Document pmJOk97GjXyjeGmORr0xg2pOX

TALC AND CHLORITE DEPOSITS IN MONTANA by - Richard B. Berg MEMOIR 45 " MONTANA BUREAU OF MINES AND GEOLOGY > A Department of M o n ta n a C ollege of M in eral S cience am i Technology 1979 EXHIBIT CAM-128 GUNTER00000814 Preface This memoir is one in a series of reports on nonmetallic mineral commodities published by the Montana Bureau of Mines and Geology. The main purpose of this study is to provide information that will be useful to those engaged in explor ation for new talc and chlorite deposits. For that reason, mapping of areas that were thought most likely to contain talc was emphasized, as was the mapping and examination of talc prospects and inactive mines that have not been de scribed in the literature, in an effort to make this publication more useful to the individual unfamiliaf w ith talc and chlorite in Montana, published information on all known talc occurrences is also included in abbreviated form. The contributions of the following field assistants were important to the completion o f the project and are much appreciated: Will Goldberg (1973i, >Hal Koechlein (1974), Roger Kuhns (1975), Leroy Swanson (1976) and Steve Czehura for a short time in 1977. I enjoyed working with each of them. Sam Maloney, Pete Womack and Bob Nolte were very helpful in showing us talc prospects. Ken Wier showed interest in our work in the Greenhorn Range and offered sug gestions for the improvement of the geologic map of that area. Review of the manuscript by Keith Papke and Willis Johns resulted in many improvements. Alice Blount of the Newark Museum, Newark, New Jersey, ran infrared scans on samples of stream sediment and soil. She, Bob Root, Dick Olson, Ed Houser and John Brady also provided me with an opportunity to discuss my ideas on the origin of Montana talc deposits. Charles Knowles of the Idaho Bureau of Mines and Geology kindly ran microprobe scans of a talc specimen in an effort to determine the composition of some very small opaque grains. Individuals involved with the operating talc mines showed us their opera tions, and perhaps most important, expressed interest in our work. They include Jim Mulryan, Don Kennedy and Max Tilford (Cyprus Industrial Minerals), Tad Dale (Pfizer, Incorporated), John Burk (formerly with Pfizer, incorporated), Peter Bixby and Van Stewart (Resource Processors, Incorporated), and Carl Hafer (owner of W illow Creek mine), Butte August 27, 1979 Richard B. Berg Economic Geologist Montana Bureau of Mines and Geology GUNTER00000815 MONTANA Memoir 45 TALC AND CHLORITE DEPOSITS IN MONTANA by Richard B. Berg GUNTER00000816 8 S 8 8 S S S 8 3 S i R S S & S ; S fc 8 8 8 8 3 3 3 8 8 8 8 8 8 - Contents PREFACE.................................................. A B S TR A C T...................................................... IN T R O D U C T IO N ........................................................ 1 L o c a tio n .....................................................................1 Previous w o rk ............................................ i Present w ork.............................................................1 COMMERCIAL ASPECTS OF TALC AND CHLORITE...............................................................2 The mineral ta lc .......................................................2 The mineral chlorite.................................................2 Commercial ta lc .......................................................3 Fibrous minerals............. . ...................................... 3 Uses of ta lc.............................................................. 3 PRODUCTION, MINING AND PROCESSING OF TALC AND CHLORITE................................... 4 Major talc producing areas..................................... 4 Mining and processing of ta lc ..................................5 Montana talc production......................................... 5 GEOLOGY AND MINERALOGY OF TALC AND CHLORITE DEPOSITS........................ .6 Pre-Belt metamorphic rocks................................... 6 M ineralogy............................................................... 7 T a lc ......................................................................... 7 C h lo rite................................................................... 7 Associated minerals................................................. 8 ORIGIN OF TALC AND CHLORITE........................ 12 Conditions of form ation......................................... 12 Time of form ation................................................... 14 Stratigraphic and structural control...................... 14 EXPLORATION FOR TA LC ......................................15 HIGHLAND MOUNTAINS..................................... 19 H-1 Golden A n tie rm in e ................................ -...19 TOBACCO ROOT MOUNTAINS............................21 TR-1 Mineral Hill prospect................................. 21 TR-2 Spuhlr'Glch occurrence........................22 TR-3 Latest Out mine......................................... 22 TR-4 Horse Creek prospect................................22 TR-5 Bivens Creek prospect................................22 TR-6 Harris Creek prospect................................24 TR-7 Grandview prospect....................................26 TR-8 Granite Creek prospect..............................26 TR-9 Bear claims (Granite Creek mine).............. 26 TR-10 Talc prospect southwest of Ennis.......... 30 RUBY RANGE.................................. 31 R-1 Ruby Peak occurrence................................ 32 R-2 Spring Creek prospect................................ 32 R-3 Gem claim ....................................................32 R-4 Whitney claims............................................33 R-5 Prospect southwest o f Whitney claims. . . . 33 R-6 Prospect north of Treasure m ine...............33 R-7 Prospect northeast o f Treasure mine......... 33 R-8 Bennett Owen claim.................................... 33 R-9 Treasure m ine..............................................34 R-10 Beaverhead mine....................................... 34 R-11 Prospect east of Beaverhead m ine............ R-12 Regal (Keystone) mine............................... R-13 American Chemet mine............................. R-14 Estelle (Sweetwater) m ine......................... R-15 Smith-Diilon mine....................................... R-16 Banning-Jones mine................................. R-17 Bozo-Zobo mine......................................... R-18 Crescent prospect (Timber Gulch d e p o s it .............................................................. R-19 Sauerbier mine................... ....................... R-20 Owen-McGovern prospect....................... Other talc occurrences........................................... GREENHORN RANGE............................................. West of the Ruby River....................................... East of the Ruby River and north of Idaho C re e k ................................................................ Between Idaho Creek and the North Fork of Greenhorn Creek......................................... GH-28 Ruby claims............................................. GH-30 Doubtful claims....................................... Area south of the North Fork of Greenhorn Creek ................................................................ GH-42 W illow Creek mine (Ruby Ridge m in e )................................................................ GH-43 Claims north of W illow Creek (Adam and Eve No. 1 and No, 2 )............................. ... GH-45 Talc occurrence south of Virginia C ity .. GH-46 Calverts claims......................................... Other prospects and inactive mines................... GRAVELLY RANGE............................................... GR-1 Tait Mountain claims................................. GR-2 Cherry Gulch prospect............................. GR-3 Yellowstone mine..................................... GR-4 Queen claim............................................... GR-5 Burlington Northern mine......................... ,GR-6 Talc-bearing conglomerate north of Johnny Gulch.................................................. MADISON RANGE................. ............................... HENRYS LAKE MOUNTAINS............................... OTHER TALC OCCURRENCES............................. 0-1 Talc mine south of Helena........................... 0-2 Lynx Creek (Mathews) talc prospect.......... OTHER AREAS OF PRE-BELT METAMORPHIC R O C K S ................ Biacktarl Range... Tendoy Mountains Snowcrest Range. Spanish Peaks area............................................... 61 Northern part o f the Gallatin Range...................... 61 Beartooth Mountains............................................. 61 Little Belt Mountains............................................. 61 Other areas of possible pre-Belt rocks.................. 61 REFERENCES........................................................... 63 GUNTER00000817 Bi f f i wBS e 6 ft g g y y 3 8 8 8 8 8 8 Figures 1--Mountain ranges and major roads in area of talc and chlorite occurrences in south western Montan a ............................................. vi 11-- Prospect southwest o f Ennis, Tobacco Root Mountains................................................30 12-- Ruby claims, Greenhorn Range................... 41 Z--Plot of compositions of analyzed ch lo rite ... .10 13-- Doubtful claim. Greenhorn Range............... 43 3-- Talc and chlorite occurrences in south 14-- Greenhorn claims, Greenhorn Range........... 44 western Montana............................. 16 15-- Geologic map of the area surrounding the 4 -- Horse Creek prospect, Tobacco Root W illow Creek mine................ 44 Mountains......................................................... 23 16-- Diagrammatic cross section o f the talc 5 -- Cut northwest of Harris Creek, Tobacco deposit at the W illow Creek mine.................... 45 Root Mountains............................................... 25 17-- Calverts claims. Greenhorn Range............... 47 6-- Prospect southeast of Harris Creek, 18-- Tait Mountain claims (northern part), Tobacco Root Mountains......................... 25 Gravelly Range..................................................50 7--Southern Granite Creek prospect, 19-- Tait Mountain claims (southern part). Gravelly Range..................................................51 Tobacco Root M ountains.,.............................27 20-- Generalized geologic map of the Cherry Gulch- 8 -- Northern Granite Creek prospect, Johnny Gulch area o f the Gravelly Range___52 Tobacco Root Mountains................................27 21 --Burlington Northern mine. Gravelly Range.. .54 9-- Northern cut at Bear claims. Tobacco 22-- Inactive pit on the Queen claim, Gravelly Root Mountains............................. 28 Range........................... y *-> ........................... 56 10--Southern cut at Bear claims. Tobacco 23-- Lynx Creek (Mathews) talc prospect, Root Mountains............................................... 29 Lincoln County..................................................60 Tables 1-- Comparison of mineralogy o f commercial talc from New York, California and Montana.......... 3 2-- Change in Montana talc production from 1970 through 1976..............................................6 3-- Chemical analyses of talc and chlorite specimens........................................................... 8 4-- Structural formulas and characteristics of analyzed chlorite................................................. 8 5 -- Analyses of ceramic and lava talc from the Yellowstone mine.............................................. 13 6 -- Talc and chlorite occurrences......................... 17 7-- Additional talc occurrences in the Ruby Range not plotted on Figure 3 .......................... 38 Plates i6 1--Photomicrographs o f analyzed talc and 7 chlorite................................. 6 9 9 9 2 --Photomicrographs of alteration sequence at . the Golden Antler m ine................................... 20 Cover-- Lava talc exhibiting dendritic patterns. Specimen No. 7330 M, courtesy: Mineral Museum, Montana Tech. Eleanor Herndon sketch, MBMG. Title Page-- Talc mines in the Ruby Range (9/1/79). Foreground: Treasure mine (Pfizer, Inc.); background: Beaverhead mine (Cyprus Industrial Minerals). Photo by Aero Tech Surveys, Inc., Riverside, CA, courtesy: Pfizer, Inc., Dillon, MT. GUNTER00000818 vsov IVfitf v-sm lARY VH [iff), n tan- 'tffcfi ,h i-si; Vf il o .4 s O ar Vl i -S Vf d-J GUNTER00000819 Abstract Talc occurrences range from small veinlets and pods less than one centimeter thick to one body of talc 30 meters thick. Chlorite (variety clinochlore) is disseminated in some talc and also forms relatively pure bodies adjacent to some talc deposits. Both talc and chlorite formed during greenschist facies retrograde metamorphism that affected the upper amphibolite facies pre-Beit metamorphic rocks. This Precambrian retrograde event has been tentatively dated at 1600 m.y. Most of the talc was produced by the reaction: 3 dolomite + 4 quartz + H20 = talc + 3 calcite + 3 CO* Because the dolomitic marble contains insufficient SiOj for this reaction, it is sug gested that SiOj was introduced by aqueous fluids of meteoric origin. A small amount of talc was produced by the replacement of tremolite. Most of the chlorite replaced rocks such as quartzofeldspatbic gneiss and diabase, which are more aluminous than the mar ble. The estimated temperature of talc formation is between 400 and 500 C. There is some evidence that talc is more abundant in areas where the marble has been tightly folded or where there are numerous faults. Fifty talc or chlorite occurrences are described and the localities of 57 additional minor occurrences are given. All except two deposits lie in southwestern Montana. Talc occurrences in the Greenhorn Range are described in detail and are plotted on a geologic map of this range. Introduction Location All of the known talc deposits of economic im portance in Montana occur in Precambrian marble within the sequence of pre-Belt metamorphic rocks exposed in the southwestern part of the state (fig. 1). In 1978 there were five active talc mines and one active chlorite mine in Montana. Most o f the talc is produced from three large mines, the Beaverhead, Treasure and Yellowstone. In 1976 Montana talc pro duction ranked second in tonnage and first in value among the talc-producing states. The paper, paint and ceramics industries are major consumers of the unusually pure Montana talc. Previous work The best compilation of information on the talc deposits o f southwestern Montana is a lengthy arti cle by Olson (1976, p. 99-143). This article empha sized the geology and talc deposits of the Ruby Range, which were described in further detail by Okuma (1971) and Garihan (1973a). The dissertation by Okuma dealt with the southwestern part of the Ruby Range, and Garihan's dissertation covered the central part of the range. Perry (1948) described the talc mines that were operating then and also men tioned tw o prospects. A description, including maps, of the Johnny Gulch talc deposit (now the Yellow stone mine), put on open-file by the U.S. Geological Survey (James, 1956), was based on field work that was done in 1943 when this large talc deposit was ex posed only by underground workings of modest ex tent and by minor surface excavations. Chidester, Engel and Wright briefly described Montana talc de posits in their summary of the talc resources of-the U.S. (1964, p. 33-35). References on the geology of specific areas are cited in the appropriate sections of this memoir. Present work Work on This project was begun late in the sum mer of 1973 when a period of several weeks was spent visiting all of the active talc mines in Montana and most of the previously described talc occur rences in the Ruby Range. A total of four months during the summers o f 1974 and 1975 was spent mapping the Precambrian geology of the Greenhorn Range and searching for talc occurrences there. The preliminary results of the work in the Greenhorn Range were put on open file with the Montana Bu reau of Mines and Geology in April 1976 (Open-File Report MBMG 19). The open-file report is super GUNTER00000820 2 seded by this memoir and a forthcoming report on the Precambrian geology of the Greenhorn Range. Mapping the Precambrian geology o f an area in cluding part o f the Henrys Lake Mountains required one and a half months during the summer of 1976. The area is approximately 45 miles (72 km) south of Ennis and next to the Idaho border. Talc occurrences are described in this memoir, but a description of the geology will be included in a report on the geology of the Centennial Valley to be published by the Mon tana Bureau of Mines and Geology. Most of the other prospects and inactive mines were mapped during the summer and early fall of 1976, but several prospects were examined during tw o weeks in the summer of 1977. This report was written during the fall and winter of 1977-1978. The geology of the Greenhorn Range was mapped directly on 714-minute quadrangle maps that had been enlarged to a scale of 4 inches eqOal 1 mile. A pocket altimeter was very useful in locating points accurately on the base map, particularly when working on heavily timbered slopes. All prospects were mapped with tape and compass, most at a scale of 1 inch equals 50 feet. A few were mapped at a scale of 1 inch equals 20 feet and then reduced for publication. ' Selected talc and chlorite specimens were ex amined in thin section. Pulverized splits of additional specimens were examined in immersion oils, and the mineralogy of many specimens was checked by x-ray diffraction using a Norelco diffractometer. Some individual mineral grains were identified by using the Debye-Scherrer camera. Representative specimens collected during the study are stored at the Montana Bureau of Mines and Geology, where they are avail able for study. Both British and metric units are given for mea surements made in the field. Where the measure ment was originally made in metric units, these are cited first followed by the British equivalent. If the measurement was made or estimated in British units, these are given first followed by the metric equivalent. Commercial aspects of talc and chlorite The mineral talc Talc is a hydrous magnesium silicate with the ideal formula Mg6 [Si802oHOH)4. Reported chemical analyses of presumably pure talc indicate that the major deviations from the formula are in the presence of Al, Fe+, and ^ e +3. Both talc and chlorite are classified as layer silicates because the arrangement o f constituent ions produces a layered structure. Be cause o f that structure, talc, chlorite, and other layer silicates such as mica can be split into very thin sheets. This ability to split into thin sheets contrib utes to the slippery feel of talc and also to the ability of small flakes to slide easily past one another, a property called "slip", which is important in some uses o f the mineral. Other physical properties of talc that are impor tant in many of its uses are its softness and light color. Talc is one of the softest minerals, and most pure talc can be easily scratched with the fingernail. The unusual softness, together with the ease with which talc splits into flakes, make it practical to pul verize talc into very small particles only a few microns in size. Many o f the uses of talc require such ex tremely fine-grained material. Also required in many applications is a material of light color, almost white. Pure talc when pulverized produces a powder that looks dead white to the eye. Montana talqthat is very pale green in hand specimen produces a white pow der when pulverized. In those applications where talc is used as a filler, its chemical inertness is an impor tant property. For example, inertness is very impor tant when talc is used as a diluent in pills. The mineral chlorite Unlike talc, chlorite has a wide range in chemical composition, as shown by the general formula (Mg, Al, Fe)12 [(SI, Al)a 0 IO] (OH)ie. Actually, chlorite is a group of minerals that can be divided into individual species on the basis of the Fe:Mg ratio, Al: Si ratio, or crystal structure. Although not nearly as valuable commercially as talc, the chlorite group is very im portant mineralogicalfy. Chlorite specimens from Montana that were identified are of the magnesian variety, clinochlore. Some chlorites possess the desirable physical properties of talc, but to a lesser degree than talc. Chlorite is slightly harder than talc, does not have as light a color as most talc when pulverized, and is not quite as slippery as talc. Like talc, it is relatively inert and can be pulverized into very small flakes. GUNTER00000821 3 acts at a Commercial talc ties is included in this category. Alabama talc is an example of this type. d at The industrial commodity talc can be very differ for ent than the mineral talc. Much of the talc used com Fibrous minerals mercially contains a large concentration of tremolite (Table 11. Other minerals that may be present in W ithin the last few years there has been great extnal commercial talc are chlorite, mica, dolomite, magnesite, pyrophyllite, serpentine, anthophyllite, concern about the carcinogenic effects of exposure to asbestos and to a lesser extent exposure to other the ray me the sns ana ail- quartz and montmorillonite. Roe (1975, P- 1134) classified commercial talc in to the following four categories based on mineralogy and physical properties. Steatite: This is massive cryptocrystalline talc that can be sawed, drilled, or machined and is used fibrous minerals. Accompanying this concern there has been much confusion as to what is an asbestos mineral. Some confusion can be avoided by applying the term asbestos only to the mineral chrysotile, the major constituent of much commercial asbestos. Be sides chrysotile, other minerals that may have a fi brous habit are the amphiboles amosite, anthophyilite, crocidolite, tremolite and actinolite. To the talc for electrical insulators. After the soft talc is ma producer or consumer, tremolite and anthophyllite eare are If ish :ric chined into the required shape, it is fired to produce a tougher product in which the talc has been changed to clinoenstatite and quartz. The fired product is called "lava", and talc suitable for this use is some times called "lava talc". Some steatite talc used for electrical insulators has been mined at the Yellow stone mine. (The designation steatite has also been are the most important minerals of this group be cause they occur in some talc. A very important dis tinction must be made between the fibrous and non fibrous habits of these minerals.,Tremolite and an thophyllite occur in both prismatic and fibrous habit. Specimens having prismatic habit produce small prisms and not fibers when crushed. Anthophyllite used to describe talc of high purity although it may has a greater tendency to occur in the fibrous habit not possess the physical properties required of "lava than tremolite, which is only rarely fibrous (Ampian, talc". Much of the talc mined in Montana has been 1976, p. 4). For more information on amphiboles in described as steatite-grade talc.) talc, the reader is referred to Goodwin, 1974. S oft p/aty talc. This variety of talc has been Uses of talc iat formed by the replacement of magnesium carbonate ;ry rocks and commonly contains minor chlorite. Almost As with many o f the nonmetallic mineral com N- all of the Montana talc could be included in this cate modities, talc has a wide range of uses. Although the ic gory. various uses of talc have specific requirements, most >r- o f them are based on the unusual physical properties ir- Tremolite talc: (Also called "hard talc" or "hard o f talc that make it possible to produce a very fine ore".) In addition to talc and tremolite, this variety of grained white powder that is chemically inert and not commercial talc may contain anthophyllite, calcite, abrasive. In 1977 the domestic consumption of talc dolomite, and serpentine. New York talc and some of was ceramics, 33 percent; paints, 24 percent; paper, the California talc are of this variety. 8 percent; plastics, 8 percent; roofing, 7 percent; al cosmetics, more than 5 percent; insecticides, 3 per 3/ Mixed talc ores: Soft friable talc that contains cent; rubber, 2 percent; and numerous other minor a dolomite, calcite, serpentine and other minor impuri uses (Clifton, 1978, p. 166). al Dr Table 1--Comparison of mineralogy of com m ercial talc from N ew York, California and le M ontana {from M ulrydn, 1974, p. 16). Mineral concentrations are expressed in percent. 