Document 44wNDRZx7XGjwEjOK9Dp5Jgvp

PLAINTIFF'S EXHIBIT SA-560 MINERALOGY AND PETROLOGY OF THE NEW IDRIA DISTRICT, CALIFORNIA A DISSERTATION SUBMITTED TO THE SCHOOL OF MINERAL SCIENCES AND THE COMMITTEE ON GRADUATE STUDY OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN GEOLOGY By Robert Griffin Coleman January 1957 I certify that I have read this thesis and that in my opinion it is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. I certify that I have read this thesis and that in my opinion it is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Approved for the University Committee on Graduate Study: Dean of the Graduate Division ii This is an authorized facsimile, made from the microfilm master copy of the original dissertation or master thesis published by UMI. The bibliographic information for this thesis is contained in UMI's Dissertation Abstracts database, the only central source for accessing almost every doctoral dissertation accepted in North America since 1861. R UMI Dissertation Services From: Pro bluest 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Michigan 48106-1346 USA 800.521.0600 734.761.4700 web www.il.proquest.com Printed in 2002 by digital xerographic process on acid-free paper TABLE OF CONTENTS Page Introduction .............................................................................................................. Purpose ............................................................................................................. Acknowledgements ........................................................................................... Laboratory techniques .............................................................................. 1 3 5 5 General Geology .................................................................................................... 7 Mineralogy.................................................................................................................. Perovskite........................................................................................................ Benitoice . . .,........................................................................................... Neptunite ............................................................................................... Joaquinite........................................................................................................ Garnet Group .................................................................................................... Pyroxene Group............................................................................................... Amphibole Group........................................................................................... Mica and Clay Group.................................................................................. Natrolite ........................................................................................................ Idocrase............................................................................. .... ........................... New Mineral.................................................................................................... Mineral List . ............................................................................................... 14 14 19 22 24 25 38 46 51 58 6l 62 63 Petrology................................................................................................................. Franciscan type rocks............................................................................. Peripheral Franciscan rocks....................... .... ............................ Tectonic inclusions ............................................................................. Jadeite bearing rocks ......................................................................... Origin of the jadeite......................................................................... Serpentines.................................................................................................... Intrusive rocks ........................................................................................... Camptonite............................................................................................... Barkevikite soda syenite................................................................ - Albitite and zeolite-rich rocks .................................................. Classification and origin of the igneous rocks .... Metasomatic rocks.......................... Chlorite rocks ...................................................................................... Calc-silicate rocks . . ",................................................................ Titano-silicates and sodium silicate rocks ....................... Origin ox' the metasomatic rocks................................................. 68 68 68 73 80 89 93 100 101 104 113 115 119 120 132 139 143 Minor and trace element study of rocks.................................................. Introduction................................................................................................... Boron........................................ Vanadium............................................................................................................ Galliun............................................. Chromium................................................................................ Titanium............................................................................................................ 146 146 148 149 149 150 150 iii TABLE OF CONTENTS Minor and trace element study of rocks (continued) Colurabium.............................................................................................. Nickel and cobalt..................................................................................... Scandium........................... Zirconium........................................................................................................ Rare earths................................................................................................... Barium and strontium............................................................................ Silver............................................................................................................ Lead................................................................................................................. Copper............................................................................................................ Other elements.......................................................................................... Summary............................................................................................................ Literature Cited ............................................................................................... LIST OF TABLES Table 1- Chemical analysis and physical properties of perovskite . ................................................................................. 2- Chemical and physical properties of benitoite ... 3- Chemical and physical properties of neptunite ... 4- Chemical and physical properties of joaquinite . . 5- Chemical and physical properties of titanian andradite.......................................................................................... 6- Determination of Ti02 content from unit cell measurements ................................................................................. 7- Refractive indices and Ti02 content of titaniferous garnets .......................................................................................... 8- Chemical and physical properties of andradite garnet ..... ........................................................................ 9- Refractive indices and densities of the New Idria garnets.............................................................................................. 10- Chemical and physicalproperties of white Jadeite . 11- Comparison of optical data on jadeites................. Page 151 152 153 153 154 155 156 157 I57 158 I59 161 18 21 23 26 28 32 33 35 37 40 42 iv LIST OF TABLES Tables (continued) Page Table 12- Comparisons of X-ray powder diffraction measure ments of Jadeite.............................................................................. 1*3 13- Optical constants of pyroxenes . ........................................ 4p 14- Chemical and physical properties of barkevikite . kj 15- Optical constants of glaucophane ...................................... 49 15- Optical constants of crossite .......................................... 50 17- Optical constants of New Idrin chlorites and related minerals ........................................................................ 5k 18- Chemical and physical properties of kamraererite . 59 19- Optical properties of kamraererite compared with Cra03 and AI2O3 content...................................................... 60 20- Variation in the optical constants of the idocrase with Ti content...................................................... 62 21- Modal analysis of camptonite ..... ........................ 105 22- Modal analysis of syenite.......................................... 108 23- Chemical analyses of camptonite and syenite . . . in 24- Norms of analysed camptonite and syenites .... 112 25- Calculation of the standard rock cell for serpentine and calc-silicate ............................................. 156 LIST OF ILLUSTRATIONS Plate I- Geologic map of the New Idria district........................In pocket II- Geologic map of calc-silicate body................................ In pocket III- Geologic map of the Gem mine.............................................. In pocket IV- Spectrcgraphic analysis of rocks andminerals . . In pocket V- Photomicrographs of perovskite crystal ...... 16 v LIST OF ILLUSTRATIONS Plates (continued) Plate VI- Photomicrographs of peripheral Franciscan schists . . 74-A VII- Photomicrographs of schist- from tectonic inclusions......................................... ................................................. 8l-A VIII- Photomicrographs of the serpentines .................................. 96-A IX- Photomicrographs of caraptonite and syenite........................ 114-A X- Photomicrographs of metasomatic rocks ................................ 1J0-A XI- Camera lucida drawings of metasomatic rocks .................. 131 XII- Photomicrographs of metasomatic rocks ................................ 131-B XIII- Camera lucida drawings of metasomatic rocks ..... l4l Figure 1- Index map showing location of the New Idria district . 2 2- Relation of unit cell size and TiC>2 content in titaniferous andraddtes............................................. .... 30 3- Relation of refractive index to density in the grossularite-tricalcium aluminum hexahydrate series . 37-A 4- New Idria chlorites plotted on Winchell's optical and chemical classification for chlorites ....................... 53 5- Camera lucida drawing of kammererite showing its formation from chromite...................... 57 6- Sketch of lens-like Jadeite body ......................................... 83 7- Geologic map of tectonic inclusion containing jadeite ......................................................................................... 85 8- Polished slab of albite-glaucophane-acmite schist . . 86 9- Sketch of Jadeite vein.......................................................... . 88 10- Geologic map of syenite intrusion..............................................106 vi 1 INTRODUCTION The area commonly referred to as the Nev Idria District is located i the southern extension of the Diablo Range of the California Coast Rangec. The rock complex studied le an elongate serpentine dome about 12 miles long and 4 miles wide, situated wholly within the New Idria, California quadrangle (Figure ]). The New Idria District has long been known as the site of one of California's most productive quicksilver deposits. Cinnabar was first discovered about 185' and since that time, up to 1944, 437,195 flask6 of quicksilver have been produced. This production is valued at about $31,000,000 (a flask of mercury now contains 16 pounds, although this weight has varied during the production history of the district). The geology of the ore deposits has been described by many writers, in cluding Becker (1888), Eckel and Ityers (1946), Lake (1929), Schutte (1931), and Yates and Hilpert (1945). These reports and accompanying maps have been consulted frequently and in part have been incorporated into the present study. Since this present investigation is not con cerned directly with the cinnabar deposits, the reader is referred to the above references for a complete description, particularly Eckel and tfyers. The district is situated in a rugged and isolated portion of the Diablo Range trending about N 40 W. The serpentine dome is charac terized by soft rounded hills, excepting San Benito Mountain (Elev. 5248) and Santa Rita Peak (Elev. 5164) which are prominent peaks of more resistant serpentine. Flanking the serpentines are resistant 2 FIGURE 1 3 sedimentary ridges marked by Sampson Peak (Elev. 49OO), San Carlos Peak (Elev. 4845), and Wright Mountain (Elev. 4560). To the south, the serpentine dome assumes a sharp ridge form which branches to the east from the main Diablo Range and is here called Joaquin Ridge. The relief within the serpentine is somewhat subdued as a result of the peculiar way in which the serpentine weathers. In contrast, the relief is sharply increased in the more rugged sedimentary rocks. The total relief of the area studied is about 3,000 feet. The headwaters of the San Benito River lie in the central and western portions of the serpentine. The river drains to the SW where it cuts a deep canyon through flanking sedimentary rocks. The northern part of the serpentine is drained by Clear Creek which also flows SW through sedimentary rocks and joins the San Benito River at Hernandez, whereas the southern part of the dome is drained by White Creek which joins Los Gatos Creek to the south. The drainage divide of the area is described by a line through San Carlos Peak to the north and Santa Rita Peak and Joaquin Ridge to the south. All of the east flowing streams are small and intermittent. ' The serpentine area is marked by poor soil development and an almost complete lack of grass and much of the area is completely bare of vegeta tion. In contrast, the ground cover in the sedimentary rock area is marked by well developed grass and open oak groves. PURPOSE This investigation was undertaken to study a peculiar suite of miner als found within the serpentine and the associated rock types; it is a well known area for the variety of minerals found within the serpentine, and the discovery of benitoite (Louderback, 1907) was the first report on 4 the peculiar and distinct type of local mineralization- Since this first account many additional discoveries of small bodies containing calcsilicates and titano-silicates have been recorded, and recently several outcrops that contain Jadeite were discovered along Clear Creek, increas ing the already complex variety of minerals present in this interesting area. An attempt was made to correlate these small isolated mineral complexes with the surrounding and enclosing rocks in order to ascertain if all of these deposits were related to a single stage of mineralization within the serpentine or if they represented several distinct periods of mineral introduction. In conjunction with the problem of origin, detailed mineralogical studies are herein reported for many of the individual mineral species to establish their exact chemical and physical character. Field work and mineral collecting extended through 1950 to 1952; no long periods of time were spent in the field although the aggregate time spent doing field work was about three months. Considerable time was spent in the laboratory purifying and analyzing the mineral species; the methods used in the laboratory investigation will be described in detail. All of the rock and mineral samples collected are now in the Stanford Univer sity collection. The samples are labeled with the author's initials (RGC) followed by the sample number and the year collected, i.e., HGC-32-50. These sample numbers are used in the text to identify the specimen and also for location. On plate I the sample numbers are plotted at points of collection in order that the reader may ascertain the geologic location of each specimen. 5 ACKH0WL5DG2MEWTS The author wishes to acknowledge the helpful guidance and counsel ing provided by Professor C. 0. Hutton during the course of this in vestigation. Laboratory facilities and equipment were provided at various stages throughout the investigation by Louisiana State Uni versity, Atomic Energy Commission, and the U. S. Geological Survey; these are gratefully acknowledged. I am indebted to Hatten Yoder and George Switzer who reviewed the manuscript and have given me many helpful suggestions. LABORATORY TECHNIQUES Those minerals which were studied in detail were carefully sepa rated from the enclosing rock by various techniques to ensure a pure mineral phase. Since these methods are not standard, they are de scribed below. The Franz Isodynamic Separator proved to be the most useful device in effecting a preliminary concentration of a desired mineral. The rock is crushed to the desired size to ensure that the individual fragments were free from inclusions, and the sized fraction is then elutriated to remove the finer dust. The coarse sized material is then passed through the magnetic separator to effect a preliminary concentration, which was then centrifuged with appropriate heavy liquid until a relatively pure concentrate was obtained. All of the mineral species analyzed in this paper were purified in this manner and in all cases the analyzed ma terial was 95^ or greater in purity. It was found that for the complex calc-silicate and titanc-silicate rocks, the various minerals could be isolated and grouped by a generalized o separatory procedure. The various steps follow: 1. Rock crushed to pass through 100 mesh sieve and then elu triated to remove fine dust particles. Further sizing was carried out so that the material used for magnetic sepa ration was between 100 and 200 mesh. 2. Abouu 50 grams of the sized material was passed through r,he magnetic separator at settings from 0.2 amps to 1.5 amps in steps of 0.2 amps. 5. Each fraction was then inspected under the binocular to deter mine the number of minerals present. 4. Those fractions which showed two or more minerals present were centrifuged first in bromoform and the heavies thus obtained were further centrifuged in methylene iodide. The methylene iodide heavies were further centrifuged in Clerici solution if additional separation was necessary. 5. Routine optical ar.d micro-chemical methods followed. This method of determining the minerals present has many advantages over thin section study, since those minerals present in minor quantities may be completely overlooked in thin section; whereas in a 50 gram sample, systematic separation almost invariably produces enough of these minor miner als for a positive identification. Further, by magnetic and density frac tionation, identification of the minerals is facilitated as the minerals are placed in definite ranges according to their density and magnetic properties. Tabulated below are the minerals commonly found to be suscep tible at a range of field strengths as determined during this investigation. The separator was set at a tilt of 5 and a slope of 10. Amperage < 0.2 0.2 - 0.4 0.4 - 0.6 0.6 - 0.8 > 0.8 ilmenite - chromite. Fe-chlorite - stilpnomelane - barkevi'^ite andrauite - melanite. Mg-chlorites - idocrase - sphene. idocrase - apatite - calcite - diopside. apatite - perovskite - zeolites - calcite. 7 Analyses of the minerals completed by the author followed the pro cedures recommended by Hillebrand and Lundell (192y). These analyzed minerals posed no particular problems and the procedures followed were strictly those recommended for the elements desired. Density determinations on purified mineral powders were made with a micro-pycnometer and CCl^ as the liquid. Other density determinations were made on the Berman Balance. Determinations of refractive indices were made by the immersion method and the combination of 'oils' which matched most closely the desired opti cal direction was measured directly by the Abbe' refractometer using sodi um light. For those minerals which have indices greater than the immersion liquids, a small prism was made along the desired optical direction and the refractive index was determined on a single stage goniometer by the minimum deviation method. The universal stage was used to relate the optical direction with crystallographic direction, and also for determi nation of feldspar compositions using the method described by Turner (19^7). The semi-quantitative spectrographic analyses of the rocks and miner als were done on 10-mg samples following the method of Waring and Annell (1953) GENERAL GEOLOGY The general geology of the New Idria District will be discussed very briefly in order to establish the geologic situation in which these peculiar raetasomatic rocks occur. The geology of the region has been described in more detail by several writers; Eckel and Hyers (19^6), Anderson and Pack (1915), Mielenz (1939)* 2-nd Phillips (1939)- Their maps and discussions 8 have been consulted freely and in part incorporated into this discussion. The geologic map of the area (PI. I) is essentially the same as that of Eckel and Myers, although the author has added or subtracted data where there was a difference of interpretation. All the major contacts were walked out and os much of the serpentine area as possible was carefully investigated by waLking out all of its drainage system. Serpentines, which make up the largest ureul unit of the mapped area, lie between the San Andreas fault on the west and the Great Valley of California on the east. The rocks in this area are folded in a series of anticlines and synclines trending about N. 70 W. Thin folding is some what oblique to the general trend of the San Andreas fault which trends N. 40 W. The serpentine is an elongate oval body which is flanked by the Franciscan formation of Jurassic age and the upper Cretaceous Panoche formation. Serpentine and flanking rocks form an asymmetric anticlinal dome. Eckel and Myers have shown that the northeast flank of the dome is marked by overturned beds and by the irregular New Idria thrust fault which follows along or near the Franciscan-Panoche contact. The remainder of the contact around the dome is marked by a shear zone which is expressed by topographic lows and where the contact can be inspected it is marked by high angle faults or shear zones. The fault bounded serpentine seems to have moved upward in relation to the Franciscan and Fanoche formations. The rocks which crop out in the area investigated are divided into six types: (l) Franciscan formation of Jurassic age, (2) serpentine, (5) Panoche formation of Upper Cretaceous age, (4) younger Cretaceous and Tertiary sedimentary rocks, (5) syenite intrusives, and (6) superficial deposits. These have been listed in their apparent chronological order and 9 will be discussed, briefly in that order. The Franciscan formation has been considered as Jurassic in age by Reed (1933) and Taliaferro (19^3) and in this particular area no fossil evidence is present to support or disprove this age. Since the Franciscan is the basement rock here, as it is in many areas, the preJurassic geologic history is obscure. The Franciscan consists mainly of greywacke sandstone with some interbedded conglomerate and shales. Minor in volume but characteristic of the Franciscan formation are thin bedded lenses of chert associated with basaltic lavas. The basaltic rocks are discontinuous and are largely altered to greenstones which are spilitic in character. The Franciscan formation is at i-.r-.s*' 5000 feet in thickness and the most extensive exposures are along the southwestern part of the dome; elsewhere it appears as a discontinuous rim around the serpentine. Large and small tectonic inclusions within the serpencine may have been formerly part of the Franciscan formation; these have been mapped as Franciscan (PI. I). Metamorphism of the Franciscan formation has been irregular and spotty in the district and has produced a grade of metamorphism similar to that of the green schist facies; glaucuphane ecnists of the Franciscan formation are found within these metamorphosed zones. The Franciscan in other parts of the Coast Ranges is generally overlain unconformably by Cretaceous and Tertiary rocks, although in the New Idria District the contacts between the Franciscan and younger formations are . usually faulted. The Franciscan formation, therefore, seems to have gained its present position by tectonic movements and a direct stratigraphic sequence with the younger sedimentary rocks cannot be established. A more 10 complete discussion of the rod; types within the Franciscan is given later in the section on petrology. The serpentine in the New Idria District as well as those serpen tine bodies 'intrusive* to the Franciscan formation in the Coast Ranges have been considered as part of the Franciscan formation and tentatively are called Jurassic in age. Detrital serpentine within the lower beds of the upper Cretaceous rocks in this district chows that seme of the serpentine is at least older than upper Cretaceous. The serpentine forms the entire central part of the district and Eckel and ttyers report small, elongate bodies of serpentine along the New Idria and other faults where these separate the Franciscan and Panoche rocks. An important feature of the serpentine is the nature of its contact with the surrounding sedimenta ry rocks. This contact is most certainly faulted around the entire serpen tine body and evidence of intrusive contact action is completely lacking. These faulted contacts suggest that the serpentine has been brought into its present position by tectonic movement and not by intrusive forces; therefore, the age of the primary ultrabasic rock cannot be determined by the present exposed contact relationships. The original movement of the ultrabasic 'magma' (probably a crystalline mush) into the earth's crust may well have been during the period of maximum downwarping of the Franciscan gccsyncline. Tectonic movements following the original emplacement of the ultrabasic rocks have probably resulted in the serpentinization and upward squeezing of the rock into its present position. The emplacement of the serpentine may have been accomplished in several distinct stages, and this view is sup ported by the presence of abundant serpentine debris in nearby sediments of three widely different ages; in the upper Cretaceous (Panoche formation). 