An XPS and SEM study of gold deposition at low temperatures on sulphide mineral surfaces: Concentration of gold by adsorption/reduction

Using X-ray photoelectron spectroscopy (XPS or ESCA) and scanning electron microscopy (SEM), we have examined the mechanism of adsorption and reduction of gold solutions on sulphides. KAu^IIICl₄ solutions are quickly adsorbed on the sulphide surfaces, and the Au(III) is quickly reduced to Au(0). Monolayer Au coverage is attained within one minute. The reduction is auto-catalyzed and Au metal grows on the surface. SEM photographs clearly show agglomerates of gold unevenly distributed on the surface. Specific sites of abnormally high Au concentration are found. We propose possible mechanisms for the adsorption and reduction. Our results suggest that adsorption of Au(III) and Au(I) by sulphide minerals, followed by reduction, could play an important role in the deposition of gold in natural systems especially at low temperature and low gold solution concentrations.