggests that TiO-> solubility may not be significantly enhanced by fluoride complexing in natural waters. At hydrothermal temperatures, Schuiling & Vink, (1967) reported 1-2 ppm dissolved Ti at 200-300C and 15-80 bars1. These investigators also showed that anatase is mote soluble than rutile, in keeping with its more positive AGf. Agapova et al., (1979) investigated the possible role of sulfate and carbonate complexes of Ti; neither of these complexing agents increased the solubility of rutile beyond the values of Schuiling and Vink under the same P T conditions. The same is true for chloride complexing; since TI concentrations do not increase with chloride concentration, we conclude that Ti-chloride complexes are not of major importance. Ayers & Watson (1993), in experiments discussed below, found an inverse relationship between chloride concentration and rutile solubility, probably due to the decrease in the activity of water. It should be pointed out that in all of these experiments, the surface properties of the solid phase and sample preparation methods of the materials used may have had a significant effect on the observed solubility. Bright and Ready (1987), for example, describe the surface properties of their materials in great detail, and conclude that the kinetics of dissolution are substantially controlled by surface reactions. The solubility of well 'Note the mistake in the literature concerning the Schuiling & Vink (1967) results: there is an apparent typographical error in a paper by Vasil'ev eLal. (1974), which recalculates the molality of the Schuiling & Vink data (133 and 137 ppm) as 2.78 x 10*8 and 338 x 10'8 mol/kg respectively, instead of the correct values of 2.78 x 10'5 and 3.28 x 10'5 mol/kg. This error has been propagated in the literature. The correct values are shown in Table 2. 6-8 crystallized rutile may be lower than that of amorphous starting materials produced in the laboratory, described as Ti02(activated) or Ti02(hydrous). These low temperature experimental data are shown in Figure 62a, together with natural waters data in Figure 6.2b to facilitate comparison. The regression lines from Figure 6.2a, defining a solubility curve, are redrawn in Figure 6.2b. In Figure 6.2b, the measurement of White (1S'63) at low pH is the only measurement which falls on the solubility curve for rutile. The data of Skiabal et al. (1992) and Orians et al. (1990), who report Ti concentrations in estuaries and the open ocean respectively, indicate that these waters are undersaturated with respect to rutile. 2.3 High Temperature Experimental Data The solubility of rutile in water increases by several orders of magnitude with increasing P and T. As shown in Table 6.2, Ayers & Watson (1991,1993) reported rutile solubility in water to be about 1% at 1100C from 10 to 21 kb. These experiments also showed increasing rutile solubility with temperature at a fixed pressure, and slightly decreasing solubility with pressure at a fixed temperature, in the interval 800-1200C and 10-30kb. In these experiments, adding C02, NaCl, and HF to the fluid each decreased rutile solubility. Schneider & Eggler (1987) found approximately 0.02 wt. % TiC^ dissolved in water at 750C and IS kb in their experiments with amphibole breakdown reactions. Phillipot and Sclverstone (1991) reported rutile daughter crystals in moderately saline fluid inclusions from eclogites, which were trapped at 550C and 10-11 kbar. A rough estimate of the concentration of Ti required to produce these daughter crystals is 0.05-0.5 wt. % Ti02 at the time of trapping. This estimate is generally consistent with the experimental data. Figure 6.3 shows rutile solubility data as a function of temperature. Solubility data at pH < 2.5 or high fluoride concentrations have been omitted. Figure 63 is a projection 6-9 Figure 6.2a - Total dissolved Ti vs pH from experiments at 1 bar, 18-25C. Data points and references listed in Table 6-2. 6-10 Figure 6.2a - Solubility o f T i Oxides co co in X a, eo c\j fl)f/|Oiu [|jJ 0o| Figure 6.2b - Ti concentrations in natural waters, 1-400 bars, 25-300C. Data points and references listed in Table 1. Ti02 solubility curve from Figure 6-2a shows poor fit to data. 6-12 Figure 6.2b - T i Concentrations in Natural Waters 2 B>/|OUJ (ijJ Oo| Figure 6.3 - Experimental Ti02 solubility in dilute, near neutral pH solutions as a function of temperature. See Table 6.2 for fluid chemistry and references. Apparent linear trend due to projection from P-T-X space onto T-X plane. Experimental pressures labelled. 6-14 0*/|oui til] 0o| Figure 6.3 - Solubility o f T i Oxides as a Function o f Temperature 200 <400 600 BOO 1000 1200 from P-T-X space onto the T-X plane. There is an essentially linear relationship between between T and log [Ti] in this projection. 2.4 Comparing the effects of Temperature and Pressure Consideration of Figure 6.3 raises the question of the relative importance of temperature versus pressure in explaining the large (3-4 orders of magnitude) increase in rutile solubility from low grade (greenschist) to high grade (eclogite) conditions. The fluid density in each of the experiments in Figure 6.3 varies between 0.7 and 1.2 g/cm3. We therefore suggest that the solubility of rutile in supercritical water is essentially temperature dependent, with the proviso that the fluid density remains high enough to show liquid-like, rather than vapor-like properties. The Ayers & Watson (1993) experiments showed an inverse relationship of rutile solubility and pressure at a constant temperature, making it difficult to argue that increasing pressure is the essential control on rutile solubility. If rutile solubility were essentially pressure dependent, then one would expect to encounter evidence of high rutile mobility under blueschist conditions. Unfortunately no experiments have reported rutile solubility data under blueschist conditions, but neither have any field studies of blucschists noted significant Ti mobility. Under high T, low P (volcanic) conditions,'in which the fluid phase has low density and vapor-like properties, the ability of water to transport dissolved substances is generally very low, and one would expect insignificant rutile solubility. 2.5 Speciation of Titanium in Aqueous Fluids Ti complexes in geologic fluids remain a subject of great dispute. Ti as a small, highly charged cation hydrolizes easily, and docs not appear as the Ti4+ion except possibly at extremely low pH. Below pH 2.5, titanium is probably present as a dipositive mononuclear complex, either Ti02+ or Ti(OH>22*, a conclusion supported by good 6-16 experimental evidence (Babko et al., 1962; Liberti et al., 1963). In the pH range 3-7, some experimental evidence (Nabivanets & Lukachina, 1964) supports the presence of a neutral complex. Baes & Mesmcr (1986) accepted this evidence and supposed that the neutral species was either Ti(OH)4 or TiO(OH)2. Schmets et al., (1966), however, proposed that above pH 5.6 a negatively charged complex HTi03* becomes dominant; if true this would enhance rutile solubility in strongly alkaline waters. Schmets et al. also described the phase T30(0H)2 as a white solid, not an aqueous species: this solid may be the substance referred to in the literature as TiOjfactivated) or TiC^fhydrous), which is known to have a higher solubility than well-crystallized rutile or anatase. Qearly, further experiments are needed. ' Although there is little agreement about titanium speciation in water, theoretical speciation calculations using the correlation methods of Helgeson et al. (1981) show promise. Recent calculations'using these methods (e.g. Willis et aL 1990) have proposed a variety of hydroxyl complexes (including some with trivalent titanium) at hydrothcnnal temperatures over a range of Hh-pH values, but experimental data confirming these predictions are not yet available. Most workers agree that if the fluoride activity is high in solution, then fluoride-species such as TiF$2- are present This means that high fluoride activity may be a sufficient, but not a necessary, condition for mobilizing "II in natural waters. As noted earlier, sulfate, carbonate, and chloride complexes appear to be of minor importance in geological fluids. In the sedimentary environment, organic complexes of Ti may be significant (e.g. Skrabal et al., 1992, Milnes & Fitzpatrick, 1989). 3. Titanium mobility observed in low grade metamorphic rocks 3.1 Alteration of silicate minerals 6-17 If all the silicate minerals in a rock are Ti02 saturated, then one or more of the saturating phases rutile, titanite (sphene), ilmenite or rarely, perovskite appear. Otherwise, titanium substitutes as a minor or trace element in silicate minerals, especially in the biopyriboles. In terrestrial silicates, Ti is almost invariably found in the +4 valence state, occupying octahedral sites where it substitutes for Al, Mg, and Fe. The distribution of Ti among silicate minerals may change as a function of mctamorphic grade (Guidotti et al., 1977). Alteration or breakdown reactions of biopyriboles, which may release Ti, are of particular interest. A common breakdown reaction occurs during retrograde metamorphism of Ti-rich biotite, producing chlorite intergrown with masses of fine rutile with preferred orientation in chlorite crystals. Plate 6.1 shows an example of the products of this reaction in a pelitic schist. Based on petrographic examination of the progressive replacement of biotite by chlorite and exsolved rutile, one can say that the Ti in the rutile crystals came from the preexisting biotite, and has moved at most a millimeter. By contrast, potassium apparently was highly mobile in this environment, as shown by the near-total lack of white mica or other sinks for the potassium released during the biotite breakdown reaction. 