Defining IOCG signatures through compositional data analysis: a case study of lithogeochemical zoning from the Olympic Dam deposit, South Australia

Abstract

The Olympic Dam Cu-U-Au-Ag deposit is dominantly composed of mineralised hematite-breccias and occurs entirely within the Roxby Downs Granite. Multivariate statistical analysis of a large whole-rock, 15 m-interval geochemical dataset (10,565 samples) was undertaken to identify geochemical signatures characteristic of iron-oxide copper gold (IOCG)-style mineralization and constrain the conspicuous lithogeochemical zonation observed at Olympic Dam. Statistical analyses include principal component analysis on centred logratio (clr)-transformed data coupled with hierarchical clustering. Certain groups of elements that can be interpreted in terms of an evolving hydrothermal system relative to host lithologies are derived from data analysis: granitophile (U-W-Sn-Mo); siderophile (Ni-Co); chalcophile (Ag-Bi) and related elements (As-Sb and Au-Te). The distributions of elements within each group are investigated through three vertical cross-sections and are compared with known lithological and Cu-(Fe)-sulphide zonation. Throughout the Olympic Dam Breccia Complex, the IOCG signature is defined by multi-element combinations of the commodity metals Cu, U, Au, and Ag, coupled with a range of trace elements. Overall, the IOCG signature overlaps well with Fe-metasomatism despite mismatch which is likely due to discrete styles of mineralisation found only on the margins of the deposit and also to the presence of mineralised domains within Fe-poor zones. The IOCG signature is composed of two geochemical associations, which exhibit distinct spatial distributions. The first group, Cu-U3O8-Se-S, shows concentric zonation whereas the second group, Au-W-Mo-Sb-As, forms a vertical ∼1800 m deep corridor in the southeastern lobe of the deposit. The specific Au-W-Mo-As-Sb signature could potentially be generic within IOCG systems across the Olympic Cu-Au province and if so, would provide a proxy model for near-mine exploration.