Cu isotopic fractionation associated with oxidation of Cu sulfide with and without T. ferrooxidans

摘要

The Cu isotopic compositions of dissolved and solid Cu-rich mineralswere monitored in batch oxidative dissolution experiments with and without Thiobacillus ferrooxidans. Aqueous Cu in leach fluids released during abiotic oxidation of chalcocite ore are isotopically heavier (δ~(65)Cu = 5.3 per thousand) than the initial chalcocite starting material (δ~(65)Cu = 2.6 per thousand). This fractionation is attributed to the precipitation of secondary Cu minerals. In the abiotic experiments, the heavier isotopic signatures of aqueous Cu can be explained by mass balance between the starting material, aqueous Cu and the new precipitates. In contrast, the Cu isotopic composition of aqueous Cu in the inoculated experiment is similar to the starting material throughout the leach cycle; however, the residual materials become significantly lighter for the last period of leaching (δ~(65)Cu = 0.6 per thousand). This change in the isotopic signature of precipitated Cu minerals can only be explained by incorporating a sink for isotopically heavy Cu (δ~(65)Cu = 5.6 per thousand) in the mass balance: this sink is inferred to be uptake into the bacteria cells. Mass balance is achieved for the biotic experiments when the bacteria, the precipitates and the aqueous Cu are considered. The generation of 'heavier' leach solutions during oxidation in these experiments would suggest that minerals precipitating in later stages of supergene enrichment processes (in naturally oxidative environments) would possess heavier Cu isotopic signatures. The average δ~(65)Cu of five supergene samples analyzed are isotopically heavy and these results are in accord with other analyses of supergene minerals found in the literature.