THE BULLFROG MINE, NEVADA - GOLD, BITUMEN, AND FLUID EVOLUTION

The Bullfrog Mine produced 2.4 million ounces of gold from a 10-Ma low-sulfidation vein deposit in highly extended 11- to 15-Ma volcanic rocks. Although modestly sized for Nevada, the deposit is interesting because it generally has very low base and toxic metal contents, gold and silver correlates well with beryllium, and some veins contain bitumen. The low salinity hydrothermal system produced at least ten vein types ranging from early chlorite-pyrite-bitumen to late comb quartz. Based on fluid inclusion and bitumen reflectivity data, the system cooled from initial peak temperatures of about 300° C to below 200° C. Vein textures indicate that early fluids included live oil, probably from a local source. The precious metals were deposited during a relatively late vein stage. Calculated boiling depths indicate that if boiling was responsible for its deposition, most of the gold was deposited at a temperature of about 250° C. Precious metal deposition mostly accompanied a late temperature spike that corresponded with the presence of CO2; adularia; and slight increases in fluid salinity and äO. Although base metals are generally low, the ore mineral, electrum, was deposited with galena, chalcopyrite, and stromeyerite. Local occurrences of extremely high-grade ore, which contains as much as 1% gold by weight, also contain anomalously high amounts of metals such as tellurium. These data are consistent with a late incursion of magmatic or highly exchanged water during the evolution of the system.