QUANTITATIVE MODELING OF STRATIFORM ORE FORMATION: THE GIANT MCARTHUR RIVER ZN-PB DEPOSIT, NORTHERN AUSTRALIA

Stratiform zinc-lead ore deposits form by metal precipitation from brine discharged to the sea floor and/or by diagenetic/epigenetic replacement reactions at shallow depths below the sea floor in muddy sediments in extensional sedimentary basins. Thermal convection and thermohaline convection can account for upward flow and discharge of brine in these settings. Fault zones can control the patterns of upward fluid flow and discharge of convecting brines. For the McArthur Basin, diagenetic ore mineralization has been simulated with the reactive-transport code RST2D which couples equilibrium-controlled reactions with fluid flow, heat, and mass transport. The model region is 0.5-km thick and includes a 100-m sequence of the host Barney Creek Formation comprised of dolomite (45 %), pyrite (20 %), calcite (10 %), quartz (5 %), porosity (20 %), and minor amounts of albite and K-feldspar. The host rock is underlain by dolomitic limestone. Upward flow of brine is imposed as uniform distributed seepage or is concentrated along a fault zone to represent convective upflow. The brine contains 170,000 mg/l of chloride and has a pH of 5.0, zinc from 20 to 200 mg/l, lead from 1 to 10 mg/l, and low sulfur content. The diagenetic pyrite of the host rock provides a source of reduced sulfur to react with the base metals in the brine to produce significant ore mineralization. The rate and extent of mineralization depend on the flow rate of the brine, the abundance of pyrite in the host rock, and the metal concentrations in the brine. The paragenetic sequence includes progressive decrease of pyrite, calcite, dolomite, albite, and K-feldspar, and formation of sphalerite, galena, quartz, and muscovite and later formation of dolomite. Reaction of brine with pyrite-bearing host rock appears to be a viable mechanism for the formation of the McArthur River deposit within the time frame of the host-rock accumulation to produce the mineralogic composition and abundances of the ore.