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 (< 5 mol% of total Cu leached), dissolved Cu was enriched in isotopically heavy Cu (65Cu) relative to the solid, with an average apparent isotope fractionation (Δaq − min = δ65Cuaq − δ65Cumin0) of 2.20 ± 0.25‰. When > 20 mol% Cu was leached from the solid, the difference between the Cu isotope composition of the aqueous and mineral phases approached zero, with ∆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 ∆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.

Research highlights
► Bornite dissolution contributes to Cu isotope variations in low-T ore deposits.
► Heavy Cu (65Cu) is initially leached into solution during bornite dissolution.
► Isotope fractionation factors decrease as more Cu dissolves from bornite.
► Isotope trends are attributed to oxidation coupled with distillation or diffusion.
► Cu isotopes may be useful in determining the extent of ore rock weathering.