HYDROGEOLOGIC MODELING OF REACTIVE FLOW, FAULTS, AND PB-ZN ORES IN EXTENSIONAL BASINS

Three giant ore provinces in extensional basins provide good examples of structural control on reactive mass transport: 1) shale-hosted Pb-Zn ores of the southern McArthur basin, Australia, 2) shale-hosted Pb-Zn-Ba ores of the Kuna basin, Alaska, and 3) carbonate-hosted Pb-Zn ores of the Lennard Shelf, Canning basin, Australia. For the McArthur basin, hydrogeologic simulations of thermally-driven free convection suggest a structural control on flow caused by the north-trending fault systems which dominate this Proterozoic extensional basin. Fluids descended to depths of a few kilometers along the western side of the basin, migrated laterally to the east through the permeable clastic and volcanic aquifers of the upper Tawallah and McArthur Formations, and then ascended along the eastern side via the Emu Fault zone. Reactive transport simulations are shown to test the ability of metal-bearing brines to flow up the Emu fault zone and cause mineralization during synsedimentation of the Barney Creek Shale. A similar hydrogeologic scenario for sediment-hosted ore mineralization occurs in the Late Paleozoic-age Kuna Formation of northwestern Alaska, host to the world’s largest zinc deposit, but with replacement from below.

On the Lennard Shelf of Western Australia, epigenetic Pb-Zn mineralization occurs in Middle Devonian evaporitic dolomites which were part of a barrier reef system. Ore mineralization exhibits a strong structural control at the basin scale and normal faults controlled pathways for brine and petroleum migration that affected ore deposition (Solomon and Groves, 1994). For the Canning basin, hydrologic simulations show that compaction was the most important process for creating overpressures and driving basinal fluids in this thick extensional basin. Reactive flow simulations will test a petroleum-reservoir model for mineralization whereby metal-bearing brines mix with accumulated hydrocarbons.