Geology and geochemistry of the Vantage gold deposits, Alligator Ridge-Bald Mountain mining district, Nevada

Geologic relationships, major element data, and isotopic geochemistry of a group of Carlin-type 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 jasperoid 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%o) and a decrease in their oxygen isotope values of up to 12 per mil (to 12.7%o) in comparison with unaltered samples. Vein calcite had even more extreme values of up to 3.1 per mil for §13C and 3.8 per mil for 5180. Sulfur isotope measurements on stibnite and orpiment varied from 5.5 to 10 per mil and from 2.5 to 12.3 per mil, respectively. Vein alunite and barite associated with intense oxidation had sulfur isotope compositions that generally varied from 3.8 to 13.4 per mil, and from 21.0 to 29.2 per mil, respectively. Barite that appeared to be sedimentary in origin yielded values of 25.9 to 47.1 per mil.

Hydrothermal fluids that formed the deposits ascended from depth along the Vantage fault. Near the level of the deposits, fluids were displaced into minor faults in the footwall limestone. At the contact between the Devils Gate Limestone and the Pilot Shale, fluids spread laterally and deposited Au, Ag, Sb, and As and quartz. Following Au deposition, hypogene oxidizing fluids using the same conduits destroyed reduced components in the Pilot Shale and produced major textural modifications. The distribution of the elements within the deposits appears to be related to (1) proximity to hydrothermal conduits, (2) composition of the host lithologies, and (3) extent of postmineralization oxidation. Oxygen isotope data of jasperoid and vein quartz indicate that the ore fluids were evolved meteoric waters. Isotopic data for epigenetic sulfides indicate that sulfur was derived from sedimentary sulfides; local stratigraphic considerations suggest that Cambrian siliciclastic rocks were the source. The deposits were geo-chemically similar to other sediment-hosted gold deposits in the Great Basin (e.g., Carlin), but they were uncomplicated by extraneous geologic features such as Paleozoic thrusts or premineralization intrusions. Thus, the Vantage deposits may be more easily understood examples of this class of ore deposits.