Petrography, Mineralogy, and Geochemistry of the Nkamouna Serpentinite: Implications for the Formation of the Cobalt-Manganese Laterite Deposit, Southeast Cameroon

摘要

The Nkamouna Co-Mn-Ni laterite is an advanced exploration project located in southeast Cameroon. The deposit comprises a deep and complex laterite profile with high Co (~0.25 wt %) and Mn (1.33 wt %) contents, subeconomic Ni grade and a Ni:Co ratio of less than 3. This makes Nkamouna an unusual type of laterite deposit as it will be primarily mined for Co. The Co-Mn mineralization occurs at the interface between an overlying allochthonous felsic rock-sourced laterite and a lower in situ serpentinite-derived laterite. The serpentinite is massive with a nonpseudomorphic texture and consists of two genetically related phases: Serpentinite-I and Serpentinite-II. Serpentinite-I is characterized by antigorite, magnetite, quartz, ferritchromite, and magnesite, and Serpentinite-II by antigorite and magnesite. Both serpentinite types are extremely low in sulfides. Serpentinite-I is the dominant and oldest phase and was derived from a subcontinental mantle (pyroxene-rich) peridotite emplaced in the crust during the Pan-African orogeny. The Ni concentration in the serpentinite is variable (0.07–0.30%) and heterogeneously distributed, partly reflecting the present dominance of pyroxenes in the protolith. However, Ni is also hosted by magnetite and ferritchromite. The differential weathering of these Ni-hosting minerals is also responsible for the lower Ni grade of the deposit. Importantly, the serpentinite is depleted in Co and Mn and the enrichment of these elements in the laterite profile suggests that these elements may be derived from an exotic source.