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 being 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 conditions 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 that is present in different mineral associations. Micro X-ray absorption near edge structure (𝜇-XANES) data were collected on apatite at the Advanced Photon Source. Collected spectra indicate varied paleo-redox conditions of ~FMQ + 2 at the most oxidized and ~FMQ + 0.3 at the most reduced. Reduced S-XANES signatures, however, were increasingly more frequent when observed in evolved samples and textures. Swales Mountain apatite included within late-stage crystallizing phases, i.e., plagioclase, orthoclase, and quartz, tended to present reduced signatures. Some apatite also exhibit low intensity and unclear spectra owing to the fact that sulfide does not substitute into the apatite structure as readily as the sulfate ion. A regionally extensive, organic carbon-rich, deep marine shale, the Vinini Formation, is a potential reducing assimilant. When assuming a high magmatic S concentration (2000 ppm) and a low TOC of the Vinini Formation of 1 wt%, we calculate that a maximum assimilation of just ~5% is needed to achieve the observed oxygen fugacity change. Assuming lower S concentrations in the magma and higher TOC results in ~1% or less assimilation that is needed to account for the change in observed S speciation measured in apatite. We use these observations to argue for late late stage reduction of Swales Mountain plutons and to further provide evidence of reduced crustal contamination leading to the mineralization of Au in Eastern Nevada.