METALLIC ORE DEPOSITS OF MONTANA

Christopher H. Gammons,1 Stanley L. Korzeb,2 and Phyllis A. Hargrave2

1Department of Geological Engineering, Montana Technological University, Butte, Montana 2Montana Bureau of Mines and Geology, Butte, Montana

INTRODUCTION ARCHEAN AND PALEOPROTEROZOIC DEPOSITS Montana, the “Treasure State,” has a rich history of placer and hard rock mining for metallic commodi- Stillwater Complex ties. It contains two world-class ore bodies, Butte and Metamorphic rocks of Archean and/or Paleopro- Stillwater, and hundreds of smaller deposits that have terozoic age crop out extensively on the Beartooth been mined from the 1870s to the present day. The Plateau, and form the backbone of several mountain purpose of this chapter is to give an overview of the ranges in southwest Montana, including the Ruby, metallic mineral deposits of Montana excluding the Highland, Tobacco Root, Gravelly, and Madison Butte and Stillwater mines, which are the focus of pa- ranges. By far the most significant metal deposits that pers elsewhere in this volume. Included under the cat- date to this time period are found in the Stillwater egory of “metallic deposits” are concentrations of base Complex, a 2.7 Ga layered mafic/ultramafi c intrusion metals (Cu, Pb, Zn), precious metals (Au, Ag, Pt, Pd), exposed on the north flank of the Beartooth Moun- iron and ferro-alloy elements (e.g., Fe, Mo, Co, Cr), tains. As discussed in another chapter of this volume rare-earth elements, uranium, thorium, and a few other (Boudreau and others, 2020), the Stillwater Complex rare metals of economic importance. In the 1960s, the contains some of the world’s richest deposits of plati- U.S. Geological Survey (USGS) and Montana Bureau num-group elements in the thin but laterally extensive of Mines and Geology (MBMG) compiled a review of J-M Reef (the target of the active Stillwater and East the metallic and industrial mineral mines and resourc- Boulder underground mines), extensive layers of chro- es of Montana (MBMG Special Publication 28, 1963), mite (mined by Anaconda in WWI and WWII), and and organized their compilation by commodity. The scattered bodies of Cu- and Ni-rich massive sulfide current paper takes a diff erent approach. Deposits are at the base of the intrusion (explored but never mined discussed partly by geologic time (beginning in the on a large scale). Because of its unique geology and Archean) and partly by ore-deposit type. This is not economically important mineralization, the Stillwater the order in which the various mines were discovered Complex has been the subject of intense scrutiny by and exploited by prospectors. Indeed, the fi rst mineral both industry and academia since its discovery, and deposits to draw miners to Montana were placer gold continues to be to this day. deposits in Tertiary or Quaternary gravels. The oldest The only other known layered mafic/ultramafi c mineral deposits of Montana, such as Stillwater and complex in Montana is the Lady of the Lake Com- the Cu-Ag mines near Troy, would not see large-scale plex, located in the central Tobacco Root mountains mining until the mid-20th century. (Horn and others, 1991; Sarkar and others, 2009). The There have been thousands of papers, reports, and complex exhibits centimeter-scale layering, is rich student theses on the individual deposits and mining in ortho- and clinopyroxene, and contains up to 20% districts in Montana. This review does not come close modal olivine near its base (Sarkar and others, 2009). to citing all of them, although eff ort has been made The age of the Lady of the Lake Complex, at 74.9 ± to cite most of the principal sources that discuss the 0.2 Ma (Sarkar and others, 2009), is much younger geology and ore-forming characteristics of the dif- than Stillwater, and overlaps with that of the Tobacco ferent deposit types. In citing the literature, emphasis Root Batholith. The complex is reported to contain has been placed on more recent work published after locally elevated gold, platinum, and palladium values MBMG Special Publication 28. (John Childs, written commun., 2018) but has never been drilled. 1 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics Other Chromite Occurrences period when the world’s oceans were thought to have In addition to Stillwater, a few outlying chromite been anoxic and rich in dissolved Fe2+ at the same time deposits of presumed Archean age exist in Montana. that the Earth’s atmosphere was slowly becoming oxy- The more signifi cant occurrences are near Red Lodge genated through the evolution of photosynthetic mi- in Carbon County and near the towns of Sheridan, croorganisms: BIF formations formed on continental Pony, and Silver Star in Madison County (Jackson, shelves where the Fe-rich ocean water was oxidized to 1963). At both locations, chromite forms irregular precipitate hematite (e.g., Robb, 2005). pods and lenses in sill-like and/or faulted bodies of Jardine District serpentinite. These deposits are examples of “Al- Although the Archean and paleo-Proterozoic rocks pine-type” chromite deposits, and diff er from the of Montana host many lode gold and polymetallic vein stratiform chromite layers of the Stillwater Complex. deposits, in many cases it is not known whether miner- Although they are dwarfed in tonnage by the deposits alization occurred in the Precambrian or, more likely, at Stillwater, the smaller bodies reportedly contain during Cretaceous–Tertiary hydrothermal activity. An chromite of higher purity and grade. A mine near Sil- exception is the Au-W vein deposits of the Jardine ver Star produced several thousand tons of Cr-concen- district (Reed, 1950; Smith, 1996). These deposits trate in WWII (Jackson, 1963). However, in today’s are hosted in Archean metamorphic rocks and can be global economy, the Montana deposits cannot compete grouped into two categories (Seager, 1944): (1) quartz with large reserves of high-purity chromite from over- veins with high scheelite and relatively low gold con- seas mining operations. tent cutting quartz–biotite schist and quartzite; and (2) Banded Iron Formations replacement bodies rich in arsenopyrite (±pyrrhotite, Large deposits of Precambrian banded iron for- pyrite) and gold cutting quartz–grunerite schist and mation (BIF) exist in southwest Montana (Bayley and meta-turbidites. The origin of the gold and associated James, 1973). For example, the Carter Creek deposit, tungsten mineralization at Jardine is disputed, with in the western Ruby Range, contains an estimated most theories falling into either a syngenetic-exhala- 95 million tons of ore grading 28–29% Fe, mainly tive or a metamorphic-hydrothermal model. Evidence as magnetite (James, 1990). The smaller Kelly iron for the former includes: (1) textures; (2) mineraliza- deposit, in the northeast Ruby Range, contains an tion being preferably developed in silicate-facies and estimated 15 million tons at 33% Fe (James, 1990). sulfi de-facies banded iron formation; and (3) trace el- Other BIF deposits have been mapped in the Gravelly, ement profi les consistent with a sea-fl oor volcanic-hy- southern Highland, and southern Tobacco Root Moun- drothermal system (Foster and Childs, 1993). Howev- tains (James, 1981). According to Immega and Klein er, later structural, mineralogical, and fl uid inclusion (1976), the metamorphic grade of BIFs in the Tobac- studies (e.g., Smith, 1996) provided evidence for gold co Roots is higher than in the southern Ruby Range. remobilization during prograde metamorphism and James (1990) described BIFs in the Ruby Mountains deformation, in a manner similar to so-called “orogen- as belonging to the Christensen Ranch Metasedimen- ic” gold deposits. Orogenic gold deposits form over a tary Suite (CRMS), a sequence of shallow platform continuum of pressure-depth-temperature conditions, sediments now metamorphosed to quartzite, marble, but most are thought to develop from prograde meta- schist, gneiss, and BIF, with intervening, mostly con- morphic fl uids at depths near the brittle–ductile transi- cordant, layers of amphibolite. Although James (1990) tion in the Earth’s crust (Groves and others, 1998). assigned a late Archean age to the CRMS, more re- Most historic production in the Jardine dis- cent interpretations argue these sediments were laid trict has come from the Jardine mine, fi rst operat- down on a ~2.0–2.1 Ga passive margin of the North ed in the 1940s, and more recently reopened in the American craton and later metamorphosed during 1960s–1980s as the Mineral Hill mine. Smith (1997) the 1.7–1.8 Ga Big Sky Orogeny (Sims and others, indicated a total, pre-mining reserve of 510,000 oz 2004; Condit and others, 2015). With this younger age Au for Jardine/Mineral Hill. Additional deposits with assignment, the deposits of the Ruby Range would be- similar geology are known to occur in the area, and long to the clan of “Superior-type” BIFs that are found sporadic exploration has continued to the present day. worldwide in shallow marine sediments dating to the These eff orts have been legally and environmentally early Proterozoic (i.e., 2.5–2.0 Ga). This was a time challenged given the district’s close proximity to Yel- 2 Gammons and others: Metallic Ore Deposits of Montana