Document b5z2x1Y15K7Ng4Qw2oXByOnk1

TALC AND CHLORITE DEPOSITS IN MONTANA by Richard B. Berg MEMOIR 45 !MONTANA BUREAU OF MINES AND GEOLOGY A Department of 1979 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 exploration 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 described in the I~terature. In an effort to make this publication more useful to the individual unfamiliar with 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 of the project and are much appreciated: Wilt Goldberg (1973~,4-1al Koechiein (1974), Roger Kuhns (1975), Leroy Swanson (1976) and Steve Czehu~a for a short time in 1977. I enjoyed working with each of them. Sam Maloney, Pete Womack and Bob Notte were very helpful in showing us talc prospects. Ken Wier showed interest in our work in the Greenhorn Range and offered suggestions 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, led Houser and John Brady also provided me with an opportunity to discuss my ideas on the origin of Montana talc deposits. Charles Knowies of the Idaho Bureau of Mines and Geology kindly ran microprobe scans of a talc specimen in an effor~ to determine the composition of some very small opaque grains. Individuals involved with the operating talc mines showed us their operations, 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 Car~ Hafer (owner of Willow Creek mine). Butte August 27, 1979 Richard B. Berg Economic GeologiSt Montana Bureau of Mines and Geology G U NTE R00000815 (NERAL SCIENCE AND TECHNOLOGY MONTANA Memoir 45 TALC AND CHLORITE DEPOSITS IN MONTANA by Richard B. Berg 1979 G U NTE R00000816 Contents PREFACE ..................................... i ABSTRACT ................................. 1 INTRODUCTION ............................. 1 Location .................................... Previous work ............................... 1 Present work ................................ COMMERCIAL ASPECTS OF TALC AND CHLORITE ................................. 2 The mineral talc ............................. 2 The mineral chlorite ....... Commercial talc ............................. 3 Fibrous minerals ........ : .................... 3 Uses of talc ................................. 3 PRODUCTION, MINING AND PROCESSING OF TALC AND CHLORITE ................... 4 Major talc producing areas .................... 4 Mining and processing of talc .................. 5 Montana talc production ...................... 5 GEOLOGY AND MINERALOGY OF TALC AND CHLORI.TE DEPOSITS ............. ~ .... 6 Pre-Belt metamorphic rocks ................... 6 Mineralogy ................................. 7 Talc ...................................... 7 Chlorite ................................... 7 Associated minerals .......................... 8 ORIGIN OF TALC AND CHLORITE ............. 12 Conditions of formation ...................... 12 Time of formation ........................... 14 Stratigraphic and structural control ............ 14 EXPLORATION FOR TALC .................... 15 HIGHLAND MOUNTAINS .................... 19 H-1 Golden Antler mine ................. ~...19 TOBACCO ROOT MOUNTAINS ............... 21 TR-1 MineraJ Hill prospect .................. 21 TR-2 Spuhl~r'Gt~lch 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 of Whitney claims .... 33 R-6 Prospect north of Treasure mine ......... 33 R-7 Prospect northeast of Treasure mine ...... 33 R-8 Bennett Owen claim .................... 33 R-9 Treasure mine ......................... 34 R-10 Beaverhead mine ..................... 34 R-!1 Prospect east of E]eaverhead mine ....... 35 R-12 Regal (Keystonel mine ................. 35 R-13 American Chemet mine ................ R-14 Estelle ! Sweetwater) mine .............. 36 R-t5 Smith-Dillon mine ..................... 36 R-16 Banning-Jones mine .................. 36 R-17 Bozo-Zobo mine ...................... 37 R-18 Crescent prospect (Timber Gulch deposit ................................. 37 R-19 Sauerbier mine ..........." ............ 37 R-20 Owen-McGovern prospect ............. 38 Other talc occurrences ....................... 38 GREENHORN RANGE ........................ 39 West of the Ruby River ..................... 39 East of the Ruby River and north of Idaho Creek .................................. 40 Between Idaho Creek and the North Fork of Greenhorn Creek ...................... 40 GH-28 Ruby claims ........................ 40 GH-30 Doubtful claims ..................... 42 Area south of the North Fork of Greenhorn Creek .................................. 42 GH-42 Willow Creek mine (Ruby Ridge mine) .................................. 43 GH-43 Claims north of Willow Creek (Adam and Eve No. 1 and No. 2) ................ ...46 GH-45 Talc occurrence south of Virginia City..46 GH-46 Calverts claims ...................... 46 Other prospects and inactive mines ........... 48 GRAVELLY RANGE ................ ; ......... 49 GR-1 Tait Mountain claims .................. 49 GR-2 Cherry Gulch prospect ................ 50 GR-3 Yellowstone mine .................... 53 GR-4 Queen claim ......................... 55 GR-5 Burlington Northern mine .............. 55 ,GR-6 Talc-bearing conglomerate north of Johnny Gulch ........................... 55 MADISON RANGE .......................... 57 HENRYS LAKE MOUNTAINS ................. 58 OTHER TALC OCCURRENCES ................ ~ O-1 Talc mine south of Helena ............... 5~ 0-2 Lynx Creek |Mathews) talc prospect ...... 59 OTHER AREAS OF PRE=BELT METAMORPHIC ROCKS .................................. 60 Blacktail Range ............................. 60 Tendoy Mountains .......................... 60 Snowcrest Range ........................... 60 Spanish Peaks area ......................... 61 Northern part of the Gallatin Range ............ 61 Bear~ooth Mountains ........................ 61 Little Belt Mountains ........................ 61 Other areas of possible pre-Belt rocks .......... 61 REFERENCES ............................... 63 GUNTER00000817 Figures 1- Mountain ranges and major roads in area 11--Prospect southwest of Ennis, Tobacco of talc and chlorite occurrences in southwestern Montana ........................ 2--Plot of compositions of analyzed chlorite .... 10 3--Talc and chlorite occurrences in south- western Montana. " .16 Root Mountains ......................... 30 12-- Ruby claims, Greenhorn Range ............ 41 13-- Doubtful claim, Greenhorn Range .......... 43 14--Greenhorn claims, Greenhorn Range ........ 44 15--Geologic map ofthe area surrounding the 4--Horse Creek prospect, Tobacco Root Mountains .............................. 23 5-Cut northwest of Harris Creek, Tobacco Root Mountains ......................... 25 6-Prospect southeast of Harris Creek, Tobacco Root Mountains ................. 25 7--Southern Granite Creek prospect, Tobacco Root Mountains..~ ............... 27 B--Northern Granite Creek prospect, Tobacco Root Mountains ................. 27 9--Northern cut at Bear claims, Tobacco Root Mountains. " .28 Willow Creek mine ....................... 44 16-- Diagrammatic cross section of the talc deposit at the Willow Creek mine ........... 45 17-- Cal~erts claims, Greenhorn Range .......... 47 18--Tair Mountain claims (northern part), Gravelly Range .......................... 50 19--Tait Mountain claims (southern part), Gravelly Range .... :.. .................... 51 20--Generalized geologic maP of the Cherry Gulch- Johnny Gulch area of the Gravelly Range .... 52 21--Burlington Northern mine, Gravelly Range...54 22--Inactive pit on the Queen claim, Gravelly Range ............... :,- = .-.. .............. 56 10--Southern cut at Bear claims, Tobacco 23--Lynx Creek {Mathews) tatc ~rospect, Root Mountains ......................... 29 Lincoln County .......................... 60 42 Tables 46 1--Comparison of mineralogy of commercial talc 5--Analyses of ceramic and lava talc from the 46 from New York, California and Montana ...... 3 Yellowstone mine ........................ 13 46 2--Change in Montana talc production from. 6--Talc and chlorite occurrences ............... 17 48 1970 through 1976 ........................ 6 7--Additional talc occurrences in the Ruby 3'Chemical analyses of talc and chlorite 49 Range not plotted on Figure 3 .............. 38 ~.9 specimens ...... ......................... 8 ~0 4--Structural formulas and characteristics of i3 analyzed chlorite .......................... 8 ~5 Plates Photomicrographs of analyzed talc and chlorite .................................. 9 2--Photomicrographs of alteration sequence at. the Golden Antler mine ................... 20 0 0 0 1 | Cover-- Lava talc exhibiting dendrftic patterns. Specimen No. 7330 M, I Courtesy: Mineral Museum, Momana Tech, FJeanor Herndon I sketch, MI]MG~ I T~tle Page--Talc mines in the Ruby Range (9/1/79). Foreground: | Treasure mine (Pfizer, lnc.); background: Baeverhesd mine (Cyprus Industrial Minerals). Photo by Aero Tech Surveys, Inc., Riverside, CA., courtesy: Pfizer, inc., Dillon, MT, GUNTER00000818 vi N MOUNTAINS win .ges Ai RANGE'~ Virginia Bozemsn Z HENRYS LAKE MO U NTA I N S .'~H_ MONTANA 20 MILES 20 KILOMETERS Figure 1--Mountain ranges and major roads in area of talc and chlorite occurrences in southwestern Montana. GUNTER00000819 Abstract Talc occurrences range from small veinlets and pods less than one centimeter thick to one body of talc 30 meters thick. Chlorite tvariety ctinochlore) 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-Belt 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 + H=O - talc + 3 calcite + 3 Because the dolomitic marble contains insufficient Si02 for this reaction, it is suggested that SiO~ 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 quartzofeldspathic gneiss and diabase, which are more aluminous than the marble. 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 ex*cept two deposits lie in southwestern M~n~a~a. 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 importance in Montana occur in Precambrian marble within the sequence of pre-Belt metamorphic rocks exposed in the southwestern part of the state (fig, 1). fn 1978 there were five active talc mines and one active chlorite mine in Montana. Most of the talc is produced from three large mines, the Beaverhead, Treasure and Yellowstone. in 1976 Montana talc production 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 of southwestern Montana is a lengthy article by Olson (1976, p. 99-143). This article empha-sized the geology and talc deposits of the Ruby Range, which were (~escribed 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 ran.qe. Perry (1948) described the talc mines that were operating then and also mentioned two prospects. A description, including maps, of the Johnny Gulch talc deposit (now the Yellowstone 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 larg~ talc deposit was exposed only by underground workings of modest extent and by minor surface excavations. Chidester, Engel and Wright briefly described Montana talc deposits 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 15reject was begun late in the summer 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 occurrences in the Ruby Range. A total of four months during the summers of 1974 and 1975 was spent mapping the Precambdan 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 Bureau of Mines and Geology in April 1976 (Open-File Report MBMG 19). The open-file report is super- GUNTER00000820 sealed by this memoir and a forthcoming report on the Precambrian geology of the Greenhorn Range. Mapping the Precambrian geology of an area including part of the Henrys Lake Mountains required one and a half months during the summer of 1976. The area is approximately 45 miles (72 kin) 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 Montana 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 two 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 7-minute quadrangle maps that had been enlarged to a scale of 4 inches equal t 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 examined 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 available for study. Both ]]ritish and metric units are given for measurements made in the field. Where the measurement 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 si]icate with the ideal formula Mge [SisOzo](OH)4. Reported chemical analyses of presumably pure talc indicate that the major deviations ~from the formula are in the presence of AI, Fe=, and "F~+z. Both talc and chlorite are classified as layer silicates because the arrangement of constituent ions produces a layered structure. Because of that structure, talc, chlorite, and other layer silicates such as mica can be splk into very thin sheets. This ability to split into thin sheets contributes 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 of the mineral. Other physical properties of talc that are important 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 sptits into flakes, make it practical to pulverize talc into very small particles only a few microns in size. Many of the uses of talc require such extremely fine-grained material. Also required in many applications is a material of light color, almost white. Pure talc when pu verized produces a powder that looks dead white to the eye. Montana talqthat is very pale green in hand specimen produces a white powder when pulverized. In those applications where talc is used as a filler, its chemical inertness is an important property. For example, inertness is very important when talc is used as a diluent in pills. The mineral chlorite Unlike talc, ct~lorite has a wide range in chemical composition, as shown by the general formula (Mg, AI, Fe)t= [(Si, AI)a Ozo] (OH)I~. Actually, chlor~te is a group of minerals that can be divided into individual species on the basis of the Fe: Mg ratio, AI: Si ratio, or crystal structure. Although not nearly as valuable commercially as talc, the chlorite group is very important mineralogicalty. 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 ea- ff ish :tic " Commercial talc The industrial commodity talc can be very different thanthe mineral talc. Much of the talc used commercially contains a large concentration of tremolite {Table 1). Other minerals that may be present in commercial talc are chlorite, mica, dolomite, magnesite, pyrophyllite, serpentine, anthophyllite, quartz and montmorillonite. Roe (1975, p. 1134) classified commercial talc into 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 for electrical insulators, After the soft talc is machined 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 sometimes called "lava talc". Some steatite talc used for electrical insulators has been mined at the Yellowstone mine. IThe designation steatite has also been used to describe talc of high purity although it may not possess the physical properties required of "lava talc". Much of the talc mined in Montana has been described as steatite-grade talc.) Soft platy talc: This variety of talc has been formed by the replacement of magnesium carbonate rocks and commonly contains minor chlorite, Almost all of the Montana talc could be inctuded in this category. Tremolite talc: (Also called "hard talc" or "hard ore".) In addition to talc and tremolite, this variety of commercial talc may contain anthophyllite, calcite, dolomite, and serpentine. New York talc and some of the California talc are of this variety. Mixed talc ores'. Soft friable talc that contains dolomite, calcite, serpentine and etherminor impuri- ties is included in this category. Alabama talc is an example of this type. Fibrous minerals Within the last few years there has been great concern about the carcinogenic effects of exposure to asbestos and to a lesser extent exposure to other 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. Besides chrysolite, other minerals that may have a fibrous habit are the amphiboles amosite, anthophyllira, crocidolite, tremolite and actinofite. To the talc producer or consumer, tfemolite and anthophyllite are the most important rdinerals of this group because they occur in some talc. A very important distinction must be made between the fibrous and nonfibrous habits of these mine[als. Tremolite and anthophyllite occur in both prisn~'a~i~ and fibrous habit. Specimens having prismatic habit produce small prisms and not fibers when crushed. Anthophyllite has a greater tendency to occur in the fibrous habit than tremolite, which is only rarely fibrous (Ampian, 1976, p. 4). For more information on amphiboles in talc, the reader is referred to Goodwin, 1974. Uses of talc As with many of the nonmetallic mineral commodities, talc has a wide range of uses. Although the various uses of talc have specific requirements, most of them are based on the unusual physical properties of.talc that make it possible to produce a very fine grained white powder that is chemically inert and not abrasive. In 1977 the domestic consumption of talc was ceramics, 33 percent; paints, 24 percent; paper, 8 percent; plastics, 8 percent; roofing, 7 perce~t; cosmetics, more than 5 percent; insecticides, 3 percent; rubber, 2 percent; and numerous other minor uses (Clifton, 1978, p. 166). Table 1--Comparison of mineralogy of commercial talc from New York. California and Montana {from Mulryan, 1~74, p. 16). Mineral concentrations are expressed in percent. New York Source Silver Lake, Ca|ifomia Panamint, California Yellowstone mine, Montana Beaverhead mine, Montana Talc 62 54 62 Carbonate 3 12 Tremol~te 30 43 At~thophy'llite 5" Quartz 3 Other Trace <0.5 2 montmorillonite, 4 mica ! chlorite GUNTER00000822 Talc is used in many ceramic products, especially 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 som6 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 Js also added to paint to decrease gloss. A very important use of talc in the paper industry is in the control of pitch. Pitch from the wood pulp may adhere to the equipment and may also produce brown spots in the finished paper. Fine]y pulverized 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 of talc to some plastics increases their strength and t~ugb,ness. Talc increases the physical stability and resiat~nce to weathering of roofing materials. A small amount of talc dusted on the surface of roofing also helps to prevent layers of the roofing from sticking together during storage. Because of its chemical inertness and fine particle size, talc is used as a carrier for insecticides. Talc of extremely high purity (both chemical and mineralogical) is used in the cosmetic and pharmaceutical industries. Atthough 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 Lapor~e 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 whicl~ is reported to have rheological properties superior to natural hectorite. 