Midas epithermal gold-silver deposit, Ken Snyder mine, Elko County, Nevada

The Midas gold deposit is the largest known high-grade Au-Ag bonanza vein system located along the northern Nevada rift. Formed in N- and NW-trending faults that dissect bimodal basalt-rhyolite volcanic rocks, the Colorado Grande and Gold Crown veins comprise the bulk of ore at the Ken Snyder Mine (Midas). New 40Ar/39Ar dates for volcanic rocks and veins and additional isotopic dates throughout the region show that the Midas hydrothermal system developed during felsic volcanism, folding, and faulting beginning about 15.6 Ma. High-grade veins formed in fault zones throughout the region depositing at least 4 million ounces of gold and 40 million ounces of silver at 15.4 Ma. The age of unaltered tuff that unconformably overlies opalized sediments establishes that tilting of units and the hydrothermal system had ceased by 15.2 Ma. The temporal and spatial coincidence of rhyolite volcanism, faulting, and high-grade mineralization may reflect the importance of contributions from deeper fluid reservoirs containing magmatic components or highly exchanged meteoric waters.; Patterns of alteration reflect effects of increasing temperature and water:rock interactions towards the Colorado Grande and Gold Crown veins and with time. Distal weak propylitic alteration (calcite-chlorite) gives way to moderate propylitic alteration (chlorite-calcite-pyrite-smectite) and increased replacement and veining closer to the main veins. An epidote isograd (first appearance of epidote) forms the contact between moderate and intense propylitic alteration (chlorite-pyrite-epidote-smectite-albite +/- prehnite +/- adularia) and drapes over the paleo-upflow zone. Haloes of potassic alteration (adularia-chlorite-pyrite/marcasite-smectite/illite-quartz) extend 30 m from veins. Formation of Au-rich-veins overlapped silicic-potassic alteration.; Ore (Stage II veins) is composed of metal-rich quartz-calcite-adularia-rich veins. Principal metal-bearing phases include naumannite, chalcopyrite, electrum, pyrite, sphalerite, and galena. Anomalous Hg, As, and Sb occur above ore-grade veins. Antimony is most enriched in middle stages of vein formation, and Hg and As in latest. Stable isotopes from calcite in veins suggest that buffering capacity of rocks along flow paths decreased with continued fluid flow, decreasing pH of fluids after Stage I. Positive Eu anomalies in most stages of veins may reflect a deep source region for the precious-metal-bearing fluids, and provide an important tool for identification of paleoupflow zones.