Formation of the Oligocene manganese deposits of the Paratethyan basins: a review

A series of early Oligocene Paratethyan manganese deposits occur in the shallow-marine sediments of the Maikop Series that surround the present-day Caspian Sea and Black Sea basins. These deposits are located either in former shallow-water seaways between the anoxic basins, where they typically occur as world-class deposits, or as much smaller deposits in shallow-water, shelf settings around the margins of the anoxic basins. The host rocks of the deposits are similar and are composed mainly of laminated marls, clays, and carbonates. The major-oxide compositions of the Paratethyan manganese deposits are similar and reveal a distinctive enrichment in Mn, Ca, and P, relative to Post-Archaean Australian Shale (PAAS). In addition, compared to the PAAS, the deposits typically show a trace element enrichment of Co, Ni, As, Ba, Sr, and strong depletion of Nb, Ta, Cr, Zr, Th, and Hf, which can be attributed to their similar processes of formation. Detrital-sourced elements of the Paratethyan deposits such as Nb, Ta, Cr, Zr, Th, Ti, and Hf are characteristically extremely low relative to the PAAS and indicate syn-sedimentary Mn-oxide and -carbonate deposition under conditions of low detrital input. Furthermore, a PAAS normalized positive Eu anomaly typically present in most deposits together with high contents of Co, Ni, As, Ba, and Sr in ores indicates a hydrothermal contribution into the basins. Besides the introduction of dissolved Mn and associated elements from rivers, this study shows that these elements may have been also discharged into the Paratethyan basins along the basin-margin growth faults. According to the geological and geochemical findings, Mn deposition is likely to have occurred from oceanic upwelling currents during periods of the high sea-level stand with syn-genetic precipitation of Mn-oxides and carbonates close to paleohighs between the deep anoxic basins.