Equilibrium isotopic fractionation of copper during oxidation/reduction, aqueous complexation and ore-forming processes: Predictions from hybrid density functional theory

Copper exists as two isotopes: Cu-65 (similar to 30.85%) and Cu-63 (similar to 69.15%). The isotopic composition of copper in secondary minerals, surface waters and oxic groundwaters is 1-12 parts per thousand heavier than that of copper in primary sulfides. Changes in oxidation state and complexation should yield substantial isotopic fractionation between copper species but it is unclear to what extent the observed Cu isotopic variations reflect equilibrium fractionation. Here, I calculate the reduced partition function ratios for chalcopyrite (CuFeS2), cuprite (Cu2O), tenorite (CuO) and aqueous Cu+, Cu+2 complexes using periodic and molecular hybrid density functional theory to predict the equilibrium isotopic fractionation of Cu resulting from oxidation of Cu+ to Cu+2 and by complexation of dissolved Cu. Among the various copper(II)complexes in aqueous environments, there is a significant (1.3 parts per thousand) range in the reduced partition function ratios. Oxidation and congruent dissolution of chalcopyrite (CuFeS2) to dissolved Cu+2 (as Cu(H2O)(5)(+2)) yields (65-63)delta(Cu+2-CuFeS2) = 3.1 parts per thousand at 25 degrees C; however, chalcopyrite oxidation/dissolution is incongruent so that the observed isotopic fractionation will be less. Secondary precipitation of cuprite (Cu2O) would yield further enrichment of dissolved Cu-65 since (65-63)(delta)(Cu+2-Cu2O) is 1.2 parts per thousand at 25 degrees C. However, precipitation of tenorite (CuO) will favor the heavy isotope by +1.0 parts per thousand making dissolved Cu isotopically lighter. These are upper-limit estimates for equilibrium fractionation. Therefore, the extremely large (9 parts per thousand) fractionations between dissolved Cu+2 (or Cu+2 minerals) and primary Cu+ sulfides observed in supergene environments must reflect Rayleigh (open-system) or kinetic fractionation. Finally the previously proposed (Asael et al., 2009) use of delta Cu-65 in chalcopyrite to estimate the oxidation state of fluids that transported Cu in stratiform sediment-hosted copper deposits is refined. (C) 2013 Elsevier Ltd. All rights reserved.