Effects of hydrothermal alteration on mafic lithologies at the Olympic Dam Cu-U-Au-Ag deposit

Two groups of mafic to ultramafic lithologies occur at the Olympic Dam copper-uranium-gold-silver deposit, South Australia. The first group consists of olivine-phyric basalt and mafic dykes belonging to the ca. 1590 Ma Gawler Range Volcanics; the second group is the Olympic Dam dolerite affiliated with the ca. 820 Ma Gairdner Dyke Swarm. Both groups of mafic rocks occur within and adjacent to the Olympic Dam Breccia Complex, the immediate host to the deposit, and have been variably altered. They contain a number of secondary minerals, mainly including chlorite, sericite, iron oxides and carbonates. Drill core assays of the ca. 1590 Ma olivine-phyric basalt reflect the variable abundance of secondary minerals such as carbonates and iron oxides (e.g. up to 26 wt.% CO₂ and 50 wt.% Fe₂O₃). Positive correlations between compatible Cr and other incompatible elements (e.g. Ti and Zr) in the olivine-phyric basalt suggest whole-rock mass and/or volume loss due to hydrothermal alteration, in accordance with the previously proposed role of the olivine-phyric basalt as an important source of Cu using Sm-Nd isotope constraints. However, rigorous mass balance calculation is not feasible due to a lack of fresh equivalents. Geochemical comparisons between the least-altered ca. 820 Ma Olympic Dam dolerite and the more-altered equivalents reported in this study reveal that the concentrations of some elements have been affected by hydrothermal alteration. In particular, Mg, K, Cs, Ba, Rb, Pb, U, and heavy rare earth elements have mostly been enriched; Ca, Sr and light rare earth elements have mostly been depleted; and Si, Fe, and Na have been affected both ways. Elevated Zn and Pb but depleted Cu concentrations in some Olympic Dam dolerite samples suggest that these metals were also mobilized. Petrographic observation and drill core assays are consistent with sodic alteration having been responsible for the Cu depletion in some dolerite samples. However, there is currently no evidence that this had any significant impact on the Cu content and distribution of the Olympic Dam deposit. Further, alteration of both groups of mafic rocks implies multiple hydrothermal events at the Olympic Dam deposit.