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 know~edge, neither process has yet been used commercially. Prices reported for talc in the January 1978 issue of Engineering and Mining Journal (McGraw-Hilt, New York] range from $10.50 to $151 I~er ton, depending mainly on the quality of the talc and the degree to which it has been processed. Although prices were not given for Montana talc, it is probabty 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 o.f 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 of talc, soapstone and pyrophyllite, but provides no annual ranking for talc alone. A large amount of talc is mined in the Balmat-Edwards area of northern New York, where talc occurs in marble of the Grenville Series (Precambrian). Talc ore bodies are in the same stratigraphic unit as zinc ore mined in the district. Most of the talc in this district is of the tremotitic variety. Talc dopes= its in Vermont that have been formed by the alteration 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 Atlamoore 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 distdct near Death Valley in California, which extends northeast into Nevada. Alabama and North Carolina both produce talc, but production is substantially less than that of the states just mentioned. OIson (1976, p. 101-108) gave a more detailed discussion of 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. GUNTER00000823 ~ sur- f the Be- size, f exrical) dusily a best ales ring ~igh ;~76, ~ra ous lure t to ~ec~ )r a :lay ge, sue ~e- ~y he Mining and processing of talc Talc deposits in the United States are mined by conventional methods, both openpit and underground. Some of the talc mined in New York, Vermont and California is produced from underground mines, but Montana production is now (1978) entirely from open pits. In openpit mines it is a common practiceto 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 sequence 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 produce 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 of 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 contains minor iron oxide, pyrrhotite and gersdorffite (NiAsS). Froth flotation is employed to produce high-purity talc from this ore. Froth flotation supplemented 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 with country rock to such an extent that some waste must be mined with the talc. tn this situation, removal of the waste rock by hand sorting has proved to be a practical means of beneficiating the talc ore. Mechanical sorting, using an electro-optical s~rter that rejects material of low reflectivity, has been tried on an experimental basis for this type of ore. As deposits of high-purity talc are depleted, more sophisticated beneflciation techniques will be used to upgrade impure talc. Much of the present market requires talc of high brightness and of extremely fine grain size. Because of its softness talc can be finely pulverized (micronized) in fluid-energy or jet mills. After initial pulverization in roller mills, the talc is fed into the fluid-energy mill, where it is pulverized by attrition in a circutar 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 central port while the coarser particles continue to travel around the periphery of the chamber. A fluid-energy mill can produce a product in which the talc particles are in the range of 0.5 to 10/~m. 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 determination, this characteristic is commonly termed GE brightness. Montana talc production The Montana talc industry has grown to its present size mainly since the late !940s. 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 (Clifton, 1978, p. 1311). Montana was the leading state in the vatue of talc produced and second to Vermont in the amount of talc produced during 1976. Many of the currently active talc mines in Montana were initially underground mines. The ore bodies at the Yellowstone, Beaverhead, Regal and Smith-Dillon mines were all developed by underground 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 Minerals 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 talc 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. Cyprus' 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 consumption of Montana talc are for 1972 when the distribution 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. GUNTER00000824 Although the annual production of talc in Montana has increased greatly over the last 30 years, production increases have not been at a uniform rate. "Fable 2 shows the changes in talc production for the period from 1970 through 1976. From all indications, the talc industry in Montana will continue to grow, and there will be continu- ing exploration for new deposits as well as reevaluation 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 With the exceptions of the talc deposit southwest of Helena (O-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-Beh 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 overlying 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 town on the northeast flank of the Tobacco Root Mountains. The Cherry Creek Series included the metasedimentar~y rocks of the "Cherry Creek beds" as originatly de~s~ri~ed by Peale. Tansley, Schafer and Hart recognized that there was no evidence for Table 2--Change in Montana talc production from 1970 through 1976, cornpi~ed 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, t9730 p, 447. "substantial increase" Welch, 1974, p. 433. 48% increase in tonnage West, 1976, p. 426, 27% increase in value S2.% decrease in t~nnage 84% increase in tonnage 96% increase in value Krempasky and Lawson, 1977, p. 421. Krempasky and Lawson, 1978, p. 45-7. Krernpasky 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 re.y,), 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 t_o 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, between Pony and Cherry Creek depends not on peculiarities visible in every outcrop, but rather depends on the aggregate of rock types present in a rather thick section. Because of possible lateral variation in rock composition, the criteria listed above for distinction may be valid only in the immediate vicinity of the map area. Further, the ~ge relationship between the Pony Series and the Cherry Creek Series is not without uncertaintY. Reid (1957, p. 14) reported that in the northern Tobacco Root Mountains, rocks of the Cherry Creek Series dip under rocks of 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. Heinrich and Rabbitt (1960) described rocks of the Cherry Creek Group and pre-Cherry Creek gneiss from the Ruby Range southwest of the Tobacco Root Mountains. 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 preBelt metamorphic rocks into the Pony and Cherry Creek Series seems premature at this stage in our GUNTER00000825 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 from southwestern Montana presented by Olson (1976, p. 111 ) show that they contain between 0.51 and 1.51 percent total iron expressed as FezO~. Also talc analyses presented in Deer, Howie and Zussman (1962, p. 122, 123), although incomplete in terms of analysis for FeO.and FezO~ in the same specimen, Cherry Creek lithology. show high values of 2.46 percent FeO and 1.49 per- cent FezO3 for different specimens. Both specimens Mineralogy of talc reported in Table 3 contain opaque grains 1 to 2/~m across that were first thought to be a possible source of iron. Those grains are not abundant 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 of the chlorite content of the talcose rock; the greater the concentration of chlorite, the darker the color of the rock. The 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 contamination 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. 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 specimen 3317-5b likewi~se contains a significant concentration of iron, 2.24 percent FeO and 0.51 percent Fe~O=. This specimen contains an estimated 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- Talc from southwestern Montana is relatively lyzed chlorite from the same deposit (Table 3, speci- pure. The major impurity in some of the talc is fine- men 3316-6), it would not contribute as much iron as ) grained chlorite, which is not recognizable in hand reported in the analysis. Because the concentration specimen nor even in some thin sections. X-ray dif- of opaque grains is too tow to account for the iron re- fraction analysis showed chlorite present in concen- ported in the analyses, it can be concluded that most trations of a few percent in many of the light-green of the iron is in the talc lattice. talc specimens. In thin section, talc shows a variety of textures. The block or lava talc (now known afso as carving talc) from the Yellowstone mine and vicinity consists of talc grains only 1 to 2/~m across and of random orientation. At the other extreme is dolomitetremolite-talc schist from the Tait claims (GR-1) that consists of well oriented talc flakes several millimeters across. The grain size of most Montana talc falls between these two extremes. Some talc displays a feathery texture caused by sheaves of talc grains as much as 0.6 mm long that are surrounded by talc grains onty 10/~m across. Several specimens exhibit a mosaic texture that is produced by 0.2- to 4-ram patches of fine-grained talc with almost uniform 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- ~rained talc in which individual flakes are several millimeters across. Chemical analyses of two talc specimens (Table show that both specimens contain a significant Chlorite " Most of the chlorite-rich rock can be distinguished from talc in hand specimen by its darker green color and greater hardness; most rocks consisting mainly of chlorite cannot be scratched by the fingernail. Although generally the greater the concentration 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 analysis of many specimens of chloritic rock showed that almost all contain talc. Zircon, apatite and futile are trace constituents of much chlorite. Except for rare silvery-gray chtoritized biotite crystals 1 to 2 cm across, all of the chlorite is microcrystalline. Two and possibly three distinct varieties of chlorite can be recognized in thin section by differences in color and texture. The most abundant variety is clinochlore, identified by chemical analyses of two specimens in which it is the only chlorite (Table 3). The grain size of most clinochlore is between 2 and 100/~m, and radial sheaves of grains form a common GUNTER00000826 texture (Plate 1). In plane light clinochlore is co!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 co,ors, forms grains as much as 0.5 mm long and vermiform growths. This chlorite, although colorless to pale green in plane ~ight, tends to be greener than the clinochlore. The third possible variety of chlorite also forms coarse flakes but is grayish green when viewed with crossed nicols and is colorless 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 specimens. SiO~ TiO= AI=O~ Fe=O= FeO MgO CaO Na~O K,O H~O+ H~O- Total Talc 83 331~5b 62.~4 0.11 0.37 0.34 1.02 30.43 0.04 0.04 0,02 4,51 0.29 100.0~ 62.42 0.04 0.45 0.51 2.24 2~.35 0_01 0.02 0.0t 4.79 0.09 99.93 Chlorite 145t 3316-6 31.54 1.47 18,59 0.74 3.74 31,70 0.17 0.01 0.02 12.16 0.62 100.76 32.41 n.d, 18.16 0.61 6.16 31.24 0.04 0.01 0.01 12,73 0.31 101.68 Two of the purest specimens of chlorite were a~alyzed 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 c~inochlore field. Structural formufas caicul.ated from these analyses are shown in Table 4. K20, CaO, and TiOz were excluded from these formulas because it is probable that those constituents where present can be attributed to impurities in the chlorite. The high concentration of TiO2 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 chlorite specimens from localities H-I, TR-7, TR-9, TR-IO, GH-28, GH~42, GR-1, and R-10. As would be expected from the geologic environment, all of these specimens are the Ilb polytype. Brown and Bailey (1962, p. 834~.835} found in their survey of chlorite from different environments that apl~roximerely 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 G~lch prospect (GR-2). 3317-5b: Talc from the Willow Creek mine (GH-42). 1451 : Chlorite from R~by claims IGH-281. 3316-6: Chlorite from the Willow Creek mine 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 Wegelin, Frank P. Jones and Gayfe 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. Descriptions of the other five were taken from the literature. Some have little genetic significance for the formation 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 contemporaneous with the talc or chlorite and thus can help in the understanding .of the conditions of formation of the talc or chlorite. Structural formula Variety Index of refraction Color Grain size Impurities Origin Table 4--Structural formulas end characteristics of analyzed chlorite. 1451 from Ruby claims (GH-28) (AI,.. Fe*Z0.~ Fe+==.== Mg~.~=} (AI~.. Si~.,,) O~= Clinochlote I|b polytype 1.583 0,005 Light olive gray 5Y 6/1 3 to 700 #rn Rutile grains 5 to 52/~m long Replacement of diabase dike 3316-6 from Willow Creek mine (GH-42) (Al~.0~ Fe ~ ~ m~' Fe'==~,. Mg, a~)(Ai~l,a SI).~) Ot~ (OH), Clinochlore lib polytype 1.~ 0.~ Dark greenish gray 5G 4/1 ~ to 1~ ~m None det~t~ Repl~ement of quanzofeldspath~c gneiss GUNTER00000827 32.41 18,16 0,61 6.16 31.24 0,'04 0.01 0,01 12.73 0.31 101.68 I A. Specimen 83 of fine-grained talc from Cherry Gulch prospect. Polars crossed, B, Specimen 3317-5b of talc from Willow Creek minePolers crossed. with , the De:eraforther talc zion p in ~ of C. Specimen 1451 of chlorite that has replaced diabase dike et the Ruby claims. Relict ophitic texture shown by masae~ of pure chlorite that has replaced plagioclase and dark areae where chtorite-ruti|e mixture has replaced a roxene. Plane light. 0.1 mm D. Specimen 3316-6 of unusually pure chlorite from the Wiltow Creek mine. Polars crossed. Plate 1 Photomicrographs of analyzed talc and chlorite. GUNTER00000828 10 Ankerite: James (1956, p. 2) reported crystals of ankerite as much as 1 inch (2.54 cm) across found in vugs in siderite at the Yellowstone mine (GR-3). Both siderite and ankerite were described as secondary carbonates. Apatite: Pale greenish-blue apatite crystals < 1 cm long have been identified in chlorite from the Beaverhead mine (R-10). Milky white apatite crystals several millimeters long are a trace constituent of some of the ch]orite from the Willow Creek mine tGH-42). On the basis of indices of refraction t~ = 1.6353, ~ = 1.6397) this is fluorapatite. Brucite: Mitlholland (1976, p. 50) reported the occL~rrence of brucite in marble north of Cherry Gulch in the Gravelly Range. Brucite grains < 0.5 mm long are intimately associated with fine-grained dolomite and talc. Calcite: Calcite coats fractures and fills vugs in some of the talc deposits. The mode of occurrence suggests that the calcite was deposited after the time of talc formation and thus was not produced by the reaction: 3 dolomite + 4 quartz + H20 = talc + 3 calcite + 3COz. Chalcopyrite: Chatcopyrite is a rare constituent of talc, having been reported from only two 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). Dolomite: Coarse-grained dotomite in which some individual rhombs are more than 5 cm across is associated with some talc occurrences. Because dolomite grains this large are found only next to talc bodies, it is inferred that such coarse-grained dolomite is related to the formation of the talc. Graphite: Graphite in the marble was not affected by replacement of the marble by tatc and is now a minor constituent in some talc. Gypsum: Both Okuma (1971, p. 93) and Garihan (1973a) reported the rare occurrence of gypsum in talc in the Ruby Range. Garihan specifically described gypsum in the talc at two prospects north of the Treasure mine (R-6 and R-7} and at the Spring Creek deposit (R-2). Gypsum has not been reported in talc from other mountain ranges. Hematite: Millho!land (1976, p. 50) identified minute hexagonal plates of hematite in some talc from the Gravelly Range. Limonite: Most fractures in pyrite-bearing talc are coated with limonite durived from the weathering of pyrite. Limonite pseudomorphs after pyrite can be found in some talc, for example, the Grandview prospect {TR-7). Magnesite: A maroon rock exposed at the Burlington Northern prospect (GR-5) consists of magnesite accompanied by minor dolomite and talc. Millholland {1976, p. 50) also mentioned magnesite in I TI~URINGITE RIPIDOLITE CLINOCFI LORE PENNINIT~ Figure 2--Plot of compositions of analyzed chlorite. O 20 GUNTER00000829 11 ch marble in this general area. Another talc locality Sepiolite: Masses of splintery sepiolite as much where magnesite is reported is in the NW sac. 13, as 10 cm long are exposed in the lowest cut at the IO- T. 8 S., R. 4 W., in the Greenhorn Range, where a prospect north of Willow Creek (GH-43). Alice specimen of marble contains magnesite, dolomite, Blount (personal communication, 1977) has also I0- talc and minor chlorite. Magnesite also occurs at the identified sepiolite from marble just south of the pit at Treasure mine (R-9). the Willow Creek mine. Malachite: Weathering of chalcopyrite has formed malachite at the Cherry Gulch prospect (GR-2). Garihan {1973a, p. 164) described malachite along fractures at the Bennett Owen claim (R-8). Muscovite (Sericite): Feldspars in quartzofefdspathic gneiss and schist next to many talc or chlorite occurrences have been altered to sericite. Sitlimanite in pelitic schist at the Bivens Creek talc deposit (TR-5) also has been replaced by'sericite. Prehnite: Prehnite has been identified at .only one locality, a small prospect in the Greenhorn Range (GH-38) where prehnite grains as much as 1.5 mm across are associated with fine-grained chlorite and coarser-grained calcite. Serpentine: According to Garihan (1973a, p. 180) thin chrysotile veinlets cut some of the talc at the Gem claim (R-3). Siderite: Siderite is reported to be a secondary carbonate at the Yellowstone mine (GR-3) (James, 1956, p. 2}. Tremolite: Tremolite in talc is reported only from the Ruby Peak occurrence (R-l). At some other localities tremolite blades as much as 3 cm long have been completely replaced by talc. An example can be seen in the longest cut at the Harris Creek prospect (TR-6) where tremolite was formed-in the marble during an earlier stage of metamorphism and was later replaced by talc. Pyrite:, Pyrite is found only rarely in talc or in quartz veins associated with talc. Most of the pyrite pyritohedrons have been replaced by limonite. Pyrotusite(?): Black dendrites in the block talc from the Yellowstone mine (GR-3) and vicinity are probably pyrolusite; they are found only in the block talc., Quartz: Irregular masses of quartz are locally abundant in some talc deposits. Unidentified mineral: Extremely small ( 1 to 3/~m) grains of an opaque mineral are scattered in cloudlike concentrations in some of the talc, Efforts to concentrate the mineral and identify it by x-ray diffraction analysis were unsuccessful. Vermiculite: Vermiculite was identified in talc at only one locality south of Virginia City (GH-45). The size and shape of the vermiculite grains suggest that they were formed by the replacement of phlogopite 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 futile intergrown with 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-28L Z~'rcon: Zircon is a trace constituent of some chlorite. Most zircon grains are subrounded to euhedral 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 replacement of those rocks by chlorite. GUNTER00000830 Origin of talc and chlorite Conditions of formation The following discussion of the genesis of talc and chtorite applies only to those occurrences in the pre-Belt metamorphic rocks of southwestern Montana and not to the deposit northeast of Troy; the deposit southwest of Helena; or those associated with metagabbro dikes in the Henrys Lake Mountains. The chlorite at the Golden Antler mine (H-l) also may be of different origin than the deposits discussed here. Most of the talc in southwestern Montana was formed by the reaction: 3 dolomite + 4 quartz + H=O = talc + 3 calcite + 3 C02. A minor amount of talc has been formed b.y the replacement of t~emolite. Possible equations for this reaction are presented in the following sections. The rarity of calcite in close association with talc indicates that this product of 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 for the complete replacement of dolomite by talc. Quartz is only a minor constituent of the marble. For example, in the Greenhorn Range the marble is estimated to contain tess th&n 10 percent quartz. The other constituent that must be added to produce,ta!c is H20, and it is suggested that silica-rich aqueous solutions caused the replacement of marble by talc. The general lack of quartz in association with talc deposits suggests that the availability of SiOz was the limiting factor in the replacement of marble at a particular locality. Obviously dolomite was plentiful, and the presence of hydrous minerals (sericite, chlorite and serpentine) in rocks adjacent to talc deposits indicates