HYDROTHERMAL FLUIDS IN AND AROUND THE EOCENE HARRISON PASS PLUTON IN NEVADA: ARE THERE CLEAR LINKS TO THE GENESIS OF CARLIN-TYPE GOLD DEPOSITS?

A critical component in the formation of Carlin Type Gold Deposits (CTGD’s) in northeast Nevada has been proposed as widespread Eocene magmatism at about 10 km depth in the crust (Cline et al., 2005; Muntean et al., 2011). This magmatism either provided a source of magmatic-hydrothermal ore fluids or served as the heat engine that drove amagmatic (surface-derived or metamorphic) fluid circulation. The polyphase ~ 36 Ma Harrison Pass Pluton in the Ruby Mountains core complex is approximately along strike of the Carlin trend of deposits and has been suggested as an exposed example of Eocene granitic magma that stalled at the depth proposed. Quartz- and pegmatitic-veins in the pluton give evidence for widespread exsolution of magmatic-hydrothermal fluids from each but the earliest phase of the pluton. The bulk compositions of fluid inclusions are low salinity and generally CO₂ bearing and are hence similar to those recorded or implied for ore fluids in CTGD’s. Multi-element Laser-Ablation – ICP –MS fluid inclusion analyses show however no marked concentrations of the suite of elements enriched in the ores (As, Sb etc.). Additionally, no hypersaline fluids that have been proposed as the immiscible counterpart to low-salinity “Au-only” magmatic-hydrothermal fluids are observed in the pluton. In the country rock within about 2 km of the pluton contacts, veins show that low-salinity, CO₂-bearing fluids circulated at temperatures slightly higher than those of ore formation (200-250 °C), but there is no evidence of zones or horizons of focused fluid migration. The field and fluid inclusion evidence for fluid circulation in and around the pluton thus give evidence for aspects of fluid generation and migration processes proposed in both the magmatic and the amagmatic models for CTGD ore fluids, but give neither persuasive evidence for one or other ore fluid sources, nor distinguish between them. It is interpreted that the Harrison Pass Pluton was neither a fluid nor a heat source for the genesis of large, high-grade gold deposits.