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

Abstract

This study examines the fractionation of iron isotopes as a reflection of processes that govern the co-magmatic formation of a hydrothermal ore system of a classic porphyry copper deposit. We present iron isotope analyses, measured using multi-collector ICPMS, of magmatic intrusive rocks and coeval hypogene ore minerals from the Batu Hijau porphyry copper-gold deposit in Sumbawa, Indonesia. Isotopic analysis were made of the intrusive magmatic host rocks to the deposit, ranging from sub-volcanic andesite and quartz diorite to tonalite, and of hypogene chalcopyrite, bornite and magnetite ore mineral separates.Iron isotope values of andesite and quartz diorite range from δ57Fe=0.17±0.05‰ to 0.26±0.05‰. The more felsic, differentiated dacite and tonalite have heavier iron isotope values (δ57Fe=0.27±0.08‰ to 0.32±0.08‰) than the intermediate and mafic rocks. Heavy iron isotopes are positively correlated with SiO2 and negatively correlated with MgO, CaO, TiO2 and V suggesting that crystallisation of clinopyroxene, amphibole and magnetite is the primary control on isotopic evolution of the melt. These isotopic trends are supported by thermodynamic (rhyolite-MELTS) modelling of crystal fractionation using published mineral-melt fractionation factors, and demonstrate that the isotopic evolution of the Batu Hijau melts was controlled by crystal fractionation.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.