Document B8OegvND06oJVwG27KZbEveBo

MaRdiisvoenr BlaMckotuaniltains TMeonudnotyains MaRdiivseorn 45o301' 13o 45' 30' 15' 112o 15' 45o STUDY AREA 40 22 18 14 21 20 12 17 23 1619 1 24 2,5 27 28 8 9,32 10 33 34 36 37 38 39 13 35 3 25 26 29 30 31 7 11 6 15 51 42 41 43 46 44 45 47 50 48 49 45' 44o301' 13o 45' Base from U.S. Geological Survey, 1:250,000, Ashton, 1972; Bozeman, 1972; Dillon, 1977; Dubois, 1978. 1927 North American Datum. Projection: Transverse Mercator. 30' 12 45 113 Twin Bridges 12 287 8 15 Sheridan 41 287 7 14,15 Alder Virginia Ennis City 287 5 Dillon 6 6 2 1 9 10 12 4 3 11 8 9 15 Monida 9 MONTANA IDAHO 11 13 17 287 87 16 11 0 10 Miles 0 10 Kilometers 112 Index Map Showing Sources of Geologic Map Data 5 5 15' SCALE 1:250 000 0 5 10 112o 15 20 MILES 0 5 10 15 20 25 30 KILOMETERS CONTOUR INTERVAL 200 FEET SUPPLEMENTARY CONTOUR INTERVAL 100 FEET IN DUBOIS QUADRANGLE NATIONAL GEODETIC VERTICAL DATUM OF 1929 Sources of Geologic Map Data 1 Berg, R.B., 1979, Precambrian geology of the west part of the Greenhorn Range, Madison County, Montana: Montana Bureau of Mines and Geology Geologic Map 6, 2 sheets, scale 1:28,800, 12 p. pamphlet. 2 Garihan, J.M., 1979, Geology and structure of the central Ruby Range, Madison County, Montana--Summary: Geological Society of America Bulletin, v. 90, part I, no. 4, p. 323-326. 3 Hadley, J.B., 1969, Geologic map of the Cameron quadrangle, Madison County, Montana: U.S. Geological Survey Geologic Quadrangle Map GQ-813, scale 1:24,000. 4 ______1969, Geologic map of the Varney quadrangle, Madison County, Montana: U.S. Geological Survey Geologic Quadrangle Map GQ-814, scale 1:24,000. 5 Heinrich, E.W., and Rabbitt, J.C., 1960, Pre-beltian geology of the Cherry Creek and Ruby Mountains areas, southwestern Montana: Montana Bureau of Mines and Geology Memoir 38, 40 p. 6 James, H.L., 1990, Precambrian geology and bedded iron deposits of the southwestern Ruby Range, Montana, with a section on The Kelly iron deposit of the northeastern Ruby Range: U.S. Geological Survey Professional Paper 1495, 39 p., 2 plates. 7 Karasevich, L.P., 1981, Geologic map of the northern Ruby Range, Madison County, Montana: Montana Bureau of Mines and Geology Geologic Map 25, scale 1:24,000. 8 Kellogg, K.S., and Williams, V.S., 1997, Geologic map of the Ennis 30' x 60' quadrangle, Madison and Gallatin Counties, Montana: U.S. Geological Survey Open-File Report 97-851, scale 1:100,000. 9 Klepper, M.R., 1950, A geologic reconnaissance of parts of Beaverhead and Madison Counties, Montana: U.S. Geological Survey Bulletin 969-C, p. 55-85. 10 Okuma, A.F., 1971, Structure of the southwestern Ruby Range near Dillon, Montana: University Park, Pennsylvania State University, Ph.D. dissertation, 122 p. 11 O'Neill, J.M., U.S.G.S., 1997, personal commun., unpublished geologic map of the Hebgen Lake 30' x 60' quadrangle. 12 Ruppel, E.T., O'Neill, J.M., and Lopez, D.A., 1993, Geologic map of the Dillon 1o x 2o quadrangle, Idaho and Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-1803-H, scale 1:250,000. 13 Sonderegger, J.L., Schofield, J.D., Berg, R.B., and Mannick, M.L., 1982, The upper Centennial Valley, Beaverhead and Madison Counties, Montana, with a section on The Madison Valley thermal springs by G.J. Weinheimer: Montana Bureau of Mines and Geology Memoir 50, 53 p., 4 plates. 14 Tysdal, R.G., 1970, Geology of the north end of the Ruby Range, southwestern Montana: Missoula, University of Montana, Ph.D. dissertation, 187 p. 15 ______1976, Geologic map of northern part of Ruby Range, Madison County, Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-951, scale 1:24,000. 16 Witkind, I.J., 1972, Geologic map of the Henrys Lake quadrangle, Idaho and Montana: U.S. Geological Survey Map I-781-A, scale 1:62,500, 2 sheets. 17 ______1976, Geologic map of the southern part of the Upper Red Rock Lake quadrangle, southwestern Montana and adjacent Idaho: U.S. Geological Survey Miscellaneous Investigations Series Map I943, scale 1:62,500. 