Exsolution of chalcopyrite from bornite-digenite solid solution: an example of a fluid-driven back-replacement reaction

Sulfide minerals host most of the world’s supplies of metals such as Cu, Ni, Co, Zn, Pb, or Mo, and understanding their crystallization, dissolution, and textural evolution is key to understanding the formation of mineral deposits, metal recovery via metallurgy, and their environmental impact. Despite the prominence of hydrothermally formed sulfide minerals, the interpretation of textures in sulfide petrology relies mainly on comparison with results from experiments conducted under dry conditions. Here, we show experimentally that the exsolution of chalcopyrite (CuFeS 2 ) lamellae from a bornite (Cu 5 FeS 4 )-digenite (Cu 9 S 5 ) solid solution ( bdss ) is a back-reaction , which occurred during low temperature annealing (150 °C) following initial replacement of parent chalcopyrite by bdss at 300 °C. The back-reaction is rapid (days), and its progress is catalyzed by small amounts of fluid present in the porosity within the bdss . This porosity initially results from the formation of bdss via the replacement of parent chalcopyrite by an interface-coupled dissolution-reprecipitation mechanism. During annealing, bdss first breaks down into bornite and digenite via solid-state exsolution. The bornite-digenite assemblage then exsolves chalcopyrite in local patches. We discovered strikingly similar textures in iron oxide copper gold (IOCG) deposits from South Australia; in these samples, the fluid-driven nature of the exsolution reaction is reflected by the fact that the chalcopyrite lamellae propagate around cracks and fractures within bornite. As in our experiments, most of this natural bornite formed via replacement of chalcopyrite. These reactions are controlled by kinetic factors (e.g., relative nucleation rates of Cu-Fe-sulfides; presence of porosity) rather than equilibrium thermodynamics, but result in final assemblages and textures that may not be distinguishable from those evolving from equilibrium processes under dry conditions.