Western Ruby Range Near Dillon, Montana

A"\ OVERVIEW OF ARCHEAN AND PROTEROZOIC CRYSTALLINB BASBMENT ROCKS IN THE WESTERN RUBY RANGE NEAR DILLON, MONTANA

\irginia B. Sisson -*rerican Museum of Natural History, 4l l8 Lanark Lane, Houston, TX 77025-l I l5

Introduction these, the Great Falls tectonic zone, is interpreted to coincide with the northern boundary To the east of Dillon lies the Ruby Range, of the Wyoming Province (O'Neill andLopez, nmed after the occuffence of garnet (early 1985; O'Neill, 1995; Fig. 1). The western prrospectors originally thought these were ru- boundary of the Wyoming Province is poorly bi6) in alluvial soils. The Ruby Range is un- known as it is disrupted and covered by Lara- derlain by exposures of cratonic basement crys- mide and Tertiary events. Harms et al. (2004) telline rocks. Many geology field camps proposed that the most of the western edge of lUniversity of Pennsylvania/YBRA, Southern the Wyoming province is part of the Protero- fiinois University, Pennsylvania State Univer- zoic Big Sky orogen. To the west of the Great s$', University of Washington, etc.) have used Falls tectonic zone, Foster et al. (2006) defrne rhis area for structural as well as igneous and the Proterozoic Selway terrane as an accreted metamorphic petrology field exercises. This juvenile Paleoproterozoic arc-like terrane. aea is one of several crystalline-cored, block- frdted mountain ranges in southwestern Mon- Ruby Range Geologic Setting - (Fig. l). Another extensive area with crys- Blline basement is the Tobacco Root Mountains The Ruby Range covers about 260 square kilo- nea Virginia City and Ennis (also home to the meters (100 square miles) and can be divided Indiana University field station: Burger et al., into three roughly parallel northeast trending l0O4; Harms et al., 2004). belts (James, 1990). The eastern belt is pre- dominantly Early to Middle Archean (?) Regional Geologic Setting gneisses and schists. The central belt is Middle to Late Archean (?) quartzofeldspathic gneiss. Th€re is some debate these days about whether The central belt makes up the crest of the Ruby 6ese uplifted fault blocks are part of the Ar- Range. These have both sedimentary and chean Wyoming Province (e.g., Houston et al., syntectonic igneous precursors. Both of these 1993; Mueller and Frost, 2006) or part of flank- belts are the basement for the westem metasedi- ing Proterozoic collisional belts (O'Neill, 1995; mentary belt. Roberts et al., 2002; Harms et al., 2004; Harms, lmQ. The Wyoming Province is a unique as- Most of the westem Ruby Range near Dillon, semblage of Late Archean rocks (e.g., Mueller MT, is composed of highly deformed Archean and Frost 2006, and references therein). It has to Proterozoic metasedimentary and metaigne- m unusually thick and strong lithosphere ous rocks. These are part of the Christensen (Henstock et al., 1998; Dueker et al., 2001). Ranch metasedimentary suite, which includes The northwestern Wyoming Province is known dolomitic marbles, calc-silicates, amphibolite, as the Montana Metasedimentary terrane qtartzite, serpentinised meta-ultramafic pods, (MMT). To the south is the tonalite- quartzofeldspathic gneiss, biotite gneiss, pelitic trondjhemite suite of the Beartooth-Bighorn (garnet-sillimanite bearing) schist, metamor- mrgnatic terrane (BBMT; -2900 Ma) and the phosed banded iron formations, and anthophyl- Wyoming greenstone terrane (2500-2800 Ma). lite gneiss (James, 1990). Some units have dis- The Wyoming Province is bounded on three tinctive near-IR spectra; thus, they are easy to sides by Proterozoic collisional orogens. One of distinguish in remote sensing studies (Crowley

\orthwest Geology, v.36r2007 p. 1-8 The Journal ofthe Tobacco Root Geological Society T ! F T .-o a .&k / T / T -.$*:c / a ! a BILLINGS C o a a I Montana I g T N q T

