Experiments on Cu isotope fractionation between chlorine bearing fluid and silicate magma: implications for fluid exsolution and porphyry Cu deposits

Hydrothermal fluid is essential for transporting metals in the crust and mantle. To explore the potential of Cu isotopes as a tracer of hydrothermal-fluid activity, Cu-isotope fractionation factors between Cl-bearing aqueous fluids and silicate magmas (andesite, dacite, rhyolite dacite, rhyolite and haplogranite) were experimentally calibrated. Fluids containing 1.75-14 wt.% Cl were mixed together with rock powders in Au95Cu5 alloy capsules, which were equilibrated in cold-seal pressure vessels for 5-13 days at 800-850 degrees C and 2 kbar. The elemental and Cu-isotopic compositions of the recovered aqueous fluid and solid phases were analyzed by (LA-) ICP-MS and multi-collector inductively coupled plasma mass spectrometry, respectively. Our experimental results show that the fluid phases are consistently enriched in heavy Cu isotope (Cu-65) relative to the coexisting silicates. The Cu-isotope fractionation factor (Delta Cu-65(FLUID-MELT)) ranges from 0.08 +/- 0.01 parts per thousand to 0.69 +/- 0.02 parts per thousand. The experimental results show that the Cu-isotopic fractionation factors between aqueous fluids and silicates strongly depend on the Cu speciation in the fluids (e.g. CuCl(H2O), CuCl2- and CuCl32-) and silicate melts (CuO1/2), suggesting that the exsolved fluids may have higher delta Cu-65 than the residual magmas. Our results suggest the elevated delta Cu-65 values in Cu-enriched rocks could be produced by addition of aqueous fluids exsolved from magmas. Together with previous studies on Cu isotopes in the brine and vapor phases of porphyry deposits, our results are helpful for better understanding Cu-mineralization processes.