THE MANGANESE FORMATION OF THE NEOPROTEROZOIC PENGANGA GROUP, INDIA -REVISION OF AN ENIGMA

A thin but laterally extensive stratiform manganese formation interbed ded with limestone, of the Neoproterozoic Penganga Group near Adilabad in Andhra Pradesh, India, which has been described in literature as p rimary sedimentary oxide deposit, has recently been found to have deve loped fron a manganese carbonate protore composed of laminated and mic rooidal rhodochrosite and kutnohorite. Models for the origin of the or e thus have to be revised. Sedimentological evidence suggests that the manganese was deposited on the outer shelf of a carbonate platform fo llowing a major transgression. Petrographic and geochemical evidence i s provided showing that the carbonate microoids originated immediately below the sediment-water interface during very early diagenesis. Micr obially mediated suboxic diagenesis and reduction of hydrogenetically precipitated Mn4+ oxihydroxides by organic carbon were instrumental in the formation of the manganese carbonates.