Geochimica et Cosmochimica Acta, Oct 1, 2013, Vol.118, p.85(13)
To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2013.04.030 Byline: David M. Sherman Abstract: Copper exists as two isotopes:.sup.65Cu ([approximately equal to]30.85%) and.sup.63Cu ([approximately equal to]69.15%). The isotopic composition of copper in secondary minerals, surface waters and oxic groundwaters is 1-12a[degrees] 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 (CuFeS.sub.2), cuprite (Cu.sub.2O), tenorite (CuO) and aqueous Cu.sup.+, Cu.sup.+2 complexes using periodic and molecular hybrid density functional theory to predict the equilibrium isotopic fractionation of Cu resulting from oxidation of Cu.sup.+ to Cu.sup.+2 and by complexation of dissolved Cu. Among the various copper(II) complexes in aqueous environments, there is a significant (1.3a[degrees]) range in the reduced partition function ratios. Oxidation and congruent dissolution of chalcopyrite (CuFeS.sub.2) to dissolved Cu.sup.+2 (as Cu(H.sub.2O).sub.5.sup.+2) yields.sup.65-63[delta](Cu.sup.+2-CuFeS.sub.2)=3.1a[degrees] at 25[degrees]C; however, chalcopyrite oxidation/dissolution is incongruent so that the observed isotopic fractionation will be less. Secondary precipitation of cuprite (Cu.sub.2O) would yield further enrichment of dissolved.sup.65Cu since.sup.65-63[delta](Cu.sup.+2-Cu.sub.2O) is 1.2a[degrees] at 25[degrees]C. However, precipitation of tenorite (CuO) will favor the heavy isotope by +1.0a[degrees] making dissolved Cu isotopically lighter. These are upper-limit estimates for equilibrium fractionation. Therefore, the extremely large (9a[degrees]) fractionations between dissolved Cu.sup.+2 (or Cu.sup.+2 minerals) and primary Cu.sup.+ sulfides observed in supergene environments must reflect Rayleigh (open-system) or kinetic fractionation. Finally the previously proposed (Asael et al., 2009) use of [delta].sup.65Cu in chalcopyrite to estimate the oxidation state of fluids that transported Cu in stratiform sediment-hosted copper deposits is refined. Article History: Received 11 July 2011; Accepted 30 April 2013 Article Note: (miscellaneous) Associate editor: Edwin Schauble
Copper Mining ; Copper Oxides ; Groundwater ; Rain ; Sulfides
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