45' 111o1455' o30' EXPLANATION Exposures of Archean marble (Permissive host rock for talc deposits)--Consists predominantly of dolomitic marble, with lesser amounts of calcite-rich marble and marble with abundant calc-silicate minerals 1 Active talc mine (Number refers to list at right) 55 Inactive talc mine, prospect, or occurrence (Number refers to list at right). Talc occurrences may plot outside of Archean marble where the host marble body is too small to show at the scale of this map 15' 52 53 57 4,56 55 54 45o 45' 45' 30' 111o4145o' 30' 45 113 Range Highland Mountains Twin Bridges Tobacco Root Mountains 287 Sheridan 15 41 287 Alder Virginia City Ennis Dillon Ruby Range 287 Madison Greenhorn Range Range Gravelly Range 287 Snowcrest 15 MONTANA IDAHO Monida Centennial Henrys Lake Mountains Mountains 0 10 Miles 0 10 Kilometers 112 Index map showing the study area (stippled) 87 CAM-133 TALC MINES, PROSPECTS, AND OCCURRENCES ACTIVE MINES (1997) Map Mine name No. 1 Beaverhead 2 Treasure State 3 Regal (Keystone) 4 Yellowstone Location NW SE sec. 14, T.7S., R.6W. NW sec. 14, T.7S., R.6W. NW NE sec. 2, T.8S., R.7W. sec. 4, T.9S., R.1W. Operator Luzenac America, Inc. Barretts Minerals, Inc. Barretts Minerals, Inc. Luzenac America, Inc. Mining method; Production; Published reserves Underhand (cemented backfill using borrow material); producing 9,000 short tons of ore per year; 150,000 short tons of proven reserves Open pit with significant talc production Open pit; current mine opened in 1997; production & reserve estimates not yet available; mined by a shaft 60 ft deep in the 1940's, mined from an open pit in the 1960's and 1970's Open pit; producing 300,000 short tons of ore per year; ore reserves of 6,000,000 short tons References Garihan (1973a); Olson (1976); Berg (1979b); McCulloch (1996); Kaas (1996); U.S. Geological Survey (1996) Garihan (1973a); Olson (1976); Berg (1979b); Kaas (1996) Perry (1948); Olson (1976); Berg (1979b); Kaas (1996) Perry (1948); James (1956); Olson (1976); Berg (1979b); McCulloch (1996); Kaas (1996); U.S. Geological Survey (1996) Map Name No. 5 Treasure Chest mine 6 Sauerbier mine Location INACTIVE MINES, PROSPECTS AND OCCURRENCES Status; Extent of development Comments NW sec. 14, T.7S., R.6W. NW NW sec. 25, T.8S., R.7W. Ruby Range Inactive open pit mine Inactive mine Although recently mined, the Treasure Chest is now inactive Mine established in 1973 7 American Chemet mine (Apex mine) NE sec. 12, T.8S., R.7W. Inactive; three open pits, several bulldozer trenches Once operated by American Chemet Corp. of Chicago, Illinois 8 Banning-Jones mine SW sec. 13, (Banjo mine) T.8S., R.8W. Inactive mine Same belt of marble as at SmithDillon mine one mile to southwest 9 Bozo-Zobo mine 10 Smith-Dillon mine NE sec. 19, T.8S., R.7W. center sec. 23, T.8S., R.8W. 11 Sweetwater (Estelle) mine 12 Bennett Owen claim 13 Crescent (Timber Gulch) prospect E sec. 13, T.8S., R.7W. NW sec. 12, T.7S., R.6W. SW sec. 1, T.9S., R.8W. Inactive mine; large open cuts up to 50 ft deep Inactive mine; originally an underground mine (1940's); worked from an open pit in the 1970's Inactive open pit mine Bulldozer cuts Mid-1960s prospect of Pfizer, Inc. Shipped 8,000 tons of talc ore in the mid-1960's Important producer of high-grade talc in the 1940's Prospect pits follow same marble unit as in adjacent NW NE sec. 24 Large body of uniformly dark green, low-purity talc Small 1940's era shaft and pits expose graphitic talc to a depth of 10-20 ft 14 Gem claim SE sec. 34, T.6S., R.6W. Two small bulldozer cuts 15 Owen-McGovern prospect SE sec. 23 & E sec. 26, T.8S., R.7W. Minor bulldozer cuts and drill holes 16 unnamed prospect SE sec. 11, T.7S., R.6W. Bulldozer cuts 17 unnamed prospect SW sec. 11, T.7S., R.6W. Two bulldozer cuts 18 Spring Creek prospect SE SE sec. 32, T.6S., R.6W. A number of cuts and drill holes Small bodies of dark green, graphitic talc are exposed Includes the Badger #1-8 mining claims; exposed talc layers 1-2 ft thick Graphitic, limonitic talc body 10 ft wide exposed in cut Talc body 7-10 ft thick exposed in cuts; underlain by dark-green graphitic talc Zone of talc extends for 5,000 ft along northeast strike of dolomitic marble 19 Ruby View prospect SE NW sec. 13, T.7S., R.6W. Small prospect pit Along strike with marble unit of Beaverhead mine; talc masses up to 3 ft thick 20 Whitney claims SW sec. 2, T.7S., R.6W. Bulldozer cuts Talc body discordant to marble layering; limonite and pyrite locally abundant 21 unnamed prospect SE SE sec. 3, T.7S., R.6W. Bulldozer cuts Talc bodies 15-20 ft thick; locally abundant graphite 22 Ruby Peak occurrence W sec. 16, T.6S., R.5W. Scattered chips of palegreen to green talc in soil Talc pieces over large area 23 occurrence SW sec. 17, T.7S., R.6W. Occurrences Lenses and layers of talc an inch thick and less than 3 ft long 24 occurrence NW SW sec. 18, T.7S., R.5W. Talc