lowstone National Park and the Beartooth Wilderness. Cominco conducted an aggressive regional explora- There are interesting similarities between the tion program in the Montana portion of the Belt Basin Jardine deposit and the world-class gold deposits of in the 1970s. At least two signifi cant sulfi de deposits the Homestake mine, South Dakota. At Homestake, were defi ned, the bigger of the two being Black Butte gold occurs along with abundant arsenopyrite and (originally called Sheep Creek), located near White pyrrhotite in the hinges of tight folds, and is strati- Sulphur Springs. Descriptions of the geology and min- graphically restricted to the Homestake Formation, a eralization at Black Butte include Himes and Petersen cummingtonite–grunerite schist of paleo-Proterozoic (1990), Zieg and others (1993), Graham and others age (Slaughter, 1968). The Homestake Formation, like (2012), and White and others (2014). Unlike most the grunerite schist at Jardine, is thought to represent SEDEX deposits that are mined primarily for zinc and a metamorphosed iron formation (Slaughter, 1968; lead, the Black Butte deposit is rich in copper with im- Smith, 1996, 1997). Rostad (1997) pointed out that the portant cobalt, nickel, and silver, and relatively minor best gold ore in the original Jardine mine was located amounts of Pb-Zn. The mineralization is dominated by in the hinges of tight folds in the grunerite schist, a chalcopyrite with subordinate tennantite and is con- situation that is directly analogous to Homestake. tained within several stratiform, massive to semi-mas- sive pyrite lenses in the Newland Formation, a marine MESOPROTEROZOIC DEPOSITS OF black shale with dolomite and debris fl ow interbeds. THE BELT BASIN Barite is locally abundant, especially in the so-called upper sulfi de zone of the Strawberry Main deposit, The Belt–Purcell Basin of Montana, northern and the enclosing shales are silicifi ed where copper Idaho, and southeastern British Columbia is host to a grades are highest (Graham and others, 2012). The number of important metal deposits (fi g. 1). A useful copper-rich zones locally contain disseminated Co-Ni summary of the metallogeny of the Belt–Purcell Basin sulfi des and sulfarsenides (White and others, 2014). can be found in Lydon (2007). By far the biggest past producer is the Sullivan mine, in southern British Columbia, one of the largest lead–zinc–silver deposits in the world. Sullivan is an example of a “sediment-hosted exhalative” or “SE- DEX” hydrothermal deposit. SEDEX deposits form in active rift basins near continental margins and accumulate where metalliferous brines vent on the sea fl oor or replace footwall sediments. Whereas Sullivan is in southern British Columbia, at least two SEDEX-style deposits are known to exist in the Mon- tana part of the Belt Basin, and these are described below. Second in importance to Sullivan are the sediment-hosted “red bed” copper–silver deposits contained in the Revett Formation of northwestern Montana, including the Troy (Spar Lake), Montanore, and Rock Creek deposits, also discussed below. SEDEX Deposits Operated by Cominco for over 100 yr, the rich ores of Sullivan were eventu- ally depleted and mining ceased in 2001. Figure 1. Generalized map of the mid-Proterozoic Belt–Purcell Basin showing the location of mines and prospects discussed in the text (modifi ed from Lyons and Searching for new Sullivan-style deposits, others, 2000). 3 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics The upper zone of the Johnny Lee deposit at Black one of the largest silver deposits in the U.S. (surpassed Butte has a measured resource of 2.66 Mt grading only by Couer d’Alene, Idaho, and Butte, Montana) 2.99% Cu, 0.12% Co, 16.3 g/t Ag, and an indicated and the 20th largest stratabound copper deposit in the resource of 6.52 Mt at 2.77% Cu, 0.13% Co, and 15.5 world (Boleneus and others, 2005). Although initially g/t Ag (White and others, 2014). Additional mineral- owned by two separate mining companies, the Rock ized zones exist on the property and are currently be- Creek and Montanore deposits are part of the same ing evaluated. Hematitic gossan formed by weathering contiguous, shallow-dipping ore body that can be of a massive pyrite lens in the Newland Formation has traced from one side of the Cabinet Mountains to the been mined at a small scale for iron ore at a location other. roughly 4 km west of Black Butte (Reed, 1949). As a group, most sediment-hosted copper deposits A second SEDEX-style sulfi de deposit explored form near a redox interface between oxidized (red) by Cominco is located up Soap Gulch in the western and reduced (grayish, green, or black) sedimentary Highland Mountains. The deposit, also marked by layers (e.g., Hitzman and others, 2010). In the case of an extensive gossan at the surface, is hosted by dark the Revett deposits, overprinting low-grade metamor- argillite and dolomite of the LaHood Formation (Mc- phism has muted the color diff erences that may have Donald and others, 2012), previously mapped as the existed in the Precambrian, obscuring redox relation- Newland Formation by O’Neill and others (1996). Al- ships (Hayes and Einaudi, 1986). Currently, the oxi- though no papers could be found on the geology of the dized horizons in the Revett have a lavender-gray hue Soap Gulch deposit, the prospect is said by local geol- and contain disseminated specular hematite while the ogists to be primarily enriched in zinc. SEDEX-style reduced horizons are gray-white or gray-green with massive sulfi de mineralization in lower Belt sediments sparse pyrite, calcite, and chlorite (Hayes and Einau- has also been reported from Wickiup Creek (Thorson, di, 1986; Boleneus and others, 2005). Ore minerals, 1984), a few kilometers to the east from Soap Gulch, mainly chalcocite and bornite, are found exclusively and at the “Calico Prospect” in Cardwell, a few kilo- in the reduced horizons, and are concentrated along meters east of the Golden Sunlight mine (Bruce Cox, bedding planes and fractures in coarse-grained sand- oral commun., 2017). A prominent gossan on the sum- stone. Disseminated pyrite is paragenetically early mit of Mount Evans, in the Anaconda Range, may be and is mostly replaced by Cu-sulfi des in the ore zones. the metamorphosed remnant of a SEDEX-style sulfi de Even in the ore-bearing strata, sulfi de minerals rarely deposit in lower Belt metasediments (Eastman and exceed a few percent of the rock. This is a contrast to others, 2017a). the SEDEX-style, Cu-rich ore bodies at Black Butte (discussed above), which are contained in zones of “Red Bed” Copper–Silver Deposits nearly massive pyrite. Sediment-hosted copper mineralization (“red-bed Lange and Sherry (1983) and Hayes and Einaudi copper”) can be found in many of the sedimentary (1986) presented similar models in which oxidized, formations of the Belt Basin (Harrison, 1972). Earhart metalliferous brines sourced from deeper in the Belt and others (1981) reported as many as 16 sequences Basin moved upwards along steeply dipping faults and of copper enrichment in the Spokane Formation, each then migrated laterally through the permeable quartz hosted by green interbeds within what is predominant- sandstones of the Revett, depositing Cu-sulfi des and ly a red siltite/argillite. However, all of the economi- other ore minerals at the transition from oxidized to cally important sediment-hosted deposits of this type reducing conditions. More recently, Hayes and others in Montana are found in white or gray quartzites of (2012) have suggested that the reductant that triggered the Revett Formation of the Ravalli Group (Harrison, copper sulfi de deposition was a reservoir of sour (H S- 1972). These include the recently closed Spar Lake 2 rich) natural gas that had migrated into the Revett mine near the town of Troy, and the Rock Creek/Mon- sandstone during early diagenesis of the Belt Basin. tanore deposit, currently in the permitting stage. Total The only remaining evidence of this ancient natural metal endowment (production + reserves) at Spar gas fi eld is the presence of methane and other hydro- Lake is estimated at 550 million lbs Cu and 65 million carbons trapped in fl uid inclusions in gangue minerals oz Ag (Boleneus and others, 2005). The same source within the deposits. cites total reserves of 5 billion lbs Cu and 679 million oz Ag at the Rock Creek/Montanore deposit, making it 4 Gammons and others: Metallic Ore Deposits of Montana Stratabound Gold Deposits in the mide orogenies brought deformation, metamorphism, Greyson Formation and plutonic activity to much of western and central In the vicinity of the historic York placer district, Montana, along with a rich and diverse inventory of on the west fl ank of the Big Belt Mountains, the mid- hydrothermal mineral deposits. dle member of the Greyson Formation of the lower The following sections are organized more by ore Belt sequence contains a number of gold deposits deposit type than by age, so that deposits that share associated with stratabound, erosion-resistant “reefs.” geologic characteristics can be discussed together. The reefs contain abundant epigenetic K-feldspar with Porphyry Cu-Mo Deposits sparse pyrite and Fe-carbonates along bedding