11 the late middle Miocene (Big Blue member of \.ne Vaqueros formation), and the Pliocene (TVlare formation) as pointed out by Eckel and ^ers. A more detailed description of the petrology of the serpentine is given later in the text. The. Panoche formation, of upper Cretaceous age, consists of shaly beds and sandstones. It completely surrounds the body of serpentine and Franciscan rocks, extending to the northwest and southwest away from the mapped area. The Panoche formation is 20,000 feet thick at the type section, (Anderson and Pack, 1915), although in the southern portion of the district only 10,000 feet are present and near the New Idria mine it thins to 5,COO feet. As mentioned earlier, the contacts between the Panoche formation and the older Franciscan formation are faulted and it is presumed that the pre-faulting relationships were unconformable. The basal layers of the Panoche formation are conglomeratic and contain abundant serpentine and Franciscan debris cemented with magnesite. The basal conglomerate grades upward into shale and concretionary sandstone in about equal proportions. Most of the sandstone is in the upper half of the formation although sandstone lenses are commonly interbedded in the lower shaly member. The Panoche formation is important with respect to the cinnabar deposits of the district,as most of the larger ore deposits are found within the Panoche formation. The Panoche formation is overlain conformably by the Moreno shale of upper Cretaceous age. The Moreno crops out in a continuous belt near the north edge of the area and is 5>000 feet in thickness in the northwest corner of the district but thins to about 1,000 feet near New Idria. The 12 Moreno is characterized by chocolate-brown to maroon or purple platy organic shale which generally serves to distinguish it from the Panoche formation and the overlying Tertiary beds. The Moreno forma tion is made up of clay shale of clastic origin, organic siliceous shale with foraminifera and diatoms, and some lenses of arkosic sandstone. Large scale calcareous concretions with megascopic fossils are commonly found in the shales. The Tertiary beds range in age from Eocene to Pliocene and represent several thousand feet of thickness. The beds are undivided since they have little bearing on the present investigation. These beds shown only in the northern part of the map (PI. I), consist largely of soft gray clay which have been interpreted as lake deposits by Mielenz (1939) and Eckel and layers (1946). ' Within the serpentine three small intrusive bodies of syenite and camptonite are located in the southeastern portion of the dome. Two of these bodies are clearly intrusive into the serpentine and the third body along the extreme southern border at tne headwaters of White Creek is part of a large landslide mass which pushed out over the Panoche formation obscuring the original relationships.. Eckel and Myers (1946, p. 91) state that the syenite is intrusive into the Panoche shales at this locality, but there is no field evidence to support this. The syenite has actually arrived at its present position across the serpentine-Panoche contact by landslide action and not by intrusive action. The age of these intrusions can only l>a inferred, although they are definitely later than the serpentine and by their lack of deformation appear to be of late Tertiary age. These intrusive rocks 13 are unique in that they have no correlatives in other parts of the Coast Ranges. Tertiary volcanic and intrusive rocks are common in the Coast Ranges but none of these rock types bear any resemblance to these syenites and camptonite3. The plastic nature of the serpentine and its deep weathering in the district combined with rather sharp relief have produced large landslide masses within the serpentine; the largest of which is in the southern portion of the area whera about four square miles of it have been mapped (PI. l). These masses are composed of the same serpentine material that is found along the ridges at their heads. The larger ones have traveled several miles crossing the serpentine sedimentary rock contacts and filling valley bottoms of the streams draining away from the dome. Some of the older landslides show con siderable erosion of the original hummocky surface. Several have forced streams to cut new valleys along their edges producing a peculiar topography, with the stream valleys having double drainage around the center filled in by serpentine debris. Many of these slides are still active, moving slowly during the rainy seasons and probably more rapidly when triggered by local earthquakes which are common in this region. 14 MINERALOGY The New Idria District has an extensive suite of rare and peculiar minerals, and this study has attempted to investigate and describe systematically the species present, exclusive of the mercury deposits. The minerals characteristic of the metasomatic bodies within the serpentine were studied in the greatest detail in addition to those from other rock types which were poorly known or exhibited exceptional development. PEROVSKITE Perovskite was first reported from the New Idria District by Bolander (1950sl) and later by Pabst (1951) and Murdoch (1951). The locality for the original discovery is given as the SE l/4 of sec. 25, R. 12 E., T. 18 S. Four additional locations have been discovered in this study, all of them within section 25- These localities are numbered 34, 37, 5^ (original discovery), 57, and 83: see Plate I. In these five localities perovskite assumes several habits and ex hibits a range of color and crystallinity. Perfect euhedral crystals are found at locality 56 and here perovskite, associated with black euhedral melanite and reddish prisms of idocrase, forms drusy surfaces and veins within a chlorite rock. Perovskito from localities 34, 37, and 57 crystallized as anhedral grains or as distinct veins within chlorite rocks, and is sometimes completely surrounded by melanite or forms discontinuous veins mixed with melanite. Localities 56 and 83 are the only two exposures which produced euhedral crystals of perovskite. 15 Euhedral perovskite is dominantly shiny black grading to dull yellow, and forms crystals up to one centimeter square. The crystals show cubic form and are commonly modified by (Oil) and (ill) faces. Occasional single octahedra are noted, particularly from locality 83, and Pabst (1951) and Murdoch (1951) report additional forms; (025), (034), (045), (311). The following interfacial angles were determined on u pic.--.-d crystal from locality 56. 100 A 010 = 6959'30" 111 A 110 = 3507'00" 100 A 110 = 4500'00" Some question regarding the symmetry of perovskite has been raised by Bowman (1908), Zedlitz (1939), and Murdoch (1951)> since the external symmetry of perovskite conforms to the isometric system, although X-ray study and the optical character suggest either orthorhombic or monoclinic symmetry. Interfacial angles measured on the New Idria perovskite certainly suggest an isometric character, although optical examination shows it to be anisotropic with biaxial optics. Perovskite is listed as pseudo-cubic in Dana's seventh edition (Palache, et al., 1944) and comparison of unit cell dimensions (calculated on the basis of isometric structure) of the New Idria perovskite with the cell edge of other occurrences showsfeir agree- ment. New Idria - 7*63 A + 0.01 Urals - 7-645 A + 0.015 Zermatt 7-590 A ? Crestmore - 7-64 A + 0.01 16 HATE V Photomicrographs of perovskite crystal cut perpendicular to the c-Axis. (X 40) Figure 1 Figure 2 Plain light Crossed nicois 17 Murdoch (1951) has suggested that perovskite crystallizes in the isometric form and on lowering of temperature inverts to a lower symmetry. This view is supported by Msgaw (1946) who has shown that synthetic double oxides of the perovskite type assume different symmetries as a result of changing temperature. Controlled heating accompanied by X-ray studies could possibly show that the inversion in perovskite may be a function of temperature. Optical determinations by universal stage show that perovskite from locality 56 is biaxial positive with an optic axial angle of 90 and ex treme dispersion, r > v. Following Bowman's (1908) interpretation of orthorhombic symmetry, Y - c and X = a. Two types of twinning were noted, polysynthetic with lamellae parallel to (00l) and the composition plane parallel to [110]. The twin lamellae show extinction at 45 to the lamination. The second type is also polysynthetic with broader lamellae parallel to {00lj and the composition plane parallel to (ill), with paral lel extinction to the lamination. Plate V shows the twinning of perov skite on a section cut perpendicular to the c-axis. Pleochroisra is narked, in the darker varieties by Z > X. A quantitative chemical analysis was made on purified perovskite from locality 56 using both light and dark material, since there was no apparent difference in their physical makeup. The analysis is set out in Thble 1 with density and refractive index data and it shows that New Idria perov skite is almost pure CaTiOs- A semi-quantitative spectrographic analysis reveals the presence of Ce and La in the range 0.1-0.5$ and Nb and Nd in the range 0.01-0.055t. Zirconium was also detected in the range of 0.005-0.01^, the complete spectrographic analysis is given in Plate IV. The lack of extensive substitution for Ca or Ti suggests that l*oo Table 1.--Chemical analysic and phyoical properties of perovskite (RGC-56B-51). ' Ti02 Si0a AI2O3 CaO MgO FeO Weight percent 58.16 0.53 0.45 40.73 0.06 0.15 100.04 Metal atoms .985 .010 .010 .935 .001 .005 ' > _ ' j 1.01 .99 Analyst, W. H. Herdsman. Calculated density = 4.oS (aQ - 7.625). Density = 4.l8 + 0.005 (Berman Balance). H = 2.585 + 0.002 (minimum deviation on prism cut parallel to c-axis). Birefringence = 0.002. xhe titanium bearing solutions were derived from a magma of basic nature rather than an alkalic type since Hb may become strongly enriched in the acid alkalic rocks. Perovskites from alkalic rocks such as those at Magnet Cove, Arkansas, and Kniserstuhl, Baden, Germany contain several percent Nb. BENITO ITE Benitoite was first described by Louderback (1907) and (1509) and since then no additional work has been reported on the genesis of this singular occurrence. Furthermore no other reports of benitoite, in place, have appeared, since the original naming and description. Reed and Bailey (1927) recognized benitoite as a detrital mineral in oil-well cuttings from wells located in the Great Valley of California, and it is assumed that the source was probably the New Idria locality. Benitoite is found in a single mineralized vein within a rather large tectonic in clusion vitliin the serpentine (PI. III). The vein is variable in width, shows numerous bifurcations, and is composed essentially of natrolite impregnated with fibrous liair-like bluish amphiboles varying in compo sition from crossite to glaucophane (PI. XIII). Benitoite accompanied by neptunite and joaquinite is always found implanted or imbedded in a natrolite gangue in open cavities of veins as drusy surfaces projecting inward to the central cavity. The vein walls are composed of pure white cockscomb natrolite which varies in thickness from fractions of an inch to several inches. The zeolite grades imper ceptibly into a bluish-green wall rock which is composed of intergrown natrolite and fibrous blue amphibole. 20 The mineral forms euhedral ditrigonal-dipyraraidal crystals with fOlll} prominent and (OOOlj , {lOlll , {l010} , Coliol common. The largest crystal reported is about two and a half inches across, although the average crystal is less than an inch. Color varies from sapphire-blue to light blue; occasionally benitoite is colorless. A strong bluish fluo rescence is produced in benitoite under ultra-violet light using both long and short wave sources. Benitoite is unique in that it is known to occur at only one locality in the world and it is the only naturally occurring example of a mineral in the ditrigonal-dipyramidal symmetry class. The unit cell and space group of benitoite have been worked out by Zachariasen (1930) as follows: 2_ space group: D j n = C 6 c Trigonal Z =2 a- 6.60 A t 0.01 c- 9.71 & t 0.01 c:a - 1.471 : 1 Louderback (1909) had duplicate analyses of benitoite made but minor or trace elements were not determined. A pttrified concentrate of benitoite was analyzed by spectrographic (PI. IV) and gravimetric (Thble 2) methods. The optical constants, determined on the material used for analysis, are given in Thble 2, where it will be noted that A1 and some Ti must be appor tioned to Si in order to give six metal atoms for this group. Ferrous iron is grouped with Ti and the Ca with Ba. In general this analytical data agrees with that given by Louderback except for those additional elements determined in the most recent analyses. It was suggested that the color variation might be due to small amounts of some unrecognized element which 21 TABU3 2. CHEMICAL AM) PHYSICAL PROPERTIES OF BENTTOITE FROM THE GEM MINE. S102 Al203 Ti02 FeO BaO CaO HgO *f* 1 43.56 20.18 36.34 - 2 43.79 - 20.00 - 36.31 - 3 43.61 - 19.50 - 37-01 - 4 43.40 0.11 20.04 0.22 36.60 0.04 nil METAL ATOMS 5-93 16.00 .01 f .06 2.06i ' (2.00 2.02 .02 J 1.96 .01 ' ii.97 * Ha0 - 100.08 100.10 - 100.12 0.19 100.60 PROPERTIES OF NUMBER 4 Omega = 1.753 i .002 Epsilon- = 1.800 i .002 (minimum deviation on cut prism) UNIAXL1L POSITIVE , (2V of 20'3 noted in strained crystal) X = colorless Z = light to deep blue DENSITY = 3.67 t .003 (Berman Balance) 1, 2, and 3 - Analyses taken from Louderback (1909), analyst Blasdale. 4 - Analyst, R. G. Coleman. 22 could produce the coloration. Spectrographic analysis of the blue mate rial shows 0.001-0.005?, V, 0.01-0.05?- tto, and 0.001-0.005? Cu and it would seem that the blue coloration might result from these trace elements. Further investigation along spectrographic lines to determine what trace elements are characteristic of the different colored varieties should be carried out. NEPTUNITE Neptunite is the most abundant Ti-silicate in the natrolite veins of the Gem mine, where it forms single, elongate crystals with distinct prismatic habit, embedded or implanted on natrolite and associated with benitoite and joaquinite. Neptunite prisms are usually attached at one end only and have grown outward with complex terminations on the pro jecting end. Tne dominant forms developed on these crystals are {10$, llo}, {ooi}, and {ill}. Ford (1909) has completely described the crystal lography of neptunite from thi3 locality. Hie neptunite is black to dark reddish-brown with a splendent luster and makes a striking specimen when implanted on the pure white cockscomb natrolite. The neptunite is found in the same relative position in the natrolite veins as the benitoite, that is, forming drusy surfaces projecting inward to the central vein cavity. Along parts of the mineralized natrolite veins, neptunite com pletely covers the surface of the vein walls to the exclusion of other minerals. Neptunite has been found only within the mineralized veins of the Gem mine. Louderback's (1909) analysis of neptunite together with optical data determined on other than analyzed material are presented in Table 5. A S5 TABLE 5- CHEMICAL AND PHYSICAL PROPERTIES OF NEPTUNTE FROM THE GEM MINE WEIGHT Si02 53.44 TI02 17.18 FeO 11.23 MgO 1.82 CaO 0.25 MnO 1.78 Wa20 9.14 k2o 5.34 100.18 ALPHA = 1.692 + .002 GAMMA = 1.708 _OPTIC ANGLE = 66 1 2 (+) EXTINCTION ANGLE, Zac = ].6 OPTIC PLANE IS PARALLEL TO (010) WITH Y = b DENSITY = 3-21 t .005 (Berman Balance) Chemical analysis from Louderback (1509), analyst, Blasdale. Optical and density determinations (by Coleman) were not made on the analyzed material. . 24 serai-quantitative spectrographic analysis for optically studied material is listed on Plate IV. A perfect prismatic cleavage is developed parallel to [lid) forming an intercleavage angle of 80; this can be seen in sections cut normal to c-axis. X-ray crystallography by Gossner and Mussgnug (1931) shows neptunite to be monoclinic with unit cell dimensions given below; Sq - 16.54 A bD - 12.64 X cD - 10.04 % beta - 11524' Further notes on neptunite from the Gem mine have been published by Arnold (1908, p. 512), Sohaller (1911, p. 55), and Buttgenbach (1937-38, p. 525). JOAQUINHE Joaquinite is one of the rarest minerals found within the New Idria District and like benitoite its occurrence here is unique. Louderback (1909) described joaquinite as a new mineral from the Gem mine and Palacba and Foshag (1932) restudied it. No later work on joaquinite has appeared in the literature. Joaquinite is sparingly associated with benitoite and neptunite as small euhedral crystals (up to 2 mm), honey yellow to brown in color and with pyramidal and flat tabular forms predominating. A second locality for Joaquinite was discovered during this investi gation along the southeastern flank of Santa Rita Peak, where it occurs 25 as extremely small crystals (less than 1.5 mm) implanted on natrolite druse within a large tectonic inclusion (see locality RGC-81-51) in serpentine. Benitoite and neptunite were not found associated with Joaquinite, although the mineralization of the host rock is similar to that found at the Gem mine. Hie chemical and physical properties are summarized in Table 4. GARNET GROUP Garnet is quantitatively an important mineral in many of the metasomatic bodies within the serpentine, and in restricted parts of the serpentine garnet is an important mineral locally. Titanian andradite (melanite as used by some authors), is the most abundant garnet in those metasomatic rocks which have a high titanium content, where it is associated with perovskite, sphene, apatite, idocrase, and chlorite. Andradite is commonly found in the unaltered serpentine or in the periph ery of the metasomatic rocks where the Ti content is low. Uvarovite is commonly associated with small pods of chromite within the serpentine and may be intergrovn with kammererite. Grossularite and hydrogarnet are found associated with the Jadeite bodies, and spessartite is present in the syenite as euhedral crystals. The low-grade metamorphic rocks peripheral to the serpentine contain garnets in some facies. A detailed study was carried out on a jet black titanian andradite from RGC-42-50 (see Plate I for location), a metasomatic rock within the serpentine. The titanian andradite occurs as euhedral dodecahedra (0.1 to 0.5 cm) implanted on chlorite (rumpfite). In thin section, the 26 TABLE 4. CHEMICAL AND PHYSICAL PROPERTIES Of JOAQUINITE FROM THE GEM MIKE." WEIGHT PERCENT Si02 36.4 Ti02 FeO BaO Na20 50.5 5-5 24.7 4.6 MgO 0.3 100.0 ANALYST - W. F. Foshag ALPHA-= 1.748 Z>Y>X ABSORPTION IN YELLOW BETA = 1.767 GAMMA =1.823 OFTIC SIGN = {+) 2V = 50 .DISPERSION - R<V PERCEPTIBLE DENSITY =3.89 * All data in this table taken froa Palache and Foshag (1952). 28 TABLE 5- CHEMICAL AND PHYSICAL PROPERTIES OF TITANIAN ANDRADITE (RGC-42-50). Wt. $ METAL ATOMS Si02 30.74 ai2o. 4.73 ^203 Ti02 17.67 11.36 FeO MnO 99 76 CaO MjO H20 + 33.51 25 nil H20 - .08 100-14 2-55 .47 1.10 .71 .07 .04 2.98 03 - f ] .02 1 > 3-00 > I.83 > 3.12 N = I.857 to 1.860 i .002 DENSITY = 3.625 1 0.005 (pycnometer, 25C) LATTICE CONSTANT = aQ 12.10 A t .01 ANALYST - R. G. Coleman IDEAL 3-00 2.00 3.00 29 placed in the octahedral positions and cost of the A1 was apportioned to the 3i positions in order to balance the excess positive charge produced by Ti in the Fe3+ positions. If the excess Ti is considered to replace Si a satisfactory balance is found, although reasons for the present apportionment will be discussed later. A titanian andradite (scnorlomite) analyzed by Zedlitz (1935) contained 17.30 percent TiOa and only 26.83 percent SiGz\ thus in this garnet much of the titanium must be lr tetrahedral coordination. In calculating the formula for this garnet it is necessary to apportion 0.75 of the Ti for Si to bring the amount of metal atoms in tetrahedral coordination up to the theoretical value of 3-00. Since the size of the Ti ion is 0.64 2 and that of A1 is 0.57 A, both larger than Si o (0.39 A), there must be some increase in the cell edge of the garnet lattice to accommodate Ti and/or A1 in the Si positions. Zedlitz (1935) has shown that this is truly the case in those titaniferous garnets analyzed by him. Zedlitz plotted the unit-cell dimensions of the analyzed garnets against the molecular percent of Ti02 and it was shown that there was an almost straight line increase of unit cell size with increasing Ti02. In the present study, Zedlitz's values have been supplemented with additional garnet analyses and all are plotted in a similar fashion (see Figure 2). In order to determine if only Ti apportioned to the tetrahedral Si positions was responsible for the lattice expansion, another plot was made (not shown) using the Ti apportioned to the Si positions plotted against the unit cell size. This plot shows a stronger divergence from a straight line function than that in Figure 2, and it seems that the total titanium in the 30 O A SOURCE 1- NEW IORIA 2*MAGNET COVE, 3- JIVAARA, 4- KAISERSTUHL, ZEDLITZ ll <1 (1933,1935). II il 5-REZBANYA, 6-SDARTA , 7-OBERBERGEN, 8'MWAARA, MACKOWSKY ( 1939). It II ll ll il ll 9-IRON HILL, LARSEN (1942). TiO % II. 36 4. 60 17. 30 12 10 NIL 0.75 9.38 12.77 5.08 UNIT CELL IN A 12 .10 12 019 12 143 12 1 04 12 .001 1 1 988 12 1 04 12 1 3 9 12 . 05 FIGURE 2 RELATION OF UNIT CELL SIZE AND Ti02 CONTENT IN TlTANIFEROUS ANDRADITES 31 structure controls the expansion of the garnet unit cell. Tlie titanium content of several garnets from the Hew Idria District was determined indirectly by measuring the unit cell and estimating the TiOa content from Figure 2. The TiOa content determined in this manner suggests that all the garnets found in the replacement veins must carry ranging amounts of titanium (Table 6). The first two garnets listed are from different zones within the same garnet. The core is a lighter color than the rim with an apparent increase in TiOo from the core out ward. The titanium andradite (RGC-5o-51) ic associated with perovskite, chlorite, and idocrase. The last two garnets in Table 6 are from the same metasocatic rock (RGC-3^-50) and a spectrographic analysis of the light green andradite shows 1-5;j Ti, or 2-8^ TiO^. This value agrees satisfactorily with the TiC>2 content determined from Figure 2 and it would appear that the color of the Hew Idria garnets is not a true index of smaller quantities of Ti substituting in the garnet structure. Comparison of the refractive index of analyzed Hew Idria titanian andradite with that of other titaniferous garnets from the literature shows that the former has a lower refractive index than that ordinarily found and it is suggested that in this instance the excess Ti4+ mainly replaces Fe3+ and not Si. Thus the refractive index would remain about the same as that of andradite. From the data presented in Table 7 it is apparent that the refractive indices of titaniferous garnets are not necessarily a function of their titanium content, although a--more complete study of these garnets could possibly produce a satisfactory explanation for the anomalies produced by Ti substitution in the andradite lattice. 32 TABLE 6. DETERMINATION OF Ti02 CONTENT FROM UNIT CELL MEASUREMENTS. SAMPLE No. MINERAL RGC-U2-50 TITANIAN ANDRADITE, BLACK (RIM) RGC-42-50 TITANIAN ANDRADITE, BROWNISH BLACK (CORE) RGC-56-5I TITANIAN ANDRADITE, BLACK RGC-34-50 TITANIAN ANDRADITE, BROWNISH BLACK ROC-34-50 ANDRADITE, LIGHT GREEN UNIT CELL IN K 12.12 12.08 12.11 12.06 12.06 Unit cell measurements made by A. Pabst. TIOp 4 14 10 13.5 8.5 3-3 53 TABLE 7- REFRACTIVE INDICES AND Ti02 CONTENT OF TITAHIFEROUS GARNETS. LOCALITY 1- NEW IDRIA, CALIF. 2- IRON HILL, COLO. 3- TURGA, FINLAND 4- ROCCA, ITALY 5- MAGNET COVE, ARK. 6- JIVAARA, FINLAND N 1.86 1--907 1.90 1.94 1.94 2.01 T102 $ 11.36 5.08 6.34 8.70 16.90 19-00 2- Larsen (1942), 5- Krar.ck (1928), 4,5,6- Winchell (1951). The close similarity of the optical and. physical properties of the dark garnets to the light colored andradite garnets led to a more detailed study of the andradite garnets found in the metasomatic bodies. In the contact zone of one of these bodies (RGC-109-52) a colorless to light green andradite was found that showed well developed dodecahedral form up to 0.1 cm in diameter. Diopside, idocrase, and chlorite are associated with the garnet. The light colored andradite grades into a dark titaniferous garnet in the central portion of the metasomatic calcsilicate body. An analysis of the andradite is presented in Table 8 together with the physical properties. Tnis andradite shows weak birefringence similar to that in titanian andradite described earlier but does not exhibit the twinning characteristics of that mineral. !he analysis shows excellent agreement with that of other analyzed andradites found in the literature. In comparing this analysis with the analyzed titanian andradite from New Idria, it can be readily seen that ferric iron was replaced to a greater extent than silica in the formation of the titanian andradite. Therefore, if the excess alumina replaces silica and all of the titanium replaces ferric iron, the physical constants of the titanian andradite, i.e., density, refractive index, should not be radically different from andradite; as the atomic weights of titanium and iron are compara ble as are their ionic refractivities. If this generalization holds true, it might well explain the similarities (optics and density) between the andradite and titanian andradite of mhe New Idria District. On the other hand if Lha titanium enters the andradite structure and replaces silicon in preference to ferric iron it should be expected The close similarity of the optical and physical properties of the dark garnets to the light colored andradite garnets led to a more detailed study of the andradite garnets found in the metasomatic bodies. In the contact zone of one of these bodies (RGC-109-52) a colorless to light green andradite was found that showed well developed dodecahedral form up to 0.1 cm in diameter. Diopside, idocrase, and chlorite are associated with the garnet. The light colored andradite grades into a dark tit-aniferous garnet in the central portion of the metasomatic calcsilicate body. An analysis of the andradite is presented in Table ft together with the physical properties. This andradite shows weak birefringence similar to that in titanian andradite described earlier but does not exhibit the twinning characteristics of that mineral. The analysis shows excellent agreement with that of other analyzed andradites found in the literature. In comparing this analysis with the analyzed titanian andradite from New Idria, it can be readily seen that ferric iron was replaced to a greater extent than silica in the formation of the titanian andradite. Therefore, if the excess alumina replaces silica and all of the titanium replaces ferric iron, the physical constants of the titanian andradite, i.e., density, refractive index, should not be radically different from andradite; as the atomic weights of titanium and iron are compara ble as are their ionic refractivities. If this generalization holds true, it might well explain the similarities (optics and density) between the andradite and titanian andradite of the New Idria District. On the other hand if the *itanium enters the andradite structure and replaces silicon in preference to ferric iron it should be expected 35 TABLE 8. CHEMICAL AND PHYSICAL PROPERTIES OF ANDRADITE GARNET (RGC-42-50). Wt. #_____________ METAL ATOMS S102 A1203 Fe203 35-33 2.98 25.48 2.96 .04 3.00 30 .26 1.60 Ti02 FeO 57 .04 52 .04 > 1-96 MnO .22 .02 y 3.11 CaO 33.76 3.02 MgO H20+ .74 nil .09 HoO- .38 99-98 N = 1.879 t .002 DENSITY = 3.717 i 0.003 (pycnometer, 24C) ANALYST - W. H. Herdsman IDEAL 3.00 2.00 3-00 that the density and the refractive indices would ce radically different, as the atomic weight ar.d ionic refractivities of silicon and titanium are not similar. Or. this basis, the titanium in the titanian andradite was apportioned to ferric iron because the refractive index indicated very little substitution of titanium for silicon. When the indices and densities of the garnets from the metasomatic and serpentine rocks are compared, their constant nature is remarkable considering the amount of substitution (Table S) Grossularite has been reported from the Hew Idria District by Pabst (1951, P- 482) and Yoder and Chesteriran (1951, p. 5)- Although the author was unable to find grossularite, minor concentrations of hydro garnet were found associated with the jadeite pods in a prehnite-rich rock. The hydrogarnet makes up to 10>L- of the rock and occurs as granular masses intergrown with prehnite. Individual grains of the hydrogarnet show distinct zoning which is manifested by differences in refractive index. From the core outward the refractive indices increase in value. A portion of the rock showing a high concentration of hycLrogarnet was ground to minus 500 mesh and centrifuged in liquids of known densities and the refractive index of each density fraction was determined in order to establish the compositional range of the zoned hydrogarnet. The determinations show that the zones of hydrogarnet fit nicely into the grossularite-tricalciun aluminum hexahydrate series as described by Hutton (1943, p. 174) and by Yoder (1950b, p. 243). These data plotted on the diagram (Figure 3) published by Yoder indicate that the com position of the inner cores extends below that of plazolite and hioschite 37 TABLE Q.