float Dark-green talc float, locally graphitic 25 occurrence SW NW sec. 2, T.8S., R.7W. Talc float Talc float traced for 200 ft along strike, 100 ft in width 26 occurrence SW SW sec. 3, T.8S., R.7W. Zone of talc float Float of high purity, light-green talc over strike length of 600 ft 27 occurrence SW SE sec. 5, T.8S., R.7W. Talc occurrence Shown on plate 2 of Heinrich and Rabbitt (1960) 28 occurrence SE NW sec. 8, T.8S., R.7W. Talc occurrence Shown on plate 2 of Heinrich and Rabbitt (1960) 29 occurrence E sec. 10 & NW sec. 11, T.8S., R.7W. Talc in outcrop and as abundant float Talc exposed across over 7,000 ft of strike length 30 occurrence NE sec. 15 & SW sec. 11, T.8.S. R.7W. Abundant talc float on each side of dolomitic marble ridge Green talc in float extends for 4,000 ft along strike 31 unnamed prospects NE sec. 21 & NW sec. 22, T.8S., R.7W. Several talc prospects in dolomitic strata One talc zone extends for about 500 ft along strike 32 occurrence SW NW sec. 19, T.8S., R.7W. Talc in outcrop Dark talc exposed for about 1,350 ft of strike length 33 occurrence NE NW sec. 26, T.8S., R.8W. Talc occurrence Shown on plate 2 of Heinrich and Rabbitt (1960) 34 occurrence SE SE NE sec. 26, T.8S., R.8W. crest Talc in outcrop along ridge Green talc along strike for 1,200 ft, 5-30 ft wide 35 Valley View prospect SW sec. 25, T.8S., R.8W. Bulldozer cuts Body of dark talc about 90 ft wide and at least 150 ft long 36 unnamed prospects NW NW sec. 36, T.8S., R.8W. Bulldozer cuts Dark-green talc exposed in two cuts 37 occurrence W NE sec. 35, T.8S., R.8.W Talc in outcrop and shallow trenches Light green talc traced over strike length of about 900 ft; some zones 10 ft thick 38 occurrence SE SE sec. 35, T.8S., R.8W. Talc occurrence Shown on plate 2 of Heinrich and Rabbitt (1960) 39 occurrence secs. 1 & 2, T.9S., R.8W. Talcose zones in marble Well-developed talcose zone traced for 5,000 ft along strike (east-northeast) 40 occurrence SW SW sec. 12, T.6S., R.6W. Talc float Small amount of talc float in marble Greenhorn Range References [The ten-digit numbers below refer to unique sequence numbers in Kaas (1996)] Garihan (1973a, p. 149-156); Olson (1976, p. 121-124); Berg (1979b, p. 34) Okuma (1971, plate 1); Olson (1976, p. 126-127); Berg (1979b, p. 37); 0300570858 Okuma (1971, p. 108-111); Olson (1976, p. 127); Berg (1979b, p. 35-36); 0300570857 Geach (1972, p. 161-162); Olson (1976, p. 127); Berg (1979b, p. 36-37); 0300010553 Olson (1976, p. 127-128); Berg (1979b, p. 37); 0300570814 Perry (1948, p. 4-6); Okuma (1971, p. 99-102); Olson (1976, p. 128); Berg (1979b, p. 36); 0300010237 Okuma (1971, p. 106-107); Olson (1976, p. 128); Berg (1979b, p. 36);0300570447 Garihan (1973a, p. 162-166);Olson (1976, p. 128); Berg (1979b, p. 33);0300570855 Perry (1948, p. 6); Okuma (1971, p. 105); Olson (1976, p. 129); Berg (1979b, p. 37); 0300010381 Garihan (1973a, p. 180-181); Olson(1976, p. 129); Berg (1979b, p. 32);0300570852 Okuma (1971, p. 107-108); Olson (1976, p. 129); Berg (1979b, p. 38);0300570859 Garihan (1973a, p. 166-169); Olson(1976, p. 129); Berg (1979b, p. 33);0300570856 Garihan (1973a, p. 169-171); Berg (1979b, p. 33); 0300570871 Garihan (1973a, p. 177-180); Olson (1976, p. 129-130); Berg (1979b, p. 32); 0300570747 Garihan (1973a, p. 160-162); Olson(1976, p. 129); Berg (1979b, p. 35);0300570250 Garihan (1973a, p. 171-174); Olson(1976, p. 130); Berg (1979b, p. 33);0300570853 Garihan (1973a, p. 174-175); Olson(1976, p. 130); Berg (1979b, p. 33);0300570252 Berg (1979b, p. 32; 1987a; 1987b, p. A19); 0300570851 Garihan (1973a, p. 181-182); 0300570869 Garihan (1973a, p. 182); 0300570870 Olson (1976, p. 131); 0300570872 Olson (1976, p. 131); 0300570873 Heinrich and Rabbitt (1960, plate 2); 0300010341 Heinrich and Rabbitt (1960, plate 2); 0300010328 Olson (1976, p. 131); 0300570874 Olson (1976, p. 131); 0300570875 Olson (1976, p. 131); 0300010558 Olson (1976, p. 130); 0300010559 Heinrich and Rabbitt (1960, plate 2); 0300010323 Olson (1976, p. 131); 0300010560 Olson (1976, p. 130-131); 0300010561 Olson (1976, p. 130); 0300010562 Olson (1976, p. 130); 0300010287 Heinrich and Rabbitt (1960, plate 2); 0300010563 Olson (1976, p. 131); 0300010300 Berg (1987b, p. A19) 41 Ruby No. 1 & Ruby No. 2 claims NE NW sec. 14, T.8S., R.4W. Pit 8 ft deep and shallow bulldozer cuts Talc and chlorite exposed in pit & cuts associated with a diabase dike Berg (1979b, p. 40-42) 42 unnamed prospect SW NW sec. 