Development of nickel-rich lithiophorite in khondalite-hosted Mn-ore deposits of South Odisha, India

The Precambrian Eastern Ghats Group of rocks consisting of a series of regionally metamorphosed rocks such as khondalite, charnockite, calc-granulite and quartzite hosts several pockets to lensoidal Mn-ores in Odisha, Eastern India. The Mn-minerals identified in these deposits are Mn-silicates and lower Mn-oxides with Mn-oxyhydroxides. Lithiophorite, an aluminous Mn-oxyhydroxide mineral, is conspicuous with other secondary Mn-phases. It generally occurs in two habits, such as cryptocrystalline/crystalline aggregates either as vug fillings or as replacement of other minerals like cryptomelane/romanechite/goethite/graphite. Two morphological types of lithiophorite viz. globular and platy were examined in detail by optical and electron microscopy. The electron probe micro-analyses (EPMA) of these two morpho-types reveal the cryptocrystalline (globular) type to be rich in both CoO and NiO, while the platy crystalline type is preferentially enriched in NiO and depleted in CoO. Compositional map for both the types also supports such selective entrapment. Binary plots of electron probe data for MnO2 and Al2O3 versus CoO and NiO in the globular type show strong positive relation confirming to their presence in adsorbed state. In contrast, the NiO in platy type show strong positive relation with Al2O3 only indicating desorption of Co and a part of Mn from the structure during crystallisation and its subsequent diadochic substitution in the alumina lattice.