Lithostratigraphy, geochemistry, geochronology and structural geology of the Nkout iron deposit (South Cameroon)

Abstract

The Nkout banded iron formations (BIFs) is located in the northwestern margin of the Congo Craton (CC) and is hosted within the high-grade metamorphic Ntem complex. Nkout iron deposit is subdivided into three prospects namely; East, Centre and West deposits. Petrographically, the Nkout iron oxide facies is composed of four types of which are strongly deformed and were metamorphosed under relative low (greenschist) to medium (amphibolite) metamorphic grades. The gneissic rocks are composed of four types namely which were subjected under relative medium (amphibolite) to high (granulitic) metamorphic grades. The litho-package from the top to the base of Nkout comprises of iron-rich member and the gneissic member containing nine litho units. The upper unit is 200m deep and consists of oxidized (haematite) and un-oxidised (fresh BIF) ore types. The middle unit is approximately 150m thick and consists of fresh BIF with minor intercalation of biotite gneiss. The lower unit is made up of gneiss which represents the footwall of the deposit. The units reveal both thicknesses and depth variations from West to the East, inferring a folded structure of the Nkout deposit. Their occurrence supports the fact that BIFs deposition took place on a basin-wide scale. The rocks have undergone polyphase tectonic evolution with four stage of deformation. The deformation phase 1 (D1) comprises of F1 fold and S1 mineral foliation with E-W trend. This stage took place under low grade green-schist metamorphic facies condition. The later D2 event affected the previously deformed E-W (D1) structures accompanied by the development of F2 folds and S2 schistosity and C2 shear plane. The F2 folding is marked by inclined similar folds verging towards north. The C2 shear planes show both dextral and sinistral shear movement. We interpreted this tectonic event as the major stage of deformation. The D3 event is a phase of tectonic superimposition with NW-SE trend. It is accompanied with the development of schistosity (S3), shear planes (C3) and fold (F3). The F3 was as a result of refolding of the F2 folds. The resulting geometry is type 1 and 3 interferrence pattern. D4 is a phase of brittle tectonism. Structural evolution is the main controlling factor in the localization and preservation of high grade ore. The area comprise of granite, granodiorite and tonalite. These plutonic rocks are peraluminous and metaluminous. They show characteristic I-type igneous rock affinity that has evolved by fractional crystalisation of the mantle source with crustal contamination. The negative Nb, Zr, Ti, Sr and Yb anomalies in all the rock types interpret the contribution of the continental crust to the genesis of the rock. They are characterized with low REE abundance, positve Eu anomalies, LREE enrichment and HREE depletion. The gneissic rocks are derived from mafic igneous protolith characterised with gabbroic to dioritic composition. They show low REE abundance, negative to no Eu anomalies, LREEenrichment and HREE-depletion. Their strong negative Ti, Yb, Ta and Zr anomalies indicates that the continental crust and the mantle participated in the genesis of this rock. The meta-sedimentary are from Fe-derived sand and quartz arenitic protolith represented by magnetite BIF and garnet BIF. The magnetite BIF shows high SiO2 and Fe2O3 and low Al2O3, TiO2, HFSE and ΣREE contents similar to detritus-free chemical sediments. In contrast, their high concentrations in garnet BIF indicate clay-rich-contaminated input. The BIFs are depleted in CaO + MgO, suggesting a near absolute absence of calcite and dolomite. Whole-rock Pb isotope composition is roughly on average of 2.0 ppm, suggesting a hydrothermal Pb source. The Pb-Pb dating yielded an isochron age of 2679 Ma interpreted as the Neoarchean metamorphic event, attributed to high-K granitoid emplacement in the Ntem complex