Mineralogy and geochemistry of profiles through lateritic nickel deposits at Kansa, Sukinda, Orissa

In the Sukinda ultramafic belt of Orissa, significant deposits of nickeliferous laterite have formed by weathering of large chromiferous dunite-peridotite suite of rocks. Two typical laterite profiles obtained by drilling (BH-1 = 50 m and BH-2 = 46 m) from Kansa sector of Sukinda belt were studied in order to establish petrochemical relationships between supergene Ni-laterites and the parent rock. In BH-1, ferruginous layers comprise the profile where goethite is intimately admixed with quartz, kaolinite and gibbsite at various depths. In BH-2, a complete laterite profile from bed rock (altered dunite) to top pisolitic laterite with transitional clay-serpentinite zone is encountered. The altered dunite is essentially composed of olivine (Fo(93.08) congruent to 35% vol.) and serpentine. The cf ay-serpentinite zone consists of varying proportions of smectite clay (nontronite) and antigorite with minor amount of chromite, goethite and quartz. This zone is enriched in Ni with a maximum value of 1.09% Ni. In the ochreous ferruginous zone overlying clay-serpentinite zone, serpentine and nontronite disappear completely and goethite constitutes the principal mineral with varying proportions of secondary quartz. This zone is rich in Fe, Ni, Co, Mn, Al and Cr and is formed by residual concentration of stable oxides. Mass-balance equation assuming Cr2O3 as constant chemical constituent reveals that nickel is strongly accumulated in the clay-serpentinite zone. A second maximum in nickel concentration is found in ochreous ferruginous zone with almost complete removal of magnesium. Normalised major and trace element data generated on the bore hole samples have been processed by using R-mode factor analysis of correlation matrix to understand the natural process of weathering. Leaching studies of the samples suggest that nickel occurs in adsorbed state within amorphous hydrated iron oxide and in weakly bonded and lattice bound states within goethite and secondary silicates.