planes and in cross-cutting quartz–carbonate veins (Foster Porphyry Cu-Mo deposits are some of the largest and Childs, 1993; Tysdal and others, 1996; Thorson metallic mineral deposits in the modern mining indus- and others, 1999; Whipple and Morrison, 1999; Chris- try. Some porphyry deposits are also rich in precious tiansen and others, 2002). Although most reefs are metals, including gold (e.g., Bingham Canyon, Utah) parallel to bedding, they locally cross-cut the stratig- and silver (Butte). Although many subtypes of por- raphy at a low angle. East of Trout Creek, as many as phyry deposits have been recognized in the literature, 10 or more individual reefs up to 30 m thick have been the majority can be classified as either: (1) Cu-domi- recognized, the largest having strike lengths of >1 km. nant with byproduct Mo ± Au, (2) Mo-dominant (also Although the overall gold concentrations are low, the known as “Climax-type”) with no significant Cu; and large and laterally persistent nature of the reefs led (3) Sn-(W) dominant. Mineralized intrusions that form Christiansen and others (2002) to report a resource of porphyry-style deposits may release metal-rich fluids 12.5 M oz of gold at grades >0.3 g/ton. The origin of that migrate into the country rock to form other types the stratabound gold mineralization is debated. Early of hydrothermal ores, such as skarns, polymetallic workers (Thorson and others, 1999) felt that the reefs veins and lodes, carbonate replacement deposits, and formed in the Precambrian by either a syngenetic-ex- epithermal deposits. halative or a diagenetic-replacement model. However, The Butte porphyry/lode deposit, mined from the Christiansen and others (2002) obtained a K/Ar date of 1870s to the present day, is one of the top 10 copper 89 ± 3.2 Ma for hydrothermal K-feldspar, and reported mining districts in the world, and has produced enor- that unpublished U/Pb and Rb/Sr data also support a mous quantities of other metals, including silver, zinc, Cretaceous age for the reefs. lead, molybdenum, gold, and manganese. The history of mining of the Butte ore bodies is interwoven with PHANEROZOIC METAL DEPOSITS the history of the State of Montana, and many books From a metallic ore deposit point of view, the have been written on this subject. Butte is unusual late Proterozoic and Paleozoic Eras, as well most of compared to most porphyry deposits in that a majority the Mesozoic Era, were a bust in Montana. During of the valuable metals (with the exception of molybde- this time, most of what is now the northern Rockies num) were mined from lodes that crosscut the earlier was a vast, nearly fl at continental shelf, receiving porphyry-style mineralization. Butte also has a more mostly marine sediments during extended episodes diverse mineral and metal assemblage than the aver- of sea-level rise (e.g., the Cambrian), and erosion of age porphyry deposit, which is one reason it has been those sediments during times of sea-level decline (e.g., the subject of intense study by economic geologists for during the Ordovician–Silurian). The only rocks of over a century. A review of the geology and geochem- economic importance that formed between 1,000 and ical evolution of the Butte deposit is given in a sepa- 100 million years ago were vast deposits of limestone, rate chapter of this volume (Reed and Dilles, 2020). building stone, hydrocarbon source rocks, coal, and in- More recently, Butte has become synonymous with the dustrial minerals. The latter include the huge reserves word “Superfund.” The legacy of 150 years of mining, of phosphorite in the Permian Phosphoria Formation milling, and smelting the great ores of Butte have left of east Idaho, west Wyoming, and southwest Mon- an enormous environmental burden. A review of some tana. This extended period of tectonic inactivity ended of these issues, focusing on the Berkeley Pit lake, is when the eff ect of the distant collision of the Farallon given by Gammons and Duaime (2020). Plate with western North America reached Montana in mid-Cretaceous time. The ensuing Sevier and Lara- 5 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics Exploration drilling by various companies has others, 2002) and is roughly 10 million years younger documented that a number of porphyry Cu-Mo and than the main host rock of the district, the Butte Gran- porphyry Mo deposits exist in Montana outside of the ite. It is not known if other porphyry Cu-Mo deposits Butte district. The largest such deposit for which in the batholith are of similar age. Porphyry Cu depos- grade-tonnage data are available is the Heddleston its outside of the batholith to the north and west (Hed- deposit (Miller and others, 1973), located near the dleston, Copper Cliff ) and to the southeast (Emigrant headwaters of the Blackfoot River. According to Gulch, New World) are Eocene. At least two of the Singer and others (2005), Heddleston contains a total Eocene deposits (Copper Cliff and New World) carry resource of 302 million tons at 0.36% Cu, 0.005% Mo, signifi cant gold along with copper. 0.053 g/ton Au, and 5.2 g/ton Ag. McClave (1998) Porphyry Mo Deposits reported a mineable reserve of 93 million tons aver- aging 0.48% Cu. The geology of Heddleston consists A number of “Climax-type” porphyry-Mo depos- of Eocene quartz monzonite stocks and dikes cutting its exist in Montana, although none have been mined. argillite and siltite of the Belt-aged Spokane Forma- Based on a review by Worthington (2007), the largest tion and a late Proterozoic diorite sill. Porphyry-style known deposits are Cannivan Gulch in the Pioneer mineralization is found in the quartz monzonite, in the Mountains (300 Mt at 0.06% Mo with some higher Spokane Formation, and in the sill. A date (K/Ar) for grade zones) and Big Ben in the Little Belts (376 Mt hydrothermal sericite of 44.5 Ma (McClave, 1998) at 0.098% Mo). Armstrong and others (1978) obtained makes Heddleston roughly 20 m.y. younger than the a K/Ar date of 59 ± 2 Ma for hydrothermal muscovite Butte deposit. A number of late, polymetallic veins at Cannivan Gulch, indicating an early Tertiary (Paleo- surround the central quartz monzonite stock, including cene) age. Rostad and others (1978) reported ages of the historic Mike Horse mine, notorious for its 1975 49.5 and 48 Ma for the Big Ben and Bald Butte por- tailings dam failure that polluted the upper Blackfoot phyry-Mo deposits, respectively. These deposits are River (Spence, 1997). Unlike the Main Stage veins of situated at the NE end of the “White Cloud–Cannivan the Butte district (Reed and Dilles, 2020), the late porphyry Mo belt” of Armstrong and others (1978), veins at Heddleston were relatively impoverished in also known as the “Idaho–Montana porphyry belt” copper. A recent study (Schubert and Gammons, 2018) or “transverse porphyry belt.” This belt of deposits is has shown that the S-isotopic compositions of pyrite in roughly parallel to the northeast–southwest-trending porphyry-style and lode-style mineralization in the Colorado mineral belt, home to the world-class Cli- Heddleston district are similar to each other, and max and Henderson porphyry-Mo deposits. Rostad overlap with the δ34S of stratabound Cu-Ag deposits in and others (1978) pointed out that the Idaho–Montana the surrounding Belt sediments (Byer, 1987). The iso- belt exhibits a close spatial and genetic association be- topic values also overlap with the δ34S of pyrite from tween porphyry-style Cu-Mo deposits, Au-Ag-Cu-W the Butte deposit (Lange and Cheney, 1971; Field and skarn deposits, and epithermal-style Au-Ag miner- others, 2005). Thus, although separated by at least 20 alization. Spry and others (1996) describe the main m.y., the two porphyry systems may have inherited breccia pipe at the Golden Sunlight gold mine as being their sulfur, and possibly much of their metal endow- underlain by a Mo-rich porphyry deposit, although de- ment, from a similar source region. tails have not been published. It is interesting to note that all of the known porphyry deposits in the Pioneer Other porphyry Cu (± Au, Mo) deposits of Mon- and Flint Creek mountains are Mo-rich, whereas most tana are shown in fi gure 2. Most of these have exten- of the deposits in the Boulder Batholith are Cu-rich sive areas of porphyry-style alteration but no publicly with minor Mo. documented grade/tonnage information. Some of the deposits (e.g., Emigrant Gulch, Golconda, Lowland Skarn Deposits Creek, Radersberg, Turnley Ridge) need more explo- Most skarn deposits form at the contact of a car- ration drilling to assess their economic importance. All bonate rock with a mineralized, water-rich igneous of the porphyry Cu-Mo deposits of the Boulder Batho- intrusion. Skarns that develop in limestone or in dolo- lith are inferred to be late Cretaceous or early Tertiary mite are referred to as “calcic skarns” or “magnesian (Paleocene), although Butte is the only such deposit skarns,” respectively (Meinert and others, 2005). Most with reliable dates. Porphyry-style, pre-Main Stage of the magnesian skarns in Montana are hosted by the mineralization at Butte is dated at 64.5 Ma (Lund and 6 Cambrian Pilgrim/Hasmark or the Devonian Jeff erson Gammons and others: Metallic Ore Deposits of Montana