that H20 permeated these rocks beyond the limits of talc bodies. The source of this SiOz-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 piutons had crystallized and been metamorphosed, thus precluding such plutons as sources of hydrothermal fluids at the time of talc format;on. Evidence for the time of formation 0f talc and chlorite is presented in the next section. Another possible source of hydrothermal solutions 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 Greenhorn Range, pegmatite dikes are most prevalent in the quartzofeidspathic gneiss. With the exception of the Golden Antler chlorite deposit (H-l), which is quite possibly of different origin than the talc and chlorite deposits being discussed 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 of cations in solution between marble and the other rock types such as quartzofe~dspathic gneiss. Without 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 H~O 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 quart~,ofeldspathic 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 ch!orite (clinochlore} and talc from the Willow Creek mine (Table 3) shows that the main differences are in the content of SiO~ and AIzO~. The talc contains approximately tWice as much SiO2 as the chlorite and almost no AI=O3 as compared to approximately 18 percent At20~ 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 quartzofeldspsthic gneiss, as at the Grandview prospect (TR-7). A crude comparison between compositions of quartzofeldspathic gneiss in southwestern Montana as reported in the literature and that of chlorite shows that magnesium 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 of 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 elements could not be accommodated in the chlorite Tremotite 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 have been added to the marble to make talc, and some magnesium from the dofomitic marble could 3 tremolite + 6 CO= + 2 H=O = 5 talc + 6 ~alcite + 4 quartz. haye been used in the replacement of quartzofetdspathic gneiss by chlorite. The same transfer of cations may have occurred at the Ruby claims (GH-28) Under the same conditions the temperature range for this reaction is approximately 400 to 460C. where a diabase dike was reptaced 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 estimate of the temperature of talc formation to be made, The same temperature range is inferred for the formation of chlorite, because of its close association with talc. On the basis of thermodynamic extrapolations from experimentally determined points, Slaughter, Kerrick and Wall (1975) showed the effect of mole fraction of COz (XCO2) on the temperature of reaction when fluid pressure (H=O and COz) remains constant. Several of the reactions are pertinent to the replacement of trernolite by talc. These curves were established for flUid pressures of 1 kb, 2 kb and 5 kb, and it is likely that the talc and chlorite formed within that range of pressure. Several possible reactions may explain the replacement of tremolite by talc, one of which is: 3 dolomite 4 quartz + H20 = talc + 3 calcite + 3 COz. Again. according to the curves of Slaughter, Kerrick and Wall, the maximum temperature of the reaction is approximately 460C and the minimum temperature is below 4000. .., : Consideration of the possible reactions shows that the talc in these deposits probably formed at temperatures between 400 and 500C if the fluid pressure was less than 5 kb. Pressure greater than 5 kb during the retrograde metamorphic event seems unlikely. Burger (1967, p. 10) suggested that a pressure of 3 kb was attained during almandineamphibofite facies metamorphism in the Tobacco Root Mountains. tremolite + dolomite + HzO + COz = In the vicinity of the Yellowstone mine (GR-3), 2 talc + 3 calcite. lava or block talc is found in addition to the typical h waxy green talc (ceramic grade). A comparison of Changes in mole fraction of CO= and fluid pressure the chemical compositions of block talc and typical do not cause a large change in the temperature at talc from the Yellowstone mine shows no significant which this reaction occurs. Within the range of con- difference between the two (Table 5). James (1956, ditions for which Slaughter, Kerrick and Wall pre- p. 5) stated that most of the block talc at the Yellow- sented data, the reaction takes place at temperatures between approximately 410 and 470C. stone mine is in a zone of deep weathering. Exami6ation of other occurrences of block talc in this area also shows that block talc is underlain by waxy green A second possible reaction is: talc at depth, suggesting that the block talc has formed by weathering of the usual waxy green talc. tremolite + 4 CO2 = The formation of block or lava talc would be an inter- 2 dolomite + talc + 4 quartz. esting problem for further study. Table 5--Analyses of ceramic and lave talc from the Yellowstone mine (GR-3) (from James, 1956, p. 5). Ceramic grade (typical talc} from pit 5 Ceramic grade {typical talc] from pit 7 Lava grade {b~ock talc) from pit 9 SiO~ 61.70 61.29 61.76 Fe~O= 1,29 1.36 1.33 AI= O~ 1.55 1.34 1.51 CaO trace trace trace MgO 32.44 31.26 31.95 Na20 0.37 0,26 0,27 ' 0.3~ 0.32 .! GUNTER00000832 14 Time of formation Because of the close association of talc and chlorite at many localities, these two minerals are thought to have formed at the same time. It is suggested that the talc and chlorite were produced during greenschist-facies metamorphism caused by a retrograde event tentatively dated as having occurred 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 (Garihan, 1973b; Olson, 1976, p. 113; Okuma, 197!, p. 42). The work described here adds further support to this hypothesis. All of the known talc deposits in southwestern Montana are in dolomitic marble of the pro-Belt sequence of metamorphic rocks. Although there are dolomitic formations of Cambrian, Ordovician and Devonian age in the area, there are no reported occurrences of talc in these rocks. For this reason a Precambrian age is inferred for the talc deposits. Before discussing further the time of talc formation it is necessary to review the metamorphic history of the pro-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 assemblages preserved in these areas are indicative of upper amphibolite-facies metamorphism. Minerals indicative of a later greenschist-facies metamorphic event are reported in the Tobacco Root Mountains and Ruby Range. In the Greenhorn Range, sericitization of feldspa.rs# chloritization of biotite, serpentinization of c'alcsilicate minerals, and alteration of 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 amphibolke metamorphism (Mueller and Cordua, 1976, p. 33). These authors suggested that dates of approximately 1600 re.y. obtained by Giletti (1966) on biotite, muscovite and K-feldspar by K-Ar and Rb-Sr methods represent the later greenschist metamorphic event. It is reasonable to conclude that a greenschiet assembiage 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 comptetion locally. Talc and chtorite are thought to be retrograde rather than prograde minerals because if the talc and chlorite that we see now were formed during prograde metamorphism, they would have been converted to a higher-temperature assemblage of miner- r als during the granulite- and amphibolite-grade metamorphism. Also both talc and chlorite replaced higher temperature minerals; talc replaced tremolite and chlorite replaced biotite. Sericite replaced sillimanite in politic schist next to some talc deposits. tn addition to the dates clustered around 1600 m.y. (!410 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 metagab= bro dikes and are of different origin than the replacement bodies being described here, which have no igneous 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 partly reset Precambrian dates over this large area, then it is reasonable to think that talc and chlorite were formed at the same time. There are two major objections to this idea. The first is that if the talc and chlorite were formed in pro-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 marbte in the proBelt 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 tatc 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 marble. has been tightly folded. The same tendency is noted for some of the talc occurrences in the Greenhorn Range, where minor occurrences of talc are numerous within one layer of marble that has been strongfy 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 mapby 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 p, receded 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 products calcite'and COz have been removed from the system, as is the case in Montana talc deposits. If the assumptions are made that no magnesium was introduced by the talc-forming solutions and that all of the SiO= and H=O were introduced, the final volume 15 of talc would be 71 percent of the initial volume of dolomite. If all of the SiOz required for this reaction was present in the marble initially, then the final volume of talc would be 48 percent of the volume of the starting material. The actual case lies somewhere between these two extremes, and because of the purity of most dolomitic marble, probably closer to the example in which all the SiOz was introduced. The surrounding marble may have adjusted to a volume decrease by flowage and by recrystaltization in the vicinity of small talc pods. Where large masses of marble have been replaced by talc, the decrease in volume may have caused local faulting. Many of the faults now exposed in talc mines may have developed 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 of Laramide age and were concentrated in the talc because of its mechanical weakness as compared to the surrounding marble or gneiss. Probably the faults se~h'i[~ talc mines are of atl three types. Talc-forming solutions followed faults, shear zones, or minor fractures in the marble, a voiume decrease caused post-talc movement along those same surfaces, and they were also sites of Laramide movement. I~ecause of good exposures in talc mines, the abundance of faults in these talc bodies may be overemphasized as compared to the abundance in surrounding metamorphic rocks. Exploration for talc The most direct way to find talc is to examine marble outcrops for taic veins and pods. The most promising areas are seemingly those where the marble 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 for many years, and there are few talcbearing outcrops that do not show signs of having been recognized by the prospector. During field work in the Greenhorn Range, only one talc-rich outcrop war discovered that did not show signs of previous discovery, and that outcrop was in thick timber. Talc is more stable chemically at surface conditions than most minerals, so there is a tendency for it to be concentrated in the soit. Many talc occurrences have 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 andmanganese 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 identify the grains is to try to streak them on a piece o.f steel. Because of its softness talc will easily leave ~1 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 Jooking for talc in the soil is not very effective. Exposures 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 considered. Although soft and easily reduce~t to finegrained sediment by abrasion, tafc should persist in stream sediments because of its chemical stability. Three samples of stream sediment were collected from Jasmine Creek (NE SE sec. 10, T. 8S., 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 GUNTER00000834 16 mesh ( < 44 #m) size fraction of these samples, talc was positiveJy identified in the < 10 #m fraction by Alice Blount of the Newark Museum, New Jersey. Blount also identified talc in these samples by infrared spectroscopy. A sample of stream sediment was collected from Harris Creek in the southern Tobacco Root Mountains approximately 1 mile downstream from two talc prospects (TR-6 and TR-7). Talc could only be tentatively identified by x-ray diffraction analysis of the < 10 #m size fraction of this sample and was not identified by infrared spectroscop~. 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 concentrated in a fine fraction such as < 2/~m. 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 consideration in the search for talc. For orderly description, occurrences of talc and chlorite are grouped by mountain range. The occurrences are shown in Figure 3. Information on the occurrences is summarized in Table 6. Figur~ 3--Talc and chlorite occurrences in southwestern Montana, GUNTER00000835 ined, of ~cen, w~ll hods and 3CUr- the 17 Table 6--Talc and chlorite occurrences. Except as noted, occurrences are plotted on Figure 3. Name Major mineral Extent of development Comments Page reference in 1hie memoir Additional H-1 Golden Antler mine Chlorite TR-! TR-2 TR-3 Mineral Hill ipegmatite Sp~hier Gutch o~u~rence Latest O~ mine Tal Talc Talc(?) TR-4 HO~ C~k pm~t Ch~e TR-5 B~ Creek pros~ct Talc TR-6 TR-7 Ha~ C~k Wo~t G~ pro~e~ Ta~ ~ Talc TR-8 Gr~ Cr~k pm~ Talc TR-9 TR-10 B~r lai~ (Grange C~ mi~e) Pm~ ~uth~ of Enn~ Talc T~ and ~lo~e Ruby Peak occurrence Talc R-2 Spring Creek prOspect Talc Gem claim Talc R-4 WhPmey claims Talc Pro~peat southwest of Talc Prospect north of Tre~ure mine Talc R-7 Prospect northe~sl of Talc Tn~um mine R-8 Bennett Owen c~aim Talc R-9 Trea~Jrs mine Talc R-10 Beav~nbead mine Talc R-11 R-12 R-13 R-14 Pfo~oect east of Beaverhead mine Regal (Keystone) mine Talc Talc .American Chemet mine Ta~ ~b (Sw~ater) mine Ta~ Active mine P~x~tion of d/lk:m)te begl~ lit 1977. Tobecao Roo~I Mmmtsins Short edit Unknown Inacti~ pr~iousm~ei ~ine Ca~ edit Little published information, Tal~:~,e rock Contains serpentine, diopside, graphite end other minerals. Occurrence of talc ha~ not Linen confirmed. Small body of chlorite exposed, p. 19 p. 21 p. 22 p, 22 p. 22 Inclin~ sha~ and L~e cut Adk and ~aflow Talc pods over a large area, minor graphite. Concordant talc layer 4 It. (~.3 m) thick. Teic pods exposed over a large ame. p. 22 ' p. 24 "p. 2~ Indin~ shaR a~d Concordant layer of talc 2 ft. (0~7 m) thick, p. 26 Inactive mine C~s Numerous talc pods in two large cuts, Talc disseminated throughout marbie, p. p, 30 Ruby R=ng'e Shallow prospect pit Talc chips in soil over a large area. Minor trm~ollte in one spec;men. Prospect cute and Talc occurrences exXend for 1.5 km (5,000 ddll holes ft.) a~ong strike. Much variation in color. Two cuts Grsphitic talc, Cuts Irregukar body of generally light green talc. Cms Cuts Cut. Cute Active mine Talc bodies in zone 20 m (64 ft.) wide, Main body of talc ia 2 to 3 m (7 to "~0 ft.) thick. Talc body of 3 m (10 ft.I exposed width. Dark-green talc body 5 by 35 m [16 by 112 Major talc mine owned by Pfizer, Incorporated. Active mine Major talc mine owned by Cyprus industfiei Minerals. Cut Pods of talc exposed in cut_ Active mine Ina~ive mine I~i~ mine Increased production in recent years, IVrme owned by Pfizer, Incorporated. Talc was mined from three pits by Amsrio can Chemat Corporation. Concordan~ layam of green tale exposed in open cut, p, 32 p. 32 p, 32 p. 33 D, 33 p. 33 p. 33 p. 34 p. 34 p. 35 p, 35 p. 35 p. 36 Berg, 1979, p, 266, Reid. 1957. p. 7. 23. Reid, 1957, p. 23. Levandowskl, 1956, p. 271~288, Levsndowski, 1956, p. Z23. Levandowski, 1956, p, 222-224. Garihan, 1973a, p. 1"F/180; Ot~on, 1976, p. 12~-130. Gadhen, 1973a, p. 180181; Olson, 1970, p, 129. Gadhan, 1973a, p. 174; O|~.On, 1976, p. 130. Gerihsn. 1973a, p. 174175; 04son, 1976, p, 130. Gerihan, 1973a, p. t69171. Garihan, 1973~, p, 166168; Olson, 1976, p, 129, Garil~fl, 1973a, p. 162- t66; Olson. I976, p. 128. Gart~an, 1973a, p. 14~158; Olson, 1975, 121-125. Garlhan, 19",/3a, p. 156- 1,80; Olson, 1976, p. 125-126. Gan'Tmno 1973a, p. t60162; Obon, t976, p, 129, Obon0 1976, p. 126; Pen"y, 1948, p. 6. Oku.ma, 1971, p. 108111. Okuma, 197t, p. 106107; Ofson, 1976, p, 128. GUNTER00000836 18 Table 6--Talc and chlorite occurrences. xcept as noted, occurrences are plotted on Figure 3, (continued] Name Meier mineral Extent of development Comments Page reference in this memoir Additional references R-15 Smith Diilon mine Talc Ruby Range (continued) Inactive mine Talc was first mined underground and then in an open pit. R-t6 R-17 R 18 Banning-Jones mine Talc Bozo-Zobo mk~e Talc Crescent prospect (Timber Gulch deposit} Talc Inactive mine Inactive mine Shallow inclined shah, cuts Tatc lan~s in marble scattered over an area 250 by 300 ft. (75 by 90 ml. In 1960s about 8,000 tons of talc ore mined, Graphite abundant in talc. R-19 R-20 Sauerbier mine Owen-McGovern p(ospe~ Talc and chlorite Talc Inactive mine Cuts and dril ho~es Talc mined by Resource Processors, inorporeted, in 1974: Talc layers 1 to 2 ft. IO.3 to 0,6 m) thick are exposed in cut. Nots: Eighteen edditional talc occurrences in the Ruby Range are li~ted in Table 7. Greenhorn Range GH-28 Ftuby claims GH-30 GH-41 Doubtful claim Greenhorn claims Chlorite TaJc Tatc Shallow cuts and small pit Cuts Cuts Mainly tan chlorite, minor talc. Unus~aily soft talc, some limon~e. Sheared ~nd contorted talcose mattde exposed in cut, GH-42 Willow Creek mine GH-43 GH-45 Claims north of Willow Creek South of Virginia City Talc and chlorite Talc Talc GHr4~ Calver~s claims Talc Active mine Six cuts One cut Five cuts Openpit mine operated by Resource Pro ces.sors, Incorporated. Talc veinlots and pods exposed in four cuts. Minor talc in mprble adjacent to quartz vein. Talc Veinlets and pods exposed in four cuts. Gcavelly Range GR-1 GR-2 GR-3 GR-4 GR-5 GR-6 Tail Mountain claims Chsrry Gulch prospect Yellowstone mine Queen otail;p Burlington Northern mine Talc-bearing congtom- Chlorite and talc Talc Talc Talc Talc Talc Shaf~ and cuts Cuts Active mine Inactive mine Inactive mine None Light-colored chlorite and minor talc m'e exposed in cuts. Some block talc exposed in cuts. Major talc mine owned by Cyprus Industdal Minerals. Small open pit owned by Cyprus Industrial Minerals. Talc is exposed in several cuts. Source of talc in conglomerate inferred to be deposit at Yellowstone mine. HL*I HL-2 HL-3 HL-4 Occurrence Occurrence Occurrence Occurrence Talc Talc Talc Talc Henwe Lake Mountains No development Minor occurrence, no economic potential. No development No development No development Minor occurrence, no economic potential, Minor occurrence, no ecOnOmiC potential. Minor occurrence, no economic potential. Other Montana talc occurrences O-1 O-2 Talc mine south of Helena* Lynx Creek IMathews) ta|c prospect* Talc Talc Inaclive mine and prospect cuts Ddll holes Reported bodies of talc 6 ~. (2 m] thick. Seric~tic talc in shades of yellow and gray. *Not plot'~ed on Figure 3. p. 36 p. 36 p. 37 p. 37 p. 37 p. 3~ p, 42 p. 42 p. 43 p. 46 p. 46 p. 6 p. 58 p, 58 p. 58 p. 58 p, 59 p. 5~ Okume, 1971, p. 99-102; Olson, 1976, p. 128; Perry, 1948, p. 4-6. Geach, 1972, p. 16!-162; Olson, 1976, p: 127. OIs~n, 1976, p. 127-128. Okuma, 1971, p. 105; Olson, 19~, p. 129; Perry, 1948, p. 6. Okuma, 1971, Plate 1; Olson, 1976, p. 126-127. Okuma, 1971, p. 107108, Plate 1; Olson, 1976, p. 129. James, 1956. Perry, 1948, p. 10-11, Johns, 1970, p. 1~2-153. GUNTER00000837 19 Highland Mountains Pre~Belt metamorphic rocks are exposed only in the southern part of the Highland Mountains. The northern part of the range is underlain by ptutons of the Boulder batholith, sedimentary rocks of the Belt Supergroup and Paleozoic and Mesozoic sedimentary rocks. The pre-Belt rocks have been intruded by two plutons of the Boulder batholith, which have been dated in the range of 72 to 77 m.y. by the K-Ar method (Robinson, Klepper and Obredovich, 1968, p. 564L Duncan (1976) mapped the pre-Belt metamorphic rocks in the Highland Mountains during his structural study and was able .to distinguish three major lithologic units: quartzofeldspathic gneiss, gametiferous gneiss and micaceous gneiss. Thin layers (maximum thickness 2 m) of .