| The nature of Cu isotope fractionation in natural Cu-Fe-S minerals was investigated through acid ferric sulfate leaching of copper ore from Morenci, Arizona. Copper isotope composition of the derived solutions varies from δ 65Cu = 0.47‰ to 5.21‰ over the course of progressive copper extraction. High δ65Cu values characterize solutions collected in the first half of the leach, while the solutions collected between 35% and 45% copper recovery exhibit lower δ65Cu values. This general pattern was observed for both bacterially-mediated and abiotic leaching.; Sulfate solutions derived from dissolving pure djurleite show variable Cu isotope compositions as well, although the range is protracted from δ 65Cu = 0.01‰ to 1.21‰. As the Cu:S ratio of the remaining sulfide decreases, crystal structure parameters change as mineralogy passes through a series of nonstoichiometric copper sulfides. Mineralogy converges to yarrowite near 44% copper dissolution.; Crystal chemical studies show that distribution of the two copper-sulfur bond coordination geometries, triangular planar and tetrahedral, in the copper sulfides, approximately corresponds to changes in δ65Cu of the leachates. In particular, the proportion of CuS3 relative to CuS4 groups decreases from Cu/S = 2.00 (chalcocite) to 1.40 (geerite). Between Cu/S = 1.40 to 1.00 (covellite), the relative proportion of CuS3 groups increases slightly. Connection between coordination number and Cu isotope fractionation implies affinity of CuS3 groups for the heavier, 65Cu, isotope. This can be justified through bond length-bond strength arguments.; Solutions from bornite dissolution vary from δ65Cu = −0.79‰ to 1.14‰, with the largest values associated with solutions from early stage of reaction (up to 15% copper removal). Around 25% dissolution, δ65Cu of the solution approaches that of the original bornite (δ65Cu = 0.02‰). This is explained by disappearance of all remaining CuS3 groups.; Sulfur isotope compositions of solutions and sulfides derived from djurleite leaching were determined to investigate the possibility of intra-mineral fractionation. Very soon after reaction initiation, δ34S of both sulfur reservoirs reach a steady-state with sulfate solutions about 2‰ enriched in 34S relative to residual sulfide. Unlike the case of Cu isotopes, the main partitioning affecting S isotopes is exchange between sulfate and sulfide. |