Porphyry deposits Cu Adel Mountain Highwood (Mo,Au) Mo Volcanics Mountains satellite plutons plutons Eocene late Cret./ Heddleston Paleocene

SkarnLolo Hotdeposits Springs Copper Cliff Big Ben Batholith Skookum Butte Bald Butte Au (Cu) Batholith W Little Belt Mountains plutons FB Royal Stock SH TL SG DH Philipsburg EG W E Castle Pluton Stock Cb Mt Powell R Sapphire Batholith Batholith LC 46° G Boulder Butte Idaho Batholith Batholith C B Golden Sunlight BH Sliderock CG Mountain Tobacco Root plutons SS Batholith Pioneer Absaroka Batholith plutons BL,L,LC New World 45° Emigrant 114° 110°

0 40 80 km

02550 mi 112°

Figure 2. Location of porphyry and skarn deposits of western Montana. Abbreviations and references: B, Beal (Wilkie, 1996); Bald Butte (Tysdal and others, 1996); Big Ben (Olmore, 1991); BH, Butte Highlands (Ettlinger and others, 1996); BL, Brownes Lake (Pattee, 1960); C, Calvert (Messenger, 2016); Cb, Cable (Cox and Bogert, 2017); CG, Cannivan Gulch (Darling, 1994); Copper Cliff (Fairbanks, 2009); DH, Diamond Hill (Pearson and others, 1992); E, Elkhorn/Golden Dream (Steefel, 1982); EG, East Goat (Ericksen and others, 1981); Emigrant (Worthington, 2007); G, Golconda (Greenwood and others, 1990); Golden Sunlight (Spry and others 1996); H, Heddleston (McClave 1998); LC, Lowland Creek (Fess Foster, oral commun.); LC, Lost Creek (Collins, 1977); New World (Johnson and Meinert, 1994); R, Radersberg (Tysdal and others, 1996); SG, Stewart Gulch (Antonioli, 2009); TL, Tolean Lake (Ericksen and others, 1981); W, Wickes/Beavertown (Elliott and others, 1986; Tysdal and others, 1996).

Formations, whereas the calcic skarns are mainly in information. All skarns developed in the Boulder the Cambrian Meagher Formation or the Mississippian Batholith are rich in gold, whereas most skarns in the Madison Group. Both types have similar skarn min- Pioneer and Flint Creek mountain ranges are rich in erals, usually including garnet (grossular, andradite), tungsten. Some of the bigger gold skarns in Montana clinopyroxene (diopside, hedenbergite, augite), epi- include the Beal Mountain, Butte Highlands, Silver dote, quartz, and magnetite/hematite. Mg-skarns may Star (Madison Gold), Elkhorn (Golden Dream), and also contain forsterite and serpentine (alteration prod- Diamond Hill deposits, all of which occur around the uct of forsterite). Retrograde alteration forms lower fringe of the Boulder Batholith, and the New World temperature minerals such as calcite, tremolite/actin- skarn near Cooke City. The New World deposit is olite, and micas (biotite, phlogopite, chlorite, musco- unlikely to be mined in the foreseeable future given vite). Selected photographs of skarns from Montana a 1996 Federal decision to terminate exploration and are shown in fi gure 3. development in the district because of its proximity to In terms of metal endowment, the skarns of Mon- Yellowstone National Park. The skarn at New World is tana fall into two categories: (1) Au (± Ag, Cu)-skarns closely associated with porphyry-style Cu-Au and car- and (2) W-skarns. Locations of individual deposits bonate replacement Pb-Zn-Ag mineralization contem- mentioned in the text are shown in fi gure 2, and table porary with intrusion of Eocene stocks into Paleozoic 1 summarizes some geological and grade/tonnage metasediments (Johnson and Meinert, 1994). A similar 7 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics

AB

CD

E

Figure 3. Selected photographs of Montana skarn de- posits. (A) Contact between hedenbergite skarn (black) and marble, Black Pit, Silver Star; (B) Massive pyrite–pyr- rhotite with minor chalcopyrite, Madison Gold deposit, Silver Star; (C) Jasperoid breccia cut by irregular calcite veins (Madison Gold, Silver Star); (D) diopside–andradite skarn cut by late calcite with bismuth minerals and gold (not visible in photo) (Elkhorn district); (E) grossular–epi- dote–actinolite skarn with coarse scheelite (Calvert mine). Photo credits: A,B,C, Sotendahl (2012); D, C, Gammons, unpublished; E, Messenger (2016).

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9 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics association exists at Elkhorn, where several gold-rich senger (2016) and Messenger and Gammons (2017) skarns and breccia pipes are found near the contact completed a mineralogy, fl uid inclusion, and stable with the Late Cretaceous Turnley Ridge porphyry isotope study of the Calvert scheelite skarn in the intrusion, a host to low-grade Cu-Mo mineralization northern Pioneer Mountains, one of the smaller but (Steefel and Atkinson, 1984). higher-grade W-skarns in Montana. Most gold skarns in Montana share a similar ore Polymetallic Vein and Carbonate mineralogy, being rich in pyrrhotite and magnetite, Replacement Deposits with lesser amounts of chalcopyrite, arsenopyrite, and Polymetallic (Ag-Pb-Zn±Au±Cu) vein deposits pyrite. Bi-sulfi des or Bi-Ag-tellurides are accessory constitute the majority of historic hard-rock mines in minerals. Gold may occur as electrum or as nearly Montana. Most polymetallic veins have an abundance pure gold, the latter being more common in retrograde of pyrite, sphalerite, and galena, sometimes with skarn or sulfi de-bearing skarns that have later been chalcopyrite, arsenopyrite, tetrahedrite, and Pb-Sb-As- oxidized (Gammons and others, 2010). Because of the Ag sulfosalt minerals, in a quartz or quartz–carbonate relative immobility of gold in the weathering environ- gangue. Black tourmaline (schorl) is abundant in some ment compared to silver and base metals, hematitic veins, especially those that cut the Boulder Batholith gossans or jasperoids developed in the oxidized zone in the Wickes and Basin districts (Cox, 2015). Gold of many skarns carry good Au grades, with the ad- may or may not be present in economic quantities. If ditional bonus of being easier to mill and extract the so, it is often residing as electrum (Au-Ag alloy) or gold. Typical grades of Au in the Montana deposits are tiny inclusions in pyrite or arsenopyrite. The veins (or 0.1–0.3 opt, and Au mineralization is spotty, leading lodes, the term for a vein exceeding 3 ft in width) are to challenges with grade control and mine design. The often continuous along strike and dip for hundreds largest gold producer to date, Beal Mountain (German to thousands of feet, and may have any orientation, Gulch), is hosted by Cretaceous clastic sediments of although steep dips are more common. Veins may the Blackleaf Formation. Nonetheless, it is considered terminate or bifurcate, and many mines in Montana a skarn based on its gangue mineralogy (pyroxenes, followed multiple veins. Some veins are strongly garnets, vesuvianite) and proximity to the German sheared. Host rocks can be igneous, sedimentary, or Gulch stock. Another signifi cant gold producer, the metamorphic, although veins cutting carbonate rocks Diamond Hill mine, is developed in andesite of the are less common, owing to the tendency of hydrother- Elkhorn Mountains Volcanics. Like Beal Mountain, mal fl uids to replace limestone rather than to break it. the Diamond Hill deposit has typical skarn mineralogy despite being hosted by a non-carbonate lithology. Carbonate replacement deposits are so-named because the ore-forming fl uids dissolved the carbon- Tungsten skarns of Montana contain scheelite ate matrix and deposited sulfi de minerals in its place. (CaWO ) as the only metallic mineral of economic 4 Such deposits diff er from skarns (preceding section) importance, and most have low concentrations of sul- in that they lack evidence of high-temperature metaso- fi de minerals such as pyrrhotite, pyrite, chalcopyrite, matism of the wall rock (e.g., presence of prograde and molybdenite (Walker, 1960, 1963). Typical grades skarn minerals, such as garnets, pyroxenes). The for the Montana deposits range from about 0.1 to 1.0 carbonate replacement deposits in Montana are similar wt% WO . Two of the larger deposits are Lentung in 3 in mineralogy and tenor to the fi ssure veins, but have the east Pioneer Mountains, developed in the Amsden a more complex geometry with ore distributed along Formation, and Finlay Basin in the Flint Creek Moun- bedding planes, in the hinges of folds, along faults, tains, which is hosted in the Madison Limestone. The and sometimes as sinuous, irregular deposits (i.e., Lentung deposit is an underground resource between chimneys or mantos; see fi g. 4). The important mines similar skarn mineralization at Brownes Lake, to the of the Hecla and Elkhorn districts as well as many of north, and Lost Creek, to the south. The published the mines in the Philipsburg district fall into this cate- resource is 3.3 Mt at 0.48% WO (Deboer, 1991), 3 gory. although the deposit has not been completely drilled out and is open along strike. There is also a possible The distinction made in this paper between poly- market for byproduct garnet from the scheelite skarns metallic vein/replacement deposits and epithermal (E.E. Nelson, oral commun., February 2018). Mes- deposits (next section) is based on the inferred con- ditions of depth and sources of ore fl uids. Whereas 10 Gammons and others: Metallic Ore Deposits of Montana