14, T.8S., R.4W. Shallow bulldozer cut Trace of coarse-grained silvery talc in sheared and altered marble Berg (1979b, p. 42) 43 Doubtful claim NW SW sec. 14, T.8S., R.4W. Small, shallow pit and two shallow cuts One pod of limonite-stained talc less than 6 ft in horizontal extent Berg (1979b, p. 42-43); 0300570860 44 unnamed prospect SE NW sec. 23, T.8S., R.4W. Shallow prospect pit Minor talc in two marble layers (4 inches thick) exposed in pit Berg (1979b, p. 42) 45 unnamed prospect SE NE sec. 23, T.8S., R.4W. Shallow cut Minor talc along two shear zones exposed in cut Berg (1979b, p. 42) 46 unnamed prospect SE NE sec. 23, T.8S., R.4W. Shallow cuts Dark-green talc poorly exposed for 45 ft in one cut & 50 ft in a second cut Berg (1979b, p. 42) 47 Greenhorn claims SW NW sec. 30, T.8S., R.3W. Bulldozer cuts Sheared and contorted talcose marble Berg (1979b, p. 42-44); 0300570861 48 Willow Creek mine (Ruby Ridge mine) (Talc Ridge mine) SW NE sec. 30, T.8S., R.3W. Inactive mine Only significant production was from 1970 to June 1979 Olson (1976, p. 136); Berg (1979b, p. 43-46); 0300570740 49 Adam and Eve No. 1 SE SW sec. 30, and No. 2 claims T.8S., R.3W. (Pettus No. 1 prospect) Six prospect cuts Talc pods 2 inches long and talc veinlets 1 inch thick Chidester and Worthington (1962, p. 4); Olson (1976, p. 136-137); Berg (1979b, p. 46); 0300570862 50 Pettus No. 2 prospect SW NW sec. 31, T.8S., R.3W. Prospect trench Trace of talc in bottom of prospect trench Chidester and Worthington (1962, p. 4); Berg (1979b, p. 46) 51 Calverts claims SW SE sec. 32, Bulldozer cuts Irregular veinlets and pods of talc, Berg (1979b, p. 46-48); 0300570864 T.7S., R.3W. most less than 4 inches thick [Berg (1979b) noted and described 35 relatively minor talc occurrences in the Greenhorn Range that are in addition to the mines and prospects listed above. These occurrences include talc chips in the soil, as well as small pods and veinlets in outcrops of dolomitic marble; they are too numerous and closely spaced to show on this map. Instead, refer to Berg (1979b, p. 39-42, 46, and plates 2 and 3; Berg sites GH-1 to GH-27, GH-31 to GH-37, and GH-45), where the occurrences are described and plotted individually at a map scale of 1:28,800.] Gravelly Range 52 Tait Mountains claims NW SE sec. 5, T.8S., R.1W. A number of trenches; prospect pits; a pre-1948 shaft 25 ft deep Minor talc on dumps and in float, associated with abundant chlorite Perry (1948, p. 8); Olson (1976, p. 136); Berg (1979b, p. 49-51); 0300570439 53 Cherry Gulch prospect NE SW sec. 31, T.8S., R.1W. Caved adit, small pits, shallow trenches Talc 1.5-2 ft thick exposed in pits; talcose zone 5 ft thick in other pit Berg (1979b, p. 50-53); 0300570865 54 Queen claim SE NE sec. 8, T.9S., R.1W. Small inactive mine in bulldozer cut; other cuts also Small amount of pale-green talc layers 4-10 inches thick mined from one cut Olson (1976, p. 136); Berg (1979b, p. 55-56); 0300570866 55 Burlington Northern mine NW sec. 3, T.9S., R.1W. Inactive open pit mine Several thousand tons of talc removed in early 1960's from a small pit Olson (1976b, p. 135-136); Berg (1979b, p. 54-56); 0300570867 56 Johnny Gulch mine N sec. 9, T.9S., R.1W. Inactive open pit mine Acquired in 1994 by Luzenac America and now part of Yellowstone mine Olson (1976, p. 136); McCulloch (1994, p. 60); 0300570941 57 Talc-bearing conglomerate S sec. 1 & N sec. 12, T.9S., R.2W. Poorly exposed conglomerate Talc pebbles form a minor constituent of an Oligocene conglomerate Henrys Lake Mountains Berg (1979b, p. 56-57); 0300570868 [During geologic mapping in the Henrys Lake Mountain area (Sonderegger and others, 1982), four small talc occurrences were noted (Berg, 1979b, p. 58). The occurrences are in dolomitic marble in contact with metagranodiorite, diabase and gabbro.] Introduction For the last several years, Montana has been the leading talc producing state in the United States (U.S. Geological Survey, 1996). For example, in 1992 Montana supplied about 40 percent of the U.S. mine production of talc (Virta, 1992). All of this production has come from the large deposits of high purity talc in the southwestern part of the state. All Montana talc is currently (1997) extracted from four mines, each within the study area of this map--the open pit operations of the Treasure State, Regal, and Yellowstone mines and the underground operation of the Beaverhead mine (see map numbers 1-4 on list and map to