Exploration potential of Cu isotope fractionation in porphyry copper deposits

摘要

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 S~(65_Cu‰=((~(65)Cu/~(63)Cu_(sample)/~(65)Cu/~(63)Cu_(NIST 976 standard))-1) * 10~3. 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 δ ~(65)Cu values of 1‰= to - 1‰ whereas secondary minerals formed by low temperature reveal a range of δ ~(65)Cu values from - 16.96‰ to 9.98‰. Secondary chalcocite is relatively heavy with δ~(65)Cu 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., 2003; Mathur, R. et al., 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.