FAULT-CONTROLLED PB-ZN-BA MINERALIZATION IN PETROLEUM SOURCE ROCKS, BROOKS RANGE, ALASKA: FINITE ELEMENT MODELING OF REACTIVE FLUID FLOW IN THE LATE PALEOZOIC

Organic-rich siliceous mudstone and finely-laminated black shale of the Kuna Formation in the western Brooks Range, Alaska, is host to the largest deposit of zinc yet discovered in the Earth's crust, containing ore reserves in excess of 140 million tons, averaging about 16% Zn and 5% Pb in the Red Dog ore district. The ores are known to have formed in the late Paleozoic, within the anoxic mud-rich rift when adjacent carbonate platforms were drowned by extension and tectonic subsidence. Fluid inclusion studies of ore and gangue minerals indicate sub-seafloor mineralization between 125 to 140 oC, during a period of diagenesis, extensional faulting, and submarine fluid expulsion. We present numerical simulations of reactive transport to test genetic models for fluid venting along faults. A finite element grid was adapted for a geological section of the basin, structurally restored to latest Mississippian time. The organic-rich Kuna Formation was deposited in 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. Thermally driven convection cells drive fluid migration to km-depths in the submarine basin, at rates of about 1 to 10 m/yr within permeable normal faults, which focus metal discharge near the sea floor. Mostly lateral brine flow is predicted to occur in the underlying clastic formations of the Endicott Group. 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 petroleum-bearing muds, in which fluid mixing and TSR processes helped formed massive zinc-lead sulfide ores. Paleoheat flows of 100 to 300 mW/m2 and focused fluid discharge along normal faults are required to explain the sulfide mineralization below and within massive deposits of barite which served as a caprock. The reactive-flow simulations illustrate the extent of mineralization associated with fluid mixing and mineral replacement and help evaluate geochemical models for fluid transport in this giant ore district.