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

In the Sukinda ultramafic belt of Orissa, significant deposits of nickelife rous laterite have formed by weathering of large chromiferous dunite-perido tite 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 i n order to establish petrochemical relationships between supergene Ni-later ites 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 (altere d dunite) to top pisolitic laterite with transitional clay-serpentinite zon e 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 antigorit e 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 o verlying clay-serpentinite zone, serpentine and nontronite disappear comple tely and goethite constitutes the principal mineral with varying proportion s of secondary quartz. This zone is rich in Fe, Ni, Co, Mn, Al and Cr and i s formed by residual concentration of stable oxides. Mass-balance equation assuming Cr2O3 as constant chemical constituent reveals that nickel is stro ngly accumulated in the clay-serpentinite zone. A second maximum in nickel concentration is found in ochreous ferruginous zone with almost complete re moval of magnesium. Normalised major and trace element data generated on th e bore hole samples have been processed by using R-mode factor analysis of correlation matrix to understand the natural process of weathering. Leachin g studies of the samples suggest that nickel occurs in adsorbed state withi n amorphous hydrated iron oxide and in weakly bonded and lattice bound stat es within goethite and secondary silicates.