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Precambrian Deposits |
Department of Earth Sciences, University of Sheffield, Sheffield S3 7HP, UK
The fractionation of manganese from iron through geological time has previously not been well understood. Iron deposits were considered to be abundant in the Precambrian and have declined in the Phanerozoic whereas manganese deposits appeared to show the opposite trend.
It is now seen that the Proterozoic banded iron formations contain about 200 times the amount of manganese in land-based manganese deposits and an order of magnitude more manganese than in deep-sea manganese nodules. The great abundance of manganese in these banded iron formation appears to resolve this problem.
In the Archaean and Proterozoic oceans, hydrothermal activity played a much more important role than at present due to the higher geothermal gradients. Upwelling of hydrothermally-derived iron, silica and manganese from anaerobic oceanic basins to more oxygenated shallow platform areas may have provided the unique conditions required for the formation of the Proterozoic banded iron formations which constitute the largest mineable reserve of any metal. An important factor in the formation of these deposits was the low sulphate ion content of the ocean waters. This ensured that the iron did not precipitate dominantly as pyrite under the reducing conditions then prevalent in the ocean waters. Manganese and iron could therefore display similar aqueous geochemistries. The reason for the separation of manganese from iron to form giant manganese deposits such as the Kalahari manganese deposit remains uncertain but variable redox conditions resulting from transgression and increasing atmospheric oxygen content or the influence of extensive hydrothermal activity may have been involved. Nonetheless, the similarity in the geochemistries of manganese and iron in the Palaeoproterozoic contrasts with the situation encountered in Phanerozoic manganese deposits in which iron became immobile in adjacent anoxic basins as pyrite thus ensuring the fractionation of manganese from iron. Oxidation of the ferrous ion could have taken place as a result of photosynthesis or of inorganic photochemical reactions. The formation of oxygen by these processes could have been the rate-determining step in the formation of the banded iron formations. It is possible that blooming by blue-green algae played a key role in the oxidation of the ferrous iron in the seawater. The seasonability of this blooming would then have resulted in the characteristic iron-rich and iron-poor microlaminae of these deposits.
