Petrographic Studies of a Part of East Coast Bauxite Deposits, Odisha, Eastern India and Implication on their Genesis.

The East coast bauxite deposits of Odisha are well known in the mineral map of India. Some such deposits exposed at Nunapaimali, Sasubohumali and Baphlimali overlying khondalite of the Precambrian Eastern Ghats belt have been studied in respect of petrology, mineralogy and microstructure with a view to find out their genetic evolution. Bore hole samples from two former deposits and mine profile samples from Baphlimali revealed a good profile differentiation, in terms of three horizontal zones of variable thickness, which in descending order are: 1 to 29m laterite zone, 2 to 28m bauxite zone, 9 to 36m saprolite zone, comprising of lithomarge and partially lateritised khondalite [PLK] developed over host khondalite. Petrographic and mineralogical difference between these three zones, brought out through optical microscopy and XRD studies, further confirms the differentiation. Genetic evolution of bauxite is interpreted from the study of a large number of samples and results are collated from the photomicrographs appended in the paper. The authigenic bauxitisation was brought about by gradual dissociation of Al-silicate to Al-hydroxide minerals through different weathering cycles as indicated from their microstructural manifestations and mineral assemblages. Primary gibbsite is formed in the initial stage of weathering directly from Al-silicate minerals. Advanced stage of prolonged weathering converts the Al-silicate to kaolinite and then to secondary gibbsite. Grain size enlargement and recrystallisation showing mosaic habit, grain boundary migration and grain orientation are the outcome of digenesis. The weathering/lateritisation process continues, resulting in colloidal precipitation of gibbsite from pore water of lateritic residuum and is in progress even in recent period The Fe-oxide phases, represented by goethite and hematite are youngest in the paragenesis, and formed due to solution, precipitation and dehydration. Iron is mostly contributed by almandine garnet present in the host khohdalite. During chemical leaching, Fe gets released from garnet and occur either as FeOOH along its fractures or fill the weak planes/inter-granular space as secondary goethite. The goethite dehydrates to hematite during digenesis. Goethitisation and limonitisation particularly in the laterite zone are the results of supergene alteration. [ABSTRACT FROM AUTHOR]