Supergene alteration of sulphides,VI. The binding of Cu,Ni, Zn,Co and Pb with gossan (iron-bearing) minerals

The results of experimentally coprecipitating the base metals Cu, Ni, Zn, Co and Pb with iron as it is oxidised and hydrolised are presented in conjunction with the data from anodically weathering 19 different sulphide ore electrodes.At pH-values above 6 the base metals are shown to be retained in the solid precipitates with Fe“Ni, Fe“Zn and Fe“Co pyroaurite-type phases forming. A 7–8-Å interlayer spacing predominates at higher pH while at the pH of 6–7 where the base metals become more soluble, a 10–11-Å spacing forms. The Fe“Cu system gives posnjakite and brochantite phases that are likely to have a high Fe content, as well as the oxides tenorite and cuprite. The base metals can be leached out of these precipitates if they are acted on by weakly acid leachants, but alkaline leaching shows retention of the base metals.Below pH 6 the base metals are more soluble, with Cu and Pb being retained to some degree in the precipitates as Fe-oxyhydroxide minerals form.In the light of experiments that show the mechanisms involved in the formation of all of these phases, two mechanisms for the formation of iron minerals during the weathering of sulphides to gossans are discussed. One at higher pH that gives rise to retention of base metals with the iron minerals, and the other giving rise to a low pH and the solubilising of the base metals.