~ REFRACTIVE INDICES AND DENSITIES OF THE HEW IDRIA GARNETS. SAMPLE No. MINERAL RGC-5^-50 HGC-109-52 LIGHT GREEN ANDRADITE DARK BROWNISH BLACK MELANITE -/ RGC-109-52 ANDRADITE (ANALYZED) RGC-67-51 ANDRADITE, LIGHT GREEN RGC-92-52 ANDRADITE, LIGHT GREEN RGC-37-50 MBLANITE, BROWNISH BLACK RGC-42-50 MELANITS, BLACK (ANALY2ED) RGC-56-51 MELANITE, BLACK RGC-3^-50 MELANITS, BROWNISH BUCK RGC-14-50 ANDRADITE, LIGHT YELLOW EGC-27-50 ANDRADITE, LIGHT GREEN II 1.83 I.863 1-879 1.863 I.865 I.863 I.857 1.866 1.866 1.869 1.862 DENSITY 4.00* - 3.717 - 3.625 3.72 3.71 3.68 3.78 * High density may be due to chromium, spectrogranhic analysis shows 1-5?- Cr. ` l/Melanite is used here as a synonym of titanian andradite. 37-A r e f r a c t iv e in d e x -Points token from Voder (1950). FIGURE 3. RELATION OF REFRACTIVE INDEX TO DENSITY IN THE GROSSULARITE - TRICALCIUM ALUMINUM HEXAHYQRATE SERIES. SIX SEPARATE ZONES OF NEW IDRlA HYDROGARNETS PLOTTED; NUMBER ONE, INNER ZONE GRADING OUTWARD TO NUMBER SIX, THE PERIPHERAL ZONE. and the peripheral zone extends up to hydrogarnets. According to Hutton no natural minerals of the series more hydrous than plazolite have been found, and therefore this appears to be the first recorded case of a natural hydrogarnet containing almost 14;J HaO. Yoder (personal communication) has found that synthetic hydrogarnets assume a zoning similar to the naturally occurring material described. Since this synthetic material was produced in open tubes at constant temperatures and pressures there may have been some leaching of silica, therefore, the zoning of the hydrogarnets cannot, as yet, be explained experimentally. PYROXENE GROUP The New Idria District is particularly noteworthy for the variety of pyroxenes present in the different rock types. The serpentine is characterized by the orthopyroxenes, the dynamothermal metamorphic rocks by Jaueite-acmite, the metasomatic rocks by diopside, and the late stages of the syenite intrusion by aegirine-augite. In many cases, pyroxene is the dominant mineral and therefore is quantitatively important in the petrology of the district. The first authenticated outcrop of jadeite in California was estab lished by Yoder and Chesterman (1951) along Clear Creek within the serpen tine. It seemed desirable to make a complete study of the jadeite since there has been considerable interest in this particular locality. The jadeite is found in small veins cutting albite-glaucophane-acmite schist and as larger lens -shaped pods within serpentine. A more complete description of the occurrence is given in the section deuling vith the petrology of the district. The vein jadeite is coarsely crystalline and almost pure white whereas jadeite from the pods is vari egated vith fractured green jadeite healed by anastomosing veins of white jadeite. Prehnite, hydrogarnet, thomsonite, and minor sphene are associ ated with jadeite pods, while vein jadeite is found with albite, analcite, natrolite, and thomsonite. The detailed study of the jadeite was carried out on the white vein material which is almost pure jadeite (Coleman, 195*0. The analyzed material was very carefully purified by centrifuging in heavy liquids and the resultant concentrate is 99*5.^ pure with possibly a minute amount of analcite present as inclusions. Two additional analyses of jadeite collected by Yoder and Chesterman from Clear Creek were obtained from George Switzer. The analyses of the white jadeite accompanied by physical properties are presented in Table 10. These two independent analyses of the white jadeite from Clear Creek definitely establish this material as an almost pure jadeite. Recalcu lations of these analyses on the basis of six oxygen to the unit cell show a close correspondence to the ideal formula for jadeite (Table 10), In analysis number 2, sodium appears to be somewhat low, although this may indicate differences in the two analytical methods. The high water, in number 2, may be accounted for by the presence of analcite as an im purity. Some of the aluminum in analysis number 1 proxies for Si, although according to Yoder (1950a) most of the aluminum must be in six-fold coordination to allow for the closer packing and thus higher 40 TABLE 10. CHEMICAL AND PHYSICAL PROPERTIES OF WHITE JADEITE FROM CLEAR CREEK. 1 METAL ATOMS 2 METAL ATOMS Si02 AlaOa T102 F-2O3 59.06 24.62 .08 .41 1-99 r .01 .98 < .97 .002 L .018 j> 2.00 > 1.00 59-38 25.82 .04 .45 2.00 j* 2.00 1.02 >1.03 .01 FeO .18 .006 trace - MnO .03 - -- MgO 17 .008 .12 .003 CaO Na20 35 14.95 .012 .983 > 1.00 13 13.40 .005 >0.88 .876 K20 .01 - .02 - h2o+ .07 - .16 - h2o .03 - .22 - Cr203 - 99.96 PHYSICAL PROPERTIES OF No. 2 ALPhA = 1.654 t .002 BETA = 1.657 GAMMA = 1.666 DENSITY = 5.43 i .005 (Berman Balance) 11 X .01 99.75 - OPTIC ANGIE = 70 EXTINCTION ANGLE = 34 = Z COLORLESS NORM #1 Jd 37* di 2* ac 1* #2 98* 4* 1 - From Switzer (Analyst - E. H. Oslundj TJ. of M. Lab.) 2 - Analyst - W. H. Herdsman. density of jadeite. The optical properties of the jadeite compare favorably with those of other published data (Table 11). A further comparison of the analyzed material from Clear Creek was made on the powder diffraction pattern with the published data of Yoder (1951). The indexing of the lines was kindly done for the author by Daphne Riska of the U. S. Geological Survey (see Table 12). The close agreement between these X-ray powder diffraction measurements indicates without a doubt chat tr.e Clear Creek material is one of the world's better localities for pure jadeite. Wolfe (1955) has described a second locality of Jadeite from Cloverdale, California and his careful crystallographic study shows well developed crystals of jadeite (9*+$ jadeite - 6$ diopside) formed in two generations in small veins cutting glaucophane schist. Optical determinations on the green jadeite associated with the white jadeite indicate that considerable Ca, Mg, and Fe are admixed with this variety. A chemical analysis of the green material kindly put at the disposal of the writer by George Switzer, shows about 10$ diopside and 14$ acmite. Jadeite has also been found as a crystalloblastlc constituent in one of the metamorphic rocks (RGC-77-51) forming a large tectonic inclu sion within the serpentine about 5/4 of a mile north and east of the Clear Creek locality. Mielenz (1959) was the first to recognize that jadeite was present in the schists of this area. In contrast to the Jadeite from the metamorphic rocks, the calciummagnesium end member, diopside, dominates in the calc-silicate metasomatic rocks. Diopside from these rocks, white to very faint pale green, 42 TABLE 11. COMPARISON OF OPTICAL DATA ON JADEITES. ALPHA BETA GAMMA 2v Zac 1- 1.654 1.657 1.666 70 34 OO 2- 1.655 1.659 1.667 34-5 3- 1.640 1.645 1.652 67 40 4- 1.664 1.671 1.694 - - 1- Clear Creek, Calif. 2- From H. S. Washington (1922). 3- Cloverdale, Calif., from C. V,'. Wolfe (1955)- 4- From Larsen and Berman (1934). I. - TABLE 12. COMPARISONS OF X-RAY POWDER DIFFRACTION MEASUREMENTS ON JADEITS 6 C2h hkl 110 020 021 220 221 510 221 4oo 512 550 ` o4i 241 441 uuo JADEITE - CLEAR CREEK d(X) 6.2 4.27 5.24 5.10 5.06 2.82 2.41 2.25 2.21 2.07 1.96 1.88 1-57 I.55 jadeite - burma' _^5 6.19 4.29 5.25 5.11 2.92 2.82 2.42 2.24 2.19 2.069 1.971 1.892 1-577 1.555 * Data taken from Yoder (1950 a) ^4 forms thin bladed elongated crystals. Two habitats have been noted: (1) as an early mineral within the metasomatic rocks associated with ldocrase and chlorite, and (2) as a late mineral forming delicate crys tals in cavities and vugs. Optical determinations on diopside from eight localities (Table 15) show little variation from essentially pure (CaMgSi20e) with little or no substitution of iron or sodium. Occurrence of diopside in such rocks as these is particularly noteworthy since the diopside has formed contemporaneously with hydrous minerals, such as chlorite, and is commonly in juxtaposition with andradite and titanian andradite. Diopside is a common mineral in 'skarn' type metamorphic deposits, but at Hew Idria, diopside has formed during metasomatic replacement of serpentine by introduction of Ca, Al, Fe, and Ti. Acmite is found within the syenite as a late deuteric mineral in veins associated with natrolite and analcite, and also may be observed as rims to barkevikite in the syenite near these late deuteric veins. An interesting but isolated occurrence of acmite (RGC-59-50) was found in the Franciscan formation near the contact on the eastern edge of the serpentine. Here veins of albite up to three inches across cut glaucopnanic rocks and contain vugy areas implanted with bladed crystals of deep green acmite (Table 15, RGC-59-50). Acmite is a common constituent in schists associated with jadeite veins. Other optical determinations were made for pyroxenes in metamorphic rocks (Table 15) of the Franciscan type, and most were found to belong to the acmite-aegirine-augite series. 1 TABLE 1 3 . OPTICAL CONSTANTS OF PYROXENES FROM THE NEW IDRIA DISTRICT s y e n ite , and the la s t th re e are from metamorphic ro ck3 . M CVO) xa 300 CCOO DaC 3OCJ CCOO 2a5 3OO c30fOc3* 3OCJ ICV6kT--OJ3*4 3OCJ CCOO a5 030 CtoO 2g 300 CCOO 232 C3OJ aQ2 W1 MxOE-i 2 3 a Q2 wa Oni r X CCOO a<X 3OO CCOO 3a5 O3a CCOO cac 3OO CCOO a05 3OO CCOO a25 30CJ fCOO a05 3OCJ CCOO Qa25 OCJ CCOO 2a5 3OO 0*M*-44 X*EOM4H CCOO a25 300 wS41 X>EO`-h4H C32O4 2wy 5 CCOO a 03CJ CtoO a25 3OO CCOO 0<*0Ca--35J CCOO ax C3OJ CCOO a25 3OO CCOO a05 3COJ CCOO aCG 3OCJ CCOO 2a5 3C0J CXO 0 2M 2a 6XO4.4-h4 W2 a01 iXr4 55 X1a X8 XC00O* rr-d<S(OuuHOHH0ptH3J u0ue KQOO4 WIMH 5WVs.J-4- rEM*h3 W tXZMH33 tMVHH 5 &>E--J4 5 y *-- 2MW20 W20O45 M2M O 2yHXCJ Xt<Cc0o1D) fpc p C>MN U < 0 s 0'COM OOjN\ OCinO O'QO 0vrOH 0'rOH 0V0O 0C25 t-4 0CO54 v-4 OOrHH 0CKON 0COn Oimr- 0cKONn Om OO O-OzT O>rH5" OmON 0on\ OK\ ON 0v0o 00 0H)) --fpCqcO.1 P3OttHo r-i | CW<EQ- CM VO vvOOCn tO-- VO C2--M `O vO VO v5O0 VO CCOM 'O VVCOOO ciOow CCr'O--M CrO*H--n rH r--{ f--1 1--1 rH rH rH rH rH rH VCXOO) rH d0c1) -dUOtHHo ,4 H5 QCQMC0HXOOH Ca0MC0xMOJ MQM QCOOMOa QCnO aMC00O4 CQMO QC0wXO WQM CXO Q0M QCMQKMPOOJi ar> fMCOL, QMO U <H2CJ. EH <a05 e <a0 .KtO-T*N1l\ Jt!03Tn1*1\ 0\ vlOT1 \ Cm0arO1Il\ CmMt O0rH1N vrmhoH11- mcK0^N11- CCmOMM11N iOn1 *1 Ci:0nMn11 Om011Nn 805 1 8 25 M85 8 M-t 285 8 25 8 05 8 C5 085 8 X p0 * rdH K'rfHnNN11 8 CiHO iOPCH0XO oU X* AMPHIBOLE GHO:,P Several unusual amphiboles are found i' the Ifew Idria District and these were given a more detailed study. A dark brown to black amphibole is an essential constituent of the comptonite and syenite intrusives and makes up from 10 - 60~: of the rock; shown to be barkevikite by optical ar.d chemical study. It forms elongate prisms ranging in size from several millimeters to 24 cm in length. Barkevikite is extremely fine grained in the camptonite whereas in the syenite the crystals become much larger and in some spots form large radiating clusters of prisms several feet across. Optical determinations on the amphibole from the different intrusive facies indicate an extremely consistent composition; however where changes in composition of the crystallizing magma had taken place, the barkevikite reflected this change. In the late deuteric stage, the barkevikite became rimmed by aegirinaugite or by glaucophane and crossite near the igneous-serpentine contact. Complete analyses have been made of barkevikite (Table 14) and of unweathered White Creek syenite (RGC-48-50) from which the amphibole was separated (Table 23). Recalculation of analysis in Table 14 on the basis of 24 (0,0H,F) to the unit cell was made in order to determine if the results fit the standard formula for amphiboles as established by Warren (1930) Almost one quarter of the theoretical amount of silicon is proxied for by aluminum to bring the four coordinated metal atom positions up to eight. The remainder of the metal atoms fit approximately the accepted formula except for the low percentage of water, 1.06 (OH) instead of the theoretical 2.00. A separate determination was made for fluorine 47 TABLE 14 . CHEMICAL AND PHYSICAL PROPERTIES OF BARKEVIKITE (RGC-48-50). WT. <f, METAL ATOMS Si02 38.74 5-97 AI2O3 14.02 2.56 Ti02 3.96 0.46 FeO Fe203 16.27 2.32 2.09 0.26 MgO 9.26 2.12 MnO 33 0.05 CaO 11.28 1.86 Na20 1.16 0.34 K20 -89 0.16 h2o+ 1.02 1.06 h2o- .20 99-45 ANALYST - W. H. Herdsman [ 2.03 1 '55 l * 8.00 L 5.46 . >. 2.41 1 > 1.06 J1 ALPHA = 1.701 t .002 Y = LIGHT REDDISH-BROWN II H O BETA = 1.695 GAMMA = 1.679 ZAc X = LIGHT YELLOW-BROWN Z = DARK REDDISH-BROWN 2V = 60 DENSITY = 3-40 t .005 (pycnometer, 24C) IDEAL 8.00 5.00 2.00 2.00 Fluorine determination on a separate portion gives 0.02^. and combined water was rechecked in order to establish definitely the low (OH) content of the oinphibole. Comparison with other barkevikite analyses shows that low (OH) content is a common characteristic of these amphiboles and Sundius (19^6) includes barkevikite with the oxyhornblendes stating: 'The barkevikite may conveniently be included in the same group (oxyhornblende). The members of the group have also been named oxyhornblende because of their high content of Fe2C>3 being as cribed to a reduction of OH and a contemporaneous oxidation of Fe.' This may explain the low water content; hovever the Fe2Q3 content is too low to compensate for the low combined water and therefore, the water determination is felt to be in error. The optical cliaracter of the New Idria material when compared with other published data on barkevikites shows good agreement. The metamorp'nic rocks from the district are largely characterized by glaucophane or crossite as an essential mineral. These two amphi boles usually form as a result of low-grade metamorphism of the greywackes or basic igneous rocks of the Franciscan formation and do not form coarse crystals that can be distinguished in the hand specimen but impart a characteristic bluish cast to the rock. Extremely fine nairlike crystals of crossite were found in the vugy cavities of the natrolite vein at the Gem mine and here again the individual crystals could not be distinguished except under the microscope. Time did not permit a comprehensive study of these amphiboles and the optical properties of the crossites and glaucophanes are given in Tables 15 and 16. The opti cal constants of these blue amphiboles compare favorably with the published 49 TABLE 15. OPTICAL CONSTANTS OF GLAUCOFHANE FROM THE NEW IDRIA DISTRICT. ir\ O t lT\ cn KV II t-- VO 1 0 O rH to cu ITS tO*I- CO K"\ C\ II rOHl O VO rH O CO PS 0 ,0 0 1 II cu ---- X 9 E* 9 o ca co co 9 s0 0 r--*. 1 & 11 K"\ JH Moo w m CVS IT\ s oI K"\ VO rH II O CQ ons r-t ir1\ irv CO ITS II OI VO rH O O O rH t 1 11 lf\ >4 O .O O 1U KV >--- >< E-< 9 >-1 p 1--I tr\ -it vO s KV II 1 VO V o rH CO co n O t 0u C\1 --- >< ccno 9 I oo i: w RJ 9PQ ir\ sot VO orun a PS ocoroHu1 CO CO 9 s oo joMz: s s R CO PoS CO 50 *(OA I rH ocmI On VO VO ? u X o o LTV OII *4 Pu GJ DJ 5 o pq co aCO oo o IA ia VO VO oii o o CM oIT1V HIA O VKO"\ ri-TuHv VO u O r-5 O K o o CM VVOO oflOAfII\ oMn 5w-4 cacoo pq oo: pH H C4 a CO o pq t4 ot*- CM VO vo HOII CM co aCO s C4 oo aPH ^W wDOC o 2 s VO pH <* a O>J 25 R CO a cq l/ L ir a it 6 o f accuracy 0 .0 0 2 .t 51 data for crossite and glaucophane (Switzer, 1950), although the am;<hiboles from (RGC-50-50) and (RGC-21-50) show anomalous optic character that fits neither glaucophane nor crossite. Further chemical and optical study of these amphiboles is needed, since the present published data are not complete enough to characterize these species by optics alone. MICA AND CLAY GROUP The sheet structure minerals are quantitatively the most important group within the serpentine. Individual localities will not be listed except where analytical work was performed, since these minerals arc sc common. The mica and clay minerals of this district may be divided into two distinct groups, (l) those making up the serpentine, (2) those found within the metasomatic bodies that have formed at the expense of the serpentine. It is assumed, that antigorite and chrysotile making up the serpentine, crystallized during the change of the ultrabasic rocks to serpentine; whereas the chlorites included in the second group crystallized during the metasomatic replacement of the serpentine. Antigorite is the most abundant mineral in the serpentine making up 80 to 95? of the rock. Antigorite produces a variety of textures within the serpentine, where it does not pseudomorph olivine; however, where pyroxenes are present, bastite assumes the form of the original pyroxene. Coarsely crystalline antigorite characteristically forms flaky fan-like aggregates whereas the finer-gTained antigorite commonly shows felt-like aggregates composed of minute flakes (PI. VIII, Fig. 3) Rarely large blade-like antigorite areas are found within the fine-grained facies. Shearing.is commonly present in the antigorite masses and a distinct 52 fabric is formed with the schistosity parallel to the alignment of the antigorite. Identification of the antigorite is facilitated by its characteristic crystalline form and optical character which is distinct from chrysotile. Antigorite was not found in any of the metasomatic chlorite-rich bodies within the serpentine formed under static conditions. Brindley (1951) points out that antigorite does not have a structure similar to chlorites but more closely resembles that of the kaolin group minerals. Since X-ray techniques were not used in the identification of these minerals, antigorite is included in Figure 4 with the other chlo rites to compare their composition. No analytical work was completed on the antigorite (Table 17). Chrysotile is included in this group although it cannot be con sidered a true chlorite-type mineral. Chrysotile is most commonly found as a product of the serpentinization of olivine, forming small veins in the fractures within olivine with a fibrous sturcture normal to the vein walls. Where olivine is completely replaced by chrysotile, it assumes a web-like structure roughly pseudomorphing olivine (PI. VIII, Fig. 2). Chrysotile also commonly forms irregular veins, up to four inches in width, that cut the serpentine and in this case approaches an asbestosform character. Identification of chrysotile was verified by its opti cal properties, and mean indices determined on several specimens are: alpha 1.562, and gamma 1.569. The chlorites received a more intensive study, since they make up a large bulk of the interesting metasomatic bodies. Winchell's (1951) classification was used to chemically categorize the various chlorites, since it was possible to determine their chemical composition by their 53 FERROANTIGORITE H^Fe^SigOg IOO 80 DAPHNITE H4FeaALSi09 60 40 20 0 j 20 60 40 yr*.i:C "A SOMATIC ROCKS 40 / ' 60 ^/ 1 * 20 _ ___ m. - - -- 80 \1 / \ 1 . / B-SERPENTINE O H, Mg Si 0 4 ^3 2 9 ANTIGORlTE 20 1 4 0 Mol. % 60 80 100 H4Mg2AI2Si09 AMESITE FIGURE.4. NEW IDRIA CHLORITES PLOTTED ON WINCHELL'5 OPTICAL AND CHEMICAL CLASSIFICATION FOR CHLORITES. THE OPTICAL DATA FOR THESE CHLORITES IS GIVEN IN TABLE IT. 54 KKKKKCKKK hi J (X e CO OOOOCSOOOO * CjE^Jm o O till | ^ |J ie.cufLi9<ii,iOiJi4^HStOfi<j(i<ofl(a(oo K...(..t.K....K....(..5....C... ...................................o o a a o a PC a P3K< oo pKq J J iJ J J J * * p-3 | p-3 *-0 IX OUOOUUiJhJJJJJUOOOOMliOH C 3 c! C fl) H *H > 01 H Pp O J-. OJ H CO > > O o O +j pH Jpj X! J3 H t-p CO U) (U 3 H H >>T3 (-jJKKK . E S* hi O C . O p-3 X pH p-3 J p-3 d p-i . . . . . . d . OOOOOUiJhWlJJl-IOiJOOOOoEo OC O pXHi bEi _ f0f5l # ; :pJk i-3 pH X C3 >Kr>C>*>fEfi>*KA>^AK>AE>AK>Er>t,W>V>KV>KA>KVE>vK>VC>Af>fVi>CV>KV>KV I cm ir\ co 88 OI po, "\ ru . K"V CU W 01 W W Ov <J\^t _TT O lOQOO i IQO i lOOOQ i OOOIA Otf'pOOOOH.rfOOHHOOOOt-OOOO + + + + + +I I I+ + + + + + + +I I I I ooCU ooCU ooPI ooCVIO oorio oiTolHooIT\Qt"Co-fg"oC-oP-rp-Co-CooUO o oi--ii oli--lifot--ioO*--irimlUtA--ir\lclulirl\lirlMlrl\iirlMirO\tr\irO,\--O< HOH 3 > rH O rH r-i QJ r-i >> <D Pi >> C 0) (D OJ tJ H H rH aa o to t II t ^ i J hi K 9 >< >* K (^0(^0 -3 co _g- Jco co C-cO O-i-CO co t~-a5 oo O a\ -u5v tw Ov CO o i--l CO CO H O Ov Ovco 1oAv Ot--vOo ONfco p<-\ KMlMAIAin ITVVO VO lf\ tf\ lTV VO VO lT\lf\ IfMfMTv IT\ ITVVO (--! i--1 i--1 i--1 i--1 i--{ i--1 t--l i--! t--1 p--1 pH i--i i--1 i--i i--I H --[ rl rl O OOO co co co co co co 5S33 o4 qpj wiqy kkkkpc S' cH co CVl CM CM pU"INHUpI-HMAIHlTpVHlIAtITOiQVV?\QIOfMOI TOOVI flI^ATV HirvI HtrvI QirSIOirSIOtrIvOirv ovovooovovor-- o nviacuy) j- ,, .Ciriv-t.Qi--riv OKuivViOOAiv Oir5ivQtir-iv C--I t--I r--I C--I I ITVIKIV lTI \ pIH CDt COI _d1- ITVI KVI KVI VOI toI pHI HI ifIMAI oooooooooooooocjocjocjoo OKSPSBKSK5K93KSSBK!K3S!3teSPlSeKPKCiS3rlSii3KoKaKoSp cvi 8 d +i ax <D C 0) *H Vi Pi m to o -p 0) 0) J2 rH H UJ V gl h o p< h ^ h 13 O I -rt O Pp ' KO ME t >> CJ al o u cj Cm O O P g T-l p-3 tj . , v u a. h4 E Hi optical properties. More recent worl: on chlorites by Hey (ly|?-*) has produced a better classification but chemical analyses are needed in order to utilize this system. Hovever several semi-quantitative spectrographic analyses on these chlorites (PI. IV) show that Winchell's system can be used with some degree of confidence to reflect the compo sition of the chlorites. Table 17 gives the recorded optical data on chlorites and related species, which in turn are plotted on Winchell's diagram (Figure 4). Picked samples were also used for X-ray diffrac tion study in order to establish a difference in their respective patterns; however.no sharp differences could be detected. Brindley (1951) suggests that the difference in intensity of the basal spacings of the higher order (001) reflections might be used to distinguish the different chlorite species, although this approuen did not produce conclusive results in this study. The chlorites in the metasomatic bodies are well developed and they make up 80 - SOj> of some specimens. Some bodies, viz., (RGC-79-51)> (RGC-70-51), and (RGC-p6-50) have well developed crystals of penninite or clinochlore, up to a centimeter in diameter, that form perched 'books' lining cavities. The texture of the chlorite-rich rocks is quite variable and grades from rocks showing a strong preferred orienta tion to others where the chlorite forms in a vugy cavernous rock with well formed platelets of chlorite at random orientation. Penninite and clinochlore, the only species found as well developed crystals, show twinning in all specimens examined and this is manifested .ar table -osit'cr. of ike ~~i? r_"ia2 n *n v* , o**' *cn3ci -- n2.ii i)s V./lnn ir.j *.*2.3 .Tonii'i crlv i,r. ti'.oc-c cv'ycr.n'1 c ' r. ogjr. cavities acrcas these ckl.ori tee fcur/d viih:n the more dense recks show r.o 'winning but produce a marked cthis '.os: ay with cue (CC1) faces parallel ao '.he direction of schistosiay. The chlorihe-: 1 ch rocks are assumed ao represent mecasomaaic renlacc- me-at rocks within ~kc serpentine, and therefore the chloriec cornices i a ion should reflect, in part, the compos 1 lion of the metasomatic fluids. Jhe composition as determined from h'ir.chell's diagram (Fir. !t) shows teat fre:: anti.gorile (essentially the ccmnosiaion of T.ke original serpentine) to t.ne cnlori.e in th.e metasomatic bodies, there has been an increase in both. A1 and Fe via:: a concomitant decrease in 3i ar.d My. Pennir.ice ar.d clinochlore are ahe common chlorites found in the least mineralised neaa- somatic rocks, followed by rumpfite and prochloriae as the intensity of the mi neraliaatior. increases, '.fncre iron is exceptionally high, delcssitc torms ana sometimes it is accompanied by stilpr.s.aelane (FI. XI, Fig. 2). Optical daaa for ahis sailpnorr.elane corresponds wit-; that of Hutton's (1958) ferrostilpnonelane (see Table if). Speetrogranhic analyses (PI. IV) of so.ee of these chlorites show that Hi, Co, Cu, Cr, Fb, and Mr. ire commonly camouflaged in the structure of ahe Hew Idria chlorites. The widespread occurrence in this district of the rare chromium chlorite, kammererite, associated wir.h small local ccncenarations of chromiae and uvarovite led to a complete study of this species. Kammerer ite was r.oa found in ary of ahe chlorite-rich rocks ar.d if seems to be an alteration product of chromite, probably formed during serpsntiniaation (Fig. 5)* The mineral is easily identified ir. the field by its c'rarac- 57 Figure 5- Camera lucida drawing of kammererite showing its formation from chromite. Note small patches of uvarovite within the kammererite. (X 4o) 58 teristic pink to lavender color. A chemical analysis of the kammererite and optica] determinations are given in Table 18. The structural formula of the kammererite was calculated on the basis of l8 (0,0H,F) atoms to the unit cell and the analysis shows fair agreement with the ideal formula; the OH group is slightly low and the octahedral positions are above the theoretical value of 6.00. Substitution of A1 for Si in the tetrahedral positions is necessary to completely fill the four Si positions assigned to the ideal formula. Cr substitutes for A1 in the octahedral positions and the tabulated analyses of kammererite from the literature (Table 19) show a general decrease in A1 with an increase in Cr. Ihe optical properties of the tabulated kammererites do not show a distinct correlation with their Cr content. KATROLITB IJatrolite is a common mineral in three distinct localities within the district and at all three it forms stubby equant crystals--a somewhat rare form, since natrolite is characterized by its acicular habit. The vein-material from the Gem mine is composed mostly of natrolite where it forms beautiful cockscomb coatings in the vugy parts of the vein. Almost perfect single crystals of natrolite are present in late deuteric veins within the syenite where they are associated with analcite and acmite. Jadeite veins contain well developed fan-shaped aggregates of natrolite associated with thomsonite, albite, and analcite. The natrolite crystals from the syenite were examined in detail and the physical properties are listed below. The crystals from the syenite and Gem mine show twinning 59 TABLE 18. CHEMICAL AND PHYSICAL PROPERTIES OF HAMMERERITE (RGC-50-50) WT. <f, METAL ATOMS Si02 AI2O3 Cr203 32.12 13-13 5.56 3.10 1.43 .45 f -90 J i -53 > 4.00 Fe203 FeO 1.27 1.50 .09 .12 >6.30 MgO 55.08 5.05 CaO .05 .01 NIO .04 - * nao+ h2o- 11.12 .04 100.27 7-16 > 7.16 J ANALYST - -R. G. Coleman ALPHA = I.585 + .002 X = PINK On CO --1 1 BETA = Y = COLORLESS GAMMA = 1.589 Z = COLORLESS KO1 O 2V = OPTIC SIGN = ( -) DENSITY =2.56 t .01 (pycnometer 25.2C) IDEAL 4.00 6.00 o.CC TABLE 1 9 . OPTICAL PROPERTIES OF KAMMERERITE COMPARED WITH C r203 and A1203 CONTENT. <x VO VO f-H 0 -* tO tO 0 CO O OO t'- rH ON VO t rH O OJ O rH to + to O - ^ to to r-1 rH OJ a 3n O to VO to UN rH 0 -=t- * 0 lO to rH H -if to 0J Ov CO to O CVJ rH rH Cvj 3 2 lO E. torH rH to O to 1--1 rH CO H co c-- V-sOr to rH ON tn -0li eu rH a(--1 04 rH < (V u B0 syto O>J .L rH 60 t*" to Lf ^ rH OJ <8 to CO to I Ov C-- c-- + to vo rH OJ ON CO VO t to to rH OJ + s. -+ S to & t to rH On t-- to , to rH < tEMo- VO t- f ON f0-- OJ to MO 1 1 - Sakok Ruopsak, Sweden, Du R ie tz (1955) 2 - Deer P a rk, Wyoming, Shannon(1920) 3- K raubath, Kopetzliy (1948) 4 - New I d r ia , C a lifo r n ia 5-Togo, French West A fric a , C reel (1925) 6-W ebster-Addie, North C a ro lin a , M ille r (1953) 7-Piem onte, I t a ly , Sanero (1955) 8 -Togo, French West A fric a , O rcel (1925) ol vnich is unusual for natrolite. The gamma index for both the Gem mine and .iadeite vein-material is 1.463 t 0.0C2, identical with the gamma index for the syenite natrolite. Qualitative spectrographic examination of these three natrolites shows no minor or trace elements which might produce this unusual crystal form. f.'atrolite from syenite Alpha - 1.476 t 0.0C2 Beta - 1.478 Z=c Gamma - 1.488 Birefringence - 0.012 110 A 110 - 8617' Optic plane parallel (010) Twinning - Composition plane (110) with twin axis J- to the (110) plane. Ill A 110 - 6310' Density - 2.23 to 2.25 (Berman Balance) IDOCRASS Idocrase has been identified in four calc-silicate bodies within the hew Idria District: (RGC-37-50), (RGC-56-51)/ (RGC-92-52), and (RGC-109-5?). Two distinct types of idocrase are present, (1) pale green massive mate rial intergrown with chlorite, calcite, and diopside; and (2) dark reddishbrown euhedral crystals present in open veins and vugy areas of the calcsilicate rocks. The massive pale green material has normal optical con stants and the X-ray pattern matches the standard for idocrase, although the reddish-brown material has higher refractive indices and is biaxial. Spectrographic determinations on three of these samples show varying a mounts of Ti which can be correlated with the optical anomalies (Table 20). 