3.1.1 Hydrothermal Rutile In hydrothermal systems Ti is generally conserved; the occurrence of ore grade deposits of hydrothermal rutile is readily explained by reactions of the type proposed by Force, (1991): silicate + sulfur = silicate + rutile + pyritc (Ti-rich) (Ti-free) Analogous reactions involving C02 or 02 as volatiles in place of sulfur yield carbonates and/or oxides. Plate 6.2 illustrates this type of occurrence in a hydrothermal rutile vein 6-18 from the Titanium Corp. of America mine at Magnet Cove. Arkansas. According to Force (pcrs.commun.. 1992), the length scale of Ti transport in typical hydrothermal systems is very short, perhaps millimeters in the case of the large deposits at Bingham, Utah. Leucoxene, a finely divided material consisting of Ti-oxides, also results from a breakdown reaction of ilmenite in hydrothermal environments (e.g. Conrad, 1990). 3.2 New Idria, California Titanium has been mobilized on a meter scale at New Idria, California, USA (Van Baalen, 1991, Van Baalen & Zbinden, 1989). The New Idria district is a fault-bounded, diapirically emplaced serpentinite body roughly 20 km long, which is considered to be part of the Coast Range ophiolite (Hopson et al,, 1981). The serpentinite body consists mainly of highly sheared and crumbly fragments of serpentine minerals (Mumpton & Thompson. 1975). It also contains numerous tectonic inclusions of various lithologies, with a size range from 1 to 1500 meters (Coleman, 1986). Some of these tectonic blocks, particularly those at the southern end of the district, contain titanium-bearing minerals, including schorlomite garnets, as well as perovskite, titanite (sphene), benitoite, and neptunite. Schorlomite garnets contain > 5 wt. % Ti02: schorlomite garnets at New Idria have Ti02 contents as much as 16 wt. %. These high-Ti minerals are only found within the tectonic blocks or in alteration rinds surrounding the blocks. Some small intrusive stocks of syenite are associated spatially with the blocks containing these minerals. Most of the Ti minerals are euhedral to subhedral and have vein-filling textures. The veins crosscut all other fabrics in the rock, and probably formed as tension cracks during uplift of the massif. A fluid inclusion study indicated that the garnets grew in low salinity fluids at temperatures around 300C (Van Baalen, 1991). Other than in association with these tectonic blocks, the New Idria serpentinite shows a total absence ofTi minerals, and whole-rock analyses of the serpentinite show consistently low levels of titanium (<1 ppm Ti by XRF analysis, this study). - 6-19 Plate 6.1 - Exsolution of rutile needles from chlorite that has formed after titanium-rich biodte during retrograde metamorphism of a politic schist. Note that the orientation of the rutile appears to be cr/stallographically controlled within the chlorite crystal. Field of view 2 mm. Sample PJ-11E courtesy of J. Sclverstone. " 6-20 Plate 6.2 - Vein of hydrothermal rudle in a porous matrix of K-feldspar, calcite and pyrite. Sample from Titanium Corporation of America pit. Magnet Cove, Arkansas. Width of sample 5 cm. 6-22 Minerals limited to the altered rinds of a tectonic block are well displayed at the Melanite Mine, located in the New Idria district Here, a tectonic block crops out at the surface as a north-south trending cliff ca. 2 m high and 15 m wide. Near the north and south ends of the cliff one finds boundary layers of the type described in Figure 6.1. The phase assemblage within the block, "rock type 1", consists of chlorite + diopsidc schorlomite garnet. The phase assemblage outside the block, "rock type 2", consists of serpentine andraditc garnet (Ti-free). At the Melanite Mine, "rock type 2" extends for at least several hundred meters, so that distance CD is effectively infinite. The boundary layer BC, about 1 meter in width, consists of serpentine plus a remarkable garnet, which has a schorlomite core overgrown with andradite, as shown in Plate 4.9 in Chapter 4. Garnets of this type are limited to the north and south boundary layers at this locality, and found nowhere else in the region. The garnets in this boundary layer document titanium mobility on a length scale of 1 meter. Fluids containing dissolved Ti moved through the tectonic block, precipitating schorlomite garnets within the block and in its boundary layer, under greenschist facies conditions. Both pervasive flow through the rock and channelized flow through the veins occurred. Titaniferous garnets are only found within the tectonic block or its boundary layer, but fluids outside the tectonic block precipitated Ti-free andradite garnet over an area much larger than the zone of titanium mineralization. Elsewhere within the New Idria district there are numerous additional instances ofTi mobility: these are always related to the tectonic blocks and their margins. There arc no fluoride minerals at New Idria; this suggests that fluoride complexing was not a major factor in Ti mobility there. 3.3 Sierra Blanca, Texas Titanium has been mobilized on a 4 meter scale at Siena Blanca, in the Trans Pecos region of West Texas, USA. At Sierra Blanca, mildly peraluminous and rare-metal-enriched Oligocenc 6-24 rhyolites have formed a series of laccoliths which intrude and dome up the Buda limestone of mid-Cretaceous age (Rubin et ah, 1989). Fluorspar occurs along the rhyolite-limestone contact, and both Be and Ti mineralization are hosted by this fluorspar. Since much of the exposure is subsurface, Rubin et al. relied on samples taken from drill cores, bulldozer cuts, and an adit intersecting the contact Mineralization is extensive at Sierra Blanca, and rutile is merely an accessory phase. The mineralization extends several meters into the limestone, the distance controlled in part by the dip of the contact Rubin et al. do not give full chemical analyses of the altered and unaltered limestone, but they do give trace element analyses, which show a relative enrichment of Tt by one order of magnitude 4 m from the contact It is interesting to note that all of the titanium group elements, also Al and Y, are enriched by about this same factor. Sierra Blanca thus shows mobility of titanium on a length scale of a few meters, in an environment where the activity of fluoride was high, as demonstrated by the presence of extensive fluorite deposits. The temperature of mineralization is estimated by Rubin et al. to have been 250-400C. The pressure is not stated, but is assumed to be low (J. Rubin, pers. commun., 1991). Unlike at New Idria, fluoride complexing with Ti in the hydrothermal solutions seems possible at Sierra Blanca. 3.4 Adamello, Italy Titanium and zirconium have been mobilized on a scale of 10 meters at Valle di Breguzzo, Italy, at the margin of the Adamcllo batholith. Adamello, located in northern Italy in the Provincia di Trento, 50 km south of the borders with Austria and Switzerland, is the largest Tertiary igneous complex in the Alps. Along the eastern margin of the batholith, an intrusive contact of tonalite against dolomite is exposed in the Valle di Breguzzo. According to Giere 6-25 (1990, 1992), veins containing Ti minerals extend up to 10 meters from the contact into the dolomite, which has been contact metamorphosed to marble. Similar veins have been observed by this author at Val Daone, which is also located on the eastern contact of Adamello. Gicrf observed four distinct mineralogical zones in the veins at Valle di Breguzzo, with sharp boundaries separating them. Several Ti and Zr-bearing minerals occur there, including rutile, titanite (sphenc), zirconolite, geikelite, and titanian clinohumite. Whole rock analyses of the unaltered dolomite show negligible Ti and Zr (< 0.08% TI and < 16 ppm Zr) (Gicrd, 1990). Textural features show that the Ti and Zr in the vein minerals were advectcd from the tonalite. Whole rock Nd-isotope data for the veins and for the tonalite suggest Nd-isotopic equilibrium between the vein-forming fluid and the intrusive tonalite (Gicrti, 1992). The Ti and Zr-bearing minerals were formed as fluids associated with the intrusive tonalite penetrated the dolomite wallrock. These fluids carried Ti and Zr. The length scale of transport for both 'll and Zr was about 10 meters along the veins, but only a few centimeters perpendicular to the vein walls. Pressure and temperature estimates for these veins are 550600C and 2 Kb, with high activities of C02, H2S, and F\ the latter suggested by the abundance of idiomorphic fluor-apatite as an accessory mineral in the veins (Gier, 1992). As at Sierra Blanca, but unlike at New Idria, fluoride complexing potentially enhanced Ti mobility. 