Potential for High-Grade Shallow-Marine Manganese Deposits in North America

Many large high-grade manganese deposits occur in marine-transgressive sedimentary sequences deposit fed on stable platforms, although the manganese mineralogy varies from carbonate to oxide facies and host lithologies vary from carbonate- to clay-dominated sections. Examples include the Nikopol and related deposits of the Soviet Union and Groote Eylandt, Australia. Recent work has shown that high sea-level stands are synchronous with stratified and stagnant oceans. We suggest that during transgressions and consequent anoxic events, the anoxic deep waters rich in dissolved manganese were dilute ore-forming fluids. Manganese was precipitated on the oxidized side of oxidation and alkalinity interfaces on shallow shelves. Where the precipitate was preserved, manganese ac- cumulated into ore deposits. Sedimentary manganese deposits have not been widely explored for in North America, nor are they always obvious in outcrop. Large areas of North America are underlain by rocks that we consider broadly favorable for such manganese deposits, and the disparity between the high potential but significant reserves of the United States is difficult to reconcile. Important undiscovered deposits seem likely. An area of Cretaceous rocks in South Dakota is presented as a possible location for a United States deposit analogous to foreign deposits described herein.