71 Source Talc Carbonate Tremolite Anthophyllite Quartz Other New York 62 30 5 3 Silver Lake, California 54 3 43 al Panamint, California 62 12 s Yellowstone mine, Montana 99 1 >t 7 Beaverhead mine, Montana 98 1 Trace <0.5 2 montmorillonite, 4 mica 1 chlorite GUNTER00000822 4 Talc is used in many ceramic products, especi ally in wall and floor tile and dinnerware. It is also used in some ceramic glazes. Talc improves the firing characteristics of the ceramic product, and in some applications it permits reduction of the time required for the firing cycle. Synthetic lava talc for electrical insulators can be made from ground steatite talc. Electrical insulators can also be made of synthetic cordierite produced by firing a mixture of talc and clay. Talc is used as a filler and extender in paint. It can be used to replace some of the more expensive pigments such as titanium dioxide and in this use would be regarded as an extender. The addition of talc to paint helps to prevent settling of the pigment when paint is stored and to avoid sagging when paint is applied. Talc is also added to paint to decrease gloss. A very important use of talc in the paper in dustry is in the control of pitch. Pitch from the wood pulp may adhere to the equipment and may also pro duce brown spots in the finished paper. Finely pul verized talc adsorbs the resins, thus helping to keep them dispersed in the pulp. Talc of high purity and whiteness is required for this market. Talc can also be substituted for titanium dioxide, which is added to pulp to increase the whiteness of the finished paper. In addition to the major uses of talc in ceramics, paint and paper, there are many other uses, which together account for almost half the market. In the manufacture of plastics and rubber, talc is used as a filler, to reduce the cost of the finished product, and to give desirable physical properties to the product. The addition o f talc to some plastics increases their strength and toughness. Talc increases the physical stability and resistance to weathering of roofing materials. A small amount of talc dusted on the sur face of roofing also helps to prevent layers of the roofing from sticking together during storage. Be cause of its chemical inertness and fine particle size, talc is used as a carrier for insecticides. Talc of ex tremely high purity (both chemical and mineralogical) is used in the cosmetic and pharmaceutical indus tries. Although talcum powder accounts for only a miniscule fraction of the market, it may be the best known use of talc. Talc can be used to produce other layer silicates that are similar to hectorite, the lithium-bearing smectite mined in California and valued for its high swelling and gelling characteristics. United States patent number 3,954,943 was granted May 4, 1976, to Laporte Industries Limited, Luton, England, for a process utilizing talc in the manufacture of a hydrous magnesium silicate, which has a crystal structure similar to that of hectorite but which is reported to have rheological properties superior to natural hec torite. Another U.S. patent (number 3,666,407) was granted May 30, 1972, to Pfizer, Incorporated, for a process for producing synthetic hectorite-type clay from talc. To the best of the author's knowledge, neither process has yet been used commercially. Prices reported for talc in the January 1978 issue of Engineering and Mining Journal (McGraw-Hill, New York) range from $10.50 to $151 per ton, de pending mainly on the quality of the talc and the de gree to which it has been processed. Although prices were not given for Montana talc, it is probably most closely represented by the California talc, which ranged from $37 to $104 per ton, with very finely ground (micronized) talc of unusual whiteness at the high end. Cosmetic-grade steatite from California ranges from $44 to $65 per ton. Production, mining and processing of talc and chlorite Major talc producing areas In recent years New York, Vermont, Texas, Montana and California have been major domestic sources of talc. The U.S. Bureau of Mines ranks the states in terms of total production o f talc, soapstone and pyrophyllite, but provides no annual ranking for talc alone. A large amount of talc is mined in the Balmat-Edwards area o f northern New York, where talc occurs in marble of the Grenville Series (Precam brian). Talc ore bodies are in the same stratigraphic unit as zinc ore mined in the district. Most o f the talc in this district is o f the tremolitic variety. Talc depos its in Vermont that have been formed by the altera tion of ultramafic bodies consist of an impure mixture of talc and other minerals. Froth flotation is used to beneficiate the ores. The ceramic industry is a major consumer of talc from the Allamoore district, in west Texas. Talc from this district ranges from white to black. Although the black rock consists mainly of talc, it looks more like black argillite than talc. Both tremolitic talc and high-purity talc are mined from an extensive district near Death Valley in California, which extends northeast into Nevada. Alabama and North Carolina both produce talc, but production is substantially less than that o f the states just men tioned. Olson (1976, p. 101-108) gave a more detailed discussion o f talc producing areas in the United States. Some other countries that produce talc are Canada (Ontario), Italy, France, Finland, Australia, China, Japan, Russia, South Korea, India and Brazil. \ I GUNTER00000823 i sur f the Besize, f exlical) dusify a best ates ring iigh ates 376, Dr a ous iure i to iecvas >r a :lay ge. sue till, de de- ;es DSt ch ely he n'a B Dr st 0 h n d 5 1 i Mining and processing of taic Talc deposits in the United States are mined by conventional methods, both openpit and under ground. Some of the talc mined in New York, Ver mont and California is produced from underground mines, but Montana production is now (1978) en tirely from open pits. In openpit mines it is a common practice to sample the talc ore body by closely spaced holes drilled after removal of the overburden. Cuttings are analyzed for purity, and the brightness of the pulverized talc is determined. Analysis of the drill hole samples enables the operator to plan the se quence of mining to provide talc of various grades for different markets. By stockpiling talc of different grades, it may also be possible to blend talc to pro duce the maximum amount of material of a particular grade. The processing of talc consists essentially of beneficiation and pulverization. The methods used to beneficiate talc differ from area to area, depending on the nature o f the ore as well as the purity required for the finished product. For example, Vermont ore consists of-a mixture of talc and magnesite and con tains minor iron oxide, pyrrhotite and gersdorffite (NiAsS). Froth flotation is employed to produce high-purity talc from this ore. Froth flotation supple mented by high-intensity magnetic separation is used on some other talc ores. In some deposits, including most of those in Montana, the talc is of high purity but blocks of talc are mixed w ith country rock to such an extent that some waste must be mined w ith the talc. In this situ ation, removal o f the waste rock by hand sorting has proved to be a practical means of beneficiating the talc ore. Mechanical sorting, using an electro-optical sorter that rejects material of low reflectivity, has been tried on an experimental basis for this type of ore. As deposits o f high-purity talc are depleted, more sophisticated beneficiation techniques will be used to upgrade impure talc. Much of the present market requires talc of high brightness and o f extremely fine grain size. Because of its softness taic can be finely pulverized (micronIzed) in fluid-energy or jet mills. After initial pulveriza tion in roller mills, the talc is fed into the fluid-energy mill, where it is pulverized by attrition in a circular chamber in which either compressed air or steam propels the talc particles through a circular path. The finer talc particles leave the chamber through a cen tral port while the coarser particles continue to travel around the periphery of the chamber. A fluid-energy rnilt can produce a product in which the talc particles are in the range o f 0.5 to 10 ^m. 5 Brightness of the pulverized talc is an important property for many uses. The brightness is a measure of the reflectivity of the talc sample as compared to an MgO standard. Because General Electric has developed carefully calibrated instruments for its de termination, this characteristic is commonly termed GE brightness. Montana talc production The Montana talc industry has grown to its pres ent size mainly since the late 1940s. Chidester and others (1964, p. 35) stated that total Montana talc production through 1956 was approximately 200,000 short tons and that most of it was produced after 1949. In 1976 Montana talc mines produced 224,753 tons of talc with a reported value of $2,960,000 (Clif ton, 1978, p. 1311). Montana was the leading state in the value of talc produced and second to Vermont in the amount of talc produced during 1976. < *; Many of the currently active talc mines in Mon tana were initially underground mines. The ore bodies at the Yellowstone, Beaverhead, Regal and Smith-Dillon mines were all developed by under ground workings, but more recent production has been from open pits. (For an interesting account of the history of talc mining in Montana, see Olson, 1976, p. 108-109.) Production of talc in 1977 was from five mines operated by three companies. Cyprus Industrial Min erals of Cyprus Mines Corporation produced talc from the Yellowstone and Beaverhead mines. Pfizer, Incorporated mined most of its Montana talc from the. Treasure mine and a much smaller amount from the Regal mine. Resource Processors mined taic at the Willow Creek mine. A small quantity of chlorite, which was mined at the Golden Antler mine, was sold to Cyprus Industrial Minerals. ' Both Cyprus and Pfizer have plants equipped with fluid-energy mills for fine grinding of talc. Cy prus' plant is situated at Three Forks, and Pfizer's plant is at Barretts siding south of Dillon. Cyprus also ships crude talc to its plants at Grand Island, Nebraska, and Gent, Belgium. Crude talc from the Willow Creek mine is shipped to Resource Processor's mills in the eastern United States. The most recent figures available for the con sumption of Montana talc are for 1972 when the dis tribution was as follows: paper, 36 percent; paint, 29 percent; ceramics, 8 percent; toilet preparations, 6 percent; and other uses, 21 percent (Welch, 1974, p. 433). In that year, 5 percent of Montana talc was exported from the United States. !!#A S ! :l f ' T1' % 1 M GUNTER00000824 6 Although the annual production of talc in Mon tana has increased greatly over the last 30 years, pro duction increases have not been at a uniform rate. Table 2 shows the changes in talc production for the period from 1970 through 1976. From all Indications, the talc industry in Mon tana will continue to grow, and there will be continu ing exploration for new deposits as well as rvalua tion of inactive mines and raw prospects. Of possible significance in the future market of Montana talc is the projected growth in the paper industry, a major market for the talc mined in Montana. The U.S. paper industry, which used 80,000 short tons of talc in 1973, is predicted to use 250,000 short tons of talc in the year 2000 (Wells, 1976, p. 1087). Geology and mineralogy of talc and chlorite deposits Pre-Belt metamorphic rocks W ith the exceptions of the talc deposit south west of Helena (0-1) and the deposit northeast of Troy (0-2), all known talc occurrences of Montana are in marble of Precambrian age within a sequence of metamorphic rocks. These metamorphic rocks, now known as pre-Belt metamorphic rocks, were divided into Archean gneiss and the overlying "Cherry Creek beds" by Peale (1896) during his work in the Tobacco Root Mountains, Madison Range and Gravelly Range. The "Cherry Creek beds", described by Peale as marble, mica schist, quartzite and gneiss, were named for exposures between Cherry Creek and Wigwam Creek in the Gravelly Range. Later Tansley, Schafer and Hart (1933, p. 8) divided the Precambrian metamorphic rocks of the Tobacco Root Mountains into the Pony Series and the overly ing Cherry Creek Series. The Pony Series included the Archean gneiss of Peale and was so named because it is exposed in the vicinity of Pony, a small tow n on the northeast flank of the Tobacco Root Mountains. The Cherry Creek Series included the metasedimentary rocks of the "Cherry Creek beds" as originally described by Peale. Tansley, Schafer and Hart recognized that there was no evidence for T able 2 --Change in M ontana talc production from 1970 through 1976. com piled from U.S. Bureau of Mines Minerals Yearbooks referenced below. ________ Year 1970 1971 1972 1973 1974 1975 1976 Change from previous year Reference 8% decrease in tonnage West, 1972, p. 435. 6% decrease (in tonnage?) Welch, 1973, p. 447. ''substantial increase" Welch, 1974, p. 433. 48% increase in tonnage West, 1976, p. 426. 27% increase in value Krempaskv and Lawson, 1977, p. 421. 52% decrease in tonnage Krempasky and Lawson, 1978, p, 457. 84% increase in tonnage 96% increase in value Krempasky and Lawson, in press. designation of the Pony Series as Archeozoic and the Cherry Creek Series as Proterozoic (Algonkian). For this reason and because the Cherry Creek Series is overlain by sedimentary rocks of the Belt Supergroup (Precambrian), they designated the Pony Series and Cherry Creek Series as pre-Beltian. This designation, frequently shortened to pre-Belt, is now in general use, The Belt Supergroup is a thick section of sedimentary rocks of Precambrian Y age (800 to 1600 m .y.l, which is exposed in western Montana and northern Idaho. In western Montana most rocks of the Belt Supergroup are no higher than the biotite grade of metamorphism. The distinction between rocks belonging to the Pony Series and those belonging to the Cherry Creek Series is difficult. Reid (1957, p. 6,7) in his discussion of the metamorphic rocks of the northern Tobacco Root Mountains stated: It must be emphasized that the distinction be tween Pony and Cherry Creek depends not on peculiarities visible in every outcrop, but rather depends on the aggregate o f rock types present in a rather thick section. Because o f possible lateral varia tion in rock composition, the criteria listed above for distinction may be valid only in the immediate vicinity o f the map area. Further, the age relationship between the Pony Series and the Cherry Creek Series is not without un certainty. Reid (1957, p. 14) reported that in the northern Tobacco Root Mountains, rocks of the Cherry Creek Series dip under rocks o f the Pony Series and he therefore concluded that unless there is major overturning of the entire section, the Pony Series is younger than the Cherry Creek Series. Hein rich and Rabbitt (1960) described rocks o f the Cherry Creek Group and pre-Cherry Creek gneiss from the Ruby Range southwest of the Tobacco Root Moun tains. In the Ruby Range the rocks of the Cherry Creek Group are separated from the pre-Cherry Creek rocks by the Dillon granite gneiss, a thick layer of quartzofeldspathic gneiss. The division of the pre Belt metamorphic rocks into the Pony and Cherry Creek Series seems premature at this stage in our GUNTER00000825 understanding of those rocks. Because of isoclinal folding, the establishment of a stratigraphic section even within one mountain range is difficult, and the correlation of units from one range across a valley to another range is extremely uncertain. Talc occurs in marble layers within a sequence of rocks, mainly metasedimentary, which can best be described as Cherry Creek lithology. Mineralogy Talc Most of the talc from southwestern Montana is pale green to white in hand specimen, exhibits a waxy luster, and when pulverized yields a powder,that to the naked eye, appears white. The variation from light green to dark green in material from some deposits is a function o f the chlorite con tent of the talcose rock; the greater the concentra tion of chlorite, the darker the color of the rock. The apparent hardness of talc varies considerably. Some of the very fine grained massive talc can be scratched by the fingernail only with difficulty, whereas coarser-grained talc can be easily scratched with the fingernail. ' Talc from southwestern Montana is relatively pure. The major impurity in some of the talc Is fine grained chlorite, which is not recognizable in hand specimen nor even in some thin sections. X-ray dif fraction analysis showed chlorite present in concen trations of a few percent in many of the light-green talc specimens. In thin section, talc shows a variety of textures. The block or lava talc (now known also as carv ing talc) from the Yellowstone mine and vicinity con sists of talc grains only 1 to 2 /im across and of ran dom orientation. A t the other extreme is dolomrtetremolite-talc schist from the Tait claims (GR-1) that consists of well oriented talc flakes several milli meters across. The grain size of most Montana talc falls between these two extremes. Some talc dis plays a feathery texture caused by sheaves of talc grains as much as 0.6 mm long that are surrounded by talc grains only 10 ^m across. Several specimens exhibit a mosaic texture that is produced by 0 .2- to 4-mm patches o f fine-grained talc with almost uni form extinction position (Plate 1). This is possibly a relict texture inherited from marble that was replaced by talc. Fine-grained talc may be veined by coarse grained talc in which individual flakes are several millimeters across. Chemical analyses of two talc specimens (Table 3) show that both specimens contain a significant concentration of iron. Analyses of other talc samples from southwestern Montana presented by Olson (1976, p. 111) show that they contain between 0.51 and 1.51 percent total iron expressed as Fe20 3. Also talc analyses presented in Deer, Howie and Zussman (1962, p. 122, 123), although incomplete in terms of analysis for FeO and Fe20 3 in the same specimen, show high values of 2.46 percent FeO and 1.49 per cent Fe20 3 for different specimens. Both specimens of talc reported in Table 3 contain opaque grains 1 to 2 (im across that were first thought to be a possible source of iron. Those grains are not abundant enough to explain the iron reported in the talc analyses, however, even if the unidentified mineral were an iron oxide. Because the talc specimens were prepared in a ceramic mortar and pestle, iron con tamination during sample preparation can be ruled out. The iron reported in these analyses must then be a constituent of the mineral talc. Specimen 83 also contains a few small grains of apatite. *- * s Talc specimen 3317-5b likewise contains a sig nificant concentration of iron, 2.24 percent FeO and 0.51 percent Fe20 3. This specimen contains an esti mated 2 to 4 percent chlorite and small opaque grains that appear similar to those in specimen 83. If the chlorite has about the same composition as the ana lyzed chlorite from the same deposit (Table 3, speci men 3316-6), it would not contribute as much iron as reported in the analysis. Because the concentration of opaque grains is too low to account for the iron re ported in the analyses, it can be concluded that most of the iron is in the talc lattice. Chlorite Most of the chlorite-rich rock can be distin guished from talc in hand specimen by its darker green color and greater hardness; most rocks con sisting mainly of chlorite cannot be scratched by the fingernail. Although generally the greater the con centration of talc in the chloritic rock the lighter the shade of green and the softer it is. Some specimens that are almost pure chlorite are light tan and softer than some specimens of talc. X-ray diffraction analy sis of many specimens of chloritic rock showed that almost all contain talc. Zircon, apatite and rutile are trace constituents of much chlorite. Except for rare silvery-gray chioritized biotite crystals 1 to 2 cm across, all of the chlorite is microcrystalline. Two and possibly three distinct varieties of chlo rite can be recognized in thin section by differences in color and texture. The most abundant variety is clinochlore, identified by chemical analyses o f two specimens in which it is the only chlorite (Table 3). The grain size of most clinochlore is between 2 and 100 fim, and radial sheaves of grains form a common GUNTER00000826 8 texture (P late 1). In plane light clinochlore is col orless to very pale green and with crossed nicols the interference colors are gray to yellowish orange depending on the grain size. Pennine!?), which shows bright blue and violet interference colors, forms grains as much as 0.5 mm long and vermiform growths. This chlorite, although colorless to pale green in plane light, tends to be greener than the clinochlore. The third possible vari ety of chlorite also forms coarse flakes but is grayish green when viewed with crossed nicols and is color less in plane light. All three varieties of chlorite can occur within the area covered by a single thin section. Table 3--Chemical analyses of talc and chlorite apecimens. SiO, no. ALO, FeO MgO CaO Na,0 K,0 H 0 + H,0" Total Talc_______ 83 62.84 0.11 0.37 0.34 1.02 30.43 0.04 0.04 0.02 4.51 0.29 100.01 33175b 62.42 0.04 0.45 0.51 2.24 29.35 0.01 0.02 0.01 4.79 0.09 99.93 Chlorite 1451 3316-6 31.54 1.47 1859 0.74 3.74 31.70 0.17 0.01 0.02 12.16 0.62 32.41 n.d. 18.16 0.61 6.16 31.24 0.04 0.01 0.01 12.73 0.31 100.76 101.68 Two o f the purest specimens of chlorite were analyzed for major elements (Table 3). A plot of these chlorite samples on Foster's classification scheme (fig . 2) shows that both specimens lie within the clinochlore field. Structural formulas calculated from these analyses are shown in Table 4. Na20 , KjO, CaO, and T i0 2 were excluded from these for mulas because it is probable that those constituents where present can be attributed to impurities in the chlorite. The high concentration of TiOa in specimen 1451 is caused by the abundant rutile. Specimen 3316-6 is, except for its unusual purity, typical of fine-grained clinochlore associated with talc. The chlorite polytype was determined for chlo rite specimens from localities H-1, TR-7, TR-9, TR-10, GH-28, GH-42, GR-1, and R-10. As would be expected from the geologic environment, all of these specimens are the lib polytype. Brown and Bailey (1962, p. 834,, 835} found in their survey of chlorite from different environments that approxi mately 80 percent of the 303 chlorite samples tabulated are the lib polytype and that this polytype is characteristic of chlorite from metamorphic rocks and high-temperature ore deposits. 83. Talc from the Cherry Gulch prospect (GR-2). 3317-5b: Talc from the Willow Creek mine (GH-42). 1451: Chlorite from Ruby claims (GH-28). 3316-6: Chlorite from the Willow Creek mine (GH-42). See Plate 1 for photomicrographs of these specimens. Analyses performed in the Analytical Laboratory of the Montana Bureau of Mines and Geology. Analyses by Larry Wegeiin, frank P. Jones and Gayle LaBlanc. Associated minerals The following 25 minerals are associated with either talc or chlorite. Twenty were observed in the deposits examined during the present study. De scriptions of the other five were taken from the litera ture. Some have little genetic significance for the for mation of talc or chlorite because they are either relict phases remaining from the rock replaced by talc or chlorite or they were deposited after the formation of talc and chlorite. Others seemingly are contempo raneous with the talc or chlorite and thus can help in the understanding p f the conditions of formation of the talc or chlorite. 1 / I Structural formula Variety Index of refraction Color Grain size Impurities Origin Tabla 4 --Structural formulas and characteristics o f analyzed chlorite. 1451 from Ruby claims (GH-28) 3316-6 from Willow Creek mine (GH-42) (A l,,, Fe+V o , Fe+S, ,, Mg, n) ( A U , Si, ,,,) 0 ,, (OH), Clinochlore lib polytype 1.583 0.005 Light olive gray 5Y 6/1 3 to 700 Rutile grains 5 to 52 im long Replacement of diabase dike (Ali-os 4* Mg* jtMAIg.m Si*.os) 0o (OH)o Clinochlore lib polytype 1.586 0.005 Dark greenish gray 5G 4/1 20 to 100 jim None detected Replacement of quartzofeldspathic gneiss C d ri tl ri GUNTER00000827 :imens. a 3316-6 32.41 n.d. 18,16 0.61 6.16 31.24 0,04 0.01 0.01 12.73 0.31 101.68 intana anc. 9 h i C- Specimen 1451 of chlorite th a t has replaced diabase D. Specim en 3316-6 of unusually pure chlorite fro m the __ "ike a t the Ruby claims. Relict ophitic texture shown by W illo w Creek mine. Polars crossed. . masses of pure chlorite th a t has replaced plagioclase and dark areas w here chlorite-rutile mixture has replaced a py roxene. Plane light. Plate 1 Photomicrographs of analyzed talc and chlorite. tf GUNTER00000828 10 9 Ankerite: James (1956, p. 2) reported crystals of Dolomite-. Coarse-grained dolomite in which * ankerite as much as 1 inch (2.54 cm) across found in some individual rhombs are more than 5 cm across is !' vugs in siderite at the Yellowstone mine (GR-3). Both associated with some talc occurrences. Because do siderite and ankerite were described as secondary lomite grains this large are found only next to talc carbonates. bodies, it is inferred that such coarse-grained dolo mite is related to the formation of the talc. Apatite-. Pale greenish-blue apatite crystals < 1 9 cm long have been identified in chlorite from the Graphite: Graphite in the marble was not af Beaverhead mine (R-10). Milky white apatite crystals fected by replacement of the marble by talc and is several millimeters long are a trace constituent of now a minor constituent in some talc. some of the chlorite from the W illow Creek mine (GH-42). On the basis of indices of refraction (e = Gypsum-. Both Okuma (1971, p. 93) and Garihan 1.6353, to = 1,6397) this is fluorapatite. (1973a) reported the rare occurrence of gypsum in talc in the Ruby Range. Garihan specifically de Brucite: Millholland (1976, p. 50) reported the scribed gypsum in the talc at two prospects north of occurrence of brucite in marble north of Cherry the Treasure mine (R-6 and R-7) and at the Spring Gulch in the Gravelly Range. Brucite grains < 0.5 Creek deposit (R-2). Gypsum has not been reported mm long are intimately associated with fine-grained in talc from other mountain ranges. ` dolomite and talc. Hematite-. Millholland (1976, p. 50) identified Calcite: Calcite coats fractures and fills vugs in some of the talc deposits. The mode of occurrence minute hexagonal plates of hematite in some talc from the Gravelly Range. suggests that the calcite was deposited after the time o f talc formation and thus was not produced by the reaction: 3 dolomite + 4 quartz + H20 = talc + 3 calcite + 3C02. Limonite: Most fractures in pyrite-bearing talc are coated with limonite derived from the weathering of pyrite. Limonite pseudomorphs after pyrite can be found in some talc, for example, the Grandview pros Cha/copyrite: Chalcopyrite is a rare constituent pect (TR-7). of talc, having been reported from only tw o locali ties. Garihan (1973a, p. 164) mentioned scattered chalcopyrite grains in talc at the Bennett Owen claim (R-8). Minor chalcopyrite occurs in talc at the Cherry Gulch prospect (GR-2). Magnesite-. A maroon rock exposed at the Bur lington Northern prospect (GR-5) consists o f magne site accompanied by minor dolomite and talc. Mill holland (1976, p. 50} also mentioned magnesite in J 1j I." * I ) j i t 1 >' ]ou THURINGITE CHAMOSITE t i - i O.BO RIPIDOLITE BRUNSV1GITE DIABANTITE 0.60 Ur 0.40 - -0.20 . SHERPDANITE CLINOCHLORE PENNINITE 3316-6O 1451O ____ 1____ ____ 1_________ L... ... ..... L...-------- 2 00 220 2.40 2.60 2.80 3-00 320 3.40 3 .6 0 3 .8 0 4.00 FORMULA POSITIONS OCCUPIED BY Si Figura 2 --Plot of compositions of analyzed chlorite. GUNTER00000829 11 ch marble in this general area. Another talc locality Sepiolite: Masses of splintery sepioiite as much is where magnesite is reported is in the N W 'A sec. 13, as 10 cm long are exposed in the lowest cut at the lo- t . 8 S., R. 4 W ., in the Greenhorn Range, where a prospect north of W illow Creek (GH-43). Alice jIc specimen o f marble contains magnesite, dolomite, Blount (personal communication, 1977) has also lo- talc and minor chlorite. Magnesite also occurs at the identified sepiolite from marble just south of the pit at Treasure mine (R-9). the W illow Creek mine. if- Malachite: Weathering of chalcopyrite has Serpentine: According to Garihan (1973a, is formed malachite at the Cherry Gulch prospect p. 180) thin chrysotile veinlets cut some of the talc at (GR-2I. Garihan 11973a, p. 164) described malachite the Gem claim (R-3). along fractures at the Bennett Owen claim (R-8). an Siderite: Siderite is reported to be a secondary in Muscovite (Senate): Feldspars in quartzofeld- carbonate at the Yellowstone mine (GR-3) (James, e` spathic gneiss and schist next to many talc or chlorite 1956, p. 2). of occurrences have been altered to sericite. Sillimanite '9 in pelitic schist at the Bivens Creek talc deposit Tremolite: Tremolite in talc is reported only from >d (TR-5) also has been replaced by'sericite. the Ruby Peak occurrence (R-1). A t some other lo calities tremolite blades as much as 3 cm long have Prehnite: Prehnite has been identified at .only been completely replaced by talc. An example can be *d one locality, a small prospect in the Greenhorn seen in the longest cut at the Harris Creek prospect ic Range (GH-38) where prehnite grains as much as 1.5 (TR-6) where tremolite was fortned'-in the marble dur mm across are associated w ith fine-grained chlorite ing an earlier stage of metamorphism and was later and coarser-grained calcite. Ic replaced by talc. 9 Pyrite:. Pyrite is found only rarely in talc or in Unidentified mineral: Extremely small (1 to 3 /m) *6 quartz veins associated with talc. Most of the pyrite grains of an opaque mineral are scattered in cloudlike 3 pyritohedrons have been replaced by limonite. concentrations in some of the talc. Efforts to concen trate the mineral and identify it by x-ray diffraction Pyrolusitei?): Black dendrites in the block talc analysis were unsuccessful. from the Yellowstone mine (GR-3) and vicinity are probably pyrolusite; they are found only in the block Vermiculite: Vermiculite was identified in talc at I- talc., only one locality south of Virginia City (GH-45). The size and shape of the vermiculite grains suggest that Quartz: Irregular masses o f quartz are locally they were formed by the replacement of phlogopite abundant in some talc deposits. present before replacement of the marble by talc. Rutile: Garihan (1973a, p. 151, 171) reported a trace of rutile in some of the talc at the Treasure mine (R-9) and also mentioned secondary euhedral rutile intergrown w ith talc and chlorite at the Whitney claims (R-4). Rutile is a trace constituent of much of the chlorite and is particularly abundant in chlorite at the Ruby claims (GH-28). Zircon: Zircon is a trace constituent o f some chlorite. Most zircon grains are subrounded to euhe dral and between 0.1 and 0,2 mm in length. A few are as much as 0.5 mm long. The zircon grains were originally present in the quartzofeldspathic gneiss or schist and were not destroyed during the replace ment of those rocks by chlorite. Pin S3! P & f:ceai % 155 St 1 i\ .si f f I- -3? :'4 ^t I ill Jk GUNTER00000830 12 Origin of talc and chlorite Conditions of formation The following discussion of the genesis of talc and chlorite applies only to those occurrences in the pre-Belt metamorphic rocks of southwestern Mon tana and not to the deposit northeast of Troy; the de posit southwest o f Helena; or those associated with metagabbro dikes in the Henrys Lake Mountains. The chlorite at the Golden Antler mine (H-1 ) also may be o f different origin than the deposits discussed here. Most o f the talc in southwestern Montana was formed by the reaction: 3 dolomite + 4 quartz + H20 talc + 3 calcite + 3 C02. A minor amount of talc has been formed by the replacement of tremolite. Possible equations for this reaction are presented in the following sections. The rarity of calcite in close association with talc indicates that this product o f the above reaction was flushed from the area of talc formation by a large quantity of water passing through the deposits. There is not sufficient quartz in the dolomitic marble to satisfy the above reaction, which requires 32 volume percent quartz fo r the complete replace ment o f dolomite