SUBDUCTION RELATED VOLATILITY OF METAL(LOID)S, FERTILE LITHOSPHERE, AND THE IMPLICATIONS FOR MID-MIOCENE EPITHERMAL AU-AG DEPOSITS IN THE WESTERN UNITED STATES (Invited Presentation)

Recent research dealing with the links between prior subduction and precious metal mineralization have provided constraints on the larger-scale issues related to how metal(loid)s behave during subduction and what processes allow them to reside in the subduction-affected mantle wedge (e.g. Richards: Geology, 2009; Richards: Ore Geol. Rev., 2011; Saunders and Brueseke: Econ. Geol., 2012). These studies focus on enrichment of volatile elements (e.g. Au, Ag, Cu, Se, Te, etc.) by subduction processes (e.g. devolitilization, magmatism, and crystal accumulation) in and adjacent to the mantle wedge. The end result of these subduction processes is fertile mantle (asthenosphere or lithosphere) that can be the source of economically significant metals. For example, Pettke et al. (Earth Planet. Sci. Lett., 2010) and Muntean et al. (Nature Geosci: 2011) both invoke a lithospheric source for the metals in deposits at Bingham (UT) and in the Carlin-trend (NV) respectively; the long history of subduction in western North America provided a mechanism for endowing the underlying mantle in metals. Along with the older ores mentioned above, the Great Basin (U.S.A.) also hosts a suite of bonanza Au-Ag epithermal deposits that are spatially/coevally associated with mid-Miocene flood basalt volcanism. To constrain the sources of metals in these deposits we have undertaken a detailed study of the Silver City mining district (SCD), Owyhee Mountains, ID. New ⁴⁰Ar/³⁹Ar geochronology demonstrates that SCD ores and magmatism overlap temporally with regional deposits and volcanic systems (e.g. Santa Rosa-Calico, McDermitt volcanic field, etc.). Furthermore, Pb isotopic data from coeval ores, gangue, volcanic rocks, and Mesozoic crust suggest that the source of the precious metals is not crustal. These Pb isotope results combined with regional Pb data from ores (Kamenov et al.: Econ. Geol., 2007 and this study) imply a mantle source for the metals (e.g. Precambrian North American lithosphere and potentially the upwelling Yellowstone plume), consistent with the models of subduction processes discussed above. Thus, this study provides an example of how subsequent magmatism (e.g. mantle upwelling associated with the Yellowstone hotspot) can “liberate” metal(loids) from fertile mantle lithosphere previously affected by subduction.