Proterozoic basins, fluids and giant ore deposits of Northern Australia

Giant ore deposits of lead, zinc, copper and uranium occur in the McArthur Basin, a Proterozoic rift system in northern Australia. Earlier studies by Raffensperger and Garven (1995) showed how free convection of oxidized brines mobilized uranium to deposit uranium ores at the basement unconformity where fluids were reduced during reactions with graphitic shear zones. In current work, we are using finite element models to understand the hydrogeologic controls on submarine venting and ore genesis for the HYC deposit in the Proterozoic McArthur Basin. Base metals were acquired in the thick rift clastics by density-driven convection of slightly oxidizing brines. These fluids were then driven up the Emu Fault to form over 230 Mt of stratiform Zn-Pb-Ag ore in the adjacent Barney Creek Shale, perhaps in a submarine sea floor basin. The models consider a number of fluid-drive scenarios, and our numerical results indicate a strong effect of faults on fluid convection within porous sediments and volcanics of the McArthur rift system. Assuming major faults act as fluid flow conduits, cool seawater recharges the deep basin via the Tawallah Fault on the western side of the rift while hot brines discharge along the Emu Fault on the eastern side. (author)