the left). The related mineral chlorite is mined at the Antler mine, located nearby, but outside of the study area in the Highland Mountains. Montana talc has at least two market advantages: (1) some deposits are very large and near surface, allowing economic mining by open pit methods; and (2) the deposits are of high purity and lack tremolite or other amphibole mineral contaminants (such as absestos) that occur in some other talc-rich deposits. Talc from southwest Montana is used in ceramics, paint, paper, plastics, cosmetics, rubber, roofing, flooring, caulking, and agricultural applications. The talc is also used in the processes of recycling paper and plastics. Talc was first discovered in the early 1900's at the present site of the Yellowstone mine (Perry, 1948, p. 9). Modest production began in 1942 from shallow pits and adits, supplying steatite (massive, compact, high-purity) talc that was used to make ceramic insulators. The southwest Montana talc industry grew to become a significant part of the region's economy; this history is described by Perry (1948), Olson (1976), and Berg (1997). Exploration and development are likely to continue for the foreseeable future for several reasons: (1) mines are active in the area at present and an infrastructure for talc processing exists; (2) large changes in domestic and export talc markets are not expected in the next few years based on recent market trends (Virta, 1997); (3) the talc of this region is especially pure and asbestos-free; and (4) except for potential ground stabilization problems and land disturbance associated with largescale open pit mining, no significant environmental impacts are associated with talc mining. Location of the Study Area The study area is entirely within Montana and includes the eastern part of Beaverhead County and the western part of Madison County. The study area, designated the "Gravelly's Landscape Analysis Unit" by the regional land managers of the Bureau of Land Management (BLM) and U.S. Forest Service (USFS), is defined as follows: (1) The western boundary is U.S. Interstate Highway 15 from Monida to Dillon, and State Highway 41 from Dillon to Twin Bridges; (2) The northern boundary is State Highway 287 from Twin Bridges to Ennis; (3) The eastern boundary is the Madison River from Ennis to Raynolds Pass (2.5 miles west of Earthquake Lake) and State Highway 87 from Raynolds Pass south to the Montana-Idaho border; and (4) The southern boundary is the Montana-Idaho border (the Continental Divide) between Monida and State Highway 87. This study area includes rugged mountains of the Blacktail, Centennial, and Henrys Lake Mountains and the Gravelly, Greenhorn, Ruby and Snowcrest Ranges, as well as the intervening basins. Within this study area, Archean dolomitic marble is the only rock type known to host talc deposits of economic size and this marble occurs within the study area only in the Gravelly, Greenhorn and Ruby Ranges and the Henrys Lake Mountains. Purpose of the Study In response to requests from the BLM and the USFS, the U.S. Geological Survey conducted a mineral resource assessment of the Gravelly's Landscape Analysis Unit. These agencies use mineral resource data in developing land-use management plans for the reasonably foreseeable future for federal lands in the region. Because the Gravelly's Landscape Analysis Unit includes the area that leads the U.S. in commercial talc production (U.S. Geological Survey, 1996), talc resource exploration and development potential must be evaluated and considered in planning. This map shows areas of surface exposure of Archean-age marble, which is the only rock type within the study area known to host talc deposits of economic size. Only exposed marble is shown because it is most probable that talc bodies will be discovered in this area by recognizing talc alteration in surface exposures or in the excavations of mines or prospects. Subsurface extensions of promising talc bodies are delineated by exploratory drilling and trenching. The subsurface extensions of the host marble units, and thus possible buried talc deposits, are not projected on this map. Identication of specific targets for potential mines is beyond the scope of this study. Multiple economic and social considerations (for example, purity of talc, mining and processing costs, proximity to markets, transportation costs, talc markets and prices, and environmental and reclamation