by talc. Quartz is only a minor con stituent of the marble. For example, in the Greenhorn Range the marble is estimated to contain less than 10 percent quartz. The other constituent that must be added to produce,talc is H20 , and it is suggested that silica-rich aqueous solutions caused the replacement o f marble by talc. The general lack of quartz in asso ciation with talc deposits suggests that the avail ability of S i02 was the limiting factor in the replace ment of marble at a particular locality. Obviously dolomite was plentiful, and the presence o f hydrous minerals (sericite, chlorite and serpentine) in rocks adjacent to talc deposits indicates that H20 perme ated these rocks beyond the limits of talc bodies. The source of this S i02-bearing water is not definitely known. A probable source is meteoric water that has dissolved silica from the overlying rocks during its downward and perhaps lateral movement. It has been suggested (Olson, 1976, p. 113) that hydrothermal fluids from Precambrian plutons, now metamorphosed to quartzofeldspathic gneiss, were a source of silica for talc formation. There is good evidence that both talc and chlorite formed long after any Precambrian plutons had crystallized and been metamorphosed, thus precluding such plutons as sources of hydrothermal fluids at the time o f talc for mation. Evidence for the time o f formation of talc and chlorite is presented in the next section. Another possible source of hydrothermal solu tions might be post-metamorphic pegmatite dikes that are found in the pre-Belt rocks of southwestern Montana. These dikes seem an unlikely possibility because they are rare in the marble and are not reported in the vicinity of talc deposits. In the Green horn Range, pegmatite dikes are most prevalent in the quartzofeldspathic gneiss. i With the exception of the Golden Antler chlorite deposit (H-1), which is quite possibly of different origin than the talc and chlorite deposits being dis cussed here, all chlorite deposits are associated with talc deposits, but not all talc deposits are associated with chlorite deposits. This association of chlorite with talc can be explained by the movement o f cat ions in solution between marble and the other rock types such as quartzofeldspathic gneiss. W ithout the addition of magnesium, quartzofeldspathic gneiss cannot be replaced by chlorite, because the gneiss does not contain sufficient magnesium for the growth of chlorite (clinochlore). if HaO was available to promote the movement of magnesium from the dolomitic marble to the gneiss, it could also have caused the replacement of dolomitic marble by talc, perhaps using silica from the quartzofeldspathic gneiss in this reaction. Chlorite, because of its close association with talc, is thought to have formed at the same time as the talc. A comparison of the chemical analyses of chlorite (clinochlore) and talc from the W illow Creek mine (Table 3) shows that the main differences are in the content of S i02 and Al20 3. The talc contains approximately twice as much Si02as the chlorite and almost no Al20 3 as compared to approximately 18 percent A!20 3 in the chlorite. Aluminum present in impurities in the dolomitic marble could not be accommodated in the talc lattice, so a minor amount of chlorite formed in the talc body as a result. Field relationships indicate that some of the rock consisting almost entirely of chlorite was produced by the complete replacement of quartzofeldspathic gneiss, as at the Grandview prospect (TR-7). A crude comparison between compositions of quartzofeld spathic gneiss in southwestern Montana as reported in the literature and that of chlorite shows that mag nesium must have been added and silicon removed if the gneiss was completely replaced by chlorite. The removal of some silicon from the gneiss would in- GUNTER00000831 13 crease the aluminum concentration to that o f chlo The temperature range for this reaction under the rite. Sodium, calcium and potassium in the gneiss described conditions is 420 to 575C. or must also have been removed because these ele ate ments could not be accommodated in the chlorite Tremolite may also have been replaced by talc lattice. If it is assumed that aluminum was immobile, according to the following reaction: then silicon from the quartzofeldspathic gneiss could lu have been added to the marble to make talc, and 3 tremolite + 6 C02 + 2 H20 = ces some magnesium from the dolomitic marble could 5 talc + 6 calcite + 4 quartz. jrn ity re have been used in the replacement of quartzofeld spathic gneiss by chlorite. The same transfer of cat ions may have occurred at the Ruby claims (GH-28) Under the same conditions the temperature range for this reaction is approximately 400 to 460C. in in where a diabase dike was replaced by chlorite when talc replaced the adjacent marble. The reaction for the replacement of dolomite by talc is: The replacement of tremolite by talc allows an 3 dolomite + 4 quartz + H20 = te estimate o f the temperature of talc formation to be talc + 3 calcite + 3 C02. nt made. The same temperature range is inferred for the s- formation o f chlorite, because o f its dose association Again, according to the curves of Slaughter, th with talc. On the basis of thermodynamic extrqpola- Kerrick and Wall, the maximum temperature o f the 3d tions from experimentally determined points. reaction is approximately 460C and the minimum te Slaughter, Kerrick and Wall (1975) showed the effect temperature is below 400C. _ it- of mole fraction of CO* (XQOa) on the temperature of ck reaction when fluid pressure (H20 and C02) remains Consideration of the possible reactions shows te constant. Several o f the reactions are pertinent to that the talc in these deposits probably formed at the replacement of tremolite by talc. These curves temperatures between 400 and 500C if the fluid 3$ were established fo r fluid pressures o f 1 kb, 2 kb and pressure was less than 5 kb. Pressure greater than 3S ie 5 kb, and it is likely that the talc and chlorite formed 5 kb during the retrograde metamorphic event seems ie within that range of pressure. unlikely. Burger (1967, p. 10) suggested that a ie <e Several possible reactions may explain the re placement of tremolite by talc, one of which is: pressure of 3 kb was attained during almandineamphibolite facies metamorphism in the Tobacco Root Mountains. tremolite + dolomite + H20 + C02 = 2 talc + 3 calcite. Changes in mole fraction o f C02 and fluid pressure do not cause a large change in the temperature at which this reaction occurs. Within the range of con ditions for which Slaughter, Kerrick and Wall pre sented data, the reaction takes place at temperatures between approximately 410 and 470C. A second possible reaction is: tremolite + 4 C02 = 2 dolomite + talc + 4 quartz. In the vicinity of the Yellowstone mine (GR-3), lava or block talc is found in addition to the typical waxy green talc (ceramic grade). A comparison of the chemical compositions of block talc and typical talc from the Yellowstone mine shows no significant difference between the two (Table 5). James (1956, p. 5) stated that most of the block talc at the Yellow stone mine is in a zone of deep weathering. Examina tion of other occurrences of block talc in this area also shows that block talc is underlain by waxy green talc at depth, suggesting that the block talc has formed by weathering of the usual waxy green talc. The formation of block or lava talc would be an inter esting problem for further study. Table 5 --Analyses o f ceram ic and lava talc from the Yellowstone mine (GR-3) Ifrom Jam es, 1956, p. 5). Ceramic grade (typical taic) from pit 5 Ceramic grade (typical talc) from pit 7 *"ava Qrade (block taic) from pit 9 SiO, 61.70 61.29 61.76 FerO, 1.29 1.36 1.33 AliOi 1.55 1.34 1.51 CaO trace trace trace MgO 32.44 31.26 31.95 Na2Q 0.37 0.26 0.27 KjO 0.46 0.33 0.32 i :^ t* i .r. v.i GUNTER00000832 14 Time of formation Because o f the close association of talc and chlorite at many localities, these two minerals are thought to have formed at the same time. It is sug gested that the talc and chlorite were produced dur ing greenschist-facies metamorphism caused by a retrograde event tentatively dated as having oc curred 1600 m.y. ago. This is not an original idea. Other workers also have suggested that talc in the Ruby Range formed by retrograde metamorphism during the Precambrian (Ganhan, 1973b; Olson, 1976, p. 113; Okuma, 1971, p. 42). The work de scribed here adds further support to this hypothesis. All of the known talc deposits in southwestern Montana are in dolomitic marble of the pre-Belt se quence of metamorphic rocks. Although there are dolomitic formations of Cambrian, Ordovician and Devonian age in the area, there are no reported oc currences of talc n these rocks. For this reason a Precambrian age is inferred for the talc deposits. Before discussing further the time of talc for mation it is necessary to review the metamorphic history of the pre-Belt rocks in this region. Scattered occurrences of granulite-facies rocks in the Ruby Range, Tobacco Root Mountains and Greenhorn Range have been interpreted as relicts of an earlier metamorphic event. Most of the mineral assem blages preserved in these areas are indicative of upper amphibolite-facies metamorphism. Minerals indicative o f a later greenschist-facies metamorphic event are reported in the Tobacco Root Mountains and Ruby Range. In the Greenhorn Range, serializa tio n o f feldspars^, chloritization of biotite, serpentinization o f calcsilicate minerals, and altera tion o f ultramafic rocks to talc and serpentine are evidence for this event. Rb-Sr whole-rock age dates from the southern Tobacco Root Mountains indicate a minimum age of 2667 66 m.y. for the upper am phibolite metamorphism (Mueller and Cordua, 1976, p. 33). These authors suggested that dates of ap proximately 1600 m,y. obtained by Giletti (1966) on biotite, muscovite and K-feldspar by K-Ar and Rb-Sr methods represent the later greenschist metamor phic event. It is reasonable to conclude that a greenschist assemblage was produced during the waning stages of metamorphism and that with the introduction of water along fractures the alteration to hydrous minerals could go to completion locally. Talc and chlorite are thought to be retrograde rather than prograde minerals because if the talc and chlorite that we see now were formed during pro grade metamorphism, they would have been con verted to a higher-temperature assemblage o f miner als during the granulite- and amphibolite-grade meta morphism. Also both talc and chlorite replaced higher temperature minerals; talc replaced tremolite and chlorite replaced biotite. Sericite replaced sillimanite in pelitic schist next to some talc deposits. In addition to the dates clustered around 1600 m.y. (1410 to 1790 m.y.), Giletti (1966) also reported dates of 2130 to 3270 m.y. A northeast-trending boundary separates the region of the 1600 m.y. dates to the northwest from the much older dates to the southeast, and all of the talc occurrences lie within the area of 1600 m.y. dates except those in the Henrys Lake Mountains. Talc occurrences in the Henrys Lake Mountains are related to the metagabbro dikes and are of different origin than the replace ment bodies being described here, which have no ig neous association. The necessary dolomitic marble host rock is exposed in the area southwest of the "date" boundary. This correlation between 1600 m.y. dates and talc occurrences further supports the hypothesis that talc mineralization occurred about 1600 m.y. ago. An alternative explanation for the 1600 m.y. dates is that the dated minerals were partly reset by the heating effect of the Boulder and Tobacco Root batholiths farther to the northwest, both of which were emplaced between 68 and 78 m.y. ago (Tilling, Klepper and Obradovich, 1968). If those plutons part ly reset Precambrian dates over this large area, then t is reasonable to think that talc and chlorite were formed at the same time. There are tw o major objec tions to this idea. The first is that if the talc and chlorite were formed in pre-Belt rocks during late Cretaceous time, the same minerals should also have been formed in Paleozoic dolomite in this area. With the'exception of the Helena deposit, talc and chlorite deposits in Montana are confined to Precambrian rocks. The second objection is that it is unlikely for a Cretaceous event to partly reset Precambrian dates over a large area to within the relatively small range of 1410 to 1790 m.y. Stratigraphic and structural control Talc is confined to dolomitic marble units, which are more abundant than calcitic marble in the preBeit sequence of metamorphic rocks. Heinrich and Rabbitt (1960, p. 20) reported that dolomitic marble is about twice as abundant as calcitic marble in the Ruby Range. Both Garihan (1973a, p. 183-186) and Okuma (1971, p. 95) suggested that specific marble layers were particularly susceptible to replacement by talc. For instance, in the Ruby Range many of the talc occurrences are in one unit, the Regal marble. GUNTER00000833 All of those who have described talc deposits in the Ruby Range state that there is a tendency for talc to occur at the hinge lines of folds or where the mar ble has been tightly folded. The same tendency is noted for some of the talc occurrences in the Green horn Range, where minor occurrences o f talc are numerous within one layer of marble that has been strongly deformed. Faults also exerted a control on the localization of talc in the Ruby Range, according to Garihan |1973b) and Okuma (1971, p. 96-98). A map by Okuma (1971, p. 97) particularly points out a spatial relationship between northwest-trending faults and talc occurrences in the southern Ruby Range. Faults are evident in all talc prospects and mines where the talc is well exposed. Displacement of talc along these faults indicates that some of the movement occurred after formation of the talc. Whether these faults pre ceded and controlled talc formation or whether all movement occurred after talc deposition is more difficult to establish. The replacement of dolomite by talc will result in an appreciable reduction of volume if the prod ucts calcite and C 02 have been removed from the system, as is the case in Montana talc deposits. If the assumptions are made that no magnesium was intro duced by the talc-forming solutions and that all of the Si02 and H20 were introduced, the final volume 15 of talc would be 71 percent of the initial volume of dolomite. If all of the S i02 required for this reaction was present in the marble initially, then the final vol ume of talc would be 48 percent of the volume of the starting material. The actual case lies somewhere be tween these two extremes, and because o f the purity of most dolomitic marble, probably closer to the ex ample in which all the SiOz was introduced. The sur rounding marble may have adjusted to a volume de crease by flowage and by recrystallization rn the vi cinity of small talc pods. Where large masses of mar ble have been replaced by talc, the decrease in vol ume may have caused local faulting. Many of the faults now exposed in talc mines may have devel oped in response to talc formation, and thus those faults did not control talc formation. There is also another possibility, namely, that faults in the talc are o f Laramide age and were con centrated in the talc because of its mechanical weak ness as compared to the surrounding marble or gneiss. Probably the faults seen` in talc mines are of all three types. Talc-forming solutions followed faults, shear zones, or minor fractures in the marble, a volume decrease caused post-talc movement along those same surfaces, and they were also sites of Laramide movement. Because of good exposures in talc mines, the abundance of faults in these talc bod ies may be overemphasized as compared to the abundance in surrounding metamorphic rocks. Exploration for talc The most direct way to find talc is to examin marble outcrops for talc veins and pods. The most promising areas are seemingly those where the mar ble is strongly folded or where faults are abundant. The recognition of areas in which the marble is highly deformed may be difficult without good exposures and the presence of distinct lithologic layering in the marble. Talc prospecting in southwestern Montana has continued fo r many years, and there are few tale bearing outcrops that do not show signs o f having been recognized by the prospector. During field work in the Greenhorn Range, oniy one talc-rich outcrop was discovered that did not show signs of previous discovery, and that outcrop was in thick timber. Talc is more stable chemically at surface condi tions than most minerals, so there is a tendency for it to be concentrated in the soil. Many talc occurrences bave been discovered by recognizing chips of talc in the soil. By careful examination of loose soil, talc chips only a few millimeters across can be identified, although a surface coating of iron and" manganese minerals on both talc and quartz granules weathered from the marble can make the visual identification of these two minerals difficult. The easiest way to iden tify the grains is to try to streak them on a piece of steel. Because of its softness talc will easily leave a white streak on the steel, whereas quartz, because of its greater hardness, will not leave a distinct streak. In heavily timbered areas where the soil is thick, especially on north-facing slopes, prospecting by looking for talc in the soil is not very effective. Expo sures on these slopes are partly hidden by timber and are not very abundant. Because of the difficulty of finding talc in these areas, the alternative possibility of examining stream sediments for talc was con sidered, Although soft and easily reduced to fine grained sediment by abrasion, talc should persist In stream sediments because of its chemical stability. Three samples of stream sediment were collected from Jasmine Creek (NE% SE14 sec. 10, T. 8 S., R. 4 W .) in the Greenhorn Range at a point less than 1 mile downstream from localities'where talc chips had been found in the soil. Although talc could not be identified by x-ray diffraction analysis of the < 325 fc'TTjW o i f oh i.rZXaZ;* .i ly, GUNTER00000834 16 mesh (< 44 im) size fraction of these samples, talc was positively identified in the < 10 un fraction by Alice Blount of the Newark Museum, New Jersey. Blount also identified talc in these samples by in frared spectroscopy. A sample o f stream sediment was collected from Harris Creek in the southern Tobacco Root Moun tains approximately 1 mile downstream from two talc prospects (TR-6 and TR-7). Talc could only be tenta tively identified by x-ray diffraction analysis of the < 10 iim size fraction of this sample and was not iden tified by infrared spectroscopy. Perhaps the analysis of stream sediment for talc would be a useful method of talc exploration in areas of thick timber cover, but obviously further testing of the method is required. For example, the optimum size fraction for talc analyses should be determined. This will of course depend on the size distribution of the sediment, but it is likely that talc will be concen trated in a fine fraction such as < 2 im. In the future, more sophisticated methods will be used in the exploration for talc deposits that are not exposed. Geophysical and geochemical methods have been suggested by some as worth considera tion in the search for talc. For orderly description, occurrences of talc and chlorite are grouped by mountain range. The occur rences are shown in Figure 3. Information on the occurrences is summarized in Table 6, Dl ----- -------ID1MILE q 10 KILOMETERS X ftoipect ot occutrenot jneUmorphictoeks Figure 3 --Talc and chlorite occurrences in southw estern M ontana. GUNTER00000835 if r mum ined. 3n of icen- * will t are hods Jera- and :cur- the No. H-1 TR-1 TR-2 TR-3 TR-4 TR-5 TR-6 TR-7 TR-8 TR-9 TR-10 R-1 R-2 R-3 R-4 n*6 n-e R-7 R-8 R-9 R-10 R-11 R-12 R-13 R-14 17 Table 6 --Talc and chlorite occurrences. Except as notad. occurrences are plotted on Figure 3. Name Major mineral Extent of development Comments Page reference in this memoir Additional references Golden Antler mine Chlorite Highland Mountains Active mine Production of chlorite began in 1977. p. 19 Berg, 1979, p. 266. Tobacco Root Mountains Mineral Hill pegmatite Spuhter Gulch occurrence Latest Out mine Tale Talc Talc(?l Horse Creek prospect Chlorite Bivens Creek prospect Talc Harris Creek prospect Grandview prospect Talc Taic Granite Creek prospect Talc Bear claims (Granite Creek mine) Prospect southwest of Ennis Talc Talc and chlorite Short adit Unknown Inactive preciousmetal mine Caved adit Little published information. Talcoee rock contains Serpentine, diopside, graphite and other minerals. Occurrence of talc has not been con firmed. Small body of chlorite exposed. Inclined shaft and Cuts Large cut Adit and shallow cuts inclined shaft and cuts Inactive mine Talc pods over a large area, minor graphite. Concordant talc layer 4 ft. (1.3 m) thick. Talc pods exposed over a large area- Concordant layer of talc 2 ft. (0.7 m) thick. Numerous talc pods in two large cuts. Cuts Talc disseminated throughout marble. p. 21 p. 22 Reid. 1957, p. 7. 23. Reid, 1957, p. 23. p. 22 p. 22 p. 22 p. 24 p. 26 Levandowski, 1956, p. 278-288. Levandowski, 1956, p. 223. Levandowski, 1956, p. 222-224. p. 26 p. 26 ` p. 30 Ruby Peak occurrence Talc Spring Creek prospect Talc Gem claim Talc Whitney claims Talc Prospect southwest of Whitney daims Prospect north of Treasure mine Prospect northeast of Treasure mine Bennett Owen daim Talc Talc Talc Talc Treasure mine Talc Beaverhead mine Talc Prospect mat of Beaver head mine Regal (Keystone) mine Talc Talc American Chemet mine Talc Estelle (Sweetwater) mine Talc Ruby Range Shallow prospect Talc chips in soil over a large area. Minor pit tremolite in one specimen. Prospect cuts and Talc occurrences extend for 1.5 km 15,000 drill holes ft.) along strike. Much variation in color. Two cuts Graphitic talc. Cuts Irregular body of generally light green talc. Cuts Talc bodies in zone 20 m (64 ft.) wide. Cuts Cut Main body of talc is 2 to 3 m (7 to 10 ft.) thick. Talc body of 3 m (10 ft.) exposed width. Cuts Active mine Dark-green talc body 5 by 35 m 116 by 112 ft.). Major talc mine owned by Pfizer, Incorporated. Active mine Major talc mine owned by Cyprus Indus trial Minerals. Cut Pods of talc exposed in cut. Active mine Inactive mine Inactive mine Increased production in recent years. Mine owned by Pfizer, Incorporated. Talc was mined from three pits by Ameri can Chemet Corporation. Concordant layers of green talc exposed in open cut. p. 32 p. 32 P- 32 p. 33 p. 33 p. 33 p. 33 p. 33 P. 34 p. 34 p. 35 p. 36 p . 35 p. 36 Garihan, 1973a, p. 177 180; Olson, 1976, p. 129-130. Garihan, 1973a, p. ISO181; Olson, 1976, p. 129. Garihan, 1973a, p. 171 174; Olson, 1976, p. 130. Garihan, 1973a, p. 174 175; Oteon, 1976, p. 130. Garihan, 1973a, p. 169 171. Garihan, 1973a, p. 166 168; Olson, 1976, p. 129. Garihan, 1973a, p. 162 166; Olson, 1976, p. 128. Garihan, 1973a, p. 149 156; Olson, 1976, p., 121-125. Garihan. 1973a, p. 156ISO; Olson, 1976, p. 125-128. Garihan, 1973a, p. 1GO162; Olson, 1976, p. 129. Olson, 1976, p. 126; Perry, 1948, p. 6. Okuma, 1971, p. 108 m. Okuma, 1971, p. 106 107; Olson, 1976, p. 128. s I Lt 6%- t * * ,} i! \ [ ! GUNTER00000836 18 Table 6 --Talc and chlorite occurrences. Except as noted, occurrences are plotted on Rgure 3. (continued} Major Extent of No. Name mineral development Comments Page reference in this memoir Additional references Ruby Range (continued) R-15 Smith-Dillon mine Talc Inactive mine Tate was first mined underground and p. 36 then in an open pit. R-16 R-17 R 18 Banning-Jones mine Talc Bozo-Zobo mine Talc Crescent prospect Talc (Timber Gulch deposit) Inactive mine Inactive mine Shallow inclined shaft, cuts Talc lenses in marble scattered over an area 250 by 300 ft, (75 by 90 m). In 1960s about 8,000 tons of talc ore mined. Graphite abundant in talc. p. 36 p. 37 p. 37 R-19 R-20 Sauerbier mine Owen-McGovem prospect Talc and chlorite Talc Inactive mine Cuts end dril holes Talc mined by Resource Processors, In corporated, in 1974, Talc layers 1 to 2 ft. 10.3 to 0.6 m) thick are exposed in cut. p. 37 p. 38 Note: Eighteen additional talc occurrences in the Ruby Range are listed in Table 7. Greenhorn Range GH-28 Ruby claims Chlorite GH-30 GH-41 Doubtful claim Greenhorn daims Talc Talc GH-42 GH-43 GH 45 Willow Creek mine Claims north of Willow Creek South of Virginia City Talc and chlorite Talc Talc GH-46 Calverts claims Talc Shallow cuts and small pit Cuts Cuts Active mine Six cuts One cut Five cuts Mainly tan chlorite, minor talc. Unusually soft talc, some limontte. Sheared and contorted talcose marble exposed in cut. Openpit mine operated by Resource Pro cessors, Incorporated. Talc veinlets and pods exposed in four cuts. Minor talc in marble adjacent to quartz vein. Talc veinlets and pods exposed in four cuts. Gravelly Ranga GR-1 GR-2 GR-3 GR-4 GR-5 GR6 Talt Mountain claims Cherry Gulch prospect Yellowstone mine Queen daiqn * T '| Burlington Northern mire Talc-bearing conglom erate Chlorite and talc Talc Talc Talc Talc Talc Shaft and cuts Cuta Active mine Inactive mine Inactive mine None Light-colored chlorite and minor talc are ' exposed in cuts. Some block talc exposed in cuts. Major talc mine owned by Cyprus Indus trial Minerals. Small open pit owned by Cyprus Indus trial Minerals. Talc is exposed in several cuts. Source oi talc in conglomerate inferred to be deposit at Yellowstone mine. HL-1 HL-2 HL-3 HL-4 Occurrence Occurrence Occurrence Occurrence Henrys Lake Mountains , Talc No development Minor occurrence, no economic potential. Talc No development Minor occurrence, no economic potential. Talc No development Minor occurrence, no economic potential. Talc No development Minor occurrence, no economic potential. Other Montana talc occurrences 0-1 Talc mine south of Helena* Talc 0-2 Lynx Creek (Mathews) Talc tate prospect* Inactive mine Reported bodies of talc 6 ft. (2 m) thick. and prospect cuts Drill holes Seridtic talc in shades of yellow and gray. ` Not plotted on Figure 3. p. 40 p. 42 p. 42 p. 43 p. 46 p. 46 p. 46 p. 43 p. 50 p. 53 p. 65 p. 55 p. 66 p. 58 p. 58 p. 58 p. 58 p, 59 p. 59 Okuma. 