HE, ND, AND STABLE ISOTOPE CONSTRAINTS ON CARLIN-TYPE ORE FLUID COMPONENTS, GETCHELL, NV, USA

Petrographic, fluid inclusion, and isotopic studies were conducted to determine the sources of ore fluid components in a Carlin-type gold system. Petrographic studies indicate that deposition of gold-bearing pyrite was followed successively by ore-stage quartz; orpiment and galkhaite; fluorite; and late ore-stage realgar and calcite. Fluid inclusion homogenization temperatures parallel the paragenesis and indicate a temperature decline of ~ 180-220°C (quartz) to 115-155°C (calcite). Ore-stage inclusion fluids liberated by decrepitation and crushing were analyzed using quadrupole mass spectrometry and are aqueous with CO₂ reaching 4 mole %.

H and C isotopic ratios of fluids and O isotopic ratios of fluids or minerals indicate the presence of a deep-seated metamorphic or magmatic fluid in ore-stage minerals. Inclusions in some ore-stage minerals and late-ore stage calcite indicate dilution of the deep-seated fluid by an evolved meteoric fluid. Fluorite yields present-day eNd values between -13 and -16 and Nd model ages of 1750 and 1850 Ma that suggest ore fluids moved through Proterozoic crust or clastics derived from them. Ion probe analyses yielded in situ d³⁴S values for gold-bearing pyrite from -1 to 3, consistent with a magmatic S source.

He isotope ratios were analyzed from inclusion fluids extracted by in vacuo crushing. Pre-ore quartz veins from a Cretaceous pluton have ³He/⁴He of 6.6 Ra, indicating that He is mainly mantle-derived. Inclusion fluids from ore-stage orpiment (1.1 – 1.5 Ra) and fluorite (0.21 - 0.55 Ra) result from mixtures of mantle and crustal radiogenic He, although the low He content of orpiment is indicative of post-crystallization He diffusive loss. The presence of mantle-derived He in the ore fluids is consistent with coincident extensional tectonism, deep crustal degassing of a mantle-derived melt, and generation of a magmatically driven hydrothermal system.