Mineralogy and sulfur isotope geochemistry of polymetallic, porphyry-epithermal mineralization peripheral to the Golden Sunlight gold mine, Montana
Graphical abstract
Introduction
A clear association exists worldwide between low sulfidation, telluride-rich epithermal gold deposits and alkalic porphyry intrusions (Mutschler and Mooney, 1993, Richards, 1995, Jensen and Barton, 2000). Examples of gold deposits that fit this description include Porgera and Ladolam, Papua New Guinea (Richards and Kerrich, 1993, Müller et al., 2002), Emporer, Fiji (Ahmad et al., 1987), Cripple Creek, Colorado, USA (Thompson et al., 1985, Kelley et al., 1998), as well as the Golden Sunlight deposit in Montana, USA (Spry et al., 1996), which is the focus of the present paper. Whereas epithermal deposits above shallow, calc-alkaline magmas may show low- or high-sulfidation affinities, shallow deposits over alkaline magmas tend to display only low-sulfidation alteration and ore mineral assemblages (Sillitoe, 2002). This is so, even if there is evidence (e.g., isotopic) for inheritance of fluids, metals and volatiles (e.g., S or Te species) from the underlying magma chamber (Sillitoe, 2002). Details of the physico-chemical transition from the porphyry to the epithermal environment are challenging to interpret unless the mineral deposit is exposed over a large vertical extent or has been drilled to great depth. Likewise, lateral zonation in porphyry-epithermal deposits may be difficult to decipher, especially if limited exploration has occurred distal to the central, defining ore body.
This study investigates the Golden Sunlight deposit of Montana, an epithermal, breccia-pipe hosted, Au-Ag telluride deposit that is underlain by at least one alkaline porphyry Mo-(Cu-Au) system. Previous workers in the Golden Sunlight district have focused on the main breccia-pipe ore body (Porter and Ripley, 1985) and its relation to a deeper porphyry (Spry et al., 1996). This study investigates several mineralized areas that are peripheral (up to 2 km lateral) to the central breccia pipe that was the focus of historical mining in the district. New information on the mineralogy and S-isotope compositions of the satellite deposits is compared with published data on the Golden Sunlight deposit to help determine whether mineralization in the district is genetically related, or the product of separate mineralizing events. An updated genetic model is presented that involves formation of hydrothermal fluids from an oxidized magma with SO2 > H2S.
The Golden Sunlight Mine (GSM) is in Jefferson County, southwest Montana, 50 km east of the famous Butte porphyry/lode deposit, and roughly 20 km east of the eastern border of the late Cretaceous Boulder Batholith (Fig. 1). With total historical gold production exceeding 3.6 million oz, it is the largest gold mine in the State of Montana. Although mining began as early as 1890, the modern mine operated from 1982 to 2019 from one large open pit, as well as two smaller pits and several small-scale underground operations. Previous papers addressing the geology, geochemistry, mineralogy, and stable isotopes of the GSM include Porter and Ripley, 1985, Foster and Childs, 1993, DeWitt et al., 1996, Spry et al., 1996, Spry et al., 1997, Spry and Thieben, 2000, and Oyer et al. (2014). The mine lies within the northeast-trending Great Falls Tectonic Zone (GFTZ), a Precambrian terrain boundary between the Wyoming Province to the southeast and the Medicine Hat Province to the northwest (Sims et al., 2004). Previous workers have noted a concentration of porphyry and epithermal style mineral deposits in the GFTZ (Foster and Chadwick, 1990), which includes the world class polymetallic deposits at Butte.
The geology of the Golden Sunlight district consists of a horst of Precambrian to Paleozoic sedimentary rocks intruded by late Cretaceous latite porphyry (Fig. 2). Quartzite, siltite, and argillite of the LaHood and Greyson Formations, part of the lower Belt Supergroup of Mesoproterozoic age, are unconformably overlain to the north by the Cambrian Flathead Quartzite and younger formations (Fig. 2). Dikes, sills, and stocks of latite porphyry cut all sedimentary units and are locally well-mineralized. Although the term latite is used in this paper and is entrenched in the literature of the district, DeWitt et al. (1996) argued that these rocks are rhyolites based on the chemistry of unaltered samples. A younger set of lamprophyre dikes and sills cuts the latite. Most of the lamprophyres are relatively unaltered (mainly propylitic) and none are known to host gold mineralization.
Although several Au-bearing veins were mined in the early 1900 s, almost all of the gold production in the district has come from the Mineral Hill Breccia Pipe (MHBP). The MHBP is roughly 200 m in diameter near the top and plunges at an average angle of 35° to the west. Clasts in the breccia pipe are a mix of Belt-aged sedimentary rocks and latite porphyry, and latite constitutes an increasing proportion of the matrix of the breccia pipe at depth (DeWitt et al., 1996). Hydrothermal alteration, along with disseminated gold and abundant pyrite, is found in the clasts, in the matrix, and extending into the surrounding metasediments. In addition, stratiform pyrite, presumably Belt-aged, is locally abundant in the metasediments surrounding the MHBP. Some of the clasts of latite porphyry in the MHBP contain molybdenite, and there are strong indications that the breccia pipe is underlain by a porphyry Mo-(Cu) system (Spry et al., 1996).
