Coupling between mineral replacement reactions and co-precipitation of trace elements: An example from the giant Olympic Dam deposit

The mechanisms of scavenging of minor elements in reactive, fluid-dominated systems control the distribution, nature, concentration, and reactivity of economically valuable or deleterious elements in many environments, ranging from hydrothermal ore deposits to sedimentary basins. Here we show that in a high-grade Cu-U ore from the super-giant Olympic Dam ore deposit, remobilized U was scavenged during the fluid-driven replacement of chalcopyrite (CuFeS₂) by bornite (Cu₅FeS₄). Hydrothermal experiments show that the pseudomorphic replacement of chalcopyrite by bornite proceeded via an interfaced coupled dissolution reprecipitation (ICDR) reaction mechanism, with further bornite growing via overgrowth. Addition of U(VI) to the experimental fluid resulted in scavenging of U in the form of nano- to micro-particles of uraninite. Uraninite precipitation occurred concurrently with replacement, resulting in abundant uraninite decorating the initial surface of the parent chalcopyrite grain, as well as the coarse porosity that developed within overgrowth bornite. Our results highlight the importance of disequilibrium or local equilibrium in controlling the endowment of U in complex ores. Under far from equilibrium, fluid-dominated conditions, e.g., during seismic or eruptive events, fast fluid movements associated with brecciation and/or faulting can lead to fast mineral reactions. The resulting ultra-local conditions (e.g., surface of reacting mineral; reaction-induced porosity) play an important role in controlling the precipitation of minor elements, U in this case, and interpretation of precipitation mechanisms as reflecting the evolution of bulk fluid may be erroneous.