Exploration potential of Cu isotope fractionation in porphyry copper deposits

We examined the copper isotope ratio of primary high temperature Cu-sulfides, secondary low temperature Cu-sulfides (and Cu-oxides) as well as Fe-oxides in the leach cap, which represent the weathered remains of a spectrum of Cu mineralization, from nine porphyry copper deposits. Copper isotope ratios are reported as δ65Cu‰ = ((65Cu/63Cusample/65Cu/63CuNIST 976 standard) − 1) ⁎ 103. Errors for all the analyses are ± 0.14‰ (determined by multiple analyses of the samples) and mass bias was corrected through standard-sample-standard bracketing. The overall isotopic variability measured in these samples range from − 16.96‰ to 9.98‰.Distinct Cu isotopic reservoirs exist for high temperature hypogene, enrichment, and leach cap minerals. Chalcopyrite from high temperature primary mineralization forms a relatively tight cluster of δ65Cu values of 1‰ to − 1‰ whereas secondary minerals formed by low temperature reveal a range of δ65Cu values from − 16.96‰ to 9.98‰. Secondary chalcocite is relatively heavy with δ65Cu varying from − 0.3‰ to 6.5‰. Leach cap minerals dominated by Fe-oxides (jarosite, hematite and goethite) are relatively light ranging from − 9.9‰ to 0.14‰. Although the dataset is relatively small (n = 50 total minerals analyzed), a combination of these data with values from previously published reports [Zhu, X.K., O'Nions, R.K., Guo, Y., Belshaw, N.S. and Rickard, D., 2000. Determination of natural Cu-isotope variation by plasma-source mass spectrometry; implications for use as geochemical tracers. Chemical Geology, 163(1–4): 139–149.; Larson et al., 2002 and Mathur, 2005. Cu isotopic fractionation in the supergene environment with and without bacteria. Geochimica et Cosmochimica Acta, 69(22): 5233–5246.; Markl et al., 2006, and Maher et al., 2007], show a distinct pattern of heavier isotopic signature in supergene samples and a lighter isotopic signature exists in the leach cap and oxidation zone minerals. The pattern could be used as a tool for exploration geology by providing the following information: 1) Identification of highly fractionated copper isotope ratios in copper sulfide and Fe-oxide samples that indicate supergene processes and the extent of leaching and enrichment copper 2) Identification of highly fractionated copper isotope ratios in surface and/or groundwaters that indicate the active weathering copper sulfides that experienced significant enrichment.