REDOX EVOLUTION OF EOCENE GREAT BASIN MAGMAS: IMPLICATIONS FOR AU MINERALIZATION IN NEVADA

Eocene arc magmatism is recognized to be responsible for the great quantities of porphyry Cu and Carlin-type Au mineralization in the Great Basin. However, an enigmatic spatial discontinuity exists in the metallogenic character of these ore deposits with Au and Cu predominantly found in eastern Nevada and western Utah respectively. This east-west variability can be explained by diverging magma-fluid evolutionary paths with reduced and oxidized end members producing the mineralization observed in eastern Nevada and western Utah. The agent of such divergence is hypothesized to be contamination via crustal material of markedly different redox states in the two regions. Here, we add to a basin-wide analysis of redox conditions of Eocene plutons associated with Au and Cu mineralization by analyzing sulfur speciation in Swales Mountain intrusive apatite inclusions present within different mineral hosts using micro x-ray absorption near edge structure (𝜇-XANES). Collected spectra indicate varied paleo-redox conditions of FMQ + 2 at the most oxidized, and FMQ + .3 at the most reduced. Notably, 𝜇-XANES spectra varied systematically depending upon the identity of the hosting mineral phase. Swales Mountain apatite included within minerals considered to reflect advanced magmatic differentiation (i.e., albite, orthoclase, quartz, iron oxide) tend to present more reduced signatures. The Vinini Formation, a regionally extensive, organic carbon-rich, deep marine shale, is a potential reducing assimilant. In a mass balance framework, if we conservatively assume a high magmatic S concentration (2000 ppm) and a low total organic carbon (TOC) content of the Vinini Formation of 1 wt%, we calculate a maximum assimilation of just ~5% is needed to achieve the observed oxygen fugacity change. Assuming lower magmatic S concentrations and higher TOC, we calculate ~1% or less assimilation, indicating only an otherwise geochemically invisible amount of reduced material is needed to produce the observed oxygen fugacity change. Through these results, we argue for late stage reduction of Swales Mountain plutons and provide further evidence of reduced crustal contamination leading to the mineralization of Au in Eastern Nevada.