Figure 4. Simplifi ed cross sections through three polymetallic, carbonate-replacement ore bodies in Montana. (A) Philipsburg (Hope Hill area, from Emmons and Calkins, 1913); (B) New World (from Johnson and Thompson, 2006); (C) Hecla (from Karlstrom, 1948). Ore bodies are shown in red. epithermal deposits form in the top 1–2 km of the and Hope-Bullion mines in the Basin mining district. Earth’s crust with major involvement of heated mete- Although grades of the polymetallic lode deposits oric waters, the polymetallic deposits discussed in this of Montana were often high, tonnages were typically section are thought to have formed at greater depths low, with most historic mines producing fewer than from primarily magmatic fl uids. However, very few of 1,000 tons. A large mine might have been >10,000 the polymetallic lode deposits of Montana have been tons, and a very large mine (by pre-1960s standards) examined by modern methods of ore deposit study. might have processed a million tons over its lifetime, Zimmerman (2016) and Korzeb and others (2018) which may have stretched from the late 19th to the conducted detailed fl uid inclusion and stable isotope mid-20th century. Table 2 lists a few of the larger studies of the Emery (Zosell) district, east of Deer polymetallic vein and carbonate replacement deposits Lodge, and concluded that the ore fl uids were domi- of Montana. This list excludes Butte, which would be nantly magmatic and that the veins formed at depths off the scale of comparison. A major stumbling block exceeding 4 km. The Emery veins cut basaltic andesite in compiling this type of list is the lack of accurate of the Elkhorn Mountains Volcanics (EMV), and have production records prior to 1900. For example, the a classic polymetallic mineral suite of pyrite, arsen- Alta mine, one of the larger Pb-Ag deposits in Mon- opyrite, galena, sphalerite with minor chalcopyrite, tana (excluding Butte), is said to have produced more Ag-rich tetrahedrite, electrum, and boulangerite (a wealth in the 25 years prior to 1902, most of it as Pb-Sb sulfosalt) in a quartz–carbonate gangue. Korzeb silver, than in all subsequent years, but detailed pro- 40 39 and others (2018) obtained a Ar/ Ar date of 77.93 duction fi gures are nonexistent. Another consideration ± 0.20 Ma on sericite from a previously unmapped when scanning the data in table 2 is that, prior to about altered granodiorite stock along the extension of the 1920, zinc was rarely recovered from the polymetallic Emery vein system. This date overlaps with U-Pb ages deposits. This explains why some early mining camps obtained on zircons from the EMV (Korzeb and oth- (e.g., Hecla, Alta, Neihart) have anomalously low zinc ers, 2018). Similarly, Lund and others (2002) obtained production fi gures despite an abundance of sphalerite 40 39 Ar/ Ar dates ranging from 73.9 to 77.0 Ma for hy- in the ores. drothermal sericite in altered wall rock at the Eva May 11 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics 5HIV .OHSSHUDQG RWKHUV 7\VGDODQG RWKHUV RWKHUV 7\VGDODQG RWKHUV RWKHUV 7\VGDODQG RWKHUV 6KHQRQ /RHQDQG 3HDUVRQ (DVWPDQDQG RWKHUV -RKQVRQ RWKHUV 6WHHIHO 5REHUWVRQ  5RE\ &DONLQV 3ULQ] 0HLQHUW -RKQVRQ  7KRPSVRQ =LQF WRQV  D QD /HDG WRQV  WRQV  &RSSHU R]  6LOYHU  R]  *ROG 

12 Gammons and others: Metallic Ore Deposits of Montana The Philipsburg district, in the southern Flint A distinct cluster of polymetallic vein deposits Creek Mountains, is unique in that it contains im- exists in far western Montana (Mineral County) along portant deposits of both fissure vein and carbonate structures related to the Lewis and Clark Line. These replacement types in close proximity. It also has a Ag-Pb-Zn±Au-bearing veins have long been consid- more complex metal endowment, including tungsten ered an eastern extension of the famous Coeur d’Alene (as huebnerite) and manganese (as Mn-oxides, rhodo- mining district of northern Idaho (e.g., Moore, 1910). chrosite, and rhodonite). Emmons and Calkins (1913) The “east Coeur d’Alene veins” are hosted in highly noted the similarity in mineralogy and metal ratios in deformed sedimentary rocks of the Belt Supergroup the granite-hosted and sediment-hosted lodes, and con- and occur on regional-scale faults that can be traced cluded that both sets of veins had a common origin, directly into the principal mines in Silver Valley, Idaho i.e., fluids that were expelled from the granitic batho- (Lonn and McFaddan, 1999). The most notable exam- liths of the Flint Creek Mountains. These observations ples in Montana are the Iron Mountain mine north of apply at a district scale as well. Thus, the grades and Superior (Campbell, 1960) and the Tarbox and Black proportions of ore minerals in the carbonate replace- Traveler mines north of Saltese, both of which are ment ores at Hecla and Elkhorn are similar to those of located on a strand of the Osburn fault zone. Assum- the fissure veins and shear zones of the Basin, Wick- ing that these deposits are an extension of the Couer es, Neihart, and Argenta districts. In virtually every d’Alene district, the origin of the Montana deposits is case, there is compelling geologic evidence to link closely tied to the origin of the Silver Valley deposits, the polymetallic mineralization to nearby porphyritic which itself is a topic of long-standing controversy intrusions. Thus, the Ag-Pb-Zn carbonate replacement outside the scope of this volume. ores at Elkhorn are thought to be the distal expression Epithermal Gold–Silver Deposits of ore fl uids emanating from the nearby Turnley Ridge porphyry (Steefel and Atkinson, 1984), and a buried Montana has a number of precious metal deposits porphyry is suspected beneath the Hecla structural that are inferred to be epithermal in origin (fi g. 5, table dome (Gignoux, 2000). Eastman and others (2017b) 3), that is, having formed in the top 1–2 km of the used fl uid inclusion and stable isotope methods to con- Earth’s crust. In the economic geology literature, it has clude that the carbonate replacement deposits at Hecla become customary to subdivide epithermal deposits formed from magmatic fluids at temperatures exceed- into high sulfi dation (HS) and low sulfi dation (LS) ing 300˚C, not from basinal brines of the Mississippi subtypes (e.g., Simmons and others, 2005). HS epi- Valley Type association as previously speculated by thermal deposits contain prominent vertical zones of McClernan (1983). The shear and fissure veins of vuggy quartz and advanced argillic alteration (quartz– Basin and Wickes, which cut granite of the Boulder alunite–clay) that often grade with depth to genetically Batholith and overlying Elkhorn Mountain Volcanics, related porphyry Cu±Mo±Au deposits. In contrast, are distributed around the periphery of an unmined LS deposits display a more pH-neutral alteration style porphyry Cu-Mo deposit (Beavertown deposit, Tysdal (adularia–sericite with carbonate gangue minerals and others, 1996). The rich Ag-Pb veins of Neihart, present), usually with no direct association with por- hosted by Archean metamorphic rocks, are close to the phyry deposits. However, some of the biggest epither- Big Ben porphyry-Mo deposit in the Little Belt Moun- mal deposits of Montana are diffi cult to classify using tains. A large but low-grade porphyry-Cu system is this scheme. Also, whereas most epithermal deposits associated with the Ag-Pb veins of the Argenta district, are hosted by volcanic rocks, many of the deposits in in the southern Pioneer Mountains (Ted Antonioli, oral Montana are not. For example, at Zortman–Landusky, commun.). A genetic link is well established between auriferous quartz–pyrite veins cut an altered syenite the incredibly rich polymetallic Main Stage veins intrusion and adjacent metamorphosed country rock. and lodes of Butte and earlier porphyry-style Cu-Mo At Drumlummon, quartz–carbonate veins and lodes mineralization (see Reed and Dilles, 2020), as well as cut Belt-aged metasediments near the contact of the the high-grade carbonate replacement deposits at New Marysville Stock. At Kendall and the deposits of the World and nearby skarn and porphyry-style ore Judith Mountains, base and precious metals occur as (Johnson and Meinert, 1994; Johnson and Thompson, veins and replacements in Paleozoic carbonate rocks 2006). near contacts with Tertiary intrusions.

