Multiple modes of sulphur cycling within a mineralised orogen: A case study from the Fraser Zone, Western Australia

Sulphur isotope analysis can be used to fingerprint sulphur sources within mineral systems and trace sulphur cycles through geological time. In this work we present petrographic and in-situ sulphur isotope analyses of sulphide minerals from the Zn-Cu-Au-Ag Andromeda volcanogenic massive sulphide (VMS) prospect, Fraser Zone, Albany-Fraser Orogen, Australia. Sulphide mineral assemblages of pyrrhotite, pyrite, chalcopyrite and sphalerite occur within deformed lenses of sulphide breccia at Andromeda, with the presence of sphalerite contrasting with typical magmatic sulphide mineral assemblages recorded elsewhere within the Fraser Zone. The delta S-34 values of Andromeda sulphides range from 4.50 parts per thousand to 6.70 parts per thousand, characteristic of Archaean and Proterozoic VMS-style deposits worldwide. The Delta S-33 data show evidence of mass-independent fractionation of sulphur (MIF-S), which indicates that sulphur from Archaean sediments was included within Andromeda sulphides, which formed in close proximity to the Fraser Shear Zone (FSZ). This contrasts with the findings of previous studies of sulphur isotopes within magmatic sulphides elsewhere in the Fraser Zone, which are distal to major shear zones, and in which sulphur derived from Archaean sediments is absent. The absence of a MIF-S signature from the magmatic sulphides is interpreted to reflect cycling of surface-derived sulphur through the ocean prior to incorporation into the magmatic sulphides. The presence of Archaean sedimentary sulphur at Andromeda is attributed to transport from a reservoir of sulphur recording MIF-S within the older Biranup Zone, which is in close proximity to the FSZ, a lithosphere-scale structure. Fluid movement through the FSZ is a viable mechanism that might have transferred sulphur recording MIF-S from Archaean sulphur-bearing pyrite in the Biranup Zone to sulphides at Andromeda. These findings support multiple modes of sulphur cycling within a single Proterozoic mineralised setting and imply that similar polymodal sulphur cycling might have occurred on other Proterozoic continental margins worldwide, with implications for mineralisation and global sulphur cycle systematics.

Automated summary

Sulphur isotope analysis can be used to track sulphur sources within mineral systems and cycles through geological time. This study in the Fraser Zone of Australia suggests the presence of multiple modes of sulphur cycling within a single Proterozoic mineralised setting, with potential implications for similar polymodal sulphur cycling on Proterozoic continental margins worldwide.