Mineralization-alteration footprints in the Olympic Dam IOCG district, South Australia: The Acropolis prospect

Abstract

The Acropolis prospect, 20 km southwest from the Olympic Dam Cu-U-Au-Ag deposit, South Australia, is a vein-style magnetite (±apatite ±hematite) system. A whole-rock dataset comprising 4864 core samples from 14 drillholes was analysed using multivariate statistical analyses to understand and identify geochemical signatures of mineralization, as well as the expressions and extents of hydrothermal alteration. Statistical analysis included unsupervised (principal component analysis, hierarchical and k-means clustering) and supervised (random forests) machine learning algorithms. The geology of the Acropolis prospect is presented as a 3D geological model complemented by cross-sections. The results of statistical analyses are overlaid and interpreted relative to the geological model, and encompass a projection of sodic and propylitic alteration as PC3, and mineralization signature as PC1. Although the mineralization footprint of the Acropolis prospect partially overlaps with a Hiltaba Suite granite, it is not centred on the granite body. A distinct ‘magnetite’ signature of Fe-V-Ni-Co is developed in the southwestern part of Acropolis and represents samples containing >60 wt% Fe. In contrast, the ‘hematite’ signature displays an association of REE, W, Sn, Sb, U, Th, Ca and P, and is present throughout the Acropolis prospect with the exception of drillhole ACD5, which is non-mineralized. Interpolated values of Cu (> 200 ppm) indicate an offset from Fe-rich veins, thus supporting a genetic model in which Cu-bearing mineralization overprints Cu-Au-deficient magnetite-dominant veins. The results obtained provide insights into the evolution from magnetite to hematite-dominant IOCG systems and may provide a proxy for exploration of shallow and economically significant IOCG deposits in the eastern Gawler Craton.