Factors controlling free thermal convection in faults in sedimentary basins: implications for the formation of zinc-lead mineral deposits

Abstract

Stratiform sediment-hosted Zn-Pb-Ag mineral deposits constitute about 40% of the Earth's zinc resources (Allen 2001), and in most cases their genesis involves the discharge of basinal brines near or on the seafloor through syndepositional faults (Sangster 2002). From the point of view of base metal exploration, it is therefore essential to identify all possible faults that formerly carried the upwelling ore-forming solutions during mineralising events. This paper presents a numerical investigation of the relative importance of various physical parameters in controlling fluid discharge, recharge and heat transport in faults. A two-dimensional, free convection of pure water, hydrogeological model is developed for the McArthur basin in northern Australia based on the surface geology, known stratigraphic and structural relationships and regional geophysical interpretations. Numerical experiments and sensitivity analyses reveal that faults with strong initial heat input, due to depth of penetration or magmatic activity, are the most likely candidates to carry discharge fluids to the sites of metal precipitation. Deeper, wider and more permeable faults are more likely to behave as the fluid discharge pathways, whereas shallow, narrow or less permeable faults act as marine water recharge pathways. Compared with these fault-related factors, aquifer physical properties are less important in determining fluid flow patterns and the geothermal regime. These results are an important step in understanding hydrothermal fluid flow in sedimentary basins in order to develop effective exploration criteria for the location of stratiform Zn-Pb-Ag deposits.