3,5 Magnet Cove, Arkansas Titanium mobility on more than one length scale has occurred at Magnet Cove, Arkansas, USA. The Magnet Cove district is in the west central pan of Arkansas, and contains several remarkable mineralogical and petrological localities associated with the intrusion in the Cretaceous of a scries of nepheline syenites and carbonatites into Paleozoic sedimentary 6-26 rocks (Erickson & Blade, 1963). These intrusions form a series of concentric ring dikes 5 km in diameter. The intruded country rocks consist mainly of shales and a slightly metamorphosed chert unit known as the Arkansas Novaculite. There are two examples of titanium mobility at Magnet Cove of interest here: one is the hydrothermal rutile at the mine of the Titanium Corporation of America, which is interpreted as a hydrothermal alteration of a phonolite. Whole rock analyses of the altered phonolite show 2-3% Ti02, which is similar to that of the unaltered phonolites in the vicinity. Titanium in the unaltered phonolite is concentrated in titanitc (sphenc) and ilmenite. The simplest interpretation is that the rutile deposit is an in-situ accumulation of the breakdown products of the titanite (sphene) and ilmenite. The brookite deposits in and around the Christy pit on the east side of the Magnet Cove complex are, however, a different matter. At the Christy pit, and at the nearby Moses Hill locality, cuhedral brookite is found intergrown with quartz crystals filling veins and vugs in the novaculite (quartzite) unit a few hundred meters beyond the eastern margin of the igneous complex. Willis et al., (1991) report in a fluid inclusion study that these quartz crystals formed at temperatures of 300-500C Unaltered, pure novaculite has an extremely low Ti content, < 0.008 % (Erickson & Blade, 1963). It has been suggested (Williams, 1891. Willis ct al., 1990) that the brookite ciystals were precipitated from hydrothermal fluids2, which caused the breakdown of igneous 11 minerals (mclanite garnet and perovskite) in the nepheline syenite 300 meters away, and transported Ti in solution to the place where brookite was preciptated. This is certainly a possibility, which if true would demonstrate that Ti moved for a distance of a few hundred meters. Another possibility worth investigating is that 2Williams (1891) stated "The most plausible theory to account for the transformation of this clement (titanium) from its original condition._ in the syenite, to that of brookite _. is that it was dissolved by the hot water and steam under pressure._ and was recrystallized." 6-27 the source of the Ti is in the Missouri Mt. Shale, which underlies the novaculite. The Missouri Ml Shale, contains about 0.5 wt % Ti02,. The contact between the novaculite and this shale crops out in the wall of the Christy pit. It is possible that the shale is the source of Ti for the brookite, and that Ti was leached out by hydrothermal fluids and transported only a few meters to the site of the brookite deposits. If this hypothesis turns out to be correct, then Ti mobility is still demonstrated at the Christy pic, but on a length scale of a few meters rather than hundreds of meters. 3.6 The Alpine Clefts The Alpine Clefts refer to a large number of mineralized cavities or vugs, located throughout the Western and Central Alps (Niggli, 1940). Many clefts are filled with euhedral, often museum quality, mineral specimens. These minerals include titanite (sphene), rutile, and other Ti-bearing phases; the environment of deposition suggests precipitation from Ti-bearing fluids. The metamorphic conditions were estimated by Poty et al. (1974) to have been 340505C and 2.5-3 kb. Titanium was mobilized in metamorphic fluids probably released by dewatering reactions in the Alpine nappes during the Miocene. The length scale of Ti mobility in the vicinity of the clefts is uncertain, and has not been specifically studied. These clefts represent an intermediate case between the low pressure environments described above, and the high pressure examples to follow. 4. Titanium mobility observed in high grade metamorphic rocks The systematic depletion ofTi relative to major elements and REE in island are basalts (TAB) is well established (e.g. Perfit et al., 1980). Ryerson & Watson (1987) showed that the Ti depiction must be a characteristic of the LAB source region. One mechanism for depiction of the source region might be leaching of Ti by aqueous fluids, which then transport Ti out of 6-28 the source region, presumably to be precipitated elsewhere, e.g. Ayers & Watson (1993). Direct evidence from subduction zones comes in two forms: one is the study of volcanic products above subduction zones; the other is the study of metamorphic rocks, especially blueschists and eclogites, which have experienced subduction zone conditions. The experimental evidence suggests that significant Tt could be present in fluids under cclogite conditions. But high solubility of rutile does not necessarily imply effective transport of Ti. 4.1 Monviso, France Philippot & Selverstone, (1991), in a study of eclogites from Monviso, France, in the western Alps, report evidence for high Ti concentration in fluids associated with these rocks. Monviso lies in the French-Italian Western Alps, and is thought to be a part of the Neotethys basin separating the South Alpine plate from the European plate. The unaltered eclogitic rocks are preserved within a horizon of metagabbros which contain 10 x 1 m boudins associated with a shear zone. The boudins contain shear fractures, tension gashes and dilatant fractures. Philippot & Selverstone (1991) estimate the P-T conditions during the metamorphism of these rocks to have been 500 50C and at least 10-11 kbar. Omphacitic pyroxenes in the veins and shear fractures contain fluid inclusions. These contain several daughter crystals, including rutile, titanitc (sphene), and baddeleyite. The presence of these crystals indicates that the concentration of Ti and Zr was significant in the brines from which the omphacitcs grew. As noted earlier, a rough estimate by this author of the concentration ofTi and Zr required to produce these daughter crystals is 0.05-0.5 wt % for both Ti and Zr, at the time of trapping. Decrepitiation of the inclusions on heating frustrated attempts to redissolve the daughter minerals. 6-29 Rutile and ritanitc (sphene) are found in both the veins and the mylonites. The authors suggest that the veins were caused by hydraulic fracturing during reduction of the bulk rock permeability due to dynamic recrystallization. If so, the titanium required to precipitate rudle and dtanite (sphene) in the veins need not have been advected from a great distance. Selverstone (pers. commun., 1992) believes that the length scale of Ti migration in these rocks is at most a few centimeters. Philippot & Selverstone also point out that the crack seal geometry of the veins suggests that fluid flow occurred in pulses, and that the minerals in the veins probably grew during opening of the veins as a result of these pulses. At Monviso the solubility of rutile was probably significant, yet transport of Ti seems to have been limited to a centimeter scale due to the limited movement of metamorphic fluids in these low permeability rocks. 4.2 Santa Catalina Island, California A second locality which sheds some light on titanium mobility under high P-T conditions is Santa Catalina Island, California, USA. The Catalina Schist, exposed on that island, is interpreted as a fossil subduction zone in which three distinct tectonic units are preserved: amphibolfte-, grcenschist-, and blueschist-facies rocks (Sorensen, 1986,1988). Tectonic blocks of various sizes included in the amphibolite unit were metamorphosed under high P and T conditions, and developed centimeter scale reaction rims or "rinds" (Sorensen & Grossman, 1989). These authors estimate the P-T conditions during formation of these rinds to have been 11 kbar and 640-750C The tectonic blocks of interest to the present discussion are described by Sorensen & Grossman as "non-migmatitic blocks with rinds". These blocks contain rutile, and the rinds contain rutile and titanite (sphene). The matrix between the blocks is described as meta-ultramafic. Whole rock analyses for all rock types 6-30 arc given: the Ti02 content of the blocks, rinds, and matrix are 1.4-2.5%, 0.2-2.4%, and 0.03-0.18% respectively. During metamorphism of these blocks under eclogite conditions, Ti migrated from the inner edges of the rinds towards the outer. Ti moved some 5-15 cm, the width of the rinds. This is a rather small distance, considering the significant concentration of Ti expected in metamorphic fluids at 700C and 11 kbar. It is possible that there was not much fluid circulation in this system. However. Bebout & Barton (1989) found evidence for massive fluid flux in a shear zone, and have proposed a model involving channelized flow along the top of the subducted slab. Bebout & Barton documented extensive mobility of major elements, including A1 and the alkalis, but did not present any evidence for significant mobility of titanium. The documented Ti mobility of 5-15 cm probably represents a minimum value, and further work at this locality may well furnish evidence for movement of Ti for greater distances. This example, as well as the next, suggests the importance of fluids in deep crustal shear zones. 