GEOLOGY AND GEOCHEMISTRY OF THE VANTAGE GOLD DEPOSITS, ALLIGATOR RIDGE-BALD MOUNTAIN MINING DISTRICT, NEVADA

Geological 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 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. 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. 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. -from Author