Trace element distributions among Cu-(Fe)-sulfides from the Olympic Dam Cu-U-Au-Ag deposit, South Australia

Abstract

Chalcocite, bornite, and chalcopyrite are the main copper minerals in the world-class Olympic Dam Cu-U-Au-Ag deposit, South Australia. Olympic Dam is characterized by systematic, inwards and upwards zonation of Cu-Fe-sulfide assemblages, encompassing chalcopyrite-pyrite, bornite-chalcopyrite, bornite-chalcocite and chalcocite-only zones. Trace element analysis of Cu-(Fe)-sulfides (~ 3500 spot analyses) by laser ablation inductively coupled plasma mass spectrometry on samples from across the deposit identifies the role of spatial position, protolith, and the presence/absence of co-existing sulfides (sphalerite, tetrahedrite-tennantite and carrollite) in control of trace element endowment. Cu-(Fe)-sulfides host concentrations of precious metals (Ag, Au), potential value-add elements (Se, Te, Bi, As, Sb, In) and deleterious elements (Pb, Hg). Where bornite-chalcocite co-exist, Ag is partitioned into chalcocite and Bi into bornite; in the absence of either bornite or chalcocite, chalcopyrite is a significant host for both elements. Chalcocite from the chalcocite-only zone is depleted in Bi-Te-Ag-Au compared to the bornite-chalcocite zone, demonstrating the role of bornite as an initial scavenger of these elements. A distinct inherited Cr-Ni-Zn signature is identified in chalcopyrite hosted by banded iron formation derived lithologies and proximal to crosscutting dykes. Despite some variation, Cu-(Fe)-sulfides generally contain more Bi and lesser Se towards deeper levels. The concentrations of these elements in paired bornite-chalcocite assemblages show promise as ore vectors, whereas Ag/Te in brown bornite and Se/Ag in chalcopyrite are prospective lateral vectors. Results carry implications for understanding deposit evolution, provide insights towards developing reconnaissance exploration vectors, and offer guidance on trace element deportments likely to impact ore quality and geometallurgical performance.