As the open pit centered on the MHBP approached the end of its mine life, attention shifted to exploration for mineralized zones peripheral to the breccia pipe. Four outlying areas of interest were examined in this study: the Apex, Bonanza, North Area Pit, and 102/Sunlight Vein zones (Fig. 2). The Apex gold prospect is located 1.5 km north of the MHBP and straddles the contact between the Greyson and Flathead Formations. Disseminated, stockwork-style vein mineralization is found in both units. Bonanza is a polymetallic (Au-Ag-Cu-Mo) deposit hosted by a hydrothermally altered porphyry stock that intruded into the Greyson Formation (Fm). The North Area Pit, a small open pit mined in 2014–2015, lies midway between the MHBP and the Bonanza deposit where a body of latite porphyry cuts Belt strata. The 102 Zone and Sunlight Vein trend are located immediately southeast of the Mineral Hill Breccia Pipe (Fig. 2). The Sunlight Vein is a north-trending, steeply west-dipping tabular zone of gold-enriched pyrite seams, stockworks and disseminations, originally targeted by underground miners in the late 1800s and early 1900s. The 102 Zone represents a second strand of mineralization parallel to and west of the Sunlight Vein. Detailed mapping of the 102 Zone shows NNW-trending, en echelon quartz-pyrite veins with elevated gold grades. Gold is also hosted in siltite that lacks significant brecciation and silicification, and minable stope widths in the 102 Zone were greater than stope widths along the Sunlight Vein.
DeWitt et al. (1996) reported an 40Ar/39Ar age of 76.9 ± 0.5 Ma for an unaltered lamprophyre dike, as well as an imprecise 206Pb-238U whole rock age of 84 ± 18 Ma for latite (rhyolite) porphyry near the MHBP. A more recent U-Pb date of 81.9 ± 1.9 Ma was obtained for zircon in latite at the MHBP (Barrick Gold, pers. commun; see supplementary data file). This compares to recent 40Ar/39Ar dates of 78.6 ± 1.5, 77.5 ± 1.1, and 78.0 ± 0.4 Ma for biotite from the latite porphyry at the Bonanza prospect (Barrick Gold, pers. commun; see supplementary data file). Excluding the whole-rock U-Pb date of DeWitt et al. (1996), the age of mineralization in the Golden Sunlight district is constrained to have taken place between 81.9 and 76.9 Ma. The apparent gap in time between the MHBP porphyry and the Bonanza porphyry could be real, implying multiple porphyry-epithermal events in the district, or it could be an artifact of thermal resetting of the Ar systematics in biotite within the latter intrusion. The latite intrusions at GSM overlap in age with the voluminous Elkhorn Mountains Volcanics (EMV) of southwest Montana, suggesting the two are genetically related. In contrast, the latites and associated mineralization at GSM are >10 m.y. older than the porphyry-lode mineralization at Butte (66 Ma, Lund et al., 2002).
Section snippets
Sample collection and petrography
Representative samples from the Apex, Bonanza, and North Pit deposits were collected from drill core stored at the Golden Sunlight mine. Sampling was guided by the presence of veins, hydrothermal alteration, visible sulfide minerals, as well as assay data supplied by the mine. Hand samples of higher grade gold ore from the Sunlight and 102 zones were collected from the underground mine ribs and faces by company geologists. Approximately 40 samples were sliced and mounted in 1″ epoxy plugs and
Vein descriptions
Within the Apex deposit, narrow quartz veins and hydrothermal breccias cut the Mesoproterozoic Greyson Fm and the overlying Cambrian Flathead Quartzite (Fig. 2). In the Greyson Fm, thin (<1 cm) quartz veins with dark, siliceous alteration envelopes form an orthogonal stockwork spaced roughly 5–10 veins per meter (Fig. 3A). Pyrite and other sulfide minerals fill the veins and penetrate along bedding planes in the host argillite. The Greyson Fm also locally contains bedded, syngenetic pyrite. The
Mineral assemblage
The similarity in ore and gangue mineralogy between Apex, Bonanza, 102/Sunlight, and the MHBP is consistent with the idea that the peripheral mineralized areas in the Golden Sunlight district are part of the same magmatic-hydrothermal system as the main breccia pipe ore body. The presence of molybdenite-bearing porphyry intrusions at Bonanza and at deeper levels in the MHBP suggests that magmatic fluids played an important role in mineralization at a district scale. The relative lack of quartz
Conclusions
The major findings and interpretations from this study that bear on the origin of porphyry-epithermal mineralization at Golden Sunlight are:
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Although the geologic setting of the peripheral deposits surrounding the central Mineral Hill Breccia Pipe is different, the ore and gangue minerals are very similar. Pyrite is ubiquitous, with lesser chalcopyrite, galena, sphalerite, tetrahedrite-tennantite, with trace amounts of gold, Ag-sulfosalts, Bi-S-tellurides, and electrum.
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Alteration and gangue
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements:
We thank Barrick Minerals and the Golden Sunlight Mine for supporting this project. Jeremy Vaughan (Barrick) was instrumental in providing the raw data for previously unpublished age dates from the district. Additional support came from the Montana Bureau of Mines and Geology and the Stan and Joyce Lesar Foundation. The manuscript was improved by the reviews of John Dilles and Franco Pirajno. We also thank Gary Wyss (Center for Advanced Mineral and Materials Processing) for assistance with the
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