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Geologic relationships, major element data, and isotopic geochemistry of a group of Carlintype Au deposits in the Alligator Ridge-Bald Mountain district of east-central Nevada were investigated to help constrain the origin and relative timing of Au mineralization and associated alteration. The Vantage gold deposits were the largest of 18 known sediment-hosted, disseminated gold deposits and prospects that are distributed over a strike length of 40 km. The district consists predominantly of Paleozoic carbonate and siliciclastic sedimentary rocks. Minor amounts of Tertiary volcanic and volcanic-related sedimentary rocks and a small granitic stock in the northern end of the district are also present. The intrusion and its surrounding aureole also host gold mineralization, but most gold deposits in the district are not spatially associated with intrusive rocks.The Vantage Au deposits formed at the contact between the Devonian Devils Gate Limestone and the Devonian-Mississippian Pilot Shale adjacent to a major fault zone, herein referred to as the Vantage fault. Gold mineralization was accompanied by silver, antimony, and arsenic and showed a strong preference for the Pilot Shale. Alteration related in time and space to Au mineralization was the replacement of host-rock carbonates by quartz. Replacement in the Pilot Shale closely followed the ore-waste boundary, whereas replacement in the Devils Gate Limestone occurred as a stratiform jasperold not confined to the ore zones. Quartz + kaolinite + stibnite veins were common within the central, high-grade portion of each orebody. Peripheral to silicification, diagenetic dolomite in the Pilot Shale was replaced by hydrothermal calcite, and calcite veins constituted as much as 10 percent of the rocks. Two oxidation events that destroyed organic matter and sulfides occurred after Au deposition and silicification. The early oxidation event was accompanied by the destruction of detrital illite in the Pilot Shale and deposition of alunite _ barite veins. This event was barren of Au but did redistribute metals introduced during silicification. The spatial distribution of this alteration was similar to that of the silicification. K-Ar dating of this alunite indicates an age of 11 Ma for its formation. Following a period of erosion, weathering-related oxidation overprinted much of the earlier alteration and deposited jarosite and goethite, but it did not affect metal distribution patterns or the matrix of the Pilot Shale.Whole-rock chemical analyses indicate that silica and sulfur were the only major components introduced during silicification and that very little material other than iron and organic matter was removed during the intense oxidation. Oxygen isotope compositions determined for jasperoid and vein quartz varied from 15.9 to 18.2 per mil, and from 16.1 to 20.1 per mil, respectively. Carbonate from whole-rock samples peripheral to silicification showed an increase in their carbon isotope values of up to 3 per mil (to 1.4%0) and a decrease in their oxygen isotope values ...

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Pyrite occurs in veins and as wall rock disseminations in three lithologic units inborehole CCM-2. These are, from top to bottom: (1) Creede Formation lacustrine sediments; (2) volcanic megabreccia and, (3) Snowshoe Mountain ash-flow tuff. In the Creede Formation disseminated pyrite is concentrated at the tops and bases of turbidite beds. Pyrite also forms stalactites near travertine bodies of the Creede Formation marking the paleoshoreline of the caldera lake. Pyrite, without calcite, is found over a vertical span of 52 m in fractures and replacing magnetite in wall rock in the upper portions of the volcanic megabreccia. In the Snowshoe Mountain ash-flow tuff and the base of the megabreccia, pyrite occurs in veins with calcite and as replacements of magnetite and biotite in wall rocks up to one centimeter from veins. Framboidal pyrite is common in Snowshoe Mountain veins. Calcite veins without pyrite occur in the Creede Formation whereas veins in the volcanic megabreccia and the Snowshoe Mountain tuff have both calcite and pyrite.Sulfur isotope variations from -18% to +70‰ δ 34 S CDT have been measured in situ with an infrared laser, sulfur isotope microprobe. A large range of sulfur isotope values is observed both in pyrite stalactites collected near travertine deposits on the paleoshoreline of Lake Creede and in veins from the Snowshoe Mountain tuff. Spatially resolved analyses demonstrate that sulfur isotope zoning becomes progressively enriched in 34 S with decreasing depositional age in individual veins and stalactites.Calcite veins have values of 0 to -17‰ in δ 13 C VPDB and +4% to +22‰ in δ 18 O VSMOW . Carbon and oxygen isotope values of calcite veins from the Snowshoe Mountain tuff show a broad antipathetic relationship with higher δ 13 C correlating with lower δ 18 O.Our interpretation of sulfur isotopic systematics is based on two working hypotheses. The first states that sulfur in the Creede Caldera originated as volcanic H 2 S. According to this hypothesis, pyrite precipitated directly from reactions between H 2 S and iron-bearing minerals. The alternative hypothesis considers that sulfate from volcanic eruptions was the primary source of sulfur in the Creede Caldera. The reduction of sulfate to sulfide as required for pyrite precipitation was catalyzed by enzymes of anaerobic bacteria with an accompanying kinetic sulfur isotopic fractionation. On balance, available evidence favors the latter hypothesis.Ilchik, R. P., and Rumble, D., III., 2000, Sulfur, carbon, and oxygen isotope geochemistry of pyrite and calcite from veins and sediments sampled by borehole CCM-2, Creede caldera, Colorado, in Bethke, P. M., and Hay, R. L., eds., Ancient Lake Creede: Its volcano-tectonic setting, history of sedimentation, and relation to mineralization in the Creede