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The Hydrothermal Footprint in Siliciclastic-Volcanic Rocks above Carbonate-Hosted High-Grade Ore at the Turquoise Ridge Carlin-type Gold Deposit, Getchell District, Nevada
AuthorJohnson, Robert David
AdvisorMuntean, John L
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High-grade mineralization located within the carbonate-rich sedimentary rocks at the underground Turquoise Ridge (TR) Carlin-type gold mine has been previously described in detail by numerous geoscientists. This study lays the foundation for characterizing the expansive footprint of the Carlin-type hydrothermal system within the 1,000-2,000 feet of siliciclastic and volcanic rocks overlying the TR deposit. Two large ore controlling structural zones, the BBT and TR Corridor fault zones, can be tracked from the deposit to the surface and are the focus of this research. Petrographic and geochemical investigations were the primary means in characterizing the hydrothermal system above TR. Petrographic characterizations involved detailed logging of 16,800 feet of core and subsequent plotting of alteration, mineralization, and lithology in cross section, along with selectively sampling the core to describe the alteration and mineralization in hand sample and polished thin sections with short-wave infrared (SWIR) spectroscopy, transmitted and reflected light, and backscattered electron imaging with EDS analyses. Select ore-stage sulfides were analyzed with an electron microprobe. Geochemical characterization of the TR Carlin-type hydrothermal system above the TR ore body utilized ~125,000 samples with multi-element geochemistry, which were statistically analyzed across different elevations and hand contoured in level plans across six elevations. Hydrothermal features above the TR ore body can sometimes be ambiguous in their interpreted relationship to ore formation at depth but were categorized as being either "healthy", representing a fluid still capable of forming strong acidic alteration, widespread mineralization by sulfidation, and containing geochemistry typical of the TR ore body, or as being "exhausted" representing hydrothermal fluids ascending after ore formation at depth. Healthy hydrothermal features above TR are manifested as 1) widespread, structurally controlled argillization dominated by kaolinite and illite from the deposit to the surface (~1km vertical extent), 2) kaolinite-illite-marcasite sulfidation veinlets that are semi-planar, parallel hydrothermal fluid pathways, and correlate near perfectly with the acidic alteration footprint, 3) arsenian marcasite associated with the sulfidation of ferroan silicates or ilmenite in basaltic rocks or tuffaceous muds, often expressed as arsenian marcasite rimming remnant TiO2 from the dissolution of ilmenite, and 4) concentrations of Au, As, Hg, Sb, Te, Tl, W, and Ag consistently in the 28th percentile of available data extending from the base of the deposit to the surface. There was no clear trace geochemical fingerprint for arsenian marcasites analyzed by EMPA, possibly due to sample selection and the large area of research. However, sulfidation veinlets, regardless of As content, commonly were auriferous. Outside of carbonate-bearing rocks, Fe-rich rocks, such as basaltic volcanic rocks and mudstones with a tuffaceous component, express these hydrothermal features more overtly than the Fe-poor rocks. Hydrothermal features observed above the TRJV ore body that appear to be indicative of exhausted hydrothermal fluids due to ore formation at depth include: 1) incomplete argillization of pre-Eocene biotite with accompanying strong arsenian marcasite and Au mineralization, indicating a more neutralized hydrothermal fluid still capable of strong mineralization, 2) the presence of disseminated Eocene scheelite in acidically altered rocks along with a related W-rich halo (>380 ppm) hundreds of feet above the TR ore body, indicating hydrothermal calcium saturation due to mass dissolution of limestone at depth, 3) a loosely defined halo of dull blue montmorillonite/illite overprinted with calcite on fracture surfaces in otherwise unaltered wall rocks indicating the partial neutralization of acidic Carlin fluids, 4) cogenetic adularia, quartz, and arsenian pyrite/marcasite suggesting a more neutral ore-stage fluid, 5) the presence of arsenopyrite and native arsenic indicating a low-sulfidation state caused by a decrease in sulfur in the rising hydrothermal fluids due to ore depositing at depth, 6) relatively pure tetrahedrite, indicating low concentrations of aqueous As in the upwelling hydrothermal fluids, possibly due to widespread arsenian pyrite formation at depth, 7) banded calcite, ferroan dolomite, and ankerite plus clear quartz veins containing arsenopyrite, indicating dissolved carbonates at depth, more neutral fluids, and a lower sulfidation state of the hydrothermal fluids, and 8) minor ammonium-bearing illite in argillized volcanic rocks within the center of the BBT fault zone, suggesting dissolution of carbonaceous sediment dissolution at depth. Many Carlin-related hydrothermal features were identified above the TR deposit, and those features dwarf the footprint of economic Au mineralization and the more cryptic lateral hydrothermal footprint identified by previous researchers. Healthy hydrothermal characteristics comprise most of this large hydrothermal footprint above TR, with exhausted hydrothermal characteristics being relatively less widespread but still identifiable with specific petrographic and geochemical techniques outlined in this paper.