The Supergene Mn-minerals in Gangpur Group, Eastern India and their Genesis

Abstract Manganese mineralization in the Precambrian Gangpur Group of rocks in Sundargarh district of Odisha occur as pockets or small ore bodies associated with metapelites and metapsamites. Different varieties of hypogene metamorphic manganese minerals belonging to Mn-oxides, Mn-carbonates, Mn-silicates, Mn-titanates have been reported from this Group. Although the Mn-bearing rocks and the Mn-ores host several supergene Mn-minerals, no detailed and comprehensive study on these minerals has been carried out so far. The present paper describes the microstructural and microchemical characteristics of the supergene Mn-minerals viz., vernadite, manganite, cryptomelane, romanechite, Fe-Mn sepiolite, nsutite, pyrolusite, and lithiophorite with a view to establish their paragenetic sequence and genetic evolution. These minerals have formed due to solution-remobilization-precipitation, oxidation and/or hydration, diagenesis and as a product of direct alteration. It is inferred that only one variety of manganite is formed due to retrograde metamorphism. While vernadite, cryptomelane and romanechite precipitated from Mn-rich solutions at low temperatures, Fe-Mn sepiolite formed as a direct alteration product of tephroite. Manganite and nsutite formed both as solution-precipitates as well as oxidation products of other Mn-minerals. While further oxidation leads to the formation of pyrolusite at a later stage, lithiophorite is the last mineral to form during weathering and lateritization.