2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 5
Presentation Time: 9:20 AM


CLINE, J.S.1, STUART, F.M.2, HOFSTRA, A.H.3, TRETBAR, D.R.4, RICIPUTI, L.5 and PREMO, W.3, (1)Geoscience, UNLV, 4505 Maryland Parkway, Las Vegas, NV 89154, (2)Scottish Universities Environmental Research Centre, East Kilbride, United Kingdom, (3)USGS, Denver, CO, (4)Barrick Goldstrike Mines, Elko, NV, (5)Oak Ridge National Lab, Oak Ridge, TN, jcline@nevada.edu

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.