3D thermal convection in the Proterozoic McArthur River Zn-Pb-Ag mineral system, northern Australia

The sediment-hosted McArthur River Zn-Pb-Ag deposit is located in the southern McArthur Basin of northern Australia. 3D numerical models are used to explore the role of thermal convection in the mineral system that formed the McArthur River deposit. The model geometry is a simplified representation of the area, comprising gently dipping hydro-stratigraphic units intersected by a steeply dipping fault. An aquifer represents the source of mineralising fluid, while the fault provides a pathway for this fluid to reach the site of mineralisation, either on the seafloor (syngenetic mineralisation) or within sedimentary rocks adjacent to the fault (diagenetic/epigenetic mineralisation). Two fault permeability scenarios are investigated, representing an open fault with high permeability, and a closed fault with low permeability in the top 300 m of the model and high permeability below. In both scenarios, thermal convection occurs within the fault and aquifer due to their high permeability, with upwellings of hot fluid spaced at ~ 14 to 23 km along the fault. The results show that convection results in sufficient fluid exchange between the aquifer and fault to account for known mineralisation at McArthur River (~20 Mt Zn). In the open fault scenario, the convective upwellings provide sufficient flow of hot fluid onto the seafloor to form a 20 Mt syngenetic deposit in ~ 0.4 to 0.9 Myr. Syngenetic mineralisation would be accompanied by minor diagenetic mineralisation within the sediments adjacent to convective upwellings in the fault. In the closed fault scenario, some of the hot upwelling fluid flows into sediments adjacent to the fault at ~ 300 m depth, sufficient to create a 20 Mt diagenetic/epigenetic deposit in ~ 2.3 Myr. The rate and temperature of fluid flowing onto the seafloor or into the host rocks increases with fault permeability and heat flux, with higher heat flux and/or permeability being required to generate a diagenetic/epigenetic deposit than a syngenetic deposit within a geologically reasonable timeframe. Geometric complexities (e.g. fault intersections, bends or offsets) and areas of anomalously high heat flow or permeability are likely to focus convective upwelling, and are therefore suggested as targets for mineral exploration.