MID-MIOCENE MAGMATISM AND MINERALIZATION IN THE NORTHERN GREAT BASIN AND OREGON PLATEAU: THE LINK BETWEEN BONANZA EPITHERMAL ORE DEPOSITS AND THE YELLOWSTONE HOTSPOT

The northern Great Basin (NGB) and its boundary with the Oregon Plateau (OP) are characterized by a long history of diverse geologic processes. In the mid-Miocene, these regions were affected by four concurrent geologic phenomena that are often linked to the inception of the Yellowstone hotspot: [1] regional flood basalt volcanism; [2] dominantly silicic volcanic field development; [3] focused lithospheric extension (e.g. the northern Nevada rift, Oregon-Idaho graben and related features) and [4] widespread epithermal bonanza Au-Ag mineralization. Across this region, flood basalt volcanism occurred from 16.7 to 14 Ma, with the main phase of voluminous activity occurring between 16.5 to 15.5 Ma. Regional silicic magma production appears to have been directly linked to local mafic upwelling and many of the silicic products and eruptive loci host epithermal deposits. In these magmatic systems, localized extension helped drive anatexis and enabled magmas, magmatic fluids, and magmatic volatiles (± Au, Ag, Se, As, Sb, and Hg) to ascend toward the surface. Recent geochronologic data obtained on adularia from many of the ores indicate that regional epithermal mineralization overlapped with the most voluminous mafic and silicic magmatism. Furthermore, Pb and Os isotope data from Au in the ores provide a link to the tholeiitic flood basalt magmas and indicate that the precious metals were derived from the mantle. Current work is underway in the Owyhee Mountains (ID; Silver City District) and in the Jarbidge Mountains (NV; Jarbidge District) with the goal of better understanding the interplay between local mid-Miocene magmatism and Au-Ag mineralization. Integrated field and laboratory observations are being used to provide a comprehensive view of how magmatic processes influence the formation, types, and styles of precious metal mineralization in the Silver City district, whether disparate host lithologies control the major element geochemistry of veins and style of mineralization in low sulfidation ores, and to provide additional constraints on the insufficiently studied spatial-temporal pattern and physical nature of mid-Miocene, OP-NGB mafic and silicic magmatism.