Invisible gold: Comparison of Au deposition on pyrite and arsenopyrite

X-ray photoelectron spectroscopy (XPS), field emission scanning electron mi croscopy (FESEM), and open circuit potentials were used to compare the size , chemical state, and distribution of adsorbed and reduced gold from Au3+ c hloride solution on pyrite and arsenopyrite. Many small AuO particles grow on the arsenopyrite surface, whereas few, much larger, gold particles appea r on pyrite. These results mimic the differences in distribution of gold in some coexisting natural pyrites and arsenopyrites. The rate-limiting step in deposition of gold from Au3+ chloride solutions is the reduction of Au3 to Au+, whereas the open-circuit potential for deposition is determined by the reduction of Au+ to Au-0. The open-circuit potential of pyrite or arse nopyrite is a corrosion potential. Presence of Au3+ shifts the corrosion po tential to a value that depends on the relative rates of the reduction of A u3+ and the oxidation of the mineral. Open-circuit potential measurements i ndicate that the rate of deposition of gold on pyrite is controlled almost entirely by the rate of reduction of Au3+. By contrast, the rate of reducti on of gold on arsenopyrite is controlled significantly by the rate of oxida tion of arsenopyrite.