REACTIVE TRANSPORT MODELS FOR ORE GENESIS IN THE RED DOG DISTRICT, KUNA BASIN, ALASKA

The upper Kuna Formation, a finely laminated, black, organic-rich siliceous mudstone and shale in the Red Dog ore district, Alaska, is host to one of the largest resources of zinc yet discovered in the Earth’s crust, containing ore reserves in excess of 140 Mt averaging about 16% Zn and 5% Pb. The ores are thought to have formed in the late Carboniferous, within the anoxic mud-rich Kuna Basin when adjacent carbonate platforms were drowned by rifting and tectonic subsidence. Fluid inclusion studies of ore and gangue minerals indicate sub-seafloor mineralization between 125 to 140 oC, during a period of sediment diagenesis and extensional faulting. To test genetic models for hydrothermal convection, fluid migration, and ore deposition, we present 2-D numerical models of coupled reactive transport. A finite element grid was adapted for a geological section of the Kuna Basin, structurally and stratigraphically restored to latest Mississippian time. Hydrologically, the Kuna Basin was a 200-km wide, rifted asymmetric basin layered with mudstones and carbonates overlying thick conglomerate and sandstone aquifers, which were structurally thickest and vertically displaced by normal faulting near Red Dog (Ikalukrok graben). Buoyancy-driven free convection cells drive fluid migration to km-depths in the submarine basin, at rates of about 5 m/yr within permeable normal faults, which focus metal discharge. Mostly lateral brine flow is predicted to occur in the deep clastic formations and fractured basement. The clastic aquifers and older metasedimentary basement rocks appear to be the principal reservoirs for metal-bearing brines that ultimately discharged near the seafloor within slightly permeable, highly porous and organic rich mud, where sulfate was reduced to form massive zinc-lead sulfide ores. High paleoheat flows of 150 to 160 milliWatts per square meter and focused fluid discharge along normal faults are required to explain the hot fluid flux. The reactive flow simulations illustrate the extent of mineralization associated with fluid mixing and replacement processes and help evaluate geochemical models for mineralization and fluid transport in this giant ore district.