Factors controlling the precipitation of copper and cobalt minerals in sediment-hosted ore deposits: Advances and restrictions

Abstract

An assessment is presented of existing data on the most important copper and cobalt complexes and sulphides applied to reactive transport modelling. The most important complexes in ore-forming hydrothermal solutions at 150°C are CoCl(H2O)5+, CoCl4−2 and CoCl2(H2O)2(aq) for cobalt, and CuCl2− and CuCl3−2 for copper. Reactive transport modelling was carried out to simulate the transport and precipitation of copper and cobalt sulphides in sediment-hosted stratiform ore deposits, such as the Central African Copperbelt. A limitation is the lack of thermodynamic data for carrollite (CuCo2S4). Initially chalcopyrite (CuFeS2) and cattierite (CoS2) precipitate in a reducing host rock. Pyrite dissolves in the horizon where both minerals form and it precipitates in adjacent rocks. The continuous supply of copper and cobalt causes formation of the metal-rich sulphides bornite (Cu5FeS4) and linnaeite (Co3S4). Chalcopyrite and cattierite precipitate further in the flow direction. In this model, the dissociation reaction of chalcopyrite and bornite are:CuFeS2(s)+2H+=Cu2++Fe2++2HS−Cu5FeS4(s)+2H+=4Cu++Cu2++Fe2++4HS−However, if the dissociation reactions only include Cu+ and Fe3+, simulations show the precipitation of only chalcocite (Cu2S), which is explained by the low concentration of trivalent iron in solution. The dissociation reactions are:CuFeS2(s)+2H+=Cu++Fe3++2HS−Cu5FeS4(s)+2H+=5Cu++Fe3++4HS−