THE MANGANESE FORMATION OF THE NEOPROTEROZOIC PENGANGA GROUP, INDIA—REVISION OF AN ENIGMA—A DISCUSSION

Abstract

Sir : Gutzmer and Beukes (1998) have revised the sedimentary-diagenetic mode of origin suggested earlier for the todorokite-rich (with birnessite, manganite, braunite, and bixbyite) manganese deposits of the Penganga Group, India (Roy, 1981; Roy et al., 1990; Bandopadhyay, 1996), concluding that these Mn oxide deposits were entirely derived by supergene alteration of a Mn carbonate protolith which they discovered in one deep quarry section (near Nipani) below the Mn oxide ore horizon. I congratulate the authors for this discovery of the Mn carbonate horizon that eluded us in spite of thorough search (Roy et al., 1990) during the 1980s, that perhaps was limited by the depth-extension, at that time, of the section by mining operations. Credit must also go to these authors for the very detailed characterization of these Mn carbonates including stable isotope and REE geochemistry. I have, however, strong misgivings on several conclusions drawn in this paper. In a 40-km-long belt of Mn oxide deposits, this study was confined to only two spots (p. 1091) near Nipani (Mn carbonate rock) and Jamdapur (supergene ore) and yet a generalization is made that the entire Mn oxide ore horizon was formed by supergene alteration of a Mn carbonate protore rejecting our conclusion (Roy et al., 1990), based on studies over the entire ore belt, that the Mn oxide ores are sedimentary-diagenetic in character. Our studies showed that the …