issues and expenses) are factors that will effect the economic viability of any future development. Geologic Setting of the Southwest Montana Talc Deposits All of the economically important talc deposits of southwest Montana are found as replacements of dolomitic marble of Archean age. Most of this talc [ideal formula Mg3Si4O10(OH)2] formed through hydrothermal processes, which introduced significant amounts of Mg+2 and SiO2 in solution and replaced dolomite [CaMg(CO3)2] (Anderson and others, 1990). In smaller amounts, talc also replaced magnesite, quartz, tremolite, serpentine, and calcite. Talc occurrences range from thin veinlets and pods a few inches thick to extensive thick masses, such as the 95-ft-thick talc body at the Treasure Chest mine (map number 5). Talc lenses parallel lithologic layering in some places; in others (Yellowstone mine), talc lenses appear to cut across layering. Most of the talc bodies are lenticular and elongate (Chidester and others, 1964). The origin of these deposits is not completely understood; they all appear to have formed during the Precambrian and are restricted to Archean dolomitic marbles. While host rock lithology (dolomitic marble) was clearly a control in their formation, the roles of regional metamorphism, structures, and hydrothermal systems are much less certain. The origin and timing of talc formation in southwest Montana are discussed by Anderson (1987); Anderson and others (1990); Berg (1977, 1979b, 1987b, 1995); Blount and Parkison (1991); Brady and others (1991); Garihan (1973a, 1973b); Kovaric and others (1996); Okuma (1971); Olson (1976); Piniazkiewicz (1984); Smith (1980); Whitehead (1979); and Wilson (1981). Dolomitic marbles in the Gravelly, Greenhorn, and Ruby Ranges occur within an Archean-age sequence of marble, quartzite, schist, amphibolite, gneiss, and iron-formation. Marble layers in the sequences range from about 10 ft to over 1,600 ft thick and individual layers can be traced for distances of up to 10 miles (Berg, 1979a). Berg (1987a, 1987b) identified three different types of marble in the sequence of the Ruby Range: (1) a dolomitic variety that forms slabby and angular outcrops, weathers to a tan color, and is white on fresh surfaces; (2) a calcite-rich variety; and (3) a variety bearing calc-silicate minerals. Talc occurrences in the northern Ruby Range are most common in the dolomitic marbles, but do occur in the latter two varieties. Geophysical and Geochemical Signatures of Talc Deposits No gravity or magnetic signatures are associated with these talc deposits. Airborne imaging spectrometer data proved useful for detecting talc-rich soils in a small-scale study (about 6 miles of ground track) in the Ruby Mountains (Crowley and others, 1989). Remote sensing data acquired by NASA's "Airborne Visible and Infrared Imaging Spectrometer" (AVIRIS) system could be used to map exposed talc deposits in the area. The AVIRIS system is a NASA (National Aeronautics and Space Administration) instrument flown in an ER-2 aircraft (a modified U-22 spy plane) at about 65,000 ft altitude, collecting spectral data in 224 channels. Minerals have unique spectral signatures, which are measured in the laboratory from known samples and compared to the remote AVIRIS measurements. This method of mapping specific minerals with AVIRIS data is described by Clark and others (1993). Geochemical methods have not proven successful for talc exploration in southwestern Montana (Berg, 1987b). Regional surveys of fine-grained stream sediment fractions and fine-grained soil fractions for talc (readily determined by X-ray diffraction) may be useful exploration tools in this region (Blount and Parkison, 1991; McHugh, 1985; Nelridge, 1987; Nelridge and Blount, 1991; Piniazkiewicz, 1984). Summary of Areas Permissive for Talc Deposits The northeast part of the Gravelly Range and much of the Ruby Range are favorable for talc deposits of significant size, whether such deposits represent new discoveries or subsurface extensions of known occurrences. In these areas the abundance of talc attests to the past existence of obviously large hydrothermal systems. These areas include world class mines exploiting high grade talc deposits of considerable size and they contain a number of abandoned shallow mine workings or prospects in talc and other known talc occurrences. Talc alteration is also present in the northern Greenhorn Range; this area