PALEOHYDROLOGY OF A 1640 MA STRATIFORM ORE DEPOSIT, MCARTHUR BASIN, AUSTRALIA

The McArthur River (HYC) zinc-lead-silver deposit (237 Mt @ 9.2% Zn, 4.1%Pb, 41 g/t Ag) of northern Australia formed in a hydrothermal system in which brines vented near the Proterozoic seafloor, but the hydrogeochemistry of ore formation is not well understood. Eight separate and finely laminated ore lenses formed in the 1640 Ma Barney Creek Formation, a carbonaceous dolomitic mudstone. HYC developed in a brine pool and/or by replacement of reduced sediments adjacent to the Emu Fault system which served as a conduit for upward migration of fluids. HYC is located on the eastern side of the Batten Trough, a deeply faulted section of the intracratonic McArthur Basin. At Barney Creek time, about 2 km of carbonates (lower McArthur Group) and 3 to 6 km of siliciclastic and volcanic rocks (Tawallah Group) comprised the underlying section in the Batten Trough.

Mathematical modeling of subsurface fluid flow in the McArthur Basin shows that basinal brines within the Tawallah Group could undergo thermal convection. Convection is strongly controlled by the geometry of the basin-bounding normal faults: sea-floor discharge of 140°C brine occurs along the Emu Fault zone while cold recharge of basin fluids occurs across the Batten Trough. Complex patterns of flow are predicted at HYC: brines discharge on the seafloor, seep down through the McArthur Group sediments, and recirculate back to the Emu Fault. While precipitation of ore minerals probably occurred in the brine pool, vertical reflux of the brine-pool fluids through the basinal muds adjacent to the Emu Fault zone provides a mechanism for synsedimentary and early diagenetic replacement-type ore genesis by an "inhalative" process in a classical SEDEX tectonic and stratigraphic setting. Chemical modeling conducted with the EQ3/6 software is being used to evaluate the ore-mineral paragenesis.