Copper Isotope Fractionation at High-Temperatures during Magmatic and Hydrothermal Processes: Insights from the Post-Collisional Dabu Porphyry Cu-Mo Deposit in Southern Tibet

Dabu is a typical porphyry Cu-Mo deposit formed in a post-collisional setting in the Gangdese porphyry copper belt, Southern Tibet. The porphyry contains whole rock mafic microgranular enclaves (MME) with delta Cu-65 values ranging from +0.22 parts per thousand to +0.71 parts per thousand. These delta Cu-65 values gradually decrease as SiO2, Eu/Eu* and Sr/Y increase, suggesting that Cu isotope fractionation was likely related to the plagioclase accumulation. Because the alkali element ions (eg. Na+) in plagioclase can combine with Fe3+ ions to form (FeNa)(4+) polycations, which can cause the redox equilibrium Cu+ + Fe3+ = Fe2+ + Cu2+ toward larger amounts of Cu+ (isotopically lighter) in the residual melt. The host monzogranite porphyries (HMP) have homogeneous delta Cu-65 contents with a mean value of 0.39 +/- 0.14 parts per thousand (2SD, n=4), close to the most fractionated member of the MME (0.22 +/- 0.05 parts per thousand, 2SD). The Cu isotope compositions of mineral separates of chalcopyrite gradually decrease (-0.37 parts per thousand to +1.12 parts per thousand) from deep to shallow levels, probably as a result of Cu partitioning between vapor and liquid fluid phases during hydrothermal evolution. Taken together, the samples display a wide range of delta Cu-65 values. This demonstrates that Cu isotope fractionation can indeed occur during high-temperature magmatic and hydrothermal processes.