STRUCTURAL CONTROLS ON REGIONAL FLOW AND GEOCHEMICAL MASS TRANSFER IN EXTENSIONAL SEDIMENTARY BASINS

This paper surveys recent efforts in applying simple Darcy flow theory to characterize the effects of normal faults on hydrothermal mineralization in sedimentary basins. Two Australian ore provinces provide superb case studies of structural control: 1) shale-hosted SEDEX-type ores of Proterozoic age in the Batten Fault Zone, southern McArthur basin, and 2) carbonate-hosted MVT-type ores of Paleozoic age in the Lennard Shelf, Canning basin. In the McArthur basin, finite element simulations were developed to compare driving forces for brine migration. Buoyancy-driven free convection permits fluids to descend to depths of a few kilometers along the western side of the Batten Trough, migrate laterally to the east through aquifers of the Tawallah and McArthur Groups, and then ascend along the eastern side of the fault zone. This remarkable flow system suggests Na-Ca-Cl brines acquired metals in the deepest levels of the McArthur basin as the fluids migrated eastward through the basal aquifer. Upward flow was relatively rapid along the Emu Fault, so much so that hydrothermal temperatures develop in the muddy sediments at the sea floor due to venting. We conclude that large-scale free convection, punctuated by episodes of pressure-released flow, characterized notably long periods of mineralization during the Proterozoic in the McArthur basin. In the Canning basin, Pb-Zn ores formed in Middle Devonian evaporitic dolomites, which were part of a barrier reef system. Ore deposition appears to be controlled by major normal faults that acted as conduits for multiphase fluid flow. Hydrologic simulations show that compaction was the most important hydrodynamic process for creating overpressures and driving mass transport. Pore fluids were driven across the Fitzroy Trough through deeply buried Silurian-Ordovician aquifer units. These fluids then migrated during periods of episodic extension along fracture zones like the Cadjebut and Pinnacle Faults which channeled flow. Reactive-flow simulations test a petroleum reservoir model for mineralization, whereby metal-bearing brines mix with accumulated hydrocarbons.