Abstract The adsorption and deposition of Au on the surface of naturally occurring pyrite under ambient conditions has been studied by scanning electron microscopy (SEM), X-ray photoelectron (XPS) and Rutherford backscattering spectrometry (RBS). Gold is reduced on the pyrite surface from either Au (I) or Au (III) solutions and tends to nucleate as small spheres at microfractures on the mineral surface; each gold cluster is surrounded by a reaction halo of unknown composition. The rate of Au deposition is approximately inverse first order in Cl − and is independent of AuCl 4 − and pH. Pyrite is found to be the reducing agent for Au (III) ; it reacts with surface ferrous iron to produce Au (I) and ferric iron. The Au(I) in turn reacts with pyrite again to form metallic gold. The need for careful interpretation of the XPS binding energy data is shown; the Au 4 f 7/2 binding energy is a function of not only oxidation state but also Au particle size.