Effects of ocean chemistry, sea level, and climate on the formation of primary sedimentary manganese ore deposits

Abstract

The mineralogy, geochemistry, and in some cases, the mode of origin of many Phanerozoic sedimentary manganese orebodies are fairly well known, but the effects of the important variables represented by changing ocean chemistry, sea level, and climate on ore formation are only recently being considered. The high degree of mobility of manganese, particularly through redox processes, insures that it can change phase during weathering, transport, deposition, and diagenesis.During chemical weathering, manganese is solubilized in acid, reducing conditions and carried in surface and subsurface waters to the coastal zone, where many of the exploitable deposits can form in slightly reducing (carbonate ores) to oxidizing conditions (oxide ores). Rainfall controls ground-water acidity through fostering vegetation growth. Major deposits at Groote Eylandt, Australia, and the extensive deposits of the Paratethys seaway in eastern Europe and the Soviet Union accumulated as a result of large-scale delivery of dissolved Mn to restricted black shale basins during marine transgressions. The deposits show compositional zoning and display evidence of having been deposited in shallow-marine environments following accumulation of a dilute ore solution in the basin interiors. Manganese was precipitated when the environments of the basin became oxidized, apparently during the early stages of marine regressions.During the Phanerozoic the concentrations of dissolved oxygen and carbon in the oceans varied significantly, in response to large changes in the burial rate of organic carbon. When the rate was high, there was a drawdown of atmospheric CO 2 , which led to global reverse-greenhouse cooling. Later, upon oxidation of sea-floor organic matter, CO 2 was released and greenhouse warming eventuated. Manganese is now known to have precipitated rapidly as crusts in the oceans late in the glacial and early in the interglacial stages of the Quaternary, probably as a result of extensive sea-floor oxidation accompanying the release of CO 2 . This same set of processes, operating over geologic time, may have played a significant role, first, in generating large reservoirs of dissolved manganese in intracratonic basins and in the oceans, and second, by providing the metal for the formation of ore deposits when conditions changed.