The Relationship Between Eocene Magmatism and Gold Mineralization in the Great Basin, USA: Insights from the Phoenix-Fortitude Porphyry-Skarn System and Regional Intrusions Associated with Mineralization

Abstract

Late Eocene (~42-34 Ma) ore deposits of the Great Basin, U.S.A. have contributed the majority of the region’s precious- and base-metal production. Mineralization is strongly partitioned, with Au-rich systems in the west in Nevada, and Cu-rich systems to the east in Utah, however, both share a strong spatial and temporal link to the broad southwestward sweep of Eocene subduction-related, high-K magmatism. Coupled with the bimodal metal distribution is a distinct shift in the Paleozoic passive margin from relatively oxidized subaerial to platform carbonate units in the east, to up to 10 km thick reduced carbonaceous slope and basin units in the west. The character of mineralization associated with Eocene magmas mirrors this change in crustal redox, with dominantly oxidized porphyry Cu, Ag-Pb-Zn carbonate-replacement, and high-sulfidation Au-Cu epithermal systems in Utah, and reduced, intrusion related Au (Cu-Ag-Bi-Te), Au-Ag-Pb-Zn carbonate replacement, and various styles of Au-rich Carlin-type mineralization. Further, Eocene magmas have been shown isotopically to have interacted with significant volumes of reduced crust in Nevada, potentially impacting the magmatic redox state, which has profound control on Au/Cu in magmas and subsequent fluids. Here, this magmatic redox hypothesis is tested by: detailed field and analytical studies of the Phoenix-Fortitude reduced, Au-rich porphyry-skarn and redox evolution of the associated intrusive complex in Nevada, and analytical studies of 21 regional intrusions associated with mineralization throughout the Great Basin to understand province-scale variability in redox and volatile contents. Detailed study of the Phoenix-Fortitude system has revealed a time-space sequence of early, more oxidizing P1 porphyry intrusions (~fayalite-magnetite +quartz, FMQ -1 to +1; Ce4+/CeTotal dominantly >90% determined from zircon) associated with Au-Cu pyrrhotite-chalcopyrite mineralized garnet-pyroxene skarn in carbonate rocks and quartz-chalcopyrite-pyrrhotite veinlets and biotite-K-feldspar alteration in aluminosilicate units. A second, later pulse of more evolved and reduced P2 porphyry intrusions (dominantly <FMQ; Ce4+/CeTotal <90%) associated with widespread Au-As-Sb-Bi-Te -rich pyrite-arsenopyrite mineralized actinolite-chlorite skarn in carbonate rocks and quartz-sericite-pyrite-arsenopyrite veins in aluminosilicate rocks. This new understanding of alteration and mineralization was coupled with a novel application of hyperspectral imaging to production blast hole cuttings at the Phoenix Mine to derive geometallurgical models capable of optimizing mill recovery and throughput. Hyperspectral data were also useful for defining detailed bench-scale alteration zoning for improved exploration targeting.To understand the regional variation of magmatic redox and volatiles, apatite and zircon from 21 regional Eocene, Eocene-early Oligocene, and one unexpected Miocene intrusion associated with mineralization were studied in detail. Analyses of Ce speciation (Ce3+/Ce4+) and determination of magmatic oxidation state from igneous zircon show initially oxidized, mantle-derived arc magmas (FMQ +1-2) were systematically reduced to ~FMQ -1 as they evolved in Nevada, whereas magmas in Utah retained, and in some cases increased their oxidation state (~FMQ +2-3). Nevada intrusions exhibit a wide range of 0-100% Ce4+/CeTotal, while Utah magmas are dominantly >70% with substantially more 100% Ce4+/CeTotal measured in zircon. In conjunction, igneous apatite in magmas from Utah is shown to be more enriched in S, dominantly 0.1-1 wt. % SO3, than in Nevada where apatite is dominantly <0.1 wt. % SO3 – likely a function of the elevated magmatic oxidation state in Utah and coupled increase in S solubility due to the transition from S2- to S6+ dominance at ~FMQ+0.5. We conclude the assimilation of reduced, carbonaceous sedimentary units in Nevada is a fundamental control on the Au-rich metallogeny of Nevada due to the modification of magmatic redox conditions to optimal levels (~FMQ) for the highest melt solubility and extraction efficiency of Au and reduced S into fluids. We propose continental arcs emplaced into thick slope and basinal sedimentary sequences are a necessary component to form reduced, Au-rich, Cu-poor intrusion-related deposits. Due to the abundance of reduced S in magmatic-hydrothermal fluids and the buffering capacity of wallrock, these geologic settings are ideal for the transport of Au to low temperatures typical of Carlin-type gold deposits and should be targeted in regional exploration programs for new Carlin-type districts.