4.3 Tauern Window, Austria A third locality which sheds light on the mobility of titanium in high grade rocks is at the Stillup Tal, in the west central portion of the Taucm Window, Austria. At this locality, ductile shearing of a granodiorite transformed this rock into a Si02-undersaturated staurolite schist at approximately 550C and 11 kbar (Selverstone et al,, 1991). Using the "conventional wisdom", the authors used AI andZras the basis of an immobile reference frame during the shearing event Relative to this reference frame, Ti, Fe, and Mg were added while Si, Ca, and Sr were removed, and there was a 60% volume loss. Alteration profiles show that fluid flow was channelized in the shear zone and did not significantly penetrate the wallrock to a distance of more than 1 meter. 6-31 Unfortunately for the purposes of the present paper, no information is preserved on the length scale of the presumed Ti mobility at Stillup Tal. Since the Ti concentration in the fluids was probably high during metamorphism in this area, a significant quantity of Ti may well have come from a considerable distance. The assumption that Zr was immobile in this case may be questioned, given the experimental evidence on zircon solubility (Ayers & Watson, 1991) and the fluid inclusion evidence from Monviso. More probably, both Ti and Zr were mobile, perhaps to varying degrees. 5. Conclusions . 1. The total body of experimental data for the solubility of Ti-oxides is rather thin; many of the experiments have been performed under geologically unrealistic conditions. 2. It is likely that the concentration of Ti in geologic fluids is limited by the solubility of Ti-oxides. TiC^ occurs in several forms, including the crystalline polymorphs and a metastable hydrated phase which has a greater solubility than the well-crystallized oxide. The rate of dissolution of these phases depends on their surface properties.3 3. The solubility of Ti02 is pH dependent. At pH < 2.5, a hydroxyl complex, either TiO2"' or Ti(OH>22\ is the dominant Ti species in solution. The nature of the dominant Ti complex in solutions at pH > 2J is in dispute. In much of the literature it is assumed that a neutral hydroxyl complex, either Ti(OH)4 or TiO(OH)20. is the dominant species. Chloride complexing is probably not important, and a high chloride activity may even decrease rutile solubility by lowering the activity of water. Experiments with other complexing agents show that they do not play a major role in the concentration of Ti in most geologic fluids. 6-32 4. TiOt solubility increases with temperature to about 1 wt. % near 1000C. The solubility of rutile in supercritical water is much more a function of temperature than of pressure, in fluids of density 2: 0.7 g/cm3. 5. Titanium mobility on a length scale of meters has been demonstrated in low grade metamorphic rocks related spatially to shallow intrusives. Studies of eclogite facies rocks have demonstrated Tt mobility on a length scale of only a few centimeters in spite of the much greater concentration ofTi expected in fluids under these conditions. There are no reports of extensive Ti mobility under blueschist conditions. Studies of mantle xenoliths (e.g. Haggerty, 1987) suggest Ti mobility and mantle metasomatism, but the evidence is not yet clearcut: a full discussion of that field is beyond the scope of this paper. 6. Evidence regarding 'll mobility in deep crustal shear zones is somewhat equivocal, but these zones must play a major role in Ti movement if significant quantities of Ti are leached from island arc basalt (IAB) source regions. Rycrson & Watson (1987) suggested that the IAB source region has been depleted in these elements, due possibly to a previous episode of partial melting, or to removal of H-group elements through interaction with aqueous fluids. If the removal is by aqueous fluids, then the scale of this process must be large. To date, study of metamorphic rocks has not been able to document such a large scale process. 6-33 ii i i I i i i CJ c >w V) E .e 3 C 0) E c 2 ow > ca c 3 CB t C cn CA C o ca e o cE G> O c o o 3 > ca *o 5 aJ3 o ca >- 1i I ii i i 1 i CM CM cn O O CA o o o> o O) n (O a> .... __ * * CO CO CO ca CO -- - CO . *. o> O) 15 15 <A > > JD .T5 oo chaa. a* w h> jC CO .2 6 JO w O 2 5 c c CXI o o to JO n oooo c .c c c Q IA CA CA O o o ooo c cc CA o c CA TJ T> c c 2 w> $ > %r = E u i CA CA (A o w t: * 3o .0 Eto a> ~ a * 5 *5 *w ca c c CA . Cco E ECA o o O o o JS s cn r*. CM cn ^r O) O) CO i o CO CM o to in CO o <r* **" o o o o O o CM CO ^t CM CM r o CO f 2E sa a E E5 5 c a a a aajCM o c fs. CO 2 o o CM in CL CM -D CM ii i tij I . j____ T) C w 15 .. CO X o w sz o c if o s> X c JD o Q N CO ZD U 1^ CA o 03 a. aE X* c w C a. 5- CA CO n si to o CA c 11 F 15 O O 3 CA N f Oo b aO -J 1 m CM *a o Qo CO CL 3 CO o Q. *- o XO CO Q CM o CO X o' >o e O O in E a]XCO CM CA K H .. ,. .a o T3 ffl Ooo O o ccec Table 6.1 CSi o roa> Chapter 7 Discussion and Conclusions Chapter 7 - Discussion and Conclusions Opening Statement............................................................................................... 1 I. Minerals and Mineral Associations..........................................................1 A. Serpentine......................................................................................... 3 B. Olivine, Pyroxene and Chromite.......................................................5 C. Chlorite and Diopside........................................................................5 D. Garnets............................................................................................. 6 E. Sodic Amphibolc...............................................................................8 II. Tectonic Blocks.............................................. ....................................... 8 III. Intrusive Rocks.......................... 12 IV. Element Mobility During Metamorphism.................................................13 V. Progressive Serpentinizadon.................................................................. 14 VI. Formation of Short Fiber Chrysotile Asbestos................................. 15 VII. Regional Implications............................................................................. 21 A. Tectonic History....................... 21 B. Jurassic Tectonics........................................................ 21 C. Cretaceous and Neogene Tectonics................................................... 22 D. Miocene Plate Rearrangements in the Eastern Pacific.........................22 E. Miocene Events................................................................................. 22 F. Multiple Effects of Triple Junction Passage at New Idria.................. 23 VIII. New Idria and the San Andreas Fault Today........................................... 28 A. New Idria as a Tectonic Window............................................. 28 B. Recent Seismicity....................................................................... 28 Closing Statement.......................................................................... A. Comparative Studies of Ophiolites................................... 29 B. Areas for Future Research........................................................ 30 Tables ......................................................................................................... 31 29 Chapter 7 - Discussion and Conclusions Opening Statement In this chapter I attempt to assemble all of the facts and ideas of the preceding chapters and combine them into a larger picture. This chapter is organized around 8 major themes; detailed conclusions from the earlier chapters are discussed in the context of these themes. The themes are arranged from the specific to the general. The mineralogy and petrology of the New Idria Serpendnite are seen to be complex, yet not without parallels in other Alpine serpentinites and ultramafic bodies. The bulk composition and relict textures in the serpendnite suggest that the ultramafic protolith was a peridotite consisting of dunite and harzburgite, together with minor Iherzolite and pyroxenite. The very high degree of alteration and weathering has obscured much of the evidence at New Idria, but there remain telltale signs of the evolution of this body in the form of relict minerals and rock textures. I. Minerals and Mineral Associations At least five generations of minerals arc present in the New Idria Serpendnite. These generations represent an overall time progression; they record one or more stages in the evolution of the serpendnite. The minerals of these generations may appear in more than one rock type, but their occurrence is consistent with their relative timing. 