Electrochemical accumulation of visible gold on pyrite and arsenopyrite surfaces

In galvanic cell arrangements gold is electrochemically deposited on semiconducting sulfide minerals (pyrite, arsenopyrite, chalcopyrite) from aerated as well as H₂S-saturated, gold-bearing 1 M KCl solutions. Observed cell potential differences of about 0.4–0.6 V in setups with one sulfide in aerated (cathode) and the other in H₂S-saturated (anode) solutions are comparable with known self-potentials of natural sulfide ore bodies. Gold preferentially accumulates on the cathode, i.e. under oxidizing conditions. Linked sulfides of variable composition in the same environment, either oxidizing or reducing, yield potential differences up to 20 mV. Such assemblages simulate conditions typically occurring at surfaces of chemically inhomogeneous single crystals (e.g. zonation). Depending on chemical composition, sulfide minerals show either n- or p-type conductivity. Visible gold is preferentially accumulated on individual domains of sulfide surfaces that act as cathodes, i.e. p-type conductors in n-p junctions. The experimental results are discussed in view of electrochemical accumulation of visible gold on sulfides in nature. Arsenic is the most important element in establishing p-type conductivity of pyrite and arsenopyrite. This feature may explain why As is such a powerful pathfinder in gold exploration.