Bighorn . /,t -f t T t Mountains It T Figure l. Generalized geologic rnap of the northem Wyoming Province shorving location of the Ruby Range, Tobacco Root Mountains, Gravelly Range, Beafiooth Plateau, and Bighom Mountains. Also shown a1'e outcrop areas of the Mon- I! tana Metasedimentary terrane (MMT - wavy lines), Beartooth-Bighom rnagmatic terrane (BBN4T - scattered lines) and t adjacent Proterozoic island arc telralle (Selway terrane - unpattemed). The Great Falls tectonic zone is shown as a gray shaded zone. Modified from Mueller et al. ( 1998; 2005). t et al., 1989). Originally they were mapped as students have found abundant garnet in some t equivalents of Cherry Creek Series in the Grav- amphibolite units: these are often semi- t elly Range to the southeast. However, James euhedral and typically several cm across but (1990) redefined them, as there was significant can be up to 30 cm across. TF uncertainty with this correlation. r! Geochronologic Constraints The metamorphosed banded iron formations F and ultramafic pods are minor to rare portions The crystalline basement rocks were defonned I of the lithologic section. The rest of the litholo- and metamorphosed to upper amphibolite fa- t! gies (arnphibolite, gneiss, dolomitic marble, cies conditions probably at one or more times calc-silicates, and pelitic schist) are present in including 3300,2700,2400, or 1800 Ma. James t near equal abundance. One important mineral (1990) proposed two periods of metamorphism: F in this range is gameU locally it is mined in Ml granulite to amphibolite facies at2150 Ma placer deposits in Alder Gulch and Ruby Creek and later M2 amphibolite to greenschist facies t regions. In 1996, the Sweetwater Gamet placer metamorphism at 1800 Ma. The peak metamor- I mine started operation abott22 km from Dillon phism is believed to have occurred at -2700 t! (Van Gosen et al., 1998a). Much of the crystal- Ma based on Rb-Sr dating of gneiss samples line basement rock could be the source for the from the Ruby Range (James and Hedge, t gamet. It is likely to be derived either from the 1980). Giletti (1966) obtained K-Ar and Rb-Sr r! amphibolites or gneisses and schists. Various ages of 1400 to 1700 Ma and concluded that a University of Pennsylvania/YBRA field camp Proterozoic thennal event aff-ected the area. r! T T I*l C Ho*ever, James (1990) postulated that this as metamorphic. This is similar to the 1800 Ma risht indicate the timing of greenschist facies zircon growth reported in the Highland dome to :etrogression. the west. This combined with the monazite results implies a pre-1820 Ma depositional age R-ecent geochronologic results help constrain for the Ruby Range sediments. Depending on 'roth the age of metamorphism as well as depo- whether the monazite in the gamet formed dur- ..ition of the sediment. U-Pb geochronology on ing metamorphism or is a detrital zircon, depo- zircon from one sample of orlhoquartzite may sition either occurted between 2185-1820 Ma indicate metamorphic growth at 1910 Ma or between 21 85-247 5 }da. r\Iue1ler et al., 1998). Dahl et al. (1998) ob- 207Pb-206Pb tained a date of 1820 Ma from Another important result from the detrital zir- monazite inclusions in garnet. Roberls et al. con geochronology is the scarcity of 3300 Ma 207Pb-206Pb r1002) obtained additional data con- zircons: Mueller et al. (1998) postulate the linning peak metamorphism between 1820 and Montana Metasedimentary terane (MMT) was 17,10 Ma. This spread in metamorphic ages not adjacent to the Beartooth Bighorn Moun- or-er -200 m.y. in the Ruby Range may indicate tain terrane (BBMT) at the time of deposition. either one prolonged event or several tectonic Thus, these were originally deposited in small events. In the adjacent Tobacco Root Moun- fault-bounded basins. The source of the detrital tains, there is now evidence for two distinct zircons may have been Archean basement. Paleoproterozoic events (Mueller et a1., 2005). Roberts et al. (2002) propose that these rocks One involves collision with the Medicine Hat were al1 deposited onto Middle Archean base- terrane in the Great Falls tectonic zone at ment during Late Archean to Early Proterozoic .