A. R. Wallace
2003
<p><span>The mercury-gold deposits of the </span>Ivanhoe<span> mining </span>district<span> in </span>northern<span> </span>Nevada<span> formed when middle </span>Miocene<span> rhyolitic </span>volcanism<span> and high-angle </span>faulting<span> disrupted a shallow lacustrine environment. Sinter and replacement mercury deposits formed at and near the paleosurface, and disseminated gold deposits and high-grade gold-silver veins formed beneath the hot spring deposits. The lacustrine environment provided abundant meteoric water; the rhyolites heated the water; and the faults, flow units, and lakebeds provided fluid pathways for the hydrothermal fluids. A shallow </span>lake<span> began to develop in the </span>Ivanhoe<span> area about 16.5 Ma. The </span>lake<span> progressively expanded and covered the entire area with fine-grained lacustrine sediments. Lacustrine sedimentation continued to at least 14.4 Ma, and periodic fluctuations in the size and extent of the </span>lake<span> may have been responses to both climate and nearby </span>volcanism<span>. The eruption of rhyolite and andesite flows and domes periodically disrupted the lacustrine environment and produced interfingered flows and </span>lake<span> sediments. The major pulse of rhyolitic </span>volcanism<span> took place between 15.16 ± 0.05 and 14.92 ± 0.05 Ma. High-angle </span>faulting<span> began in the basement about 15.2 Ma, penetrated to and disrupted the paleosurface after 15.10 ± 0.06 Ma, and largely ceased by 14.92 ± 0.05 Ma. Ground motion related to both </span>faulting<span> and </span>volcanism<span> created debris flows and soft-sediment deformation in the lakebeds. Mercury-gold </span>mineralization<span> was coeval with rhyolite </span>volcanism<span> and high-angle </span>faulting<span>, and it took place about 15.2 to 14.9 Ma. At and near the paleosurface, hydrothermal fluids migrated through tuffaceous sediments above relatively impermeable volcanic and Paleozoic units, creating chalcedonic, cinnabar-bearing replacement bodies and sinters. Disseminated gold was deposited in sedimentary and volcanic rocks beneath the mercury deposits, although the hydrologic path between the two ore types is unclear. Higher-grade gold-silver deposits formed in massive rhyolites and Paleozoic quartzites at deeper levels, and these mineralized zones possibly represent the feeder zones for the higher-level deposits. Fluctuations in the ground-water table locally produced hydrothermal oxidation of the near-surface mercury and disseminated gold deposits. The locus of </span>mineralization<span> shifted with time, moving south and east from its inception point in the west-central part of the </span>district<span>. Thus, although </span>mineralization<span> in the </span>district<span> took place during a span of 300,000 years, the duration of </span>mineralization<span> at any one place probably was much shorter. The low-sulfidation deposits of the </span>Ivanhoe<span> </span>district<span> formed at the same time and under similar conditions as those in the nearby Midas </span>district<span>, 15 km to the northwest, which includes the large, high-grade Ken Snyder gold-silver </span>epithermal<span> vein deposit. The exposures in the </span>Ivanhoe<span> </span>district<span> are interpreted to represent the near-surface example of the paleosurface that originally was present above the Midas mineralizing system. The resulting combined </span>Ivanhoe<span>-Midas model provides an exploration guide for </span>epithermal<span> deposits in similar geologic environments in </span>northern<span> </span>Nevada<span>.</span></p>
application/pdf
10.2113/gsecongeo.98.2.409
en
Society of Economic Geologists
Geology of the Ivanhoe Hg-Au district, northern Nevada: Influence of Miocene volcanism, lakes, and active faulting on epithermal mineralization
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