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 tremofite-actinotite, phlogopite, plagioclase, diopside, apatite and quartz. No talc occurrences have been reported, in the pre-Belt rocks of the Highland Mountains. The chforite veins at the Golden Antler mine are in quartzofeldspathic gneiss. H- 1. Golden Antler mine Location: SW sec. 14, T. 2 S., R. 6 W., Madison County. Approximately 2.5 miles (4 kin) southwest of Silver Star. Twin Bridges 15-minute quadrangie. Accessibility: Newly constructed road follows an indirect course 1.1 miles (1.8 kin) from Montana Highway 41 to the claims. OWnership: Golden Antler claims ~ocated by Robert Noite and Sylvan Donegan, both of Twin Bridges, Montana Description: Chlorite veins crosscut pre-Belt biotitequartz-feldspar gneiss, a layer of amphibolite, and another of metagabbro (Sheet l-A). Foliation of the gneiss strikes approximately west and dips to the north. The chlorite veins parallel wel!developed 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 of small chips of chlorite a centimeter or two 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 microscopic examination of the chlorite are zircon in a trace concentration (~ 1 percent) and futile, also in trace concentration, in a few specimens. This mineral must be a relict from the biotite-quartzfeldspar gneiss, and remained unaltered during the chloritization of the gneiss. Chlorite clearly was produ~ced by the alteration of the biotite-quartz-feldspar gneiss and probably also by the alteration of amphibolite and metagabbro {l~erg, 1979, p. 266). Photomicrographs (Plate 2) show a typicat sequence of alteration from gneiss to chlorite. The specimens photographed 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 surrounded by a quartz-sericite=chlorite zone, which is recognized in the field by its ~ighter 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 chlorite. Propylitic alteration of the quartzofeldspathic gneiss was observed adjacent to the quartzsericite-chlorite zone. There is no obvious genetic relationship between the gold-bearing veins of the Silver Star district and the chlorite veins. No sulfide minerals or their alteration pro~lucts 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. 1610 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 two plutons of the Boulder batholith suggests that the GUNTER00000838 2~ 0.1mm A. Slight sericitic alteration of feldspars, minor epidote and chlorite. J B. More intense alteration. Sericite, epidote, and chlorite much more abundant than in A. C. Quartz surrounded by fine-grained chlorite. D. Fine-grained chlorite. 1 0.1 mm Plate 2 Photomicrographs of alteration sequence at the Go~den Antler mine. GUNTER00000839 replacement of gneiss by chlorite may have been caused by hot water, possibly meteoric water heated by one of those plutons. The Hell Canyon pluton is exposed 1.5 miles (2.4 kml 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 replacement 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 solution of do{omite and calcite. Convective circulation of this heated water along fractures in the quartzofeldspathic gneiss could have produced the local replacement of gneiss by chlorite. From field relationships 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 Nolte (oral communication, 1977), one of these is situated less than 1 mile (1.6 kin) 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 batholith 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 of 118 re.y. (McDowell, 1971, p. 9}. Paleozoic and Mesozoic sedimentary rocks are exposed on the'north and west flanks of the mountain 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 studied than most other areas of pre-Bett rocks in Montana. 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 (t961), Koehler (1976), Levandowski (1956), Reid H1957, 1963), and Tansley, Schafer and Hart (1933). Evidence has been presente.d. I~y 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 Iater greenschist-facies metamorphism. Although interpretations of the Precambrian structural history of the Tobacco Root Mountains differ, there is ample, evidence for large isoclinal folds (Burger, 1967). Hantey (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 th~ Tobacco Root Mountains are in the southern part of the range, which is an area of relatively moderate relief~ Most of the metar mining has been farther north in the northeastern and western parts of the range. The pre-Bel~ rocks of the Tobacco Root Mountains are similar to those exposed in other mountain ranges in southwestern Montana. Rocks described from the Tobacco Root Mountains include quartzofeldspathic gneiss, hornblende gneiss, amphibolite, marble, aluminous schist, quartzke, iron formation, anthophyltite (gedrite) gneiss and metamorphosed rnafie and ultramafic intrusive rocks. Diabase dikes of Precambrian age are much more abundant in the Tobacco Root Mountains than in the Greenhorn Range to the south. There are also numerous PreCambrian pegmatite dikes in the Tobacco Root Mountains. TR- 1 Mineral Hill prospect Location: WV= sac. 26, T. 1 S., R. 3 W., Madison County. Approximately 8 miles (13 kin) northwest of Harrison. Harrison 15-minute quadrangle. Accessibility: The Carmichael Canyon road, which goes between the South Boulder River road and Harrison, is within I 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 tl4 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 kin) 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 kin) northeast of Twin Bridges. Waterloo 15-minute quadrangle. Accessibility: The closest road is the Wisconsin C.reek road that passes.approximately t.5 miles , (2.4 kin) west and many feet lower than the talc occurrence. Ownership: Not kno.wn. Description: This deposit was not visited during the present investigation, and all of the information 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 serpentine (antigorite), 25 percent diopsidic augite, 5 percent magnetite, 5 percent graphite and a trace of spinel (pleonaste). TR-3 Latest Out r~in#~. Location: Sac. 32, T. 4 S., R. 4 W., Madison County. Copper Mountain 7 -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 sac. 5, T. 4 S., R. 4W. 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) of the collar of the shaft. It is mentioned here onty because several samples were described as talc on an assay sheet (Levandowski, 1956, p. 286287). 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: SE SWl/~ NW sec. 5, T. 5S., R. 4W,, Madison County. Sheddan 7-minute quadrangle. Approximately 4 miles (6.5 kin) 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 4). Malachite occurs in some of the schist and on pieces of vein quartz found at the adit. Finegrained material from the malachite-bearing schist was identified by x-ray diffraction analysis as a mixture of sericite and kaolinite, probably formed by alteration of feldspar. Some pale-green partly altered sillimanite occurs in the biotite-quartzfeldspar schist. The only talc recognized is in a layer of talcose marble 2 feet 10.7 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 futile. The rutile forms feathery clusters of needles within the chlorite. A trace of clinozoisite occurs in the chlorite. The presence of ruti]e in the chtorite and the suggestion of a relict ophitic texture observable in thin section indicate that the chlorite may have formed by alteration of a basic dike within the marble, similar to the chlorite occurrence at the Ruby claims (GH-28). TR-5 Bivens Creek prospect Location: NY2 NE sec. 14, and S SE sac. 11, T. 5 S., R. 4 W., Madison County. Copper Mountain 7 -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, Montana. GUNTER00000841 Minor 23 EXPLANATION Garnet-biotite-quartzfeldspar gneiss B iotite-q uartz-feldspa r schist Coarse-grained dolomitic marble Disseminated talc Green chloritesericite rock Attitude of foliation Contact; dashed where inferred Figure 4--Horse Creek prospect, Tobacco Root Mountains (R. B. Berg, October 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 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 vicinity of the shaft, where it has an inferred thickness of 135 feet (45 m), and it can be traced for a distance of 10000 feet (330 m) along the strike. Judging from the talc piled near the shaft, the greatest concentration of talc is in the,shaft and in the shallow 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- nized in several exposures. The talc is white, pale green and Iight gray. Minor talc is exposed in the cuts just northwest of the shaft, but the two northwesternmost cuts contain more talc, and m~ch milky white quartz is found in this vicinity. Alteration of the sillimanite schist has produced green chlorite. In most exposures relatively pure waxy green chtorite 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 need~ies 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 exl~ose,s a small area of chloritic alteration of hornblende gneiss. The area of chlorite is 3 feet by 3 feet (I m by I m). The geology of this part of the southern Tobacco Root Mountains was described by Cordua (1973), and Levandowski 11956) described the Bivens Creek prospect, which he incorrectly reported 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 incfined shaft. TR-6 Harris Creek prospect Location: SW sec. 13, T. 5 S., R. 4 W., Madison County. Copper Mountain 7 -minute quadrangle. Six miles (10 kin} northeast of Alder. Accessibility: The Harris Creek road branches from the California Creek road in the S 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 prospects 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 atong the northwest side of Harris Creek (fig. 5). Layerin9 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 intertayered 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 of 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 tremolite_that was formed in the marble during earlier metamorphism. 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 typically gray, although some is pale green. A chrysoti~e 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 {I.3 m) thick is in cont,act with an overlying chlorite layer 6 inches (15 cm) thick. The chlorite grades into biotite-quartz-feldspar gneiss to the west and undoubtedly has been produced by alteration of the gneiss. Malachite coats some fractures in the talc. Another talc occurrence is exposed by shallow scraping Ifig. 61 situated 900 feet (300 m) N. 68 E. from the large cut on Harris Crook first described in this section. Dolomitic marble, and biotite-quartzfeldspar gneiss are exposed. Some of the biotite quartz-feldspar gneiss is chioritized 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 Amphlbolite 25 Limonit~ stained and leached marble along ]0m Hornblende gneiss and in bottom of cut Downhill to Harris Creek EXPLANATION Garnet-biotite-quartz-feldspar gneiss ~ Hornblende gneiss ~ Biotite schist ~:=~--~".~ Dolemitic marble Attitude of fnliation Contact- dashed where inferred Fault - dashed where inferred Minor diopside D iabase dike gneiss Brecciated talc To G~lifornia Creek road Dolomiti Diagrammatic cross section A - A' Figure 5--Cut northwest of Harris Creek. Tobacco Root Mountains [R. B. Ber~, October 1976}. Garnetiferous biotitequartz-feldspar gneiss Approximate outline of scraped area /~,~~' // EXPLANATION (locally garnetiferous) a~. ~[ii:~:i!i~!,!-!~.:~,~i!i; DCholloormitiitcicalmtearrabtileon L 50 feet L~ 10 m I _-~ Attitude of foliation ~ Fold axis ----Coinnftearrcedt) (dashed where Figure 6--Prospect southeast of Harris Creek, Tobacco Root Mountains (R. B. Berg, October 1976|. GUNTER00000844 26 TR- 7 Gr~ndview prospect Location: SE SW1/~ sec. 13, T. 5S., R. 4W., Madison County. Copper Mountain 7-minute quadrangle. Six miles 110 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 tatc along a fault. Talc also is exposed in a shallow cut northwest toward Harris Creek. tt is unlikely that any surface shows of talc on this prospect have not been investigated by scraping away the soil. Interlayered biotite-quartz-feldspar gneiss and dolomitic marble, with northwest strike and nearvertical attitude, are in contact 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 atilt. The greatest concentration of talc at this prospect is in the marble just west. of the fault. Because the fault seems to be only a short structure timited to the area of talc mineralization, and because post-talc movement on it is obvious, it seems likely that the talc layer influenced the toc~tion of the fault rather than that the fault control[e~ ~he formation of talc. The talc is white except for some material piled near ti~e mouth of the atilt, which is stained with limonite along fractures. Chlorite from the prospect contains trace concentrations of zircon, aparite and futile. This prospect provides a good opportunity to study the relationship between talc and chlorite. The talc, with the exception of the large body west of the fault, forms small pods a few feet (! m) across within the dotomitic 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 determining 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 marb}e. TR-8 Granite Creek prospect Location: SW sec. 3, T. 6 S., R. 3 W., Madison County. Virginia City 15-minute quadrangle. Three miles (5 km) northwest of Virginia City. Accessibility: A ranch road from Granite Creek goes past the prospect. Ownership: W. D. Conklin, Alder, Montana. Description: The main occurrence of talc is in the SW SW sec. 3, where a small amount has been mined from a shallow inclined shaft (fig. Shallow pits near the shaft expose talcose marble and talc pods within the marble. Two other prospect 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 impurities. Cordua (1973) showed that the marble layer exposed at this prospect extends S. 70 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 NW SW s~c. 3, T. 6 S., R. 3 W. Ifig. 8), but slumping in the cut has obscured the relationship between the talc layer and chlorite. A 2-foot (0.7-m) layer of talc containing minor graphite is exposed in the cut. X-ray diffraction analysis of the chlorite shows that it contair~s a small amount of talc. A pale-green rock from the same cut consists of serpentine and minor quartz and contains a trace of 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 Us situated across a small g~ich 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 SW NE sec. 25, T. 5 S., R. 3 W., Madison County, and the southern cut is in the NE NE ,SW of the same section. Virginia City 15-minute quadrangle. Approximately 6 miles (10 kin) northeast of Virginia City. GUNTER00000845 Tal~ marble expo~d i~ pit Talc pods exposed in 27 Much talc on dump, some I~monite stained E XP LANAT I0~ Figure 7--Southern Granite Creek prospect, Tobacco Root Mountains (R. B. Bergt September 1976). SSOE Tan, cosrse-grai~ed dolomitic marbl e Very white medium-grained tatc with minor graphite N50W Road EXPLANAT 10N Talc Chlorite feet Figure 8--Northern Gren|te Creak 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 r~orthern cut is 1.1 miles (t.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 of the fo!iation 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 tayer 17 feet (5.6 m) long and 2.2 feet (0,7 m) in verticat dimension, which is surrounded by dolomitic marble |fig. 9). Although the talc 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 tatc and chlorite are the only othe~ recognized occurrences of talc in the cut. Biotite-quartz-feldspar gneiss and pegmatitic pods within the gneiss have been partly ahered to green chlorite. Both talc and chlorite are exposed in the southern 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 marbie is exposed near the north end of the cut and also near the south end, where a high-angle fault separates 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 chtoritized quartzofeldspathic gneiss. X-ray diffraction anatysis of a specimen of the chlorite shows the only impurity to be a trace of quartz. A thin section of the quartzofeldspathic gneiss shows that the biotite has been altered to chlorite and the feldspar to sericite. Apatite and zircon are trace constituents. At some localities this altered quartzofeldspathic gneiss grades into a rock that in a hand specimen is judged to consist entirely of chlorite. Exposures along the road between the two cuts are mainly in biotite-quartz-feldspar gneiss; minor talcose marble is also exposed. Abundant chlorite in quartz-feldspar gnei~t Figure 9--Northern cut at Bear claims, Tobaoco Root Mountains (R. B. Berg r September lg761. GUNTER00000847 ruthin of ititeItion two 10|: is at also sep3 by cut. n of hlolion the eis~ lrite are :red tt in f of 29 Coarse-grained white dotom itic mar'ole overlies altered biotitequartz-feldspar gneisS; Concordant talc pod 1 by 3 ft (0.3 by lm} within marble; s~ricitic and chloritic alteration of gneiss Concordant layer of white talc 2 ft, [0,6m) thick 72 / / Vertical laver of chlorite 6 ft. (2m Impure talc Partly chloritized biotite-quartz feldsaar Altere~l quartzofel~lspathic gneiss Gray, coarse-grained calcitic marble with minor talc ~"~.,. White talc with fimonite exposed in bottom ~f cut EXPLANATION Precambrian ~ Biotite:q~artz-feldspar gneiss ~ Talc ..' .:':!" Oisseminated talc ~ Ch Iorite ',. ". " Disseminated chlorite --'%z Attitude of foliation -- Contact ' Fault ~h. Drill hole; no fu~her information ~0~o Loose blocks Figure 10--Southern cut at Bear claims, Tobacco Root Mountains JR, B. Berg, September 1976). i GUNTER00000848 30 TR- 10 Talc prospect southwest of Ennis Location: SE1/. NE sec. 9, T. 6 S., R. 2W., Madison County. Virginia City 15-minute quadrangle. Approximately 6 mffes (10"kin) southwest of Ennis. Accessibilky: The prospect is 0.5 mile (0.8 kin) 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 Ifig. 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 kin) to the north. Several concentrations of float of quartz-chlorite rock within the marble layer are interpreted to be small pegmatite lenses in which the feldspar has been altered to chlorite. Pink microdine 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 prospect consists mainly of chlorite but contains minor sericite, and a trace of quartz was detected by x-ray diffraction analysis. A specimen of talc contains, in addition to talc, a minor concentration of quartz and a trace of chlorite. Flomblend gneiss a==d amphibolite Predominan~!y coarse-grained dolomitic marble, minor calc~tic marble B iotile muscovite quartz feldspar Disseminated talc or talc floa~ D is~erninated chlorite Attitude of foliation .... Contact; dashed where inferred Gradational contact Downhill Talc pods a few'i Talco~ a~d chloritic /~ / SECTION 10 .J Figure 11--Prospect southwest of Ennis, Tobacco Root Mot;ntains (R. B. Berg, September 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 I]eaverhead,. 