Sulfidisation mechanisms for heavily oxidised sulfide minerals

The sulfidisation mechanism for heavily oxidised sulfides involves the chemisorption of hydrosulfide ions followed by an electrochemical reaction to produce the sulfide surface, unlike that for basemetal "oxide" minerals. In the case of heavily oxidised pyrrhotite, the adsorbed hydrosulfide ions react with ferrous hydroxides to produce FeS. After further adsorption of hydrosulfide ions, an electrochemical reaction causes the formation of a pyrite surface. In the case of heavily oxidised pentlandite, it is proposed that ferrous hydroxides are preferentially sulfidised and at higher sulfidisation potentials, nickel hydroxide is finally converted to nickel sulfide. However the nature of species that formed on the sulfidised oxidised pentlandite was unclear. For heavily oxidised chalcopyrite, an anionic exchange reaction resulting in covellite formation appears likely, although copper in the +1 valency state was identified. Elemental sulfur/polysulfides were also found and most likely result from electrochemical reactions coupled with dissolved oxygen reduction.