-1860 Ma. There is also evidence for a sepa- time. rate, younger event at -1770 Ma. Metamorphic Assemblages and Pres- The exhumation of the Ruby Range can be con- sure-Temperature Constraints strained by obtaining temperature-time (T-t) ooAr/'eAr data from homblende and biotite geo- The conditions of peak metamorphism can be chronology. Brady et al. (1998) suggest cooling ascertained oC by mineral assemblage information through -525 between 1764 and 1716 Ma. as well as geothermobarometry. Key minerai In contrast, Roberts et al. (2002) report a mean assemblages are the occuffence of garnet- 'oAr/'nAr age of 1771 + 36 Ma for biotite sillimanite-biotite-quartz-plagioclase in the pe- rvhich records cooling between -300-350 "C. garnet-clinopyroxene a0Ar/3eAr lititc schists as well as in Whereas their amphibole samples the banded iron formations. Based on mineral gave meaningless ages of 1668 to 1266 lla. assemblage geothermobarometry, Dahl (1979, Together these T-t data from monazite and bio- 1980) determined that peak metamorphic con- oC oC tite indicate a time-integrated cooling rate of ditions were 675 to 145 and 500 to 800 -350 mm per million years (- I mm per 2.5 MPa. Many workers have also noted the pres- J.ears; Roberts et al., 2002). This exhumation ence of migmatite zones that may be evidence rvas followed by localized late-stage metasoma- for partial melting during metamorphism. How- tisrn at 1360 Ma and formation of talc deposits ever, migmatites are not extensive indicating in some of the Archean marbles (Brady et al., that the regional temperature did not greatly 1 e9B). exceed the [muscovite + quartz + plagioclase + sillimanite + K-feldspar + meltl curve (Kenick, The provenance of the metasedimentary rocks 1972). At 700 MPa, this curve passes through can be determined by detrital zircon geochro- -610'C. This is similar to the temperature and nology. Mueller et al. (1998) analyzed one pressure indicated by recalculated garnet- sample of orthoquartzite from the central Ruby clinopyroxene geothermobarometry (Ravna Range. Their reported ages ranged from 1910 Krogh, 2000). There is some minor retrogres- to 3520 Ma. The 1910 Ma zircon is interpreted sion to greenschist facies conditions.

^l t t t Pelitic assemblages include garnet-sillimanite- The marbles are typically coarse grained and t biotite-quartz-plagioclase schist as well as cor- have the assemblage calcite-diopside-forsterite- dierite and muscovite. This assemblage is key phlogopite-graphite. Some have thin layers of l! to geothermobarometric studies. Many studies pure tremolite. The bright orange lichens that t have reported peak pressure-temperature esti- cover exposed rocks often identiff marbles. t mates including: Karasevich et al. (1981) who There are generally three types of marble: dolo- estimated peak conditions of 675 + 45 "C and mitic marbles, calcitic marbles, and a variety of t 560 + 20 MPa; Mogk and Henry (1988) re- calc-silicate assemblages. Brady et al. (1998) t ported garnet-biotite geothermometry of 700- report calcite-graphite carbon isotopic geother- oC 800 'C with pressure estimates fiom garnet- mometry of 600-700 for some dolomitic t sillimanite-quartz-plagioclase geobarometery marbles near talc deposits. The calc-silicate t of 700 MPa; Duke et al. (1990) analyzed some mineral assemblage can contain scapolite, diop- garnet-sillimanite schists at the Crystal Graph- side, clinozoisite, homblende and rarely garnet. t ite Mine and reported garnet-biotite tempera- A1l three types of marble may host late stage t tures of 750-800 "C at 540 to 770 MPa. Finally, talc (Van Gosen et al., 1998; and Cerino et al., t Gerwin (2006) analyzed one sample with P-T this volume). In recent years, 40% of the U.S. conditions for mineral rims of 475-685 "C at production of talc has originated from the Ruby t pressures of 180 to 580 MPa. The low pres- Range (Van Gosen et al., 1998b). Another late- t sures and temperatures probably indicate garnet stage hydrothermal event resulted in asbestos r! re-equilibration during cooling. None of these (chrysotile) in marble in contact with diabase studies are sufficient to document regional dikes. The final hydrothermal event produced t trends in either temperatme or pressure. Dahl brown silicification (asper) along northwest t (1979) observed cordierite coronas around gar- trending faults. Minor amounts of azurite and net, which is typically indicative of isothermal malachite are sometimes associated with this t decompression during cooling (e.g., Selver- silicification. t stone and Hollister, 1980). Amphibolites occur throughout the area with t The banded iron formations are regionally variable thickness up to 4 km and are traceable ! metamorphosed and typically contain amphi- for many kilometers along strike. They vary bole, quartz, and magnetite in altemating lay- from massive to salt and pepper textured. Most t ers. These range in thickness from 0.3 to 30 m. have a