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 summarized here for 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 (t973a) on the centrai 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 of the southwestern part of the range, and James, Wier and Shaw (1969) mapped the geology of the Christensen Ranch 7 minute quadrangle and surrounding area in the southern part of the range. A detailed map of the Carter Creek "iron deposit, which lies within the Christensen Ranch quadrangle, has been published by James and Wier (1972). The geology of the northern part of the Ruby Range has been mapped by Tysdal (1976), who concentrated his efforts on the Paleozoic, Mesozoic and Cenozoic formations and did notr map individual rock types in the sequence of pre-Belt rocks. Larry Karasevich, a graduate student at Pennsylvania State University, is now (1978) working on the pre-Belt metamorphic rocks of the northern part of the range. Th6 Ruby Range is a northeast-trending uplifted block, principally of pre-Belt metamorphic rocks. With the exception of a small "bridge" of metamorphic rocks connecting the Ruby Range with the Greenhorn Range to the east, the Ruby Range is surrounded by intermontane basins partly filled with Tertiary sediments. Paleozoic and younger sedimentary rocks are exposed only in the northern part of the Ruby Range, where they generally flank the Precambrian 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 dofomitic marble layers that are the host rock for all of the talc deposits occur in the Cherry Creek-type rocks. These rocks, whk;h 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 northwest flank of 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 of meters to 400 meters. One layer of marb|e, 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 middle of the Ruby Range and separates Cherry Creektype rocks" to the northwest fron~- i~r~-Cherry Creek rocks to the southeast. The Dillon Granite Gneiss consists mainly of quartzofeldspathic gneiss but contains fesser 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 quartzofeldspathic gneiss was produced by metamorphism of a synkinematic batholith emplaced during the Precambrian (Heinrich and Rabbitt, 1960). On the other hand, Garihan and Okuma (1974) cited evidence that the Dillon Granite Gneiss could have been formed by isochernical metamorphism of arkosic rocks. Their most convincing argument for a sedimentary precursor is the presence of thin layers of marble weft within the Ditton Granke Gneiss and traceable for 6 km (3.6 mi.). They also noted that compositional variation within the Dillon Granite Gneiss is more neady compatible 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, sillimani~e 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 Digon Granite Gneiss of the Ruby Range. The metamorphic history of the pre-Belt rocks of the Ruby Range is similar to that of the metamorphic rocks exposed in the other mountain ranges GUNTER00000850 32 of southwestern Montana. The rocks were subjected to multiple periods of Precambrian deformation, which produced i~oclinal fotds and a mineral asssmblage of the amphibolite facies. Local granulite facies assemblages may be relicts of an eartier 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 of 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 eartier investigations. The sources of information are included in the description of each deposit. Ruby Peak occurrence Location: W sec. 16, T. 6 S., R. 5 W., Madison County. Laurin Canyon 7=minute quadrang!e. Approximately 6 miles (10 kin) west of Alder. Accessibility: A private road up Hinch Creek goes within 0.5 mile (0.8 kin) 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 preBelt rocks of thee northern Ruby Range in the summer of 1977. Tgsi~at (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 e~end southwes~ 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 smart prospect pit in the smal! 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 tremotite. R-2 Spring Creek prospect Location: SE sec. 32,_SW sec. 33, T. 6 S., R. 6 W., and NE sec. 5, T. 7 S., R. 6 W., Madison County. Beaverhead Rock SE 7-minute quadrangle. Approximately 14 miles (23 kin) northeast of Dilton. 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 of a synform. Talc occurrences in this marble can be traced t.5 km (5,000 ft.) (northeast) to the point where the marble is overlain by Paloozoic 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 this deposit shows an unusually wide variation in coior, including white, green, pink, purple and yellow varieties. Limonite, gypsum and graphite are reported to occur in the talc. Other minerals in the marble or very impure talc are serpentine(?), 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. Sources of information: Garihan, 1973a, p. 177-180; Olson, 1976, p. 129-130. R-3 Gem claim Location: SE sec. 34, T. 6 S., R. 6 W., Madison County. Approximately 16 miles (26 kin) northeast of Dillon. Beaverhead Rock SE 7-minute quadrangle. Accessibility: The road along Spring Creek goes within 1 mile of the claim. Ownership: Pfizer, Incorporated. Description: This deposit is in the Regal marble, which is exposed on the southeast limb of a northeast-trending synform. The Spring Creek prospect (R-2) is on the northwest limb of the same structure. A zone of micaceous, graphitic talc 2 to 3 meters (6 to 9 ft.) wide is exposed in the lower cut. Lenses of 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 during the same event that produced talc in the enclosing dolomitic marble. Sources of information: Garihan, 1973a, p. 180-18!; Olson, 1976, p. I29. GUNTER00000851 R-4 Whitney claims Location: SW sec. 2, T. 7 S., R. 6 W., Madison County. Mine Gulch 7-minute quadrange. Approximately 16 miles (26 km) northeast of Dillon. Accessibility: A road to the claims branches off 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 alongthe Left Fork of Stone Creek. Ownership: Pfizer, Incorporated. Description: Garihan described a talc body exposed in bulldozer cuts as being digitated, locally discordant to layering in the onclosing marble, and variable in thickness. Faulting has extensively fractured the talc. Limonite and pyrite are abundant in some of the talc. Sources of information: Garihan, 1973a, p. t71-'174; Olson, 1976, p. 130. R-5 Prospect southwest of Whitney claims Location: SE sec. 3, T. 7 S., R. 6 W., Madison County. Mine Gulch 7-minute quadrangle. Approximately 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 marbte 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 of dark-blue and light-green talc. In another cut irregular talc bodies form a zone reported to be 20 meters (65 ft.) wide. Sources of 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 Gufch 7-minute quadrangle. Approximately 16 miles (26 kin) 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 sec. 14, T. 7 S., R. 6W., passes within 0.25 mile (0.4 kin) 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 17 to 10 ft.) thick and is underlain by a layer of dark-green graphitic talc 2 meters (7 ft.) thick. The talc is concordant to layering in the enclosing marble and can be traced for 25 meters (82 ft.) along strike. In addition there are discordant stringers, layers and small lenses of talc in the surrounding marbfe. Garihan identified talc, graphite, chlorite, serpentine and gypsum in the enclosing dolomitic marble. Source of information: Garihan, 1973a, p. 169-171. Prospect northeast of Treasure mine Location: SE sec. 11, T.7 S., R. 6 W., Madison County. Mine Gutch 7-minute quadrangle. Approximately 17 miles (28 kin) northeast of Dillon. Accessibility: A road that bra0c,h.es from the Left Fork of Stone Creek in the NW~ sec. t3, 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 exposed 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: NW sec. 12, T. 7 S., R. 6 W., Madison County. Mine Gulch 7 -minute quadrangle. Approximately 17 miles (28 km} northeast of Dillon, Accessibility: The closest road to the prospect is a road along Cottonwood Creek, which continues to w~thin a mile of the prospect. Ownership: Not known. Description: The talc body has an outcrop of approximately 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 replaced olivine and pyroxene in the adjacent marble. Sources of information: Garihan, 1973a, p. 162-166; Oison, 1976, p. 128. GUNTER00000852 34 R-9 Treasure mine Location: N sec. 14, T. 7 S., R. 6 W,, Madison County. Mine Gulch 71~-minute quadrangle. Approximately 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 of 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 kin) south of Dillon, whereit is sorted, pulverized and bagged for shipment. The ore body at the Treasure mine is a tabular body of 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 forths 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 I1,188 ft.) long, and the dip ranges between 45 and 65 N. The Treasure State ore body is reported 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 45o 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 extensively a~tered, the biotite being altered to chlorite, the plagiocLase to white mica. The altered footwall schist presents a problem in s~ope 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. R- 10 Beaverhead mine Location: SE sec. 14, T. 7 S., R. 6 W., Madison County. Mine Gulch 7-minute quadrangle. Approximately 16 miles (26 kin) east of Dillon. 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 wa!.l at the Beaverhead mine, suggesting the possibility that the deposits are on opposite limbs of an isoctinal 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. The talc body at the Beaverhead mine is at least 800 feet (244 m) long and is offset by relatively minor faults. Ols,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 substantially 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 kin) to the northeast, where it is GUNTER00000853 35 washed and sorted before either being shipped directly or being hauled to Three Forks for pulverizing and bagging. Sources of information: Garihan, 1973a, p. 156-160; Olson, 1976, p. 125-126. The talc ore lies within a layer of dolomitic marble 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 of the R-I ! Prospect east of Beaverhead mine lack of soil cover, the talc zone can be traced beyond the limits of the pit. The zone consists of talc Location: NW sec. 13, T. 7 S., R. 6 W., Madison County. Mine Gulch 7-minute quadrangle. Approximately 17 miles (28 km) east of Dillon. 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 Accessibility: This prospect is several hundred feet north of the haul road to the Beaverhead mine and is easily visible from the road. 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. Ownership: Not known. A near vertical diabase dike 60 to 100 feet (18 to 31 m) thick trending at right angles to the strike of Description: Layers and irregular masses of talc are exposed in a cut in dolomitic marble along strike to the east of the marble at the Beaverhead mine. 3"he thickest body of talc is approximately 1 meter the marble is exposed at the western end of the area of major talc concentration. Marble adjacent to the dike has been altered, t,o:a fine-grained rock that consists of talc and serpentine. (3 ft.) thick and dips 50 to 60 N. Garihan noted the occurrence of small pods of 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 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. solutions or alternatively were of such composition Sources of information: Okuma, 1971, p. 102-104; that they were more susceptible to replacement by Olson, 1976, p. 126; Perry, 1948, p. 6. talc. Planes of rhombohedral cleavage are still f recognizable in some of the coarse-grained rhom- R- 13 American Chemet mine bohedral carbonate (dolomite?) that has been replaced by talc. Most of the talc exposed in this cut is dark green or grayish green. Location: SE sec. 1, NE sec. 12, T. 8 S., R. 7W., Madison Counter. Christensen Ranch 71~-minute quadrangle. Approximately 12 miles (20 kin) south- Sources of information: Garihan, 1973a, p. 160-162; east of Dillon. Olson, 1976, p. 129. Accessibility: A road that branches to the nor[h R-12 Regal (Keystone) mine from the Sweetwater road in the N sec. 13, T. 8 S., R. 7 W., leads directly to the mine, a distance Location: N sec. 2, T. 8 S., R. 7 W., Madison County. Christensen Ranch 7-minute quad- of 1.3 miles (2.1 kin). Ownership: Not known. rangle. Approximately 11 miles (17 kin) southeast of Dillon. Description: This mine, now inactive, was operated by American Chemet Corporation, East Helena, Accessibility: The mine is adjacent to the Sweetwater road. Montana. Development consists of three pits, from which talc was mined, and several bulldozer cuts. The mine is situated in a complexly deformed Ownership: Pfizer, Incorporated. layer of marble that in the vicinity of the mine strikes northeast and dips 45 to 75 NW. Although 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 (t38 m) long, 50 to 100 feet (15 to 31 m) wide, and 20 to 30 feet (6 to 9 m) deep. the marble is generally surrounded by Dillon Gram ite Gneiss, there are layers of amphibolite southwest 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 uttramafic 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 of information: James, Wier and Shaw, 1969; Okuma, 1971, p. 108-111; Olson, 1976, p. 127. R-14 Estelle (Sweetwater) mine Location: E sec. 13, NE sec. 24, T. 8 S., R. 7.W., Madison County. Christensen Ranch 7-minute quadrangle. Approximatefy 13 miles (21 kin) southeast of Dillon. Accessibility: The main pit is situated on the northeast side of the Sweetwater road. Prospect pits southwest of the Sweetwater road can be reached by taking a road that branches from the Sweet.water road in the SE sec. 13, T. 8 S., R. 7 W. The prospects are 0.5 mile {0.8 kin) 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 Sweetwater Creek and the Sweetwater road. The mine and prospect cuts are in a layer of dotomitic marble of unusually unifom 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 Sweetwater mine. Abundant shear zones subparallel to the marble layer are prominent in the pit. The talc generally forms concordant layers and pods within the dotomitic~rnarble. Most of the talc is various shades of gree# d[ gray. Sources of information: James, Wier and Shaw, 1969; Okuma, 1971, p. 106-107; Olson, 1976, p. 128. R- 15 Smith-Dillon mine Location: E sec. 23, T. 8 S., R. 8 W., Beaverhead County. Ashbough Canyon and Dillon East 7minute quadrangles. Approximately 8 miles (t3 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-Digon 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 exposed 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 workings. 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 Banning~Jones mine Location: SW sec. 13, T. B S., R. 8 W., Beaverhead County. Dillon East 7 -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 approximately 1 mile (1.6 km). Ownership: State section. Description: In 1964 the property was leased to WaF 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 kin) 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 of talc within this 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 (12 m) 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 of aqueous solutions that reacted with the dolomitic marble to form talc. Sources of information: Geach, 1972, p. 161-162; Olson, 1976, p. 127. R- 17 Bozo-Zobo mine. Location: NE sec. 19, T. 8 S.~R. 7 W., Beaverhead County. Dillon East 7-minute quadrangle. Approximately 9 miles (15 km) southeast of Dillon. Accessibility: The road up Axes Canyon passes within 0.25 mile (0.4 kin) 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 American 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 exposed 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 usua~ markets. The talc zone is in a layer of marble that strikes northeast and dips northwest. Source of information: Olson, 1976, p. 127-128. R-18 Crescent prospect (Timber Gulch deposit) Location: SW sec. 1, T. 9 S., R. 8 W., Beaverhead County. Ashbough Canyon 7-minute quadrangle. Approximately 11 miles (!8 km) southeast of Dilion. 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 two layers of 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 excavations prove the continuation of talc to a depth of 10 to 20 feet(3 to 7 m). Perry reported that intermittent exposures of talc can be followed for 800 to 1,000 feet (248 to 3t0 m} along strike. An unusually high concentration of graphite in the talc probably has been the main deterrent to the development 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. 7 W., Madison County. Elk Gulch 7-minute quadrangle. Approximately 13 miles (22 km} southeast of Dilton. Accessibilky: 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 kin). Ownership: Karl L. Sauerb=er, &lder, Montana. Description: The Sauerbier mine, although now (1978) inactive, was operated by Resource Processors, Incorporated, in 1974. More recently the property has been leased to Cyprus Industrial Minerals. Both the Sauerbier mine and the OwenMcGovern prospect fie 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 marbie body is bordered on the southwest and northeast by major northwesttrending faults, and it is surrounded by Dillon Granite Gneiss. The mine consists of a north-south cut 100 feet (31 m) wide, 300 feet (93 m) long, and 50 feet (16 m) deep at the ~outh end where it is deepest. Talc pods within dolomitic marble are most abundant in the southern half of the cut, where evidently most of the talc was mined. Numerous steeply inclined faults are exposed in th~ pit. Chlorite tayers within both the talc and the dolomitic 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 ~of information: Okuma, 1971, Plate 1; Olson, 1976, p. 126-127. R-20 Owen-McGo vern prospect Location: SE sac. 23, E sac. 26, T. 8 S., R. 7 W., Madison County. Elk Gulch 7-minute quadrangle. Approximately 13 miles (22 kin) southeast of Dillon. Accessibility: The road to the Sauerbier mine, which branches from the Sweetwater road in the SW sac. 20, T. 8 S., R. 6 W., goes.within 0.25 mile (0.4 kin) of the Owen-McGovern prospect at a distance of approximately 2.5 miles (4 krn) 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 bordered to the northeast and southwest by mbjor northwest-trending faults. The marble strikes northeast and generally dips 50 to 80 NW. Pegmatite and diabase dikes intrude the marble, and amphibolite is also exposed at this prospect. Exploration 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} concluded: 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 published, 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 described by Olson and are referenced to his 1976 article because it provides the most detailed locality information. Table 7--Additional talc occurrences in the Ruby Range not plotted on Figure 3. Location SW ~. 17, T. 7 S., R. 6W. SW1/, sac. 18, T. 7 S., R. SW. SW sac. 11, T. 7 S.0 R. 6W. SW NW~ sec, 2, T. 8 S., R. 7W, sw~ SW~ ~.3, T. 8 S., SW~ SE~ ~ . 5, T. ~S~, "~ R. 7W. SE~ NW~ ~. 8, T. 8 S., R. TW, E% ~. 10 end NW~ ~c. tl, T. 8 S., R. 7W. NE~ ~. 1E a~ SW~ ~c. 11, T. 8 S., R. 7W. NE% ~. 21 a~ NW~ ~. ~, T. 8 S,, R. 7 W. SW~ ~ ~. 19, T, 8 S., R. 7W. NE~ ~ ~. ~, T, 8 S., R. BW. SE~ SE~ NE~ ~, ~, T. 8 R, BW. SW~ ~. ~, T, 8 5., R, 8 W. NW~ NW~ ~. ~, T, 8 S., R. BW, W~ NE~ ~. ~, T. 8 S., R. 8W. SE SE~ ~. 35, T. 8 S., R, BW. sac. I and 2, T, 9S.0 R.8W. D~c~ipdon Lenses and layem of talc e few centimeters thick end ~ than 1 rn long. Dark-green talc float, locally graphitic. Medium- to light-green taic float and some graphite. Light-cOlOred ~lc float can I:~ traced for 200 ft, (62 rn| along strike over e width of 100 ft. (31 Light-green talc flosS can be traced for 600 ft. (186 m) maximum width of 825 ft. |10t m). No available informallon. No eveilable information. Talc in outcrop and float over a t~tal strike le~gth of 7,(X)0 ft. (2.2 kin), Green talc float of ~poradic d~stdbuti0n over e strike length of 4.000"ft, (1,2 kin), One talc zone extends for approximately 500 ft, (15~ m) along stdke. Zone of dark talc expo~d for 1,350 ft. (419 m) ~ong strike, No available InformaCdofl. Grin talc e~posed for several hur~rad feet along stdke, widths of 5 to ~ ~. (1.6 to 9 mL Vall~ ~ p~ ~ of ~ ta~ 1~ ~ {47 m) ~ng ~ ~ ~. {~ m) ~. Oa~-~ ~ ex~ in ~1~ c~. light-green taJc can be traced for approximately 900 ft. (2~0 m| along strike. Some talc tayam t0 ft, (3 m) thick. No aveilabte Infon-natio~. Zone of talcoee ro~ks can be ~ for 5,000 ft. (t.6 kin) along stn~e leastnortheast|. Refsra~ce Garihan, 1973a, p. 181-1B2. G~rihan, 1~'/3a, p. 182, Gedhan, 1~T3~, p, lt~. Otson, 1975, p. 131, Olson, 1978, p~ 131. Hendch and Rabbit't, 1960, pl. 2. Henrich and Ral~tt, 1~60, pL2. Olson, ,1976, p. 131. Olson, 19-/6, p. 131. Olson, 1976, p. 131. Oloon, 1976, p. 130. Heinrich end Rabble, lg60, ~.2. Olson, 1976, p. 131. Olson, 1976, p. 131. OIs0n, 1976, p. 130, 1976; p. 13o. Hendc~ end Rel0bitt, 1960, Id, 2; O!,~;~r 1976, p. 131, GUNTER00000857 Greenhorn Range 39 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 publication. The Greenhorn Range is structurally the northwest 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 ]imb of the same structure but their outc.rops are separated from those of the pre-Belt rocks in the Greenhorn Range by outcrops of Pateozoic and Mesozoic sedimentary formations. Pre-Belt rocks extend west'from the Greenhorn Range in the vicinity of Ruby Dam to connect with 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, preBelt rocks" are overlain by Eocene andesite-dacite porphyry (K-Ar age of 50 m.y.), which is in turn overlain by Oligocene basalt (K-Ar age of 33 to 34 m.y.) as reported by Marvin, Wier, Mehnert and Merritt (1974). 