PALEOZOIC SHALE-HOSTED BASE METAL DEPOSITS OF THE NORTHERN CORDILLERA WITH EMPHASIS ON ALASKA

Sedimentary basins in the northern Cordillera host some of the largest and most economically important Zn-Pb-Ag deposits in the world. Deposits in the Kuna basin, Alaska (e.g., Red Dog district) and those in the Selwyn Basin, Yukon/northern BC, contain at least 60 Mt Zn. Sulfides are hosted in carbonaceous mudstone, shale, chert or calcareous units. In addition to Pb-Zn sulfides, correlative strata may host barite and/or phosphate deposits. The basins show evidence of periodic tectonic instability +/- minor mafic to felsic alkaline rocks and are interpreted as extensional passive continental margin basins that range in age from Cambrian to Late Mississippian.

Recent work in northern Alaska highlights the importance of coincident factors in the evolution of the basin that were favorable for deposit formation. The reconstructed Carboniferous Kuna basin was large (200 x 600 km) and was bordered by carbonate platforms on three sides, which limited input of siliciclastic material and preserved organic carbon that likely served as a reductant during mineralization. Mississippian extension and horst-and-graben architecture is manifested by facies variability between coeval units. The deposits at Red Dog are superimposed upon a NNW-trending facies boundary defined by abrupt lateral thickness changes in shale and turbidite units. The climate may have been arid, leading to evaporation of seawater, production of brine, and availability of marine sulfate for barite formation. Initiation of barite, phosphate, and zinc mineralization coincided with regional drowning of the outboard carbonate platform in Late Mississippian time, presumably as a result of extensional tectonism/subsidence. Methane was transported with barium into shallowly buried sediments, and H2S was produced by anaerobic methane oxidation. By analogy with cold methane seeps on the modern seafloor, the production of H2S may have been orders of magnitude faster than during normal organic matter decomposition. The main stage Zn-Pb sulfide mineralization followed barite deposition, with some deposition occurring at the seafloor in unconsolidated muds, and the majority of the sulfides forming subseafloor relacements of barite or carbonate. The implied high H2S production rates may have led to efficient precipitation of metals as sulfide minerals and, thus, may explain the high grades of the zinc deposits.