OXYGEN ISOTOPE CONSTRAINTS ON MID-MIOCENE SILICIC MAGMA PRODUCTION IN THE OWYHEE MOUNTAINS (ID)

As part of an effort to better understand the relationship between mid-Miocene magmatism and epithermal Au-Ag mineralization across the northern Great Basin (NGB), we have obtained O isotope data from igneous rocks in the Silver City district (SCD), Owyhee Mountains, ID. Ongoing work in the SCD has been focused on the petrogenesis and eruptive styles of mid-Miocene rhyolites. To constrain their petrogenesis, a representative suite of upper crust (e.g. Cretaceous Silver City granite, part of the ID batholith; Kolb et al., 2010) have also been collected and analyzed for their bulk chemistry and radiogenic isotope characteristics. The purpose of this particular study is to specifically examine the O isotope characteristics of SCD samples, in an effort to fully understand the petrogenesis of SCD rhyolites, their relationship to the local crust, and also any hydrothermal processes that have affected these rocks. Quartz and feldspar phenocrysts were separated from 15 samples (10 rhyolites and five granitoids) and analyzed. Granitoid feldspars yield values between 3.86-10.01‰ and quartz ranges from 9.78-12.19‰. SCD rhyolites have feldspar values between 2.13-9.93‰ and quartz between 7.84-10.97‰. Low feldspar values in the rhyolites appear to reflect interaction with post-eruption hydrothermal fluids (δ¹⁸O values = -6.5 to -9.7‰; Cupp, 1989) related to 15.5 Ma Au-Ag mineralization. Mineral veins cut the granitoid exposures, portions of some rhyolites are pervasively mineralized, and the proximity of SCD rhyolites that contain the most strongly altered feldspar phenocrysts to mineralization, support this conclusion. Overall, the relationships between granitoid and rhyolite O values are consistent with radiogenic isotope data and indicate that at least some of the rhyolites were derived via melting of the Silver City Granite. Conversely, the mid-Miocene mineralization lowered the δ¹⁸O feldspar values of the SCD rhyolites and presumably, their δ¹⁸O whole rock compositions.