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 Merritt, 1974). Tertiary sediments are exposed in the upper Ruby Valley, w!~ich 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 quartzofetdspathic gneiss exposed in other mountain ranges of southwestern Montana. Most of the gneiss shown on Sheets 2, 3 as quartzofeldspathic gneiss is biotite-quartz-feldspar gneiss, Some hornblende-quartz-feldspar gneiss occurs within the quartzofeldspathic gneiss. The amphibolite assemblage contains hornblende gneiss, granulite and metagabbro in addition to the predominant amphibolite. Marble, mainly dotomitic, 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 abundant in the northern part of the range. The Snowcrest fault extends west from the Gravelly Range into the Greenhorn Range, where pre-Bett rocks have been thrust over Paleozoic formations in the vicinity of the Willow Creek talc mine. Farther to the south pre-Belt metamorphic rocks have been thrust over Paleozoic rocks along the Greenhorn fault. The north end of the Greenhorn Range is partly bounded by a highangle 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 fragments in the soil or in outcrop. The numbered localities are shown on Sheets 2, :3, with the exception of GH-45 and GH-46, which lieoutsidethe 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 *~he .outcrop and in the soi! at several localities (numbers 12 to 20) within a marble layer exposed north of the North Fork of Greenhorn Creek. There are also several occurrences of 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 concern tration of talc in the Greenhorn Range is the deposit at the Willow Creek mine (locality GH-42). Prospects and abandoned metal mines encountered during mapping are listed in a section following the list of talc and chlorite occurrences. The following is a description of the occurrences, the first 27 of which are described only briefly. We~t of the Ruby River GH-1 SE SW sec. 32, T. 6 S., R. 4 W. Minor talc in soil below exposure of calcitic marble. GH-2 SW SW 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 NE NW sec. 4, T. 7S., R.4W. Coarsegrained silvery talc in one piece of dolomitic marble. GH-4 SE NE 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 NE NE sec. 8, T. 7 S., R. 4 W. Coarsegrained silvery talc and chlorite in one piece of marble float, which also contains graphite. GUNTER00000858 4O East of the Ruby River and north of 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 Dave Creek. GH-6 SEX SW sec. 35, T. 6 S., R. 4 W. Several smalt fragments of fine-grained talc in soil. GH-7 SW SE sec. 35, r. 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 SW NE sec. 2, ~-. 7 S., R. 4 W. A few small fragments of fine-grained talc in soil. GH-9 SWl/~ NW sec. 1, T. 7 S., R. 4 W. One fragment of fine-grained tatc in soil. (~H-10 NW SW sec. 1, T. 7 S., R. 4W. Several pieces of marble float that contain silvery, coarse-grained talc. Between Idaho Creek and the North Fork of Greenhorn Creek GH-11 SE NE sec. 6, T. 8 S., R. 3 W. Fine- grained talc veinlet about 8 mm thick. GH-12 SE SW sec. 6, T. 8 S., R. 3 W. Minor fine-grained talc in outcrop, GH~13 NE NW sec. 7, T. 8 S., R. 3W. Minor talc in float. GH-14 SE NWl/, sec. 7, T. 8 S., R. 3W. Minor fine-grained talc in float. GH-15 SW NE sec. 7, T. 8 S., R. 3 W. Small (1 to 2 mm) ~blebs of white to light-green talc in outcrop. GH-16 SW NE sec. 7, T. 8 S., R. 3 W. Marble float contains veintets and pods of finegrained talc. Talc float concentrated in an area 2 by3 m. GH-17 SWNEsec. 7, T. 8S.,R. 3W. Trace of talc float below outcrop. GH-18 NW NE sec. 7, T. 8 S., R. 3W. Minor talc in float. GH-19 NW NE sec. 7, T.8 S., R.3W. Abundant talc veintets in outcrop of intensely deformed marble. Talc constitutes approximately 10 percent of the outcrop, which is 3 by !0 m. GH-20 NW NE sec. 7, T. 8 S., R. 3 W. "[race of talc in soil. GH-21 SW NW sec. 11,-F. 8 S., R.4W. Very small (tess than 1 cm} chips of fine-grained talc in soil found for 200 m in a north-south traverse. GH-22 NW SW sec. 11, T. 8 S., R. 4 W. Minor fine-grained talc in soil. GH-23 SE NW sec. 11, T. 8 S., R. 4 W. Minor fine-grained talc in soil here and belween 22 and 23. GH-24 SW NE sec. 11, T. SS., R.4W. Trace of fine-grained talc in soil. GH-25 SE SEI/~ sec. 11, T. 8 S., R. 4 W. One small fragment of talc in soil. GH-26 NW NE sec. 14, T. 8 S., R. 4 W. Trace of talc in outcrop of marble. GH-27 NW NE sec. 14, T. 8 S.~ R. 4 W. Sev- eral small fragments of talc in soil. GH-28 Ruby claims Location: NE NW sec. 14, T. 8 S., R.4W., Madison County. Ruby Dam 7-minute quadrangle. Approximately 14 miles (22 km} south of Alder. . !. .(. 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. I" 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 tt~e alumina-poor marble. 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 chlorite 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 of chlorite were produced. Petrographic examination of the fresher diabase shows some alteration of the plagioclase to sericite in addition to secondary chlorite, epidote and actinolite. A photomicrograph IPlate 1) shows the relict ophitic texture characteristic of some of the chlo- GUNTER00000859 41 Marble with talc pods as long as 5cm and ffmonite after pyrite exposed on southwest side of pit Siliceous marble with dotomite crystals in cavities (float only) Precem!~rian in chlOriteexposed in pit - 8 ft. [2.4m) White vuggy siliceous dolomitic marble EXPLANATION Diabase Medium to coarse-grained dolomitic marble Chlorite Dissemin~]ted ~;h Iorite or chlorite float Attitude of fault Inferred conta~ in area of no outcrops ,!. Downhill Dolomiti marble Figure l:~--Ruby claims. Greenhorn Range (R. B. Berg, September 19761. Most of the chforite north of A is tan and most of the chlorite south of A is yellowish green. GUNTER00000860 42 rite. The clear relict plagioclase laths, now completely replaced by the chlorite, are surrounded by chlorite that contains many small futile grains. Presumably 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 exposed in the large prospect pit on the southwest side of the dike. No other tatc was found in the marble adjacent to the dike. GH-29 SWl/~ NW sec. 14, T. 8 S., I~. 4 W. Sheared and altered marble exposed in shallow cut. Trace of coarse-grained silvery talc in marble. GH-30 Doubtful claim Location: NW SW sec. 14, T. 8 S., R. 4 W., Madison County. Ruby Dam 7-minute-quadrangle. 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 t 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 because 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 iimonitestained silicifiec} ~n~rble 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 of clasts 1 ram across of fine-grained talc (grain size 4 #m) surrounded by coarser-grained talc (grain size 30 to 150 #m). This texture suggests that brecciation followed the formation of finegrained tatc and provided fractures in which coarser-grained talc was deposited. This brecciated talc, although not common, is seen in many of the talc deposits in southwestern Montana. A layer of chlorite containing blue apatite is poorly exposed in the southern cut. Microscopic examination of a sample from the southern cut shows that it consists of chlorite containing trace concentrations of apatite, sphene, zoisite, zircon and talc(?). Veinlets of chlorite 1 to 10 cm thick are exposed in small pits dug in the knob above the talc pod. Talc mineralization is confined to the one pod. GH-31 NW SE sec. 14, T. 8 S.0 R. 4 W. Trace of talc in soil. GH-32 SW SE sec. 14, T. 8 S., R. 4 W. Minor concentration of talc in float. GH-33 NE SW sec. 13, T. 8 S., R. 4 W. Small blebs of talc in some of the marble float. GH-34 SW SW sec. 13, T. 8 S., R. 4 W. Minor talc in soil. GH-35 SW SW sec. 13, T. 8 S., R. 4 W. Minor talc in soil here and between 34 and 35. GH-36 SE SW sec. 13, T. 8 S., R. 4 W. Trace of telc in soil. GH-37 NW NE1/~ sec. 23, T. 8S., R.4W. Talc pod 2 by 10 cm in outcrop; talc float in soil. GH-38 SE NW sec. 23, T. 8 S., R. 4 W. Minor talc in two marble layers each approximately 4 inches (10 cm) thick, exposed in a shallow prospect pit. Prehnite and chlorite have been identified from this prospect. GH-39 SE NE sec. 23, T. 8 S., R. 4W. Minor talc along two shear zones exposed in shallow cut. GH-40 SE NE sec. 23, T, 8 S., R. 4 W. Darkgreen talc poorly exposed for a distance of 45 feet (t5 m) in cut, which trends N. 25 W. Another Cut on the ridge to the east trends N. 75 W. and exposes fragments of dark-green talc in the marble saprolite for approximately 50 feet (16 m). Loose blocks of massive white quartz contain, smart talc pods. Most of the talc from this prospect is a darkgreen variety, probably chloritic. Area south of the North Fork of Greenhorn Creek ,Several talc prospects are present in this area in addition to the Wiltow Creek talc mine. GH-41 GREENHORN CLAIMS Location: SW NWl/~ sec. 30, T. 8 S., R. 3 W., Madison County. Home Park Ranch 7-minute quadrangle. Approximately 16 miles (26 kin) southeast of Alder. Accessibility: A bulldozer trail leads from the haul road for the Willow 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 of approximately 7,160 feet (2,183 m). This hau1 road is not shown on the topographic map. Ownership: Mr. and Mrs. Carl Hafer, Sr., Butte, Montana. GUNTER00000861 | 43 EXPLANATION ~ Tatc Dissem inated ch Iorite 8o Attitude of foliation Downhill 50 ft I IOta Soft talc with lirnonite along fractures; poorly exposed in shallow pit Marble Covered 8~ Marble Limonite pseudomorphs after pyrite in marble Marble Limonite-stained marble mphibolite and quartzite exposed in Chlo_ritized schist sh~1~oW cut downhill -- 1 ~- ~--~ / Marble or rk- ,~et ,/ NOTE: Loose material and slumping in th~s cut ma~e interpretation very b~e tentative Figure 13--Doubtful claim, Greenhorn Range (R. B. Berg and L. Swanson, June 1976). rk- 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: Atthough a small amount of talc was 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 Willow 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 marbte (fig. 16). and after sorting is trucked to Alder for rail ship- ~aul GH.42 Willow Creek mine (Ruby Ridge mine) ment. test The mine is situated within a small body of mar- ~ast Location: NE1A sec. 30, T. 8 S., R. 3 W., Madison ble, which is surrounded by quartzofeldspathic 183 County. Home Park Ranch 7 -minute quadrangle. gneiss Ifig. 15). Pre-Bek metamorphic rocks have 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 Willow Creek road, and the haul road to the talc the Willow Creek mine is only approximately mine 9ees through the sorting yard. known, tn the mine, metamorphic units strike con- GUNTER00000862 44 275 ft. (83m) to hauf road to Willow Creek mine EXPLANATION ~ A:[titude of foliation ;,..'.,. T;~lcose marble and '.',',': irreguSar talc len~,es Graphitic scl~is~ and tal~ose marble exposed in cut Figure 14--Greenhorn claims, Greenhorn Range (R, B. Berg and L. ,Swanson, July 1976|, I MILE I KILOMETER Figure ll~--Geo|ogic map of the area surrounding the Willow Creek mine (GHJ,2). The symbols used on this map are the same as those used on Sheets 2, 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 planes cutting talc, chlorite and country rock are abundant at the mine. The large isolated body of calcitic marble west of the mine is barren of talc. Figure 16 isa diagrammatic cross section through the Wiltow 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 ultimate alteration product being chlorite. The first effect of the alteration was the replacement of biotite by chlorite. Further alteration resulted in the sericitization of the feldspar, and more intense alteration produced a rock that consists of chlorite and quartz. The final stage in the alteration sequence was dark-greenish-gray chlorite. A chemical analysis of a typical specimen of the chlorite is given in Table 3. Grain size of the chlorite varies considera.bly within the area of one thin section, for example from 10 to 150 iLm. The chlorite contains rare local concentrations of idiomorphic zircon and idiomorphic crystals of apatite. On the basis of~ = 1.6353 + 0.0005 and~ = 1.6397 + 0.0005, one specimen of apatite is estimated to be approximately 76 percent fluorapatite, 12 percent hydroxyapatite and 12 percent chlorapatite. Some ~arge flakes of silvery chlorite a centimeter across presumably have been formed by the replacement of coarse-grained biotite in the quartzofetdspathic gneiss. Rutile needles occur in some of the coarsegrained 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-feldspar gneiss also resulted in the formation of talc from the subjacent dolomitic marble. Large blacks of marble have been completely replaced by fine-grained white to palegreen talc. Some of the talc is medium green, colored by small grains of dark-green chlorite. Most of the talc is very fine grained, some grains are less than 2 #m across, but even within a specimen 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 ! cm across in one specimen from a quartz vein are rimmed by a layer of talc about I mm thick:,.Pl~r{qaps talc formed by reaction of chforite with silic'a-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 creases to the southeast away from the ore body. Calcitic marble that contains abundant forsterite, mainly in grains smaller than 5 mm across, is exposed on the ridge just south of the mine. Some porphyroblasts of brown forsterite are several centimeters across, and a few of these are cut by chr.ysotile 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 P~ecambrian'J--_~--'~ Biotite-quartz.feldspar gneiss ~ Sericitic and chloritic alteration of biotite-quart2.-felclspar gneiss m Talc ~ Chlorite ....... Gradational contact feet 50 m Figure 16--Diegrammatlc cross section of the tel deposit at the Willow Creek mine showing the probable relationship between taic and host rock before poet-talc faulting. GUNTER00000864 46 placement of forsterite. Phlogopite and tremolite also occur in the calcitic marble. Tremoli!;e is typically gray to black because of abundant included graphite, and is concentrated in almost monomin= eralic layers a centimeter or two in thickness. No talc was identified either optically or by x-ray diffraction analysis from the calcitic marbie. Sepiolite has been identified from this locality (Alice Blount, personal communication, 1977). GFI-43 Claims north of Willow Creek (Adam and Eve No. 1 and No, 2) Location: SW SE sec. 30, T. 8 S., R. 3 W., Madison County. Home Park Ranch 7-minute quadrangle. Approximately 16 miles (26 kin) southeast of Alder. Accessibility: The road to the prospect branches from the haul road for the Willow Creek mine at a sharp switchback above Little Willow Creek at an altitude of 6,920 feet (2,110 m). The haul road for the Willow Creek mine joins the Willow Creek road at the site of the talc-sorting plant. Ownership: Sam and Goldie Maloney of Alder, Morn taRa. Description: Marble, quartzofeldspathic gneiss and sillimanite-biotite-garnet schist are exposed in six prospect cuts, all of which trend nearly perpendicular 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 th~rrS,m 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 sma]l 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 of sepiolite as long as 10 cm were found in the lowest cut. The sepiotite resembles splintered wood that has weathered grayish tan. A few thin chrysotile veinlets 2 mm thick occur in catciticmarble. GH-44 SW NW sec. 31, T. 8 S., R. 3 W. A trace of talc was found in the bottom of a prospect trench, which exposes marble for a distance of 45 feet (15 m). GFI-45 Talc occurrence south of Virginia City Location: NW NW sec. 23, T. 7 S., R. 3 W., Madison County. Varney 15-minute quadrangle. Approximately 5 miles (8 kin) south of Virginia City. Accessibility: This area is 100 feet (33 m) south of the road between Barton Gulch and Alder Gulch where this road descends into Alder Gulch. Ownership: Not known. Description: A nearly vertica! 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, tremotite and a trace of vermiculite. The bright-green vermiculite grains are less than 5 mm across and have presumably been produced by.alteration of phtogopite. The bright green color suggests that this vermiculite is nickel bearing. GFI-46 Calverts claims Location: SW SE sec. 32, T. 7 S., R. 3 W., Madison County. Varney 15-minute quadrangle. Approximately 8 miles (13 km) southwest of Virginia City. Accessibility: The claims may be reached by following 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 (l~illings, Montana), and,Carl Haler, Sr. (Butte, Montana). Description: A dolo,mitic marble layer with an exposed width of approximately 250 feet (76 m) strikes northeast and dips northwest (fig. 17}. Amphibolite lies northwest of the marble, and amphibolite and quartzofeldspathic gneiss lie to the southeast. Irregular veinlets and pods of talc, most of which are less than 10 cm thick, are exposed in alt 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 chlorite is intermingled with the talc and there is no evidence of replacement of quart~ofeldspathic 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. GUNTER00000865 Denm timber, no outcrops 47 Amphibolite float / Metagabbro / Amphibolite and q,=artzofeldspathi~ gneiss float / Very ~arse o~ain~_ dolomitic marble with many clu~ers of quartz crystals;/./, minor chlorite ~lcitic marble with thulRe, zolsite, and tremolite L " .//,// 50 feet Shallow cut/ E XPLANAT I0 N ~ Quar[zofeldspathic gneiss Precambrian ~ Dulomitic marble // ~ Amphibollte -~ Talcose marble in place : :.-1-.: Talco~ marble in float (or loose pi~e~ in cut) ~ Chlorite" ~ A~itude of foliation -- Contact da~ where inf~r~, queried where doubtful [ Oownhit[ Figure 17-- Calvert$ teims, Greenhorn Range (R. B. Berg, September 1977|. GUNTER00000866 48 Very coarse-grained dotomitic marble containing clusters of quartz crystals is exposed west of the claim notice. Some individual dolomite crystals are 8 cm across. Minor green chlorite (but no talc) occurs in this rock. Thulite (pink} 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 catciticmarble that contains the thulite and zoisite. Other prospects and inacdve mines Several prospects and inactive mines presumably 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 workings there are many shallow prospect pits, als.o mainly in shear zones in the quar~zofeldspathic gneiss. NW SE1i, sec. 5, T. 7 S., R. 4 W. Vertical shaft at least 30 feet (10 m} deep into sheared quartzofeldspathic gneiss and some malachite on dump. SEI/~ SE1/~ sac. 3, T. 7 S., R. 4 W. Severat 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 sac. 14, [. 7 S., R. 4 W. Two caved adits and a prospect p~t ln:a steeply inclined shear zone in schist and quartzofeldspathic gneiss. NW SE sec. 13, T. 7 S., R. 4 W. Prospect pits in sheared biotite schist. Sheared and limoniteo stained quartzofeldspathic gneiss is present in this area. NW NE sac. 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 uphitl. 