Iron-isotope systematics from the Batu Hijau Cu-Au deposit, Sumbawa, Indonesia

Magnetite from the Batu Hijau ore deposit is isotopically heavier (δ57Fe range from 0.24 ± 0.14‰ to 0.74 ± 0.14‰) than coexisting chalcopyrite (from − 0.62‰ ± 0.04‰ to − 0.16 ± 0.05‰) and bornite (from − 0.72 ± 0.23‰ to − 0.08 ± 0.03‰), consistent with theoretical fractionation factors derived from spectroscopy. Comparison of our mineral data with data from other deposits defines a range of δ57Fe values for hypogene chalcopyrite from porphyry copper deposits from − 1.26‰ to 0.29‰. Models of Rayleigh type fractionation assuming an initial hydrothermal fluid composition which is isotopically light (δ57Fe between − 0.5 and − 0.1‰), produces mineral compositions that match our analytical data. The modelled isotope values are in agreement with experiments demonstrating that hypogene chalcopyrite has isotopic compositions reflecting the isotopic value of the hydrothermal fluid. However, hypogene chalcopyrite from this study is significantly lighter than values of chalcopyrite separates from the Renison tin deposit which span a range of 0.28 ± 0.06‰ to 1.32 ± 0.06‰. We propose that the presence or absence of pyrrhotite influences the relative differences in δ57Fe values of chalcopyrite between porphyry copper and tin deposits, because of its preference for isotopically light iron, which in turn is controlled by both oxygen and sulfur redox conditions in the fluid.