Three-dimensional numerical modeling of salinity variations in driving basin-scale ore-forming fluid flow: Example from Mount Isa Basin, northern Australia

This paper presents a fully 3-D numerical investigation into the effect of salinity on ore-forming hydrothermal fluid flow, and develops a highly conceptualized hydrological model to simulate the fluid plumbing system early in the history of the Mount Isa basin, northern Australia when lead–zinc deposits were formed therein. Our numerical modeling results indicate that the active synsedimentary faults and clastic aquifer form a favourable hydro-framework for regional-scale fluid flow, and variations in salinity have important implications for fluid migration and heat transport. When salinity is constant throughout the basin, hydrothermal fluid flow mainly circulates within the more permeable faults as 2-D convection cells, unless the contrast in permeability between the faults and aquifer is less than one order of magnitude. Enhanced salinities on the basin floor due to evaporation of seawater facilitate the development of full 3-D thermohaline flow systems. The 3-D convection rolls established by the evaporitic conditions tend to be stretched if only longitudinal faults exist, but become ‘mushroom-shaped’ when a cross fault is added to intersect the longitudinal ones. The intersection of the two sets of the faults enables seawater to channel downwards to a greater depth and circulate through larger volumes of the source rocks to leach more metal content, and eventually the heated basinal brines ascend to the basin floor via a more ‘localized’ discharge zone at higher venting fluid velocity and temperature, potentially forming giant ore deposits.