MINERALOGY, CHEMISTRY AND GENESIS OF NISHIKHAL MANGANESE ORES OF SOUTH ORISSA, INDIA

Manganese ores of Nishikhal occur as distinctly conformable bands in t he khondalite suite of rocks belonging to the Precambrian Eastern Ghat s complex of south Orissa, India. Manganese minerals recorded are cryp tomelane, romanechite, pyrolusite, with minor amounts of jacobsite, ha usmannite, braunite, lithiophorite, birnessite and pyrophanite. Goethi te, graphite, hematite and magnetite are the other opaque minerals and quartz, orthoclase, garnet, kaolinite, apatite, cellophane, fibrolite , zircon, biotite and muscovite are the gangue minerals associated wit h these ores. The mineral chemistry of some of the phases, as well as the modes of association of phosphorous in these ores have been establ ished. The occurrence of well-defined bands of manganese ore; co-foldi ng of manganese ore bands and associated metasedimentary country rocks ; the mineral assemblage of artite-sillimanite-braunite-jacobsite-haus mannite; the geochemical association of Mn-Ba-Co-Ni-Zn together with t he Si versus Al and Na versus Mg plots of the manganese ores suggest t hat the Nishikhal deposit is a metamorphosed Precambrian lacustrine de posit. Continental weathering appears to be the source for manganese a nd iron. After deposition and probable diagenesis, the manganese-rich sediments were metamorphosed along with conformable psammitic and peli tic sediments under granulite facies conditions, and subsequently unde rwent supergene enrichment to produce the present deposit.