1971, p.99-102; Olson, 1976, p. 128; Perry, 1948, p. 4-6. Geach, 1972, p. 161-162; Olson, 1976, p. 127. Olson, 1976, p. 127-128. Okuma, 1971, p. 105; Olson, 1976, p. 129; Parry, 1948, p. 8. Okuma, 1971, Plate 1; Olson, 1976, p. 126-127. Okumit, 1971, p. 107 108, Plate 1; Olson, 1976, p. 129. James, 1956. Perry, 1948, p. 10-11. Johns, 1970, p. 152-153. GUNTER00000837 r^tl-; tH JfW O roaS b 19 Highland Mountains Pre-Belt metamorphic rocks are exposed only in the southern part o f the Highland Mountains. The northern part of the range is underlain by plutons o f the Boulder batholith, sedimentary rocks of the Beit Supergroup and Paleozoic and Mesozoic sedimen tary rocks. The pre-Belt rocks have been intruded by two plutons o f the Boulder batholith, which have been dated in the range of 72 to 77 m.y. by the K-Ar method (Robinson, Klepper and Obradovich, 1968, p. 5641. Duncan (1976) mapped the pre-Belt metamor phic rocks in the Highland Mountains during his structural study and was able Jto distinguish three major lithologic units: quartzofeldspathic gneiss, garnetiferous gneiss and micaceous gneiss. Thin layers (maximum thickness 2 m) o f .amphibolite, magnetite gneiss, anthophyllite gneiss and calcitic marble are found only in the garnetiferous gneiss. Duncan (1976, p. 26-27) reported that the marble occurs at only a few localities, is no thicker than 2 m, and has a maximum exposed strike length of approximately 10 m. In addition to calcite, the marble contains tremolite-actinolite, phlogopite, plagioclase, diopside, apatite and quartz. No talc occurrences have been reported in the pre-Belt rocks of the Highland Mountains. The chlo rite veins at the Golden Antler mine are in quartzo feldspathic gneiss. H-1 Golden Antler mine Location: S W 'A sec. 14, T. 2 S., R. 6 W ., Madison County. Approximately 2.5 miles (4 km) south west of Silver Star. Twin Bridges 15-minute quadrangle. Accessibility: Newly constructed road follows an in direct course 1.1 miles (1.8 km) from Montana Highway 41 to the claims. Ownership: Golden Antler claims located by Robert Nolte and Sylvan Donegan, both of Twin Bridges, Montana Description: Chlorite veins crosscut pre-Belt biotitequartz-feldspar gneiss, a layer o f amphibolite, 8nd another of metagabbro (Sheet 1-A). Foliation f the gneiss strikes approximately west and dips to the north. The chlorite veins parallel welldeveloped near-vertical joints that strike north to a few degrees east of north. Some joint surfaces are coated with epidote, The purest chlorite is pale green on a fresh surface and some breaks into thin plates a few centimeters thick, which are translucent on thin edges. Chlorite veins are eroded more rapidly than the unaltered gneiss and can be found by the abundance o f small chips of chlorite a centimeter or tw o across in the soil. The chlorite veins are broken by perpendicular fractures spaced less than 1 centimeter apart. Rare veins of milky quartz are the only impurity recognized in the chlorite in the field. The only impurities recognized by micro scopic examination o f the chlorite are zircon in a trace concentration { 1 percent) and rutile, also in trace concentration, in a few specimens. This mineral must be a relief from the biotite-quartzfeldspar gneiss, and remained unaltered during the chloritization of the gneiss. * Chlorite clearly was produced by the altera tion o f the biotite-quartz-feldspar gneiss and prob ably also by the alteration of amphibolite and metagabbro (Berg, 1979, p. 266). Photomicro graphs (Piate 2) show a typical sequence of altera tion from gneiss to chlorite. The specimens photo graphed in this sequence were collected along a traverse 1.5 meters long parallel to the foliation of the gneiss and extending a few centimeters into the chlorite vein. Most of the chlorite veins are sur rounded by a quartz-sericite-chlorite zone, which is recognized in the field by its lighter color (white to very pale green) as compared to the darker green of pure chlorite and also by its tendency to break into larger fragments than the pure chlo rite. Propylitic alteration o f the quartzofeldspathic gneiss was observed adjacent to the quartzsericite-chlorite zone. There is no obvious genetic relationship be tween the gold-bearing veins of the Silver Star dis trict and the chlorite veins. No sulfide minerals or their alteration products have been recognized in the chlorite deposit, and only a minor amount of sericite and lesser chlorite are reported from the metalliferous veins of the district (Fritzsche, 1935, p. 60). Metalliferous veins strike west to northwest as compared to the north strike of the chlorite veins, and the closest metalliferous vein to the chlorite deposit is 2,000 ft. (610 m) west at the Golden Rod mine. This gold-bearing vein within pre-Belt gneiss strikes N. 85 W. and dips 45 SW. (Sahinen, 1939, p. 49). The proximity of the chlorite deposit to tw o plutons of the Boulder batholith suggests that the r GUNTER00000838 20 r* A Slight sericitic alteration of feldspars, minor epldote B. M ore intense alteration. Serlcite. epidote, and chlorite much more abundant than m A. C. Quartz surrounded by fine-grained chlorite. D - Fine-grained chlorite. Plate 2 Photomicrographs of alteration sequence at the Golden A ntler mine. GUNTER00000839 replacement of gneiss by chlorite may have been caused by hot water, possibly meteoric water heated by one of those piutons. The Hell Canyon pluton is exposed 1.5 miles (2.4 km] southwest of the chlorite deposit, and the Rader Creek pluton, also of the Boulder batholith, is exposed 2 miles {3.2 km) northeast of the deposit. A possible source of the magnesium required for the replace ment of quartzofeldspathic gneiss by magnesian chlorite is the Jefferson Limestone {Upper Devo nian). The Jefferson Limestone, which is known to contain dolomitic beds, is exposed about 3 km northeast of the Golden Antler mine. Ground water passing through the Jefferson Limestone 21 could have acquired Mg as well as Ca by the solu tion of dolomite and calcite. Convective circulation of this heated water along fractures in the quartzo feldspathic gneiss could have produced the local replacement of gneiss by chlorite. From field rela tionships it can be concluded only that the chlorite is younger than the youngest Precambrian metamorphic event that affected the gneiss. Besides the chlorite veins at the Golden Antler, there are other chlorite occurrences in the vicinity. According to Bob Noite (oral communica tion, 1977), one of these is situated less than 1 mile (1,6 km) northwest of the Golden Antler mine. Tobacco Root Mountains The Tobacco Root Mountains are an uplifted block of pre-Belt metamorphic rocks that have been intruded by the Tobacco Root batholith. The batho lith is of quartz monzonite composition, and biotite from this body has been dated by the K-Ar method to be 72 m.y., whereas hornblende dated by the same method gives an age o f 118 m.y, (McDowell, 1971, p. 9). Paleozoic and Mesozoic sedimentary rocks are exposed on the north and west flanks o f the moun tain range. The pre-Belt metamorphic rocks extend south into the Greenhorn Range and east into the Madison Range. Because the Indiana University Geologic Field Station is in the northern part of the Tobacco Root Mountains, this area has been more thoroughly stud ied than most other areas of pre-Belt rocks in Mon tana. Some sources of information on the geology of the range, done by Indiana students and others, are: Burger (1967), Cordua (1973), Gillmeister (1972), Hanley (1975), Hess (1967), Johns (1961), Koehler (1976), Levandowski (1956), Reidt1957, 1963), and Tansley, Schafer and Hart (1933). The pre-Belt rocks of the Tobacco Root Mounlatns are similar to those exposed in other mountain ranges in southwestern Montana. Rocks described from the Tobacco Root Mountains include quartzo feldspathic gneiss, hornblende gneiss, amphibolite, marble, aluminous schist, quartzite, iron formation, anthophyllite (gedrite) gneiss and metamorphosed nriafic and ultramafic intrusive rocks. Diabase dikes of recambrian age are much more abundant in the obacco Root Mountains than in the Greenhorn ar*ge to the south. There are also numerous Pre Cambrian pegmatite dikes in the Tobacco Root Mountains. Evidence has been presented by some authors (Cordua, 1973; Reid, 1963) for three episodes of Precambrian metamorphism. Metamorphism of granulite grade was followed by metamorphism of amphibolite grade, which did not completely destroy the granulite assemblage. Alteration of these rocks is attributed to later greenschist-facies metamorphism. Although interpretations of the Precambrian struc tural history of the Tobacco Root Mountains differ, there is ample evidence for large isoclinal folds (Burger, 1967). Hanley (1975, p. 272) found that Precambrian rocks along the northwest-trending Mammoth fault have been displaced more than Paleozoic formations, thus indicating Precambrian movement on the fault, which is a major structure in the northern part of the range. Most of the talc occurrences in the Tobacco Root Mountains are in the southern part of the range, which is an area of relatively moderate relief.' Most of the metai mining has been farther north in the northeastern and western parts of the range. TR-1 Mineral Hill prospect Location: W 4 sec. 26, T. 1 S., R. 3 W ., Madison County. Approximately 8 miles {13 km) northwest of Harrison. Harrison 15-minute quadrangle. Accessibility: The Carmichael Canyon road, which goes between the South Boulder River road and Harrison, is within 1 mile of the prospect. Ownership: Not known. Description: Reid (1957, p. 7, 23) described a body of almost pure talc at the west end of the Mineral ! . 3n3 l S 3 !s :: ernvi > ::C ."J z I GUNTER00000840 22 Hill pegmatite. Although Reid did not mention the size of the talc body, he reported that a small adit had been driven into the talc. The Mineral Hill pegmatite was described by Reid as a mixture of pegmatitic material, gneiss, amphibolite, biotite schist and serpentine-pyroxenite layers. This talc prospect was not visited during the present investigation. The Gilliam vermiculite deposit is less than 1 mile (1.6 km) south of the talc prospect. TR-2 Spuhler Gulch occurrence Location: NE% sec. 21, T. 3 S., R. 4 W., Madison County. Approximately 11 miles (18 km) northeast of Twin Bridges. Waterloo 15-minute quadrangle. Accessibility: The closest road is the Wisconsin Creek road that passes approximately 1.5 miles , (2.4 km) west and many feet lower than the talc occurrence. Ownership: Not known. Description: This deposit was not visited during the present investigation, and all of the informa tion on this deposit is from Reid (1957, p. 23). The talc body, which is described as bluish-gray talcgraphite rock, is exposed on the south wall of Spuhler Gulch. The talc layer is 40 feet (12 m) thick and perhaps 1,000 to 1,500 feet (305 to 460 m) in length. The estimated mineralogical composition of one specimen is 40 percent talc, 25 percent ser pentine (antigorite), 25 percent diopsidic augite, 5 percent magnetite, 5 percent graphite and a trace of spinel (pleonaste). TR-3 Latest Out mine* Location: Sec. 32, T. 4 S., R. 4 W ., Madison County. Copper Mountain 7 'A-minute quadrangle. Approximately 4 miles (6 km) southeast of Sheridan. Accessibility: The road to the mine branches to the north at the Horse Creek road in sec. 5, T. 4 S., R. 4 W. Ownership: Not known. Description: All of the following information is from Levandowski (1956, p. 278-288). This mine produced a small amount of gold and silver, and in 1956 it was reported flooded to within 30 feet (9 m) o f the collar of the shaft. It is mentioned here only because several samples were described as talc on an assay sheet (Levandowski, 1956, p. 286 287). The samples may be fault gouge mistakenly identified as talc. Because marble, the host rock for talc deposits, is present in the biotite schist at the mine, the occurrence of talc is a reasonable possibility. TR-4 Horse Creek prospect Location: SE1/ SW14 N W K sec. 5, T. 5 S., R. 4 W , Madison County. Sheridan 714-minute quad rangle. Approximately 4 miles (6.5 km) southeast of Sheridan. Accessibility: The prospect is visible from the Horse Creek road. Ownership: Not known. Description: This prospect was explored by an adit, now caved, in biotite-quartz-feldspar schist (fig. 4), Malachite occurs in some of the schist and on pieces of vein quartz found at the adit. Fine grained material from the malachite-bearing schist was identified by x-ray diffraction analysis as a mixture of sericite and kaolinite, probably formed by alteration o f feldspar. Some pale-green partly altered sillimanite occurs in the biotlte-quartzfeldspar schist. f The only talc recognized is in a layer of talcose marble 2 feet (0.7 m) thick exposed just west of the biotite-quartz-feldspar schist. Chlorite is much more abundant at this prospect, but is poorly exposed where it occurs within the marble east of the schist. A sample of this green rock consists mainly of chlorite but contains minor quartz and sericite and a trace of rutile. The rutile forms feathery clus ters o f needles within the chlorite. A trace o f clinozoisite occurs in the chlorite. > j [ The presence of rutile in the chlorite and the suggestion o f a relict ophttic texture observable in thin section indicate that the chlorite may have formed by alteration o f a basic dike within the mar ble, similar to the chlorite occurrence at the Ruby claims (GH-28). 77T-5 Bivens Creek prospect Location: N>4 NEV4 sec. 14, and S 1/4 SE'A sec. 11, T. 5 S., R. 4 W ., Madison County. Copper Moun tain 7 'h -minute quadrangle. Approximately 8 miles (13 km) southeast of Sheridan. Accessibility: The Bivens Creek road to Copper Mountain passes within 1,500 feet (500 m) of the prospect. A road that goes directly to the prospect branches from the Bivens Creek road at the west boundary of sec. 14, T. 5 S., R. 4 W . Ownership: James G. McLaughlin, Sheridan, Mon tana. GUNTER00000841 23 3* K{rO'-CT~'H-lJ?+i;;-: ^2. it; ; ; <3 M Figure 4--Horse Creek prospect. Tobacco Root Mountains (R. B. Berg, October 1976). I! iij GUNTER00000842 24 Description: This prospect has been explored by a shallow inclined shaft 20 feet (7 m) deep, now partly caved, and also by many cuts (Sheet 1-B), Coarse-grained, white dolomitic marble strikes northwest and is in contact with sillimanite schist to the northeast and garnetiferous amphibolite to the southwest. The marble is thickest in the vicin ity o f the shaft, where it has an inferred thickness of 135 feet (45 m), and It can be traced for a dis tance o f 1,000 feet (330 m) along the strike. Judg ing from the talc piled near the shaft, the greatest concentration of talc is in the shaft and in the shal low cuts 75 feet (25 m) southeast of the shaft. Minor green chlorite is associated with the talc where the chlorite has formed by alteration of the sillimanite schist and hornblende gneiss. Graphite occurs in some of the talc, and talc pseudomorphs after tremolite blades 1 to 2 cm long were recog ' nlzed in several exposures. The talc is white, pale green and light gray. Minor talc is exposed in the cuts just northwest of the shaft, but the tw o northwesternmost cuts contain more talc, and much milky white quartz is found in this vicinity. Alteration of the sillimanite schist has produced green chlorite. In most exposures relatively pure waxy green chlorite is confined to shear surfaces within the schist. Partial alteration of sillimanite to sericite can be recognized in most cuts, where this alteration changed the typical white sillimanite needles in the schist to pale green needles. Biotite in the sillimanite schist also altered to chlorite. A shallow prospect pit 1,200 feet (400 m) south of the shaft exposes a small area of chloritic altera tion o f hornblende gneiss. The area o f chlorite is 3 feet by 3 feet (1 m by 1 m). The geology of this part of the southern Tobac co Root Mountains was described by Cordua (1973), and Levandowski (1956) described the Bivens Creek prospect, which he incorrectly re ported as being situated In sec. 13, T. 5 S., R. 4 W. At the time of Levandowski's work, evidently talc was exposed only in the northernmost cut and in the inclined shaft. TR-6 Harris Creek prospect Location: SW% sec. 13, T. 5 S., R. 4 W ., Madison County. Copper Mountain 7 'A-minute quadrangle. Six miles (10 km) northeast o f Alder. Accessibility: The Harris Creek road branches from the California Creek road in the S 1/4 sec. 23, T. 5 S., R. 4 W. The major cut is at the end of this road, approximately 1.2 miles (1.9 km) from its junction with the California Creek road. The other pros pects on Harris Creek are within 2,000 feet (650 m) of this cut. Ownership: James E. Katz, Sheridan, Montana. Description: The largest body of talc exposed in these prospects is exposed in a cut along the northwest side of Harris Creek (fig . 5). Layering of the metamorphic units strikes northeast and dips northwest. The exposed sequence from southeast to northwest is garnet-biotite-quartzfeldspar gneiss, hornblende gneiss, biotite schist, dolomitic marble and talc. A fault at the top of the talc layer separates it from the overlying sequence of interlayered marble and biotite-quartz-feldspar gneiss. The talc layer can be traced for 160 feet (49 m) in the cut and is 4 feet (1.3 m) thick at its thickest point. The overlying impure marble has been altered to a rock that consists of chlorite and lesser quartz and graphite and traces of zircon and rutile. The most distinctive feature o f the talc at this cut is.the abundance of talc pseudomorphs after tremolite, which are as much as 1 cm long. Talc has completely replaced the trem olite.that was formed in the marble during earlier metamor phism. , Talc also is exposed in a shallow prospect cut at an altitude of 6,180 feet (2,026 m) at a bearing of N. 52 E. from the cut just described. The small amount of talc, which is poorly exposed, is typi cally gray, although some is pale green. A chrysottle veinlet, which has a maximum thickness of 2 cm, cuts the dolomitic marble. Talc is exposed in a small pit approximately 400 feet (130 m) southeast from this cut. The attitude of layering is N. 53 E., 45 NW. A conformable talc layer 4 feet (1.3 m) thick is in contact with an overlying chlorite layer 6 inches (15 cm) thick. The chlorite grades into biotite-quartz-feldspar gneiss to the west and un doubtedly has been produced by alteration of the gneiss. Malachite coats some fractures in the talc. r* ] Another talc occurrence is exposed by shallow scraping (fig. 6) situated 900 feet (300 m) N. 68 E. from the large cut on Harris Creek first described in this section. Dolomitic marble and biotite-quartzfeldspar gneiss are exposed. Some of the biotite quartz-feldspar gneiss is chloritized. Small talc pods in the dolomitic marble are poorly exposed where the bedrock has been partly exposed by scraping. The marble layer could not be traced to the north of this prospect but was traced south to the Grandview prospect. GUNTER00000843 Amphibolite and hornblende gneiss float 25 Limonite-stained and leached marble along fault Hornblende gneiss and schist in bottom of cut E X P LA N A T IO N I / / 'x -I Garnet-biotite-quartz-feldspar gneiss Hornblende gneiss \ Biotite schist Dolomitic marble m | Talc I. Attitude o f foliation -------------Contact - dashed where inferred ------ --- Fault - dashed where inferred Impure marble interlayered with biotite-quartz-feldspar gneiss Dolomitic marble To California Creek road Diagrammatic cross section A - A r Fig u re 5 -- C u t n o rth w e s t o f H arris C re ek. T o b a cco R oo t M o u n ta in s (R. B, B erg, O c to b e r 1976). TO--"cfTl. '. cC->- i:3 1 3 , E X P L A N A T IO N c(0 *k. 11 ! 1jl B iotite-quartz-feldspar gneiss L*-->--I (lo ca lly garnetiferous) E sa> D o lo m itic marble CL C hloritic alteration Small talc pods poorly exposed A ttitu d e of foliation i ~ - Fold axis ----------Contact (dashed where inferred) Figure 6 - Prospect southeast of Harris Creek, Tobacco Root Mountains (R. B. Berg, October 1976). ! GUNTER00000844 26 77?-7 Grandview prospect Location: SE1/* S W A sec. 13, T. 5S ., R. 4 W ., Mad ison County. Copper Mountain 7 A-minute quad rangle. Six miles (10 km) northeast of Alder. Accessibility: The prospect is 6.5 miles (10 km) up the California Creek road from Montana Highway 287. Ownership: George Davies, Butte, Montana. Description: The prospect has been explored by four trenches perpendicular to the strike of the marble, by many small pits, and by an adit downhill to the northeast (Sheet 1-C). The adit is now partly caved where it intersects brecciated talc along a fault. Talc also is exposed in a shallow cut northwest to ward Harris Creek. It is unlikely that any surface shows of talc on this prospect have not been in vestigated by scraping away the soil. Interlayered biotite-quartz-feldspar gneiss and dolomitic marble, w ith northwest strike and near vertical attitude, are in oontact with amphibolite to the southwest. A steeply inclined fault, also of northwest strike, separates dolomitic marble and talc from biotite-quartz-feldspar gneiss. This fault and the brecciated talc along its west side are the cause of caving in the adit. The greatest concen tration of talc at this prospect is in the marble just west, o f the fault. Because the fault seems to be only a short structure limited to the area of talc mineralization, and because post-talc movement on it is obvious, it seems likely that the talc layer in fluenced the location of the fault rather than that the fault controlled the formation of talc. The talc is white except for some material piled near the mouth of the adit, which is stained with limonite along fractures. Chlorite from the pros pect contains trace concentrations of zircon, apa tite and rutile. This prospect provides a good opportunity to study the relationship between talc and chlorite. The talc, with the exception o f the large body west of the fault, forms small pods a few feet (1 m) across within the dolomitic marble. The chlorite occurs in the gneiss and is commonly adjacent to talc pods in the marble, thus illustrating the effect of the composition of the host rock in determin ing whether talc or chlorite forms. In the gneiss, where sufficient aluminum was available, chlorite formed; but in the marble, where aluminum was not available in sufficient concentration, talc formed. It is likely that the excess silica left from the alteration of gneiss to chlorite contributed to ^ the formation of the talc in the marble. j TR-8 Granite Creek prospect Location: S W A sec. 3, T. 6 S., R. 3 W ., Madison County. Virginia City 15-minute quadrangle. Three miles (5 km) northwest of Virginia City. \ r' [ j Accessibility: A ranch road from Granite Creek goes past the prospect. Ownership: W. D. Conklin, Alder, Montana. j t Description: The main occurrence of talc is in the S W A SW'A sec. 3, where a small amount has been mined from a shallow inclined shaft (fig. 7). Shallow pits near the shaft expose talcose marble and talc pods within the marble. Two other pros pect pits were excavated in altered pegmatite along a jasper vein. The feldspar in the pegmatite has been altered to a fine-grained mixture of quartz, clinozoisite, and chlorite. Blades of tremolite in marble show evidence of having been partly replaced by talc. X-ray diffraction analysis of a specimen of talc failed to show the presence of im purities. Cordua (1973) showed that the marble layer exposed at this prospect extends S. 70 W. from Granite Creek for 3,500 feet (1,150 m) to a point where it is covered by Tertiary volcanic rocks. Both talc and chlorite are exposed in a prospect cut along the road in the N W A S W A sec. 3, T. 6 S., R. 3 W . (fig. 8), but slumping in the cut has obscured the relationship between the talc layer and chlorite, A 2-foot (0.7-m) layer o f talc contain ing minor graphite is exposed in the cut. X-ray dif fraction analysis of the chlorite shows that it con tains a small amount of talc. A pale-green rock from the same cut consists of serpentine and minor quartz and contains a trace o f calcite. Talc and chlorite are also exposed in a small pit along the road north of the cut described above. The talc occurs in a marble layer or pod less than 10 feet (3 m) thick, which is surrounded by quartzofeldspathic gneiss. The talc layer is less than 3 feet (1 m) thick. An adit 20 feet (7 m) long is situated across a small gulch south of the talc. No talc was seen in the adit, only yellowish-green serpentine. !. TR-9 Bear claims (Granite Creek mine) Location: The northern cut is in the S W A NE A sec. 25, T. 5 S., R. 3 W ., Madison County, and the southern cut is in the NEA NEA S W 'A of the same section. Virginia City 15-minute quadrangle. Approximately 6 miles (10 km) northeast of Vir ginia City. GUNTER00000845 inTalcose marble exposed pit limonite pseudomorphs after pyrrte 5mm across Talc pods exposed in , shallow cut; 70 ft. , (21 m l N .20 E. j Talc layer I ft. (0 3ml thick minor graphite exposed in shallow pit 0 /65 White, coarse-grained dolomitic marbie w ith coarse-grained talc exposed in pit 27 J./WV yi Figura 7--Southern Granite Creak prospect. Tobacco Root M ountains {R. B. Berg, Septem ber 1976). S 50 E Tan, coarse-grained dolomitic marble Very white medium-grained talc with minor graphite N 50 W z feet Road E X P L A N A T IO N Talc Chlorite Figure 8 --Northern Granite Creek prospect, Tobacco Root Mountains (R. B. Berg, September 1976). ^ GUNTER00000846 28 Accessibility: The road to the mine branches from the Granite Creek road 4.4 miles {7 km) from the junction of the Granite Creek road with Highway 34. The northern cut is 1.1 miles (1.8 km) by road from the Granite Creek road. Ownership: Albert Kingrey, Virginia City, Montana. Description: The claims have been developed by two cuts on the southeast slope of a ridge, which is parallel to the strike o f the foliation of the metamorphic rocks. The cuts are 1,500 feet (450 m) apart. Talc has been mined from the cuts, and more recently (1973) some exploratory drilling has been done in the southern cut. The claims are within the area mapped by Cordua (1973). The largest talc body exposed in the northern cut is a layer 17 feet (5.6 m) long and 2.2 feet (0.7 m) in vertical dimension, which is surrounded by dolomitic marble (fig . 9). Although the tale is generally fine grained, a few flakes are 3 to 5 mm across. Pyrite pyritohedrons in some of the talc have been replaced by limonite. Loose blocks of a rock consisting of both talc and chlorite are the only other recognized occurrences of talc in the cut. Biotite-quartz-feldspar gneiss and pegmatitic pods within the gneiss have been partly altered to green chlorite. Both talc and chlorite are exposed in the south ern cut. The talc has been formed by alteration of marble, and the chlorite by alteration of biotitequartz-feldspar gneiss. The greatest concentration of talc is at the northern part of the cut, where two concordant layers of talc are exposed (fig. 10)* One layer is 2 feet (0.7 m) thick and the other is at least 1 foot (0.3 m) thick, but its true thickness is obscured by slumped material. Talcose marble is exposed near the north end of the cut and also near the south end, where a high-angle fault sep arates it from gneiss. A talc pod 1 by 2 feet (0.3 by 0.7 m) is exposed at the southern end of the cut. In the central part of the cut a near-vertical vein of green chlorite 6 feet (2 m) thick cuts strongly chloritized quartzofeidspathic gneiss. X-ray diffraction analysis of a specimen of the chlorite shows the only impurity to be a trace of quartz. -i ( f t i i i A thin section of the quartzofeidspathic gneiss shows that the biotite has been altered to chlorite and the feldspar to sericite. Apatite and zircon are trace constituents. A t some localities this altered quartzofeidspathic gneiss grades into a rock that in a hand specimen is judged to consist entirely of chlorite. Exposures along the road between the tw o cuts are mainly in biotite-quartz-feldspar gneiss; minor talcose marble is also exposed. Partly chloritized blotite qnartz feldspar gneiss I Figure 9 --Northern cut et Bear claims. Tobacco Root M ountains (fl. B, Berg, Septem ber 197G). GUNTER00000847 uthm of >titeition two 10), ts at ss is le is also sep3 by cut. n of hlo tion the t v , eiss rite are red it in f of :uts nor ) 29 Figure 10--Southern cut a t Bear cfaims. Tobacco Root M ountains [R, B. Berg, Septem ber 1976) I GUNTER00000848 30 TR-10 Talc prospect southwest o f Ennis ' Location: SE'A NE% sec. 9, T. 6 S., R. 2 W ,, Madi son County. Virginia City 15-minute quadrangle. Approximately 6 miles (10 km) southwest of Ennis. Accessibility: The prospect is 0.5 mile (0.8 km) north of Montana Highway 34. Ownership: Mr. Schulz of Sheridan, Montana. Description: Talc is disseminated or in small pods within a layer of coarse-grained dolomitic marble (fig. 11), which is of variable attitude and has an average thickness of 120 feet (36 m). Cordua (1973) traced the marble layer for 3 miles (5 km) to the north. Several concentrations of float of quartz-chlorite rock within the marble layer are in terpreted to be small pegmatite lenses in which the feldspar has been altered to chlorite. Pink micro cline and quartz in one specimen are surrounded by a fine-grained matrix of chlorite. The biotite-muscovite-quartz-feldspar schist southeast of the marble layer locally contains sillimanite and garnet. Biotite altered to chlorite whereas feldspar altered to sericite and possibly chlorite in the schist exposed in the southern part of this prospect. A green chloritic rock at this pros pect consists mainly of chlorite but contains minor sericite, and a trace of quartz was detected by x-ray diffraction analysis. A specimen of talc con tains, in addition to talc, a minor concentration of quartz and a trace o f chlorite. EXPLANATIO N j Hornblend gneiss aiul am phibolite p>-----Predom inantly coarse-grained [ s ' ), d o lo m itic marble, m inor calcitic marble i- r ru --I B otite muscovite-quartz- feldspa ' ^ 1 schist ! - Disseminated talc or talc float ' v : Disseminated chlorite Attitude o f foliation -------- Contact; dashed where inferred Gradational contact \ Downhill Q uartz veins and f . pods in this area 7f SECTION IQ Figure 11 --Prospect southw est o f Ennis, Tobacco Root M ountains (R. B. Berg, Septem ber 1976). GUNTER00000849 31 Ruby Range Talc occurrences are more numerous in the western part of the Ruby Range than in any other area in southwestern Montana. Besides the many talc prospects and several inactive mines, there are three operating mines in this range, the Beaverhead,. Treasure and Regal mines. Although the geology and talc occurrences in the Ruby Range have been well described by Olson (1976, p. 115-133), they are sum marized here fo r completeness. A dissertation by Okuma (1971) on the geology and structure of the southwestern part of the Ruby Range, and a similar dissertation by Garihan (1973a) on the