Using Petrographic and Geochemical Analyses to Elucidate the Genesis of Au-Ag Epithermal Deposits on Florida Mountain, Silver City District, Idaho

Abstract

Florida Mountain (FM) is classified as a low-sulfidation epithermal deposit and has a rich history of Au-Ag economic mining. It 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 Florida Mountain, a deposit that has not been the subject of extensive study. Previous research focused on high-grade veins and information from historical mining to 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. Samples were characterized by petrographic analysis prior to geochemical analyses. Electron microprobe analysis was used to measure trace element concentrations in sulfide minerals associated with high metal grades, and to better evaluate mineralogical textures and relationships. Important mineral phases identified include electrum, silver selenide, and pyrite, which were analyzed for their Au, Ag, Ti, Cu, S, Fe, As, and Se content. Elevated Au and Ag concentrations were measured 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 (e.g., rim vs. core) correlating with different mineralogical textures and associations. Preliminary Ar-Ar geochronology on adularia crystals from FM samples determined 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 sequence of events that resulted in the formation of the FM Au-Ag epithermal deposit.