simple mineralogy of hornblende and t None of these metamorphosed banded iron for- plagioclase with small amounts of quartz, bio- t mations have any economic value despite the tite, and garnet. presence of layers with magnetite. Many of t these contain two coexisting amphiboles such Quartzite ranges from a few cm to 30-m-thick t as cummingtonite with actinolite or hornblende beds that are resistant to weathering. Some (Labotka and Papike, l99L). Some samples have minor fuchsite (Cr-rich muscovite). Most t also have apatite and rarely orthopyroxene or have a sugary texture. Other minerals include t clinopyroxene. The amphiboles typically dis- altered biotite, plagioclase, and diopside. I play exsolution features in thin section. The geochemistry of the banded iron formations in Graphite deposits occur in the southwest end of T the nearby Gravelly Range shows a positive the Ruby Range about 15 miles from Dillon. T europium anomaly and low total abundances of There are three graphite claims and mines: (1) rare-earlh elements, which is suggestive of the Faithful claim with seams of graphite paral- T deposition in a volcanically active ocean basin lel to marble; (2) the Lucky Boy claim with T whereas other banded iron formations in the seams in pelitic schists as well as in veins and Dillon region have different light rare-earth along faults; and (3) high-grade graphite depos- I patterns. All the geochemical characteristics are its occur at the Crystal Graphite Co. clairns I similar to other Archean banded iron forma- (Winchell l9l4). The Crystal Graphite mine is q tions (Hammarstrom and van Gossen, 1998) the only mine that produced graphite (Weis, qI I . r:-l r. This deposit was worked intennittently They also found several opposite shear indica- .'.:r 1902 and total production was about 2,200 tors, suggesting top-to-the-north or nonnal :-:.s. \Iost of the material was mined during sense of motion that appears to be later than the ',\ rrld War I; no production is reporled after thrusting. . -:S I \\/eis, I 963). Igneous Activity Drke et al. (1990) postulate that the graphite in -:e Cr1'stal Graphite mine was deposited from The metamorphic sequence is crosscut by sev- :-.uids produced by devolatilization reactions in eral generations of igneous rocks including :re dolornitic marble. They analyzed fluid in- pegmatites and diabase dikes. There are two ;lusions in qrartz-graphite veins and found that generations of pegmatites: the earliest are con- :rost are CO2-CH4 mixtures. Many contain >20 cordant sheets with a simple mineralogy equir-alent mol%o CH+ and up to 97 mol% CHa. (quartz-microcline-plagioclase). The younger \-irtually all inclusion compositions are incom- ones are small bodies that may be zoned and patible with computed graphite equilibrium and contain gamet, tourmaline, and./or large blue- their isochores likewise do not pass through green apatite. Rose quartz can occur in their estimated metamorphic conditions. Thus, most core zones. The early pegmatites are slightly inclusions have been modified subsequent to foliated and thus were probably intruded during original trapping, possibly through HzO loss. the waning stages of tectonism. The younger pegmatites are undeformed. Their age is 1762 -{ srnall number of mafic and ultramafic bodies Ma (magmatic monazite U-Pb; Dahl,2005). intrude Archean rocks in the region (e.g., Des- marais, 1981; James, 1990). Some of these The diabase dikes as well as northwest trending have been prospected for nickel, chromium and faults crosscut the pegmatites. The diabase vermiculite (weathered biotite; see Garverich, dikes are now altered and poorly exposed. They this volume). None of these prospected bodies mnge up to 100 m thick and may be followed are economic except for vermiculite for several kilometers. These dikes are dated at (Hammarstrom et al., 1999). Compositionally, 1100 and 1400 Ma (Wooden et al., 1978). the bodies include serpentinized metaperidotite Similar dikes also occur in the Tobacco Root tharzburgite) and metapyroxenite. Many of Mountains. There, Harlan et al. (2005) distin- these bodies are very small and deformed. They guished two distinct generations of diabase are often aligned with the regional foliation. dikes intruded at 1450 Ma and 780 Ma. The Desmarais (1981) estimated conditions of younger