includes an abandoned talc mine that had moderate of production in the 1970's--the Willow Creek mine (map number 48). Federal land managers should expect continued talc exploration in these areas in the foreseeable future; trenching and exploratory drilling will be used to evaluate targets. The amount of known and estimated talc reserves in the region of the Ruby, Greenhorn, and Gravelly Ranges has been increased significantly in recent years, over past estimates, due to the exploration and development efforts of Barretts Minerals, Inc., and Luzenac America, Inc. For example, Chidester and others (1964) estimated inferred reserves of more than 1 million (short) tons of talc rock for this area. Now a single mine, the Yellowstone mine, has known reserves of 6 million short tons of ore. For comparison, in 1995 the total U.S. mine production of crude talc was 1.17 million short tons (1.06 million metric tons) (Virta, 1997). Although the Henrys Lake Mountains area contains a large amount of rock that is permissive to host talc--dolomitic marble of Archean age--it appears much less prospective for undiscovered deposits. In the course of mapping the geology of this area (Sonderegger and others, 1982), Berg examined the dolomitic marbles for talc. Only four small occurrences of talc were found. These occur in dolomitic marble in contact with a small body of Precambrian metagranodiorite and with Precambrian dikes of diabase and gabbro. The large-scale hydrothermal systems that formed talc deposits in the northern mountain ranges apparently did not operate in the Henrys Lake Mountains area. References Cited Anderson, D.L., 1987, Timing and mechanism of formation of selected talc deposits in the Ruby Range, southwestern Montana: Bozeman, Montana State University, M.S. thesis, 90 p. Anderson, D.L., Mogk, D.W., and Childs, J.F., 1990, Petrogenesis and timing of talc formation in the Ruby Range, southwestern Montana: Economic Geology, v. 85, no. 3, p. 585-600. Berg, R.B., 1977, Origin of talc deposits in the Greenhorn Range, Madison County, Montana [abs.]: Geological Society of America Abstracts with Programs, v. 9, no. 6, p. 708-709. ______1979a, Precambrian geology of the west part of the Greenhorn Range, Madison County, Montana: Montana Bureau of Mines and Geology Geologic Map 6, 2 sheets, scale 1:28,800, 12 p. pamphlet. ______1979b, Talc and chlorite deposits in Montana: Montana Bureau of Mines and Geology Memoir 45, 66 p., 3 plates. ______1987a, Potential for talc deposits within the BLM Wilderness Study Area of the northern part of the Ruby Range, Madison County, Montana: U.S. Geological Survey Open-File Report 87-001, map with text, scale 1:24,000. ______1987b, Talc, in Tysdal, R.G., Lee, G.K., Hassemer, J.H., Hanna, W.F., and Schmauch, S.W., Mineral resources of the Ruby Mountains Wilderness Study Area, Madison County, Montana: U.S. Geological Survey Bulletin 1724-A, 22 p., 1 plate, scale 1:24,000. ______1995, Evidence for the direct precipitation of hydrothermal talc in southwestern Montana [abs.]: Geological Society of America Abstracts with Programs, v. 27, no. 4, p. 2. ______1997, Montana's changing industrial minerals industry, in Jones, R.W., and Harris, R.E., eds., Proceedings of the 32nd Annual Forum on the Geology of Industrial Minerals, Laramie, Wyoming: Wyoming State Geological Survey Public Information Circular 38, p. 167-175. Blount, A.M. and Parkison, G.A., 1991, Hydrothermal alteration haloes and enlarged soil anomalies over concealed talc bodies, southwestern Montana: Ore Geology Reviews, v. 6, nos. 2/3, p. 185-193. Brady, J.B., Cheney, J.T., Duvall, M.L., Green, Chris, Kaufman, Lewis, Kogut, A.I., Larson, A.C., and Vasquez, Angela, 1991, Metasomatic talc deposits in southwestern Montana--Geochemical evidence for deep circulation of water-rich fluids [abs.]: Geological Society of America Abstracts with Programs, v. 23, no. 5, p. A263. Chidester, A.H., Engel, A.E.J., and Wright, L.A., 1964, Talc resources of the United States: U.S. Geological Survey Bulletin 1167, 61 p., 7 plates. Chidester, A.H., and Worthington, H.W., 1962, Talc and soapstone in the United States, exclusive of Alaska and Hawaii: U.S. Geological Survey Mineral Investigations Resource Map MR-31, scale 1:3,168,000, includes 9 p. pamphlet. Clark, R.N., Swayze, G.A., and Gallagher, Andrea, 1993, Mapping minerals with imaging spectroscopy [chapter N], in Scott, R.W., Jr., Detra, P.S., and