Anatomy of a complex mineral replacement reaction: Role of aqueous redox, mineral nucleation, and ion transport properties revealed by an in-situ study of the replacement of chalcopyrite by copper sulfides

The fluid-driven transformation of chalcopyrite (CuFeS₂) into Cu-rich sulfides (e.g., digenite, Cu_1.8S; covellite, CuS; and chalcocite, Cu₂S) is a complex mineral replacement reaction where the reaction pathway is controlled by the interplay between evolving mineral make-up, texture/porosity, and solution chemistry. This transformation was investigated in CuCl₂ + H₂SO₄ solutions under mild hydrothermal conditions (180 to 300 °C); the reaction kinetics, nature of minerals formed, and oxidation states of aqueous Fe and Cu were followed in-situ in real-time using synchrotron powder X-ray diffraction (PXRD) and X-ray absorption spectroscopy (XAS). These results are corroborated by an analysis of the textures of reaction products from comparative ex-situ quench experiments. The in-situ and ex-situ experiments revealed that: (i) aqueous Cu²⁺ quickly reduced to Cu⁺ during chalcopyrite replacement in all experiments, and Fe dissolved as Fe²⁺; (ii) covellite was the first mineral to form, followed by digenite-high with delayed nucleation; and (iii) a non-quenchable hydrated Fe sulfate mineral (szomolnokite, FeSO₄.H₂O) formed at 240 °C at relatively low concentrations of added CuCl₂, which supressed the formation of digenite-high. The quantitative mineral phase evolution retrieved using in-situ PXRD was modelled using a novel dual power law (dual Avrami approach). Avrami exponents revealed that chalcopyrite replacement proceeded initially via a 3-dimensional growth mechanism, followed by diffusion-controlled growth. This is consistent with initial formation of a porous covellite rim around chalcopyrite, confirmed by the observation of the ex-situ reaction products, followed by a second reaction stage where the transport properties of aqueous Fe (released from the chalcopyrite) and aqueous Cu (added from the initial solution) to and from the reaction front become the rate-limiting step; and these two kinetic stages exist even where covellite was the only replacement product. The activation energies calculated for these two kinetic stages were 42.9 ± 7.4 kJ mol⁻¹ and 39.3 ± 13.1 kJ mol⁻¹, respectively. We conclude that (i) the replacement of chalcopyrite by covellite and digenite proceeds via an interface coupled dissolution and reprecipitation mechanism; (ii) availabilities of aqueous Cu⁺ and of Fe²⁺ play a critical role in covellite nucleation and on the sequence of mineral precipitation during chalcopyrite replacement; the Cu⁺ to Cu²⁺ ratio is controlled by the kinetics of Cu²⁺ to Cu⁺ reduction, which increases with increasing temperature, and by the transport of Cu²⁺ through the daughter mineral to the reaction front, while Fe²⁺ availability is limited at high temperature by the formation of insoluble ferrous sulfate; and (iii) this reaction evolves from a bulk fluid-chemistry controlled reaction (initial formation of covellite) to an interface-controlled reaction (digenite-high or further transformation to covellite). The current findings highlight the complex feedback between Cu²⁺/Cu⁺ aqueous redox, mineral nucleation, and ion transport properties during replacement reactions, and the applicability of combined in-situ PXRD and XAS techniques in deciphering complex fluid-driven mineral replacement reactions.