Abstract
Nickeliferous limonite occurs over a wide area above tbe serpentinized ultramafic rocks of Sukinda Valley (85°45'E: 21°4'N - 85°52'E: 21°4'N), Orissa, India. It is overlain by a thin mantle of laterite, and is itself underlain successively by silicified ultramafics and serpentinites with layers of chrornitite, followed by unaltered or partly altered dunite-peridotite. Nickeliferous limonite profile extends from 1.22 metres up to 7.0 metres on an average below the laterite cover. NiO content in limonite varies from 0.32 to 1.17 per cent; in the primary ultramafic rocks (e.g., dunite-peridotite) from 0.44 to 0.75 per cent, and in the chromitite from 0.13 to 0.40 per cent. Thin laminations of the chromite layers are preserved in the limonite on the hanging and foot-wall sides, indicating its formation in situ. No separate nickel mineral was identified.
It is concluded that nickel was fixed in the lattice positions of olivine and chromite in dunite-peridotite and chromitite respectively. The element was released during breakdown of these minerals by residual weathering, and was precipitated in the limonite profile at the base of the laterite mantle in an acid environment.
