MIOCENE MAGMATISM IN THE SILVER CITY DISTRICT, OWYHEE MOUNTAINS, IDAHO (USA): ORIGIN AND IMPLICATIONS FOR FLOOD BASALT MAGMATISM LINKED TO THE INCEPTION OF THE YELLOWSTONE HOTSPOT AND Au-Ag EPITHERMAL PRECIOUS METAL MINERALIZATION

The inception of the Yellowstone hotspot is associated with a period of areally extensive mid-Miocene volcanism that is associated with coeval bonanza epithermal Au-Ag mineralization. This relationship, between magmatism and mineralization, exists in the Silver City district, Owyhee Mountains ID (SCD), which is also the “birthplace” of modern studies of low-sulfidation Au-Ag epithermal ores. To better constrain the evolution of the SCD and work focused on sources of the SCD precious metals, we performed detailed sampling of local crust and mid-Miocene igneous units to constrain their physical, geochemical, isotopic, and geochronological characteristics, as well as provide constraints on the petrogenesis of the mid-Miocene igneous package. New ⁴⁰Ar/³⁹Ar ages show that SCD magmatism occurred from ~16.7 to ~14.5 Ma, overlapping with Au-Ag mineralization, ca. 15.7 – 15.4 Ma. The SCD mid-Miocene magmatic suite is compositionally diverse and ranges from Steens Basalt (e.g., Columbia River basalt group) through high-Si rhyolite. Steens lavas erupted locally, based on the presence of dikes, intrusives, and pyroclastic deposits. Stratigraphically overlying this lower basalt suite is a complex package of rhyolite lavas/domes, felsic pyroclastic units, additional basaltic lavas, intermediate lavas, and mafic/felsic shallow intrusives. We identified two intermediate units and six locally erupted felsic units (Hayden Peak latite, quartz latite, Glass Mountain dacite, Glass Mountain rhyolite, Silver City rhyolite and the tuff of Flint Creek) based on petrography and bulk rock geochemistry. Petrologic modelling and Sr-Nd-Pb-O isotope data shows that SCD felsic units formed via melting of Cretaceous granitoid upper crust, while the intermediate units formed via mixing of mafic and felsic units, plus crustal assimilation. SCD rhyolites are compositionally, more like other mid-Miocene rhyolites that formed via melting of quartzofeldspathic upper-crust (e.g., Jarbidge Rhyolite, Santa Rosa-Calico volcanic field rhyolites), than to younger, Snake River plain rhyolites that contain a greater mantle component. The SCD provides an archetypal example of how a diverse suite of magmas can form and erupt in a continental flood basalt province, as well as how magmatism is linked to local precious metal mineralization.