Alteration, geochemistry, and paragensis of the Midas epithermal gold-silver deposit, Elko County, Nevada

The Ken Snyder mine in the Midas district is located in north-central Nevada
along the eastern margin of the northern Nevada rift. Epithermal precious metal- and
selenium-rich quartz-adularia-calcite veins formed in N- and NW-trending faults at
15.4 Ma, roughly coeval with rhyolitic magmatism. Patterns of alteration reflect
effects of increasing temperature and water:rock reactions towards the Colorado
Grande and Gold Crown veins and with time. Distal weak propylitic alteration (calcitechlorite)
gives way to moderate propylitic alteration (chlorite-calcite-pyrite-smectite),
and increased wall rock 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).
Haloes of potassic alteration (adularia-chlorite-pyrite/marcasite-smectite/illitequartz)
extend 30 m from veins. Fe-rich chlorite and increased abundances of pyrite
occur closer to the main veins, and adularia, illite, and kaolinite are present in wall
rocks. Formation of banded veins coincided with potassic alteration. High-grade crustiform
veins are composed of alternating metal-rich and quartz-calcite-adularia-rich
bands. The principal metal-bearing phases include naumannite, pyrite, chalcopyrite,
electrum, sphalerite, and galena. Pathfinder elements typical of epithermal systems,
including Hg, As, and Sb, as well as K form haloes above ore-grade veins. Base metals
show no significant enrichment over the vertical interval studied.