central part of the range, are the other main sources of information on the Precambrian geology and talc deposits of this area. Talc occurrences in a small area in the southern part of the Ruby Range are described in a thesis by Whitehead (1979). In addition, Heinrich and Rabbitt (1960) described the geology o f the southwestern part of the range, and James, Wier and Shaw (1969) mapped the geology of the Christensen Ranch 716minute quadrangle and surrounding area in the southern part o f the range. A detailed map of the Carter Creek "iron deposit, which lies within the Christensen Ranch quadrangle, has been published by James and W ier (1972). The geology of the north ern part of the Ruby Range has been mapped by Tysdal (1976), who concentrated his efforts on the Paleozoic, Mesozoic and Cenozoic formations and did not map individual rock types in the sequence of pre-Belt rocks. Larry Karasevich, a graduate student at Pennsylvania State University, is now (1978) work ing on the pre-Belt metamorphic rocks of the north ern part of the range. Th Ruby Range is a northeast-trending uplifted block, principally o f pre-Belt metamorphic rocks. With the exception o f a small "bridge" of metamor phic rocks connecting the Ruby Range with the Greenhorn Range to the east, the Ruby Range is sur rounded by intermontane basins partly filled with Tertiary sediments. Paleozoic and younger sedimen tary rocks are exposed only in the northern part of the Ruby Range, where they generally flank the Pre Cambrian core. The pre-Belt metamorphic rocks of the Ruby Range have been divided into three major categories, Cherry Creek-type rocks, Dillon Granite Gneiss, and Pre-Cherry Creek rocks. The Cherry Creek-type rocks are the most significant group for this study because the doiomitic marble layers that are the host rock for all of the talc deposits occur in the Cherry Creek-type rocks. These rocks, which are exposed along the northwestern flank of the range, are truncated to the northwest by the range-front fault. The general strike of layering is northeast, roughly parallel to the north west flank o f the range. Numerous steeply inclined faults that strike northwest cut across the pre-Belt metamorphic rocks. Marble, quartzite, calc-silicate rock, sillimanite schist, chlorite schist, actinolite schist, corundum schist, muscovite schist, biotite schist, amphibolite, hornblende gneiss, magnetite-bearing iron formation and anthophyllite gneiss have been recognized within the sequence of Cherry Creek-type rocks. The marble layers range in thickness from a few tens o f meters to 400 meters. One layer of marble, informally designated the Regal marble, can be traced more than 16 km (10 mi.) from a point south of Carter Creek north to Spring Creek. The Dillon Granite Gneiss crops out in the mid dle of the Ruby Range and separates Cherry Creek type rocks to the northwest from [3re-Cherry Creek rocks to the southeast. The Dillon Granite Gneiss consists mainly of quartzofeldspathic gneiss but con tains lesser pegmatite and aplite. Layers or stringers of the Dillon Granite Gneiss are present in both the Cherry Creek-type rocks and the pre-Cherry Creek rocks. Unlike the Cherry Creek-type rocks, which are for the most part clearly metasedimentary, the Dillon Granite Gneiss has a less obvious precursor. Some workers have concluded that this mass of quartzofeldspathrc gneiss was produced by metamorphism of a synkinematic batholith emplaced during the Pre cambrian (Heinrich and Rabbitt, 1960). On the other hand, Garihan and Okuma (1974) cited evidence that the Dillon Granite Gneiss could have been formed by isochemical metamorphism of arkosic rocks. Their most convincing argument for a sedimentary precur sor is the presence of thin layers of marble well within the Dillon Granite Gneiss and traceable for 6 km (3.6 mi.). They also noted that compositional variation within the Dillon Granite Gneiss is more nearly com patible with a sedimentary than an igneous origin. The pre-Cherry Creek rocks, which lie southeast of the Dillon Granite Gneiss, have generally been judged to be older than the Cherry Creek-type rocks because they lie stratigraphically below them. Units described in the pre-Cherry Creek sequence are biotite-quartz-feldspar gneiss, hornblende gneiss, amphibolite, sillimanite gneiss and chlorite schist. Ultramafic bodies and diabase dikes thought to be of Precambrian age also occur within rocks belonging to the pre-Cherry Creek, Cherry Creek and Dillon Granite Gneiss of the Ruby Range. The metamorphic history of the pre-Belt rocks of the Ruby Range is similar to that of the meta morphic rocks exposed in the other mountain ranges 1 .3 ry r> .4 $ I GUNTER00000850 32 of southwestern Montana. The rocks were subjected to multiple periods o f Precambrian deformation, which produced isoclinal folds and a mineral assem blage o f the amphibolite facies. Local granulite facies assemblages may be relicts of an earlier metamorphic event. Evidence of retrograde greenschist facies metamorphism is widespread in the Ruby Range. Most of the descriptions of talc occurrences and mines in the Ruby Range are summarized from the publications o f Okuma (1971), Garihan (1973a), or Olson (1976). Although most of the prospects and mines were visited during the early stages of this project, they are not mapped or described in detail, because of the above earlier investigations. The sources of information are included in the description of each deposit. R~1 Ruby Peak occurrence Location: W s e c . 16, T. 6 S., R. 5 W ., Madison County. Laurin Canyon 7!4-minute quadrangle. Approximately 6 miles (10 km) west o f Alder. Accessibility: A private road up Hinch Creek goes within 0.5 mile (0.8 km) of the summit of Ruby Peak, within 0.2 mile of the southernmost talc occurrence. Ownership: State school section. Description: This occurrence of talc was found by Larry Karasevich during his mapping of the pre Belt rocks of the northern Ruby Range in the sum mer o f 1977. Tysdal (1976) had previously mapped the Paleozoic, Mesozoic and Cenozoic rocks of this part of the range. Both dolomitic marble and calcitic marble are well exposed over a large area surrounding Ruby Peak. Scattered chips of palegreen to green talc can be picked up along the ridges that extend southwest and northeast from Ruby Peak. Although talc chips are scattered over a large area, no concentration of talc was found here. Some talc found southwest of Ruby Peak is poorly exposed in a small prospect pit in the small saddle just east of the point where the southwest-trending ridge crosses the western edge of section 16. An old discovery post shows that prospectors may have located a claim here without realizing that they were on a state section; and in order to mine they would have to lease the mineral rights. Tremolite grains, most less than 1 mm in length, were observed in a thin section of one specimen of talc, but there is no evidence that the talc replaced the tremolite. R-2 Spring Creek prospect Location: SEVi sec. 32...SWJ4 sec. 33, T. 6 S., R. 6 W., and NEV4 sec. 5, T. 7 S., R. 6 W ,, Madison County. Beaverhead Rock SE 7V4-minute quadrangle. Approximately 14 miles (23 km) northeast of Dillon. j ^ j f v Accessibility: The prospect can be reached by a road that goes up Spring Creek. Ownership: Not known. Description: The talc deposit is in the Regal marble on the northwest limb o f a synform. Talc occurrences in this marble can be traced 1.5 km (5,000 ft.) (northeast) to the point where the marble is overlain by Paleozoic formations. A body of talc 3 to 7 meters (10 to 22 ft.) wide is exposed for 20 meters (64 ft.) in one of the cuts. Talc from thisdeposit shows an unusually wide variation in color, including white, green, pink, purple and yellow varieties. Limonite, gypsum and graphite are re ported to occur in the talc. Other minerals in the marble or very impure talc are serpentinet?), chlorite, chrysotile, tremolite, diopside, rutile, scapolite, phlogopite and garnet. The Spring Creek prospect is unusual in its long strike length, which led Olson to conclude that it is worthy of further exploration. I ! i \ | j ) j j I i j Sources of information: Garihan, 1973a, p. 177-180; Olson, 1976, p. 129-130. N ! R-3 Gem claim * Location: SE% sec. 34, T. 6 S., R. 6 W ., Madison County. Approximately 16 miles (26 km) northeast of Dillon. Beaverhead Rock SE 7 Vi-minute quadrangle. f Accessibility: The road along Spring Creek goes within 1 mile of the claim. _ Ownership: Pfizer, Incorporated. j f Description: This deposit is in the Regal marble, which is exposed on the southeast limb of a north east-trending synform. The Spring Creek prospect (R-2) is on the northwest limb of the same struc ture. A zone of micaceous, graphitic talc 2 to 3 meters (6 to 9 f t . ) wide is exposed in the lower cut. Lenses o f graphitic talc only a few centimeters across are exposed in the upper cut. Garihan noted that a pegmatite dike that cuts the talcose marble has been partly altered to chlorite, presumably dur ing the same event that produced talc in the enclosing dolomitic marble. [ ^ i Sources of information: Garihan, 1973a, p. 180-181; Olson, 1976, p. 129. GUNTER00000851 Whitney claims Location: SW14 sec. 2, T. 7 S., R. 6 W ., Madison County. Mine Gulch 7%-minute quadrange. Ap proximately 16 miles (26 km) northeast of Dillon. Accessibility: A road to the claims branches o ff to the north from the road along the Left Fork of Stone Creek in the NW% sec. 14, T. 7 S., R. 6 W. The claims are 1.6 miles (2.5 km) by road from the road along the Left Fork of Stone Creek. Ownership: Pfizer, Incorporated. Description: Garihan described a talc body exposed in bulldozer cuts as being digitated, locally discor dant to layering in the enclosing marble, and vari able in thickness- Faulting has extensively frac tured the talc. Limonite and pyrfte are abundant in some of the talc. Sources o f information: Garihan, 1973a, p. 171-'174; Olson, 1976, p. 130. R-5 Prospect southwest o f Whitney claims Location: SE14 sec. 3, T. 7 S., R. 6 W ., Madison County. Mine Gulch 7y2-minute quadrangle. Ap proximately 16 miles (26 km) northeast of Dillon. Accessibility: The prospect can be reached from the road to the Whitney claims, which branches to the north from the road along the Left Fork of Stone Creek in the NW% sec. 14, T. 7 S., R. 6 W. The distance by road from the Left Fork of Stone Creek to the prospect is 1.6 miles (2.5 km). Ownership: Not known. Description: Cuts at this locality expose talc in the same marble layer as that exposed on the Whitney claims. Garihan reported that somewhat graphitic talc occurs along fractures in the dolomitic marble and also noted the presence of a body o f dark-blue and light-green talc. In another cut irregular talc bodies form a zone reported to be 20 meters (65 ft.) wide. Sources o f information: Garihan, 1973a, p. 174-175; Olson, 1976, p. 130. R-6 Prospect north of Treasure mine Location: SW% sec. 11, T. 7 S., R. 6 W ., Madison County, Mine Gulch 754-minute quadrangle. Ap proximately 16 miles (26 km) east of Dillon. Accessibility: The road to the Whitney claims, which branches from the road along the Left Fork of Stone Creek in the NW 14 sec. 14, T. 7 S., R. 6 W., passes within 0.25 mile (0.4 km) of the prospect. Ownership: Not known. 33 Description: The main body of talc, which is exposed in two bulldozer cuts, is 2 to 3 meters (7 to 10 ft.) thick and is underlain by a layer o f dark-green gra phitic talc 2 meters (7 ft.) thick. The talc is con cordant to layering in the enclosing marble and can be traced fo r 25 meters (82 ft.) along strike. In addition there are discordant stringers, layers and small lenses of talc in the surrounding mar ble. Garihan identified talc, graphite, chlorite, ser pentine and gypsum in the enclosing dolomitic marble. Source of information: Garihan, 1973a, p. 169-171. R-7 Prospect northeast o f Treasure mine Location: SE'A sec. 11, T. 7 S., R. 6 W ., Madison County. Mine Gulch 714-minute quadrangle. A p proximately 17 miles (28 km) northeast of Dillon. Accessibility: A road that branches from the Left Fork of Stone Creek in the NW !4 sec. 13, T. 7 S., R. 6 W., leads directly to the prospect. Ownership: Not known. Description: A talc body approximately 3 meters (10 ft.) wide and traceable for 10 meters (32 ft.) is ex posed in a cut. Graphite and limonite discolor the talc. Garihan mentioned that gypsum is associated with some smaller talc pods in the marble. Sources of information: Garihan, 1973a, p. 166-169; Olson, 1976, p. 129. R-8 Bennett Owen claim Location: NW14 sec. 12, T. 7 S., R. 6 W ., Madison County. Mine Gulch 714-minute quadrangle. A p proximately 17 miles (28 km) northeast o f Dillon* Accessibility: The closest road to the prospect is a road along Cottonwood Creek, which continues to within a mile of the prospect. Ownership: Not known. Description: The talc body has an outcrop o f approx imately 5 by 35 meters (16 by 112 ft.) and is dark green. Garihan described subrounded masses of talc recognized in thin section, which may be pseudomorphs after serpentine. Chlorite was also recognized within the talc. Serpentine has re placed olivine and pyroxene in the adjacent marble. Sources o f information: Garihan, 1973a, p. 162-166; Olson, 1976, p. 128. Z\9> 1; GUNTER00000852 34 R-9 Treasure mine Location: N'/2 sec. 14, T. 7 S., R. 6 W., Madison County. Mine Gulch 714-minute quadrangle. Ap proximately 16 miles (26 km) east of Dillon. Accessibility: The haul road for this mine is along the Left Fork of Stone Creek. Ownership: Pfizer, Incorporated. Description: Mining of this deposit began at the openpit Treasure State mine, which was closed when a larger body o f talc to the east was mined at the Treasure Chest mine, also an openpit mine. More recently the Treasure State and Treasure Chest ore bodies have been mined from one large pit now known as the Treasure mine, which is an important producer of high-quality talc. Talc mined here is hauled to Pfizer's mill at Barretts siding 8 miles (13 km) south of Dillon, where it is sorted, pulverized and bagged for shipment. The ore body at the Treasure mine is a tabular body o f talc formed by the almost complete replacement of dolomitic marble. Only a few blocks of unreplaced dolomitic marble remain within the talc body. The west-striking layer of talc is cut by numerous high-angle faults that trend northwest. Movement on all of these faults is such that the southwest block has moved up relative to the northeast block. One such fault, known as the Treasure Fault, separates the Treasure Chest ore body to the east from the Treasure State ore body to the west. Seemingly the same talc layer forms both ore bodies but has been offset along the Treasure Fault.- , The Treasure Chest ore body ranges In thickness from 30 to 50 meters (96 to 160 ft.), is 360 meters (1,188 ft.) long, and the dip ranges between 45 and 65 N. The Treasure State ore body is re ported to range from 20 to 30 meters (66 to 98 ft.) in width where exposed in the pit. It is more than 100 meters (330 ft.) long and dips approxiimately 45 N. The hanging wall at the Treasure mine is Dillon Granite Gneiss, and the footwall is garnet-biotite schist. Both the gneiss and schist have been ex tensively altered, the biotite being altered to chlo rite, the plagioclase to white mica. The altered footwall schist presents a problem in slope stability when the overlying steeply inclined layer of talc is removed. Olson reported that in the mid-1960s the waste-to-ore ratio at this mine was 3:1 or 4:1 and that by 1976 it had increased to something on the order of 15:1. Garihan identified graphite, limonite, chlorite, gypsum!?), apatite and rutile in talc at this mine. Although interesting mineralogically, those minerals do not occur in sufficient concentration to significantly affect the quality of the talc. Sources of information: Garihan, 1973a, p. 149-156; Olson, 1976, p. 121-125. ' ( f | { r' ' ' R-10 Beaverhead mine j Location: SE1A sec. 14, T. 7 S., R. 6 W ., Madison County. Mine Gulch 714-minute quadrangle. Approximately 16 miles (26 km) east of Dillon. | , I Accessibility: The haul road for this mine goes up Cottonwood Creek after branching from the Ruby River road. | Ownership: Cyprus Industrial Minerals. Description: The Beaverhead mine is situated just across a ridge southeast of the Treasure mine, and the talc body at the Beaverhead mine may be in the same layer of marble as the ore body at the Treasure mine. The strike of the dolomitic marble at the Beaverhead mine is roughly west and the dip is 35 to 70 N. The footwall schist at the Treasure mine is similar to the schist exposed in the hanging wall at the Beaverhead mine, suggesting the pos sibility that the deposits are on opposite limbs of an isoclinal fold. The footwall at the Beaverhead mine is dolomitic marble, which is underlain by amphibolite. The best talc at the Beaverhead mine is in the upper part of the ore body close to the contact with the overlying biotite schist, which has been severely altered. f r ? , i The talc body at the Beaverhead mine is at least 800 feet (244 m) long and is offset by relatively minor faults. Ol$on and Garihan suggested that the talc body pinches out along strike rather than being displaced by faults: The horizontal width of the talc body ranges between 25 and 100 feet (8 and 31 m) where exposed in the mine. Blocks of unreplaced dolomitic marble are numerous within the talc body. The Beaverhead mine has increased substan tially in size in recent years as the result of a major stripping program in 1974 and 1975 and additional stripping in 1976. Because the ore body dips into the hillside, much overburden must be removed as the mine is deepened. Talc from the Beaverhead mine is hauled to Alder, 25 miles (40 km) to the northeast, where it is \ ti j GUNTER00000853 washed and sorted before either being shipped di rectly or being hauled to Three Forks for pulver izing and bagging. Sources of information: Garihan, 1973a, p. 156-160; Olson, 1976, p. 125-126. R-11 Prospect east o f Beaverhead mine Location: NW14 sec. 13, T. 7 S .t R. 6 W ., Madison County. Mine Gulch 7 %-minute quadrangle. Ap proximately 17 miles (28 km) east of Dillon. Accessibility; This prospect is several hundred feet north of the haul road to the Beaverhead mine and is easily visible from the road. Ownership: Not known. Description: Layers and irregular masses of talc are exposed in a cut in dolomitic marble along strike to the east o f the marble at the Beaverhead mine. The thickest body of talc is approximately 1 meter (3 ft.) thick and dips 50 to 60 N. Garihan noted the occurrence of small pods o f talc along bedding planes, which are still recognizable in the marble, suggesting that some layers in the marble were more easily permeated by talc-forming aqueous solutions o r alternatively were of such composition that they were more susceptible to replacement by talc. Planes of rhombohedral cleavage are still recognizable in some of the coarse-grained rhom bohedral carbonate (dolomite?) that has been replaced by talc. Most of the talc exposed in this cut is dark green or grayish green. Sources of information: Garihan, 1973a, p. 160-162; Olson, 1976, p. 129. R-12 Regal (Keystone) mine Location: N14 sec. 2, T, 8 S., R. 7 W ., Madison County. Christensen Ranch 7 ,/4-minute quad rangle. Approximately 11 miles (17 km) southeast of Dillon. Accessibility: The mine is adjacent to the Sweet water road. Ownership: Pfizer, Incorporated. Description: The Regal mine, originally known as the Keystone mine, was first developed by a shaft 60 feet (18 m) deep with more than 300 feet (92 m) of drifts at the bottom. The underground workings have long been inaccessible. More recently, talc has been mined from an openpit 450 feet (138 m) long, 50 to 100 feet (15 to 31 m) wide, and 20 to 30 feet (6 to 9 m) deep. 35 The talc ore lies within a layer of dolomitic mar ble close to the core of a tightly refolded synform. The strike of the marble is N. 45 E. to S. 80 E. and the dip is 30 to 75 N. The talc zone is exposed for more than 750 feet (229 m) along strike and is more than 300 feet (92 m) wide. Because o f the lack of soil cover, the talc zone can be traced be yond the limits of the pit. The zone consists of talc lenses in a dolomitic marble unit, which is cut by numerous faults. Micaceous quartz schist forms the footwall of the deposit; the hanging wall is partly altered coarse-grained dolomitic marble. A zone of brecciated talc 1 to 2 feet (0.3 to 0.6 m) thick separates the main mass of ore from the hanging wall marble. A near vertical diabase dike 60 to 100 feet (18 to 31 m) thick trending at right angles to the strike of the marble is exposed at the western end of the area of major talc concentration. Marble adjacent to the dike has been altered to, a fine-grained rock that consists of talc and serpentine. The talc deposit at the Regal mine, although of good size, has not been mined extensively because of the dark color of the talc, at least part of which is due to limonite. Sources of information: Okuma, 1971, p. 102-104; Olson, 1976, p. 126; Perry, 1948, p. 6. R -13 American Chemet mine Location: SE'A sec. 1, NE14 sec. 12, T. 8 S ,, R. 7W ., Madison County. Christensen Ranch 7 VS-minute quadrangle. Approximately 12 miles (20 km) south east of Dillon. Accessibility: A road that branches to the north from the Sweetwater road in the N 'A sec. 13, T. 8 S., R. 7 W ., leads directly to the mine, a distance of 1.3 miles (2.1 km). Ownership: Not known. Description: This mine, now inactive, was operated by American Chemet Corporation, East Helena, Montana. Development consists of three pits, from which talc was mined, and several bulldozer cuts. The mine is situated in a complexly deformed layer of marble that in the vicinity of the mine strikes northeast and dips 45 to 75 NW. Although the marble is generally surrounded by Dillon Gran ite Gneiss, there are layers of amphibolite south west of the mine. A northwest-trending metagabbro dike cuts the marble between two of the pits. Although Okuma suggested that some of the talc-like rock exposed in the workings may have been produced by the alteration of an ultramafic rock, most of the talc is in the marble. This dark- GUNTER00000854 36 greenish-gray rock contains chlorite, quartz and minor phlogopite in a fine-grained matrix of either white mica, or talc. Sources o f information: James, Wier and Shaw, 1969; Okuma, 1971, p. 108-111; Olson, 1976, p. 127. R -14 Estelle (Sweetwater) mine Location: EV4 sec. 13, NEW sec. 24, T. 8 S., R. 7 W., Madison County. Christensen Ranch 7 'h -minute quadrangle. Approximately 13 miles (21 km) south east o f Dillon. Accessibility: The main pit is situated on the north east side of the Sweetwater road. Prospect pits southwest o f the Sweetwater road can be reached by taking a road that branches from the Sweet water road in the SEW sec. 13, T. 8 S., R. 7 W. The prospects are 0.5 mile (0.8 km) southwest of the Sweetwater road. Ownership: Pfizer, Incorporated. Description: The Estelle mine, now inactive, is an open pit just northeast of the Sweetwater road. Some bulldozer cuts and prospect pits have been dug farther southwest, in the area between Sweet water Creek and the Sweetwater road. The mine and prospect cuts are in a layer of dotomitic marble of unusually unrfom thickness and attitude, which is enclosed by Dillon Granite Gneiss. The marble layer is approximately 400 feet (122 m) thick, strikes northeast, and dips 50 NW. at the Sweet water mine. Abundant shear zones subparallel to the marble layer are prominent in the pit. The talc generally forms concordant layers and pods w ith in the dolomitic marble. Most of the talc is various shades of green dr'gray. Sources of information: James, Wier and Shaw, 1969; Okuma, 1971, p. 106-107; Olson, 1976, p. 128. R -15 Smith-Dillon mine Location: EW sec. 23, T. 8 S., R. 8 W ., Beaverhead County. Ashbough Canyon and Dillon East 7Wminute quadrangles. Approximately 8 miles (13 km) southeast of Dillon. Accessibility: The mine is adjacent to the road in Axes Canyon, which crosses private land and may have a locked gate. Ownership: Pfizer, Incorporated. Description: The Smith-Dillon mine, now inactive, originally was an underground mine, but more recently talc has been mined from an open pit. The underground mine consisted of 1,500 feet (465 m) of adits and drifts on the main haulage level 30 feet (9 m) above creek level and another 400 feet (124 m) of drifts on a level 60 feet (19 m) lower. Perry noted that the talc holds well in underground workings and that very little timbering was required in exploratory work. The talc is in a layer of dolomitic marble estimated to be 1,300 feet (400 m) thick. The marble strikes N. 30 to 40 E. and dips 60 to 70 NW. To the west of the mine the marble is covered with alluvium, and to the southeast the footwall of the deposit is a thin layer of amphibolite with many slickensided surfaces. Dillon Granite Gneiss is ex posed farther to the southeast- The lenticular ore body is approximately 750 feet (233 m) long and has a horizontal width of approximately 100 feet (31 m). Even within the talc zone, dolomitic marble veined by milky white quartz is more abundant than talc. Perry observed clay gouge and crushed zones within the talc zone where it was exposed in the underground work ings. Left-lateral displacement along a weststriking fault at the north end of the open pit has displaced the talc zone approximately 70 feet (22 m) to the west. Talc from the mine is light greenish gray to light bluish gray and only rarely contains graphite. Sources of information: Okuma, 1971, p. 99-102; Olson, 1976, p. 128; Perry, 1948, p. 4-6. R -16 BanningJones mine Location: SW% sec. 13, T. 8 S., R. 8 W ., Beaverhead County. Dillon East 7 1/4-minute quadrangle. Approximately 8 miles (13 km) southeast of Dillon. Accessibility: A road to the Banning-Jones mine branches from the Axes Canyon road near the center.of sec. 23, T. 8 S,, R. 8 W. The distance by road from Axes Canyon to the mine is approxi mately 1 mile (1.6 km). Ownership: State section. Description: In 1964 the property was leased to Wal lace Banning and Lester Jones of Dillon. Although some talc was mined from this deposit in the years since 1964, the property is now inactive. The mine is situated on both sides of a gully 1 mile (1.6 km) northwest of the Smith-Dillon mine, and the talc is in the same layer of dolomitic marble as at that mine. A sketch map by Geach illustrates well the relationship between marble and talc at the Banning-Jones mine. A talc zone approximately 40 feet (12 m) wide is exposed in a cut on the east side of the gully. On the basis of talc float and outcrops, the talc zone may be more than 100 feet (31 m) in width. The thickest layer o f talc within this f f ; r` f f i i f ! } ' f I f i I ) j' , j | > i | S : j1 j GUNTER00000855 zone is approximately 10 feet (3 m) wide. On the west side of the gully there are indications of three lenticular bodies of talc, the largest of which is probably 100 feet (31 m) wide and 200 feet (62 m) long. Talc from this deposit is gray green and of steatite grade. A granite dike 40 feet (12m) thick and trending a little north of east has intruded the marble just south of the talc deposit. Geach suggested that this dike was a source o f aqueous solutions that reacted with the doiomitic marble to form talc. Sources of information: Geach, 1972, p. 161-162; Olson, 1976, p. 127. R-17 Bozo-Zobo mine Location: NE'A sec. 19, T. 8 S .,,R . 