PARAGENESIS AND DISTRIBUTION OF TRACE ELEMENTS, REES, AND STABLE ISOTOPES OF EPITHERMAL AU-AG VEINS, KEN SNYDER MINE, MIDAS, NEVADA

Epithermal Au-Ag veins at the Ken Snyder Mine (Midas) in north-central Nevada occur in a basalt-rhyolite sequence within the mid-Miocene northern Nevada rift. A study of paragenesis and geochemistry provides evidence for sources and pathways of hydrothermal fluids. Of six general stages of formation, Stage II crustiform, colloform veins, the main ore stage, are composed of dark, sulfide- and selenide-rich bands alternating with light, quartz-calcite-adularia-rich bands. The principal metal-bearing phases include naumannite, chalcopyrite, electrum, pyrite, sphalerite, and galena, with minor aguilarite, and marcasite. Precious-metal-rich bands are commonly associated with fine-grained adularia mixed with mosaic quartz, evidence for deposition due to boiling. Bladed and massive calcite is common in most stages with replacement by quartz in later stages. Stage II is enriched in precious metals, selenium, and base metals. Typical pathfinder elements, including Hg and As, show the greatest enrichment in the latest stages of veining, with variable increases during middle stages.

Comparison of chondrite-normalized REE patterns of Midas veins to those of host rocks indicates that most stages of veins do not exhibit patterns similar to those of host rocks. Stages I, II, IV, and V show similarities to coeval glassy rhyolite dikes and may point to a magmatic contribution to the veins of REEs and possibly precious metals. Stage III veins, which formed during widespread brecciation, contain higher concentrations of REEs, immobile elements, and patterns similar to those of host rocks due to lithic fragments. In Stage VI the patterns may be due to increased abundances of calcite.

Stable isotope data for carbonates suggest that veins were deposited from a parent fluid with a composition of d13C = -4‰, and d18O = -5‰. A decrease in d13C in calcite from Stage I to II may have resulted from a decrease in pH of the parent fluid as the system developed and the buffering capacity of wall rocks along the flow path decreased, or from an influx of a different parent fluid. The similarity in REE patterns for Stages I and II suggests that they were derived from the same fluid source, evidence of a constant parent fluid and change in pH.