61-A TABLE 20. VARIATION IN THE OPTICAL CONSTANTS OF HXX7RASE WITH Ti CONTENT. EPSILON OMEGA 2V Ti f, RGC-37-5C 1.719 1.721 UNIAXIAL 0 1 RGC-92-52 1.720 1.726 BIAXIAL 2-5 2 RGC-109-52 1.729 1.732 BIAXIAL 10 5 Ti determined spectrographically. The powder X-ray patterns of the two biaxial idocrases show a slight shift in the lines which can be seen visually but the aeasurenents fall within the limits of accuracy. This suggests that there is an expansion of the idocrase structure, similar to that found in the andradites, in order to accommodate the excess titanium. Further work needs to be completed to establish definite relationship between optics, unit cell, and titanium content. H5W MINERAL A new mineral species was discovered associated with benitoite and natrolite from the Gem mine. The X-ray pattern and optical character of this mineral does not match any of the described minerals in the litera ture . Unfortunately only a very small sample was found and insufficient material was available for a complete chemical analysis. The mineral is a shiny blue-black with a blue streak forming elongated crystals in small radiating clusters. The hardness of the mineral is 5-4 and density 4.15. Qualitative chemical tests indicate that Ti is a major constituent. The optical constants of the mineral are listed: Alpha - I.850 + 0.002 - colorless to light yellow Beta - I.9O0 - reddish yellow Gamma - 1.927 2V - 68 (-) - deep indigo blue Z> Y> X Birefringence - 0.097 Parallel extinction Z = c This same mineral is been found in well cuttings from the Ccaiinga oil field by C. 0. Hutton (personal communication), probably a detrital fragment derived from the Gen mine locality. Further searching of the 63 Gem mine vac unsuccessful in producing additional material for study. MINERAL LIST ' The following lists tabulate all of the minerals identified in the various rock types, i.e., serpentines, metamorphic rocks (Franciscan type), syenite-camptonite, and metasomatic rocks. The formulas given for each species are taken from Key's (1950) classification. SERPENTINE Magnetite Chromite Picrochromite Calcite Magnesite Hydromagnesite Olivine Antigorite Bastite Chrysotile Penninite Kammererite Andradite Uvarovite Glaucophane 8(Fe304) 8(FeCr204) 8(MgCr204) 2(CaC03) 2(MgC03) 2(Hfc(CQ3)4(OH)2*4HaO 4( (Mg,Fe)2Si04 ) - Fo^Fa^o to FoeoFa2Q l6(Mg3Si205 (0H)4 ) Near (Mg,Fe)Si034-5H20 2(Mg3Si20s (0H)4 ) 2( (Mg,Fe",Al)12(Si,Al)B022(0H)aS) Near Mg2.s(Al,Cr)Si1.sOs(OH)4 8(Ca3Fe'"2Si3012) 8(Ca3Cr2Si30i2) 2(Na2(Mg,Fe")3Al2SiQ022(0H)2) Albitc Quartz Pumpellyite Pyrite Lawsonite Giaucophane Crosaite Actinolite Treraolite Pigeonite Acmite Jadeite Epidote Zoisite Clinozoisite Prehnite Muscovite Chlorite Stilpnomelane Garnet Thomsonite Natrolite Analcite Hydrogarnet 64 METAMORPHIC ROCKS (FRANCISCAN TYPE) 4(NaAlSi30a) - up to Anl0. 5(SiOa) Ca4(Al ,Mg,Fe )nSif;0?3 (0H)3 "2^0 4(FeS2) i4(CaAl2Si20s(0H)4) 2(Ha2(Mg,Fe")3Al2Sis022(OH) 2 ) 2(Na2(Mg,Fe,,)3(Fe'" ,Al)2Si6022(OH)2 ) 2(Ca2(Mg,Fe)5SiQ022(0H)2) 2(Ca2HgsSi8022(0H)2) 8( (Mg,Fe,Ca)Si03 ) 4(NaFe'" Si206) 4(HaAlSi206) 2(Ca2(Al,Fe)3Si30i20H ) 4(Ca2Al3Si30i20H ) 2(Ca2Al3Si30120H ) 2(Ca2Al2Si30lo(0H)2 ) 4(KAl3Si30io(0H)2) - includes sericite. Species not identified. Near K(Fe",Fe"1,Al)loSi1203o(0,0H)i2 Species not identified. 4 (!,raCa2Al5Si502o'6H20 ) 8(Na2Al2Si30iO-2H20 ) l6(NaAlSi20e-H20 ) 8(Ca3Al2Si3_x0i2_4x(0H)4x), x up to 5 Plagioclase Analcite Natrolite Prehnite Zoisite Clinozoisite Magnetite ILtcnite Rutile Perovckite Calcite Doloiaite Apatite Sphene Benitoite Neptunite Joaquinite Chevkinite Zircon Titanian andradite Andradite Idocrase Diopside 66 Ans to All69 16 (NaAlSisS i30lo 2II20) 8(Na2Al2Si30io * 2K20) 2(Ca2Al2Si30lo(0H)2 ) 4(Ca2Al3Si3Oi2OH ) 2(Ca2Al3Si30120II ) METASOMATIC ROCKS 8(Fe304) 2(FeTi03) 2(Ti02) 8(CaTi03) 2(CaCo3) (CaMg(C03)2) 2(Ca5(P04)3F) 4(CaTiSi04(0H,F) ) 2(BaTiSi309) 0( (Na,K)2(Fe",Mn)TiSi4Oi2) 4(NaBaTiSi4015) Near (Fe",Ca)(Ce,La)2(Si,Ti)2Oa) ? 4(ZrSi04) 8(Ca3(Fe"',Ti)2(Si,Ti)3Oi2 ) Ti02 up to l4 8(Ca3Fe"'2Si3012) 4(Cai0(Ms)Fe"lFe"1 ,)2Al4SigO34(0H)4) up to % Ti substituting in structure. 4(MgCaSi20s) Natrolite Stilpnomelane Clinochlore Delessite Rumpfite (Leuchtenbergite) Penninite Pyrochlorite (Ripidolite) 8(Na2Al2Si301o2H2O) Near K(Fe",Fe'" ,Al)loSi1203O(0f0H)12 2( (Ms.Al, Fe")12(SilAl)s02o(0H)i6 ) Near (Mg,Fe")3.sAl3Si30io(0H)e 2( (t5gioAl2)(Al2Si0)O2o(OH)i6 ) 2( (He,Fe",Al)12(Si,Al)8022(0H)i6 ) 2( (l'!g,Fe")9AleSi502o(0H)ie ) 68 PETROLOGY Franciscan type rocks The Mew Idria District contains a heterogeneous group of rocks with lithologies similar to those of the Franciscan formation. From stratigraphic evidence, the oldest formation exposed in the district is probably Franciscan in age, although there is no direct fossil evidence to corroborate this assumption. The bulk of this formation is made up of sedimentary greyvackcs accompanied by minor amounts of greenstone, cherts, and schists containing glaucophane. They are found flanking the serpentine and show a universally faulted contact relationship. Many tectonic inclusions within the serpentine have lithologies similar to the peripheral Franciscan rocks; and therefore, have been included in this discussion of the Franciscan rocks. The Franciscan type rocks have been divided into three categories for convenience of discussion, (1) rocks peripheral to the serpentine, (2) tectonic inclusions within the serpentine, and (3) Jadeite bearing rocks within the serpentine in and near the tectonic inclusions. Peripheral Franciscan rocks The major rock type found in this category is a sedimentary greywacke characterised by a brown to gray cast and containing abundant secondary quartz veinlets. Many isolated lenses of thin-bedded chert are found interbedded with the greywacke. These chert beds may reach a thickness of 100 feet or more in this district. Taliaferro (19^3) has pointed out that these cherts probably formed as a result of the volcanism attendant 69 during the Franciscan time of deposition. Altered volcanic rocks are sparsely interbedded with the greywacke and are characterized by light green color and dense texture. Alteration of these rocks has produced a rock that may be best described os a spilitic greenstone. Small isolated patches of schist are commonly found within the greywackes and these appear to be concentrated along the Franciscanserpentine contact; however, nc regular spatial relationship could be established between the schist zone- and the local structure. These schists commonly contain glaucophane as a major mineral and are usually referred to as glaucophane schists. All of the Franciscan rocks within the New Idria District show evidence of low-grade dynamothermal metamor phism and it may be that these local schist zones are areas that have undergone a slightly more intense pulse of shearing and pressure. No field evidence was found in this district to support Taliaferro's (1943, p. 182) view that these schists have formed by pneumatolytic action resulting from the serpentines emplacement. It is not the purpose of this paper to discuss the origin of the glaucophane schists, one of the unsolved problems in metamorphic petrology however a brief petrographic description of the major rock types is given. The peripheral Franciscan rock types are divided into five groups as follows: Group I* Greywacke (RGC-26-50) do. (RGC-39-50) Sample numbers may be located ir. Plate I. 70 Group I (continued) Greywacke (RGC-20-50) do. (RGC-18-50) do. (RGC-1*9-50) do. Group II (RGC-4C-50) Chert (RGC-21-50) Group III Greenstone (RGC-22-50) do. (RGC-73-51) do. (RGC-21-50) Group IV - Schists Glaucophane-chlorite-muscovi '-sphene (RGC-60-51) Glaucophane-c'nlorite-garnet-pyroxene (RGC-75-51) Glaucophane-crossite-lawsonite (RGC-51-50) Glaucophane-muscovite-lavsonite (RGC-80-51) Group V - Schists Calcite-giaucophane-stilpnomelane (RGC-21-50) Quartz-stilpnoraelane-lawsonite (RGC-29-50) Albite-glaucophane-chlorite (RGC-80-51) Tremolita (RGC-62-51) Group I - The greywackes are brown to dark gray and tend to show a very crude schistosity or a platy direction of fracture. Secondary veins of white quartz conmonly cut the bedding or nay extend along the bedding planes. Angular to sub-angular quartz, the most abundant mineral, almost universally shows strong undulatory extinction and forms sheared augen 71 with mortar structure. The quartz in the secondary veins snows similar features. Plagioclase makes up 2y': or more of the greywacke; it usually retains its original twinning, although determination of its former com position is difficult due to alteration. Saussuritization is typical of all the feldspars and dense aggregates of clinozoisite and epidote form within the crystals. Index determinations on the altered feldspar shows it to be albite (An5_10K Lithic fragments of andesite and basalt are abundant and detrital chert fragments (RGC-20-50) may be locally abundant. The matrix of these rocks is 'pasty* and is molded around the larger detrital fragments. This matrix is a finely crystalloblastic, dense mass consisting mostly of clinozoisite, sericite, chlorite, sphene (leucoxene), and iron ores. Shear planes that have developed are manifested by semi opaque dark brown bands containing iron ores, sphene, sericite, and occasionally wisps of glaucophane and stilpnomelane (RGC-26-50) and (RGC-59-50). This cursory examination strongly indicates that the greywackes surrounding the serpentine have undergone a low-grade dynamothermal meta morphism, that is comparable to that in Hutton's (194o) Chlorite sub zone Chi. 1. Group II - The chert is thin-bedded, often rhythmically interbedded with shale, and it may be red or brown like jasper, but more often green, light gray, or blackish. It is composed of cryptocrystallme quartz and chal cedony with a fibrous structure. Many small euhedral platelets of chlorite are dispersed within the chert and also concentrated in thin sinuous lines with small magnetite grains. Many small secondary veins of coarsergrained quartz cut the fine-grained groundmass of these cherts. Radiolarian tests were not seen in the section studied, although Taliaferro 72 (1943, p. 147) reports that they are common in Franciscan cherts. Group III - The greenstones are dense fine-grained rocks cut by white reticulated veinn. Clinozoisite and albite form a dense crystalloblantic groundmass and in some instances relict diabasic textures are discernable, with relict pyroxenes apparently unchanged. Minor sphene, chlorite, and quartz are also present. Foliation, lineation, or schistosity is not developed within these rocks. Preserved cavities reminiscent of original vesicular volcanic rocks are not uncommon. The reticulated veins contain quartz, calcite, and albite in that order. These rocks are probably altered fine-grained volcanic rocks similar to spilitic basalts. ^ Group IV - Most of the schist zones, less than 100 square yards in areal extent, are characterized by a light blue to gray color and exhibiting a fairly well developed schistosity. The dominant glaucophane (PI. VI, Figs. 1 and 3) forms well developed crystals and produces a lepidoblastic texture in these rocks. Crossite commonly accompanies the glaucophane either as discrete crystals or zonally arranged on the glaucophane (RGC-51-50), whereas large, well crystallized plates of chlorite and muscovite (PI. VI, Fig. 4) form the interstitial areas between the amphiboles. Porphyroblastic garnets (PI. VI, Fig. 2) developed in (RGC-73-51) show some evidence of retrograde change to chlorite. Jadeitic pyroxene found rimming glaucophane in (RGC-73-51) appears to contradict the evidence of retrograde change in this rock; although it is not clear what P-T conditions are favorable to form Jadeite. Iawsonite forms colorless square tabular crystals and anhedral aggregates (PI. VI, Fig. 1) interstitial to the amphiboles, and it seems to have formed later 75 than the other cons' ituents. Idioblastic crystals of sphere are uni versally present (FI. VI, Fig. 2 and h) and may .take up 10' of the rock. These mezarr.orphic schists characterised by glaucochane-cnlorite, contain no relict minerals or textures ar.d appear to have ceen completely recryszallized by low-grade chlorite tone r.etatorphic processes. Group V - This group contains diverse rock types showing individual characteristics. (RGC-uO-51) was probably a auartz-?.lbite-glaucophane rock that has been brecciated and subsequently replaced, in part, by calcite. Stilpnomeiane has formed contemporaneously with the carbonate. An albite-cnlorite-glaucophar.e schist. (P.GC-BO-51) is similar to Group IV rocks, except for abundant albite. Tne quartz-stilpnomelanelawsonite schist (P.GC-29-50) forms a small selvage in a large mass of slightly metamorphosed greyvacke where it has completely recrystallized with a well developed schistosity. Tne tremolite schist, the only ir.onomineralic rock found, is coarsely crystalline with a lepidoblastic texture. A faint bluish tint is de veloped around the borders of the individual crystals, suggesting late introduction of Ra. Tectonic inclusions Present within the serpentine are numerous foreign rock bodies of variable size and shape; the largest mass is about five acres in areal extent and the smaller bodies are less than ICO square yards. The distri bution of these inclusions is entirely random and the observed attitude of their structural trends is also random; however, the larger bodies PIATE VI. Photomicrographs of peripheral Franciscan schists. Figure 1 . Glaucophane-crossite-lausonite schist. Prismatic gray crystals are glaucophane with borders of crossite. Iawsonite fills the interstices. Plain light (X 80). Figure 2 . Glaucophane-chlorite-garnet-pyroxene schist. Idioblastic garnet ahuws incipient alteration to chlorite. Large, high relief crystal diagonal to the field is sphene. Interlocking prisms of glaucophane are present in the upper part of the field. Plain light (X 80). ' Figure 5 . Albite-glaucophane-chlorite-rmiscovite schist. Idioblastic grains of glaucophane associated with albite and muscovite. Plain light (X 8o) . Figure 4 . Glaucophane-chlorite-muscovite-sphene schist, large warped plate of chlorite containing grains of sphene. Plain light (X 80). PLATE VI 7i ," appear to rovgxly parallel the rerional 17.1-3L structural trend (ace Pi. I). The contacts between *hc serpentines and these inclusions arc almost invar;ably sheared and faulted, ar.d where contacts show no anp-arent shearing or faulting, microscopic exar.ination reveals shearing, and recon stitution of the serpentine to chlorite and andradite. Antigorite develons al :.ag s'near planes of the serpentine up to ten feet away from the contact. Careful field examination of these contacts revealed r.c evidence of con tact me inmorphism within these bodies that slight have resulted from deep seated alteration by an uitrabasic magma. There is no '-ir.eralogical evidence to indicate magnesium metasomatism within the included rocks, alikougr. tr.ere appears to h-ave been some ...wve.v.ent of elements from the . tectonic inclusions into the serpentines as there is local development of soda-rich pyroxenes near the contacts. The nature of these foreign rock bodies suggests that they are tectonic inclusions, similar to Brother's (195^, P* 6l6) glaucophane schist bodies within the serpentine of the Berkeley Hills, California. The direction of movement of these tectonic inclusions within the serpentine is extremely hard to interpret. Tne general shape of the inclusions, usually elongate, tabular with vertical bedding or schistosity, suggests a downward movement ar.a, therefore, it appears that these inclusions have foundered from the roof of the serpentine mass or have been dragged around into parallel position with the bounding thrusts yet immersed in a mass of semi-plastic serpentine. It is difficult to imagine under what conditions the present serpentine was emplaced in the country rock,as it is evident that it was not a magma at that time. If the inclusions were brought up from depth, a higher grade of metamorphism should be present in them. The observed relation:; of these inclusions, in the field, strongly suggest that they were floated or rafted into the ulzrabasic rock during sorpentinizazion, or after serpviillwizaliori during an intense tectonic movement of the Diablo Gangs. It i? assumed ?;uar.r.e tectonic inclusions were derived fro:n the Franciscan formation, on account of similarity of mineralogy and bull: composition. Tne grade of rr.etam.orp'ni s:n is similar to that found .in the peripheral Franciscan rocks; however,the meiatorphic minerals within the .ectonic inclusions are much finer grained than those in the pcrioheral rocks (compare FIs. VI ar.ti VII). This suggests that the period of meta morphism was shorter lived on the tectonic inclusions than that exerted on the peripheral rocks . 7:.e main rock types found vi shirt these tectonic inclusions are grouse below according to their similarity in .-..iraralogy. Group I pyroxe ne-pumpcllyite (EG G-3-51) pyrcxor.c-albite-cr.lor->te (GGC-33-51) chlorite-pumpellyite-lawsoni te-spher.e (EGC-1C7-52) pyroxene-albite-chlorite (Gem mine) gabbro-saussuritized (Gem mine) Group II uarzz-albite- glav.cophane-stilpnomelar.e (E3C-7o-5l) quarzz-albite-serieite-chlorite (KC-C-53-51) quartz-albite-zoisite-chlorite (GGC-?2-p2) quartz-albi te-chlorite-actir.olite (EGG-52-pO) 77 Group II (continued) quartz-albite-chlorite-sericite (RGC-77-51) quartz-albite-chlorite-zoisite (RGC-95-52) auartz-sericite-chlorite-albite (RGC-107-52) Group III albite-quartz-stilpnomelane-tremolite (RGC-103-32) albite-quartz-crossite-acmite (RGC-32-50) albite-quartz-glaucophane-stilpnomelane (RGC-76-31) Group IV albite-glaucophane-actinolite (RGC-Sl-52) albite-glaucophane-acnite (RGC-5^-51) albite-glaucophane-acmite-stilpnoraelane (RGC-32-50) albite-glaucophane-stilpnomelane (RGC-5^-31) albi te -glaucophane -epidote ( RGC -3*+-51) albite-glaucophane-crossite (RGC-99-52) albite-glaucophane-epidote (RGC-106-52) albite-crossite-epidote (Gem mine) albite-crossite-glaucophane (RGC-50-50) Group I--These rocks are characterized by a light to dark green color and relict igneous textures that reveal the original nature of the rock. The texture and mineral composition indicate that they vere medium- to fine-grained rocks ranging from gabbro to augitites in composition. Relict pyroxenes are abundant and usually show little or no alteration. Optical and X-ray determinations on these pyroxenes establishes their composition as varying from aegirine-augite to pigeonite. Alblte has 7 formed at the expense of zha original calcic-feldspars. Pumpellyito ar.d lavsor.izc also form a- `fry expense of she calc ic-feldspars ar.d partial breakdown. of the pyroxene. Sphere is abur.ians in several of these rocks ar.d .ray be found as large wedge-shaped crystals. In the hand specimen, these rocks are extremely fine-drained, tough, greenish in color and are indistinguishable from the greenstones found in the peripheral Franciscan. Foliation, lineatior., ar.d schistosity are not found. Tnese rocks are probably slightly metamorphosed spilitic basalts and gaobres, since their calculated compositions from their modes are similar to these basic recks, group II--This group includes greyvacke-type rocks that are gray to brown, medium-grained rocks k-aving a crude foliation and schistosity. Thin quarts veins are abundant in most of those rocks. Quartz and plagioclase are the most abundant constituents accompanied by lesser amounts of altered lithic fragments and fine-grained chlorite, sericite, zoisite, ar.d o.ctinolite. A weak dynamothermal rr.ezamorphism has partially recrystallized the finer constituents, but the original character of the rocks is easily ascertained. The rocks are greyvackes, for they are composed cf clastic quartz and feldspar (25> or more) accompanied fcy lithic fragments, and the intergranular uament is made up of a paste or titurated equivalents of the .above mentioned minerals. Tne greywackes exhibit recrystallization along the shear planes and the pasty intergranular cement is crystalloblastic. Glaucophane (P.GC-77-51) has formed fine needles in the shear zones and tufts around the periphery of the detrital pyroxenes in response to dynamothermal metamorphism. The greyvackes within the tectonic in clusions are similar to those described by Hutton (1940) and could be classed in his Chlorite sub-zone C'nl. 1. These rocks show slightly higher 79 metamorphism than the peripheral greywackes. Group III--Thc rocks in this group are characterised by their bluish to grey coloration and by the moderately to well developed schistosity and foliation. The schists are fine-grained compact rocks and many contain late fractures filled by quartz, calcite, or prehnite. The quartz and albite in the schists are completely recrystallized and only rare porpnyroclasts of quartz remain. Usually tne quartz and feldspar sepa rate into folia and the crossite, glaucophane, and stilpnonelane form along the shear planes that are still recognizable by the abundance of opaque material still uncombined with the recrystallized minerals. Acmite is present as a stable relict mineral forming porphyroclasts that may or .may not have 'whiskers' of glaucophane growing out from the surface (Pi. VIII, Figs. I and 2). It is difficult to determine the original nature of these rocks although they seem to be the metamorphic equivalents of the quartzo-feldspathic greywackes. These schists show a slightly higher rank metamorphism thnn that observed in the greywackes, but still In the Chlorite tone of metamorphism (Chlorite sub-zone Chi. k, Hutton, 19;i0) . Group IV--The schists in this group are quire similar to those in Group III; fine-grained, bluish to grey, and moderately developed schistosity and foliation. Alb'te is the most abundant mineral forming crude segre gation bands. Quartz is minor or lacking in these rocks and when present it has been comdetely recrystallized. Relict calcic-plagioclase was not found in this group. Glaucophane is the dominant dark mineral forming deli cate needles along the shear planes, aggregates of fibrous bundles, or 80 as "whisker-like" mats around relict pyroxene grains (PI. VII, Figs. 1 and d). Crossite is commonly associated with glaucophane and in many instances a small needle-like crystal may have a core of glau cophane and a rim of crossite. Acmite and pigeonite appear as detrital relicts and show only incipient recrystallization to glaucophane (PI. VII, Figs. 1 and 2). Epidote forms equigranular aggregates along the shear planes and has apparently formed at the expense of calcicfeldspars and mafic minerals. These schists seem to be the metamorphosed equivalents of quartzo-feldspathic sediments that were unusually rich in feldspar and ferromagnesian minerals. Jadeite-bearing rocks Jadeite was first reported from California by Mielenz (1959; as an essential constituent in a quartz-albite-jadeite schist from the Franciscan formation in San Benito County. Bolander (1950) reported jadeite boulders along Clear Creek in the northwestern part of the serpentine (see Fiate I). Following the article by Bolander, considerable interest was aroused and in 1951 Yoder and Chesterman described the oc currence of jadeite-bearing rocks, in place, along the banks of Clear Creek near Bo.lander's original discovery. Yoder and Chesterman (1951) reported eight large exposures of jadeite in the canyon of Clear Creek. Several more bodies near the original site and one large isolated mass near Santa Rita Peak (RGC-8h-52) were dis covered during this investigation. The jadeite is found in two distinct occurrences. (1) Lens-like bodies having an irregular vugy surface and completely surrounded by sheared antigoritic serpentine. The central portion of these bodies contains 80 as "whisker-like" mats around relict pyroxene grains (PI. VII, Figs. 1 and 4). Crossite is commonly associated with glaucophane and in many instances a small needle-like crystal may have a core of glau cophane and a rim of crossite. Acmite and pigeonite appear as detrital relicts and show only incipient recrystallization to glaucophane (PI. VII, Figs. 1 and 2). Epidote forms equigranular aggregates along the shear planes and has apparently formed at the expense of calcicfeldspars and mafic minerals. These schists seem to be the metamorphosed equivalents of quartzo-feldspathic sediments that were unusually rich in feldspar and ferromagnesian minerals. Jadeite-bearing rocks Jadeite was first reported from California by Mielenz (1959) as an essential constituent in a quartz-albite-jadeite schist from the Franciscan formation in San Benito County. Bolander (1950) reported jadeite boulders along Clear Creek in the northwestern part of the serpentine (see Plate I). Following the article by Bolander, considerable interest was aroused and in 1951 Yoder and Chesterman described the oc currence of Jadeite-bearing rocks, in place, along the banks of Clear Creek near Bolander's original discovery. Yoder and Chesterman (1951) reported eight large exposures of jadeite in the canyon of Clear Creek. Several more bodies near the original Gite and one large isolated mass near Santa Rita Peak (RGC-84,52) were dis covered during this investigation. The jadeite is found in two distinct occurrences, (l) Lens-like bodies having an irregular vugy surface and completely surrounded by sheared antigoritic serpentine. The central portion of these bodies contains 81 PLATS VII. Photomicrographs of schist from tectonic inclusions. Figure 1. Albite-glaucophane-pigeonite schist. Groundmass of colorless albite containing relict grains of pigeonite. Alteration by raetamorphism has produced 'whiskers' of glaucophane around the pigeonite. Plain light (X 100). Figure 2. Albite-glaucophane-crossite schist. Groundmass of albite containing relict grains of pyroxene. Glaucophane and crossite 'whiskers' forming on the pyroxene in response to metemorphism. Plain light (X 100). Figure 5- Albite-glauecphane-epidote schist. Groundmass of albite containing 'bundles' of glaucophane. Irregular shear zones contain iron oxides and epidote. Plain light (X 100). Figure 4. Albite-glaucophane-actinolite schist. Groundmass of albite cut by sinuous shear zone where needles of glaucophane and actinolite are concentrated. Plain light (X 100). 