An XPS study of gold deposition at low temperatures on sulphide minerals: Reducing agents

The reduction of KAuCl ₄ to metallic gold by pyrite, high iron content sphalerite and galena was studied using surface analytical and solution techniques, including X-ray photoelectron spectroscopy (XPS) and atomic absorption spectroscopy (A.A.). High resolution XPS of the reacted mineral showed that the mineral surface is the Au reducing agent. On galena and high iron sphalerite, a corroded layer forms rapidly as S ^2- is oxidized to polysulphides, S _x^2-, and the metals are leached from the surface. Although Au is also reduced on pyrite, the formation of surface polysulphide is not concurrent with Au reduction. Solution analysis for the pyrite and high iron sphalerite reactions shows, however, that considerable sulphate is produced due to the oxidation of S ^2-, S ₂^2- or the intermediate polysulphide.