Residual Ore Deposits Systems Prospect, Case Study Nkamouna cobalt-nickel distribution in the laterites of Lomie, South-eastern Cameroon

Abstract

Residual deposits are the insoluble products of rock weathering which have escaped distribution by transporting agencies, and which still mantle the rocks from which they have been derived. Nickel-cobalt (Ni-Co) laterite deposits are supergene enrichments of Ni±Co that form from intense chemical and mechanical weathering of ultramafic parent rocks. They form in active continental margins and stable cratonic settings, and take as little as a million years for a laterite profile to develop.

In the Lomie´region (south-east Cameroon), strong weathering of serpentinized ultramafic rocks has produced a thick laterite cover with significant nickel and cobalt contents. The highest concentrations of these elements are located in the middle section of the laterite profiles, in the lower clay horizon, and preferentially along the slopes of the interfluves. The investigation of the composition of the laterite ores (by whole-rock analysis) and of the main components, using SEM/microprobe and XRD, reveals the presence of four main enriched facies: a non-differentiated facies, a layered smectitic facies, a quartz-rich facies and a gibbsitic nodular facies.

Nickel, with generally low concentrations (less than 2% NiO), is hosted by several secondary mineral phases (goethite, Mn-oxyhydroxides and smectite locally). Cobalt is generally of higher grade (up to 0.9% CoO), and is associated with cryptocrystalline and crystallized Mn-oxyhydroxides. SEM/microprobe observations suggest that nickel and cobalt concentration in secondary minerals is due to repeated remobilization. This has also favored the formation of mineral phases, of which the best crystallized and most richly mineralized are mainly those of the asbolan–lithiophorite group. The SEM studies indicate that these mineral phases show various morphologies related to their chemical composition: poorly crystallized nipple shaped (Fe, Mn, Ni), fine cross-bedded needles (Mn, Ni) and elongated crystals (Mn, Al, Ni, Co) occur in the layered smectitic facies, while platy and needle-like forms (Mn, Al, Ni, Co) characterize the gibbsitic nodular facies. The predominantly cobaltiferous nature of the Lomie laterite ore deposit is the result of remobilizations and transformations of elements that led to the impoverishment of both the Ni–Co contents of the laterite but most importantly of Nickel rather than Cobalt.

Keywords: Laterites; Nickel; Cobalt; Manganese; Serpentinite; Cameroon; deposit; Segregation; Residual.

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