Hydrogeologic and geochemical processes of stratiform zinc-lead ore-deposit formation, application to the McArthur Basin, Australia

Theoretical modeling of fluid flow in extensional sedimentary basins and epigenetic geochemical reactions is used to simulate processes of formation of stratiform zinc-lead ore deposits. Extensional sedimentary basins are dynamic environments for fluid-flow processes due to elevated heat flow, thick accumulations of permeable sediments, and fluid pathways along extensional faults. Thermal convection and thermohaline convection are favored by elevated heat flow and thick permeable sediments. Large-scale thermal convection can occur in a basin if the critical Rayleigh number is exceeded. The presence of fault zones with enhanced permeability can destabilize the pore fluid and enable thermal convection to occur under subcritical conditions of basin thickness, permeability, and heat flow. The size, geometry, and fluid-flow rates of thermal convection cells depend on spacings of fault zones, fault zone permeability, and other factors. Fault-bounded convection cells can have longer dimensions and control the locations of fluid discharge points to surface environments. Thermohaline convection can cause elevated-salinity basal brines to migrate upward and discharge to the sea floor. The onset of penetrative thermohaline convection can be accelerated by the presence of faults. The discharge of thermohaline plumes could be important for stratiform ore formation near faults. Reactive transport modeling with the numerical model RST2D was applied to the conditions of formation of the McArthur River deposit (McArthur Basin of Australia). An epigenetic origin of the McArthur River deposit is concluded based on the framboidal habit of the early pyrite, sulfur to organic carbon ratios of the host sediment, the preponderance of textural evidence, paleomagnetic data, the inverse relationship between pyrite and base metal sulfide concentrations, mass balance of sulfur in the host rock and the high-grade ore, and interpretation of the sulfur isotopic compositions of the sulfide minerals. The reactive-transport modeling included transport of metals to pyrite-bearing host sediment with pyrite replacement by sphalerite and galena, transport of metals and reduced sulfur together, and transport of metals with sulfate to carbonaceous host sediment with sulfate reduction. The pyrite-replacement process generated the most significant amounts of sphalerite and galena with simulated sphalerite concentrations comparable to the highest concentrations in the McArthur River deposit.