The McArthur River (HYC) Zn-Pb-Ag deposit in the Carpentaria Zn belt, northern Australia, is one of the world’s largest and most studied sediment-hosted base metal deposits, owing to its lack of deformation and preservation of sedimentary and ore textures. However, the ore formation process (syngenetic vs. epigenetic) is still a subject of controversy. In this paper we focus on key characteristics of the HYC deposit that remain unexplained: preservation of sedimentary carbonate (dolomite) and its association with Zn, and the role of thallium (Tl) and manganese (Mn) distribution in the orebody.
Our findings demonstrate a sequence of events during ore formation: Tl is hosted almost exclusively within euhedral pyritic overgrowths around early diagenetic pyrite; sphalerite mineralization occurred after Tl-bearing pyrite overgrowths, in association with acid dissolution (replacement) of laminated and nodular dolomite across the subbasin; and outer rims are enriched in Mn on preserved dolomite at the dissolution reaction front in contact with sphalerite. New thermodynamic fluid chemistry modeling demonstrates the metal distribution and paragenesis can be explained by acidic, oxidized ore fluids entering the pyrite-dolomite host lithology, allowing reduction and pH buffering by acid carbonate dissolution, resulting in stepwise metal deposition in an evolving fluid.
We argue this represents strong evidence for epigenetic ore formation at HYC. Furthermore, the primary control on ore deposition is not synsedimentary faulting in the subbasin; rather, the chemical potential of sedimentary carbonate within reduced, sulfidic lithologies appears to be of critical importance to precipitation of sphalerite.