PETROGRAPHIC AND GEOCHEMICAL CHARACTERIZATION OF GRANITOID AND METAMORPHIC BASEMENT ROCKS, OWYHEE MOUNTAINS, ID

Research is currently being conducted on the ~17-14 Ma magmatism and precious metal mineralization in the Silver City district of the Owyhee Mountains, ID to unravel their relationship. Crucial to this research is an understanding of the basement rock that lies under much of the mid-Miocene volcanic pile. In addition to hosting Au-Ag bearing mineral veins, this dominantly granitoid basement could potentially be the source of some of the silicic magmas that make up a dominant component of the mid-Miocene volcanic suite. Previous work on the granitoids adjacent to the Silver City district (Silver City Granite; SCG) have found it to be a ~66-62 Ma biotite-muscovite granodiorite, that locally grades to quartz monzonite and granite (Pansze, 1975). Additionally, the SCG has been has been tentatively classified as the southernmost portion of the Idaho batholith (Pansze, 1975; Benford et al. 2010). Also locally present are sparse and very small (<1 km²) metamorphic pendants within the SCG that are most commonly quartz-biotite-muscovite schists, but also include some sparse gneiss and quartzite. To further understand the SCG and these metamorphic rocks, we are performing field, petrographic, geochemical, and isotopic studies, with a focus on exposures in and adjacent to the Silver City district. Preliminary petrographic studies are consistent with prior work and indicated that the dominant minerals in the SCG are plagioclase, quartz, orthoclase, microcline, muscovite, and biotite. These are medium to coarse grained and are mostly equigranular, with occasional larger phenocrysts. Most of the samples are very similar to one another and are classified as biotite-muscovite granodiorite. Thus far, SCG samples analyzed for major and trace element geochemistry range from ~69-74 wt% SiO₂ and have trace element characteristics consistent with a subduction origin (e.g. LILE enrichment and Nb depletion). Additional geochemical analyses are underway and will be coupled with Sr-Nd-Pb isotope data to fully evaluate whether melting (and/or assimilation) of the SCG contributed to local mid-Miocene silicic magmatism. Furthermore, these data should also help establish whether the SCG is truly a member of the Idaho batholith.