FORMATION, PRESERVATION, AND DESTRUCTION OF MIOCENE PALEOSURFACE-RELATED EPITHERMAL DEPOSITS, NORTHERN NEVADA

Middle Miocene extensional faulting, bimodal basalt-rhyolite volcanism, and lakes combined to form numerous near-surface, low-sulfidation epithermal mineral deposits across northern Nevada, including the Ivanhoe Hg-Au deposits in SW Elko County. There, fine-grained sediments were deposited in a shallow, at times ephemeral, lake between about 16.5 Ma and at least 14.4 Ma. Rhyolite domes and andesite flows erupted around the lake margins between about 16 and 14.9 Ma, with peak rhyolite volcanism between 15.2 and 14.9 Ma. High-angle extensional faulting began at ~15.2 Ma and began to wane by 14.9 Ma. Hydrothermal activity began at ~15.2 Ma and continued for about 250,000-300,000 years, coeval with rhyolite volcanism, faulting, and sedimentation. Hydrothermal fluids formed Hg-Au sinter and near-surface replacement deposits and underlying high-grade quartz-adularia-Au vein deposits.

Various combinations of late-Cenozoic, post-mineralization volcanism, sedimentation, and uplift-related erosion variably concealed, preserved, and (or) destroyed near-surface epithermal deposits in different subregions of northern Nevada. Concealment of deposits by volcanic units and (or) sediments, in some places almost immediately after mineralization, was most common in the northwesternmost and northeastern parts of Nevada. Late Miocene and younger (~7-0 Ma) extension-related uplift and resultant erosion in other subregions (northwestern, north-central, and eastern Nevada) removed some to all of the middle Miocene near-surface epithermal mineral deposits in those areas, although epithermal mineralization may not have taken place in easternmost Nevada. Thus, the present distribution of exposed middle Miocene epithermal deposits in northern Nevada likely does not reflect the original full distribution of mineralizing centers. Mineral exploration for epithermal deposits should consider the effects of regionally variable post-mineralization processes on the preservation, concealment, and destruction of the deposits.