8l-A PIATE VII an irregular 'eye' of crushed jadeite (Figure 6). (2) Veins of .iadeite around the periphery and central portions of tectonic inclusions (Fig. 7). The lens-like bodies (Figure 6) form bold resistant outcrops within the serpentine and have a very indistinct contact with the serpentine country rock; near the contact the serpentine is blackish and glassy con taining antigorite surrounding blebs of chrysotile. The contact zone is composed of earthy weathered naterial tliat apparently has undergone ex treme crushing and shearing. Andradite garnet is present, within the ser pentine near this contact zone. Yoder and Chesterxan (1951) "eport a contact zone containing groscularite, lawsonite, punpellyite, and a green amphibolc. The outer zone of the lens is composed of a tough, greenish to brown, fine-grained rock composed essentially of fibrous prehnite, hydrogarnet, and sphene with minor amounts of biotite altering to chlorite. The sphene seems to be relict as it is in distinct bands reminiscent of an original metamorphic structure. The outer zone grades imperceptibly into a similar rock that is brownish with a cavernous weathered surface. The groundmass is composed of irregular grains of fully hydrated thomsonite containing inclusions of fibrous prehnite. Relict grains of sphene are present as residual crenulated, segregated bands.again suggesting in herited metamorphic structure. Minor amounts of chlorite may be found in the interstitial areas of the reck. Tne central portion of the lens-like body is composed of an 'eye' of greenish jadeite. Tne jadeite 'eye1 is nonomineralic except for extremely thin veinlets of biotite. Two generations of jadeite can be distinguished; (l) greenish jadeite that has been crushed and fractured, (2) white jadeite healing the fractmed and crushed Jadeite. Tne green jadeite is cloudy and SKETCH OF LENS-LIKE JADEITE BCwY sp. ' wsp 9 ^!(// covered v ^ j-f-(-sheared contact A'{;; Vil -- yv n wsp u EXPLANATION , | sp [ Serpentine outcrop. .----1| wsp | Weathered serpentine. lLB Jadeite. vc^/yl" \\\\ ff/ v wsp |>-;t-::| Thomsonite-prehnile veins. | br | I/////I Border rock, {prehnlte-hydrogamet-chloritcthomsonite rock.) Sheared contact. 0 _i___ L. Scale 5 Feet Outcrop boundory. Observed zone boundory. Approximate zone boundary. Sample location number - RGC-105-52. Figure 6 34 has abundant fine-grained opaque inclusions, In contrast, the white jadeite is clear, with very few inclusions. Optical determinations on the green .jadeite reveal that it must contain some diopside and acmite in solid solution. ' Veins, up to two inches in width, containing thomsonitc, prehnite, and minor hydrogarnet transect these various zones in the lens and are the latest stage of mineralization observed in the bodies. Iiatrolite and prehnite are present in veins from other lens-shaped bodies in the vicini ty. Tne zoned hydrogarnet from these bodies is discussed under mineralogy. Tne banded character of these rocks and the relict stringers of sphens in the outer-zone rocks suggest that these lens-like bodies may have originally been metamorphic rocks similar to others found as tectonic inclusions within the serpentine. Tne mineralogy of the lens-like boay described by Chesterman and Yoder is somewhat different than the one studied by the author, although the general characteristics are similar and cursory examination of other lens-like bodies shows a variable mineral composition. Individual veins of jadeite were found in schist bodies along Clear Creek (Fig. 7). These veins are usually along the periphery of the schist and show cross-cutting relationships to the enclosing schist (Fig. 8). The veins are quite variable in composition and color, and show irregular form. Most of these veins are less than one inch in width.vith local swellings up to eight inches. Tne larger veins contain two generations of jadeite, a dark green variety containing diopside and acmite in solid solution cut by intersecting veinlets of white jadeite accompanied by minor analcite and low albite. Tr.ese large veins pinch and swell, and TECTONIC INCLUSION CONTAINING 85 Figure 7 Somole Jocohon num ber! R G C -32-50 nG C *54-5l A nolyied J o d tllt token (10m fh .i t t m . 86 c*< Figure 8. Polished slab of albite-glaucophane-acmite schist cut by vein of white jadeite. Note folia of jadeite developed along planes of schistosity. ' 37 in many places are offset by minor cross faults. Smaller veins of almost pure white jadeite show very little fracturing or secondary veination. The detailed relationships between the smaller veins and the enclosing 3chi3t are shown in Figs. 3 and 9. The contact between the enclosing schist and the jadeite vein is always vory sharp and is marked by a thin selvage of green jadeite about 5 mm in width. In places the jadeite penetrates the 3chist along the pianos of schistosity, forming small dis continuous folia (Fig. 8). Microscopic study of these small jadeite veins shows that the jadeite forms normal to the vein walls and is grouped in semi-radiating clusters that are intimately interlocking. Albite and analcite are commonly present in the central portion of the veins. The albite (low albite from X-ray determinations) seems to have formed later than the jadeite followed by analcite which replaces both the jadeite and albite (Fig. 9). The albite in these veins is stable with respect to jadeite as it was not found replacing the jadeite. The distal ends of these veins may be completely lacking in jadeite and in its place are found albite, analcite, thomsonite, natrolite, and prehnite in variable amounts. The large block of schist shown in Fig. 7 is composed mostly of albite-glaucophane-acmite with minor variations in composition. This block is similar to the tectonic inclusions which have been described earlier and exhibits a well developed foliation and schistosity accompanied by minor small-scale folding. The contact with the enclosing serpentine is highly sheared and tremolite is developed as very fine-grained crystals in this zone. Small elongate blebs of jadeite intergrown with antigorite are common near the schist body and appear to replace the serpentine. 88 -- Schist Jadeite --Albife + Analcite i--Jadeite Schist Albite Analcite X 100 Figure 9. Sketch of jadeite vein showing relationship between jadeite, albite, and analcite. Camera lucida drawing exhibits re placement of jadeite and albite by analcite. 89 Origin of the jadeite The mode of formation of jadeite in the New Idria district presents several problems, although the observed field and raineralogical data seem to verify some of the recent experimental work on the jadeize problem. Jadeite has been considered by many writers to be formed only at high pressures and temperatures because of its high density and its apparent association with eclogites. Yoder (1950a) has shown that analyzed jadeites fall into two distinct groups, (l) those high in Na and A1 associated with albite and nepheline and (2) those lower in Na and A1 associated with garnet, i.e., eclogite. The first group of jadeites are always found associated with serpentines, the Clear Creek jadeite falling in this group. Since there are no eclogite facies in the New Idria district or in other rocks containing Na-Al rich jadeites, it would seem that the formation of Na-Al rich jadeites is separate and distinct from the eclogite problem. The jadeite veins in the schist are apparently formed in metamorphic rocks which belong to a low-grade metamorphic facies similar to the sub zones of the Chlorite zone as described by Hutton (1940). The presence of stilpnomelane in some of the schists cut by jadeite veins and contain ing folia of jadeite suggests Chlorite zone metamorphism. Quartz-albiteepidote-stilpnomelane, therefore, are here considered as a mineral facies characteristic of the Chlorite zone. Thus it would seem that P-T conditions during the formation of the jadeite were similar tc those characteristic of the Chlorite zone. 90 The presence of low albite and zeolites within the jadeite veins suggests that the pressures and temperatures following the formation of jadeite were not excessively high. Furthermore, the enclosing serpentine has not undergone any alteration which may have resulted from higher temperatures or pressures. Bowen and Tuttle (1949, p. 459) state that serpentine cannot be present in any layer of the earth's crust whoee temperature is normally above the decomposition temperature of serpentine under the pressure prevailing. Bowen and Tuttle have also shown experi mentally that serpentine inverts to forsterite and talc at a temperature above 500 C with pressure up to 45 kilobars having little effect. Thus from the following observations we have no positive evidence of extremely high pressures or temperatures involved in the formation of the jadeite from this locality, and it would appear from these observations that the P-T conditions during the formation of jadeite were similar to those characteristic of the Chlorite zone of metamorphism. Jadeite has been synthesized by Griggs, Fyfe, and Kennedy (1955) using analcite powder at elevated temperatures and pressures. The jadeiteanalcite boundary extends from the triple point (jadeite - nepheline + albite + vapor - analcite + vapor) at about 600 C and 12 kilobars to ,300 C and 18 kilobars. Combining Yoder's (1950a) measurements cn the nepheline + albite + water --> analcite equilibrium, and Adams' (1953) thermodynamic calculations on the formation of jadeite from nepheline and albite under anhydrous conditions, with that of Kennedy et al., a possible interpre tation of the conditions during the formation of the Clear Creek jadeite can be made 91 Jadeite is stable and can form from analcite at temperatures from 300 to 0OO0 C and pressures from 13 to 12 kilobars, respectively (Griggs et al,, 1955)i however if an anhydrous system is assumed, following thermodynamic calculations of Adams (1955)1 it is found that jadeite is stable at temperatures below 240 C and at pressures below 2000 bars. The petrographic observations show that the Jadeite in the veins could have formed in an anhydrous environment since there are no hydrated minerals formed at this stage. Replacement of jadeite and albite by later analcite suggests an introduction of water following the earlier formation of jadeite. If an anhydrous condition existed during the period of jadeite formation the thermodynamic calculations of Adams show thaz it would be stable at P-T conditions similar to those postulated for the Chlorite zone of metamorphism. The original reaction to form the jadeite is obscured; nepheline has not been found although albite is plentiful in these rocks. Three possible sources for fluids responsible in the formation of the jadeite must be considered: (l) fluids from the syenite intrusion, (2) fluids residual from the process which gave rise to the serpentine, and (5) fluids produced by metamorphism of the tectonic inclusions. The syenite intrusions are enriched in soda during the late stages of solidification and could have supplied fluids of the proper composition necessary for the formation of jadeite,' however, the jadeite bodies are not closely associated with the intrusions. The apparent strong meta morphism accompanying the formation of jadeite is not manifest within the 92 syenite rocks, and it seems that the intrusives are younger than the formation of the Jadeite. During the serpentinizazion of the original ultrabasic rocks, fluids may have been enriched in sodium and alumina during the late stages and these fluids could have been concentrated in the lens-like bodies and along the contacts of the more resistant schists where the greatest fracturing and shearing would provide ample open spaces. The trace element study of the serpentines revealed that sodium was below the limit of sensitivity in all samples and it seems unlikely that serpentinization could produce a concentrated Ila-Al rich fluid, unless it came from an outside source. The close association of the jadeite with the tectonic inclusions strongly indicates that the soda-rich fluids may well have come from these rocks. The minerals formed by dynamothermal metamorphism of these rocks are in some cases soda-rich and analyses of most of the rocks of supposed Franciscan age are extremely soda-rich ranging from 3 to 6 per cent Na20 (Taliaferro, 19^3 P* 136). it would appear most likely that the sodium-rich fluids have been produced during the metamorphism of these large tectonic inclusions. Higher confining pressures were probably developed in the tectonic inclusions than in the surrounding serpentine because of the resistant and brittle nature of the rock when compared to the plastic serpentine. Jadeite was identified as a metamorphic mineral in a glaucophanealbite schist (RGC-77-51) and has also been reported as a metamorphic mineral in schist from this same region by Mielenz (1939)* According to 93 de Roever (1955) Jc.dei.te has formed ir. quartz-albite schists from the Celebes associated with glaucophanic rocks. He has shown that the jadeite forms under metamorphic conditions characteristic of the glaucophane schist facies. The production of larger veins and pods of Jadeite may have resulted from mobilization by metamorphic differentiation. The metamorphism of these tectonic inclusions probably took place during the serpentinization of the ultrabasic rocks and these processes may have contributed, in part, to the peculiar set of conditions necessary in the formation of jadeite. The recent review of the jadeite problem by Yoder (1950a) has shown that jadeite (Na-Al rich) is not necessarily genetically associated with eclogites, but seems to be more closely related to serpentines. All of the Jadeite localities, in place, described in the literature are in or near serpentine, and it would appear that the tectonic environments and rock types characteristic of the serpentine belts produce a situation favorable for the formation of Jadeite. SERPENTINES The serpentine body is approximately thirteen miles long and four miles wide ana-at least 2,000 feet in thickness. Various interpretations have been given as to tne structure of this body. Taliaferro (19^5) suggests that it may be a large sill with laccolithic swelling in the thickest part of the sill, but Eckel and f-tyers (1946) conclude that it is a large plug. The contacts on all sides of the serpentine are fault contacts and nowhere can the original contact relations be observed (see Plate I). The bottom of the body is not exposed and the only basis for calling it a sill is its somewhat concordant relations with the Franciscan formation alone the southern border. The serpentine occupies the center of an elongate asymmetric dGne that forms a local bulge in the Coalinga anticline. Its emplacement and its form were controlled more by the tectonic development of the Coalinga anticline than by any other single factor. A diligent search of the entire serpentine contact demonstrates the fact, that this contact is faulted and sheared and evidence of an intrusive contact between the serpentine and enclosing rocks is completely lacking. The serpentine mass must have been brought into its present nosirion by tectonic movements even though it my have been an ultrabasic 'Snagma1' at greater depths. Ifo field evidence can be found to substantiate the hypo thesis that the serpentine was emplaced in its present position as an ultrabasic magma which had later been serpentinized in situ. The serpentine in this district has been strongly weathered to produce a terrain of low rounded hills composed of flaky serpentine debris tens of feet thick. Scattered throughout the serpentine are many small and large areas which have not been completely weathered and these stand out as bold irregular outcrops. A comprehensive study of the structure and petrography was not possible because of these large deeply weathered areas. Approximately 25 separate outcrops of unweathered serpentine were studied and the general petrography and mineralogy was established by thin section examination. One complete chemical analysis (Table 25) ar.d seven semiquantitative spectrographic analyses (PI. IV) were made to establish the composition of the serpentine. The most important feature of the serpentine is the completeness of the serpentinization in the ultra-basic body. Ihere is no .'narked difference between those specimens taken from the center and rocks taken from the edge of the body. This indicates that the serpentinizatior. was not the result of local alterations produced by intrusives, a mechanism sometimes used for the serpentinization of ultrabasic rocks (Du Rietz, 1955)- Tne serpentinization may well have taken place during the tectonic emplace ment of the rock, a time which affords increasing temperatures and a plentiful supply of water from the enclosing sediments. This mechanism has been suggested by Bowen and Tuttle (19^9) for the formation and emplacement of serpentine masses. Antigorite and chrysotile are the two most abundant minerals within the serpentine. Antigorite predominates where the serpentine has undergone more pronounced shearing and typically forms sheaf or flame-like aggregates (PI. VIII, Fig. 5). Antigorite-rich serpentines are found along the con tacts between the tectonic inclusions and serpentine, also where serpentine is in contact with sediments around the periphery. Chrysotila commonly forms a structure which pseudomorphs the original texture of olivine. The overall texture produced is the typical mesh-type (PI. VIII, Fig. 2). Chrysotile commonly forms larger cross-fiber veins within the serpentine measuring several inches across. A more detailed discussion of the antigorite and chrysotile is given in the mineralogy section. Magnetite is a universal accessory in ti.ese serpentines and two dis tinct generations can be distinguished, (l) primary magnetite formed as 96 PLATE VIII. Photomicrographs of the serpentines. Figure I. Large bastite grain surrounded by andradite within antigoritic serpentine. Plain light (X 40). Figure 2. ' Relict ^.olivine surrounded by chrysotile producing a typical serpentine mesh-texture, large opaque grain of primary magnetite surrounded by a reaction rim of chlorite. Fine-grained opaque material within the chrysotile is exsolution magnetite. Plain light (X 40). Figure 5* Antigoritic serpentine illustrating typical sheaf- and flame-like aggregate forms. Crossed nicols (X 40). Figure 4. Andradite vein following a chear zone in an tigoritic serpentine. Plain light (X 40). )1 an accessory mineral in the original ultra mafic roe):, (2) c>:solution marnetitc foraod during the serpentinization of the pyroxene and olivine (FI. '/Ill, Fig- 2). The primary magr.eai-,. ;characterized by much larger grains (up so 5 era) and is usually surrounded by a halo of chlorite. The exsolution magnetite is extremely fine-grained (less than C.Z :zm) and follows the former outline of the olivine grains cr may follow the cleavage lines in the pyroxene. 'hloritc docs not develop around the exsolution magnetite. Chromite is erratically distributed within the serpentine as a miner accessory or as infrequent ped-iike accumulations. The latter have been mined to some extenz, although r.o single pod has produced more than several tons of ore. This chromite has beer, formed by segregation from the ori ginal ultrabasic magma as suggested by its tabular form and banded nature. Chromite from these pods Ii=.s a density of 4.12 and an index of 2.045. matching very closely the density and index given for picrochromite, MgCr20.i. The chromite segregations exhibit incipient alteration to uvarovite and kammererite (Figure 5) that may have resulted from the processes giving rise to serpentinization. A detailed description of the kammererite is given under mineralogy. Olivine was detected in only three of the serpentines studied and in every case only a few relict grains were visible (PI. VIII, Fig. 2). The average ccnreosition, determined by measuring the optic axial angle, is 17>j FeaSiO^. Zoning was not detected in any of the olivines from the serpentines. The iron content of the serpentine, as determined spectrographically, ranges from 1 to 5?^ Fe and if the assumption is made that all of the iron was originally in olivine, this would give a very low amount s>e oi1 fayalite, less than 10'., somewhat lowcr than that determined by optical measurements. . Several tabular and banded zones within the serpentine contained abundant relict pyroxene altered to castite. The optical constants on the unaltered relict pyroxenes natch those of enstatite. The bastite and relict pyroxene have retained their original criss-cross texture and the interstitial areas contain chrysotile pseudonornhing olivine. Tne pyroxene hands represent a partial differentiation of the ultrabasic itagisa, however, the volume of these rocks is small and probably does not represent more than !' of the total. Many areas within the serpentine contained an isotropic serpentine r.itiers' associated with chrysotile. Andradite garnet is a prevalent mineral produced by the serpentini- 7-ation process within the serpentine. Many of the more resistant knobs ar.d ridges within the serpentine contain abundant small anastomosing veins of ar.dradite. The crest of Santa Rita Peak, the highest point in the district, is composed of ar.dradite-bearing serpentine and its survival as a prominent peak results from the garnet veination that produces a tough resistant rock. The garnet is always a late mineral and occurs in veins that follow, in part, the shear planes within the serpentine (PI. VIII, Fig. U); although in some instances the garnet is found intimately associ ated with bastite and in part pseudomorphs the original pyroxene (PI. VIII, Fig. l). The andradite is always found in antigoritic serpentines and its formation may depend upon the shearing conditions attendant in the crys tallisation of antigorite. Assuming that the garnet is formed by the break down of pyroxene (Ca-bearing) and magnetite, the following reaction shows 99 that it would take three molecules of diopside to make one of andradite. 3(Kg0*Ca0*2Si02) + Fe203 ------(3Ca0-Fe203.3Si02) -t- 3Si02 + 3MgO diopside magnetite andradite The spectrographic analyses on the average serpentine show 0.01 to 0.05% Ca, whereas the andradite-bearing serpentines show 1 to 5? Ca, a hundred fold increase. It would seem, from this data, that the formation of andradite within the serpentine must be accompanied by external source of Ca or an extremely efficient mechanism of mobilizing Ca from the pyroxenes within the serpentine. The almost complete serpentinization precludes an exact determina tion of the original ultrabasic rock types. The textures observed in the unshesfed serpentine show that olivine greatly exceeded pyroxene; therefore it seems reasonable to assume that the bulk of the parent rock was dunitic in compostion before alteration to serpentine. locally where differentiation or accumulation is manifest by the presence of tabular bastite zones the original ultrabasic rocks were harzburgites and pyroxenites. The spectrographic analyses of the serpentines, when compared to the data of Faust and Murata (1955), show that the elements character istic of serpentines arising from a 'magmatic' source are also present in the New Idria serpentine in comparable amounts. This analogy produces further evidence that the ultimate source of these serpentines was magmatic; however, it does not imply that the emplacement of the serpentine in its present position was an ultrabasic intrusion. 