7-1  Relict Igneous Minerals: such minerals are scarce, but are occasionally preserved as unserpentinized cores of grains in seipentinite rock. Olivine, onhopyroxene, clinopyroxene and chromite arc locally preserved. Chromite is unique in that it appears to be relatively unaffected by serpentinization. Serpentine Minerals: the serpentine minerals and related oxides and hydroxides form the host rock of the District, which comprises over 90% of all rock types present. New Idria differs from most other serpentinites in that it is chrysotile-dominated. Lizardite, antigorite, brucite and magnetite all occur in lesser quantities. However, serpentine recrystallization generally limits our ability to see through more than two serpentinization events. Metamorphic Minerals of the Tectonic blocks: mineral associations in these tectonic inclusions arc a valuable source of information about the metamorphic processes to which these rocks were subjected, and indirectly, about the nature of the protoliths. Each block type records an Ml and an M2 metamorphic event. The Ml generation of minerals appears to predate entrainment of the tectonic blocks, but Ml metamorphism itself may have been a multi-stage event for some of the tectonic blocks. The M2 mineral generation postdates block entrainment; the minerals for which the District is best known, benitoite and Ti-gamet, belong to this generation. Intrusive Igneous Minerals: late intrusions of an amphibole soda syenite have a unique mineralogy. A small contact aureole about the best exposed of these intrusions documents the intrusive event The original igneous minerals arc preserved, but minerals resulting from deuteric alteration 2re also found.  Surface Alteration Minerals: these are chiefly carbonates of various types, that record the interaction of meteoric waters with the rocks of the serpentinite. These minerals appear primarily in fully serpentinized igneous rocks, but are also found in the mctamorphic rocks of tectonic inclusions. Calcitc, magnesite and hydromagnesite arc the most abundant of the alteration minerals. Table 7.1 summarizes these points in the form of a correlation chart. Each column represents a distinct rock type at New Idria. Within each column, the relative timing of mineral generations proceeds from oldest to youngest. In a few cases it is possible to correlate mineral generations across rock types; the shaded bar in the Miocene suggests that all of these mineral generations record Miocene events, as discussed below. A. Serpentine 1. The phase relations among the serpentine minerals are difficult to show schematically, and are not completely understood. In general, lizandite and chrysotilc are the low temperature serpentine minerals; antigorite is the high temperature phase. Lizandite and antigorite are the high pressure phases. The most up-to-date discussion of this issue is in Wicks & O'Hanley (1988). 2. Antigorite, chrysotilc and lizardite are no longer considered to be polymorphs: they are not chemically identical (O'Hanley et al., 1989). Each mineral occupies a compositional range, and these ranges partially overlap. 3. According to O'Hanley et al. (1989), aH20 affects the formation of lizardite versus chrysotilc. Molecular water in the halloysite structure might correspond to (thus far 7-3 undetected) water in the chrysotile structure. If true, this would lead to a hypothetical chrysotiie formula of MgjSijOjfOHj^nHjO., where 0 S n 1. 4. In antigorite, the Si-O-Si bonds at the points where the tetrahedral sheets invert arc responsible for the tough, resistant nature of antigorite compared with lizardite or chrysodle. in which the adjacent layers are held together by weak bonds involving hydrogen. Antigorite is a member of a polysomatic series between lizardite and talc (e.g. Ottcn, 1993; Sanford, 1978; Thompson, 1978). The conventional antigorite formula is equivalent to 15 Liz + 1 Tic. If the wavelength of the modulation were to decrease the structure would become increasingly talc-like, while if the wavelength were to increase the structure would become increasingly lizardite-like. 5. Fe and A1 are the principal compositional variants in serpentine. Compositional variation occurs along the three exchange vectors AIjMg.jSi.jCTk); FeMg.j; Fe^AI.j. The dioctahedral vacancy substitution 0Al2Mg.3 is not known to occur in serpentine. Trace amounts of Ca, Na, and K reported in serpentine analyses may represent mixtures mixtures of other silicates with serpentine in the analyzed samples. 6. At New Idria. the dominant phase assemblage of chrysotiie + brucite indicates an Mg/Si ratio between forstcritc and enstatite, serpentinizing fluids low in C02, and a low temperature equilibrium (<250C). Antigorite knockers described in Chapter 4 record a higher temperature. On the basis of trace occurrences ofjosephinite, I conclude that the environment during serpentinization was relatively reducing. 7-4 B . Olivine, Pyroxene and Chromite 1. Relict olivine, onhopyroxene and clinopyroxene are locally preserved in lhcrzolitc veins, in the cores of serpentine grains. Their compositions are related by the inequality: 2. Accessory chromite includes two groups, a primary group and an altered group. The primary group includes chromites with appreciable contents of Cr, Fe, Mg, and Al, but only trace amounts of Ti, Mn, Ni, and Zn. The compositions are typical of chromites from Alpine-type ultramafic bodies worldwide, and are properly namedferroan magnesiochromites. 3. The altered group of chromites occurs in rocks which have clearly undergone postserpentinization metamorphism. Altered chromites are frequently found rimmed by the chromian chlorite kammererite; this chlorite overprints the surrounding serpentine fabric in these rocks. Altered chromites are cryptically zoned, with Cr-rich cores and Cr-poor rims. C. Chlorite and Diopside 1. Evidence for Fe-Mg partitioning (Xm,) between coexisting minerals is preserved in the serpentinite and in tectonic blocks. A surprising observation is that, for these bulk compositions, chlorite appears to be more Fe-rich than the coexisting Cpx, an exception to the normal rule that chlorite is less Fe-rich than a coexisting mafic mineral. In this study, the following relationships were established: 7-5 yOI(ig) ^ yOI(mei) AMg > AMg yCpx(ig) yCpi(met) AMg > AMg CpX y Chi AMg > AMg yChl(MZ) ^ yChl(Ml) AMg > AMg 2. Under the prevailing metamorphic conditions, chlorite was the only phase that could accommodate significant amounts of A1 in addition to Fe and Mg. Differences in chlorite compositions reflect differences in bulk compositions from one block to another. However, based on the relatively Fe and Al-rich nature of the Ml chlorite compared to the M2 chlorite within a single tectonic block, I conclude that the Ml event occurred at a higher metamorphic grade than the M2 (Laird, 1988). D. Garnets 1. Most of the New Idria garnets are calcic and belong to the andradite clan. Many, but not all garnets of the New Idria district are strongly anisotropic, sector twinned, and zoned. Titanium substitution occurs principally along the exchange vector TiFe.,, making these garnets titaniferous andradites. Some of the garnets are also hydrous and vary along the hydrogamet exchange vector H4Si.]. A few of the garnets are Ti-grossulars, and occupy a portion of composition space not previously explored. Compositional variation in these garnets occurs along the vector TiFe2+Al.2. The mean TiC^ content of the garnets in this study is 5 wt. %. One of the garnets contains 15.62 wl % Tt02, which corresponds to approximately one Ti atom per formula unit, i.e. the schorlomite end member. 7-6 2. Most analyses have less than 3 Si canons per formula unit (pfu), and more than 3 Ca cations pfu. The New Idria garnets are therefore similar to many other reported titaniferous garnets in that they are subsilicic and supercalcic. 3. At the present time that presence of Ti3* in garnets has not been completely ruled out, but is only supported by indirect and equivocal evidence. TiFe.! substitution may be achieved by more than one crystal chemical process: the vector is equivalent to either or both of the vectors TiFe2*Fe32* and Ti3*Fc3j*. The Ti3+Fe3]+ vector is considered the less likely of the two, but until the proper EXAFS studies arc completed, the debate will rage on. In this study using micrapiobe analytical techniques it was not possible to distinguish between these two possibilities and TiFe.j is used to describe compositional variations in garnets. 4. New Idria andradite garnets occur in a variety of hues, including yellows, reds, greens, browns, and black. The colors of garnets and other minerals relate broadly to chemical composition, both major and trace elements. The principal color series is from yellow to black, correlating well with progress along the TiFe., exchange vector. Chromian garnets are emerald green, grossular garnets are cinnamon colored. Garnets containing both Ti and Mn are a dull brown color. 5. Andradite garnets containing up to 38 mole % uvarovite component are found in slabby serpentinized harzburgite blocks. These garnets are anhedral, emerald green, and often associated with relict chromite grains. Chromian andradites arc very rare: these are believed to be the first chromian andradites reported from the United States. 7-7 E. Sodic Amphibole I. A blue amphibole was found in veins within antigorite serpentinite near the western margin of the New Idria serpentinite. The amphibole has a partially filled A*site and appears to lie approximately one-third of the distance between a winchite and eckcrmannite; the paragencsis is a new one. An approximate formula for this amphibole is: {Na.K)0j1(Na,Ca,Fe2<-)2(Mg,Fe3+,Fe2+,Al.Cr,Mn,Ti)3Si800O22(OH.Cl,F)2 II. Tectonic Blocks . 1. Three types of tectonic blocks are recognized in the New Idria Serpentinite: chloritediopside-gamet rocks, mafic schists, and antigorite knockers. 