Abstract: New perspectives on the origin and distribution of Cenozoic epithermal gold-silver deposits in the northwestern United States

Epithermal Au and Ag deposits form at <1 km depth and <300° C, mostly within the central, up-flowing parts of hightemperature hydrothermal systems that develop mainly at convergent plate margins in association with arc magmatism, as well as in intra-arc, back-arc and post-collisional rift settings. Numerous schemes for classification of epithermal deposits have been proposed based on minerals and textures observable in hand specimen, and in a few cases, distinctive igneous rock affiliations. Several classification schemes that reflect acid-base and reduction-oxidation fluid-mineral equilibria distinguished acid-pH-type deposits (acid-sulphate or alunite-kaolinite) from near-neutral pH type deposits (adularia-sericite). Epithermal deposits also can be distinguished by their ore mineral assemblages in terms of their high (HS), intermediate (IS), or low (LS) sulphidation state with HS deposits corresponding to acid-sulphate deposits and IS and LS deposits corresponding to adularia-sericite deposits. In the northwestern United States, Cenozoic epithermal AuAg deposits are widespread but unevenly distributed in time and space. They are most abundant in the northern Great Basin and sparse in the Western Cascades, northern Rocky Mountains, and Columbia Plateau. In the northern Great Basin, most deposits are Miocene and younger and are spatially related to two igneous suites: (1) bimodal basalt-rhyolite and (2) western andesite. In contrast, few epithermal deposits in this region are exposed in the more voluminous Eocene to early Miocene igneous rocks of the interior andesite-rhyolite assemblage; the largest deposit is the Round Mountain LS deposit. The bimodal suite is a reduced, high-K tholeiitic series, mostly of basalt, andesite, and rhyolite, formed during continental extension. In the bimodal suite, LS Au-Ag deposits formed in two distinct settings: (1) rhyolite flow domes and (2) mafic igneous rocks of the northern Nevada rift. The LS deposits include Midas, Sleeper, Mule Canyon, Buckskin-National, and Delamar; HS and IS deposits are absent. The western andesite suite is an oxidized, high-K calc-alkaline series of andesites and dacites formed in a subduction-related continental-margin arc along western North America. HS, IS, and sparse LS Au-Ag and porphyry Cu-Au deposits are present in the western andesite suite and include Comstock Lode, Tonopah, Goldfield, Bodie, Paradise Peak, and Rawhide. Compared to epithermal deposits in the western andesite suite, LS Au-Ag deposits in the bimodal suite formed under low ƒO2 and ƒS2 and generally have low base-metal contents, low Ag:Au ratios, and common selenide minerals. Petrologic studies suggest that these differences reflect variations in the petrotectonic settings of the associated magmatic assemblages. The contrasting types of epithermal deposits and associated igneous rocks suggest that there is a strong genetic link between epithermal Au-Ag deposits and magmatism. The dearth of epithermal deposits in the interior andesite-rhyolite assemblage, including in the northern Rocky Mountains (Challis Volcanics), and in the Eocene to Pliocene Western Cascades arc in Oregon and Washington may reflect (1) preservation, (2) eruptive style or rates, (3) basement rocks, (4) tectonic setting, (5) depth of magma generation, or (6) a combination of these or other factors. These topics are the subject of ongoing research.