Morpho-chemistry of hematite in manganese-rich formation of Gangpur Group, Orissa

Abstract

The Precambrian manganese-rich formation (Goriajhar Formation) Gangpur Group contains appreciable Fe, as reflected from the occurrence of hematite in different paragenesis. The first generation of hematite (Hm(1)) exhibits dissimilar micromorphology, contains Mn in solid solution (MnO: 1 to < 4%) and may be termed as manganoan-hematite. The different morphological types (Hm(1)) recorded are: hexagonal plate, twinned idiomorph, subhedral inclusion within quartz and braunite, blebs forming zone within spessartine garnet, crystal dusted with oriented intergrown ilmenite, haphazard platelets within Mn-aegirine, oriented grains forming alternate bands and fused grains forming large porphyroblast. The hematite formed due to martitisation is of second generation (Hm(2)) and the one that occurs as vein fillings, often traversing the other two types, is of third generation (Hm(3)). Both Hm(2) and Hm(3) are devoid of Mn content. The first generation hematite is of primary type, syngenetic with manganese minerals and developed at different stages of metamorphism. The hematite of other two generations is of hypogene origin, Hm(2) being formed due to low-temperature oxidation, while Hm(3) during solution and reprecipitation at meteoric condition.