2. All types of tectonic blocks show evidence of two metamorphic events, here defined as Ml and M2. Ml metamorphism is associated with the development of penetrative foliation in the chlorite-diopsidc-gamct rocks and the mafic schists, and the interpenetrating fabric of the antigorite blocks. M2 metamorphism is associated with the development of tension cracks and veins that crosscut the Ml foliation in chlorite-diopsidegamet blocks and mafic schists, and with crosscutting garnet veins in the antigorite knockers.3 3. Shearing during MI metamorphism is manifest by penetrative foliation of the dominant Fe-rich Ml chlorite, and by bending, fracturing and recrystallization of Ml diopside and garnet. These observations show that the Ml metamorphism was synkincmatic. 7-8 4. The M1 fabric is crosscut by veins interpreted as tension cracks. These veins are filled with M2 garnet, diopsidc, and minor chlorite. The M2 minerals in the veins have beautifully preserved delicate textures without post-mineralization deformation. Therefore the M2 mineralization was postkinematic. 5. A study of fluid inclusions in M2 minerals showed that homogenization temperatures averaged 235C. Measurements of melting temperatures showed that Tm(ice) = -3.5 to -2.0C, indicating that the salinity was equivalent to 3-5 wl % NaCl. Assuming that the pressure of M2 metamorphism did not exceed 3 kb (see Section III), the trapping temperatures were at or below 400G 6. The bulk compositions of the chlorite-diopside-gamct rocks differ from that of the bulk serpenunite, but are very similar to those of pyroxenites from other ultramafic rocks world- wide. The trace dement concentrations of the chlorite-diopside-gamct rocks fall within the normal range for pyroxenites; the concentrations are different from those of a serpentinized harzburgite sample from New Idria. 7. The chlorite-diopside-gamct rocks are shown, by means of a "spinel norm" calculation, to be the metamorphosed equivalents of olivine webstcrites. 8. The phase assemblage chlorite + diopside + Ti-gamet magnetite can be derived from the low-temperature hydration of an ultramafic precursor of pyroxenite composition. The phase assemblages found at New Idria suggest that greenschist or blueschist facies conditions accompanied this hydration. The stability of this phase assemblage is at present the only control on the temperature of Ml metamorphism. 7-9 9. The chlorite-diopside-gamet rocks of New Idria did not form as rodingites because the bulk composition of both the metapyroxenitc blocks and the enclosing serpentinite were both silica-poor. Chlorite, diopside and titaniferous andradite garnets are all stable phases in association with serpentine minerals. There is no regional scale rodingitc rind at the boundary of the New Idria Seipeminiie because the emplacement into the country rock postdated serpendnizadon. 10. Low solubilities of Ti oxides, lack of a well-developed vein system connecting chlorite-diopside-gamet blocks, lack of penetrative foliation in serpentine rock adjacent to foliated chlorite-rich rocks, and lack of apparent Ti-mctasomatism at the margin of the igneous intnisives all argue against formation of chlorite-diopside-gamet rock by the metasomatic alteration of ordinary serpentine rock by fluids from igneous intrusives. 11. Bulk compositional similarities, the observation of Ml and M2 metamorphism, rock textures, and the existence of chlorite-diopside-gamet rocks in other localities argue for essentially isochemical metamoiphism of blocks of pyroxenite to produce the chloritediopside-gamet rocks at New Idria. 12. It is possible that the mcta-pyroxeniie blocks arc tectonic inclusions from an unknown source entrained within the serpentinite: tectonic inclusions of a variety of other rock types are present It is also possible that the meta-pyroxenite blocks were primary igneous features or even that they have a cognate origin with the depleted dunites and harzburgites. They may represent cumulate layers or crosscutting veins and dikes. The minor symmetry observed at the 34-50 locality suggests, but docs not prove, an origin as a multiple injection dike. The present random orientation of the blocks may well be the result of boudinage or other tectonic dismemberment of dikes followed by block rotation during the diapiric ascent of the New Idria Serpentinite. 7-10 13. Chlorite + diopside + Ti-gamet rocks are now known from three localities outside the New Idria District: Val Malcnco, Italy; Sangabawa District, Japan; Mid-Atlantic Ridge at 43N. All of these parageneses are in serpentinized ultramafrc rocks. 14. Greenstone and mafic schist at the Gem Mine are interpreted as Franciscan rocks entrained by the serpentinite. The mineralized zone that contains benitoite lies along or near the contact between these two rock types. 15. It is proposed here that benitoite at the Gem Mine formed by metamoiphism of the contact between the hanging wall greenstone and foorwall mafic schist, in the presence of Na-rich, low silica fluids. Mineralized dnisy cavities and extensive development of vein systems suggest hydrothermal mineralization at the Gem Mine, but the chemical components needed to produce benitoite and probably the other exotic minerals are available in the adjacent wall rocks. Therefore there is no need to advect Ti, Ba and other components from a great distance. Ordinary brines associated with M2 metamorphism probably produced local mobility of the all the required chemical components of benitoite and other minerals. 16. A preliminary determination of the age of benitoite and neptunite has been made using the Rb-Sr method and a two-point isochron. The age is estimated to be 11.96 Ma. (error bar not available). This age should be compared with the age of the New Idria Syenite, 12.4 0.8 Ma reported in Section HI. A further anempt to date neptunite crystals using ^Ar/^Ar technique is currently underway. 17. Knockers of dense, tough antigorite with a suga^' texture form resistant, positive weathering knobs in the District. These knockers may be sheared or mylonitized as at 7-11 Santa Rita Peak, with rccrystallization of amigorite within the myJonite zone. LocalJy, the knockers are crosscut by veins of M2 garnet that contain fluid inclusions. Some of the fluid inclusions contain methane. The knockers have a thermal history separate in pan from that of the bulk serpentinite, and must be older than the immediately surrounding serpentinite. III. Intrusive Rocks 1. The New Idria Serpcndnite contains small syenite stocks of Miocene age, here collectively named the New Idria Syenite. The syenite is shown to have an intrusive relationship with the surrounding serpentinite. In the contact aureole around the intrusive syenite, prograde olivine replaces antigorite. The aureole grades outwards to a mixture of chrysotile and lizardite. 2. The New Idria Syenite is a soda syenite, composed primarily of sodic feldspar and kaersutite amphibole. The amphibolc is very coarse grained in places, with crystals up to 30 cm. long. Elsewhere the rock is fine-grained with only millimeter-sized amphibole crystals. 3. The age of the New Idria Syenite has been determined by Marvin Lanphere (USGS) to be 12.4 0.8 Ma, using the vMf"Ar total fusion technique on an amphibole mineral separate from the syenite. 4. The occurrence of the prehnite-zoisite pair in deuterically altered syenite provides a pressure estimate at the time of alteration. The stability fields of prehnite and zoisite overlap only between 1 and 3 kb and between 250 and 280C (Frey et al., 1991). 7-12 Therefore the outcrop presently at the surface lay at a depth of 3.5 to 10 km. at the time of alteration, assuming an average density of 2.8 g/cm3 for the rock formerly overlying the outcrop. This is the only quantitative pressure estimate obtained during the course of this study, although blueschist minerals described in Chapter 4 provide a general estimate. 5. If the 12.4 Ma age of the syenite is close to the time of deuteric alteration, which is reasonable for a small intrusion, then the unroofing rate since the late Miocene may be calculated. The average unroofing rate is calculated to be 0.3 to 0.8 mm/year for the past 12 million years. This calculation agrees well with the estimated rate of 1 mm/year for the Coast Ranges, obtained using other data (Ring & Brandon 1994). IV. Element Mobility During Metamorphism 1. Metamorphism at New Idria was accompanied by limited element mobility ofTi and other elements in metamorphic fluids. An element is considered mobile if measurable changes in its concentration relative to a specified reference frame occur during a particular process. The strongest case for significant Ti mobility is the appearance of Ti-bearing phases in a rock otherwise devoid of Ti02, when these phases are limited to a boundary layer and where textural evidence shows advection across this boundary layer. The New Idria Serpentinite is an ideal place to look for such evidence because the country rock of depleted serpentinite has less than 1 ppm Ti02, while Tl-enriched phases occur in the tectonic blocks. 