7 W ., Beaverhead County. Dillon East 7 V2-minute quadrangle. Ap proximately 9 miles (15 km) southeast of Dillon. Accessibility: The road up Axes Canyon passes within 0.25 mile (0.4 km) of the mine. The Axes Canyon road crosses private land and may have a locked gate. Ownership: Not known. Description: The Bozo-Zobo mine is now Inactive but in the mid-1960s it is reported by Olson that Amer ican Chemet Corporation shipped 8,000 tons of ore. The talc was mined from an opencut 25 to 30 feet (8 to 9 m) wide and approximately 50 feet (16 m) deep. Only about 50 percent of the marble ex posed in the cut has been replaced by talc. Some of the talc is stained by manganese oxides to such an extent that it is not suitable for the usual mar kets. The talc zone is in a layer o f marble that strikes northeast and dips northwest. Source of information: Olson, 1976, p. 127-128. R-18 Crescent prospect f Timber Gulch deposit) Location: SW% sec. 1 ,T .9 S .,R .8 W ., Beaverhead County. Ashbough Canyon 7 Vi-minute quad rangle. Approximately 11 miles (18 km) southeast of Dillon. Accessibility: A road up Timber Creek on the Brown ranch passes within several hundred feet of the prospect. Ownership: Not known. Description: This prospect is the southernmost known occurrence of talc in the Ruby Range. Talc occurs in tw o layers o f marble 10 to 15 feet (3 to 5 m) thick separated by 150 feet (47 m) of micaceous gneiss, all of which is surrounded by Dillon Granite Gneiss. Metamorphic units follow the regional trend of northeast strike and northwest dip. A 37 shallow inclined adit and a few prospect excava tions prove the continuation of talc to a depth of 10 to 20 feet (3 to 7 m). Perry reported that inter mittent exposures of talc can be followed for 800 to 1,000 feet (248 to 310 m) along strike. An un usually high concentration of graphite in the talc probably has been the main deterrent to the devel opment of the deposit. Sources of information: Okuma, 1971, p. 105; Olson, 1976, p. 129; Perry, 1948, p. 6. R -19 Sauerbier mine Location; NW% sec. 25, T. 8 S.r R. 7 W ., Madison County. Elk Gulch 7/2-minute quadrangle. Ap proximately 13 miles (22 km) southeast of Dillon. Accessibility: A road to the Sauerbier mine branches from the Sweetwater road in the SW% sec. 20, T, 8 S., R. 6 W. The distance from the Sweetwater road to the mine is approximately 2.5 miles (4 km). Ownership: Karl L. Sauerbier,1Alder, Montana. Description: The Sauerbier mine, although now ' (1978) inactive, was operated by Resource Pro cessors, Incorporated, in 1974. More recently the property has been leased to Cyprus Industrial Minerals. Both the Sauerbier mine and the OwenMcGovern prospect lie within a body of marble that has a teardrop-shaped outline. The layering in the marble generally strikes northeast and dips 50 to 80 NW. The marble body is bordered on the southwest and northeast by major northwest trending faults, and it is surrounded by Dillon Granite Gneiss. The mine consists of a north-south cut 100feet (31 m) wide, 300 feet (93 m) long, and 50 feet (16 m) deep at the south end where it is deepest. Talc pods within doiomitic marble are most abundant in the southern half of the cut, where evidently most of the talc was mined. Num erous steeply Inclined faults are exposed in the pit. Chlorite layers within both the talc and the doio mitic marble can be recognized in the southern part of the pit. The thickest layer of chlorite, which is 10 to 20 feet (3 to 6 m) thick, is exposed on the west wall of the pit, and is perhaps a basic dike or layer of quartzofeldspathic gneiss that has been completely replaced by chlorite. A specimen, thought to be impure talc when it was collected in the field, can be recognized in thin section as thoroughly altered quartzofeldspathic gneiss. Biotite has been replaced by chlorite, and feldspar has been replaced by a mixture of sericite and chlorite. Garnet grains in the gneiss have been altered to sericite along fractures. X-ray diffraction analysis shows that the specimen contains minor talc in addition to chlorite and sericite. GUNTER00000856 38 Sources o f information: Okuma, 1971, Plate 1; Olson, 1976, p. 126-127. R-20 Owen-McGovem prospect Location: SEJ4 sec. 23, E'/ sec. 26, T. 8 S ., R ,7 W ., Madison County. Elk Gulch 7 1A-minute quad rangle. Approximately 13 miles (22 km) southeast of Dillon. Accessibility: The road to the Sauerbier mine, which branches from the Sweetwater road in the S W 'A sec. 20, T. 8 S., R. 6 W ,, goes within 0.25 mile (0.4 km) o f the Owen-McGovern prospect at a dis tance of approximately 2.5 miles (4 km) from the Sweetwater road. Ownership: Not known. Description: The Owen-McGovern prospect is in the same body of dolomitic marble as the Sauerbier mine to the east. The marble body is 4,000 feet (1,2 km) long; is teardrop shape in outline; is surrounded by Dillon Granite Gneiss; and is bor dered to the northeast and southwest by major northwest-trending faults. The marble strikes northeast and generally dips 50 to 80 NW. Peg matite and diabase dikes intrude the marble, and amphibolite is also exposed at this prospect. Ex ploration includes trenching and some drilling. Talc layers exposed in the trenches are 1 to 2 feet (0.3 to 0.6 m) thick. Olson (1976, p. 129) con cluded: There is no talc deposit in Montana known to the writer whose geological relationships are so difficult to decipher as this one. Sources of information: Okuma, 1971, p. 107-108, Plate 1; Olson, 1976, p. 129. ' Other talc occurrences Many other occurrences of talc in the Ruby Range, about which little information has been pub lished, are summarized In Table 7. Also Okuma (1971, p. 97) has plotted ten new talc occurrences on a map. Most of these talc occurrences have been de scribed by Olson and are referenced to his 1976 arti cle because it provides the most detailed locality information. I ! ! r r r f r T a b le 7 - A dd itio nal ta lc occurrences in th e R uby R ange not p lotted on Figure 3. Location SWK sec 17, T. 7 S., R. 6 W. SW K sac. 1B, T. 7 S,, R. 5 W. S W K sec. 11, T. 7 S., R. 6 W. SW K NW K sec. 2, T. 8 S., B.7W, S W ft S W ft c . 3, T. 8 s .f R.7W. S W ft SE ft sec. 5, T. Sr, > R.7W. SEK N W S sec. 8, T. 8 S., R. 7 W. Eft sec. 10 and N W ft sec. 11, T. 8 S . . R . 7 W . NE Vi sec. 15 and SW ft sac. 11, T. 8 S., R. 7 W. NEK sec. 21 and N W S sec. 22. T. 8 S,, R. 7 W. SW K N W K sec. 19, T. 8 S,, R. 7 W. NEft N W ft sec. 26, T. 8 S., R. B W. SEK SEK NEK sec. 26. T. 8 S., R. B W. SW K sec. 25, T. 8 S,, R. 8 W. NW K N W K sec. 36, T. 8 S,, R. B W . W K NEK sec. 35, T. 8 S., R. 8W. SEft SEft sac. 36, T. 8 5., n, b w . sec. 1 and 2, T. 9 S., R . 8 W . Description Lenses and layers of talc a few centimeters thick and less than 1 m long. Dark-green tele Boat, locally graphitic. Medium- to light-green talc float and some graphite. Light-colored talc float can be traced for 200 ft. (62 ml along strike over a width of 100 ft- (31 m). Light-green talc float can be traced for 600 ft. (186 m) along strike and has a maximum width of 82S ft. (101 m). No available information. No available information. Talc in outcrop and float over a total strike length of 7,000 ft. <2.2 km). Green talc float of sporadic distribution over a strike length of 4,000ft. (1.2 km). One talc zone extends for approximately 500 ft. (155 ml along strike. Zone of dark talc exposed for 1,350 ft. <419 m) along strike. No available Information. Green talc exposed for several hundred feet along strike, widths of 5 to 30 ft. <1.6 to 9 ml. Valley View prospect body of dark talc 150 fL (47 ml long by 90 ft. (28 ml wide. Dark-green talc exposed in bulldozer cuts. Light-green talc can be traced for approximately 900 ft. <290 m) along strike. Some talc layers 10 ft. <3 m) thick. No available Information. Zona of talcose rocks can be traced for 5,000 ft. (1.6 km) along etrike leastnortheast). Reference Garihan, 1973a, p. 181-162. Garihan, 1973a, p. 182. Garihan, I973a, p. 182. Olson, 1976, p. 131. Olson, 1976, p. 131. Henrich and Rabbrtt, 1960, pi. 2Henrich and Rabbrtt, 1960, pi. 2. Olson. 1976, p. 131. Olson, 1976, p. 131. Olson, 1976, p. 131. Olson, 1976, p. 130. Heinrich and Rsbbitt, 1960, pi. 2. Olson, 1976. p. 131. Olson, 1976, p. 131. Olson, 1976. p. 130. Olson. 1976, p. 130. Henrich and Rabbrtt, 1960, pi, 2. Olson, 1976, p. 131. i > > I i! GUNTER00000857 39 Greenhorn Range Because much time was devoted to studying the geology and talc occurrences of the Greenhorn Range, more detailed information is presented on the geology of this range than for other areas, where only talc and chlorite prospects were mapped. The information, which is only summarized here, will be covered in more detail in a forthcoming MBMG publi cation. The Greenhorn Range is structurally the north west limb of a large syncline, the axis of which plunges to the southwest. Pre-Belt metamorphic rocks in the Gravelly Range are on the eastern limb of the same structure but their outcrops are separated from those of the pre-Belt rocks in the Greenhorn Range by outcrops of Paleozoic and Mesozoic sedi mentary formations. Pre-Belt rocks extend west'from the Greenhorn Range in the vicinity of Ruby Dam to connect w ith the large area underlain by these rocks in the Ruby Range (fig. 3). The Precambrian rocks also extend north past the Virginia City district to the Tobacco Root Mountains. East of Virginia City, pre Belt rocks' are overlain by Eocene andesite-dacite porphyry (K-Ar age of 50 m.y.), which is in turn over lain by Oiigocene basalt (K-Ar age of 33 to 34 m.y.) as reported by Marvin, Wier, Mehnert and Merritt 11974). A rhyolite plug just east of Ruby Dam, dated as 45 m.y. by the K-Ar method, intruded pre-Belt metamorphic rocks (Marvin, Wier, Mehnert and Mer ritt, 1974). Tertiary sediments are exposed in the upper Ruby Valley, which separates the Greenhorn Range from the Ruby Range to the west. Monroe (1976) described the stratigraphy and depositional history of those sediments. The most abundant rock type in the Greenhorn Range is quartzofeldspathic gneiss, which is similar to the Precambrian quartzofeldspathic gneiss ex posed in other mountain ranges of southwestern Montana. Most o f the gneiss shown on Sheets 2, 3 as quartzofeldspathic gneiss is biotite-quart2-feldspar gneiss. Some hornblende-quartz-feldspar gneiss occurs within the quartzofeldspathic gneiss. The am phibolite assemblage contains hornblende gneiss, granulite and metagabbro in addition to the pre dominant amphibolite. Marble, mainly dolomitic, is the most abundant of the rocks that are clearly meta sedimentary. Other metasedimentary units are quartzite, anthophyllite gneiss and sillimanite schist. Several small ultramafic bodies, now partly serpentinized, are exposed in the Greenhorn Range. Postmetamorphic granite and pegmatite dikes are abun dant in the northern part o f the range. The Snowcrest fault extends west from the Gravelly Range into the Greenhorn Range, where pre-Belt rocks have been thrust over Paleozoic formations in the vicinity of the W illow Creek talc mine. Farther to the south pre-Belt meta morphic rocks have been thrust over Paleozoic rocks along the Greenhorn fault. The north end of the Greenhorn Range is partly bounded by a high angle fault, and a shear zone is well developed in quartzofeldspathic gneiss along the west flank of the range. Included in the following descriptions are all localities where talc was found, either as small frag ments in the soil or in outcrop. The numbered locali ties are shown on Sheets 2, 3, with the exception of GH-45 and GH-46, which lie outside the area covered by that map. Although there are a few scattered occurrences of talc north of Davey Creek, the greatest concentration is in the area south of Idaho Creek. Talc was found both in the outcrop and in the soil at several localities (numbers *12 to 20| within a marble layer exposed north o f the North Fork of Greenhorn Creek. There are also several occurrences o f talc within the large area underlain by marble at Dunegan Mountain. Dunegan Mountain is not named on the topographic map, but it is situated in sec. 14, T. 8 S., R. 4 W. The largest known concen tration of talc in the Greenhorn Range is the deposit at the W illow Creek mine (locality GH-42). Prospects and abandoned metal mines encountered during mapping are listed in a section following the list o f talc and chlorite occurrences. The following is a description of the occurrences, the first 27 o f which are described only briefly. West o f the Ruby River GH-1 SEVi SW!4 sec. 32, T. 6 S., R. 4 W. Minor talc in soil below exposure of calcrtic marble. GH-2 S W 1/ SWVi sec. 33, T. 6 S., R. 4 W . Talc pods (5 cm maximum length) in calcitic marble adjacent to shear zone approximately 1 m thick. Prospect tunnel in malachite-stained marble along shear zone. GH-3 NE1/ NW% sec. 4, T. 7 S., R .4W . Coarse grained silvery talc in one piece of dolomitic marble. GH-4 SEVi NEVi sec. 5, T. 7 S., R. 4 W . Minor coarse-grained silvery talc in marble. Also one piece of float found that contains minor green talc. GH-5 NEVi NE'/i sec. 8, T. 7 S., R. 4 W . Coarse grained silvery talc and chlorite in one piece of marble float, which also contains graphite. GUNTER00000858 40 East o f the Ruby River and north o f Idaho Creek Although the large exposures of marble north of Barton Gulch and near the mouth of Idaho Creek were examined specifically for talc, the only talc found in the area is north of Davey Creek. GH-6 SE% SW% sec. 35, T. 6 S., R. 4 W. Sev eral small fragments of fine-grained talc in soil. GH-7 S W A SE1/* sec. 35, T. 6 S., R. 4 W. Sev eral small fragments of fine-grained talc in soil. No outcrop, but concentration of marble float here. GH-8 S W 1/* NE1/* sec. 2, f . 7 S., R .4 W . A few small fragments of fine-grained talc in soil. GH-9 S W 1/* N W /i sec. 1, T. 7 S., R. 4 W. One fragment of fine-grained talc in soil. GH-10 N W 1/* S W 1/* sec. 1, T. 7 S., R. 4W . Sev eral pieces of marble float that contain silvery, coarse-grained talc. Between Idaho Creek and the North Fork o f Greenhorn Creek GH-11 SE1/* NE% sec, 6, T. 8 S., R. 3 W. Fine grained talc veinlet about 8 mm thick. GH-12 SE'/* S W 1/*sec. 6, T. 8 S .,R .3 W . Minor fine-grained talc in outcrop. GH-13 NE1/* NW% sec. 7, T. 8 S., R, 3 W. Minor talc in float. GH-14 SE1/* N W 1/* sec. 7, T. 8 S ,, R .3 W . Minor fine-grained talc in float. GH-15 S W 1/* NE1/* sec. 7, T. 8 S., R .3 W . Small (1 to 2 mm) -blebs of white to light-green talc in outcrop. '" GH-16 S W '/* NE1/* sec. 7, T. 8 S., R. 3 W. Mar ble float contains veinlets and pods o f fine grained talc. Talc float concentrated in an area 2 by 3 m. GH-17 S W K NE1/* sec. 7, T. 8 S ., R. 3 W. Trace of talc float below outcrop. GH-18 N W 1/* NE1/* sec. 7, T. 8 S .,R .3 W . Minor talc in float. GH-19 N W 1/* NE1/* sec. 7, T. 8 S., R .3W . Abun dant talc veinlets in outcrop of intensely de formed marble. Talc constitutes approximately 10 percent of the outcrop, which is 3 by 10 m. GH-20 NW % NE1/* sec. 7, T. 8 S., R. 3 W. Trace of talc in soil. GH-21 S W 1/* N W 1/* sec. 11, T. 8 S., R .4W . Very small Hess than 1 cm} chips of fine-grained talc in soil found for 200 m in a north-south traverse. T GH-22 NW % S W 1/* sec. 11, T. 8 S., R. 4 W. Minor fine-grained talc in soil. GH-23 SE1/* N W 1/* sec. 11, T. 8 S., R. 4 W, Minor fine-grained talc in soil here and between 22 and 23. GH-24 S W 1/* NE1/* sec. 11, T .8 S ., R .4 W . Trace of fine-grained talc in soil. GH-25 SE1/* SE V* sec. 11, T. 8 S., R. 4 W. One small fragment of talc in soil. GH-26 N W 1/* NE1/* sec. 14, T. 8 S., R. 4 W. Trace of talc in outcrop of marble. GH-27 N W 1/* NE1/* sec. 14, T. 8 S., R. 4 W. Several small fragments of talc in soil. - [ f ; : | . j. ' < 1 ' ( GH-28 Ruby claims Location: NE1/* NWy* sec. 14, T .8 S ., R .4W ., Madison County. Ruby Dam 7 Vi-minute quadrangle. Approximately 14 miles (22 km) south of Alder. f ! Accessibility: The claims can be reached by taking a road that branches to the south from the Jasmine Creek road. This is the only road that branches to the south from the Jasmine Creek road east of thepoint where the road enters the timber and begins to ascend. j ^ f j Ownership: Ruby No. 1 and Ruby No. 2 claims were located by Sam Maloney of Alder, Montana. Description: A diabase dike, presumably of Precambrian age, intruded dolomitic marble at this prospect (fig. 12). The relations here illustrate well the effect of the composition of the host rock on the final alteration product. Light-tan to gray chlorite was produced by alteration of the more aluminous diabase, whereas white talc was produced from the alumina-poor marble. j 1 i , ( I j l 1 Exposures are sparse at this prospect, which is mainly within the timber, and without the benefit of shallow cuts; the geology of the occurrence would be very difficult to decipher. Talc and chlo rite are exposed only in the cuts. On the basis of the chlorite exposed in cuts made across the dike, it can be inferred that alteration of the diabase was locally of sufficient intensity to completely replace the diabase by chlorite, but in other places along the dike only pods and irregular veinlets o f chlorite were produced. Petrographic examination of the fresher diabase shows some alteration o f the plagioclase to sericite in addition to secondary chlo rite, epidote and actinolite. ! \ j . ! j n A photomicrograph (Plate 1) shows the relict ophitic texture characteristic of some of the chlo- : GUNTER00000859 41 J Figure 12--Ruby claims. Greenhorn Range (R. B. Berg, Septem ber 1976). M ost of the chlorite north o f A is tan and most of the chlorite south o f A is yellowish green. GUNTER00000860 42 rite. The clear relict plagioclase laths, now com pletely replaced by the chlorite, are surrounded by chlorite that contains many small rutile grains. Pre sumably the chlorite could not accept all of the titanium originally present in the pyroxene of the diabase and thus rutile was formed. Table 3 gives a chemical analysis of chlorite from this prospect. Talcose dolomitic marble and minor talc are ex posed in the large prospect pit on the southwest side of the dike. No other talc was found in the marble adjacent to the dike. GH-29 S W 'A NW% sec. 14, T. 8 S., R. 4 W. Sheared and altered marble exposed in shallow cut. Trace of coarse-grained silvery talc in marble. GH-30 Doubtful claim Location: N W '/i S W '/i sec. 14, T. 8 S., R. 4 W., Madison County. Ruby Dam 7%-minute quad rangle. Approximately 14 miles (22 km) south, of Alder. Accessibility: The claim can be reached by ranch roads not shown on the topographic map. It can also be approached within 1 mile from a road that branches north from the North Fork of Greenhorn Creek, in sec. 26, T. 8 S., R. 4 W. This junction is not shown correctly on the topographic map be cause changes have been made in the road since the map was made. Ownership: Sam Maloney, Alder, Montana. Description: Unusually soft, limonite-stained talc has been dug from, a small pit just below limonitestained silicified marble exposed in the northern cut {fig . 13). The body of talc was concealed by loose rock, but it is probably no more than 1 by 2 meters in horizontal dimension. A specimen of talc consists o f clasts 1 mm across of fine-grained talc (grain size 4 /xm) surrounded by coarser-grained talc (grain size 30 to 150/xm). This texture suggests that brecciation followed the formation of fine grained talc and provided fractures in which coarser-grained talc was deposited. This brecciated talc, although not common, is seen in many o f the talc deposits in southwestern Montana. A layer o f chlorite containing blue apatite is poorly exposed in the southern cut. Microscopic exam ination of a sample from the southern cut shows that it consists of chlorite containing trace concen trations o f apatite, sphene, zoisite, zircon and talc(?). Veinlets of chlorite 1 to 10 cm thick are ex posed in small pits dug in the knob above the talc pod. Talc mineralization is confined to the one pod. GH-31 N W '/i SE'A sec. 14, T. 8 S., R. 4 W. Trace of talc in soil. GH-32 SW% SE14 sec. 14, T. 8 S., R. 4 W. Minor concentration of talc in float. GH-33 NE'/i SW% sec. 13, T. 8 S., R. 4 W. Small blebs of talc in some of the marble float. GH-34 SW% SW'/x sec. 13, T. 8 S., R. 4 W. Minor talc in soil. GH-35 S W 'A SW!4 sec. 13, T. 8 S., R. 4 W. Minor talc in soil here and between 34 and 35. GH-36 SE14 SW'/x sec. 13, T. 8 S ,, R. 4 W. Trace of talc in soil. GH-37 NW% NE'A sec. 23, T. 8 S., R. 4 W. Talc pod 2 by 10 cm in outcrop; talc float in soil. GH-38 SEy* NW'A sec. 23, T. 8 S., R. 4 W. Minor talc in tw o marble layers each approximately 4 inches (10 cm) thick, exposed in a shallow pros pect pit. Prehnite and chlorite have been identified from this prospect. GH-39 SE'A NE% sec. 23, T. 8S ., R .4 W . Minor talc along tw o shear zones exposed in shallow cut, GH-40 SE'A NE% sec. 23, T. 8 S., R. 4 W . Darkgreen talc poorly exposed for a distance of 45 feet (15 m) in cut, which trends N. 25 W. Another cut on the ridge to the east trends N. 75 W . and ex poses fragments of dark-green talc in the marble saprolite for approximately 50 feet (16 m). Loose blocks of massive white quartz contain small talc pods. Most of the talc from this prospect is a darkgreen variety, probably chloritic. Area south o f the North Fork o f Greenhorn Creek .Several talc prospects are present in this area in addition to the W illow Creek talc mine. GH-41 GREENHORN CLAIMS Location: S W 'A NWVa sec. 30, T. 8 S., R. 3 W., Madison County. Home Park Ranch 7 V4-minute quadrangle. Approximately 16 miles (26 km) south east of Alder. Accessibility: A bulldozer trail leads from the haul road for the W illow Creek mine to these cuts, which are approximately 1,400 feet (430 m) west of the point where the haul road turns northeast at an altitude o f approximately 7,160 feet (2,183 m). This haul road is not shown on the topo graphic map. Ownership: Mr. and Mrs. Carl Hafer, Sr., Butte, Montana. > GUNTER00000861 1 E X P L A N A T IO N 43 Talc Disseminated chlorite ^80 A ttitu d e o f fo lia tio n J. V Downhill I/, 50 ft le V. 'V s- Am phibolite and quartzite exposed in d C hloritized schist shalfo'w cut d o w n h ill Of , Marble jt. rk- n ^ ^ ;et ;ut / " 1lM ' l l i' i' m i Yi Y " NOTE; Loose material and slumping x - in this cut make interpretation very ble tentative ise aie Figure 13--Doubtful claim, Greenhorn Range (R. B. Berg and L. Swanson, June 1976). rk- i in Description: Sheared and contorted talcose marble Ownership: The mine is owned by the Madison Min and graphite schist are exposed in the deepest part erals Corporation, Butte, Montana, and is of the cuts, but pods of pure talc are not exposed. operated by Resource Processors. . Because of a lack of exposures uphill and to the south of the area shown in Figure 14, the areal ex Description: Although a small amount of talc was N., tent of the body of marble can only be approxi mined underground many years ago, the only sig ute mately inferred, but the body is not a continuation nificant production has been that by Resource Pro th- of the talc-bearing marble at the W illow Creek cessors between 1970 and 1979. Mining ceased in mine; the two marble bodies are separated by a June 1979. Talc ore is hauled to the sorting yard large area of calcitic marble (fig. 15). and after sorting is trucked to Alder for rail ship iaul ment. its, GH-42 Willow Creek mine iRuby Ridge mine) 'est The mine is situated within a small body of mar fast Location: NE14 sec. 30, T. 8 S., R. 3 W., Madison ble, which is surrounded by quartzofeldspathic 183 County. Home Park Ranch 7% -minute quadrangle. gneiss (fig. 15). Pre-Belt metamorphic rocks have po- Approximately 16 miles (26 km) southeast of Alder. been thrust over Paleozoic formations, which are exposed north and east of the mine. Because of Accessibility: The sorting yard is adjacent to the poor exposures the position of this fault north of tte. Willow Creek road, and the haul road to the talc the Willow Creek mine is only approximately mine gees through the sorting yard. known. In the mine, metamorphic units strike con- G U N TE R 00000862 44 275 ft- (B3m) to hauE road a f NOTE: Northwest ertd of cut is 56 ft {20m ) higher in elevation than southeast end o f cut. ----rY\ w 50 fi 10m Figure 14--Greenhorn claims, Greenhorn Range (R> B. Berg and L. Swanson, July 19761, l Figure 16--Geologic map of the area surrounding the W illow Creek mine (GH-42), The symbols used on this map are the same as those used on Sheets 2 r 3. ; GUNTER00000863 sistently northeast and dip 40 to 70 NW. There is greater variation in the attitude of foliation north of the mine where outcrops are scarce on the heavily timbered north-facing slope. Shear pianes cutting talc, chlorite and country rock are abundant at the mine. The large isolated body of calcitic marble west o f the mine is barren of talc. Figure 16 is a diagrammatic cross section through the W illow Creek ore body and shows the relationship between talc and associated rock types. The effect of post-talc faulting is not shown on this simplified cross section. The hanging wall of the deposit is biotite-quartz-feldspar gneiss, which has been altered to varying extent, the ulti mate alteration product being chlorite. The first ef fect of the alteration was the replacement of bio tite by chlorite. Further alteration resulted in the sericitizatlon of the feldspar, and more intense, al teration produced a rock that consists of chlorite and quartz. The final stage in the alteration se quence was dark-greenish-gray chlorite. A chemi cal analysis o f a typical specimen of the chlorite is given in Table 3. Grain size of the chlorite varies considerably within the area of one thin section, for example from 10 to 150 /im. The chlorite con tains rare iocai concentrations of idiomorphic zir con and idiomorphic crystals of apatite. On the basis of e = 1.6353 0.0005 and w = 1.6397 0.0005, one specimen of apatite is estimated to be approximately 76 percent fluorapatite, 12 percent hydroxyapatite and 12 percent chlorapatite. Some large flakes of silvery chlorite a centimeter across presumably have been formed by the replacement of coarse-grained biotite in the quartzofeldspathic gneiss. Rutile needles occur in some of the coarse grained chlorite. Some specimens of chlorite are cut by many veinlets of talc a few millimeters to a centimeter in thickness. Most specimens of chlo 45 rite contain talc in sufficient concentration to be detected by x-ray diffraction analysis. The alteration that produced chlorite from the biotite-quartz-feidspar gneiss also resulted in the formation of talc from the subjacent dolomitic marble. Large blacks of marble have been com pletely replaced by fine-grained white to palegreen talc. Some of the talc is medium green, col ored by small grains o f dark-green chlorite. Most of the talc is very fine grained, some grains are less than 2 tm across, but even within a speci men of fine-grained material there are patches of coarser-grained feathery talc in which individual grains are 1 mm long. Talc pseudomorphs after tremolite blades are observed in one specimen. Veins of white quartz are rare in both the talc and the chlorite. Clasts of green chlorite 1 cm across in one specimen from a quartz vein are rimmed by a layer of talc about 1 mm thick.. Perhaps talc formed by reaction of chlorite with silica-rich solutions, which added the necessary silica to make chlorite from talc and removed the alumina left over from that reaction. Dolomitic marble on the footwall side of the ore body contains many small veinlets and pods of talc. The abundance of talc in this marble de creases to the southeast away from the ore body. Calcitic marble that contains abundant forsterite, mainly in grains smaller than 5 mm across, is ex posed on the ridge just south of the mine. Some porphyroblasts of brown forsterite are several cen timeters across, and a few of these are cut by chrysotile veinlets. Serpentine was also observed in thin sections of both the calcitic marble and the dolomitic marble. The serpentine occurs in small blebs, which may have formed by the complete re- EXPLANATION | 3 p [ Biotite-quartz-feldspar gneiss Sericitic and chloritic alteration of biotite-quartz-feldspar gneiss I l Talc I | Chlorite ........ Gradational contact 50 m Figure 16--D iagram m atic cross section o f the talc deposit at the W illo w Creek mine showing the probable relationship between talc and host rock before post-talc faulting. GUNTER00000864 46 placement of forsterite. Phlogopite and tremolite also occur in the calcitic marble. Tremolite is typi cally gray to black because of abundant included graphite, and is concentrated in almost monomineralic layers a centimeter or tw o in thickness. No talc was identified either optically or by x-ray dif fraction analysis from the calcitic marble. Sepiolite has been identified from this locality (Alice Blount, personal communication, 1977). GH-43 Claims north o f Willow Creek (Adam and Eve No. 1 and No. 2) ' Location: SWVi SE% sec. 30, T. 8 S., R. 3 W ., Madison County. Home Park Ranch 716-minute quadrangle. Approximately 16 miles (26 km) south east o f Alder. Accessibility: The road to the prospect branches from the haul road for the W illow Creek mine at a sharp switchback above Little W illow Creek at an altitude of 6,920 feet (2,110 m). The haul road for the W illow Creek mine joins the W illow Creek road at the site of the talc-sorting plant. Ownership: Sam and Goldie Maloney of Alder, Mon tana. Description: Marble, quartzofeldspathic gneiss and sillimanite-biotite-garnet schist are exposed in six prospect cuts, all of which trend nearly perpen dicular to lithologic layering of the metamorphic units. Marble is exposed in all but the highest cut, which is 240 feet (73 m) higher than the lowest cut. Talc pods less tharr5.cm long and talc veinlets 3 to 4 cm thick, parallel to compositional layering of the marble, are exposed in the lower four cuts. Talc chips are found in the soil between the cuts in the area underlain by marble and also in a small area just east of the cuts. Although there is no large concentration of these chips, they were found in the soil over an area of approximately 300 by 1,500 feet (100 by 500 m). Masses o f sepiolite as long as 10 cm were found in the lowest cut. The sepiolite resembles splint ered wood that has weathered grayish tan. A few thin chrysotile veinlets 2 mm thick occur in calciticmarble. GH-44 SW% NWVi sec. 31, T. 8 S., R. 3 W . A trace of talc was found in the bottom o f a prospect trench, which exposes marble for a distance of 45 feet (15 m). GH-45 Talc occurrence south o f Virginia City Location: NW% NWV4 sec. 23, T. 7 S., R. 3 W ., Madison County. Varney 15-minute quadrangle. Approximately 5 miles (8 km) south of Virginia City. Accessibility: This area is 100 feet (33 m) south of the road between Barton Gulch and Aider Gulch where this road descends into Alder Gulch. Ownership: Not known, Description: A nearly vertical quartz vein 1 to 3 feet (0.3 to 1 m) thick separates quartzofeldspathic gneiss from calcitic marble. Foliation in the marble is parallel to the vein. Adjacent to the quartz vein the marble contains talc, tremolite and a trace of vermiculite. The bright-green vermiculite grains are less than 5 mm across and have presumably been produced by alteration of phlogopite. The bright green color suggests that this vermiculite is nickel bearing. GH-46 Calverts claims Location: SW% SE1/* sec.32, T .7 S .r R .3 W ., Madi son County. Varney 15-minute quadrangle. Ap proximately 8 miles (13 km) southwest o f Virginia City. Accessibility: The claims may be reached by follow ing the Idaho Creek road to its end in the NW % of sec. 33. From that point the road to the. claims is unimproved and steep in some places. Ownership: The Calverts No. 1 through No. 4 claims were located by Frank Ludwick and Jim Ludwick (Alder, Montana), Jim Eby (Billings, Montana), and-Carl Hafer, Sr. (Butte, Montana). Description: A dolomitic marble layer with an ex posed width of approximately 250 feet (76 m) strikes northeast and dips northwest (fig . 17). Am phibolite lies northwest of the marble, and amphib olite and quartzofeldspathic gneiss lie to the south east. Irregular veinlets and pods of talc, most of which are less than 10 cm thick, are exposed in all but the easternmost cut. Talc is most abundant in the largest cut, where a small amount of waxy green chlorite is also exposed. Because the chlo rite is intermingled with the talc and there is no evidence o f replacement of quartzofeldspathic gneiss, it is concluded that both the talc and the chlorite formed by alteration of dolomitic marble. Tremolite is found scattered throughout much of the marble, and in the westernmost cut, tremolite layers 3 to 5 cm thick have been replaced by talc. JL. 'ir i i [ 1 4. GUNTER00000865 Dense timber, no outcrops 47 J IV4V ------ Contact-da^ed where inferred, queried where doubtful l Downhill Figure 17" Calverts claims* Greenhorn Range (R. B. Berg* Septem ber 19771. l GUNTER00000866 48 Very coarse-grained dolomitic marble containing clusters of quartz crystals is exposed west o f the claim notice. Some individual dolomite crystals are 8 cm across. Minor green chlorite (but no talc) occurs in this rock. Thulite Ipink) and zoisite (white) occur at several places on these claims but are most abundant in the shallow cut in the southern part of the area, where some zoisite crystals are 5 mm across. Minor tremolite and diopside occur in the calciticmarble that contains the thulite and zoisite. Other prospects and inactive mines Several prospects and inactive mines presum ably excavated for metals were encountered during the'mapping in the Greenhorn Range and are listed below. Most of these are in shear zones within the quartzofeldspathic gneiss. In addition to these work ings there are many shallow prospect pits, also mainly in shear zones in the quartzofeldspathic gneiss. NW'A SE% sec. 5, T. 7 S., R. 4 W, Vertical shaft at least 30 feet (10 m) deep into sheared quartzofeld spathic gneiss and some malachite on dump. SE% SE% sec. 3, T. 7 S., R. 4 W. Several prospect cuts and an adit (caved) in the marble. The only evidence of mineralization recognized was black manganese minerals in the cut at the adit. Fault surfaces are prominent in that cut. NW% SE% sec. 14..T. 7 S., R. 4 W. Two caved adits and a prospect pit ln"a steeply inclined shear zone in schist and quartzofeldspathic gneiss. NWy* SE1/* sec. 13, T. 7 S,, R. 4 W. Prospect pits in sheared biotite schist. Sheared and limonitestained quartzofeldspathic gneiss is present in this area. NWVi NEVi sec. 19, T. 7 S-, R. 3 W. Bull Frog mine. An adit extends at least 36 feet (12 m) into the hill. Sheared rock of the amphibolite assemblage and minor calcitic marble are on the dump. A second adit (caved) is several hundred feet uphill. Schist is predominant on the dump and contains traces of chalcopyrite. Sec. 16, 17, 21, T. 7 S., R. 4 W. Barton Gulch. The lower part of Barton Gulch, within 1 mile of its mouth, has been dredged for gold. More recently mineral collectors have recovered garnets from the gravel at the mouth of Barton Gulch. Many of these garnets have weathered out of the Tertiary sediments. SW'A S W 1A sec. 26, T. 7 S ..R .4 W. Prospect pits have been dug in sheared quartzofeldspathic gneiss, reportedly in search for uranium. SE1/* SWVi sec. 12, T. 8 S., R .4 W . Silver Bell claim. Three adits are in sheared and limonite-stained quartzofeldspathic gneiss. Two of the adits extend approximately 20 feet (7 m) into the hillside, and the third adit goes straight in for approximately 50 feet (17 m) and then curves. Malachite and minor chalcocite were found on the dump. SE% S W 1/ sec. 24, T. 8 S., R. 4 W. Adit extends approximately 80 feet (27 m) into hillside. The adit is in a sheared ultramafic body, which contains minor chalcopyrite and malachite. Sillimanite schist and altered quartzofeldspathic gneiss, in addition to sheared ultramafic rock, are found on the dump. SE% SE% sec. 25, T. 8 S., R, 4 W . Cuts above a caved adit expose intensely sheared rock over an area approximately 50 feet (17 m) high by 150 feet (50 m) long. Quartzofeldspathic gneiss,pegmatite and amphibolite are all present in this zone of shearing. Minor malachite coats some fractures. The shaft shown onthetopographic map is now filled. } ' ' ,, I ( | / * ; ! ') j j * j ` : ifv' GUNTER00000867 49 Gravelly Range The Gravelly Range lies along the east limb of a large syncline that plunges to the southwest. The outcrops o f pre-Belt metamorphic rocks of the Greenhorn Range on the west limb of the syncline are separated from those of the Gravelly Range by outcrops o f Paleozoic and Mesozoic sedimentary rocks. Pre-Belt metamorphic rocks are exposed in a north-south belt along the east fro n t of the Gravelly Range and are covered by Quaternary alluvium in the Madison Valley farther to the east, in the southern part of the range, Tertiary volcanic rocks cover the pre-Belt rocks and separate the area of exposed pre Belt rocks in the Gravelly Range from exposures of pre-Belt rocks in the Henrys Lake Mountains. The geology of the northern part of the Gravelly Range has been mapped by Hadley (1969a, 1969b), and Heinrich and Rabbltt (1960) have mapped and studied the geology of a part of this same area, infor mation on the pre-Belt rocks of the southern part of the range is much less complete. A map by Wier (1965) shows the geology of an area o f approxi mately 18 square miles (47 square km) surrounding the Black Butte iron deposit in T. 11 S., R. 1 W. Mann (1954) described the geology of a large part of the Gravelly Range but concentrated on the Phanerozoic rocks and did not describe the pre-Belt rocks in detail. Metasedimentary rocks are exposed in the northern part o f the range, and It was in the area be tween Wigwam Creek and Cherry Creek that Peale (1896, p. 2) originally described what he called the Cherry Creek beds, more recently designated the Cherry Creek Group. The Cherry Creek Group and the terminology of the pre-Belt rocks are discussed in the section on pre-Belt geology of southwestern Montana. There is much variety In the metamorphic rocks of this range, particularly in that part of the sequence that is clearly metasedimentary. Pre-Belt rock types recognized are quartzofeldspathic gneiss, amphib olite, hornblende gneiss, dolomitlc marble, quartzite, banded quartz-magnetite iron formation, phylllte, schist and metadiorite. The schist can be separated into mica, sillimanite, kyanrte, kyanlte-staurolite, andalusfte and staurollte-andalusite varieties. Greenschlst-facies metamorphic rocks are ex posed in a segment o f the Gravelly Range approxi mately 7.5 miles (12 km) long, which includes the Yellowstone talc mine. In this area fine-grained mar ble and phyllrte are exposed rather than the coarse grained marble and schist typical of pre-Belt rocks In the Ruby Range, Greenhorn Range and Tobacco Root Mountains. A very detailed study of the petrol ogy of these rocks (Millhoiland, 1976) showed that the transition from low-grade to higher-grade meta morphic rocks is abrupt and that the low-grade assemblage is not retrograde (Millhoiland, 1976). Ap proximately 10 miles (16 km) south o f the Yellow stone mine there is another transition zone from amphibolite-facies rocks on the north side of Horse Creek to greenschist-facies rocks on the south side (Jahn, 1967). Jahn dated biotite and muscovite from rocks across this transition zone and found that biotite from the rocks north of Horse Creek gave a K-Ar age of 1.6 b.y., whereas the biotite from rocks south o f the transition gave an age of 2.6 b.y. Both Millhoiland and Jahn mentioned evidence o f cataclasis in some of the rocks of their respective areas. Perhaps these abrupt changes in metamorphic grade can be explained by Precambrian faulting that juxta posed rocks of different metamorphic grade. The pre-Belt metamorphic rocks exposed farther south in the Henrys Lake Mountains are of greenschist facies. What may be the greatest concentration of talc in Montana is in the dolomitlc marble of the Gravelly Range at the Yellowstone mine and vicinity. The de posit of talc at the Yellowstone mine and the sur rounding occurrences are within a thick sequence of marble, which Is probably made up of thinner layers that have been isoclinally folded. The known talc occurrences at this locality are limited to the south eastern half of the area underlain by marble. GR-1 Taft Mountain claims Location: S>4 sec. 5, T. 8 S., R. 1 W ., Madison County. Varney and Cameron 15-minute quad rangles. Approximately 12 miles (20 km) south of Ennis. Accessibility: The claims can be reached from a ranch road that branches from the road along the west side of the Madison River in sec. 32, T. 8 S., R. 1 W. Ownership: Pete Womack, Ennis, Montana. Description: Chlorite and talc are exposed in two areas about 400 feet (130 m) apart separated by a shadow saddle in which no bedrock is exposed (figs. 18, 19). The northern area has been more extensively explored by seven trenches, and chlo rite is exposed in all trenches. It Is likely that at least some of the chlorite, that exposed In the westernmost cut, has been produced by the altera tion of pegmatite. In other prospects in southwest ern Montana there is clear evidence that chlorite GUNTER00000868 50 Altered pegmatite m Marble float Minor talc in dolomitic marble M inor tremol ite in marble E X P L A N A T IO N .2 1 '1 Biotite-quartz l-V " ' >1 feldspar gneiss E j ____ S I m Dolomitic marble et Disseminated talc , ^ | ~| Chlorite Attitude of foliation Maximum depth of cut in feet Green, tan, and red chlorite throughout pit m 5 0 feel 10 m Limonite-stai ned and silicified marble 400 ft 1120 m) S. 35 E. to shaft shown on Figure 21 Figure 18--Tait Mountain claims (northern part). Gravelly Range (R. B. Berg, September 1976), has been formed by alteration of quartzofeldspathic gneiss (for example, see the description of the Golden Antler mine). A t the Tait Mountain prospect, with the exception of the altered pegma tite, the lithology of the rock that has been re placed by chlorite is not clear. A heavy-liquid sep aration was made on tw o specimens of chlorite in an effort to separate zircons from the chlorite. No zircons were recovered, suggesting that the chlo rite replaced a zircon-free rock, presumably impure dolomitic marble. At other localities zircons can be recognized in chlorite that on the basis of field rela tionships is presumed to have replaced quartzofeldspathic gneiss. Talc is exposed in only one ex cavation. The general lack of talc in the dolomitic marble exposed in these trenches is puzzling. The chlorite in the northern area ranges from green to tan and is locally stained red by hematite along fractures. Talc is a trace constituent o f the chlorite. Tremoiite is a widespread trace constitu ent of the dolomitic marble and has been replaced by talc at some places. Because of the slumping in the cuts and the lack of exposures between them, it was impossible to trace the contacts between cuts. Judging from folds in the marble exposed north and west of the cuts, it is likely that the marble in and near the cuts has been isoclinally folded. An old prospect pit in limonite-stained and silicified marble was probably dug in an effort to find a metalliferous vein rather than chlorite or talc. In the southern area, better exposures make it possible to trace the contact of marble with quartzofeldspathic gneiss around agentle fold. Tertiary freshwater limestone (Hadley, 1969a) overlies the marble and gneiss to the southwest. Crumbly green chlorite is exposed in a prospect trench along the contact between dolomitic mar ble and quartzofeldspathic gneiss that i$ in part pegmatitic. One of the chlorite specimens that was checked in vain for zircons was from this cut. Minor tremoiite and talc are scattered throughout the dolomitic marble exposed west of this cut. A 25-foot (&-m) shaft was sunk in talc at the north end of the area of marble exposures. The work was done prior to 1948 by Tri-State Minerals, but the shaft was abandoned because of the low grade of the talc and the small quantity present (Perry, 1948, p. 8). The talc on the dump is unusually coarse grained and is accompanied by abundant tremoiite. Gft-2 Cherry Gulch prospect Location: NEVi SWV. and N W % SE1/* sec. 31, T. 8 S., R. 1 W ., Madison County. Varney 15-minute quadrangle. Approximately 18 miles (29 km)southwest of Ennis. Accessibility: The claims are located just south of the Cherry Gulch road, which branches from the road along the west side of the Madison River. Both of these roads cross private land. l f 1 j J j l i I ^ j I i j ( ! i * I GUNTER00000869 51 148 f t (45m) to shallow cut in marble float Shaft 25 ft (8m) deep Coarse-grained, white dolomitic marble, coarse-grained tremolite and coarse- EXPLANATION 0> 0c) , i . OJ I v , V ' / V ' AFreshwater limestone / _____ ' ' lU 't l ' I I / / / . / l l ', / / f\-- --| Quartzofeldspathic gneiss with minor pegmatite '/ / / / W/ / / > A" Dolomitic marble Prcambrien [; ; '. | Talc on mine dump 1 Chlorite I X Attitude of foliation __ ?_____ ____ _ Inferred contact, '' queried where doubtful ?6/ *" Pegmatite and quartzofeldspathic gneiss; minor tourmaline in gneiss 'S'* , \ f ' * -1 4 3 ft 144m| to prospect trench in dolomitic marble with minor green chlorite Figure 19--Tait M ountain claims (southern part). Gravelly Range (R. B. Berg, Septem ber 1976). ii GUNTER00000870 52 Ownership: A patented claim is owned by Albert Thexton, and adjacent claims are owned by Pete Womack, both of Ennis, Montana. Description: Layering in the Precambrian metamorphic rock strikes northeast and is near vertical (Sheet 1-D). The Flathead Quartzite (Cambrian) unconformably overlies the metamorphic rocks to the south (Hadley, 1969b, and fig . 20), and ben tonite was encountered in prospect pits dug south and east of the talc occurrence. Two layers of R- 2 W 1 R I W. medium-grained dolomitic marble are separated by a layer of staurolite schist in which many of the staurolite porphyroblasts are between 1 and 3 cm in length. Biotite-quartz-feldspar schist Is exposed in the gully west of the talc, and a thin layer of quartzite is poorly exposed at the contact between dolomitic marble and staurolite schist. Chalcopyrite, malachite, and cuprite are exposed in a prospect trench cut in the quartzite. Blades of kyanite can be found in some of the white quartz float. f EXPLANATION Pleistocene 09 Miocene or ' p Pliocene Tr Oligocene T Bouldery deposits of uncertain origin Cobble and coarse pebble gravel Rhyolite ash flow tuff Andesite flows Felsictuff Precambrian p-Cd Dolomitic marble Precambrian undifferentiated; Includes quartzofeldspathic PC gneiss, schist, quartzite,and hornblende gneiss Talc pebbles in soil Yellowstone Mine Talc prospect Paleozoic Undifferentiated Paleozoic Pol sedimentary units I KM Figure 20--Generalized geologic map o f the Cherry Gulch-Johnny Gulch area of the Gravelly Range. Geology from the more detailed geologic maps of th e Varney and Cameron quadrangles by Hadley (1969a, 1969b). GUNTER00000871 An adit, which is now caved at the portal, may have been driven in search for copper. Some talc is scattered on the dump at this adit. Chalcopyrite and malachite are trace constituents of some of the talc exposed here. A concordant layer of talc 1.5 to 2 feet (0.4 to 0.6 m) thick, some o f which is lava talc, is exposed in tw o small pits on the east side of the small gully just west o f the adit. These pits are situated along the south boundary of sec. 31, T. 8 S., R. 1 W ., close to the unconformity between the Precam brian metamorphic rocks and Flathead Quartzite. An additional shallow exploration trench 25 feet (8 m) long exposed additional hard lava talc in the same layer of dolomitic marble. Talc also is exposed in two shallow cuts in mar ble on the east side of the small gully just east of the area shown in Sheet 1-D. Concordant talc pods 2 to 4 inches (5 to 10 cm) thick are exposed in these cuts. Another poorly defined talcose zone 5 feet (1.5 m) thick is exposed in a small prospect pit north o f these cuts toward Cherry Gulch. The chemical analysis of a specimen of unusuaily hard block talc from this pit is given in Table 3. The specimen contains irregular patches of small grains (4 fim} o f an opaque mineral and a trace of apatite. Although there are some patches of relatively coarse-grained talc in the specimen, most of the talc grains are about 4 (im across. Some of the lava talc from this prospect has been selectively mined for the carving market. The presence of bentonite in this small area sur rounded by pre-Belt rocks is unusual. Presumably volcanic ash accumulated in a depression or a small pond along the ridge and subsequently was altered to bentonite. The bentonite may be equiva lent in age to the Oligocene felsic tu ff mapped by Hadley (1969b) 1.7 miles (2.7 km) to the south west. The mineralogy of the sand-size fraction of this smectite clay shows that it is of volcanic origin and not a result of hydrothermal alteration of metamorphic rocks. Biotite, plagioclase, K-feldspar, quartz, zircon, partly devitrified glass, a zeo lite and catcite were identified in the > 325 mesh <>44 tm) fraction of six samples of bentonite. None of the pits within the area of the bentonite occurrence exposed bedrock under the bentonite, but the deepest pit is only 15 feet (4.5 m) deep. GR-3 Yellowstone mine Location: Sec. 4, T. 9 S., R. 1 W ., Madison County. Cameron 15-minute quadrangle. Approximately 18 rniles (29 km) south of Ennis. 53 Accessibility: The Johnny Gulch road is the haul road for the mine. Ownership: Cyprus Industrial Minerals. Description: The Yellowstone mine, which is one of the largest talc mines in the United States, is an important producer of high-purity talc. The follow ing description of the geology of the mine is based mainly on the work done by James (1956) and to a lesser extent Perry (1948), Olson (1976) and the author's observations. Talc occurs in fine-grained dolomitic marble forming pods and layers generally concordant to the layering (relict bedding?) in the marble. Many of the pods are less than 1 foot (0.3 m) thick and are offset along small shears that are similar to those shown in the sketch of a pit face on the Bur lington Northern mine (fig. 21). James (1956, p. 4) reported talc lenses 70 feet (21 m) or more in length and 35 feet (11( m) thick in the Johnny Gulch area but pointed out that most lenses are much smaller. Because of the small size of most talc bodies at the Yellowstone mine it is impractical to selectively mine pure talc and all of the ore Is hand sorted. Talc is picked from waste at a sorter situated at the pit, and the higher-grade ore is sorted at the main facility at Johnny Gulch where the waste is removed from the talc. Ore that is of such (ow grade that it cannot be hand sorted is stockpiled and will undoubtedly be an Important source of talc in the future. Raw talc from this mine ranges in color from green through pale green and light gray to white. Unlike some o f the other taic mines, the Yellow stone mine has no chlorite in association with the talc. Impetus was given to the development of this mine during World War II because of the presence of block or lava talc here. Lava talc was in demand because it could be machined into objects such as insulators and then fired without cracking. Typical talc, sometimes designated ceramic or cosmetic talc depending on purity, contains many minute fractures and will easily break if subjected to physi cal or thermal stress. Lava talc, because of its dur ability, is also in demand by talc carvers, who are particularly interested in material that contains dendritic patterns of black manganese minerals. Most lava talc is white or cream and is not translu cent as is the typical pale-green ceramic or cos metic talc. James (1956, p. 2) reported that some of the do lomitic marble has been altered to coarse-grained GUNTER00000872 54 Spongy quartz along shear zone Dark-green __ talc w ith a few black and maroon spots 20 cm S. W. Figure 21 --Burlington Northern m ine. Gravelly Range. Projection of pit face on a vertical plane perpendicular to layering of the m arble {R. B. Berg, Septem ber 1976). i GUNTER00000873 siderite (red weathering) and ankerite. Lava talc was mined from a deeply weathered part of a zone of siderite and ankerite in the marble. Because lava talc has been found only near the surface, it pre sumably formed from typical talc by weathering, but a comparison of a chemical analysis of lava talc and analyses of ceramic talc shows no significant difference (Table 5). Although lava talc was important in the early de velopment of the Yellowstone mine, essentially all of the production from this mine has been ceramlcor cosmetic-grade talc, which has found a variety of uses from the paper industry to the plastics in dustry. The talc in this area was reportedly discovered by Lewis Clark on his homestead in the early part of the twentieth century. Significant development of the property was begun in 1942 when the 240foot (72-m) Madison Tunnel was driven by the Tri State Minerals Company and a 75-foot (23-m> shaft was sunk by the U.S. Bureau of Mines. The first shipment of lava talc was made in December 1942 arid consisted of 4,000 pounds 11,812 kg). Perry (1948, p. 9) reported that in 1943 and 1944, 127 tons (115 mt) of lava-grade talc was shipped. Sierra Talc acquired the mine in 1948 and began openpit mining of ceramic or cosmetic talc rather than the lava talc. The mine, formerly known as the Mountain Talc mine, was renamed the Yellow stone mine. The choosing of this new name as related by James D. Mulryan, Western Area Pro duction Manager of Cyprus Industrial Minerals, is quoted from Olson (1976, p. 109): The original name o f the Yellowstone Mine was the Mountain Talc Mine. Sierra Talc, largely through my father's efforts, acquired the property in 1948. The mine was renamed the Yellowstone Mine a t that time and the story behind this is that Henry Mulryan and Otis Booth, both o f Sierra Talc, were driving to Montana to took over the property after having acquired it. One commented to the other that since they had now bought the mine, they ought to figure out some sort o f name for it. A bout that time, there was occasion for a panic-type stop in the car they were driving, and a bottle o f Yellowstone whiskey, which had been under the front seat, rotted out and h it the passenger on the foot. They felt that this must have been an omen o f some sort, and the mine "as named Yellowstone Mine, it is therefore named fo r the Yeiiowstone whiskey, in spite o f the fact it is only about 50 miles from West Yeiiowstone, Montana. 55 In 1964 the Yellowstone mine changed owner ship when Cyprus Mines Corporation acquired Sierra Talc. The mine has been a significant pro ducer of high-purity talc and will continue to be a major producer in the foreseeable future. GR-4 Queen claim Location: SE% NE'A sec. 8, T. 9 S., R. 1 W ., Madi son County. Cameron 15-minute quadrangle. Ap proximately 19 miles (30 km) south of Ennis. Accessibility: The mine on this claim is 100 feet (31 m) south of the Johnny Gulch road. Owner: Cyprus Industrial Minerals. Description: A small amount of talc has been mined from the eastern end of the northern cuts, where the greatest concentration of talc is now exposed (fig. 22). Although talc is ,,exposed in other cuts, they seem to be exploration cuts. H ie host rock is fine-grained dolomitic marble exposed at the mine and in outcrops to the west. Pale-green talc at the mine is in conformable layers generally 4 to 10 inches (10 to 25 cm) thick. Some of the talc is of lava grade. GR-5 Burlington Northern mine Location: N W 1/ sec. 3, T. 9 S., R. 1 W ., Madison County. Cameron 15-minute quadrangle. Approxi mately 18 miles (29 km) south of Ennis. Accessibility: A short road leads to the prospect from the Johnny Gulch road. Ownership: Burlington Northern, Incorporated, Energy and Minerals Department. Description: The description of this deposit was pro vided by Ed Houser of Burlington Northern, In corporated. The deposit can be divided into three areas, the northern, central and southern --each showing a predominance of green talc chips in the soil. The best tal,,c is in the centra! zone, followed by the southern, then the northern. American Chemet removed several thousand tons in the early 60s from a pit in the central zone. A sketch of a pit face is shown in Figure 21. Talc in the deposit is principally o f the lightgreen variety, but all colors and shades are seen. The host rock generally is slightly siliceous lighttan to light-gray microcrystalline to fine-grained dolomitic marble. Very coarsely crystalline dolomhic marble also occurs on the property, but GUNTER00000874 rarely is it adjacent to or near the talc. Some small conformable stringers and pods of white quartz are present in the host rock, which trends northeast and dips steeply to the northwest. Most of the talc seems to be conformable, with little crosscutting of the host. Post-talc deformation is minima), as only minor displacements are seen in the ore zones. Detailed mapping, trenching, and drilling of the prospect by Burlington Northern during the sum mer of 1977 confirmed that the greatest concentra tion of talc is in the central zone. The trenching also exposed some zones of chlorite formed by the alteration of phyllite; in all cases the chlorite is bounded by deformed talc. A specimen of maroon rock from this cut was identified by x-ray diffraction as magnesite con taining traces of talc and dolomite. GR-6 Talc-bearing conglomerate north o f Johnny Gulch Location: S14 sec. 1 and NV4 sec. 12, T. 9 S., R. 2 W ., Madison County. Varney 15-minute quad rangle. Section inferred from Beaverhead National Forest map. Approximately 20 miles (32 km) south west of Ennis. Accessibility: The road along the south side of Johnny Gulch leads to this area. Ownership: Beaverhead National Forest. Description: In 1974 Pete Womack of Ennis discovered pebbles of taic in the soil in an area underlain by Lodgepole Limestone (Mississippian) (fig . 