MIDDLE MIOCENE EPITHERMAL AU-AG DEPOSITS RELATED TO CONTINENTAL THOLEIITIC BASALT MAGMATISM, NORTHERN GREAT BASIN, WESTERN USA

Numerous low-sulfidation, epithermal Au–Ag deposits are related to the early stages (ca. 16.5–15 Ma) of continental rift-related, tholeiitic, bimodal magmatism in the northern Great Basin. Although most deposits are associated with rhyolites (e.g., Sleeper, Midas), several deposits along the northern Nevada rift (NNR) are hosted by basalt and andesite lava flows (Mule Canyon, Buckhorn). The basalt-hosted deposits formed at the end of early rift-related mafic magmatism in NNW–elongated grabens filled with sediments, lakes, and mafic flows. Ores in both rhyolite and basalt have similar characteristics: low ƒO₂ and ƒS₂ mineral assemblages, low Ag/Au (£10), and enrichment in Au, Ag, As, Sb, and Se. Relative to rhyolite-hosted deposits, the basalt-hosted deposits have high Fe–sulfide contents (commonly >20 vol. %), lower Ag/Au (<3), minor silicification, and narrow and discontinuous Au–bearing siliceous veins. Stable isotope data for Mule Canyon indicate that ore fluids consisted dominantly of meteoric water that evolved by deep circulation through Paleozoic sedimentary rocks at low water-rock ratios (approx. 1). C and S isotopic data allow a magmatic component for these elements, and some C may have come from Miocene lake water. Most metals probably were leached from underlying Paleozoic sedimentary rocks. Mafic magmas of NNR are temporally equivalent and compositionally similar to the Columbia River flood basalts (CRB), and both probably are related to the Yellowstone hot spot. Epithermal Au–Ag deposits are unknown in CRB and uncommon in continental tholeiitic basalts worldwide. The formation of deposits in NNR-related mafic lavas likely resulted from: (1) deep circulation of meteoric water through dike-filled rift fractures; (2) heat from thick mafic dike complexes (3-5 km wide by 15 km deep) that drove fluid circulation; (3) shallow water-filled grabens that supplied H₂O and some CO₂; and (4) continentally-derived sedimentary sequences through which meteoric water circulated and scavenged metals. CRB has factors 1–3 but not factor 4, possibly explaining the absence of epithermal Au–Ag deposits in CRB flows. Epithermal Au–Ag deposits in mafic flow sequences similar to NNR may be present elsewhere in the Great Basin and in other continental tholeiitic basalt provinces where similar conditions were present.