2. M2 "cats eye" garnets in the lateral transition zone at the Meianite Mine show evidence for meter-scale Ti mobility. Cores of Ti-rich garnet overgrown by Ti-poor garnet are found only in the lateral transition zone. These are interpreted as the result of Ti- 7-13 bearing fluids moving out of the Ti-rich chlorite-diopsidc-gamct rock into Ti-frce serpenrinite rock. 3. Presence of M2 andradite garnets in the anrigorite knockers at Santa Rita Peak and San Carlos Peak argue for Ca mobility in fluids. Interfingering of M2 diopside and serpentine at other localities also suggests local Ca mobility during in-situ recrystallization. 4. A blue sodic amphibole in an antigorite schist near the boundary fault between the serpentinite and Franciscan rocks suggests that Na was mobile in this environment. Local mobilty of Na, Ba, Ti, and other elements occurred in the mineralized zone at the Gem Mine, within Franciscan rocks. 5. Local mobility of Fe and Mg during M2 metamorphism is suggested by the large number of vein-filling ferromagnesian minerals. On the other hand, there is no solid evidence for mobility of A1 at New Idria. V. Progressive Serpentimzation 1. Serpentine formed after dunite often has a knobby outcrop texture formed by intersecting serpentine/bmeite veins which isolate rounded kernels of unserpentinized dunite. This texture is widespread at New Idria because of the abundance of dunite in the protolith. 2. Serpentine formed after harzburgite often has a slabby texture, and the serpentinized rock fractures leaving sharp edges, rather than the rounded kernels of serpentinized dunite. This texture is found locally at New Idria. 7-14 3. Models for serpendnizadon are frequently expressed in the MSH system. A more general model for serpendnizadon of fertile ultramafic rocks in the system CMASH and CFTMASH is proposed in this study. The resulting low-temperature CMASH phases all lie inside the brucite-talc-diopside-chlorite tetrahedron. 4. Ca is considered a mobile component during serpendnizadon; Al. Fc and 'll are generally conserved. Element behavior during serpendnizadon is as follows, to an excellent approximation: Mg partitions into serpentine; Al, Mg, and Fe into chlorite; Fe into magnetite and garnet; Ca into diopside and gamed Si into all of these silicates. Mg and minor Fe form brucite. Coleman (1977) noted additionally that: VI. Formation of Short Fiber Chrysolite Asbestos 1. New Idria is one of the largest deposits of chrysodle asbestos in the world and the site of the only operating asbestos mine in the United States at the present time. New Idria is unique in that the mineralogy of the serpentinite is dominated by chrysodle. Chrysodle asbestos from New Idria is also remarkable in that the fiber length is much shorter (5 pm typical) than other occurrences. 2. Most serpentinites are lizardite-dominated. Lizardite is much less abundant at New Idria than chrysodle, possibly due to the recrystallization of earlier lizardite under shearing stress. An alternative explanation for the abundance of chrysodle is the extensive growth of cataclastic fabric due to the predominance of dunite in the protolith. 7-15 Figure 7.1a- Kinematic model for development of chiysodle asbestos by block rotation of partially serpentinized dunite kernels under dextral shear. Note that sense of shear between kernels is everywhere sinistral; a particle between kernels will be repeatedly milled; irregular kernels will develop slickensided fabric. The process leads to effective destruction of the rock fabric. 7-16 Figure 7.1b - Kinematic model for development of subparallel shear planes in siabby, partially scrpentinized harzburgite lacking kernel pattern. Note sense of shear is everywhere dextral. repeated milling of particles reduced by comparison with Figure 7.1a. Mechanical integrity of rock is preserved for a relatively long time. 7-18 cd Gu.O 3 X) K rt 33 _C cn CD C C3 C3 CD XL CO u_ 1 /I sCD Q, _o - CD > CD Q CD -a o CD C3 CD c Ct-HD 3 GO c 3. Chrysotilc asbestos is the habit of serpentine that forms at low temperatures in rocks that are being deformed. The reason for the abundance of short fiber chrysotilc asbestos at New Idria has been attributed to repeated recrystallization under shearing stress, leading to a continual size reduction (Mumpton & Thompson, 1975). However, evidence for repeated rcciystailization is ambiguous. 4. An alternative model, suggested in the field to the author by David O'Hanley, has to do with the progressive serpentinization of dunite summarized in the previous section. Once serpentinization has proceeded far enough to mechanically isolate kernels of dunite, block rotation and shearing of isolated kernels under tectonic stress could produce the slickensided textures and repeated milling of fibers in the veins between kernels (Figure 7.1a). This process would mechanically destabilize the rock and permit introduction of more serpentinizing fluids; positive feedback between progressive serpentinization and the mechanical properties of the rock would lead to effective destruction of the rock fabric. New Idria may well approximate the end-state of these chemical and physical processes. Isolated dunite kernels from New Idria show development of slickensides in a variety of orientations relative to a single kernel, suggesting a chaotic process. 5. Progressive serpentinization of harzburgite would produce a more slablike texture, in which the mechanical integrity of the rock would be maintained for a relatively long time. Asbestos would form preferentially in the shear planes, leading to slip-fiber asbestos deposits typical of Vermont and the Eastern Townships of Quebec (Figure 7.1b). 7-20 VII. Regional Implications A. Tectonic History At this point it is necessary to summarize the tectonic environment in which the New Idria Serpentinite formed. While it is beyond the scope of this study to examine the full story of the tectonic history of California, certain ideas must be kept in mind art the following description unfolds. For a more complete discussion of California tectonics, the reader is referred to Ernst (1981) or Irwin (1990). Since the oldest rocks in the District are. of Jurassic age, our discussion is confined to events since that time. B. Jurassic Tectonics During the Late Jurassic period the western margin of North America was geologically active. The Farallon Plate was being subducted under the North American Plate during this time. A forearc basin developed in front of the Sierra Nevada volcanic arc (Dickinson & Seely, 1979; Bartow, 1990). The forearc basin itself is now represented by the Great Valley of California. The Franciscan Formation, a tectonic melange of greywackes, metavolcanics and cherts was being deposited in the Jurassic as an accretionary prism near the trench. A flake of Farallon Plate oceanic crust or underlying mantle was obducted onto the North American continent in a structural position within the Franciscan accretionary prism. At roughly the same time, other ophiolidc flakes were obducted onto the continental margin; as a group these comprise the Coast Range Ophiolite (Ernst, 1981). Ages of these flakes, where established by crosscutting intrusives, range from 153-165 Ma, i.e. latest Jurassic. Blueschist metamorphism accompanied the subduction of the Farallon Plate. 7-21 C. Cretaceous and Neogene Tectonics Sedimentation in the foreaxc basin was nearly continuous from the Cretaceous through the Pliocene; these sediments are now represented by the thick clastic rocks collectively named the Great Valley Sequence. As the Tertiary continued, the shallow forearc basin became isolated from the Pacific Ocean; the Great Valley Sequence includes both marine and nonmarine sediments. (Taliaferro, 1943). During this time, the flake of oceanic crust now exposed at New Idria was buried under these sediments, and was possibly undergoing serpentinization. D. Miocene Plate Rearrangements in the Eastern Pacific In the Oligocene/Miocene there was a fundamental reconfiguration of plate movements in the Eastern Pacific. At this time, the Farallon Plate began to break up into smaller pieces, and the dominant geometry became strike-slip rather than convergence (Atwater. 