Depositional model for shallow-marine manganese deposits around black shale basins

Some of the world's largest and most valuable manganese resources formed as shallow-marine chemical sediments. We hypothesize that manganese deposition was on the margins of black shale (and related) facies of stratified seas. Manganese was deposited when deep anoxic water with high manganese solubility mixed with shallower oxygen-bearing water with low manganese solubility. The deposits show little or no evidence of volcanic contribution of manganese from nearby sources. The size of the manganese deposit varies with the vigor and dimensions of mixing and with the dissolved manganese content of anoxic deep water, the dilute ore-forming solution. The purity of the deposit depends on prior removal of dissolved iron from the system, primarily by pyrite precipitation in basinal environments, and on the absence of clastic dilution. Deposits may form one or two members of a zonal spectrum that includes sulfides, phosphorite, and barite. In many deposits there is evidence of great organic productivity, probably a by-product of vigorous mixing across stratification surfaces in the water column. Also common are primary sedimentary pisolites or oolites, glauconite, and biogenic silica.Manganese oxide facies deposition is preserved on oxic substrates in shallow water. Carbonate facies deposition can apparently be either on these oxic substrates or on reduced substrates in slightly deeper water. The reduced carbonate facies apparently forms by replacement of calcareous substrates by anoxic waters saturated with MnCO 3 , just below the water column redox interface. A single zoned deposit may show landward oxide facies and basinward reduced carbonate facies. Depositional regression may preserve the water column redox interface as a contact between overlying oxide facies and underlying carbonate facies manganese deposits.Stratigraphic evidence indicates that the deposits formed most commonly during high sea level stands in narrow time intervals when ocean anoxia was widespread. Eight manganese-precipitating modern environments and seven manganese deposits are described.