The geochemistry of the weathering sequences — Present and past — In and around the Pottangi and Panchpatmali bauxite-bearing plateaus, Orissa, India

Abstract

The so-called East Coast Bauxite Deposits of India occur in a 30-km-wide and 400-km-long belt on the Eastern Ghats as lateritic bauxite cappings over plateau tops. The characteristics of the weathering process now active over some of these bauxite-bearing plateaus are determined by way of studying the properties of groundwater and the recent weathering profile. The groundwater is characterised by low total dissolved solids (TDS) content, feebly acidic pH and moderately high Eh-value which is promoting formation chiefly of kaolinite from the parent silicates and a minor amount of gibbsite. The results of interaction between the groundwater and the parent rock (khondalite) are registered in the recent weathering profile. A probe in the relative mobility (RM) of cations reveals the fact that out of the immobile cations Fe, Ti and Al, the least mobile is Fe and that RM of Al and Ti is higher in the kaolinitic stage than in the laterite-bauxite stage of weathering. It is concluded that bauxitisation is inoperative in the area under review during the present time. A detailed study of the ancient weathering profile comprising the laterite-bauxite duricrust has been conducted. It is established from petrographical evidence that the lateritic bauxite is an in situ derivation from the Archaean parent rock (khondalite) by direct gibbsitisation. The age of formation has been assigned to the early Eocene period. The paragenesis of mineral formation and the behaviour of the various oxides have been studied. The conditions favouring bauxitisation back in early Eocene times is discussed.