Morphological and microstructural characteristics of bauxite developed over a part of Precambrian Iron Ore Group of rocks, Sundergarh District, Eastern India

Bauxite occurring as a blanket over volcanics of Precambrian Iron Ore Group in Kusumdih and Jaldih of Sundergarh district, Odisha, Eastern India exhibits four types of morphology viz. pisolitic, disseminated, spotted and massive. The volcanics/tuffs are altered to shale and is predominantly constituted of kaolinite. Microstructures documented in different morphologies of bauxite are collomorphous, framework, chain, stalactitic, reticulate, vesicle filled and foliated-platy types. Gibbsite, diaspore and kaolinite exhibit the abovementioned morphological and microstructural features. The morphological and microstructural characteristics are interpreted in terms of genetic evolution of aluminous minerals of bauxite in such a setup. Gibbsite and diaspore are formed through solution and precipitation/recrystallisation from kaolinite booklets present in parent tuffaceous shales. Some diaspores were formed at the expense of gibbsites. Small well-ordered kaolinites are of authigenic nature. Kaolinite formed inside a localized closed system containing supersaturated solution exhibits growth spirals. The textural and morphological diversities of the bauxite, abundance of well-ordered bauxite minerals and the absence of any iron/silica phase along with limited occurrence of primary kaolinite together suggest that the deposit has attained a high degree of bauxitisation in weathering environment at surface temperature.