SEDIMENTOLOGIC AND PALEONTOLOGIC CONSTRAINTS ON SETTING AND AGE OF THE RED DOG ZN-PB-AG MASSIVE SULFIDE DEPOSIT, WESTERN BROOKS RANGE, ALASKA

The upper Kuna Formation (Lisburne Group) in northwest Alaska hosts the giant Red Dog and related Zn-Pb-Ag massive sulfide deposits; our studies of Kuna sedimentology and paleontology constrain the setting, age, and thermal history of these deposits. The upper Kuna in the Red Dog area (Ikalukrok unit) is mainly finely laminated, black siliceous shale and mudstone, locally rich in sponge spicules and radiolarians. Mean total organic carbon (TOC) content is 4.6 wt %. Locally abundant carbonate layers are of two types: calcareous radiolarite (calcite-replaced radiolarians in a carbonate matrix) and fine to coarse sand-sized calcareous lithic turbidites. Turbidites (2 cm to >4 m thick) were derived from a mix of shallow- and deep-water sources; they consist of carbonate (<5 to >50 %), quartz (5-30 %), and other non-carbonate material (20-50%), including mud, lithic clasts, and phosphatic grains. Some turbidites lack carbonate. Similar lithofacies occur in the Kuna and related units ~120-200 km to the east (Howard Pass quadrangle), where TOC contents average 3.7 wt % and P₂O₅ values reach 7.7 wt %.

Sedimentologic, faunal, and geochemical data suggest that Kuna deposition in the Red Dog and Howard Pass areas took place mainly in basin and slope settings characterized by anoxic or dysaerobic bottom waters and locally high organic productivity (possibly related to upwelling). Conodonts and radiolarians indicate an age range of Osagean-Chesterian (late Early-Late Mississippian) for the Ikalukrok. Carbonate input into the Ikalukrok basin occurred chiefly during middle Osagean (Sc. anchoralis-Do. latus Zone) and late Meramecian-early Chesterian time, and ceased when adjacent carbonate platforms drowned. Conodont color alteration indices (CAI) for the Kuna in the Red Dog area are mostly 3.0 or 3.5, indicating temperatures of at least 120-200°C; tectonic burial and (or) an elevated geothermal gradient are needed to explain these CAI values.