diabase dikes are part of the Gunbarrel metamorphism were 710 "C at 500-700 MPa. mafic magmatic event related to breakup of the Rodinian supercontinent (Harlan et a1., 2003). Structural History It is not known if the younger generation of diabase dikes is present in the Ruby Range. The fabric of the Christensen Ranch metasedimentary sequence is dominated by northeast Intrusion of these dikes was coeval with depo- trending foliation expressed as lithologic layer- sition of the Belt Supergroup sedimentary rocks ing as well as fold axes (both map scale and in the Belt Basin beginning at -1400 Ma minor folds). The foliation has been refolded (Wooden et al., 1978; Schmidt and Graham, by several ductile events. Zimmennan (pers. 1986). There are three sets of lineaments ob- comm.) noted the occurence of deformed servable on satellite images (Ehmann, 1985) quartz-sillimanite pods and postulated that they oriented northwest, north-northwest, and north- may be strain markers. Recently, Olsen et al. east. The norlheast trending lineaments may be (2007) recognized a sequence of ductile shear associated with higher concentrations of metal zones with mylonitic foliations striking north- deposits (Ehmann, I 985). east with top-to-the-south sense of shear con- sistent with regional compression and thrusting. Overlying this sequence are Tertiary (Pliocene) I I I basalts presumably related to the Yellowstone Burger, H.R." 2004, General geology and tectonic volcanic trend. They were emplaced as flows at setting of the Tobacco Root Mountains: in Brady, I about 4 Ma (James, 1990). The silicification of J.B., Burger, H.R., Cheney, J.T., and Harms, T.A., I eds., Precambrian Geology of the Tobacco Root late faults probably occurred at this time. I Mountains, Montana: Geological Society of Amer- v ica Special Paper 377 , p. 1-14. Synopsis I Crowley, J.C., Brickey, D.W., and Rowan, L.C., C In summary, the western Ruby Range records 1989, Airborne imaging spectrometer data of the T Late Archean to Early Proterozoic sediments Ruby Mountains, Montana: Mineral discrimination that were probably deposited in small basins on using relative absorption band depth images: Re- I a passive margin of the Wyoming Province. mote Sensing of the Environment, v.29,p. l2l-134. a\' The source of the clastic sediments may have been the Beartooth Plateau basement. Then, Dahl, P.S, 1979, Comparative geothermobarometry I based on major element and oxygen in Precambrian during the Paleoproterozoic, the Big Sky metamorphic rocks from southwestern Montana: C orogeny may have occurred through collision American Mineralogist, v. 64, p. 1280-1293. of one or more arc terranes (e.g., the Selway I terrane and/or Medicine Hat block) with the Dahl, P.S, 1980, The thermal dependence of Fe2*- T Wyoming Province. In the Ruby Range, this Mg distribution between coexisting garnet and cli- occurred at 1910 to 1740 Ma (Mueller et al., nopyroxene: applications to geothermometry: I 1998; Dahl et al., 1998; Roberts et a1.,2002). American Mineralogist, v. 65, p. 852-866. I This resulted in upper amphibolite facies re- C gional metamorphism; P-T conditions were Dahl, P.S., 2005, Integrating geochronologic and about 650 to 750 oC at 600 to 800 MPa. Then, Earthscope investigations to constrain the role of the I Archean Wyoming craton in Precambrian supercon- isothermal decompression and exhumation oc- tinent cycles: Earth Scope in the Northern Rockies I curred. Rifting and formation of the Belt basin Workshop, http://serc.carleton.edu/files/ at Ma was accompanied by intrusion of T -1400 earthscoperockies/dahlposter.ppt (6 I 2007 ). diabase dikes parallel to northwest trending t faults. Shortly afterward at 1360 Ma, me- Dahl, P.S., Frei, R., and Dorais, M.J., 1998, When t tasomatism of marbles created extensive talc did the Wyoming Province collide with Laurentia? deposits. Other economic resources in the Ruby New clues from step-leach Pb-Pb dating of garnet t Range include garnet, graphite, and vermicu- independent of its inclusions: Geological Society of t lite. America Abstracts, v. 30, no. 7,p. 109. I Desmarais, N.R., 1981, Metamorphosed Precam- Acknowledgements brian ultramafic rocks in the central Ruby Range, C Montana: Precambrian Research, v. 16, p. 67 -101. I would like to thank my various co-teachers at the T Princeton-University