Berger, B.R., eds., Advances related to United States and international mineral resources--Developing frameworks and exploration technologies: U.S. Geological Survey Bulletin 2039, p. 141-150. Crowley, J.K., Brickey, D.W., and Rowan, L.C., 1989, Airborne imaging spectrometer data of the Ruby Mountains, Montana--Mineral discrimination using relative absorption band-depth images: Remote Sensing of Environment, v. 29, p. 121134. Garihan, J.M., 1973a, Geology and talc deposits of the central Ruby Range, Madison County, Montana: University Park, Pennsylvania State University, Ph.D. dissertation, 282 p. ______1973b, Origin and controlling factors of the talc deposits of steatite grade in the central Ruby Range, southwestern Montana [abs.]: Geological Society of America Abstracts with Programs, v. 5, no. 2, p. 164-165. Geach, R.D., 1972, Mines and mineral deposits (except fuels), Beaverhead County, Montana: Montana Bureau of Mines and Geology Bulletin 85, 194 p., 3 plates. Heinrich, E.W., and Rabbitt, J.C., 1960, Pre-beltian geology of the Cherry Creek and Ruby Mountains areas, southwestern Montana: Montana Bureau of Mines and Geology Memoir 38, 40 p. James, H.L., 1956, Johnny Gulch talc deposit, Madison County, Montana: U.S. Geological Survey Open-File Report [unnumbered, released on Feb. 1, 1956], 11 p., 3 maps. Kaas, L.M., ed., 1996, Indices to U.S. Bureau of Mines mineral resources records: U.S. Bureau of Mines Special Publication 96-2, 1 CD-ROM. Kovaric, D.N, Brady, J.B., Cheney, J.T., Grove, Marty, Jacob, L.J., and King, J.T., 1996, 40Ar/39Ar evidence for reheating events affecting basement rocks in the Tobacco Root, Ruby, and Highland Mountains, SW Montana [abs.]: Geological Society of America Abstracts with Programs, v. 28, no. 7, p. A-493. McCulloch, Robin, 1994, Montana mining directory--1993: Montana Bureau of Mines and Geology Open-File Report 329, 84 p. ______1996, Montana mining directory--1994: Montana Bureau of Mines and Geology Open-File Report 343, 70 p. McHugh, Brian, 1985, X-ray exploration for talc deposits: Cheney, Eastern Washington University, M.S. thesis, 64 p. Nelridge, R.A., 1987, X-ray diffraction data for talc content in carbonate containing soils and their use in prospecting for talc ore deposits: Newark, N.J., Rutgers State University, M.S. thesis, 94 p. Nelridge, R.A., and Blount, A.M., 1991, The use of soil anomalies to locate concealed talc bodies: Ore Geology Reviews, v. 6, nos. 23, p. 195-210. Okuma, A.F., 1971, Structure of the southwestern Ruby Range near Dillon, Montana: University Park, Pennsylvania State University, Ph.D. dissertation, 122 p. Olson, 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. Perry, E.S., 1948, Talc, graphite, vermiculite and asbestos deposits in Montana: Montana Bureau of Mines and Geology Memoir No. 27, 44 p. Piniazkiewicz, R.J., 1984, Geology of and exploration techniques for pre-Beltian talc deposits on the Malesich Range, Ruby Range, Madison County, Montana: Tucson, University of Arizona, M.S. thesis, 101 p. Sonderegger, J.L., Schofield, J.D., Berg, R.B., and Mannick, M.L., 1982, The upper Centennial Valley, Beaverhead and Madison Counties, Montana, with a section on The Madison Valley thermal springs by G.J. Weinheimer: Montana Bureau of Mines and Geology Memoir 50, 53 p., 4 plates. Smith, Kevin, 1980, Petrology and origin of Precambrian metamorphic rocks in the eastern Ruby Mountains, southwestern Montana: Missoula, University of Montana, M.S. thesis, 84 p. U.S. Geological Survey, 1996, The mineral industry of Montana, in Minerals yearbook, Area reports--Domestic 1995, v. II: U.S. Geological Survey, p. 171-172. Virta, R.L., 1992, Talc and pyrophyllite, in Minerals yearbook, 1992, metals and minerals, volume I: U.S. Bureau of Mines, p. 1,3651,374. ______1997, Talc and pyrophyllite, in Minerals yearbook, 1995, metals and minerals, volume I: U.S. Geological Survey, p. 841-846. Whitehead, M.L., 1979, Geology and talc occurrences of the Benson Ranch, Beaverhead County, Montana: Butte, Montana College of Mineral Science and Technology, M.S. thesis, 53 p. Wilson, M.L., 1981, Petrology and origin of Archean lithologies in the southern Tobacco Root and northern Ruby Ranges of southwestern Montana: Missoula, University of Montana, M.S. thesis, 92 p. This map is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards nor with the North American Stratigraphic Code. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.