MECHANISM OF GOLD TRANSFER AND DEPOSITION IN A SUPERGENE ENVIRONMENT

The possibility of gold mobility in supergene environments is suggested by numerous indirect observations of gold particles. However, presently there are no dissolution or transport data based on the chemistry of actual weathering solutions. We have investigated models of gold transport by studying the water chemistry of a stream where weathering of auriferous minerals is occurring under supergene conditions in a temperature climate. Dissolved gold concentrations in the auriferous zone are forty times higher (200 pmolL-1) than in non-auriferous zones (5 pmolL-1). We have examined the potential gold ligands: thiosulfate, hydroxide, and chloride. Speciation calculations using the field data show that gold solubility is controlled by a gold thiosulfate complex: Au(S2O3)2(3-). The oxidation of the thiosulfate ligand induces the destabilization of the aurothiosulfate complex. Gold is then reduced to the metallic state as submicronsized suspended particles. This conclusion is supported by the measured increase in gold content of suspended particulates. Mass balance calculations performed with both dissolved and particulate gold data permit the quantification of gold deposition along the stream.