of Pennsylvania YBRA field Dueker, K., Yuan, H., and Zurek,8., 2001, Thick- I camps including Laurel Goodell, Kevin Hoo- "Doc" structured Proterozoic lithosphere of the Rocky ver, Jennifer Lindline, and Rob Thomas. In addition, I Mountain Region: Geological Society of America many keen field camp students shared their observa- Today,v.11,p.109. I tions as well as curiosity about these rocks. They all have contributed to my field knowledge area. ofthis Duke, E.F., Galbreath, K.C, and Trusty, K.J., 1990, I Fluid inclusions and carbon isotope studies of C References Cited quartz-graphite veins, Black Hills, South Dakota, C and Ruby Range, Montana: Geochimica et Cosmo- Brady, J.B., Cheney, J.T., Rhodes, A.L., Vasquez, chimica Acta, v. 54, p. 683-698. I A., Green, C., Duvall, M., Kogut, A., Kaufman, L., and Kovaric, D., 1998, Isotope geochemistry of Pro- Ehmann, W., 1985, Lineaments and their associated T terozoic talc occurrences in Archean marbles of the metal deposits, Ruby Mountains, Montana: U.S. I Ruby Mountains, southwest Montana, U.S.A.: Geo- Geological Survey Open File Report, no. 85-599, l6 logical Materials Research, v. 1, n. 2,p. l-41. pp. I I I I t b I' b , s:er. D.A., Mueller, P.A., Mogk, D.W., Wooden, bacco Root Mountains, Montana: Geological Soci- - b . .. :nd Voel, J.J., 2006, Proterozoic evolution of ety of America Special Paper 377, p.227-243. b .:.- i\ estem margin of the Wyoming craton: irnplica- .- :s tirr the tectonic and magrnatic evolution of the Henstock, T, Levander, A., Snelson, C., Keller, R., b .-, ::hem Rocky Mountains; Canadian J. of Earth Miller, K., Harder, S., et al., 1998, Probing the Ar- b >:r:ncu's. r'.43, p. 1601-1619. chean and Proterozoic lithosphere of westem North America: Geological Society of Arnerica Today, v. b G.tetti. B.. 1966, Isotopic ages frorn southwestem 8, p. 1-5 and 16-17. J,Itrntanai Joumal of Geophysical Research, v.7l,p. D r rl9-+036. Houston, R.S., Erselv, E.A., Frost, C.D., Karlstrom, TD K.E., Page, N.J., Zientek, M.L., et al., 1993, The it Gcnrin. D., 2006, A regional study of metamor- Wyoming Province: in Reed, J.C., Jr., Bickford, :lt.sed pelitic rocks in southwestem Montana: 19th M.E., and Houston, R.S., eds., Precambrian Conter- D .\nnual Keck Symposium: http ://keck.wooster.edu,/ minous U.S.: Geological Sociefy of America, Boul- :-hlications, p. 193-l 97. der, CO, Geology of North America, v. C-2,p.121- !t t70. D i{ammarstrom, J.M, and Van Gosen, B.S., 1998, Selected geochemical data for the Dillon BLM Re- James, H.L., 1990, Precambrian geology and bedded D sLrurce Area (including Virginia City mining dis- iron deposits of the southwestern Ruby Range, D irict). \Iadison and Beaverhead Counties, southwest Montana, with a section on the Kelly iron deposit of rt \Iontana: Mineral-resource and mineral- the northeastern Ruby Range: U.S. Geological Sur- enrirournental considerations: U.S. Geological Sur- vey Professional Paper 7495,39 p.,2 plates. D r er Open-File Report 98-244c, 109 pp. James, H.L., and Hedge, C.E., 1980, Age of the D Hammarstrom, J.M, Van Gosen, B.S., Carlson, R.R, basement rocks of southr.r,est Montana: Geological and Kulik, D.M., 1999, Map showing the potential Society of America Bulletin, v. 9 1 , p. 1 1- i 5. D tbr mineral deposits associated with Precambrian D nral-rc and ultramafic rocks in the Blacktail and Hen- Karasevich, L.P., Garihan, J.M., Dahl, P.S., and ry s Lake Mountains and the Greenhorn and Ruby Okuma, A.F., 1981, Sumrnary of Precambriall meta- D Ranges of southwestem Montana: U.S. Geological morphic and struotural history, Ruby Rarrge, south- D Sun'ey Open-File Report 98-0224d, 1 map sheet, west Montana: iir Tucker, T.8., ed., Montana Geo- D Scale 1:250,000. logical Society Field Conference and Symposiurn Guidebook to Southrvest Montana: p.225-247 . a Harlan, S.S., Hearnan, L.W., LeCheminant, A.N., and Premo, W.R., 2003, The Gunbarrel mafic mag- Kerrick, D.M., 1972, Experimental determination of D matic event: A key 780 Ma time marker for Rodinia muscovite+quartz stability with P H2O

Weis, P.L., 1963, Graphite: Montana Bureau of I Mines and Geology Special Publication 28 - avail- C able at http://www.mbmg.mtech.edu/sp28/ I contents.htm#minl t Winchell, A.N., 1911, A theory for the origin of I graphite as exemplified in the graphite deposit near I I I I