13 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics

HH ZL

K G MM, 7UP

D MT B R

GS T = Low sulfidation M = High sulfidation = Breccia pipe

0 40 80 120 160 km

0 25 50 75 100 mi

Figure 5. Epithermal gold–silver deposits of Montana. B, Basin; D, Drumlummon; GS, Golden Sunlight; HH, Hog Heaven; K, Kendall; M, Mayfl ower; MM, McDonald Meadows; MT, Montana Tunnels; R, Ruby (Oro Fino); 7UP, Seven Up Pete; T, Tuxedo; G, Gies; ZL, Zortman-Landusky.

Several epithermal deposits in Montana are rich in MHBP include hydrothermally altered latite porphyry telluride minerals, including Golden Sunlight, May- as well as silicifi ed Belt country rock, and the matrix fl ower, Zortman–Landusky, and the Geis mine in the of the breccia grades into latite with depth. DeWitt Judith Mountains. Previous workers have suggested and others (1996) obtained an imprecise whole-rock that these deposits are part of a northeast-trending U-Pb age of 84 ± 18 Ma for the altered latite (which belt of epithermal Au and porphyry Cu-Mo deposits they re-classifi ed as rhyolite), and an 40Ar/39Ar date of associated with Late Cretaceous and Tertiary alkaline 76.9 ± 0.5 Ma for biotite phenocrysts in a post-mineral magmatism. This belt of mineralization approximately lamprophyre dike. An unpublished zircon U-Pb date coincides with the Great Falls Tectonic Zone, and is of 81.9 ± 1.9 Ma was recently obtained for latite as- similar in some respects to the Colorado Mineral Belt. sociated with the MHBP (D. Odt, Barrick Gold, writ. The Golden Sunlight district has produced more commun., July 2017). Thus, mineralization at Golden gold (>3 M oz) than any other deposit in Montana. Sunlight overlaps with emplacement of the Elkhorn Most of this gold has come from the Mineral Hill Mountains Volcanics. Geologic evidence in combina- Breccia Pipe (MHBP). The MHBP is roughly 200 m tion with sulfur-isotope studies shows that pyrite in the in diameter, plunges west at about 50 degrees, and cuts Golden Sunlight deposit, as well as base and precious lithic sandstone, siltite, and argillite of the mid-Pro- metals, is a mixture of Late Cretaceous magmatic/ terozoic LaHood and Greyson Formations (fi g. 6). hydrothermal and Precambrian sedimentary sources The MHBP is believed to be an upper-level structure (Porter and Ripley, 1985; Spry and others, 1996; Fos- associated with a deep porphyry-Mo hydrothermal ter and others, 1999; Gnanou and Gammons, 2018). system (Spry and others, 1996). Fragments in the The Montana Tunnels mine near Jeff erson City is

14 Gammons and others: Metallic Ore Deposits of Montana \S\ULWH %DUWOHWWDQG RWKHUV 6LOOLWRHDQG RWKHUV 3RUWHU 5LSOH\ 6SU\DQG RWKHUV 2\HUDQG RWKHUV /LQGVH\ .XULVRR 6KHQRQ /DQJHDQG RWKHUV )RVWHU  &KLOGV 3DUGHH  6FKUDGHU :LOVRQ  .\VHU 5XVVHOO )RVWHU  &KLOGV 7HDODQG RWKHUV :DONHU *ULIILWK   W 0 RQIURP FWL ]$XZLWK XFW$JDQG&X UHVHUYH  UHVRXUFH $JIRUGLVWULF R]$XUHVRXUFH 0R]ORZJUDGH aR]$X 0HWDO3URGXFWLRQ 5HIV EXWR]$X XQPLQHGUHVHUYH  'UXPOXPPRQPLQH 0R] R]$JWRQV 3EWRQV=Q aR]$JWRWDO PLQRU$XSURGXFWLRQ E\SURG PRVWSURGX WRQV3EWRQV&X aR]$X ! SURGXFWLRQUHVHUYHV $JSURGXFWLRQ! !0R]$XDQG!

15 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics

Figure 6. Simplifi ed geologic cross sections through the Montana Tunnels, Golden Sunlight, and McDonald (East Butte) gold deposits. Sources of data: (A) Sillitoe and others (1985); (B) DeWitt and others (1996); (C) Bartlett and others (1998).

