Gold deposition at low temperature on sulphide minerals

Abstract

Gold deposits are commonly associated with quartz veins containing variable amounts of pyrite and other sulphides. Gold is thought to be transported as AuCl⁻₂ or AuCl⁻₄ in strongly acidic and saline solutions and as AuS⁻ or Au(S₂O₃)³⁻₂ in weakly acidic and alkaline solutions by hydrothermal solutions coming from great depth and rising in rock formations through faults^1,2. In some cases gold occurs as finely divided particles, intimately intergrown with pyrite, suggesting that gold, quartz and pyrite were probably precipitated in response to a change in temperature. Fyfe and Henley³ showed that the solubility of gold increases from 10 p.p.m. at 300 °C to 1,000 p.p.m. at 510 °C. In other cases gold may be aggregated in one spot in abundance while the remainder of the ore is barren, indicating preferential precipitation in one site. Using ESCA (electron spectroscopy for chemical analysis), we have studied the reaction between KAuCl₄ in solution and various sulphide minerals. Gold(III) was rapidly reduced to metallic gold on the surface of these minerals, suggesting a new mechanism for the deposition of gold at low solution concentration and temperature.