The influence of climate, geomorphology and primary geology on the supergene migration of gold and silver

Abstract

Abstract Redistribution of secondary gold and silver through the weathering profile is dependent on the nature and stability of anionic complexes present in fluids of the oxidized zone. Through their effect on groundwater composition, and profile development, external controls such as climate and geomorphology, as well as regional and ore geology, may influence the form and concentration of complexes and hence the mobility of gold and silver during secondary processes. Secondary gold and silver in the supergene zone of the Edna May gold mine at Westonia in Western Australia, and at the Upper Ridges mine near Wau, in Papua New Guinea, have been transported under contrasting chemical and physical conditions occurring in the extreme climate and topography of these regions. Consequent differences in the nature of secondary gold and silver enrichments have been noted. At Westonia secondary gold reprecipitated by reduction at the site of iron oxidation, is of higher fineness (lower silver content) than primary gold, due to the formation of soluble AgCl0, and AgCl2− complexes. Gold migrates as the chloride complex AuCl4−, in the acid, saline groundwaters present in the near-surface iron-rich laterite profile. At the Upper Ridges mine, repeated “flushing” by heavy rainfall in a region of rugged relief, dilutes groundwater and prohibits anion concentration and/or the development of acid conditions on a regional scale. Under neutral to basic, moderately oxidizing conditions, found in the vicinity of the weathering carbonate veins, gold and silver may be complexed by thiosulphate, to form Au(S2O3)3−2 and Ag(S2O3)3−2 or a mixed complex. Secondary gold-silver alloy is reprecipitated by oxidation in the MnO2 wad assemblage. Gold of low fineness (high silver content) is reprecipitated by reduction at the water table, as both gold and silver complexes destabilize under similar pH and Eh conditions. The identification of secondary enriched gold horizons, relative to primary ore, has important repercussions for the potential viability of a drilled deposit.