16 Gammons and others: Metallic Ore Deposits of Montana also a breccia pipe deposit, but is richer in Ag and base rides, are associated with pyrite, marcasite, arsenopy- metals (Pb-Zn) than the Golden Sunlight breccia, is rite, minor base-metal sulfi des, and fl uorite (Wilson less silicifi ed, and has a gangue mineral assemblage and Kyser, 1988; Foster and Childs, 1993). Epithermal that includes carbonates (Mn-calcite, siderite) as well mineralization is developed in breccias along high-an- as quartz. Also, whereas Golden Sunlight is Creta- gle faults and in stockworks on the fault margins ceous, Montana Tunnels is Eocene (Sillitoe and others, (Russell, 1991; Foster and Childs, 1993). 1985). Production from the Montana Tunnels open pit A number of Au- and Ag-rich quartz veins occur from 1986 to 2010 is estimated at 1.7 M oz Au, 30.8 in the Marysville district, roughly 30 km northwest of M oz Ag, 400 million lbs Pb, and 1.1 billion lbs Zn Helena. The largest historic producer was the Drum- (Eastern Resources website, 2012). The Montana Tun- lummon vein, a N15°E-striking, steeply dipping lode nels breccia pipe includes pieces of volcanic rock and up to 10 m wide, 1 km long, and 500 m deep (Knopf, Butte Granite (as well as carbonized wood fragments) 1913; Walker, 1992). Other lodes stemming off the in a tuff aceous, quartz–latite matrix. The pipe is inter- Drumlummon include the North Star, Castletown, preted to be a diatreme emplaced near the paleosurface Empire, and St. Louis veins. Together, these mines during eruption of the Lowland Creek Volcanics (Silli- produced over 1 M oz of gold and over 10 M oz of toe and others, 1985). silver between 1873 and 1950 (Walker, 1992). The Montana Tunnels is located near the northeast veins are hosted by hornfels of the mid-Proterozoic end of a graben that cuts the Butte Granite and that is Empire and Helena Formations near their contact partly fi lled with felsic to intermediate volcanic rocks with the Marysville Stock, the northernmost pluton of the Lowland Creek fi eld (Foster, 1987). Other epith- of the Boulder Batholith. In 2010, renewed explo- ermal deposits associated with this graben include the ration in the district discovered the Charly vein, a Tuxedo mine near Butte (Scarberry and others, 2015), high-grade “bonanza”-style vein that was the focus of the Oro Fino district west of Basin (Korzeb, 2017), small-scale, underground mining from 2010 to 2013. and the Basin Creek (Pauper’s Dream) deposit north The Charly vein contains abundant electrum, native of Basin (fi g. 5). Korzeb and Scarberry (2018) ob- silver, stromeyerite, argenite/acanthite, pearceite, and tained 40Ar/39Ar dates of 45.8 ± 1.0 and 50.7 ± 1.2 Ma base-metal sulfi des, in a gangue of quartz, chalcedony, for hydrothermal sericite at Oro Fino. Although orig- adularia, and Mn-rich dolomite (Griffi th, 2013). inally thought to be part of the Lowland Creek Volca- The Kendall mining district, located on the east nics, Teal and others (1987) reinterpreted the pyroclas- fl ank of the North Moccasin range about 30 km north- tic–rhyolite host rocks at Basin Creek to be Oligocene. west of Lewistown, produced roughly 800,000 oz of Basin Creek was most recently mined in 1988–1990, gold from a series of open pits from 1900 to 1942, and producing 82,000 oz of gold from 3 Mt of ore at 0.033 again from 1988 to 1996 (Robertson, 1950; MBMG oz/t (Teal and others, 1987). Although much gold is archives). Most of the gold is hosted by the Mission reported to remain in the ground at Basin Creek (Tys- Canyon limestone near its contact with a Paleocene dal and others, 1996), it is unlikely to be mined, as the syenite–porphyry intrusion. Gold occurs as dissemi- property is now being used as a mine-waste repository nations in stratabound karst breccias, in hydrothermal/ for the State of Montana. intrusive breccias, in altered syenite, and in sandstone The Zortman–Landusky Mines, located on the beds of the overlying Kibbey Formation (Kurisoo, crest of the Little Rocky Mountains, were a series of 1991; Foster and Childs, 1993). The Kendall district open pits located 5 km apart that together produced bears certain similarities to “Carlin-type” gold depos- over 1 M oz of gold between 1979 and 1998. Min- its of the Great Basin, including carbonate host rocks, eralization at the two mines is similar, and has been alteration consisting of silicifi cation and carbonate dis- described as an epithermal system overprinting a solution, the occurrence of “invisible” gold associated syenite–porphyry intrusion that itself lacks typical with arsenian pyrite, and elevated concentrations of porphyry-style alteration and mineralization patterns thallium, a trace metal that is common in Carlin-type (Wilson and Kyser, 1988; Russell, 1991). Mineraliza- gold deposits but otherwise is extremely rare (Ikra- tion exists in the syenite and extends into basement muddin and others, 1986). gneiss of Archean or paleo-Proterozoic age. Ore min- The unmined McDonald gold deposit (also known erals, mainly native gold, electrum, and Au-Ag-tellu- as McDonald Meadows, fi g. 6), located near the

17 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics headwaters of the Blackfoot River 5 km west of the stones of the Pryor Mountains (Moore-Nall, 2016); (3) Heddleston porphyry Cu-Mo deposit, fi ts most of the elevated but low-grade U in lignite of eastern Montana characteristics of a typical LS style epithermal deposit. and the Dakotas (Hail and Gill, 1953; Armstrong, The deposit is hosted by Eocene to Oligocene felsic 1957); and (4) elevated but low-grade U in phosphat- volcanic rocks (ash-fl ow tuff s), and is overlain by ic black shale of the Permian Phosphoria Formation. silica sinter that locally contains plant fossils (Bartlett The only U production in Montana has been from the and others, 1998). Gold occurs in a network of quartz– fi rst and second types listed above, and this has been adularia and chalcedony veins that extend up into the minor. The Butte Granite as a whole is elevated in U surface sinter. The deposit contains an estimated 7.85 and Th, which has caused problems with radon gas in M oz of gold, and has been oxidized to a depth of homes and elevated concentrations of radionuclides 300–350 m. The mineralization at McDonald is low in drinking water wells (Caldwell and others, 2014). grade, averaging 0.67 g/ton (ppm) Au (Bartlett and Conversely, tourists can enter underground tunnels at others, 1998). At these grades, the most economically the Merry Widow (Basin) and Free Enterprise (Boul- viable means of mining and milling the entire deposit der) “health spas” and relax in comfortable chairs is by open pit, cyanide heap-leach. However, in 1998, while inhaling low levels of radon gas, thought by the people of Montana passed bill I-137, which banned some to be therapeutic (Salak, 2004). Although small, open-pit, heap-leach cyanide mining, eff ectively halt- the U deposits of the Pryor Mountains are locally of ing further development at the McDonald property. good grade, and are geologically similar to the breccia The current owners of the property are attempting to pipe hosted U deposits that cut the Redwall limestone defi ne a smaller, high-grade reserve at McDonald that (stratigraphic equivalent of the Madison Group) in could possibly be mined by gravity and fl otation. the Colorado Plateau. Uranium-bearing breccias in The only epithermal deposit in Montana that could the Pryor Mountains contain a number of rare second- be put into the high sulfi dation (HS) category is Hog ary U minerals, including tyuyamunite (Moore-Nall, Heaven, located 30 km west of Flathead Lake. Here, 2016). silver and lead, with lesser amounts of Au, Cu, and Zn, Several occurrences of minerals rich in rare earth were mined from an area of vuggy silica and advanced elements (REE) exist along the Montana–Idaho bor- argillic alteration developed at shallow level in an der. These deposits are associated with alkalic dikes Oligocene dacite lava dome (Lange and others, 1994). and stocks of the western Montana–Idaho alkalic belt Lange and others (1994) suggested that the Hog Heav- (Schissel, 1983; Gammons, 2020a), which extends en district represents an Ag- and Pb-rich end member from Lemhi Pass to the Rainy Creek Complex, host among the continuum of HS epithermal deposits. to the Libby vermiculite mine. At Lemhi Pass, thorite

