Tracking mineralisation with in situ multiple sulphur isotopes: a case study from the Fraser Zone, Western Australia

Sulphur isotopes have become a widely deployed tool to address the sources of fluids in ore systems, given the widespread interaction of H₂S, HS⁻ and S₂ ^- with metals. However, the sensitivity of sulphur isotopes to magma source composition relative to other radiogenic isotopes and the nature of processes differentially affecting isotopic systems have not been extensively explored in magmatic mineral systems. In this work we present a case study of sulphide bearing magmas within the Fraser Zone of the Albany-Fraser Orogen in Western Australia that seeks to evaluate the sensitivity of sulphur isotopes to track magma source composition in space and time. New δ³⁴S data from mineralised NRM, Plato and Octagonal localities of the Fraser Zone indicate variable degrees of assimilation of local sediments of the Snowys Dam Formation. Despite an Archean heritage in the Fraser Zone magmas being indicated by radiogenic isotopes, sparse xenocrystic zircon, and whole rock geochemical modelling, Δ³³S data from these localities reveal Archean sulphur is absent; implying decoupling of the sulphur component from other geochemical and isotopic systems. This is consistent with a mechanism by which Archean sulphur was removed from detrital material incorporated in the Fraser Zone. We conclude that sulphides were stripped from sediments during uplift, erosion and transport to the Fraser Zone, with these Archean-sulphur depleted sediments then assimilated by the Fraser Zone parental magmas. Our results indicate a coupling between variable assimilation of external sulphur by Fraser Zone magmas and the mineralisation present at the prospects studied. However assimilated sulphur cannot account for all of the sulphur within mineralised samples, indicating the involvement of additional processes such as tenor upgrading.