THE GENESIS OF AU-AG DEPOSITS ON FLORIDA MOUNTAIN EPITHERMAL DEPOSIT, SILVER CITY DISTRICT, IDAHO

Florida Mountain (FM) is classified as a low-sulfidation epithermal deposit, a type of metal deposit that forms from shallow hydrothermal activity and provides important global resources of Au and Ag. FM is one of a trio of similar deposits in southwestern Idaho within the Northern Great Basin and along the Yellowstone hotspot track. This study used petrography and geochemistry to elucidate the genesis of FM, a deposit that has not been the subject of extensive study. Limited preliminary research focused on high-grade veins and information from historical mining do provide a geologic framework for the district. During summer 2019, 60 samples from seven lithologic units were collected from drill core and retired open pits. Field maps of a neighboring deposit (DeLamar Mountain; DM) were also developed during that time. The geologic units at DM closely coincide with units at FM, so this mapping increased familiarity with both deposits. Petrographic analysis characterized the samples prior to geochemical analysis. Electron microprobe analysis was done to measure trace element concentrations and better evaluate mineralogical textures and relationships using energy dispersive spectrometry, wavelength dispersive spectrometry, and backscatter electron imagery. These analyses characterized the details of the geochemistry of the matrix, vein complexes, and mineralization. Important mineral phases present are electrum, silver selenide, and pyrite, which were analyzed for Au, Ag, Ti, Cu, S, Fe, As, and Se. Au and Ag were regularly seen in pyrite grains and varied across individual crystals. Laser ablation inductively coupled plasma mass spectrometry was also used to quantify lower amounts of the trace elements and confirmed variations in the concentrations of Au, Ag within individual pyrite grains correlating with different mineralogical textures and associations. Preliminary Ar-Ar geochronology on adularia crystals shows an age range consistent with the Yellowstone hotspot (15.352-15.95 Ma). These new data and observations provide insight into the source of fluids and metals as well as the number and sequence of events that resulted in the formation of the FM Au-Ag epithermal deposit. This study has important implications for the current exploration of FM and all low-sulfidation epithermal deposits.