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 pa~ of Bar,on Gulch, within 1 mile of its mouth, has been dredged for gold. More recently minera{ 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 SW sec. 26, T. 7 S., R. 4 W. Prospect pits have been dug in sheared quartzofeldspathic gneiss, reportedly in search for uranium. SE SW sec. 12, T. 8 S., R. 4 W. Silver Bell claim. Three adits are in sheared and limonite-stained quartzofeldspathic gneiss. "rwo 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 chaicocite were found on the dump. SE SW '/4 sac. 24, T. 8 S., R. 4 W. Adit extends approximately 80 feet (27 m) into hitlside. The adit is in a sheared ultramafic body, which contains minor chalcopyrite and malachite. Sillimanite schist and altered quartzofetdspathic 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 approximatety 50 feet (17 m) high by 150 feet 150 ~n) tong. Quartzofeldspathic gneiss, pegmatite and amphibolite are all present in this zone of shearing. Minor malachite coats some fractures. The shaft shown on the topographic map is now filled. GUNTER00000867 49 Gravelly Range The Gravelly Range lies along the east limb of a large syncline that plunges to the southwest. The outcrops of 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 of Paleozoic and Mesozoic sedimentary rocks. Pre-Beit metamorphic rocks are exposed in a north-south belt along the east front 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 preBelt rocks in the Gravelly Range from exposures of pre-Belt rocks in the Henrys L,ake Mountains. The geology of the northern part of the Gravetly Range has been mapped by Hadley (1969a, "1969b), and Heinrich and Rabbitt (1960) have mapped and studied the geology of a part of this same area. Information on the pre-Belt rocks of the southern part of the range is much tess complete. A map by Wier (1965) shows the geology of an area of approximately ~8 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 of the range, and it was in the area between Wigwam Creek and Cherry Creek that Peale (1896, p. 2) originally described what he caIfed 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 cluartzofeldspathic gneiss, amphibelite, hornblende gneiss, dolomitic marble, quartzite, banded quartz-magnetite iron formation, phyllite, schist and metadiorite. The schist can be separated into mica, sillimanite, kyanite, kyanite-staurolite, andalusite and staurolite-andalusite varieties. Greenschist-facies metamorphic rocks are exposed in a segment of the Gravelly Range approximately 7.5 miles (12 kin) long, which includes the Yellowstone talc mine. in this area fine-grained marble and phyllite are exposed rather than the coarsegrained marble and schist typical of pre-Belt rocks in the Ruby Range, Greenhorn Range and Tobacco Root Mountains. A very detaited study of the petrology of these rocks (Millhoiland, 1976) showed that the transition from tow-grade to higher-grade metamorphic rocks is abrupt and that the low-grade assemblage is not retrograde (Millholland, 1976). Approximately 10 miles (t6 km) south of the Yellowstone 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 of the transition gave an age of 2.6 b.y. Both Millholland and Jahn mentioned evidence of cataclesis in some of the rocks of their respective areas. Perhaps these abrupt chang,es, in metamorphic grade can be explained by Precambri~ faulting that juxtaposed rocks of different metamorphic grade. The pre-Bett metamorphic rocks exposed far~her south in the Henrys Lake Mountains are of greenschist facies. What may be the greatest concentration of talc in Montana is in the dolomitic marble of the Gravelly Range at the Yellowstone mine and vicinity. The deposit of talc at the Yellowstone mine and the surrounding 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 southeastern half of the area underlain by marble. GR- 1 Tait Mountain claims Location: S sec. 5, T. 8 S., R. 1 W., Madison County. Varney and Cameron 15-minute quadrangles. Approximately 12 miles {20 kin) south of Er~nis. 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. lW. Ownership: Pete Womack, Ennis, Montana. Description: Chlorite and talc are exposed in two areas about 400 feet (130 m) apart separated by a shallow saddle in which no bedrock is exposed (figs. 18, 19}. The northern area has been more extensively explored by seven trenches, and ch~orite 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 alteration of pegmatite. In other prospects in southwestern Montana there is clear evidence that chlorite GUNTER00000868 50 m 1 Altered pegmatite ! Marble float Minor talc in dolomitic marble mMinor trernolite in marble EXPLANATION /~ Biotite-quartz ~}["~ ~ : >' feldspar gneiss ~[[m~ Dolomitic marble :.:-. Dis~minated talc ~ Chlorite Attitude of foliation Maximum depth of cut in feet Green, tan, and red chlorite throughout pit m 50 feel J 1 Limonite-stained ~ and sHic~fied -~ } marble ~ 400 ft (120 m) S. 35 E. to shaft shown on Figure 21 Figure 18--Tait Mountain cla]ms (northern parl], 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). At the Tait Mountain prospect, with the exception of the altered pegmatite, the lithology of the rock that has been replaced by chlorite is not clear. A heaw-liquid separation was made on two specimens of chlorite in an effort to separate zircons from the chlorite. No zircons were recovered, suggesting that the chlorite replaced a zircon-free rock, presumably impure dolomitic marble. At other localities zircons can be recognized in chto~ite that on the basis of field relationships is presumed to have replaced quartzofeldspathic gneiss. Talc is exposed in only one excavation. The general lack of talc in the dolornitic marble exposed in these trenches is puzztin~]. The chlorite in the northe[n area ranges from green to tan and is locally stained red by hematite along fractures. Talc is a trace constituent of the chlorite. Tremolite is a widespread trace constituent of the dolomitic marble and has been replaced by talc at some places. Because of the slumping in the cuts and the tack 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, ~t 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 a gentle 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 marble and quartzofeldspathic gneiss that is. in part pegmatitic. One of the chlorite specimens that was checked in vain for zircons was from this cut. Minor tremolite 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 i948 by Tri-State Minerals, but the shaft was aban~loned 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 tremolite. GR-2 Cherry Gulch prospect Location: NE SW and NW SE sec. 31, T. 8 S., R, 1 W., Madison County. Varney 15~minute quadrangle. Approximately 18 miles 1:29 kin) 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. GUNTER00000869 148 ft {45m) to shallow cut in marble float Shaft 25 ft {8m) deep white dolomitic marble, coar~-grained tremo~ite and coarse- Figure 19--Tait Mountain claima {aouthern patti, Gravelly Range (R. B. Berg, September 1976|. 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 ISheet l-D). The Flathead Quartzite (Cambrian) unconformably overlies the metamorphic rocks to the south (Hadley, 1969b, and fig. 20}, and bentonite was encountered in prospect pits dug south and east of the talc occurrence. Two layers of 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 poody 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. Cherry Gulch Prospect J Pol Prospects Yellowstone Mine \ ".SS. T. gs. E XP LANAT IO N Pleistocene Bouldery deposits of uncertain origin Cobble and coarse pebble gravel Miocene or * Rhyolite ash flow Plio~ene ~ tuff Andesite flows Oligocene {~ Felsic tuff Undifferentiated Paleozoic Paleozoic ~ sedimentary units Precambrian X Dotomiti marble Precambrian undifferentiated; Includes quartzofeldspathic gneiss, schist, quartzite~nd hornblende gneiss Talc pebbles in soil Yellowstone Mine Talc prospect MILE J KM Rgure 20--Generalized geologic map of the Cherry Gulch-Johnny Gulch area of the Gravelly Range. Geology from the more detailed geologic mape of the Varney and Cameron quadrangles by Hadley (19~3a, 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 of which is lava talc, is exposed in two small pits on the east side of the small gufly just west of the adit. These pits are situated along the SOL~th boundary of sac. 31, T. 8 S., R. 1 W., close to the unconformity between the Precambrian 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 marble on the east side of the small gu~fy 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 nor[h of these cuts toward Cherry Gulch. The chemical analysis of a specimen of unusually hard bloc~, talc from this pit is given in Table 3. The specimen contains irregular patches of small grains (4/~m} of 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 #m across. Some of the lava talc from this prospect has been selectively mined for the carving market. ]he presence of bentonite in this small area surrounded 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 I~entonite may be equivalent in age to the Oligocene felsic tuff mapped by Hadley (1969b) 1.7 miles (2.7 kin) to the southwest. The mineralogy of the sand-size fraction of this smectite clay shows that it is of volcanic origin and not a result of hydrothermal afteration of metamorphic rocks. Biotite, plagioclase, K-reid" spar, quartz, zircon, partly devitrified glass, a zeolite and calcite were identified in the > 325 mesh (>44 /~m} 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: Sac. 4, T. 9 S., R. 1 W., Madison County. Cameron 15-minute quadrangle. Approximately 18 miles (29 kin) south of Ennis. 53 Accessibility: The Johnny Gulch road is the haut toad 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 ~alc. The following 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 sP,etch of a pit face on the Burti.ngton Northern mine ~fig. 21). James (!956, 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 tale 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 tow 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 of the other talc mines, the Yellowstone mine has no chtorite 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. Typicaf talc, sometimes designated ceramic or cosmetic talc depending on purity, contains many minute fractures and wilt easily break if subjected to physical or thermal stress. Lava talc, because of its durability, 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 translucent as is the typical pale-green ceramic or cosmetic talc. James (t956, p. 2) reported that some of the doIomitic marble has been altered to coarse-grained GUNTER00000872 ht-green talc Maroon marble bench / Maroon " dotornitic ~ green [~ Green talc .Co.a r s e.-g.r a i aed. , aOlOrnltlC rnarole Marble ~ Dark~Jreen talc with a few black and maroon spots White dolomiti marble; so me ~arkred gdoralio~rendite Coarse-grained white marble with some maroon dolomite crystals ~ Cove~ed Dark-green and maroon talc tEXPLANATION ~ ~: Disseminated talc ~ Botryoidal tatc Shear zone Unusually coarse-grained dolomite; individual ~ystals ~ 2 cm Figure 21--Burlington Northern mine, Gravelly Range. Projection of pit face on e vertical plane perpendicular to layering of the marble {R. B. Berg, September 1976). GUNTER00000873 siderite (red weathering} and ankerite. Lava talc was mined from a deeply weathered par~ of a zone of siderite and ankerite in the marble. Because lava talc has been found only near the surface, it presumably 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 development of the Yellowstone mine, essentially all of the production from this mine has been ceramicor cosmetic-grade talc, which has found a variety of uses from the paper industry to the plastics industry. The talc in this area was reportedly discovered by Lewis Clark on his homestead in the early part of the twentieth century. Significant devetol6ment of the property was begun in 1942 when the 240foot (72-m) Madison Tunnel was driven by the TriState 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 a~d consisted of 4,000 pounds (1,812 kg). Perry (1948, p. 9) reported that in 1943 and t944, 127 tons (115 mr) 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 Yellowstone mine. The choosing of this new name as related by James D. Mulryan, Western Area Production Manager of Cyprus Industrial Minerals, is quoted from Olson (1976, p. 109): The original name of the Yellowstone Mine was the Mountain Talc Mine, Sierra Talc, largely through my father's efforts, acquired the property in !948. The mine was renamed the Yellowstone Mine at that time and the story behind this is that Henry Mulryan and Otis Booth, both of Sierra Talc, were driving to Montana to look 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 of name for it. About that time, there was occasion for a panic-type stop in the car they were driving, and a bottle of Yellowstone whiskey, which had been under the front seat, rolled out and hit the passenger on the foot. They felt that this must have been an omen of some sort, and the mine Was named Yellowstone Mine, It is therefore named for the Yellowstone whiskey, in spite of the fact it is only about 50 miles from West Yellowstone, Montana. 55 In 1964 the Yellowstone mine changed ownership when Cyprus Mines Corporation acquired Sierra Talc. The mine has been a significant producer of high-purity talc and wilt continue to be a major producer in the foreseeable future. GR-4 Queen claim Location: SE NE sec. 8, T. 9 S., R. 1 W., Madison County. Cameron 15-minute quadrangle. Approximately 19 relies (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 .,e.x.p?sed in other cuts, they seem to be exploratiori cuts. The 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: NW sec. 3, T. 9 S., R. 1 W., Madison County. Cameron 15-minute quadrangle. Approximately 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 provided by Ed Houser of Burlington Northern, Incorporated.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 talc is in the central zone, followed by the southern, then the northern. American Chemet removed severa~ 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 of 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 dofomitic marble. Very coarsely crystalline dolomitic marble also occurs on the property, but GUNTER00000874 56 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 steepty to the northwest. Most of the talc seems to be conformable, with little crosscutting of the host. Post-talc deformation is minimat, as only minor displacements are seen in the ore zones. Detailed mapping, trenching, and drilling of the prospect by Burlington Northern during the summer of 1977 confirmed that the greatest concentration of talc is in the central zone. The trenching also exposed some zones of chlorite formed by the alteration of phyltite; in all cases the chlorite is bounded by ~eformed talc. .A specimen of maroon rock from this cut was identified by x-ray diffraction as magnesite containing traces of talc end dolomite, GR-6 Talc-bearing conglomerate north of Johnny, Gulch Location: SV= sec. 1 and Nf sec. 12, T. 9 S., R. 2 W., Madison County. Varney 15-minute quadrangle. Section inferred from Beaverhead National Forest map. Approximately 20 miles (32 kin) southwest of Ennis. Accessibility: The road along the south side of Johnny Gulch leads to this area. Ownership: Beaverhead Nationat Forest. Description: In 1974 Pete Womack of Ennis discow ered pebbles of talc in the soil in an area underlain by Lodgepole Limestone (Mississippian) Ifig. The pebbles range in color from white to tan to pale green and in size from 0.5 to 4 cm. Further examination by Pete Womack showed that a conglomerate exposed in the N V~ sec. 120 T. 9 S., R. 2 W., contains talc pebbles and was a likely source for the pebbles found in the soil. The pebbles recovered from the soil at this locality are more rounded (subrounded to roundedl 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 tuff mapped by Hadley (1969b). The felsic tuff overlies Lodgepole Limestone (Mississippian) and Three Forks Formation (Devonian). The tuff 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 reddishkbrown quartzite in a fine-grained matrix that is cemented with calcite. Some calcite crystals line cavities in the conglomerate. Talc pebbtes 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 with minor talc Abundant pale green talc EXPLANATION ~ Downhill Figure 22--Inactive pit on the Queen claim, Gravelly Range (R. B. Berg, September 1976), GUNTER00000875 probably derived from Paieozoic carbonate units, and the reddish-brown quartzite pebbles are typical 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 Womack, Roger Kuhns, and the author failed to find any areas of Precambrian rockother than that shown by Hadley (1969a, b) 1 mile (1.6 km) to the east. Schist and dolomitic marble similar to the pebbles 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 exposure of 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 tithology 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 t1956, p. 3) may have filled a topographic depression or valley. The rhyolite and rhyolitic tuff are designated as Mioceneor Pliocene in age by Hadley (1969b). Without the benefit of absolute age determinations on the andesite that overlies the conglomerate and on the rhyolitic welded tuff that ovedies the talc at the Yel|ws~tone mine, no precise limit can be put on the intbrval of time during which talc was eroded from this deposit. Paul Pushkar ef Wright State University collected samples of both the andesite and the welded tuff during 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 separated from the metamorphic rocks of the Gravelly Range by the intervening Madison Valley. Hadley (1969a) mapped quartzofeldspathic gneiss, quartzite, hornblende gneiss, anoF~hosite gneiss, dolomite marble, iron-rich quartzite, mica schist, phyllite and metadiorite in that part of the Madison Range that fies in the Cameron quadrangle. South of the Cameron quadrangle the geology of the Precambrian rocks i~ less well known. Eric Erslev, a graduate student at Harvard, is now (1978) working on the Precambrian geology of that part of the Madison Range. Fine-grained dolomite is abundant in the debris deposited by the 1959 earthquakeqnduced slide that partly filled the Madison River Canyon. Father 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 preBelt metamorphic ro.cks shown on Witkind's map are metagranodiorite, amphibofite, quartzite, mica schist, diabase and gabbro. Eric Erslev [personal communication, 1978) reported 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 of economic interest. The only other known occurrence of 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 description of this mine.) GUNTER00000876 58 Henrys Lake Mountains The Henrys 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 2,50.0 re.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 typically finer grained and of lower metamorphic grade th~n the pre=Belt metamorphic rocks to the northwest 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 finegrained dolomite; average grain size being between 0.05 and 0.1 ram. tn addition to dolomite, Precambrian rocks of this area are mica schist, quartzite, 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. Trois area was chosen for study because of a reported occurrence of talc and the abundance of dolomite, the host rock for t'ale deposits in southwestern Montana. 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, including 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 development. 