Epithermal gold-siver deposits in the western United States: time-space products of evolving plutonic, volcanic and tectonic environments

The western United States has been the locus of considerable subaerial volcanic and plutonic igneous activity since the mid-Mesozoic. After the destruction of the Jurassic-Cretaceous magmatic arc-trench system, subduction was re-established in the Late Mesozoic with low-angle underthrusting of the oceanic plate beneath western North America. This resulted in crustal shortening during the Late Cretaceous to Early Tertiary and removal of the mantle lithosphere west of the Rocky Mountains. Commencing in the Eocene, flat subduction ceased, the volcanic arc began to re-establish itself along the continental margin, and the hingeline along the steepening subducting plate migrated from east to west. The crust east of the migrating hingeline was exposed to hot asthenosphere, and widespread tectonics and volcanic activity resulted. Hydrothermal activity accompanied the volcanism resulting in numerous epithermal gold-silver deposits. The temporal and spatial distributions of epithermal deposits in the region are therefore systematic and can be subdivided into discrete time intervals which are related to widespread changes in magmatic activity. Time intervals selected for discussion are Pre-Cenozoic, 66-55 Ma, 54-43 Ma, 42-34 Ma, 33-24 Ma, 23-17 Ma, and <17 Ma. Many of these intervals contain both sedimentary-rock and two varieties of volcanic-rock hosted deposits (adularia-sericite and alunite-kaolinite ± pyrophyllite). Continental rifting is important to the formation of deposits, and, within any given region, it is at the initiation of deep rifting that alunite-kaolinite ± pyrophyllite type epithermal deposits are formed. Adularia-sericite type deposits are most common, being related to all compositions and styles of volcanic activity. Therefore, the volcano-tectonic context of the western United States provides a unified framework in which to understand and explore for epithermal type deposits.