Late diagenetic changes in manganese ores of the Upper Proterozoic Penganga Group, India

Abstract

Sedimentary manganese-oxide ores occur interstratified with chert and minor calcareous shale and enclosed in limestone in the transgressive Upper Proterozoic marine sequence of the Penganga Group, Andhra Pradesh, India. This sequence shows no evidence of volcanism or hydrothermal activity and is not metamorphosed. The manganeseoxide ores show unusual coexistence of the primary sedimentary minerals, todorokite and birnessite, with their diagenetic conversion products, manganite, braunite and bixbyite, The mineralogical characters and chemical compositions of the important phases as well as their texture are discussed. Braunite occurs in two generations, one earlier or contemporaneous with bixbyite and the other later than bixbyite. The different stages of conversion of todorokite and birnessite to braunite and todorokite to manganite are preserved. The general observations suggest that bixbyite was also produced by conversion of todorokite-birnessite. Pyrolusite and cryptomelane, conventionally considered as diagenetic products of Mn-oxide/hydroxide precursors deposited as primary sediments, are here rarely present and then only as supergene derivatives. This documentation of diagenetic transformation of primary todorokite and birnessite directly to braunite and bixbyite should be considered while extrapolating the progenitor mineralogy of the metamorphosed manganese-oxide ores that abound in the Precambrian.