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 SW'/~ sec. 5, T. 14 S., R. 1 E. Upper Red Rock Lake 15- minute 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 NW sec. 18, T. 14 S., R. 2 E. Upper Red Rock Lake 15-minute quadrangle, Talc blebs approximately 1 cm ~ong are scattered throughout dolomite in an area one meter square. This occurrence of talc is at the contact of dolomite and a small body of 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 of talc was recognized. GUNTER00000877 Other talc occurrences Besides the described talc occurrences in the pre-Bett metamorphic rocks of southwestern Montana, there are only two other known talc occur- rences in the state, one south of Helena and one northeast of Troy. Both are described below. the ad~t, which is within 50 feet (16 m) of the loading platform. The Hasmark Dolomite has been metamorphosed in this area to medium-grained white dolomitic marble, presumably by a small nearby granodiorite pluton. O-1 Talc mine south of Helena Location: SW SE sec. 35, T. 10 N., R. 9 W. (adit), NE SW sec. 36, T. 10 N., R. 4 W. (quarry), Lewis and Clark County. Helena 15minute quadrangte. The quarry is approximately 2 miles (3.2 kin) aouthwest of the center of Helena. AccessibiFity: 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 Dotomite (Upper Cambrian). {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 taic range in thickness from less than t inch (2.5 cm) to 6 feet (2 m) and that these bodies may have a vertical dimension of 12 feet (4 m). The largest stope in the talc was 35 feet {11 m) 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 approximately 1 mile (1.6 kin) southwest of the limestone 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. t0, 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 U.TM northing is 5372220, the easting is 592110, and the locality is in zone 1, Lincoln County. The prospect is on the northeasttrending ridge between King Mountain on the southwest and China Mountain on the northeast, Kootenai Falls 7-m.nut~e;quadrangie, 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 kin) northeast of Troy. Accessibility: The Lynx Creak road branches from the O'Brien Creek road northeast of Troy in the S sec. 32, T. 32 N., R. 33 W. The distance by road from O'Brien Creek to the deposit is approximately 6 miles (10 km). Ownership: The deposit was located by a Mr. Mathews prior to 1958. Description: ]'he host rock for the talc is the Striped Peak Formation of the 13elt Supergroup (Precambrian). At this locality the Striped Peak .Formation 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 I~ast 40 feet (12 m). The talc is gray, yellow gray, yellow brown and green gray, and is described as sericitic talc that contains disseminated pyrite; secondary iron minerafs; and some quartz. Source of information: Johns, 1970, p. 152-153. GUNTER00000878 60 To Troy rio Lynx Creek N l I ~.. EXPLANATION Talc outcrop talc~ /" orgillile Drill hole number with depth of hole Strike and dip of bedding Contact Road partly grown over in 1978 Co ke Figure 23--lynx Creek (Mathews) talc prospect, Lincoln County |modified from Johns, 1970, p. 15~). 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 lacking 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 Rabbitt, 1960, p. 3638). The major pre-Belt metamorphic rock types exposed in this range are Dillon Granite Gneiss, preCherry Creek gneiss, and an ultramafic body. Tendo Mountains Scholten, Keenmon and Kupsch (1955, p. 351) described marble, mainly calcitic, within the sequence of pre-Belt metamorphic rocks exposed in the Tendoy Mountains southwest of Dillon. The marble is a minor unit in this area, in which quartzofeldspathic gneiss dominates. They did not mention talc in the marble. Snowcrest Range A small area on the northwest flank of the Snowcrest Range, southeast from the Ruby Range, is underlain by pre-Belt metamorphic rocks. AI- GUNTER00000879 61 though most of the metamorphic rock is quartzofeidspathic gneiss, a layer of marble 30 feet (9 m) thick is reported (Heinrich and Rabbitt, 1960, p. 35-36). Tremolite and fine-grained chlorite but no tatc are found in the marble. 3"his area of Precambrian metamorphic rocks is not shown on the Geologic Map of Montana ~Ross, Andrews and Witkind, 1955). Spanish Peaks .area A large part of the Spanish Peaks area at the north end of the Madison Range is underlain by preBelt metamorphic rocks. Spencer and Kozak (1973) 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 southwest of Cherry Lake where it is 20 feet (6 m) thick. Cherry Lake is approximately 11 miles.ItS km) northeast of Ennis. The marble contains calcite accompanied by minor tremolite, antigorite and diopside, but no talc (Spencer and Kozak, 1973, p. 43}. Northern part of the Gallatin Range Pro-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), Tysdat (1966), and Weber (1965) make no mention of marble. The most -abundant rock is quartzofeldspathic gneiss, which is associated with amphibolite. Beartooth Mountains All of the known talc areas of southwestern Montana could fit within the area of pre-Belt metamorphic rocks exposed in the Beartooth Mountains, which are situated in south-central Montana just north of Yellowstone National Park. The predominant metamorphic rock types are granitic gneiss, migmatite and amphibolite (Poldervaart and Bentley, 1958). Marble is reported only in the North Snowy Block in the northwestern part of the range (Reid, McMannis and Palmquist, t975, p. 14-15). The marble layer, named the George Lake Marble, is as much as 192 feet (60 m) thick and can be traced for more than 18 miles (29 kin) around the core of a nappe. Although it is mainly dolomitic, some calcitic marble is intimately intermixed. Diopside in the marble is in some places replaced by tremolite. Reid, McMannis and Palmquist concluded that growth of coarsegrained tremolite was followed by the growth of finegrained tremolite and talc. No mention is made of the abundance of the talc. The major Precambrian rock types in the Jardine district, in the southwestern part of the Beartooth Mountains, are schist and what is thought to be a Precambrian granitic pluton (Seager, 1944, p. 21-34). No marble is described ,in this area. Little Belt Mountains The core of the Little~Belt Mountains of central Montana consists of pre-Be~ metamorphic rock, mainly schist, gneiss and the Pinto metadiorite (Catanzaro and Kulp, 1964, p. 88-93), but marble is not described. Henry G. McClernan, who has studied an area on the southwest flank of the Little Belt Mountains, reported thet no marbte was recognized within the sequence of pre-Belt metamorphic rocks (personal communication, 1978). Other areas of possible pre-Belt rocks High-rank metamorphic rocks exposed along the eastern edge of the Idaho batholith in the Bitterroot Range have been suggested by some to be pro-Belt in age. Whether they are pro-Belt or metamorphosed sedimentary rocks of the Belt Supergroup or equivalent is an interesting question, but more important to talc exploration is the lack of marble !Berg, 1977; Chase, 1973). Carbonate bodies in the metamorphic rocks of southern Ravalli County thought to bo carbonatites contain rare-earth elements. Detaitect study of the mineralogy of the carbonate rocks by Crowley (1960) failed to find tatc. GUNTER00000880 63 References Ampian, S. G., 1976, Asbestos minerals and their nonasbestos analogs: Paper presented at the Review of Mineral Fibers Session of Electron Microscopy of Microfibers, Pennsylvania State University, University Park, Pennsylvania, August 2.3-25, 1976, 30p. Berg, R. B., 1977, Reconnaissance geology of southernmost Ravalll County, Montana: Montana Bureau of Mines and Geology Memoir 44, 39 p. 1979. Chlorite deposit in Precambrian .quartzofeld- spathic gneiss, Silver Star, Montana fabs.~: Geological Society of America Abstracts with Programs, v. 11, no. 6, p. 266. Brown, B. E., and Bailey, S. W., 1962., Chlorite ~olytypisrn: I. Regular and semi-random one-layer structures: American Mineralogist, v. 47, p. 919-850. Burger, H, R., t11, 1967, Bedrock geology of the Sheridan district, Madison County, Montana: Montana Bureau of Mines and Geology Memoir 41, 2~ p. Catanzaro, E- J,, and Kulp, J. L., 1964, D[scordar;t zircons from the Little Belt (Montane], Baartooth [Montana}, and Santa Catalina (Arizona) Mountains; Geochimica et Cosmoch]mica Acta, v. 28, p. 87-124. Chase, R. ~3., t973, Petrology of the northeastern border zone of the Idaho bathotith, Bitterroot Range, Montana: Men tana Bureau of Mines and Geology Memoir 43, 28 p. Chidester, A. H., Enget, A. E. J., and Wright, L. A., 1964. Talc resources of the United States: U.S. Geologica~ Survey Bulletin t167, 61 p. Clifton, R. A., 1978, Talc and pyrophytlite, in Minerals Yearbook, 1976: U.S. Bureau of Mines, v. t, p. 1309~1315. Cordua, W. S., 1973, Precambrian geology of the southern Tobacco Root Mountains, Madison County, Montana: Bloomington, indiana, Indiana University, unpublished Ph.D.dissertalion, 258 p. Crowley, F. A., 1960, Columbium-rare-earth deposit,~, southern Ravalli County, Montana: Montana Bureau of Mines and Geology Bulletin 18, 47 p. Deer, W. A., Howie, R. A., and Zussman, J., 1962, Rockforming minerals, v. 3, Sheet sificates: New York, John Wiley, 270p. Duncan, M. S., 1976, Structural analysis of the pro Beltian metamorphic rocks of the southern Highland Mountains, Madison and Silver Bow Counties, Montana: Bloomington, Indiana, Indiana University, unpublished Ph.D. dissertation, 222p. Foster, M. D., 1962, Interpretation of the composition and a classification of the chlorites: U.S. Geological Survey Professional Paper 414-A, p. A1-A33. Fritzsche, Hans, 1935, Geology and ore deposits of the Silver Star mining district, Madison County, Montana: Butte, Montana, Montana School of Mines (now Montana College of Min* sral Science and Technology), unpublished M.S. thesis, 89 p. Gar{han, j. M., 1973a, Geology and talc deposits of the central Ruby Range, Madison County, Montana: University Park, Pennsylvania, Pennsylvania State University, unpublished Ph.D. dissertation, 209 p. --=------_ 1973b, Origin and controJling factors of the talc deposits of steatite grade in the central Ruby Range, southwestern Montana [abe.): Geological Society of America Abstracts with Programs, v. 5, r~o. 2, p. 164. Garlhan, J. M., and Okuma, A. F., 1974, Field evidence suggesting a nonigneous origin for the Dillon quartzofeldspathic gneiss, Ruby Range, southwestern Montana labs.t: Geological Society of America Abstracts with Programs, v. 6, no. 6, p. 510. Geach, R. D., 1972, Mines and minerat deposits (except fuels), Beaverhsad County, Montana: Montana Bureau of Mines and Geology Bulletin ~.5, 194 p. Giletti, B. J.0 1966, tsotopic ages from southwestern Montana: Journal of Geophysical Research, v. 71, p. 4029-4036. Gillmeister, N. M., 1972, Petrology of Precambrian rocks in the central Tobacco Root Mountains, Madison County, Montana: Cambridge, Massachusetts, Harvard University, unpublished Ph.D. dissertation, 201 p. Goodwin, Aurar, compileri 1974, Symposium on talc, Washington. D.C., May 8, 1973, Proceedings: LLS. Bureau of Mines Information Circular 8639, 102 p. Hadley, J. B., 19698, Geologic map of the Cameron quadrangle, Madison County, Montana: I~1=~. LGeological Survey Map GQ-813. __1969b, Geologic map of the Varney quadrangle. Madison County, Montana: U.S. Geological Survey Map GQ-814. Henley, T. B., 1975, Structure and petrology of the northwestern Tobacco Root Mountains, Madison County, Montana: Bloomington, Indiana, indiana University, unpublished Ph.D. dissertation, 289 p. Hair, rich, E. W., and Rabbit, J. C., t960, Pre-Beltian geology of the Cherry' Creek and Ruby Mountains areas, southwestern Montana: Montana Bureau of Mines and Geology Memoir 38, 40p. Hess, Do F., 1967, Geology of pre-Beltian rocks in the central and southern Tobacco Root Mountains with reference to superposed effects of the L~ramide-age Tobacco Root batholith: Bloomington, indiana, Indiana University, unpublished Ph.D. dissertation, 333 p, Jahn, B. M., 1967, K-At mica ages and the margin of a regional metamorphism, Gravelly Range, Montana: Providence, ,Rhode ~sland, Brown University, unpublished M.S. thesis, 37 p. James, H. L., 1956, Johnny Gulch talc deposit, Madison County, Montana: U.S. Geological Survey Open-File Report, 13 p., map. James, H. L., and Wier, K, L., 1972, Geologic map of the Carter Creek iron deposit, sac. 3, 9, and 10, T. 8 S., R. 7 W., Madison and Beaverheed Counties, Montana: U.S. Geological Survey Miscellaneous Field Studies Map MF-359. James, H. L., Wier, K. L., and Shaw, K.. W., 196~, Map showing lithology of Precambrian rocks in the Christensen Ranch and adjacent quadrangles, Madlson and Beaverhead Counties, Montana: U.S. Geological Survey Open-File Map, 1 sheet, sca~e 1:20,0(30. Johna, W. M., 1961, Geology and ore deposits of the southern Tidal Wave mining district, Madison County, Montana: Montana Bureau of Mines and Geology Bulletin 24, 53 p. 1970, Geology and mineral deposits of Lincotn and Flsthead Counties, Mor~tana: Montana Bureau of Mines and Geology Bulletin 79, 182 p. GUNTER00000881 64 Knopf, Ado|ph, 1963, Geology of the northern part of the Boulder bathylith and adjacent area, Montana: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-381. Koehler, S, W., 1976, Petrology of the diabase dikes of the Tobacco Root Mountains, Montana: th Guidebook The Tobacco Root Geological Society 1976 Fie~d Conference: Montana Bureau of Mines and Geology Special Publication 73, p. 27 36. Krempasky, G. T., and Lawson, D. C., 1977, The mineral industry 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. ~ ~979, The mineral industry of Montana, in Minerals Yearbook, 1976: U.S. Bureau of Mi~es (in press). Levandowski, D. W., 1956. Geology and mineral deposits of the Sheridan-Aider area, Madison CoUnty, Montana: Ann Arbor, Michigan. University of Michigan, unpublished Ph.D. dissertation, 3t8 pc Mann, J. A., 19r--~, Geology of Dart of the Gravelly Range, Montana: Yellowstone-Bighorn Research Project Contribution 190, '92 p. Marvin, R. F., Wier, K. L., Mehnart, H. hi., and Merritt, '~/. M., t974, K-At ages of selected Tertiary igneous rocks in south=western Montana: isechron West, no. 10, p. 17-20. McDowell, F. W., 1971, K-At ages of igneous rocks from the western United States: lsochron West, no. 2, p. 1-t6. McMannis. W. J., end Chadwick, R. A., 1964, Geology of the Garnet Mountain quadrangle, Gatlatin County, Montana: Mon tana Bureau of Mines and Geology Butletin 43, 47 0. MillbolFand, M. A., 1976, Mineralogy and petrology of Precambrian metamorphic rocks of the Gravelly Range, southwestern Montana: Bloomington, indiana, indiana University, unpublished M.A. thesis, 134 p. Monroe, J. S.. 1976, Vertebrate paleontology, stratigraphy, and sedimentation of the upper Ruby River Basin, Madison County, Montana: Missoula, Montana, University of Montana, unpublished Ph. D. dissertation, 345 p. Mueller, P. A., and Cordua, W. S., 1976, Rb-Sr whole rock ~ge of gneisses from the Horse Creek area. Tobacco Root Mountains, Montana: Iso~chron West, no. 16, p. 33-36. Mulryan, H. T., 1974;" C'h,~Facterization and occurrence of talc, in Goodwin, Aurel, compiler, Symposium on ta1c, Washington, D.C., May 8, 1973, Proceedings: U.S. Bureau of Mines Information Circular 8639, p. 16-21. Okuma, A. F,, 1971, Structure of the southwestern Ruby Range near Dillon, Montana: University Park, Pennsylvania, University of Pennsylvania, unpublished Ph.D. dissertation, 122 p. Oison, R. H., 1976, The geology of Montana talc deposits, in Eleventh Industrial Minerals Forum, Proceedings: Montana Bureau of Mines and Geology Special Publication 74, p. 99~143. Peale. A. C., 1896, Three Forks, Montana: U.S. Geological Survey Geological Atlas, Folio 24, 7 p, Perry, E. S., 1948, Talc, graphite, vermiculite, and asbestos in Montana: Montana Bureau of Mines and Geology Memoir 27, 44p. Poldervaart, Aria, and Bentley. R. D., 1958, Precambrian and Inter evolution of the Beartooth Mountains, Montana and Wyoming, in Billings Geological Society, Guidebook, 9th Annual Field Conference, August 14-16, 1958, p. 7-t5. Raid, R. R., 1957, Bedrock geology of the north end of the Tobacco Root Mountains, Madison County, Montana: Montana Bureau of Mines and Geology Memoir 36, 26 p. __ 19~3, Metamorphic rocks of the northern Tobacco Root Mountains, Madison County, Montana: Geologicai Socie~ of America Bulletin, v. 74, p. 29~-305. Reid, R. R., McMannis, W. J,, and Palmquist. J. C., 1975, Precambrian geology of North Snowy block. Beartooth Moantains, Montana: Geological Society of America Special PaPer 157, 135 p. Robinson. G. D., Klepper, M. R., and Obradovich. j, 19f~8, Overlapping pfutonism, volcanism, and tectonism in the Boulder batholith region, western Montana, in Coats, R, R., Hay, L., and Anderson, C. A., editors, Studies in votcanology: Geological Society of America Memoir 116, ~. 557-576. Roe, L. A.. 1975, Talc and pyrophyllite, in Lefond, S. J., editor, Ind~stria~ minerals and rocks: American Institute of Mining, Metallurgical, and F~etroleum Engineers. p. 1 ~27-1 ~47. Ross, C. P., Andrews, D. A,, and W~tkind, I. J., 1955. Geotogic map of Montana: U.S. Geological Survey. Sahinen. U, M., 1939, Geology and ur~ deposits of the Rochester and adjacent mining districts, Madison County, Montana: Montana Bureau of Mines and Geology Memoir ~9, 53 p. Scholten, Robert, Keenmon, K. A., and Kupsch, W. O., 1955, Geology of the Lima region, southwestern Montana and adia~ cent Idaho: Geologics~ Society of America Bulletin, v. 66, p. 345-404. Saager, (3. F., 1944, Gold, arsenic, and tungSten deposi~ of the Jardine~Crevasse Mountain district, Park County, Montana: Montana Bureau of Mines and Geology Memoir 23, !11 p. Slaughter, J., Kerrick, D. M., and We|t, V. J., 1975, ~:xperirnental and thermodynamic study of equilibria in the system CaO-MgO~SiO~o~zO-COz: American Journal of Science, v. 275, p. 143-162.. Spencer, E. W., and Kozak, S. J., 1973, Geology of the Spanish Peaks area, Montana: Montana Bureau of Mines and Geolooy, map and 99-page text on often fi;e. Tansley, Wilfred, Schafer, P. A., and Hart, L., H., 1933, A geological reconnaissance of the Tobacco Root Mountains, Madison County, Montana: Montana Bureau of Mines and Geology Memoir 9, 57 p. Tilling, R. l., Klepper, M. R., and Obradovich, J. D., 1968, K-Ar ages and time span of emplacement of the Boulder batholith, Montana: American Jm=rnal of Science, v. 266, p. 67 Tysdal, R. G., 1966, Geology of a part of the north end of the Gallatin Range, Call, tin County, Montana: Bozeman, Montana, Montana State University, unpublished M.S. thesis, 95 p. 1976, Geologic map ofnorthern part of Ruby Range, Madison County, Montana: U,S, Geological Survey Miscellaneous Geologic Investigations Map 1-951. Weber, W. M., 1965, General geology and geomorphology of the Middle Creek area, Gaflatin County, Montana: Bozeman, Montana, Montana State University, unpublished M.S. thesis, 86 p. Welch, J. R., 1973,The minerai industry of Montana, in Minerals yearbook, 1971, v. 2, p. 437-451. ~ 1974, The mineral industry of Montana, Lo Minerals yearbook, 1972, vo 2, p. 425-437. Wells, J. R., 1976, Talc, soapstone, and pyrophyllite, in Minaret facts and problems, 1975 edition: U.S. Bureau of Mines Bulletin 667, p. 1079-1090. West, J. M., 1972, The mineral industry of Montana, in Minerals yearbook, 1970, v. 2, p. 425-441. GUNTER00000882 --------1976, The mineral industry of Montana, in Minerals yearbook, 1973, v. 2, p. 417-431. Whltehead, M. L., 1979, Geology and talc occurrences of the I~enson Ranch, Beaverhead County, Montana: Butte, Montana, Montena College of Mineral Science and Technology, unpublished M.S. thesis, 53 p. Wier, K. L,, 1965, Preliminary geologic map of the Black Butte iron deposit, Madison County, Montana: U.S. Geological Survey Open-File Map. Witkind, 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 pert of the Upper Red Rock !.~ke quadrangle, southwestern Montana and adjacent idaho: U.S, Geological Survey Miscellaneous Geologic Investigations Map 1-943. Production Information Camera-ready copy prepared on EditW~iter 7500 by MBMG. Stock: Cover- 17 pt. Kivar Text -- 70 Ib, Mountie Matte Shee~s -- 60 lb. Offset Book Composition: Univers 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. Presswork; Heidelberg Sorkz Ink: Leber Order: 2,000 copies GUNTERO0000883