20). The pebbles range in color from white to tan to pale green and in size from 0.5 to 4 cm. Further ex amination by Pete Womack showed that a con glomerate exposed in the N 'k sec. 12, T. 9 S., R. 2 W ., contains talc pebbles and was a likely source for the pebbles found in the soil. The pebbles re covered from the soil at this locality are more rounded (subrounded to rounded! than the angular talc chips found in the vicinity of talc veins elsewhere. The talc-bearing conglomerate, which is only poorly exposed, is within Oligocene felsic tu ff mapped by Hadley (1969b). The felsic tu ff overlies Lodgepole Limestone (Mississippian) and Three Forks Formation (Devonian). The tu ff is overlain by andesite also suggested by Hadley to be of Oligocene age. The conglomerate consists of pebbles of pink to brown dolomitic marble, talc, schist, white quartz, limestone, dolomite and reddish-brown quartzite in a fine-grained matrix that is cemented with calcite. Some calcite crystals line cavities in the conglomerate. Talc pebbles are only a minor constituent of the conglomerate. The pebbles of marble, schist and quartz are obviously derived from Precambrian metamorphic rocks. The pebbles of limestone and dolomite were f' ; i > * <-. ' ' * ^ \ j I ( , ***-- A ttitu d e o1 foliation --* D ownhill Figure 22--Inactive pit on the Queen cleim. Gravelly Range (R. B. Berg, Septem ber 1976). GUNTER00000875 probably derived from Paleozoic carbonate units, and the reddish-brown quartzite pebbles are typi cal of the Flathead Quartzite (Cambrian) in this area. The source of the talc pebbles in the soil and in the conglomerate is unknown. Examination of the area north of Johnny Gulch by Pete W o mack, Roger Kuhns, and the author failed to find any areas of Precambrian rock other than that shown by Hadley (1969a, b) 1 mile (1.6 km) to the east. Schist and dolomitic marble similar to the peb bles in the conglomerate are exposed to the east. In addition, Flathead Quartzite is exposed to the east where it unconformably overlies the Pre cambrian metamorphic rocks. The closest expo sure o f Flathead Quartzite to the west is 12. miles (19 km) northwest. The fine-grained schist (almost a phyllite) in the conglomerate does not resemble coarser-grained schist exposed to the west in the Greenhorn Range. Thus on the basis of the lith ology of pebbles in the conglomerate, an eastern source is indicated. The most likely source of the talc is the deposit at the Yellowstone mine, which is 3 miles (4.8 km) east of the conglomerate and 57 approximately 1,320 feet (403 m) lower than the conglomerate. Although Hadley (1969a, b) showed one fault (the east side down) between the conglomerate and the Yellowstone mine, the displacement on that fault cannot approach 1,300 feet. The most reasonable Inference is that the talc deposit at the Yellowstone mine was exposed at a higher elevation during the Oligocene, perhaps 1,300 feet (397 m) above the present erosion surface near the mine. At the Yellowstone mine, talc and dolomitic marble are unconformably overlain by rhyolite and rhyolitic welded tuff, which according to James (1956, p. 3) may have filled a topographic depres sion or valley. The rhyolite and rhyolitic tu ff are designated as Miocene or Pliocene in age by Had ley (1969b). W ithout the benefit of absolute age determinations on the andesite that overlies the conglomerate and on the rhyolitic welded tu ff that overlies the talc at the Yeltoyvstone mine, no pre cise limit can be put on the Interval of time during which talc was eroded from this deposit. Paul Pushkar of W right State University collected sam ples of both the andesite and the welded tu ff dur ing the summer of 1978, and he will attempt to date these rocks by the K-Ar method. Madison Range Pre-Belt metamorphic rocks are exposed along the west flank of the Madison Range but are separ ated from the metamorphic rocks o f the Gravelly Range by the intervening Madison Valley. Hadley (1969a) mapped quartzofeldspathic gneiss, quartzite, hornblende gneiss, anorthosite gneiss, dolomite mar ble, iron-rich quartzite, mica schist, phyllite and metadiorite in that part of the Madison Range that lies in the Cameron quadrangle. South of the Cam eron quadrangle the geology of the Precambrian rocks Is (ess well known. Eric Erslev, a graduate stu dent at Harvard, is now (1978) working on the Pre cambrian geology of that part of the Madison Range. Fine-grained dolomite is abundant in the debris deposited by the 1959 earthquake-induced slide that partly filled the Madison River Canyon. Farther to the south, Witkind (1972) showed large areas underlain by dolomite at the southern end of the Madison Range just north of Henrys Lake, Idaho. Other pre Belt metamorphic rocks shown on Wrtkind's map are metagranodiorite, amphibolite, quartzite, mica schist, diabase and gabbro. Eric Erslev (personal communication, 1978) re ported that impure talc is associated with ultramafic bodies in the Madison Range. This talc contains chlorite, actinolite, serpentine and anthophyllite, and occurs in bodies that are probably too small to be o f economic interest. The only other known occurrence o f talc in the Madison Range is south of the Madison River, where a small amount of talc is exposed at the Cliff Lake mine. (See Perry, 1948, p. 39-40, for a de scription of this mine.) GUNTER00000876 58 Henrys Lake Mountains The Hertrys Lake Mountains are situated along the Idaho-Montana boundary west of Henrys Lake, Idaho. These mountains are underlain by metamorphic rocks, which are a continuation of the same units exposed in the southern Madison Range across a valley to the east. Witkind has mapped metamorphic rocks in the Henrys Lake 15-minute quadrangle and also in the southern half of the Upper Red Rock Lake 15-minute quadrangle to the west (1972 and 1976, respectively). He has designated these rocks Precambrian X (1,600 to 2,500 m.y, old), which would make them approximately equivalent in age to the pre-Belt metamorphic rocks of southwestern Montana. Metamorphic rocks from this area are typi cally finer grained and of lower metamorphic grade than the pre-Belt metamorphic rocks to the north west in the Gravelly and Greenhorn ranges. Rather than the coarse-grained marble of those other areas, the carbonate units of the Henrys Lake area are fine grained dolomite; average grain size being between 0.05 and 0.1 mm. In addition to dolomite, Precam brian rocks of this area are mica schist, quartzite, am phibolite, metagranodiorite, diabase and gabbro. During the summer of 1976 Leroy Swanson and the author mapped the geology of a 40-square-mile (104-square-km) area of pre-Belt metamorphic rocks just north of the area in the southern part of the Upper Red Rock Lake quadrangle mapped by Witkind (1976) and including a small part of the Cliff Lake and Hebgen Dam 15-minute quadrangles. This area was chosen for study because of a reported occurrence of talc and the abundance of dolomite, the host rock for talc deposits in southwestern Mon tana. The results were disappointing. Only one small talc occurrence was found in the mapped area and three others nearby but outside the mapped area. Work on the petrography of the metamorphic rocks from this area is in progress, and the results, in cluding the geologic map, will be included in a separate report by the Montana Bureau of Mines and Geology, on the geology of the Centennial Valley and surrounding area. Known talc occurrences in the Henrys Lake Mountains are limited to four localities where small amounts of talc are exposed in dolomite adjacent to dikes or other igneous bodies. None of these talc occurrences is large enough to warrant develop ment. Most of the areas underlain by dolomite in the Upper Red Rock Lake quadrangle were checked, but talc was found at only two localities (described below) within that quadrangle. HL-1 North of Hackett Creek in the S W 'A sec. 5, T. 14 S., R. 1 E. Upper Red Rock Lake 15minute quadrangle. A poorly exposed diabase dike approximately 175 meters (575 ft.) in out crop width has intruded dolomite. A few small chips of talc can be seen in the soil adjacent to the dike. HL-2 N W 'A sec. 18, T. 14 S., R. 2 E. Upper Red Rock Lake 15-minute quadrangle. Talc blebs approximately 1 cm long are scattered through out dolomite in an area one meter square. This occurrence of talc is at the contact of dolomite and a small body o f metagranodiorite. HL-3 A diabase dike trending northeast near the state line is exposed in sec. 24, 27, 33, T. 13 S., R. 2 E. Targhee Peak 7% -minute quadrangle. The dike is well exposed and ranges in thickness from 10 to 50 meters (33 to 160 ft.). Minor talc was found in the dolomite at the contact with the diabase. The greatest concentration of talc is in an area 0.5 by 1 meter (1 by 3 ft.) in which round talc blebs 5 to 10 mm across constitute approximately 50 percent of the rock. Dolomite adjacent to other diabase dikes in this area was 'not checked for talc. HL-4 Northeast side of Elk Mountain in the NE% sec. 33, T. 13 S., R. 1 E. Upper Red Rock Lake 15-minute quadrangle. Talc blebs less than 1 cm long are found scattered through dolomite within 3 meters (10 ft.) of a gabbro dike. No significant concentration o f talc was recognized. .f i II ( f ) ( ( f \ 1 l i GUNTER00000877 59 Other talc occurrences Besides the described talc occurrences in the pre-Beit metamorphic rocks of southwestern Mon tana, there are only tw o other known talc occur rences in the state, one south o f Helena and one northeast o f Troy. Both are described below. the adit, which is within 50 feet (16 m) of the load ing platform. The Hasmark Dolomite has been metamorphosed in this area to medium-grained white dolomitic marble, presumably by a small nearby granodiorite pluton. 0-1 Talc mine south o f Helena Location: SWV* SE% sec. 35, T. 10 N., R. 9 W. (adit), NE% SW54 sec. 36, T. 10 N., R. 4 W. (quarry), Lewis and Clark County. Helena 15minute quadrangle. The quarry is approximately 2 miles (3.2 km) southwest of the center of Helena. Accessibility: Both talc occurrences are adjacent to the road along Grizzly Gulch southwest of Helena. Ownership: Not known. Description: Beginning in 1935 talc was mined from what had formerly been a limestone quarry. The duration of talc mining at this quarry is not known, but there has been no mining in recent years. Perry reported (1948) that talc was first recognized as an impurity remaining after the limestone was heated to make lime. The remains of the lime kilns are still standing (1978) between the quarry and the Grizzly Gulch road. The quarry is in Hasmark Dolomite (Upper Cam brian). (The unit called the Hasmark Dolomite in the Helena area is now generally referred to as the Pilgrim Limestone.) Perry reported that irregular veinlike bodies and stringers of talc range in thick ness from less than 1 inch (2.5 cm) to 6 feet (2 m) and that these bodies may have a vertical dimen sion of 12 feet (4 m). The largest stope in the talc was 35 feet (11m) long, 6 feet (2 m) wide, and 8 feet (2.5 m) high. On the surface talc has been traced intermittently for 350 feet (109 m). Most of the talc is white, but limonite derived from the weathering of pyrite has locally stained the talc. An adit (now caved) and several small cuts have been excavated into the Hasmark Dolomite ap proximately 1 mile (1.6 km) southwest of the lime stone quarry. Although the author did not find any talc in these cuts, some pieces of coarse-grained dark-green talc were found next to an old loading platform. Perhaps some talc was encountered in Sources of information: Knopf, 1963, map; Perry, 1948, p. 10, 11. 0-2 Lynx Creek (Mathews) talc prospect Location: Because sections are not shown on the map in this area, the prospect is located by UTM coordinates. The UTM northing is 5372220, the easting is 592110, and the locality is in zone 1, Lincoln County. The prospect is on the northeast trending ridge between King Mountain on the southwest and China Mountain on the northeast, Kootenai Falls 7 %-minufe quadrangle, and is in the first major saddle northeast of King Mountain. The Lynx Creek road crosses the ridge between King Mountain and China Mountain in this saddle. The talc deposit is approximately 6 miles (10 km) northeast of Troy. Accessibility: The Lynx Creek road branches from the O'Brien Creek road northeast of Troy in the S'/j sec. 32, T. 32 N., R. 33 W. The distance by road from O'Brien Creek to the deposit is approxi mately 6 miles (10 km). Ownership: The deposit was located by a Mr. Mathews prior to 1958. . Description: The host rock for the talc is the Striped Peak Formation of the Belt Supergroup (Precam brian). At this locality the Striped Peak Forma tion consists of argillite and quartzite, strikes northeast, and dips 20 to 30 SE. (fig . 23). Johns (1970, p. 152) reported that the deposit has been investigated by 15 vertical drill holes ranging in depth from 2 to 43 feet (0.6 to 13 m). The ore body is 100 by 250 feet (31 by 78 m) and extends to a depth of at least 40 feet (12 m). The talc is gray, yellow gray, yellow brown and green gray, and is described as sericitic talc that contains dissemi nated pyrite; secondary iron minerals; and some quartz. Source of information: Johns, 1970, p. 152-153. GUNTER00000878 60 To Troy via Lynx Creek X 21 EXPLANATION X Talc outcrop X ta lc /' X arginile Drill hole number with depth of hole Strike and dip of bedding --------- Contact -------- Road partly grown -------- over in 1978 China Lake Figure 2 3 - Lynx C re ek (M a th e w s ) ta lc p ro s p e c t, Linco ln C o u n ty {m o d ifie d fr o m J o h n s , 1970, p. 152). Other areas of pre-Belt metamorphic rocks Pre-Belt metamorphic rocks are exposed in seven or possibly eight areas in Montana in addition to those mountain ranges in which talc and chlorite deposits are known to occur. Talc deposits are lack ing in these areas simply because dolomitic marble is lacking or very sparse. Blacktail Range The Blacktail Range is situated south of the talcrich Ruby Range. No marble is reported from the Blacktail Range (Heinrich and Rabbrtt, 1960, p. 36 38). The major pre-Belt metamorphic rock types ex posed in this range are Dillon Granite Gneiss, pre Cherry Creek gneiss, and an ultramafic body. Tendoy Mountains Scholten, Keenmon and Kupsch (1955, p. 351) described marble, mainly calcitic, within the se quence of pre-Belt metamorphic rocks exposed in the Tendoy Mountains southwest of Dillon. The mar ble is a minor unit in this area, in which quartzofetdspathic gneiss dominates. They did not mention talc in the marble. Snowcrest Range A small area on the northwest flank o f the Snowcrest Range, southeast from the Ruby Range, is underlain by pre-Belt metamorphic rocks. Al- GUNTER00000879 61 i McMannis and Palmquist, 1975, p. 14-15). The ma - r ,. of the metamorphic rock is quartzofeld- ble layer, named the George Lake Marble, is as much S S S S s . C r o. garble 30 M O m> . h i * is as 192 feet (60 m) thick and can be traced fo r more enorted (Heinrich and Rabbitt, I960, p- than 18 miles (29 km) around the core of a nappe. Tremolite and fine-grained chlorite but no talc are Although it is mainly dolomitic, some calcitic marble Lund in the marble. This area of Precambnan meta is intimately intermixed. Diopside in the marble is in morphic rocks is not shown on the Geologic Map o some places replaced by tremolite. Reid, McMannis Montana (Ross, Andrews and Witkind, 1955). and Palmquist concluded that growth of coarse Spanish Peaks area grained tremolite was followed by the growth of fine grained tremolite and talc. No mention is made of the A large part of the Spanish Peaks area at the abundance of the talc. north end of the Madison Range is underlain Idv pre Belt metamorphic rocks. Spencer and Kozak <19/d> described the geology of a large area that extends from the Tobacco Root batholith in the center of the Tobacco Root Mountains east to the Gallatin River. The only marble described in that area crops out The major Precambrian rock types in the Jardlne district, in the southwestern part o f the Beartooth Mountains, are schist and what is thought to be a Precambrian granitic pluton (Seager, 1944, p. 21-34). No marble is described jn this area. southwest o f Cherry Lake where it is 20 feet (6 m) thick. Cherry Lake is approximately 11 miles.08 km) Little Belt Mountains northeast of Ennis. The marble contains calcite ac companied by minor tremolite, antigorite and diopside, but no talc (Spencer and Kozak, 1973, p. 43). The core of the Little-Belt Mountains o f central Montana consists of pre-Beft metamorphic rock, mainly schist, gneiss and the Pinto metadiorite (Ca Northern part of the Gallatin Range tanzaro and Kulp, 1964, p. 88-93), but marble is not described. Henry G. McClernan, who has studied an Pre-Belt metamorphic rocks are exposed south of Bozeman in the northern part of the Gallatin Range. Descriptions of the geology of this area by McMannis and Chadwick (1964), Tysdal (1966), and area on the southwest flank of the Little Belt Moun tains, reported that no marble was recognized within the sequence of pre-Belt metamorphic rocks (per sonal communication, 1978). Weber (1965) make no mention of marble. The most abundant rock is quartzofeldspathic gneiss, which is Other areas of possible pre-Belt rocks associated with amphibolite. High-rank metamorphic rocks exposed along Beartooth Mountains All of the known talc areas o f southwestern Montana could fit within the area of pre-Belt meta morphic rocks exposed in the Beartooth Mountains, which are situated in south-central Montana just the eastern edge of the Idaho batholith in the Bitter root Range have been suggested by some to be pre Belt in age. Whether they are pre-Belt or metamor phosed sedimentary rocks of the Belt Supergroup or equivalent is an interesting question, but more im portant to talc exploration is the lack of marble (Berg, north o f Yellowstone National Park. The predomi 1977; Chase, 1973). Carbonate bodies in the'm eta nant metamorphic rock types are granitic gneiss, migmatite and amphibolite (Poldervaart and Bentley, morphic rocks of southern Ravalli County thought to be carbonatites contain rare-earth elements. Detailed 1958). Marble is reported only in the North Snowy Block in the northwestern part of the range (Reid, study of the mineralogy of the carbonate rocks by Crowley (1960) failed to find talc. GUNTER00000880 63 References a ,,a n S . G . 1976, Asbestos minerals and their nonasbestos " 'analogs: Paper presented at the Review of Mineral Fibers Ses sion of Electron Microscopy o f Microfibers, Pennsylvania State University, University Park, Pennsylvania, August 23-25, 19/0, 30 p. B ara R. B ., 1977, Reconnaissance geology of southernmost Ra valli County, Montana: M ontana Bureau of Mines and Geology Memoir 44, 39 p. ________ 1979, Chlorite deposit in Precambrian quartzofeld- spathic gneiss. Silver Star, Montana labs.): Geological Society of America Abstracts with Programs, v. 11, no. 6 , p. 266. B ro w n , B . E .. and B ailey, S . W ,, 1962, Chlorite polytypism: I. Regular and semi-random one-layer structures: American Mineralogist, v. 47, p. 819-850. B urger. H . R .. I ll, 1967, Bedrock geology of the Sheridan dis trict, Madison County, Montana: Montana Bureau of Mines and Geology Memoir 41, 22 p. C ata n zaro , E. J .. an d K u lp . J . L-, 1964, Discordant zircons from th e Little Belt (M ontana), Beartooth (M ontana), and San ta Catalina (Arizona) Mountains; Geochimica et Cosmochimica Acta, v. 28, p, 87-124, C hase. R. B ., 1973, Petrology of the northeastern border zone o f the Idaho batholith, Bitterroot Range, Montana: Mon tana Bureau of Mines and Geology Memoir 43, 28 p. C hid ester, A . H ., Engel, A . E. J ., an d W rig h t, L. A ., 1964, Talc resources of the United States: U .S , Geological Survey Bulletin 1167, 61 p. C lifto n , R. A ., 1978, Talc and pyrophyllite, in Minerals Yearbook, 1976: U .S . Bureau of Mines, v. 1, p. 1309-1315. C ordua, W . S ., 1973, Precambrian geology of the southern Tobacco Root Mountains, Madison County, Montana: Bloom ington, Indiana, Indiana University, unpublished Ph.D.disserta tion, 25B p. C ro w ley , F. A ., 1960, Columbium-rare-earth deposits, southern Ravalli County, Montana: Montana Bureau of Mines and Geology Bulletin 18, 47 p. D eer, W . A ., H o w ie , R. A ., and Zu ssm an , J ., 1962, Rock forming minerals, v, 3, Sheet silicates: New York, John Wiley, 270 p. D uncan, M . S ., 1976, Structural analysis of the pre Beltian metamorphic rocks of the southern Highland Mountains, Madison and Silver Bow Counties, Montana: Bloomington, In diana, Indiana University, unpublished P h.D . dissertation, 222 p. Foster, M . D ., 1962, Interpretation o f the composition and a classification o f th e chlorites: U .S . Geological Survey Profes sional Paper 414-A, p. A1-A33. Fritzsche, H ans, 1935, Geology and ore deposits of the Silver Star mining district, Madison County, Montana: Butte, Mon tana, Montana School of Mines (now Montana College of Min eral Science and Technology), unpublished M .S . thesis, 89 p. G arihan, J . M ., 1S73a, Geology and talc deposits o f the central Ruby Range, Madison County, Montana: University Park, Pennsylvania, Pennsylvania State University, unpublished Ph.D. dissertation, 200 p. -- -----------1973b, Origin and controlling factors of the taic depos its of steatite grade in the central Ruby Range, southwestern Montana [abe.]: Geological Society of America Abstracts with Programs, v. 5, no. 2, p. 104. G arih an , J . M ,, and O k u m a . A . F., 1974, Field evidence sug gesting a nonigneous origin for the Dillon quartzofeldspathic gneiss, Ruby Range, southwestern Montana [abs.k Geologica 8 . . ... i; R rlA fl. p. 510. G ea ch R D ., 1972, Mines and mineral deposits (except fuels), Beaverhead County, Montana: Montana Bureau of Mines and Geology Bulletin 85, 194 p. G ile tti, B. J ., 1966, Isotopic ages from southwestern Montana: Journal of Geophysical Research, v. 71, p. 4029-4036. G illm e is te r, N . M -, 1972, Petrology o f Precambrian rocks in the central Tobacco Root Mountains, Madison County, Montana: Cambridge, Massachusetts, Harvard University, unpublished P h.D . dissertation, 201 p. G o o d w in , A urel, compiler, 1974, Symposium on talc, Washington, D .C ., May 8 , 1973, Proceedings: U .S . Bureau of Mines Information Circular 8639, 102 p. H ad ley. J . B-, 1969a, Geologic map of the Cameron quadrangle, Madison County, Montana: LL&. geolo gical Survey Map GQ-813. ________ 1969b, Geologic map of the Varney quadrangle, ' Madison County, Montana: U .S . Geological Survey Map GQ-814. H an ley, T . B ,, 1975, Structure and petrology of the northwestern Tobacco Root Mountains, Madison County, Montana: Bloom ington, Indiana, Indiana University, unpublished P h.D . disser tation, 289 p. H e in ric h , E. W ,, an d R ab b it. J . C ., 1960, Pre-Beltian geology of the Cherry Creek and Ruby Mountains areas, southwestern Montana: Montana Bureau of Mines and Geology Mem oir 38, 40 p. 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W ., 1969, M ap show ing lithology o f Precambrian rocks in the Christensen Ranch and adjacent quadrangles, Madison and Beaverhead Counties, Montana: U .S . Geological Survey Open-File M ap, 1 sheet, scale 1 :20,000. Jo h n s. W . M ., 1961, Geology and ore deposits of the southern Tidal W ave mining district, Madison County, Montana: Mon tana Bureau of Mines and Geology Bulletin 24, 53 p. ________ 1970, Geology and m ineral deposits o f Lin coln and Flathead Counties, Montana: Montana Bureau of Mines and Geology Bulletin 79, 182 p. GUNTER00000881 64 K n o p f, A d o lp h , 1963, Geology of the northern part of the Boulder bathylith and adjacent area, Montana: U .S . Geological Survey Miscellaneous Geologic Investigations Map 1-381. K o eh ler, S. W ., 1976, Petrology of the diabase dikes of the T o bacco Root Mountains, Montana: in Guidebook --The Tobacco Root Geological Society 1976 Field Conference: Montana Bureau of Mines and Geology Special Publication 73, p. 27-36, K re m p a s k y , G. T., and L a w so n , D . C ., 1977, The mineral in dustry of Montana, in Minerals Yearbook, 1974: U .S . Bureau of Mines, v. 2, p. 421-432. ________ 1978, The mineral industry of Montana, in Minerals Yearbook, 1975: U .S . Bureau of Mines, v. 2, p. 449-460. ________ 1979, The mineral industry of Montana, in Minerals Yearbook, 1976. U .S. Bureau of Mines (in press). L e v a n d o w s k i, D. W ., 1956, Geology and mineral deposits of the Sheridan-Alder area, Madison Cobnty, Montana: Ann Arbor, Michigan, University of Michigan, unpublished Ph.D. disserta tion, 318 p, M a n n , J . A ., 1954, Geology of part of the Gravelly Range, M on tana: Yellowstone-Bighorn Research Project Contribution 190, 92 p. M arvin . R. F., W ier, K. L., M ehnert. H. H ., and M e rritt, V . M ., 1974, K-Ar ages of selected Tertiary igneous rocks in southwestern Montana: Isochron W est, no. 10, p. 17-20. M c D o w e ll, F. W ., 1971, K-Ar ages of igneous rocks from the western United States: Isochron W est, no. 2, p. 1-16. M c M e n n is , W . J ., and C h a d w ic k , R. A ., 1964, Geology of the Garnet Mountain quadrangle, Gallatin County, Montana: M on tana Bureau of Mines and Geology Bulletin 43, 47 p. M illh o lla n d , M . A ., 1976, Mineralogy and petrology of Precam brian metamorphic rocks o f the Gravelly Range, southwestern Montana: Bloomington, Indiana, Indiana University, unpub lished M .A . thesis, 134 p. M o n ro e , J . S ., 1976, Vertebrate paleontology, stratigraphy, and sedimentation o f the upper Ruby River Basin, Madison County, Montana: Missoula, Montana, University of Montana, unpub lished Ph.D. dissertation, 345 p. M u e lle r, P. A ., and C o rd u a , W . S ., 1976, Rb-Sr whole rock age of gneisses from the Horse Creek area. Tobacco Root Moun tains, Montana: Isochron W est, no. 16, p. 33-36. M u lry a n , H . T ., 1974,'Characternation and occurrence of talc, in Goodwin, Aurel, compiler. Symposium on talc, Washington, D .C ., May 8 , 1973, Proceedings: U .S . Bureau of Mines Infor mation Circular 8639, p. 16-21. O k u m a , A . F., 1971, Structure of the southwestern Ruby Range near Dillon, Montana: University Park, Pennsylvania, University of Pennsylvania, unpublished Ph.D. dissertation, 122 p. O lson, R. H ., 1976, The geology of M ontana talc deposits, in Eleventh Industrial Minerals Forum, Proceedings: Montana Bureau of Mines and Geology Special Publication 74, p. 99-143. P eale , A . C ., 1896, Three Forks, Montana: U .S. Geological Sur vey Geological Atlas, Folio 24, 7 p. P erry, E. S ., 1948, Talc, graphite, vermiculite, and asbestos in Montana: Montana Bureau of Mines and Geology Memoir 27, 44 p. 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G ., 1966, Geology of a part o f the north end of the Gallatin Range, Gallatin County, Montana: Bozeman, Mon tana, Montana State University, unpublished M S. thesis, 95p. --------------1976, Geologic map of northern part of Ruby Range, Madison County, Montana: U .S . Geological Survey Mis cellaneous Geologic Investigations Map 1-951. W e b e r, W . M ., 1965, General geology and geomorphology o f the Middle Creek area, Gallatin County, Montana: Bozeman, Mon tana, Montana State University, unpublished M .S . thesis, 86 p. W elch , J . R., 1973, The mineral industry of Montana, in Minerals yearbook, 1971, v. 2, p. 437-451. - ........ -- 1974, The mineral industry of Montana, in Minerals yearbook, 1972, v. 2, p. 425-437. W ells. J . R ., 1976, Talc, soapstone, and pyrophyllite, in Mineral facts and problems, 1975 edition: U .S . Bureau of Mines Bulletin 667, p. 1079-1090. W e s t, J . M ., 1972, The mineral industry of M ontana, in Minerals yearbook, 1970, v. 2, p. 425-441. GUNTER00000882 65 ________1976, The mineral industry o f Montana, in Minerals yearbook, 1973, v, 2, p, 417-431. W hitehead, M . L., 1979, Geology and talc occurrences of the Benson Ranch, Beaverhead County, Montana: Butte, Mon tana, Montana College of Mineral Science and Technology, unpublished M .S . thesis, 53 p. W ier, K. L., 1965, Preliminary geologic map of the Black Butte iron deposit, Madison County, Montana: U .S . Geological Survey Open-File Map. W itk in d , I. J ., 1972, Geologic map of the Henrys Lake quadrangle Idaho and Montana: U .S . Geological Survey Miscellaneous Geologic Investigations Map 1-781-A. ________ 1976, Geologic map of the southern part of the Upper Red Rock Lake quadrangle, southwestern Montana and adja cent Idaho: U .S , Geological Survey Miscellaneous Geologic Investigations Map I-943. Production Inform ation Camera-ready copy prepared on EditWriter 7500 by MB MG, Stock: Cover -- 17 pt. Kivar Text -- 70 lb. Mountie Matte Sheets -- 60 lb. Offset Book Com position: U n ivers type Heads -- 1st order 118 pt. theme, leaded 2 pt.); 2d order (12 pt. theme, leaded 2 pt.); 3d order (10 pt, italics theme, leaded 2 pt.) Text -- 10 pt. theme, leaded 2 pt. References -- 8 pt. theme, leaded 2 pt. P ressw ork: Heidelberg Sorkz In k: Leber O rder: 2,000 copies GUNTER00000883