However, the mine has limited vertical exposure, and (ThSiO4) and monazite (CePO4) occur with specular others have suggested that the advanced argillic alter- hematite, barite, and biotite in veins and shear zones ation at Hog Heaven might instead be a near-surface cutting Proterozoic metasediments (Anderson, 1961; phenomenon (Rostad and Jonson, 1997). The question Sharp and Cavender, 1962; Staatz and others, 1972; has exploration signifi cance, as many HS epithermal Gillerman and others, 2003). According to Van Gosen deposits occur on top of gold-rich porphyry–Cu sys- and others (2009), the veins of the Lemhi Pass district tems. A recent thesis by Kallio (2020) includes new represent the largest thorium resource in the United mineralogy and stable isotope data consistent with the States. Given that the ratio of REE to Th is roughly idea that the Hog Heaven mine was once underlain by 1:1 in the veins, there is also a substantial amount of a porphyry magma. REE in the district. The Snowbird deposit, located in the Bitterroot Mountains 55 km west of Missoula, Deposits of Uranium, Thorium, and the contains parisite (CaCe (CO ) F ) with quartz, fl uo- Rare Earth Elements 2 3 3 2 rite, and ankerite in a vein cutting the Idaho Batho- An overview of uranium deposits in Montana is lith. Owing to the unusually coarse-grained nature of given by Weissenborn and Weis (1963). These depos- the mineral intergrowths, the Snowbird deposit was its include: (1) uraninite in late chalcedony veins with initially thought to be a carbonatite dike (Clabaugh minor silver and base metals cutting the Butte Granite and Sewell, 1964). Later workers concluded that it of the Boulder Batholith (Becraft, 1956); (2) second- is the product of hydrothermal fl uids sourced from ary U minerals in brecciated Madison Group lime- the Idaho Batholith (Metz and others, 1985; Samson 18 Gammons and others: Metallic Ore Deposits of Montana and others, 2004). Other REE occurrences have been placer gold would be discovered in 30 of the 56 coun- described in the Sheep Creek area near the headwaters ties in Montana and placers are known to exist in 243 of the West Fork of the Bitterroot River. Here, a set of drainages (fi g. 7). northwest-striking quartz-carbonate pods and veins cut Records suggest a total placer gold production in metasedimentary rock at a low angle. The veins are Montana of over 17,700,000 oz of gold between 1862 locally rich in barite, apatite, the niobium-rich miner- and 1965 (Lyden, 1948). This number is probably low, als columbite, euxenite, and fersmite, as well as the since few accurate records were kept, especially in the REE-bearing minerals monazite, allanite, and ancylite fi rst few months following a new discovery. Placers, (Crowley, 1960; Heinrich and Levinson, 1961; Gam- more so than lode gold deposits, yielded gold that was mons, 2020b). The literature is unclear as to whether often not accounted for in offi cial records. Table 4 the Sheep Creek deposits are of hydrothermal, meta- gives a list of placer gold production by county, with morphic-segregation, or magmatic affi nity. major deposits named. Virginia City (Alder Gulch) The central Montana alkalic province, known to was the largest producer by far, followed by Last petrologists for its unusually diverse igneous rocks Chance Gulch (Helena) and Confederate Gulch. Float- (Irving and Hearn, 2003), hosts several carbonatite ing, bucket-line dredges were used for many of the bodies, the largest of which is the Rocky Boy stock major placer operations, including Last Chance Gulch, of the Bears Paw Mountains. Veins associated with Gold Creek, Alder Gulch, North Meadow Creek near the Rocky Boy carbonatite locally contain up to 3.5% Ennis, and Eldorado Bar east of Helena (Jennings REE (Pecora, 1962). Other carbonatites, as well as and Janin, 1916). Besides gold, sapphires and garnets kimberlites (including the diamond-bearing Home- have been recovered from placer deposits in Montana stead kimberlite, Hearn, 2004), can be found in the (see Berg and others, this volume), as well as minor Missouri Breaks and Grass Range areas. In a recent amounts of scheelite and cassiterite. synthesis, Duke (2009) proposed that the igneous Given the geographic distribution of placers in rocks of central Montana are part of a larger, 700-km- Montana (fi g. 7), it is clear that no single geologic long, N40°W-trending belt of alkaline magmatism setting is responsible for placer gold occurrences. Sig- extending from the Black Hills of South Dakota to nifi cant placer gold has been found in streams draining southern Alberta that he has named the “Great Plains Archean basement (e.g., Alder Gulch), Belt sedimen- alkalic province.” The large and possibly mineable tary rock (Confederate Gulch, Ninemile), and Paleozo- Bear Lodge REE deposit is located in carbonatite and ic–Mesozoic metasediments near contacts with granit- associated alkalic rocks in the Wyoming portion of ic intrusions (Bannack, German Gulch, Gold Creek, the Black Hills (Moore and others, 2015). Although Last Chance Gulch). In some districts there is a clear no mineable REE deposits have yet been found in the link between placer gold and a nearby bedrock source alkalic rocks of Montana, it is possible that new dis- (Woodward, 1994). For example, most of the gold in coveries of this type could be made in the future. German Gulch probably came from the Beal Mountain Gold Placers gold–skarn deposit near the headwaters of this small creek. In other cases, the link is more mysterious, as Placer gold was fi rst reported in Montana in 1852 at Bannack, Ninemile, and Alder Gulch. Alder Gulch in alluvium along Gold Creek, Powell County (Pardee, stands out as one of the districts with the highest ratio 1951), and the signifi cant placer deposits at Bannack of placer gold produced to known lode gold sources. were discovered in 1862, with Alder Gulch and Last Is this because exploration has not yet found the big Chance Gulch shortly after. Many of these early dis- lodes, or is it because the source rocks have eroded coveries were made by miners who had rushed to the away? Other districts contain large amounts of gold in California gold fi elds in 1848, and then fanned out to their bedrock but have produced relatively little placer other western territories with their newly developed gold. Examples include the Whitehall district (near exploration and mining skills (Gardner and Johnson, Golden Sunlight, Montana’s largest gold mine) and 1934). The volume of miners was augmented with Silver Bow Creek (which drains the world-class Butte the conclusion of the Civil War and the incredible porphyry–lode deposit). Roughly 3 million oz of gold displacement of people caused by the confl ict. Plac- has been mined from the lode deposits of Butte, and er mining required little capital but had the potential Houston (2015) estimated that as much as 18 million to supply a lifetime of wealth for some. Eventually, 19 MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics

D D D D D D D DD D DD DDDD DDD D DD D DD D DD DDDD 15 D DDDDD D D Major Districts D D D D D 1. AlderD Gulch D16 DD DD D 2.D BannackD DDD17DDD D DDDD 3. FrenchD Creek D DDD DDD D 4. German Gulch D 90 DDD D DDD13D DDDDD D DD D 14DDD DDDDDDDDD DDDD D DDD DD 5. Silver Bow Creek D D D DD D DDDDD D DD D DD12 D D DDDDD11DDD DDD 6. Highlands DDDD D D DD D DDDDD DDDDD DD DDDDDD DDD9 D 10 D 15D D DDDDDDD7 DD DD DDD D D D D 7. Gold Creek D D DD D DD D D D N D D D DD D8D D 8. Prickly Pear D D D DD DDDDD DD D D DD D DD DDDDDD D DDDD D D D DD D DD D D DD 9. Last Chance (Helena) D DD D DD DD D D DDD 10. Confederate Gulch D D D D DDD5D DDD 11. York/Missouri D 4 D DDD D D D DD6 D 12. Finn, Ophir (Avon) D D D 90 DDD D 3 D D DD 13. Lincoln, McClennan DD D D DD D D DD D D D 14. Garnet DDD DD DD 15. Henderson Creek D DD1 D DD DDDDD 16. Ninemile D D D D DD DD D D2 D DD D DD DDDD D DD D D 17. Trout Creek D Placer mines DDD D D D D D D DDD DDDD D DD D DDDDD D DDDD DD 062.5 125 mi 15 D D 0100 200 km

Figure 7. Location of gold–placer deposits in Montana.

oz gold might have existed as an epithermal cap on top particles (Loen, 1994, 1997), as well as their fi neness of the Butte deposit based on a comparison with other (percentage gold by mass). Because silver is more porphyry–epithermal systems globally. By compari- soluble than gold in the weathering environment, gold son, only around 120,000 oz of gold was recovered by particles, including the rims of larger gold nuggets, placer diggings in Silver Bow Creek. tend to increase in fi neness with time and transport Although there must ultimately be a bedrock distance (Desborough, 1970). Some placer districts source, whether or not a good source region produces were known for their exceptionally pure gold, includ- a rich placer also depends on the district’s history of ing the Ninemile district (96–100% fi ne). Gold fi ne- glaciation, mass wasting, and alluvial transport. Many ness is also a function of the purity of the gold in the rich placers in Montana were reworked multiple times bedrock source, which can range from Ag-rich elec- by periods of alpine glaciation and outwash, debris trum (e.g., at the Drumlummon mine in Marysville) to fl ows, and fl oods. The importance of debris fl ows, nearly pure gold. Lange and Gignoux (1999) cited the especially in narrow drainages where there is a nick high purity of gold at Ninemile, in combination with point in stream gradient, was noted by McCulloch the relatively short inferred distance of transport, as and others (2003). Such deposits can concentrate gold evidence that the source of the gold placers could be and other dense minerals due to the water-saturated, undiscovered quartz-lode deposits of the “orogenic” or turbulent nature of the fl ows. Transport of gold nug- “slate-belt” affi nity, which typically contain free gold gets downgradient can also change the shape of the with high fi neness. 20 Gammons and others: Metallic Ore Deposits of Montana

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The discovery of large gold nuggets in river grav- ACKNOWLEDGMENTS el, in addition to their monetary value, usually sparks intense mystery and excitement. Some notable gold We thank Bruce Cox and John Childs for their nuggets found in Montana include: 57 oz from Mc- helpful comments and suggestions on an early draft Clellan Creek, 27 oz from the Pineau placer at Gold of this manuscript, and Robin McCulloch, who laid Creek, 24 oz from the McFarland placer at Gold out the framework for the placer section. Technical Creek, and a record 84.48 oz nugget from California reviews were also provided by Fess Foster and Ian Creek near Alder Gulch (New York Times, 1902; Lange. Pardee, 1951). More recently (1989), the 27.495 troy oz Highland Centennial nugget was found south of Butte. This stone, believed to be Montana’s 7th largest gold nugget, is now displayed at the Mineral Museum at Montana Tech.

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