1986). A spreading center in the Gulf of California began to separate Baja California from the mainland (Stock & Hodges, 1989); the Juan de Fuca and Pacific Plates, separated by the Mendocino Fracture Zone, both lay in strike -slip contact with the North American plate, creating the San Andreas Fault. The triple junction at which the North American, Pacific, and Juan de Fuca plates all meet, named the Mendocino Triple Junction, began its northwest migration along the San Andreas Fault. E. Miocene Events By the late Miocene, the triple junction had reached a position directly under New Idria (Stock & Hodges, 1989) Triple junction passage occurred approximately 12 Ma ago (Figure 7.2). As noted in Chapter 1, the intrusion of the New Idria Syenite occurred at 12.4 0.8 Ma. It is suggested here that the intrusion is related to passage of the 7-22 Mendocino Triple Junction, possibly due to the slab window mechanism described by Dickinson & Snyder (1979) and Hole et al., (1991). The locus of points that mark the trace of the Mendocino Triple Junction as it has progressed northward is also marked by volcanic and intrusive igneous activity (e.g. Glazner & Supplee, 1982; Johnson & O'Neill, 1984; Fox et al., 1985; Stanley, 1987; Liu & Furlong, 1992). As noted in Chapter 4, the age of mineralization at the Gem Mine is about 11.96 Ma (no error bar available). It is suggested that this marks the time of M2 metamorphism at New Idria in tectonic blocks of all types. Finally, as noted in Chapter 1, the late Miocene was a period of active tectonic uplift, during which the New Idria diapir breached the surface. This event is recorded by the sudden appearance of elastic serpentine in the sedimentary record of the Great Valley Sequence, in a unique boulder conglomerate mapped as the Big Blue member of the Tremblor formation (Anderson & Pack, 1915). The Miocene Big Blue member documents the uplift and emergence of the New Idria serpentinite; the large size of the clasts in this conglomerate (up to several meters) suggests that transport was only over a short distance. F. Multiple Effects of Triple Junction Passage at New Idria The following hypothesis is proposed: tectonic uplift, mineralization resulting from lowgrade metamorphism (including mercury mineralization), and igneous intrusions all were effects of a single cause - passage of the Mendocino Triple Junction at about 12 Ma. The thermal energy required for uplift, development of tension cracks with a northeast orientation (Figure 4.1), M2 metamorphism, mercury mineralization along the New Idria Thrust Fault, and the intrusion of the New Idria Syenite came from upwelling asthenosphere caused by the slab window opened by the Mendocino Triple Junction (Dickinson & Snyder, 1979). The thermal coupling between the New Idria Syenite and M2 metamorphism was probably limited to contact aureoles such as the one described in Chapter !. Hydrothermal fluids of rather ordinary composition, with moderate salinity. 7-23 Figure 7.2 - Plate configurations in late Miocene (Anomaly 5AA, 12.9.Ma), showing position of Mendocino Triple Junction, where North American, Pacific, and Juan de Fuca Plates meet near New Idria, which is attached to North American Plate. See Stock & Hodges (1989) for plate reconstruction details. 7-24 Figure 7.3 -'Perspective drawing of tectonic windows in the Coast Ranges, after Irwin (1990), showing New Idria and other windows. 7-26 Figure 7.3 - Tectonic Windows in the Coast Ranges after Invin, 1990 probably circulated only locally, and not on a widespread basis. Mobility of chemical components needed to form the vein-filling M2 minerals was apparently local as well. VIII. New Idria and the San Andreas Fault Today A. New Idria as a Tectonic Window Today the New Idria Serpcntinite is seen as a tectonic window into the Coast Range Ophiolite (Figure 7J3). The diapir protrudes through the surrounding clastic rocks on the steep western limb of the asymmetric Great Valley synclinorium. Compressional uplift of the Coast Ranges may be due to a small orthogonal component of the strike slip movement along the San Andreas Fault. This northeast directed compression is estimated to be as much as 10% of the strike slip movement, or 5 mm/yr (Montgomery, 1993). B . Recent Seismicity The region around and immediately to the south of the New Idria District is seismically active today, as shown in Figure 2.1. There are swarms of magnitude 1-3 earthquakes and an occasional larger event (e.g. the 1926 Idria, Mw = 5.5,1982 MufS.4 New Idria, 1983 Mw=6.5 Coalinga, and 1985 Mw^.l Kettleman Hills events, ref. Stein & Ekstrom, (1992)). The 1983 earthquake of magnitude 6.5 effectively levelled the town of Coalinga, and stimulated research interest in this part of California (e.g. Rymer & Ellsworth, 1990). The 1983 event occurred near the crest of the Coalinga Anticline, a southerly continuation of the New Idria Thrust Fault The predominant mode of recent seismicity has been reverse or thrust faulting, due to crustal shortening from compressional stress normal to the strike of the San Andreas fault, (e.g. Montgomery, 1993; Bartow, 1990; Wentworth & 7-28 Zoback, 1989). Another pan of the seismic activity may be due to continued diapiric rise of the serpentinite massif, expressed as small ruptures of the boundary faults. Closing Statement A. Comparative Studies of Ophiolites Ophiolites and the chemical and physical processes that accompany their evolution are as yet incompletely understood. Every serpentinite or other pan of an ophiolite provides clues, but it is necessary to visit many such places in order to assemble a realistic and coherent picture. In this study I found visits to the Mont Albert Massif in Quebec, the Beividere Ml Complex in Vermont, and the Malenco Serpentinite in Italy to be crucial in the growth of my understanding. Much of what Alpine geologists have learned from study of Alpine ophiolites is direedy applicable to the California Coast Ranges, which do not superficially resemble the Alps. There is historical symmetry in this realization: in 1891, Gustav Steinmann, on field trip in the Coast Ranges with Andrew C Lawson, recognized the common structural features between California and the Alps that led to development of his model of ophiolite structures known as the Steinemann Trinity. Comparative studies of the Alps and the California Coast Ranges were key to this development, according to Steinemann (1906). 7-29 B . Areas for Future Research The three main areas most ripe for future research in the New Idria District are, in my opinion: 1. Subduction zone processes, including obduction of ophiolites and entrainment of tectonic blocks, and the role of fluids in metamorphism. 2. Structural processes, including modes of crustal shortening in strike slip environments such as the San Andreas Fault system. . 3. Public policy issues, related to the largely unfounded concerns about health hazards of chrysotile asbestos, have brought recent attention to New Idria. Fluvial transport of chrysotile asbestos from the New Idria District eastward via Los Gatos Creek and Cantua Creek into the San Joaquin Valley, as well as westward via the San Benito River, has been taking place for a great length of time, possibly for as much as 12 million years. Since the completion of the California Aqueduct, which interferes with normal drainage patterns, some of the chrysotile asbestos has gone into the canal, mainly under flood conditions. Therefore the downstream users of this water have found chrysotile asbestos fibers in the water. Concern about the possible health hazards of this condition has been greatly overstated; the hazard is negligible. It will be a matter for science and public policy together to find a safe and economical solution to this issue. Fin 7-30 S a *5 2cO O03 *5 -- If o JadJ <_o c a. j EO CM ' 5 o c '.Os) ac V) oj -- CV. C .'S .C .N C QO> C2 o '5) 5o N S. mC O H x 0 -H 1 s. Nw. -C Q} C^ mQ) ^ CO CO O CO c3 Eo (0 U 2 cc oO) ~u <5 a| ; 2 c! 1=5* 3 22 c JO c *5 E xo: CO a T> jjj CO Ow LL. C o CO o c ul ^ o<o 0* JoSC *c3 5> 5> O CO aO JO Xa E X *cO m H* 1+ CcO o h. c o w> a. So 5E a m 3 ot>ov*4 e "o C\J + I a c o a. nE o ofl jjj o Q. 13 3 L a CM O *to -- + o o &JS -- a CO E *wo>o* CS cm & u m <m= <a o ----- > tr tr is K1+ E + 5S o 5 c *0 &<0 CJ oo . O *- X S& O + * * QO. I1 > O, o <o X, 8^ > 2 -Hi S c3 oN *o u XI II o CO Bibliography Bibliography Agapova, G.F., Modnikov, I.S. and Shmariovich, Ye.M., 1989, Experimental study of the behavior of titanium in hot sulfide-carbonate solutions. 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Wood, B.J. & Banno, S., 1973, Gamct-orthopyroxene and orthopyroxene-clinopyroxene relationships in simple and complex systems. Corn. Min. Pet., 42, pp. 109-124. Woodward-Clyde Consultants, 1990. Characterization of disturbances related to mining and exploration in the New Idria/Coalinga/Table Mountain study region. Prepared for the U.S. Environmental Protection Agency, Region 9, San Francisco. 73 pp. Yada, K., 1967, Study of chrysotile asbestos by a high resolution microscope. Acta Cryst 23, pp. 704-710. Zigan, F. & Rothbauer, R., 1967, Neutroncnbeugungsmessungen am BruciL Neues Jahrb. Min. Monat., pp. 137-143. Bib-31 Ziglcr, J.L., Wentworth, C.M. and Bartow, J.A., 1986, Structure contour map of the top of the Kreycnhagen Formation and Cretaceous strata in the Coalinga area, Fresno and Kings Counties, California. Reston, Va: U.S. Geological Survey map MF-1843. Note to the catalogen this thesis contains 482 pages and one folded map. Bib-32 PLEASE NOTE: Oversize maps and charts are filmed in sections in the following manner: LEFT TO RIGHT, TOP TO BOTTOM, WITH SMALL OVERLAPS The following map or chan has been refilmed in its entirety at the end of this dissertation (not available on microfiche). A xerographic reproduction has been provided for paper copies and is insened into the inside of the back cover. Black and white photographic prints (IT x 23') are available for an additional charge. UMI UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY PlPi' Plate /. 7n J^`30M V !' 7>VV i o?o rr.C' SANTA RITA EAK QUADRANGLE CALIFORNIA - - AC* ..V 7.5 MINUTE SERIES (TOPOGRAPHIC) it/* (OfflA I) aUAO*NCLC v:o' 73 ' y, 1?0'70` 36'?2'3cr -////A' f. 380 OOC UFeet :: v^v -a.-v. h i* ) : v' :r.' l?0M7^0 .c *.* Mapped edited, a^d oubis^ed bv Goo'CR'Ca* Survey Csm*c* !)v USGS n-d USC^O^ '* TopoB^'C^t *y pKOnp* %* |0*i7 **0*" ***< C**C*<1 1 999 * ` P-.(v^f..r -- 19?? 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