Cu isotope fractionation during bornite dissolution: An in situ X-ray diffraction analysis

Low-temperature ore deposits exhibit a large variation in δ⁶⁵Cu (~12%), and this range has been attributed, in part, to isotope fractionation during weathering reactions of primary minerals such as chalcocite and chalcopyrite. Here, we examine the fractionation of Cu isotopes during dissolution of another important Cu ore mineral, bornite, using a novel approach that combines time-resolved X-ray diffraction (XRD) and isotope analysis of reaction products. During the initial stages of bornite oxidative dissolution by ferric sulfate (<5mol% of total Cu leached), dissolved Cu was enriched in isotopically heavy Cu (⁶⁵Cu) relative to the solid, with an average apparent isotope fractionation (Δ_aq-min=δ⁶⁵Cu_aq-δ⁶⁵Cu_min⁰) of 2.20±0.25%. When >20mol% Cu was leached from the solid, the difference between the Cu isotope composition of the aqueous and mineral phases approached zero, with Δ_aq-min⁰ values ranging from -0.21±0.61% to 0.92±0.25%. XRD analysis allowed us to correlate changes in the atomic structure of bornite with the apparent isotope fractionation as the dissolution reaction progressed. These data revealed that the greatest degree of apparent fractionation is accompanied by a steep contraction in the unit-cell volume, which we identified as a transition from stoichiometric to non-stoichiometric bornite. We propose that the initially high Δ_aq-min values result from isotopically heavy Cu (⁶⁵Cu) concentrating within Cu²⁺ during dissolution. The decrease in the apparent isotope fractionation as the reaction progresses occurs from the distillation of isotopically heavy Cu (⁶⁵Cu) during dissolution or kinetic isotope effects associated with the depletion of Cu from the surfaces of bornite particles.