REACTIVE FLOW MODELING OF FLUID MIXING & ORE GENESIS AT RED DOG, ALASKA

Giant shale-hosted Zn-Pb-Ba ore deposits in the Red Dog district of Alaska appear to have been formed by hot basinal brines migrating through organic-rich mud and replacing carbonate minerals at shallow depth below the sea floor in the Late Mississippian Kuna rift basin, but the sources of metal and sulfur, fluid migration pathways, effects of fluid mixing, and general hydrogeology are not well understood. The clustering of deposits, high ore grades and unusually large amounts of barite deposition suggests that the formation and evolution of these deposits may differ significantly from sediment-hosted ore districts in other parts of the world. This study focuses on the Anarraaq deposit at Red Dog, which is capped by the world's largest known barite deposit.

We have constructed 2-D numerical models to test a range of different scenarios to determine the necessary and sufficient conditions to facilitate ore mineralization within the hosting Kuna shale at Anarraaq (a carbonate-replaced radiolarite bed) and the subsequent accumulation of base metals. Reactive transport simulations test the effects of mixing the H2S-rich pore fluid (sourced within the radiolarite unit) with the metal-bearing basinal brine, sourced from a basal sandstone unit. The numerical results suggest that a long-lasting source of H2S in the radiolarite bed (massive ore zone) is necessary to replicate the observed sulfide mineralization to form the Anarraaq ore body in a reasonable time frame (< 1 My). A hydrothermal model based on evolving mixing scenarios likely explains the ore body zonation, from the massive ore zone closest to the fault to the banded sulfide zone and the low-grade, semi-massive ore zone further away.