100 Intrusive Rocks Igneous rocks representing a restricted period of intrusive activity are present within the serpentine. Two small bodies crop out near the Get; mine, one in the HE l/U of sec. 26 and the other in the 5W l/b of sec. 25. Along the southern border of the serpentine, a lnrge exposure of igneous rock lies ucross the serpentir.e-Fanoche shale contact near the headwaters of './hite Creek (see Plate l). The intrusive rock in section 26 is a small plug deeply weathered and covering about 25 square yards in areal extent. The weathered nature of the serpentine and igneous rod; precludes any statement of the contact relationships. Specimens procured several feet below the surface ohov the rock to be a fine-grained, dark camptonite. An elongate dike-like intrusion approximately hCO feet long and 60 feet wide crops out in section 25 (Fig. 10). Tne dike lias been well exposed by differential erosion on the north flank of a ridge forming an irregular wall-like mass extending down the ridge. Tne contact between the serpentine and intrusion is obscured by talus and weathered serpentine. Tne dike rock is a coarse-grained, brownish barkevikite syenite. A thin selvage of albitite (6 inches to 1 foot) occurs sporadically along the contact. Tne './hite Creek igneous mass covers several acres and is the most extensive exposure of this rock in the district. During the mapping of thin exposure it became apparent that this mass has been displaced and wan part of a large landslide that has ridden out over the upturned Panoche shales. Eckel and Myers (19^6) show this igneous body in place Intruding 101 och serpentine ar.d Panache shales. Entailed inspection of the contacts be '.ween the serpentine and intrusive shows a gouge none made up of both serpentine and igneous rock. Several trenches dug along the rerpentine- shale contact show a similar relationship. i(o i'.ornfels or contact alteration of the siiales could ce found although several displaced frag ments of baked scrrent'r.c were found in one of ; he t rone ho s. Tnt: igneous rock tills exposure is extremely frash lr. comparison to fix' otnor intrusive bodies. It is indeed unfortunate that this body has been dislocated so as to obscure the structural relations with the serpentine. It was decided, none che less, to use this rock for analytical and air.eralexical study, keeping in mind that the relations with the other intrusive bodies would be somewhat obscured. The roci: from this exposure is quite variable in texture, light brownish ar.d made up of oarkevikite syenite and minor cam.ptor.itc. These igneous rochs hr."e been divided into three types or. the basic of texture, mineralogy, ar.d chemical co.position. I - Camptonite fine-grained with dark brown to black r color containing: oarkevikite, olivine, pigeonitc, and csIcic-plagioclase. . II - Syenite coarse to medium-grained with phenocrysts of borkevikite in light groundless of feldspar and zeolites. Ill - Albitite vugy ar.d coarse-grained containing: albite, analcite, natrolite, ar.d acinite. Camptonite is found at two of the three exposures although its exact relationship is not clearly shown in the field. The exposure in section 2c is exclusively camptonite ana is interpreted as an intrusive plug which nas crystallized rapidly. Camptonite is found in the White Creek mass 102 only by digging along the southern border of the outcrop and here it is associated with baked serpentine; although no fragments were found which show a complete transition from baked serpentine through camptonite to syenite. The camptonite probably represents an early differentiated phase of the intrusion and has been preserved along the "chilled" margins of the intrusive. The fresh camptonite is dark brown to black and has a distinct conchoidal fracture. It is extremely fine-grained and shows a rough banding along the face of the serpentine contact. The border zone or "chilled" contact has a groundmass consisting of a felted aggregate of calcicplagioclase and needles of amphibole (PI. IX, Fig. 2). Fhenocrysts of olivine and barkevikite are "floating" in this extremely fine-grained groundmass. The groundmass feldspar is too fine-grained to determine exact composition but it probably i3 close to An^g. The amphiboles in the groundmass are variable in composition; richterite is found in cores rimmed by crossite and barkevikite crystals are rimmed by crossite and glaucophane". Tne formation of the blue Ha-Kg amphiooles in the "chilled" zone suggests contamination by Mg assimilated from the serpentine; although no partially assimilated inclusions of serpentine were found in the camptonite. The goundmass in parts of the "chilled" zone shows flow structure with a trachytic texture. The "chilled" zone is one to four inches in thickness and is followed by a thin selvage (about f inch) rich in plagioclase. This plagioclase is lath-shaped and strongly zoned (An^g core to An^g rim) exerting strong euhedralism and forming a pilotaxitic texture (PI. IX,Fig. 2). 105 Except for the chilled zone and plagioclase selvage the camptonile is uniform in composition and texture. The camptonite groundmass consists of plates of zoned plagioclase and small needles of barker;,kite. Olivine with reaction borders of barkevikite and biotite form kelyphitic clots in the feldspar groundmass. Colorless pigeonite also forms cores in the barkevikite clots, although kelyphitic rims are not developed round the pyroxene. The plagioclase has a strong continuous zoning and forms a groundmass of subhedral to euhedral plates. The cores of plagioclase are clear and fresh with an increasing cloudiness developing in the peripheral parts of the crystal, and rimming these crystals is a clouded and somewhat altered feldspar. The fresh cores of the plagioclase are An5e-.e9 ranging outwards to the periphery by continuous zoning where a composition of A1120 was determined. The "deuteric" rim surrounding the clear, zoned feldspar has an An content from 10 to 12 and contains fine-grained needles of prehnite. The plagioclase shows complex albite-ala twinning and less commonly simple albite twinning. Within the interstitial areas, minor natrolize and aualcite replace, in part, the clouded albite rims. The small euhedral crystals of brown barkevikite form small clots with olivine or augite as a central core, although it is more abundant as idiomorphic prisms in random orientation (PI. IX, Fig. 4). The miner alogy of the barkevikite has been discussed earlier. Olivine, as noted earlier, is always found with a kelyphitic rim which is composed of exsolution magnetite and fibrous pyroxene surrounded by barkevikite crystals. The olivine is strongly zoned and the 2V*.varies from 82 (edge) to 88 (core) giving Mg2Si04 78$, (edge) and 84=& (core); 104 suggesting that the olivine has crystallized from a differentiated parent basic magma. The colorless pyroxene within the barkevikite clots has a small 2V (ca. 30 to 4o), positive sign, and Zac 37; probably pigeonitic in composition (PI. IX, Fig. 4). Indistinct zoning was observed. Elongated crystals of apatite are common in the groundmass, some attaining lengths up to 1 mm. Irregular blebs of sphene are associated with ilaenite and primary pyrite. A modal point-count analysis is given in Table 21, the chemical analysis in Table 23 and the norm calculated from this analysis in Table 24. Barkevikite Soda Syenite is the dominant rock type from tne igneous mass at the White Creek locality and in the dike from section 25. The grain size and texture are quite variable ranging from fine-grained facies similar to the camptonite to a coarse-grained porphyritic facies with a granitoid groundmass and large prisms of barkevikite (up to 6 inches in length). The hornblende shows no preferred orientation although sharp changes in texture are quite common. Inclusions of foreign rock were not found within the syenite from either locality. Figure 10 shows the outcrop pattern of the syenite dike and such structural features as could be determined; no detailed map of the White Creek mass was made since it has been dislocated by landslide. In the hand specimen, the grenite is mottled black and white with extremely well developed black prisms of barkevikite set in a white groundmass of feldspar. Where soda-pyroxenes dominate over barkevikite a greenish cast is developed in the syenite. The extreme variability in 105 TABLE 21. MODAL ANALYSIS OF CAMPTONITE (RGC-45-50). MINERAL PLAGIOCLASE (An 20-69) BARKEVUCiTE OLIVINE (FOgo-Fa2o) PYROXENE APATITE BIOTITE SPHENE MAGNETITE FYRITE PREHNITE PERCENT 4o.o 59-5 5-5 1-3 2-5 4.2 3 .6 4.8 1.0 99-7 106 Figure 10 107 in size of the hornblende phenocrysts is the cost striking feature of the various facies when contrasted to the groundsmen which appears to remain medium to fine-grained. The syenite has a panidiomorphic-granular texture and where the rock shows deuteric alteration the texture becomes porphyritic with a hypidomorphic-granular groundmass. The feldspar forms almost equant tabu lar crystals which show strong zoning under crossed nicols. The inner portions of the crystals are usually clear except for incipient alteration; these cores grade out into rather thick rims of cloudy feldspar Ans_lc which in turn may grade into natrolite and/or analcite (PI. IX, Fig. 5). The composition of the fresh core is andesine (An.i5_s5) which grades out to the rim of the clear area in continuous zoning to a composition of An30. Minor perthitic intergrowths were observed in several of the cloudy albite rims. Tiie cloudy albite is the most abundant feldspar and in she syenite facies the calcic-plagioclase never exceeds 1^ of the total feldspar. Caleic-plagioclase is completely lacking in some facies and in these rocks the feldspar is albite (Ans_i0). The albite from the White Creek mass is always clouded and it seems to have formed by late deuteric or hydrothermal alteration. Under high magnification the clouding materials appear to be elongated fibers and short rods accompanied by opaque dust. Differential acid leaching of the clouded albite clears the albite and the inclusions that remain have optics similar to those of prehnite. The dissolved material is thought to be iron oxides and natrolite. MacGregor (1951) and Poldervaart and Gilkey (195*0 have shovn that clouding in feldspars may be due to a deuteric or hydrothermal alteration. 108 TABLE 22. MODAL ANALYSES OF SYENITE. MINERAL ALBITS (Ads^iq) ANDES INE-LABRADORITE (AnS5_30) BARKEVBCITS ANALCITE HATROLITE ZOISITE MICA CHLORTTE APATITE SPHERE _ OPAQUES RGC-l-50 28.8 - 37-8 - 10.3 12.2 8.4 5 2.2 7 100. 7 * Same as analysed rock RGC-47-50 RGC-48-50* 53 .6 26.2 2.3 5-3 k.9 1.6 - - 99-8 io ; I.-, contrast to the syenite from '..'kite Creek, the syenite fro.. tne dike ` r. sect'or. 2p car.*,a'".us abur.dan - clear ar.i ;'rjs!i albi.se (An5_10; ci: may have reciy sta3.1izod curing a late tectonic ,iovetcr.k that ' :t: i '.o located .t Creek syenite has a; rarer, sly oscanc-a. by land til he d' s- iocatlon. 7::e dike syenite dees no.. cental:: any calclc-phujicclasc ar.:; ere re Is ::o zoning or svi nr.ir.g In ti:e 1 recrysta3.il ted1 alb ire. The r>::- cess Ca has beer taker, up by rre hr. 1 to and cllnozohsite. Aral; To and aatroiite ir. the syenite have- io.tied by deutcric or k.yrirothc'Y.si alteration. An. ale L to see.r.s to form first a::d 1:; found cnlarying cleavage traces of aTrite ar.d as later patches replacing alb l:c. Large area; cf analctte are found surrounding oarltev Ik.i tc and .'.esc cctt.ior.ly contain ferry' b'otltc. i.'atrolito follows ar.alai.e and relec - i vc-2.y replaces albite . Iiatrolite nay comolstciy replace feldspar preserving its original texture. Barkevikite as the .v.cst abundant mafic mineral of the syenite ar.d is characterized by well-developed euhedral habit and distinctive plecchroism (rl. IX, Fig. 1). Tr.e physical properties of barkevikite are similar in all facies of the camptoniTe and syenite and it appears to be stable in both rocks except in the 'chilled1 zone of the camptonite. Tne chemical and optical properties of barkevikite fro:: the White Croc!: syenite are riven in Table 1*-. Accessory minerals fro:; the syenites are similar to those found ir. campconiles with some minor variations. Apatite is abundant, forming elongate, light brown, prisms with wea?: pleochroism. A speetrograpnic analysis of the pure apatite from the White Creek syenite is given in Plate IV. Sphene is found both as a primary mineral and as an alteration no r.roduct of '-he barkevih: . Piotite is more abundant in '.ho syenites chan ir. the camptonites p.r.d is formed during late deuter'c or hydro thermal alteration when it crystallised as wormy clots within the zeolites. Calcium. released in the alteration of the calcic feldspar has produced prehnite, zoisite, and clinozoisite. Stcall amounts of .Muscovite are present in tine syenite dike although it was not definitely identified it: one i/hi te Creek syenite, Magnetite, ilmenite, and iron sulfides are scarce in all cases. Phenocrysts of a deep red garnet (up to 1 cm) are present in a syenite boulder in the White Creek syenite and optical and density determinations (h'-l.EOO and D-4.18) on these garnets are similar to those given for spessarLite. heavy mineral separations of the norzal syenites also produced small quantities of this garnet. Tine chemical analyses of both the camptonite and syenite (Table 23) shoe that they arc undersaturated rocks with respect to silica.as repheline is present in the calculated norms (Tabic 2!i) , although, nepheline was not found in any of the rocks. Tee absence nf n.ephelinc in these rocks could be explained by the fact that in a hydrous deutevic or hydrothermal stage, nepheline is unstable. Yoder (1950a) has shown that aralcite forms in a hydrothermal system, at tem.Dsratures below 575 0. and above this, albite and r.epheline are stable. This experimental relationship becomes valuable in interpreting the presence cf analcite and natrolite in the late stages of crystallization of the syenite intrusions. Tne presence of unalcit= ar.d the absence of nepheline in an undersaturated rock, such as this, would seem, to indicate that the final stage of intrusion was characterized by low temperatures and a hydrated condition. A modal analysis of the White Creek syenite and the syenite dike in section 25 are given in Table 22 Ill TABLE 23. CHEMICAL ANALYSES OF CAMPTONITE AND SYENITE. 1 25 Si02 Ti02 AI2O3 FeO 48.56 2.01 15.72 9.65 51.42 2.20 I6.58 7.62 60.00 .42 16.88 3-02 Fe203 .36 1.43 1.83 MnO .17 23 .12 CaO MgO 7.76 6.52 6.77 3.68 3.16 1.40 k2c Na20 H20h2o+ ZrO Cr2Oa BaO SrO L120 75 5.22 .42 2.29 nil .05 - 1.06 6.04 .54 1.41 traces traces .08 - .94 9.51 43 1.53 03 .06 .02 traces P2O5 - .49 .14 co2 - 4? 59 s - - traces 99.88 99.85 99.88 1 - Cajnptonlte (RGC-45-52), analyst - W. H. Herdsman, 2 - 3yenite (RGC-47-52), analyst - W. H. Herdsman. 3 - Syenite from Arnold and Anderson (1910), analyst - W. F. Hillebrand 112 TABLE 2h . NORMS OF ANALYSED CAMPIONITE AND SYENITES. 1* ORTHOCLASE 4 .19 ALBITE ANORTHITE 30.92 17.24 NEPHELINE DIOPSIDE 8.10 17.43 OLIVINE MAGNETITE HM2NITE 15.96 .46 4.86 APATITE - CALCIUM CARBONATE - WATER 2.29 CIPW CLASSIFICATION- 1 - Class III, salfemic Order 6, lecdofelic Rang 3, alkali-calcic Subrang 5 persodic 2* 6.12 43.75 15.29 5.83 10.82 9-75 2.16 4.26 1.27 1.00 1.41 00 ON 3* 5.56 73.56 1.44 2.72 5-72 2.99 2.66 .34 1.34 1.53 2 and 3 - Class II, dosalic Order 5> perfelic Rang 2, domalkalic Subrang 5 persodic * See Table 23 for chemical analyses and identification. 113 and in Table 25 two independent analyses of the l/hite Creel: syenite are given. Albitite and Zeolite-Rich Rocks are found in minor amounts associated with the late stage hydrothermal or deuteric alteration of the intrusive. These rocks are characterized by a vugy texture and almost pure white color. A small zone, up to one foot thick, of albitite is developed in the syenite dike along the contact between the intrusive-serpentine contact. The rock is made up almost entirely of clear albite (An5_io) accompanied by clinozoisite that forms delicate needle-lii;c druses in the numerous vugs. Clear sphene is a common accessory in the groundmass. Barkevikite cores rimmed by acraite or aegirine-sugite are present in sub ordinate amounts (1 to 5,-0 and the araphibole is the only mafic mineral observed. Isolated stringers and clots of albitite are found within the syenite but here it is a very minor facies. Numerous late, vugy veins of zeolite are present in the White Creek syenite. These late veins vary in width from one to six inches and nay extend to several yards in length. The minerals within the veins are particularly well developed and a fairly strong copper sulfide mineralization in many of these veins was observed. In addition, large irregular vugs (up to a foot across) are also present in the syenite and these contain the sane well-formed crystals that are found in the veins. Hatrolite is the most abundant mineral in the veins and vugs, crystallizing as distinct prisms up to one inch in length and about 4 mm. in cross-section. The unusual morphology of this natrolite has been discussed earlier under mineralogy'. Analcite, prominent in both vugs and veins, attains exceptional size in the larger isolated vugs where crystals up to one inch or more were found. The trapezohedral faces 114 PLATE IX. Photomicrographs of camptonite and syenite. Figure 1. Barkevikite soda syenite. Large prisms of barkevikite inclosed by plagioclase feldspar. Note grains of plagioclase with clear cores rimmed by cloudy albite. Interstitial areas contain zeolites and biotite. Plain light (X 25). Figure 2. Camptonite, 'chilled zone* showing a fine grained groundmaes with euhedral crystals of barkevikite and olivine which Is surrounded by a kelyphitic rim. Note embayment of the 'chilled zone' by the plagicclase-rich selvage. Plain light (X 40). Figure 5. Alteration of calcic-plagioclase to albite. Note the fine inclusions within the albite contrasted to the clear calcic-plagioclase. Crossed nicols (X 100). Figure 4. Camptonite. Large clot of pigeonite surrounded by a fine-grained mat of barkevikite and calcic, plagioclase. Plain light (X 40). 114-A PIATE K are well developed, on colorless to opaque white crystals containing numerous hair-like inclusions of acmite. Trie larger crystals have a very faint birefringence and a rather complicated twinning. Individual crystals of tabular, prismatic acmite are abundant in the vugs and veins intergrown and implanted on the natrolite and analcite. Occasional cores of barkevikite are found rimmed successively by aegirine-augite and acmite. The copper mineralization within the veins is present as small blebs of c'nalcocite accompanied by galena. Spectrographic analysis of this sulfide mixture shows 0.05-0.1$ Hg and detailed inspection of the concentrate shows matei'ial similar to cinnabar. Classification and Origin of the Igneous Rocks Considerable difficulty is encountered in placing these rocks within present rock classifications, either on the basis of chemistry or mineralogy. Hie norms calculated in Table 24 certainly do not resemble the minerals reported in the modes (Tables 21 and 22). tlepheline was not found in any of the rocks, although all three calculated norms show nepheline; further more orthoclase is present in the norms and not in the modes. Diopside and olivine also are shown in all three norms but these mineral types have been identified only in the camptonites. Natrolite and analcite are present in all of the intrusive rocks and it seems that these minerals have proxied for nepheline. The stability relationships of nepheline-analcite as discussed earlier, suggest that in the late stages a hydrothermal lowtemperature conditions may have removed any nepheline formed in the earlier intrusive stage. This, somewhat peculiar, situation has pro duced an under-saturated rock without the expected feldspathoids, The 136 high-sodium content contrasted to the paucity of potassium is another peculiarity of these rocks that poses a difficult problem in classifi cation. The two rocks analyzed for this investigation have been classi- fied according to both chemical and mineralogicr.l methods; the results are tabulated below. RGC-U5-51 RGC-^7-51 Washington (1917) 111-6-5-5 II-5-2-5 Johannsen (1931) Family 5216 camptonite Family 2216 Kepheline-bearing diorite Shand (1927) DVK & sub-basalt 1camptonite' XUM S' essexite t'neralite Kiggii (1956) soda gabbro magma Type 2 Mugearitisch sommatische magma (sub-alkalic) Type Mclarkitisch The early facies of these intrusives, namely (RGC-U5-51)* is clearly shown to be a camptonite as it falls into the comptonite categories of Shand ana Jo'nannsen. Hie chemical classifications of Niggli and of Washington (CIPW) give erroneous placement as the normative minerals are grossly different from the modal minerals. The classification of the main intrusive rock (RGC-47-51-) is still somewhat doubtful and it appears that the rock may be a hybrid. The rock was first described by E. S. larsen for Arnold and Anderson (1910) as a soda syenite and has since been referred to as a soda syenite by Taliaferro (19!*3) and Eckel and Myers (19^6). Following the definition of a syenite given by Johannsen (1951), it appears that this could not be called a syenite, since potash feldspar is an essential constituent in the later and the New Idria rocks contain less than 1 K20; this is probably present in biotite. Following 117 Shand's classification this rock would be grouped with the theralites and Johannsen's classification shows it to be a nepheline-bearing diorite. The chemical classifications of Niggli and CIFW are of no value in determining a proper classification because of the unrealistic norms. A more descriptive petrographic name for the soda syenite would be barkevikite sodaclase diorite following Johannsen's nomenclature, but the original rock name given by Larsen is retained here to avoid confusion. The rocks formed by late stage hydrothermal or deuteric action and containing albite and zeolites could be best called albitites with qualifying adjectives. Turner (1896) first described albitites as aplitic rocks composed entirely of coarse granular aggregates of albite. The intrusives of the New Idria district seem to have arisen from a parent magma that was probably similar in composition to olivine basalt magmas. Marked zoning in feldspars and olivine of the camptonite indi cates differentiation. The camptonite border facies seems to have crystallized from a mobile and somewhat nigh-temperature magma, but the peculiar composition of the soda syenite cannot be readily explained by further differentiation of the camptonite. Several other factors may have influenced the course of crystallization and final composition; these are given below. 1. Contamination of the parent magma by soda-rich rocks. 2. Filter-pressing of residual liquor of a partially differentiated basaltic magma. 3. Selective migration of Ca, Mg, Fe, and Ti into the host serpentine during emplacement. 118 All these factors must be considered in any discussion on the origin and composition of the intrusive rocks. The genesis of the intrusive rocks bears important relationships to the origin of the metasomatic calc-silcate and chlorite rocks within the sex-pentine near the intrusives. A variation diagram of the available analyses shows that the rocks follow a differentiating trend with Ca, Mg, Fe, and Ti decreasing and Na and A1 increasing with little change ir. K. This would indicate that the intrusive material differentiated after arriving at its point of emplacement within the serpentine. The lack of inclusions within the syenite indicate that little or no contamination has taken place, al though this is based on negative evidence. Spectrographic analyses on the analyzed rocks has produced some very interesting data. The syenite and camptonite contain similar cobalt and nickel values with Ni = Co (lOO-JOO) ppm. Nockolds and Allen (195*+) have shown that in alkaiic rocks during the early stages of differentiation Ni ? Co and as differentiation proceeds Ni ^ Co with Ni< Co in the later stages. The Ni-Co relationship 'suggests that these intrusives have arisen from a partially differentiated magma. The Hi, Co, and Cr content of these rocks is similar to the values given by Nockolds and Allen and It would seem that these elements would have been enriched had there been serpentine contamination of intrusive magtna. Zr and Eb were not detected in these rocks whereas Nockolds and Allen consistently report these elements in alkaiic rocks. Barium and strontium appear in normal quantities within these rocks although the low value of K has probably controlled, in part, the Rb and Ba content. The calc-silicate bodies near the intrusions generally have matching trace 119 elements, which suggests -ha'- there was a moveme n t of me La sora cl c solutions from the intrusives into the serpentine. These metasoiratic bodies show enrichment (Table 25) in Al, Ca, Fe, and Ti and it would appear logical to derive these elements from the intrusi/cs since the late stages of the intrusive are depleted in these elements. This mechanism would result In a magma that was enriched in N'a, probably with H2O obtained from the enclosing serpentine. Metasomatic Bocks V/itnin the serpentine are a number of small rock bodies formed by direct replacement of the serpentine and small veins and zones of meta somatism are also found in the tectonic inclusions. The fluids re sponsible for these replacements are in no way connected with the processes giving rise to the serpentine or schists; since these metasomatic rocks contain a suite of calc-silicate and titano-silicate minerals that could hardly have been produced by anything but emanations from an intrusive body. The metasomatic rocks have been divided into three different types on the basis of their mineralogy. (1) Chlorite rocks characterized by dominant chlorite accompanied by minor amounts of garnet, perovskite, ar.d titanium-bearing minerals. (2) Calc-silicate rocks characterized by diopside, idocrase, and chlorite. (3) Titano-silicates in soda-rich veins within tectonic inclusions. Each of these groups will be discussed separately and the mineralogy and geology of each separate occurrence will be described. 120 The lack of po: .-mineralisation deformation is one feature that these mctasomatic re :!:s "nave in common. Thin Is important in a rogior. where numerous 3 true oral movement 3 through Tertiary time have defo-cted other rocks within i e district. Tine apparent age of tr.eae c.e v-asomat > c roc-is muse be fairly recent (late Tertiary), since they iove for.t.ed vugy do p os 1 - s within roc!-: that have undergone 6ynamethermal meia.torpkio:.: il*}T * n * Tci* : * '"irv w*'i'',^ In the discussion to fellow, :.:e3ar.ito is used as a synor.yt! for the titartar, ar/lradite described under mineralogy. T.e sequence of ..`.ir.eral- -^c.t.jor. ' *vot. f'oi oacb do3*'U'ibo'' r'oncc b^sod op r'C2t,rvr`d */"* vUTUi. r*o T **> t ** or *\ b * 0.nd OH 0 ".CP C *** 1 y V131 taciic:: a pa:a"cpctrie 3cq.uer.ee ri 2.3^ 2.2.' d 2 c"': b*pc.'' b ~j i i ,''j ~ ~eclcj;c ira.^ (PI. b*r urlr.,-- : '.r> ^d 1 c d:~lt3 cT tr.c 3arr.v>^o mpfucr Iklor:tc Rocks (RGO-TC-yl) crept out a]cr.g :he serpentir.e-Far.cche eh.e.le contact at the extreme southeastern edge of the serpentine mass, where it forms ar. nn"n-p vc * r ^bat. 3".T`'*1*tG3 G 70^ Z In ebeared ccrpcnblns. ^rc vein ^ 3 four to sin feet vide and about 50 feet in length. It is a dense, com pact., dark green rock with anastomosing veins of light brown garnet, a variegated texture and well developed foliation. Clinochlore is the most abundant mineral. The veins contain andradite with a brownish to reddish tint on outer surfaces. Tne tinting of the garnet is ascrioed to late introduction of Ti. Isolated patches of garnet within the clinochlore groundmass contain idocrase cores. Small amounts of magnetite ar.d 121 calcite form as late minerals in open cavities. Sphene formed as a further reaction with the anciradite and was seen to replace it on the rims of several crystals. The sequence of mineralization is given below. Chlorite Idocrase Garnet _____________ ______________ _____________________ Sphene __________ Magnetite ________ Calcite (RGC-93-52) is found in several small outcrops (up to six feet long and one or two feet in width) trending in a north-south direction. Tiie contact with the enclosing serpentine is marked by a distinct color change. The rock is gray i: green, and dense, with a crude schistosity cut by anastomosing veins of light brown garnet. Clinochlore is the most abundant mineral and its preferred orientation produces the observed schistosity. Associated with chlorite as an early mineral, is a pyroxene of diopside composition, that makes up about 4 to 6^ of the total rock. Small crystals of zircon and apatite are present in the chlorite ground- mass. Andradite veins cut the schistosity and are definitely later than chlorite and diopside. Small crystals of magnetite have formed on the open surfaces of the rock. The sequence of mineralization is given below. 122 Chlorite ____________ Diopside ____________ Apatite _____ Zircon ___ Garnet __________ Magnetite _____ (RGC-J6-50) crops out as a small tabular vein striking N 30 E, about 20 feet long, and 4 to 5 feet wide. The rock is vugy with a crude schistosity parallel to the strike of the vein. The color is green to grayish-green, enhanced in the vugy areas where perfect crystals of garnet and chlorite produce brilliant reflections. The contact of the chlorite rock with serpentine is gradational showing alteration of ser pentine to chlorite along shear planes. The rock is composed essentially of garnet and chlorite accompanied by minor amounts of sphene, pyroxene, and magnetite. Chlorite forms euhedral to 6ubhedral platelets that have random orientation, and dark olive green color in hand specimen; under the microscope a strong pleochroism from green to light yellow is seen. The optics are similar to those of delessite (Table 17). Garnet forms light yellow to olive-green euhedral crystals partly intergrown with the chlorite (PI. XII, Fig. 4). The cores of the garnet may be hollow or mottled with a fine-grained mixture of sphene and pyroxene. Optical determinations are similar to those given for andradite. A light brownish zone around the periphery of the garnets is due to Ti introduced during late stages of formation. The mineralization sequence is given below. 