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Copper Isotope Fractionation at the Magnetite Crisis in the Manus Backarc Basin

The formation of magmatic hydrothermal ore deposits Cu is a multi-stage process that requires a number of processes to enrich Cu to economicaly viable proportions. Copper is a highly chalcophile element and therefore closely controled by the concentration and speciation of sulfur. An important factor in concentrating Cu is the prevalence of oxidising conditions during magmatic di9erentiation that prevents fractionation of sul:des and concentrates Cu in evolved magmas. The oxidising conditions in volcanic arc magmas are therefore favourable for Cu enrichment and Cu initialy behaves as an incompatible element during di9erentiation of arc magmas. However, the onset of magnetite fractionation in di9erentiated arc magmas lowers the oxygen fugacity which causes the reduction of sulfate to sul:de. Fractionation of sul:des is coupled with dramatic decreases in Cu concentrations and has hence been termed the ‘magnetite crisis’ [1,2]. However, it remains unclear whether magnetite triggered sul:de saturation contributes to the formation of porphyry Cu deposits [1,2]. Copper isotope ratios have been suggested as tracers for Cu ore forming processes [e.g. 3]. Yet, the potential for Cu isotope fractionation during initial arc magma evolution is largely unexplored. To establish a link between magma sources and porphyry Cu ores we investigate the potential for Cu isotope fractionation at the magnetite crisis. Our samples are from the Manus basin where Cu concentrations initialy increase from 86 ppm at MgO = 9.2 wt.% to 276 ppm at MgO = 3.0 wt.% and then decrease to 27 ppm at the magnetite crisis [2]. Density functional theory predicts signi:cant fractionation of Cu isotopes during transition from sulfate to sul:de [4]. Yet, our initial results suggest only minor Cu isotope variation at the magnetite crisis which we atribute to the near quantitative transition from sulfate to sul:de and incomplete removal of (Cu-)sul:des from the melt. This alows us to establish a direct link between magma source compositions and late-stage ore formation processes.