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Precambrian Deposits |
1 Institut für Geowissenschaften, Justus-Liebig-Universität, Senckenbergstrasse 3, D-35390 Giessen, Germany
2 Department of Earth Sciences, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK
3 Postgraduate College for Geoscientific Research in Africa, University of Würzburg, Germany
The genesis of manganese and iron formations (MnF and IF) is a closely related process during the Proterozoic, in which oceanic crustal venting provided metal-rich solutions to be deposited on adjacent continental shelves. Their deposition in condensed marine sequences occurred when continental break-up initiated a major transgression and provided favourable conditions for precipitation of chemical sediments from seawater. The Neoproterozoic Rodinian supercontinental break-up shows the relationship between this tectonic process, climate, and MnF/IF genesis. Metallogenesis followed particular rift-drift transitions during supercontinental break-up. MnFs/IFs are often associated with glaciomarine sedimentary rocks. Higher O2/CO2 during continental rifting would, through inducing glacial conditions and withdrawing seawater, increase ocean salinity and thereby allow both deposition of glaciomarine sediments and precipitation of evaporites. Salinities would have been particularly enhanced in the restricted oceanic basins provided by early supercontinental rifting, where atmospheric evaporation would also have played a part. Accumulation of Mn and Fe in the Neoproterozoic may be directly related to a probably glacially derived increased ocean salinity that promoted accumulation and storage of Mn and Fe in ocean water through increased Mn, Fe, and Ba(?) solubility. Very similar processes may have been effective in Palaeoproterozoic times, although then a higher amount of Fe and Mn discharge and different ocean chemistry would have superseded the effect of ocean salinity. Chemical Mn and Fe precipitates related to supercontinental break-up in the Proterozoic as well as the Phanerozoic indicate a similar relationship between tectonics and ore accumulation. The resultant close time and space relationship between potent oceanic source and suitable continental-shelf depositional settings optimised accumulation and preservation of ancient Mn-Fe deposits. This scenario is at variance from present-day environments where source and suitable precipitation areas of long-term preservation potential are mostly widely separated in space.