125 Pyroxene Spile ne Garnet Chlorite Magnetite ______________________ ______________________ (nGC-57-?l) forms a small bench on a smooth slope of weathered flaky serpentine, the largest outcrop of which is about UO square feet and it rises only five feet above the surrounding terrain. The ser pentine and chlorite rock arc considerably broken up and altered by weathering processes obscuring the contact, between the two rocks, but it is assumed that the chlorite rock has formed by replacement of the serpentine. The outcrop is very irregular and the only structural trend observed was a set of 2-'./ joints dipping about 55 to the south. The rock is very irregular ana vugy and shows no preferred orientation of the constituents. The weathered surface of the rock is rusty brown and on the fresh surfaces the true color is dark oli''e-Green. Chlorite (possibly delessite), melanite, perovskite, and apatite are essential constituents accompanied by minor amounts of sphene and magnetite (PI. XI, Fig. 2) . Tr.e texture is porous and vugy, and magnetite and perovskite are the only euhedral minerals present in the cavities. The early stages of mineralization were richer in iron and many of the large fan-shaped aggregates of chlorite have cores of stilpnomelane that grade impercepti bly into delessite (PI. X, Fig. 2). Melanite, the next most abundant mineral, forms irregular patches within the rock and does not exevt. its 124 normal tendency to form idioblastic crystals. The Irregular patches of melanite are ccmmonly intergrown with sphene and perovskite (PI. X, Pig. 2). Strong zoning of the melanite is not found at this locality and it seems that the garnet has formed late from Ti-rich fluids. Perovskite is present in two forms, (1) perfect resinous yellow octahedra and (2) irregular granular aggregates. Pabst (1951) reported colorless and dark adamantine spinels from this locality; however, he did not report perovskite from this locality and it may be that the colorless spinel he described is the octahedral perovskite. Small elongate, colorless prisms of apatite are common within the gmundnass. Magnetite- forms euhedral octahedra and also irregular patches within the groundmass. Sphene is present within the cores of melanite and lias formed in the early stages. The mineralization sequence is given below. Sliipuomelane ______ Delessite ______________ Magnetite ________ Apatite ________ Sphene _________ Melanite _ Perovskite ____________ (RGC-83-51) is one of the larger areas of metasomatic rocks and six separate outcrops cover an area of about 30C square yards. The individual bodies are tabular in shape and show moderate contortions parallel to the shear planes ir. the serpentine. The contacts between the chlorite bodies 125 and the serpentine are gradational and are conformable with the local structure in the serpentine. The general strike of the bodies is 2-W and their dip is almost vertical. The rock is dark, bluishgreen with schistosity and elongate vwgy areas parallel to the schistosity. Magnetite, iLmenite, and perovskite are common as euhedral crystals on open fractures and vugs. This body is the only one found that shows alteration of the earlier ir.etasomatic mineral as semblage. Tne early phase of minei'alization produced a garnet-chlorite rock containing little or no titanium. Tne garnet was probably andradite, but this is difficult to establish since there are very few relict grains of garnet. A late stage of mineralization, rich in CO2, altered moss of the garnet to calcite and chlorite. Many spots, where garnet was formerly present, are now pseuiomorphed by a fine mixture of chlorite (delessite), calcite, and magnetite; and in other spots the area occupied by the garnet is now vacant (PI. XII, Fig. 1). The rock is composed of chlorite (rumpfite and delessite), calcite, magnetite, and ilmenite. Tne chlorite assumes a crude schistosity and the magnetite forms anhedral clots within the calcite-chlorite replacements. Minor clots of chevkinite (?) were identified from the thin section and these small concentrations are characterized by plecchroic halos produced in the chlorite. Sodium fluoride bead tests on this mineral show it to contain uranium. Rutile was identified from the rock in one small isolated cluster of microscopic crystals. is6 A series of thin sections across the serpentine-chlorite rock contact indicates that metasomatism of the serpentine is accomplished by alteration of antigor'te to rumpfite and delessite with little or no change in the original fabric of the serpentine, although within the center of these bodies a coarser crystallization obscures this original fabric. The large amounts of calcite produced in the late stages of mineralization obscure the replacement sequence; however, it is clear that the serpentine has been metasomatically replaced by fluids enriched in calcium, aluminum, iron, and titanium. The inferred mineralization sequence is given below. Chlorite Andradite J-iagnetite _______ . Ilmenite _________ Perovskite Calcite Cnevkinite (?) Rutile (RGC-79-51) crops out 200 feet away from the contact between the serpentine and Franciscan formation. This body is irregular in shape and about forty feet long and fifteen feet wide, no general structural trend could be determined, although a crude schistosity within the rock conforms, in part, with the shear planes in the serpentine. Tr.e ser pentine-chlorite rock contact is sharp and there is little interfingering. The surrounding serpentine is antigoritic and shows evidence of strong 127 shearing. The replacement rock is dense and compact with very few open fractures or vugs (PI. X, Fig. 3). It is intensely veined with a yellowish-brown andradite. Ciinochlore makes up most of the rock and forms a crude schistosity; in vugy areas beautiful well-developed crystals of ciinochlore are associated with crystals of andradite, calcite, and magnetite. Small clots of colorless apatite are associated with the garnet and early sphene is present as cores within the garnets. The mineralization sequence is given below. Ciinochlore ________________ Andradite _________________ Sphene ___________ Apatite ___________ Calcite _________ Magnetite _________ (RGC-5&-51) crops out as a small discontinuous body within a ' bold serpentine outcrop. This metasomatic zone is irregular in shape and about 40-feet long and 20 feet wide. The host serpentine is a dense rock which has been impregnated with thin veins of andradite. Serpentine has been strongly sheared and antigorite is the most abundant mineral in the rock. The chlorite rock is dense, grey-green and is cut by anastomosing veinlets containing melanite and perovskite (EL. X, Fig. 1). The chlorite, similar to rumpfite in sompostion, exhibits a crude schistosity within the metasomatic rock. The veined perovskite and melanite are anhedral and show extremely irregular grain boundaries 128 with the chlorite. The mel&nite is also characterized by a zoning that grades from light-colored cores to dark reddish-brown rims again manifesting the increased Ti substitution in the late stages. Apatite and sphene occur consistently, although sparingly, as early minerals within the chlorite groundraass. Several small crystals of zircon were noted. Along the veins within the metasomatic rock, there are many open fractures and vugs that are covered with a beautiful suite of crystalline material. All of the individual crystals exhibit almost perfect crystal forms, however, none of them are more than one centi meter in their longest dimension. Melanite is present as splendent black dodecahedrons producing drusy coatings on the vein walls. Yellow and metallic black perovskite has crystallized as cubes and octahedrons. A chemical analysis of the perovskite is given in Table 1. Red prisms of titaniferous idocrase are sparsely distributed on the drusy surfaces. White diopside has crystallized in delicate bladed crystals and forms small tufted clumps lining the vugy surfaces. This locality is well known to mineral collectors and has produced some of the finest mineral specimens from the district. The mineralization sequence is given below and it is considered that anhedral vein material formed during the Bane period as the drusy material. 1?9 Chlorite Apatite ___________ Sphene __________ Melanite ____________________ Perovskite ____________________ Dionside _________ Titaniferous idocrace _________ (P.GC-J;2-50) is a small body less than four square feet in size projecting out of ./Gathered serpentine as a small knob. The relation ships between the serpentine and the metasomatic rock are obscure and the author is not convinced that this rock is in place. It is coarse grained with a porous, open texture, and the constituent minerals assume no preferred orientation. The rock is nearly bimineralic composed of black melar.ite and green chlorite (rumpfite) as shown in PI. X, Fig. 4. Mslanite shows a strong tendency to euhedralism with the dodecahedral form dominant. Zoning in the melanite is distinct and is manifested by a change in color from light brownish-red to a deep brownish-red. The analyzed garnet in ife.ble 5 was taken from this body. Apatite in present as a minor constituent and has crystallized as snail clear prisms. Minor amounts of magnetite, sphene, and perovskite are also present. The mineralization sequence is given below. Magnetite _________ Chlorite Sphene Apatite Melanite Perovskite 130 PIATE X. Pnotomicrographs of metasomatic rocks. Figure 1. Chlorite rock. Groundmass of clinochlore and rumpfite cut by irregular veins of melanite garnet. Larger mass to the right of the field is perovskite. Plain light (X40). Figure 2. Chlorite rock composed of mtergrowths of delessite ana slilpnomelane in the light areas. Subhedral opaque grains of magnetite are associated with semi-opaque intergrowths of melanite garnet and perovskite. Plain light IX 40). Figure 3- Chlorite rock composed of clinochlore, light groundmass and irregular, distorted stringers of andradite garnet. Plain light (X 40) . Figure 4. Chlorite rock containing anhedral grains of zoned melanite garnet intergrown with colorless rumpfite. Opaque areas are magnetite. Plain light (X 40). 1J0-A PLATE X 151 PLATE XI. Camera lucida drawings or metasomatic rocks. Figure 1. Calc-silicate rock. Late calcite vein containing sub'nedral grains of melanite rimmed by idocrase accompanied by anhedral grains of idocrase and 'wormy' rurapfite. (X bo). Figure 2. Chlorite rock. Central area illustrates delessitestilpnomelane intergrowths surrounded by melanite garnet, perovskite, and apatite. (X 60). 131-A PIATZ XII. Photomicrographs of metasomatic rocks . Figure 1. Chlorite rock illustrating late ste.ge alter ation. Gr0ur.dr.as5 contains runpfite and anhedral magnetite grains. Altered crystal of andradite replaced by runpfite, calcite, and magnetite. Plain light (X 80). Figure 2. Calc-silicate rock composed of colorless aiopside intergrown with melanite garnet, late vein of titar.iferous idocrase cuts the diopside and melanite. Plain light (X 40). Figure 3. Chlorite rock composed of clinoehlore intergrown with andradite. Late vein of strongly zoned melanite garnet cuts the rock. Plain light (X 40). Figure 4. Chlorite rock composed of delessite intergrown with andradite garnet, note vugy openings. Fine-grained intergrowths of sphere and pyroxei nroduce dark cores in the garnet. Plain light (X 40). Ct (D FIATE XII 1J2 Calc-silicatc Rocks (RGC-57-50) crops out, as a snail layered body about fjv*' feet long and three feet thick, tabular in form and dipping ; 7- 55 2. with an exposure so poor that the contact rexation ''i*'h the terpentine is obscured. Further the serpentine is flaky and weathered to a con siderable depth, and digging was not helpful in determining the nature of the contact. However, it is assumed that this body was formed by metasomatism of serpentine. The calc-silicate rock is banded ar.d has crude schistosity and foliation, with vertical fractures normal to the schistosity healed by calcite. The light-green bands are composed of chlorite ar.d diopside and the darker green ones contain idocraoe, diopside, and chlorite. In thin section, chlorite is almost colorless prochlorite with strong preferred orientation that is responsible for the schistosity. Diopside is also colorless and usually forms radiating clusters, whereas granular aggregates of idocrase cut pre-existing structures in the rock. The calcite is a late mineral that fills interstitial spaces within the granular aggregates and follows the planes of schistosity in the chloriterich bands. Sphene is present as a minor accessory and where It is in contact with chlorite, pleochroic halos are produced in the latter. The late calcite veins contain silicates that have been enriched in titanium. For instance, melanite is commonly found with cores of andraaite or the melanite may be rimr.ed by idocrase (PI. XI, Fig. l). Small euhedral prisms of reddish-brown titaniferous idocrase are implanted in vugy 133 areas of the calcite veins. The observed mineralization sequence is given below. Chlorite ____________ Diopside __________ Idocrase __________ Tl-bearing Calcite _______________ Melanite _______________ Andradite _____ Sphene ___ (RGC-5U-50) Is the best exposed metasomatic body within the district and it was possible to work out some of the details of mineralization, that were obscured at the other metasomatic bodies. The geologic sketch map shown in Piace II piesents the geologic situation. Ihe rocks crop out in four individual knobs on the crest of a ridge ccxn- posed of flaky weathered serpentine, and since these calc-silicate rocks are much more resistant to erosion than serpentirr.-- they show remarkably good exposures as a result of differential weathering. Both the bottom and top contacts of the calc-silicate body are exposed. The body is tabular in shape with minor contortions that pars.I7.vl the structure within the serpentine, and distinct layered sequence is made obvious by color changes. A columnar section through the calc-: ilicate body, shown in Plate II, gives the detailed relationships. The body marks a strong contrast with the dark green serpentine, since it is almost white in the upper portion and light green ir. the lower bands. The serpentine-calc-silicate contact although gradational is narked by 'fingers' tliat replace the serpentine along shear planes. Diopside is the dominant mineral in the 'fingers' and in v_ugy areas light yellow andradite occurs. The sheared serpentine along the contact is composed of antigorite and relict chromite. Diopside directly re places antigorite and where diopside contacts chromite is assumes a light green color probably as a result of addition of chromium to the pyroxene lattice. Andradite garnet also assimilates chromium and spectrographic analysis of andradite from this locality shows 1 so 5'/- Cr. The contact zone is about six inches wide and within this cone a gradual change from a diopside-antigorite rock to a diopside-chlorite rock takes place. The diopside-chlorite rod: extends about one foot down from the top contact and is cliaraclerized by a well developed foliation and schictosity. Tills zone snows strong evidence of deformation during min eralization, as small folds, accompanied by vertical fractures, have developed. The fractures are, in part, healed by andradite and melanite, and in the fractures, garnet forms beautiful drusy surfaces together with radiating clusters of bladed prisms of white diopside. Ho evidence of post-mineralization movement can be shown, since these delicate drusy fractures have been perfectly preserved. The transition from the upper diopside-rich zone to the underlying chlorite-rich zone is marked by a sharp color change from white to light green. Tne lower zone is composed dominantly of chlorite (rumpfite) containing folia of diopside. Tnis zone exhibits schistosity and foliation with r.: nil scale fold!:v; . .-matures dovcloned wit-vn chi: ^po o.ro halcd mostly by moianicc wnich io sti-orgiv zoned with she periphery much darker chan the core. The lovrer scrpentir.e-calc-s ilicn-.e contact is similar to the upper contact wish diopside extending alor__: the shear planes in the serpentine. !Li nor amounts of sp''.er.c and perovskite are present in both oor.ee, with the sphene producing oleochroic halos in she chlorite. The telasort?, tic replacement of serpentine at this locality was ac companied by deformation and fracturing, but the later stages of miner alization continued beyor.d the deformation and healed the fractures. The formation of titanium-rich melanite in. the rost-clnformaticn stagseems to ce the pattern for all the metasc:tac:c bodies investigated. In order to gain some inei3'r.z concerning replacement processes, quantitative chemical analyses of the host serpentine and the calc-silicate from this location were made (Table 25). The analyzed serpentine was taken one foot above the contact and a channel cample across the calc-silicas..- body exclusive of the gradational contacts, ftts analyzed. The "eight percent were recalculated into the standard roc!: cell following ti'.e method of Barth (19-tS) in order to determine the actual ion exchange (Table 25). It can be readily aocertained that the metasomatism of the serpentine was acccmplishsd by solutions enriched in Ca, 7e, Ai, and Ti; and a concomitant removal of 3i, Mg, and "20. The mir.eralogical composition of the ether calc-silicate bodies is similar to (?.CT->';-50) ar.d it wcula scam logical that the solutions responsible for their formation came from, the same source. Tne observed mineralization seqw*>'~ce is given below. 136 TABLE 25. CALCULATION OF THE STANDARD ROCK CELL FOR SERPENTINE AND CALC-SILICATE SiOa AI2O3 FeO Fe 2O3 Ti02 MnO CaO MgO SERPENTINE WT. % CATIONS 41.47 35-6 3-35 1.4 5.57 4.0 2.62 1.7 .04 - .24 .1 nil - 36.03 46.0 CALC-SILICATE WT. CATIOfS 34.69 33*0 10.72 12.0 9.76 7-7 4.52 3*2 1.90 1.4 27 .2 12.33 12.5 16.46 26.0 K20 Na20 H20+ nil nil 12.05 34.3 05 .08 6.83 .1 21.8 n20- .76 - *39 C02 nil - nil Pzs - nil - traces BaO nil - nil 100.13 100.04 SERPENTINE ALTERS TO CALC- SILICATE By adding 0.1 ions of Ha 12.5 ions of Ca 0.1 ions of Mn 1.4 ions of Ti 5-2 ions of Fe 10.6 ions of A1 By subtracting 20.0 ions of Ms 2.6 ions of Si 25.O ions of H 2Q.9 cations 47.6 cations - Analyst - W. II. Herdsman 137 Chlorite Diopside Garnet ____________ Ti-ricn Sphene ________ Perovskite ___________ (RGG-109-52) is a small roughly lenticular outcrop, l!i feet by 5 feet, that has been mutilated by enthusiastic collectors. The contact, when exposed, is extremely sharp and is marked by a change from dark preen serpentine to an almost chalk-white chlorite rock. The surrounding serpentine shows little or no shearing and is composed mostly of chrycocile. Thin sections cut across the contact reveal an almost pencil line sharpness to the netasomatic replacement front in the serpentine. Tre chrysotile alters to clinoc'nlore which retains the original structure of the serpentine. Small rosettes of diopside have developed within the serpentine about one inch away from the sharp contact zone. The calc-silicate rock is fine-grained and made up of white necdlelike crystals of diopside. Chlorite and andradite, commonly developed near the contacts, are lacking in the central portions of the body. Diopside has a crude lineation and is cut by late veins of melanite ar.d titaniferous idocrase. Perovskite and sphene were not identified from this locality. Near the contact and implanted on the serpentine, are perfect crystals of cubic magnetite. Fabst (1951) "was fci.e first to report this rare occurrence of cubic magnetite ar.d his X-ray work in dicates that the pattern of the cubic magnetite dees not differ from the normal magnetites of octahedral habit. Tre observed mineralization sequence is given. Chlorite Diopside Garnet Idocrase Magnetite Ti-rich (P.GC-92-52) crops out as a long narrow vein-like body about two feet in width and about fifty feet long that pinches down to one or two inches in width. The contact between tr.e serpentine and calcsilicate is obscured by flaky serpentine debris, but pieces of float that contain the contact show it to be gradational. Diopside and chlorite extend out along shear planes within the serpentine. Replace ment of the serpentine by the calc-silicate rock was followed by fracturing and shearing with subsequent healing of fractures by idocrase and garnet. This calc-silicate rock is dense, heavy, with well-developed foliation and linea.tion; is composed essentially of idocrase, diopside.and chlorite (see PI. XII, Fig. 2). Diopside forms colorless elongate, ciaded crystals which produce a strong lineation, whereas colorless to green chlorite creates discontinuous folia within diopside. Idocrase is light green, forming anhedral grains cross-cutting the fabric of the rock. Andradite fills fractures and in vugy areas produces drusy surface implanted with ' bladed diopside. The late fracture-filling garnets and idocrase are once again tinted a reddish-brown, due to late introduction of titanium. The observed mineralizing sequence is presented. 139 Chlorite Diopside Idocrase ___________ AndraditeTi-rich Titano-Silicates and Sodium Silicate Rocks The Gem mine is the best known mineral locality within the New Idria district and has long been famous as the only known occurrence of benitoite and joaquinite. Of this locality and its minerals, Louderback (1907, 1909) has given an excellent description that is much more complete than the one presented here, since he had the opportunity to study this deposit while it was being mined for benitoite. At the time of this investigation many of the original features had been obs cured by slumping, and a dense brush cover precluded a detailed study. A plane table map of the area was completed and the principal rock types we:.- plotted (Plate III), Surrounded by sheared serpentine, the Gem mine is situated within a tectonic inclusion that is more resistant than .the inclosing serpentine and accordingly forms a small ridge which extends down to the headwaters of the San Benito River. The tectonic inclusion contains three distinct rock types; (1) a dense, fine-grained greenstone composed essentially of pyroxene (near aegirine-augite), albite, and chlorite, (2) saussuritized gabbro, (J) albite-crossite-epidote schist. The relationships of these rock types may be seen in Plate III, and petrographic description was treated previously in the section on tectonic inclusions. ]>0 The r.onc of metasomatism is found wholly within file alb izecrossite-epido^e schist. This zone is elongate and consists oX' many irregular natroli le veins than para] lei the elongation. The surrounding roc!', in oho mineralized none has been altered from the albi tc-crosr.; toepidete schist to a vugy and porous roc!: impregnated with nafrozize and fibrous crossitc (FI. XIII, Fig. 1). Replacement, veins of natrolite form granular aggregates that project invard to produce cockscomb drusy surfaces. The center of these veins is usually open and vugy, although many parts are completely filled with natrolite. '-imy 0f the open vugs are filled with very fine hair-li.ke runnnibola whose optics are similr.r to those of crossite, and these fibrous needles support small aggregates of natrolite. Implanted on the cockscomb drusy surfaces are euiiedral crystals of blue, tabular, pyramids of ber.itoite (PI. XIII, Fig. 2) ar.d brilliant, black crystals of prismatic neptunite. Jcaauinite is extremely rare and always is present as snail, honey-yellow crystals not more tlian 2 mm. in size. 2-reenish clots on the surx'aca of the natrolite are composed of a core of chalcocite surrounded by an alter ation halo of chrysocclla. In this present investigation, a new mineral has been found in small bluish-black fibrous aggregates within small veins cutting natrolite. Preliminary work indicates that this may also be a titanium mineral., but insufficient material is at hand to complete the description. The mineralized zone has several post-depositional faults accompa nied by shearing, although extensive deformation is not apparent,as many of the delicately implanted drusy surfaces have retained even the most 141 PLATE XIII. Camera lucida drawings of raetasomatic rocks. Figure 1. Crossite-natrolite rock from the Gem mine. (X 40). Figure 2. Benitoite crystal implanted in a crossitenatrolite rock from the Gem mine. (X 40). 142 fragile crystals. As is typical, the contact between the tectonic inclusions and serpentine is strongly sheared. The observed mineral izing sequence is presented. Natrolite ________________ Amphibole ________________ Benitoite __________________ Neptunite __________________ _ Joaquinite _________ Chalcocite ____________ On the southeast flank of Santa Rita Peak is a small tectonic inclusion (RGC-81-52) that contains numerous natrolite veins with mineralogy quite similar to that found in the Gem mine. The natrolite veins are located within an albite-glaucophane-actinolite schist and follow an elongate trend similar to the Gem mine. Natrolite forms drusy surfaces of euhedral prisms and in the more vugy spots, hair-like crossite is implanted on the natrolite. Natrolite impregnation of the host rock is not as extensive at this locality, although the metasomatism is sim ilar to that found at the Gem mine. The only titanium mineral identified was joaquinite, but it occurs very sparingly. Chalcocite with chrysocolla alteration halos is quite common. This is the only other locality in the dkfcrict which has a mineralization sequence similar to that found in the Gem mine and it would seem that the geologic conditions attendant on the formation of these rare barium-titanium silicates is indeed unique. o ic r.ocli, T*'r' ^s* ">t.iri<*>t '-`oc.ic* v*"4''n *-ii3 corcor..'n nn'i ~i*r* '%* m- ult pi'CCl geologic ii`..era .urc produced few descrir`.loro cf ierosits similar to ~.'