Time-Space Relationships between Sediment-Hosted Gold Mineralization and Intrusion-Related Polymetallic Mineralization at Kinsley Mountain, NV
AuthorHill, Tyler James
AdvisorMuntean, John L
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Carlin-type gold deposits (CTGDs) in north-central Nevada contain a large endowment of gold, making Nevada one of the world’s leading gold producers. Most CTGDs occur in four distinct clusters, known as the Carlin Trend, Cortez, Getchell, and Jerritt Canyon. These four camps of CTGDs are estimated to contain >200 million ounces of Au and share common features, including the occurrence of gold in solid solution or as submicron particles in disseminated arsenian pyrite in large carbonate replacement bodies where alteration is dominated by decarbonatization and silicification of silty carbonate host rocks. CTGDs can be confused with other disseminated gold deposits hosted by predominantly carbonate-bearing sedimentary rocks that share many features with CTGDs, but also have many distinctly different characteristics and generally have lower endowments of gold. These similar deposits, referred to as here sediment-hosted gold deposits (SHGD), include distal-disseminated deposits temporally and spatially associated with upper crustal felsic intrusions, epithermal deposits hosted by carbonate rocks, and epizonal orogenic deposits hosted by carbonate rocks. A very important question to resolve is whether CTGDs and SHGDs represent a spectrum of deposits with a shared origin, in which CTGDS are an endmember, or whether they have different origins that ultimately resulted in similar looking alteration and mineralization. Kinsley Mountain lies approximately 150 km east of the majority of large CTGDs in Nevada. SHGDs, intrusion-related tungsten skarn, and polymetallic carbonate replacement vein deposits have been mined from Kinsley Mountain, proximal to an Eocene intrusive center. Stratigraphically and structurally controlled SHGDs occur as replacement bodies in Cambrian shelf carbonates and siliciclastics. The SHGDs at Kinsley are broadly similar to the large CTGDs to the west, but key differences are highlighted in this paper. Kinsley Mountain underwent multiple episodes of contraction and extensional deformation in the Mesozoic. During the Eocene, a granodiorite stock and associated dikes were emplaced at ~40 Ma, with slightly later Eocene extension and volcanism. High-angle basin and range faulting began in the Miocene continues through the present. Quartz veins containing polymetallic mineralization cut the stock and polymetallic veins replace carbonate country rock proximal to the stock. Molybdenite in skarn near the stock returned a Re-Os age of 37.88 ± 0.3 Ma. Numerous cogenetic dikes extend up to 4 km north of the stock into the area of the SHGDs. SHG mineralization is primarily hosted by the Cambrian Dunderberg Shale and Secret Canyon Shale. These units contain interbedded shale and limestone with up to 50% ferroan dolomite in shale beds. In strongly mineralized intervals, ferroan dolomite has been replaced by quartz and pyrite. A 39.7 ± 0.6 Ma mineralized dike places a maximum age constraint on mineralization, whereas ~36-37 Ma unmineralized dacite to rhyolite flows that flank both sides of Kinsley Mountain and onlap Cambrian stratigraphy, place a minimum age on mineralization. SHG mineralization is characterized by diagenetic or hydrothermal pyrite cores surrounded by euhedral, oscillatory zoned Au-bearing arsenian-pyrite rims. Locally, pyrite cores are surrounded by stibnite or tennantite-tetrahedrite, which are enveloped by arsenian-pyrite rims. Gold strongly correlates (correlation coefficient >0.5) with Ag, As, Hg, Sb, Te, and Tl. The ore-stage pyrite at Kinsley contrasts with ore stage pyrite in CTGDs, where the arsenian-rims are typically “fuzzy” in appearance and do not display zoning. Additionally, the presence of early base metals has not been documented in Carlin-type mineralization. Sulfur isotope data on pyrite revealed δ34S values of ~3‰-8‰ for pyrite proximal to the stock, while pyrite from the SHGDs were significantly heavier (16‰-25‰). The differences suggest a magmatic source of sulfur for pyrite proximal to the stock and a sedimentary source or mixed magmatic and sedimentary source of sulfur for pyrite in SHG mineralization. The hydrothermal system at Kinsley had two sulfur sources – a magmatic source that formed the intrusion-related polymetallic mineralization and a sedimentary crustal source that formed the SHGDs. In the SHGDs, carbonate-bearing shales rocks were preferentially sulfidized by the ore-fluid, forming Au-bearing arsenian-pyrite as Fe was liberated from ferroan dolomite during decarbonatization. The presence of euhedral pyrite, carbonate in ore zones, and lack of marcasite, kaolinite, or dickite suggests ore fluids that were higher in temperature and less acidic than ore fluids that form the large CTGDS in Nevada. Aluminum in hornblende geobarometry, typical formation depths of tungsten skarn deposits, and the lack of faulting between the stock and the SHGDs suggest depths of ≥3 km for ore formation. However, such depths require significant uplift between 40 Ma and 36 Ma. Shallower depths for the SHGDs are possible if the range was tilted northward.