Cu isotopic fractionation in the supergene environment with and without bacteria

The isotopic composition of dissolved Cu and solid Cu-rich minerals [delta(65)Cu (parts per thousand) = ((65)Cu/(63)Cu(sample)/(65)Cu/(63)Cu(std)) - 1)*1000] were monitored in batch oxidative dissolution experiments with and without Thiobacillus ferrooxidans. Aqueous copper in leach fluids released during abiotic oxidation of both chalcocite and chalcopyrite was isotopically heavier (delta(65)Cu = 5.34 parts per thousand and delta(65)Cu = 1.90 parts per thousand, respectively, [+/- 0.16 at 2 sigma]) than the initial starting material (delta(65)Cu = 2.60 +/- 0.16 parts per thousand and delta(65)Cu = 0.58 +/- 0.16 parts per thousand, respectively). Isotopic mass balance between the starting material, aqueous copper, and secondary minerals precipitated in these experiments explains the heavier isotopic values of aqueous copper. In contrast, aqueous copper from leached chalcocite and chalcopyrite inoculated with Thiobacillus ferrooxidans was isotopically similar to the starting material. The lack of fractionation of the aqueous copper in the biotic experiments can best be explained by assuming a sink for isotopically heavy copper present in the bacteria cells with delta(65)Cu = 5.59 +/- 0.16 parts per thousand. Consistent with this inference, amorphous Cu-Fe oxide minerals are observed surrounding cell membranes of Thiobacillus grown in the presence of dissolved Cu and Fe. Extrapolating these experiments to natural supergene environments implies that release of isotopically heavy aqueous Cu from oxidative leach caps, especially under abiotic conditions, should result in precipitates in underlying enrichment blankets that are isotopically heavy. Where iron-oxidizing cells are involved, isotopically heavy oxidized Cu entrained in cellular material may become associated with leach caps, causing the released aqueous Cu to be less isotopically enriched in the heavy isotope than predicted for the abiotic system. Rayleigh fractionation trends with fractionation factors calculated from our experiments for both biotic and abiotic conditions are consistent with large numbers of individual abiotic or biotic leaching events, explaining the supergene chalcocites in the Morenci and Silver Bell porphyry copper deposits. Copyright (c) 2005 Elsevier Ltd.