PETROLOGY AND MAGMATIC EVOLUTION OF THE SILVER CITY DISTRICT, OWYHEE MOUNTAINS (ID)

Previous studies of the northern Great Basin have indicated that mid-Miocene epithermal Au-Ag ore deposits distributed regionally are temporally related to the magmatic activity associated with the onset of widespread extension and the Yellowstone hotspot. This study is focused on the volcanic rocks and ore deposits from the Silver City district (SCD) to address the petrogenesis and magmatic evolution that was influential in forming local precious metal deposits. Furthermore, our goal is to understand the tectonomagmatic conditions that contributed to the mineralization in numerous Au-Ag bearing veins in the SCD, as well as provide details on the relationship between coeval mid-Miocene magmatism and mineralization across the northern Great Basin and Oregon Plateau. In order to better constrain the magmatic evolution of the SCD and potential sources of the precious metals, we have undertaken detailed sampling of local volcanic units to constrain their physical, geochemical, isotopic, and geochronological characteristics. Prior studies of the local volcanism have yielded K-Ar and ⁴⁰Ar/³⁹Ar ages of ~16.6 to 14 Ma, while Hames et al. (2009) dated adularia from one SCD mineral vein and obtained ⁴⁰Ar/³⁹Ar ages of between 15.6 and 16.3 Ma. Preliminary observations are consistent with earlier work (Pansze, 1975; Bonnichsen and Godchaux, 2006; Camp and Ross, 2009) and reveal a sequence of basalt consisting of regionally prevalent Steens/Imnaha Basalt that apparently pre-dated precious metal mineralization. Some of the basalt appears to have been erupted locally, based on the presence of mafic dikes, similar to other regional mid-Miocene magmatic systems. Stratigraphically overlying this lower basalt suite is a complex package of rhyolite flows and domes, thin silicic pyroclastic units, additional basalts, and mafic/silicic shallow intrusives. Despite the confidence in mapping and field relationships in and around the SCD, the timing and petrogenesis of local intermediate and silicic magmatism is inadequately understood and needed to clarify the relationship between local mafic upwelling, concurrent extension, and SCD epithermal mineralization. Work to resolve this is currently underway; these data will be used to help place constraints on the temporal and mass fluxes associated with SCD mineralization.