.ccc found ir. `./.o How If:'.'a district; '..r.crefcrc ger.e*ic implications by analogy arc r.ot possible. Turner (1935) has described lime-silicate masses in serpentines from Hew Zealand that contain antigorite, chlorite, talc, diopside, garret, magnetite, and idocrase. These rocks, according to Turner, have been formed by alteration of the original pyroxenite by hydrothermal processes which took place under conditions of marked shearing stress. Grange (1927) has described grossulariae-pyroxene rocks--rodingiteo of Marshall (1911)--from the peridotite belt of Kelson, Hew Zealand and they have been shown to be due to garnetization of a gabbroid rock, under the influence of 'Concentrated magmatic water' acting probably at high pressures. Tyrrell (1951, P- 29) and Arshinov and Merenkov (1950) have described and discussed dikes in the Ural Mountains that contain garnet-pyroxene, garnet-idocrase, and garnet-chlor:te that invade ser- pentinized herzburgites. They consider that these calc-silicate rocks are formed by garnetization of micro-diorites and alteration of pyro xenite sehlieren by calcium-rich solutions liberated in the processes of serpeutinization. All of the rocks from these different localities are somewhat similar to the chlorite and calc-silicate rocks from the Hew Idria district, hut they have been related to alteration of pre-existing rock types within Turner (195?) has described, lime-silicate masses in serpentines from New Zealand that contain antigorite, chlorite, talc, diopside, garnet, magnetite, and ldocrase. These rocks, according to Turner, have been fonned by alteration of the original pyroxenite by hydrothermal Drocesses which took place under conditions of marked shearing stress. Grange (1927) has described grossularits-pyroxene rocks--roaingites of Marshall (1911)--from the peridotite belt of Kelson, New Zealand and they have been shown to be due to garnetization of a gab'oroid rock, under the influence ox 'Concentrated magmatic water' acting probably at high pressures. Tyrrell (1951; P* 29) and Arshinov ar.d i-ferenkov (1950) have described ar.d discussed dikes in the Ural Mountains that contain garnet-pyroxene, garnet-idocrase, and garnet-chlorite that invade serpentinized harzburgites. They consider that these calc-silicate rocks are formed by garnetizazion of micrc-dicrites and alteration of pyro xenite schlieren by calcium-rich solutions liberated in the processes of serpeniinization. All of the rocks from these different localities are somewiiat similar to the chlorite and calc-silicate rocks from the New Idria district, but they have been related to alteration of pre-existing rock types within 1M the cerpuri r><->5. The v.etasomatic rocks from this district, exclusive of the Gem mine .material, have no relict minerals or structures that could be related to reconstituted rod: types otner ^nan the serpentine itself. Furtheiv.ore, the existence of titanium minerals, zircon, apa tite, and other minerals unusual in ultrabasic rocks, indicates that the fluids responsible for the metasomatism came from a source other than those processes related to formation of serpentine. Spectrographic analyses of the serpentines and metasomatic rocks, to be discussed later, show no affinities with each other. All of these metasomatic bodies are clustered around the syenitecampeonite intrusions (Pi. I) and chemical study of these intiusives shows a progressive depletion of Ca, J-'g, Fe, and Ti and in the late stages enrichment in 11a. Tne calculation of the standard rod: cell (Table 25) shows that metasomatism of serpentine was accomplished by fluids enriched in Ca, Fe, Al, and Ti. From these facts, it would seem reasonable to effect metasomatism of serpentine by fluids which had arisen from the intrusive during its emplacement. Abundant evidence of deformation during metasomatism indicates tectonic activity during mineralization. The intrusions may well have been emplaced during a tectonic period and fluids arising from the magma probably migrated along shear zones, selectively replacing the serpentine. The titano-silicates from the Gem mine pose a special problem since the mineralogy here is quite different from that found in the calc-silicat and chlorite rocks. However, the high titanium and sodium content plus the copper mineralization favor the intrusive rocks as a source of the r.c: all siny fluids. The intrusive short an extr:...ely stror.y ha -p - - r'''--q-" j^.jv***^'* -v.0 ''i **n> o*n^^ accc^^n^ oci 'jv cocccjt i1'! ncyc.ll 22,*.. on v.i^r. "cor ocntcn" Iv. cgyctc.1 .iilncrdc oj'O'l'icoz o.n or.l~r.cs. since neither the intrusives nor serpentine contain ubr.o:'.'.al assents of barius, ar.d furtker~.o re, barium minerals re re not identified it: any of the- calc- silicate or chlorite bodies. Therefore, the intrusive rooks apparently did not sucply barium. for benitoite and the other Bu-rich minerals. The root lihely souroe of the barium is from the host rocks that sake up the tectonic inclusion. Tarite is not uncc.tr.or. in ranyanese concentrations v: J '* " '4_ r*^ T7r'-)*'rs-'' ^ c qf* cons * do3o r,2.r}^^n,"'GS within the Terr, rsir.e inclusion. Trie r.ost plausible theory as to the or by in of the Gem ."nine minerals is one '.each proposes a multiple source -V\o Clr>***rirlC 3.3 rt*v,Ol'"` 'TO f *" ~ ca T"Q ft t ~ `one* T*C C'J 1220. i'l*C*C a normal type of .mineralization. Tne author suyyests that the unique deposit at the Gen mine has resulted from the fortuitous mining of fluids fro.:: the intrusive rocks with barium-rich, zones within the metamorphosed sediments, and this has resulted ir. the crystallization of the extremely rare mineral assemblayer benitoite, joaquinite, and neptunite. Iho MINOR AND TRACS SLS-2.TT STUDY 0? T5E ROCKS AND MINERALS Int/oduction A comprehensive study of the rocks and minerals involved in the formation of the metasomatic rocks within the serpentine by spectrorrapnic methods has been very iHuminatins and helpful ir. the general problem of genesis. Sacii rock type analyzed represented approximately a two pound sample. This material was ground to pass through an 20-mcsh sieve and a representa tive split of ten milligrams was used for the analyses. The analyzed minerals were separated from the same rock specimen used for spectro graphic determinations. Quantitative chemical analyses have been made on the same material used for the spectrographic determinations. Therefore, in the discussion on the distribution of the elements in rocks and their constituent minerals, the analyses have been on material from the same specimen,eliminating the problem of variations due to poor sampling. The semi-quantitative spectrographic analyses followed the proceedure of V/aring and Annell (1953) Tne results of the spectrographic analyses are presented in Plate IV, with the values obtained given in ranges. All of the elements detected are reported in the table, but not all of these elements are discussed, particularly the major elements. Tne sensitivity for each element is given at the bottom of the Table and in the extreme left-hand column sample numbers are given. The location of each sample a1miner may be determined by using the middle digits of these numbers which are plotted on the geologic map (Plate I). Seven serpentine rock samples were taken and six of these from, the imm.eadiate area of the netasomatic rocks .whereas (P.GC-101-52) comes from 147 the western part of the serpentine. The metasomatic rocks and minerals are divided into the calc-silicate and chlorite types defined earlier. The intrusive rocks were obtained from the White Creek body and this material is a split of that used for the quantitative chemical analyses. These analyses were undertaken to determine, if possible, the source of the metasomatic fluids that formed these curious bodies within the serpentine and also to establish fundamental data on the partition of minor and trace elements in this type of mineralization. The possible sources for fluids may be enumerated at this point in order to illustrate how the following observations fit the possibilities. Since the syenite-camptonite intrusions crop out in close proximity to these metasomatic bodies, fluids produced during the time of emplacement could have migrated up along the numerous shear zones witnin the ser pentine and metasomatically replaced the serpentine and parts of the tectonic inclusions. If this had happened, we might expect to find minor and trace elements concentrated within the metasomatic bodies that are characteristic of an 'alkalic' magma. A second possible source are fluids that have formed by the processes that have given rise to the serpentine. The ultimate source of the serpentine was an ultrabasic 'magma' that had undergone crystallization and emplacement probably accoupanied by serpentinization. The fluids responsible for the metasomatic bodies may have been the end product of these processes producing solutions enriched in Ca, Ti, and other elements characteristic of the replacement rocks. Again if this were the ultimate source the minor and trace elements within the bodies should reflect a suite of elements expectable from 'ultrabasic 148 magmas'. The third possibility would be a deep seated intrusive which was not apparent on the surface, but which may have produced emanations that were carried upward into the relatively previous serpentine and metasomatically replaced the serpentine and portions of the tectonic inclusions. The minor and trace element suite from such a source again would be distinct enough to eliminate the two previous alternatives. Tne following discussion is primarily aimed at a possible correlation between the observed minor and trace element suite with one of the three possibilities. Each element is discussed in turn except where two elements are similar in their geochemistry. Boron j*. on shows above average concentrations in the serpentines. Rankama and Sahama (1950) give an average of 51 ppm (parts per million) for ultra basics vherees these serpentines range from 100-500 ppm. The calcsilicate and chlorite rocks vary considerably in boron content but generally have a high content (up to 1COO ppm). The border rock from the jadeile pods (RGC-105-50) contains almost 1 boron. An extremely high content (1000-5000 ppm) was found in the ilmenite from the chlorite rock (ROC-83-51). Idocrase and chlorite from the metasomatic rocks show an enrichment in 3 when compared to the other minerals. The camptonite and syenite show 10 ppm B which is similar to that given for average igneous rocks by Goldschmidt (195^ P- 28l). Boron was not detected in she minerals from the Gem mine. The high boron in selected rocks from this area indicates a possible outside source of boron. Goldschmidt (1954, p. 281) suggests that a high boron content in such rocks as these may result, not from a magmatic source, but by contamination from marine sediments in juxtaposition with 'intrusive' rocks. The Francsican formation, a marine sediment, is intimately associated with the ser pentine and appears to be the logical source of the abnormal boron encountered here. Vanadium cori ' c contain porr. vitnciit1. a nosit-ivs coi*j*Gia'tion between ?e, Cr, or Ti as is commonly found in other rocks. The chlorite rocks show a slightly higher concentration of V (1G0-5CC0 ppn), again with no positive correlation between Te, Cr, or 'Pi. Ilmen ice from (R0C-85-51) contains nc V, although the rock as a whole has 100-500 ppn V. This follows Rankana and Sahama (19**9* P- 59^) wr.ere they show that V does not enrich in iimenite but prefers tir.anian magnetites. The calc-silicate rocks show 100-50C ppm V consistently and in these rocks vanadium is noticeably enriched in the chlorite as compared to the garnets, idocrase, and perovskite. The V'"*+ apparently substitutes in the chlorites for Al'>+ ir. the octahedral positions. The intrusive rocks contain 100-500 ppm V; comparable to other rocks of this type (Higazy, 195*** P- 50) Gallium Gallium was not detected in the serpentine as would be expected since gallium is camouflaged in aluminum-bearing minerals: the serpentines all contain less than l Al. The calc-silicate and chlorite rocks averaged 10-50 ppm Ga. It is interesting to note that Ga is enriched in clinochlore 150 from RGC-109-52 when compared to idocrase from the same specimen even though the A1 content of the idocrase is several times greater than that of the clinochlore. The garnets also appear to favor the sub stitution of Ga over idocrase. Tne intrusive rocks and minerals averaged 50-100 ppm with the exception of the late Cu-Fo sulfide from the veins within the syenite. The sulfide shows .1-.5- Ga showing an enriclunent in Ga during the late stages of the syenite intrusion. Goldschmidt (195^, p. 329) has shown liiat. sulfides of the 'tetrahedral' group commonly contain significant amounts of Ga. Chromium The serpentines with the highest Cr content, average 0.1-0.6%- Cr, and it is Drooably contained within chromite, for the most part. Since all the other rocks analyzed are replacing, included in, or intrusive into the serpentine their high Cr content probably reflects contamination. The chlorite rocks show 100-1000 ppm with some tendency for the Cr to concentrate in the chlorites, and ilmanite tliat has crystallized late in RGC-S3-5I shows extremely high Cr (0.1-0.5%) The calc-silicate rocks have the same range in Cr as the chlorite rocks, i.e., 100-1000 ppm., although andradite near the borders of the replacement bodies contains 1-5% Cr. The intrusive rocks contain above normal Cr; canytonite 100 500 ppm and syenite 10-50 ppm, indicating some serpentine contamination. Titanium The serpentines contain 10-100 ppm Ti which is generally low for ultrabasic rocks, Faust and Murata (1955) find an average of 140 ppm Ti 151 l'or seven serpentines and Higazy (195"> P- 55) on several v.ltrabasic rocks shows almost 1$ Ti. This low Ti content summarily excludes the serpentine as a source of the high Ti in the metasomatic rocks within the serpentine since there is a hundred-fold increase from the ser pentine to the metasomatic rocks. Ihe chlorite rocks are extremely variable with respect to Ti; a high of 5$ to a low of .05$. The titanium is concentrated within the minerals perovskite, melanite, and ilmenite and these minerals may make up a large part of the rock. The calcsilicate recks are lover in Ti but more consistent than the chlorite rocks with an average content of .1-1$ Ti. In these rocks Ti prefer entially enters garnets, idocrasc, and chlorite in that order. The camptonite and syenite contain ICO-500 ppm which is considerably below the average for igneous rocks (0.6l$); however, within the syenite carkevikite contains 1-5$ Ti. The ultimate source of the Ti in the metasomatic rocks is obscured,as both the serpentine and intrusive rocks are below average in Ti when compared to rocks of a similar type; possibly the late stages of the intrusions produced fluids enriched in Ti since there were no discrete titanium minerals formed in the early stages of crystallization. Niobium Niobium was not detected in any of the rock splits (sensitivity .001$); however, in the purified mineral separates from the rocks several minerals contained significant amounts of Nb. Ferovskite shows 100-500 ppm, melanite 10-50 ppm, and benitoite from the Gem mine 100-500 ppm. Niobium is commonly found in perovskite, although this, so far as the author knows, is the first known occurrence of significant amounts of Kb in garnet. Beni'.oite might he expected to carry Kb, but neptunite does not show fib although it is rich in Ti. It is curious that Kb was net detected in the ilmenite from the chlorite rocks, in view of the fact Goldschmidt (195^, P- 503) reports several percent of Kb in ilmenite from granitic pegmatites. The close association of lib with rocks of the later stages of magmatic evolution (granites and syenites) strongly suggests that the lib found in the minerals from the metasomatic rocks has been derived from fluids produced by tre syenite intrusion. iiickel and Cobalt In the serpentine, Co ar.d Ni show a constant relationship with !ii > Co (Co- .01-.1`,., Hi- .1-.5)) and follow tag the work of Faust ar.d Mnrc-.-a (1335) this relationship is sypicai for serpentines derived from ultrabasic rocks of 'magmatic' origin, or from any igneous rock high in Mg^+ since tli^ follows i>ig^+ (k'ager ar.d Mitchell, 1952) . Tne chlorite rocks show less nickel and cobalt with IIi=Co (HI- .01-.057-) (Co- .01-.05A) In the chlorite rocks Co ar.d Hi show peferential substitution in the various minerals. Perovsicite contains 10-50 ppm Co, but Ni was not de tected. Kelanifce shows Ni>Co by a factor of ten. Tne calc-silicate rocks have a variable relationship with Hi>Co in some bodies and Hi=Co in others. Tne andradites from the calc-silicate rocks contain 100-50C ppm Hi: and Co was nor. detected. Clinochlore shows strong enrichment in Hi (1C00-50G0 ppm) and idocrase consistently has more nickel than Co. Tne syenite and camptonite are similar in nickel- and cobalt-content 153 with Hi Co (.01-.05%). Nockolds and Allen (1954) in their study of alkalic rocks show Hi Co in the early stages of differentiation with a trend during the magmatic descent for those elements to become equal and in the final stages Co is greater than Ni. Following this data, it might be said that the Hi-Co ratio in the syenite suggests that it has been intruded from a partially differentiated magma. Furthermore, the similarity of the Ni-Co ratio in the metasomatic bodies when compared to the intrusive rocks indicates a closer kinship to the intrusive rocks than to the serpentines. Scandium Although not detected in the serpentines, Faust and Murata (1955) report scandium in 7 out of 9 serpentines analyzed. Scandium was found in both the chlorite and calc-silicate rocks (10-50 ppm) and it shows concentration only within ilmenite (100-500 ppm). The low-iron chlorites did not contain detectable amounts of Sc, but the other mineral phases within the metasomatic bodies show 10-100 ppm Sc. Camptonite and syenite contain 10-50 ppm Sc with some concentration noted in the barkevikite and apatite. Zirconium Zirconium may be used in this investigation as a key trace element to establish, in part, the genesis of thg metasomatic rocks. Zr was not detected in the serpentine nor in any of the other rock splits (sensi tivity-. 0008%) ; however, in the mineral separates from the calcsilicate and chlorite rocks Zr was found in ilmenite (50-100 ppm), melanite (100-500 ppm), perovskite (50-100 ppm), and idocrase (100-500 ppm). 154 In the intrusive rocks Zr was found in albite (10-100 ppm), barkevikite (100-500 ppm) and apatite (50-100 ppm). Benitoite from the Gem mine shows 10-50 ppm Zr. The partition of Zr in these rocks shows a strong genetic relationship between the metasomatic and in trusive rocks. Hevesy and Wurstlin (1934) give 60 ppm Zr as the average for peridotites, eclogites, and dunites; and Faust and Murata (1955) find that only two of 20 magmatic and metamorphic serpentines contain 60 ppm Zr. Therefore it would seem difficult for these raetasomatic rocks within the serpentine to obtain such a high Zr content from other than solutions arising from the intrusive rocks. Elaborate separation on about 50 pounds of rock from the syenite were made in order to concentrate zircon for age determination; however, no zircon was re covered. It is assumed that the Zr present in the magma entered other silicate structures rather than forming zircon and in part may have been removed by emanations which led to the formation of the metasomatic bodies. Rare Earths Rare earths were not detected in any of the serpentines or ser pentine minerals and this agrees with t.he work of Faust and Murata (1955) who report no rare earths from 20 spectrographic analyses of serpentines. The chlorite rocks show variable amounts of rare earths with Y >Yb=La. Perovskite from these rocks concentrates significant amounts of rare earths with Ce=La >Y=Nd >Yb, and melanite from the same rocks shows Y>Ia. The calc-silicate rock splits all show rare earths with Y=LiYb. Idccranc from tnese rocks ohov/s a range of rare earth concent with Y detected in all specimens and Ce and La present in (FGC-109-52). Camptonite contains la as the only detectable rare earth whereas the syenite shows Y> Yb. Barkevikite from the syer.itc contains 10-50 ppm Y. The apatite from the syenite contains an inter esting ovite of rare earths with Ce > la > Nd > Y >Dy>r>Yb. Goldschmid (195*0 has shown that rare earths within a magma may concentrate within the apatite and, therefore, the rare earths present within the apatite from the syenite should show what rare earths may have been present in magmatic emanations available during the emplacement of the syenite. The marked similarity between the rare earths in the metasomatic rocks when compared to those present in the apatite suggest a strong genetic kinship. Barium and Strontium Barium presents a unique problem in distribution vrithin the New Idria district, since benitoite, a rare barium titano-silicate (565. BaO), is restricted to the metasomatic rocks of the Gem mine. Barium is not found as an essential constituent in other metasomatic rocks within the district; therefore, the source of the barium is important in the genesis of benitoite. The serpentines contain up to 50 ppm Ba and Sr is not present in detectable amounts (sensitivity 10 ppm). Faust and Murata (1955) show barium up to 7 ppm in 'magmatic1 ser pentines and do not report Sr from these rocks. The chlorite rocks contain 10-1000 ppx Ba and Si with the Ba-Sr ratios variable from one 156 rock to the next. In the chlorite rocks Ba is concentrated in the late minerals, viz. ilmenite (1000-5000 ppm) and perovskite (100 500 ppm) and Sr is concentrated in melanite (100-500 ppm) and perovskite (500-1000 ppm). In the calc-silicate rocks, the Sr-Ba ratio is consistent with Sr> Ba except for (RGC-105-52) where the ratio is reversed. Barium shows some tendency to concentrate in the chlorite (10-50 ppm) and strontium is enriched in idocrase (500 1000 ppm). Tne rock with the highest Ba concentration is the border rock (RGC-105-52) associcted with the jadeite pods and here Ba is 1000-5000 ppm. The cnmptonite and syenite show Sr=Ba (500-1000 ppm) and the separated minerals from these rocks have the same ratio and concentration. Ba exceeds Sr by a factor of 1000 in benitoite and neptunite. The intrusive rocks are extremely low in barium when com pared to similar rock types; however, this is to be expected, since these rocks are extremely low in potassium; and Ea in igneous rocks is to a large extent camouflaged in potassium-bearing minerals. The ultimate source of the barium in the Gem mine seems to be related to the host rock as both the serpentine and intrusive rocks are extremely low in Ba. Tne barium minerals ere restricted to metasomatic zones within the tectonic inclusions. As proposed earlier, the barium has probably been assimilated from Ba-rich zones within the metamorphosed sediments present in the tectonic inclusion. Silver Silver was detected in many of the rocks and minerals as the sen sitivity of this element by spectrographic methods is extremely high 157 (O.l ppm). Ag is present in all of the serpentines (G.l-10 ppm); however, Faust and Murata (1955) have not reported silver in such an environment. The chlorite and calc-silicate rocks contain 0.1-10 ppm with some tendency for the Ag to concentrate in the chlorites. The camptonite and syenite contain 1-10 ppnu somewhat above Goldschmidt's (I95U) average for igneous rocks (0.02 ppm). The late Cu-Fb sulfide within veins cutting the syenite contains significant amounts of Ag (500-1000 ppra). Lead Fb was detected in two serpentines (RGC-25-50) and(EGC-101-50), but it was not detected in the chlorite rocks, although melanite from these rocks has 100-500 ppm and perovskite shows 10-50 ppm. Three out of five calc-silicate rocks show 10-50 ppm Fb with prochlorite containing 500-1000 ppm and andLradite 10-50 ppm. Hie camptonite and syenite contain 10-50 ppm with a marked concentration in the barkevikite (100-500 ppm). Copper Copper was detected in every rock and mineral analyzed and this affords a good opportunity to examine the behavior of Cu in relation ship to substitution or camouflage in silicate minerals within a small province. The serpentines show a rather high background in Cu and consistently contain 10-100 ppm. The chlorite rocks have the same range as the serpentines (10-100 ppn) and the minerals from these rocks show the following partition of Cu: chlorite (50-100 ppm), melanite (100 500 ppm), perovskite (5-50 ppm) and ilmenite (50-100 ppn). The calc- 158 silicate rocks show 10-500 ppm and the minerals therein show the following partition of Cu: chlorite (10-50 ppm), andradite (10-50 ppm), and idocrase (1-100 ppm). The camptonite and syenite show a very high Cu content (100-1000 ppm) and albite and barkevikite therein show 10-50 ppm Cu. The late sulfide in the syenite is mostly chalcocite (CU2^) and there seems to be an enriciiment of copper in the late stages of the intrusive rocks. In general, the district has a fairly high background in Cu and the source for Cu in the metasomatic bodies may be multiple. Other Elements Tin is absent in the serpentine and is below the sensitivity in most of the metasomatic rocks, however, tin was detected in the ilmenite (50-100 ppm) and in idocrase (100-500 ppm) from the metasomatic rocks. (RGC-36-50) a chlorite rock located fairly close to the small camptonite plug' shows 50-100 ppm Sn. The camptonite shews 10-50 ppm Sn while the syenite contains no detectable amounts. Here again, there seems to be strong kinship between the intrusive rocks and the metasomatic bodies. Molybdenum was not detected in the serpentines; however, it was found to be present very sporadically in the other rocks and minerals. Mo is present in the chlorite rock (RGC-36-50) (10-50 ppm), prochlorite from (FOC-37-50) (10-50 ppm), andradite from (RGC-3^-50) (1C-50 ppm), caJc-silicate (RGC-109-52) (10-50 ppm), and significantly in barkevikite from the syenite (10-50 ppm). 159 The Late Cu-Fo sulfide from veins within the syenite contains significant amounts of metals; 5C0-100C ppm Rg, 10-50 ppm Au, 100-500 ppm Cd, 1-57. Sii and 1000-5000 ppm Bi. The presence of mercury in this sulfide concentration is significant, since the Few Idria district contains many mercury deposits and it may be' that these intrusives produced those fluids responsible for the cinnabar deposits. Summary The minor and trace element content of the serpentine is similar to those serpentines studied by Faust and Murata (1955) and known to have formed from ultrabasic rocks of magmatic origin. There is no evidence that the peculiar suite of elements found in the metasomatic rocks originated from processes which have given rise to the serpentine. The nickel, cobalt, and chromium present within the metasomatic rocks and intrusives may well nave been introduced by contamination from the serpentine. 3oron and barium seem to have been introduced from the sediments in juxtaposition or incorporated v;ifnin the serpentine. The striking similarity between the trace elements from the metasomatic rocks as compared to those from intrusive rocks indicates a very strong consanguineous relationship. The indicator elements within the meta somatic rocks that point to the intrusive rocks as their only apparent source are No, Zr, rare earths, Sn, and Mo. Had the metasomatic rocks formed directly from the serpentine without introduction of material, it is quite improbaoie that the indicator elements listed could have been mobilised from the serpentine, even though large volumes of rock 160 were involved. There is no indication that a deep-seated igneous mass has contributed to the formation of these rocks, unless the exceptionally high boron content is construed as coming from a granitic magma. The sparse data from the jadeite facies indicates that these rocks are a separate and distinct period of mineralization and may very well be related to the metamorphism of the tectonic inclusions either during serpentinization or in a later period of tectonic activity. The high boron and barium content combined with their close proximity to the large masses of metamorphosed sediments implies a very close association between the metamorphism of the sediments and serpentinization. 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