Aeromagnetic patterns of half-graben and basin inversion: implications for sediment hosted massive sulphide Pb-Ag-Zn exploration

Abstract

The inverted extensional basins of the Palaeoproterozoic North Australian Craton are amongst the most metallogenically endowed on earth for sediment-hosted massive sulphide (SHMS) Pb-Zn-Ag deposits. Faults that evolved during the evolution of these basins are an important exploration vector as they have behaved as conduits for mineralizing fluids, controlled facies architecture, and impacted on hydrothermal convection cells. We attempt to map out ancient normal faults using integrated structural and aeromagnetic analysis to identify faults that are either buried beneath younger cover or have been inverted during later orogenic events. This technique can be used for targeting potential conduits for mineralising fluids during SHMS Pb-Zn-Ag exploration, as well as a tool to map the architecture of extensional basins. We have constructed simplified cross-sections of half-graben and inverted half-graben and have forward modelled their magnetic response. The aeromagnetic signature of a pre-inverted half-graben shows that the tilt block is characterised by a shallow magnetic gradient related to the shallowing of the magnetic marker beneath the half-graben. Normal offset of the marker unit across a normal fault is characterised by a steeper magnetic gradient, as the depth to the marker horizon increases. Simplified cross-sections of isolated growth anticlines and the hanging wall buttressing show that the general shape of the magnetic profiles are maintained until the half-graben has been completely inverted and the magnetic marker horizon displays reverse offset along the reactivated normal fault.