UC San Diego

Gold is a noble metal that has fueled mineral resource exploration in the Northern Great Basin and profoundly affected the national economy. The origin of the gold deposits has been at the center of basic and applied research, with the goal to develop refined exploration models. For the first time, I am able to unambiguously constrain the nature of electrum from a low-sulfidation epithermal deposit in the Northern Nevada Rift. In this study, the highly siderophile elements (HSE; Os, Ir, Ru, Rh, Pt, Pd, Re and Au) serve as an innovative tracing tool due to the inherent relationship between the HSE and Au that can be used to trace the origin of electrum directly. Coupled precise 187Os/188Os compositions and isotope dilution HSE abundance data are reported for electrum grains from the Fire Creek Mine, as well as for a suite of host rocks that encompass the bimodal basalt-rhyolite volcanism of northern Nevada. The mid-Miocene bimodal host-rocks have Os isotope compositions consistent with crustal contamination observed in the Columbia River Flood Basalt (CRFB) province. The host rocks are isotopically similar to the CRFB Wannapum, Grande Ronde, and Imnaha basalt formations. Highly siderophile element concentrations are heavily fractionated for the host rocks but are less fractionated for the electrum. Rhenium-Osmium isotope analysis of electrum yields a relatively unradiogenic initial 187Os/188Os of 0.1588, demonstrating a source distinct from the crust and that can only be of mantle origin. From this, I estimate that as much as 96% of the gold in the Fire Creek deposit is of mantle origin, in contrast to evidence for much larger crustal contributions from proxy measurements.