Observations on the behavior of gold during supergene oxidation at Summitville, Colorado, U.S.A., and implications for electrum stability in the weathering environment

Abstract

The textural relations, morphology, and composition of gold grains in both oxidized and unoxidized ore from the Summitville, Colorado Au Cu Ag deposit have been investigated in this study. Gold from oxidized Summitville ore is finer grained than that from unoxidized ore, and occurs exclusively in intimate intergrowths with goethite of presumed supergene origin. These oxide-gold grains show no evidence of silver depletion and are enriched in silver relative to primary gold in a few cases. This trend can be explained using the stability relations of silver in electrum at 25°C. At this temperature, electrum with up to 20 wt % Ag may be relatively stable in the weathering environment, provided that chloride contents are sufficiently low in the coexisting aqueous phase. This is in accord with the maximum silver values observed in gold grains in the oxidized zone of the Summitville deposit, and with the negligible chloride content of present mine drainage. The absence of chloride that is implied by these electrum compositions effectively rules out gold-chloride complexes as an explanation for the local remobilization of gold in the oxidized zone at Summitville. It strongly suggests that gold-chloride complexes are not necessary to transport gold, at least on a local scale, in the acidic environment that characterizes many weathering sulfide ore bodies. Local gold mobilization in this environment is believed to reflect the formation of transient gold-thiosulfate complexes.