Copper isotopic signature of the Tiegelongnan high-sulfidation copper deposit, Tibet: implications for its origin and mineral exploration

We report the copper isotopic composition of Cu sulfides in the Tiegelongnan high-sulfidation (HS) copper deposit, Tibet, and investigate the possible application of Cu isotopes to mineral exploration. The copper isotopic values of samples from four drill holes display consistent progressive increase with depth to 400 m below surface, with an overall variation of delta Cu-65 of up to 7.60aEuro degrees (-4.76 to 2.84aEuro degrees). Such a large range is very different from that observed in Cu porphyry deposits which commonly exhibit a small delta Cu-65 range (< 1aEuro degrees), with decreasing values with depth. The large delta Cu-65 variation is likely of supergene origin and reflects strong leaching under oxidized conditions. The systematic increase of delta Cu-65 with depth can be explained by the release of isotopically heavy Cu at the top and its transport downward, which is supported by the coupling of positive delta Cu-65 values with high Cu grade and the occurrence of both negative and positive delta Cu-65 values in two drill holes. Mass-balance consideration on the basis of Cu isotopes can indicate mineralized zones which have been eroded or have not been found yet. Such mineralized targets, if buried at depth, could be located using Cu isotopes.