Understanding Copper Isotope Behavior in the High Temperature Magmatic‐Hydrothermal Porphyry Environment

Copper stable isotope geochemistry has the potential to constrain aspects of ore deposit formation once variations in the isotopic data can be related to the physiochemical conditions during metal deposition. This study presents Cu isotope ratios for samples from the Pebble porphyry Cu‐Au‐Mo deposit in Alaska. The δ65Cu values for hypogene copper sulfides range from −2.09‰ to 1.11‰ and show linear correlations with the δ18O isotope ratios calculated for the fluid in equilibrium with the hydrothermal alteration minerals in each sample. Samples with sodic‐potassic, potassic, and illite alteration display a negative linear correlation between the Cu and O isotope results. This suggests that fractionation of Cu isotopes between the fluid and precipitating chalcopyrite is positive as the hydrothermal fluid is evolving from magmatic to mixed magmatic‐meteoric compositions. Samples with advanced argillic alteration display a weak positive linear correlation between Cu and O isotope results consistent with small negative fluid‐chalcopyrite Cu isotope fractionation during fluid evolution. The hydrothermal fluids that formed sodic‐potassic, potassic, and illite alteration likely transported Cu as CuHS0. Hydrothermal fluids that resulted in advanced argillic alteration likely transport Cu as CuCl2−. The pH conditions also control Cu isotope fractionation, consistent with previous experimental work. Larger fractionation factors were found between fluids and chalcopyrite precipitating under neutral conditions contrasting with small fractionation factors calculated between fluids and chalcopyrite precipitating under acidic conditions. Therefore, this study proposes that hydrothermal fluid compositions and pH conditions are related to Cu isotope variations in high temperature magmatic‐hydrothermal deposits. Key Points Copper isotope ratios are presented for 12 well characterized hypogene sulfide samples from the Pebble Cu‐Au‐Mo deposit, Alaska The copper isotope results correlate with oxygen isotope results for hydrothermal alteration minerals in the same samples The data suggest that fluid‐chalcopyrite fractionation factors are controlled by the Cu species in the fluid and the pH conditions