Geology and geochemistry of epithermal gold mineralization in the Lake Owyhee volcanic field-Western Snake River, plain region of eastern Oregon and western Idaho

The Lake Owyhee volcanic field-Western Snake River plain region of Oregon and Idaho contains over 30 epithermal gold prospects. Mineralization occurs in a number of diverse geologic settings and hydrothermal environments.;Mineralization on Indian Head Mountain, Idaho, occurs in a lateral fluid flow regime. Fluids moving laterally within a confined aquifer discharged into overlying fault breccias during periods of high fluid flow. Quartz-pyrite veins and breccias are present within the dacite flow which served as the aquifer. Anomalous Au concentrations, accompanied by elevated As, Sb, Hg, and Mo concentrations, are found in samples containing abundant pyrite. The overlying fault breccias host quartz-pyrite veins as well as quartz-chlorite/smectite and calcite veins which contain electrum. Mineralization occurred in response to mixing of hydrothermal fluids with cooler groundwaters.;Hydrothermal activity at Red Butte, Oregon, was concurrent with deposition of coarse grained fluvial sediments. This association is typical of epithermal gold prospects in the region. Examples of many of the features of a hot spring Au deposit are exposed on Red Butte, including: siliceous sinter, a blanket of argillic alteration produced by steam-heated waters, and a hydrothermal eruption crater filled with bedded breccia deposits. Electrum is found in quartz-adularia veins below the argillic alteration. Mineralization occurred both in response to fluid boiling and mixing of the boiling fluid with steam-heated waters.;Geochemical comparison of 5 epithermal gold prospects from the region reveals that, in general, there is more variation within a given prospect than there is among prospects. The Katey and Bannock prospects, which are spatially related to high-silica rhyolite domes, are enriched in Ag, Se, and Mo when compared to the other prospects. Mineralized areas containing large amounts of pyrite are enriched in As, Ag, Sb, and Mo.