stratigraphie et analyse tructurale des formations ferrifères du secteur de Toko-Nlokeng, Sud-Cameroune TOKO-NLOKENG

Abstract

The Toko-Nlokeng area, located in the Southern Region, belongs to the Nyong complex, which defines the NW limit of the Congo Craton in Cameroon. The main objective of this work is to contribute to the understanding of the geological formations (iron formation and host rocks) and structural framework of the Toko-Nlokeng sector of the Nyong complex. Investigations focused on petrographic, structural, and geochemical studies on total rock. Petrographically, the area comprises a metamorphic assemblage of hornblendites, mafic and ultramafic granulites, garnet and amphibole gneisses, leucocratic gneisses, garnet amphibolites and Iron Formations (IFs) comprising banded (BIFs) and granular (GIFs) rocks. All rocks have heterogranular granoblastic microstructures. These lithological entities are characterized by (i) granulitic paragenesis with Grt + Cpx + Pl ± Op (mafic granulites), (ii) amphibolitic Hbl + Cpx + Bt (hornblendites), and (iii) greenschist facies: Mag + Qtz ± Chl (BIFs), and Mag + Qtz ± Hem (GIFs). Morpho-structural analysis shows, on the one hand, the configuration of the leading ENE-WSW strike-slip network with associated satellite faults and a configuration compatible with the Riedel system; and, on the other hand, it shows the predominantly NE-SW direction of foliation paths in the study area. From a structural point of view, the structural elements recorded enable us to distinguish four phases of deformation. The first phase of deformation is characterized by S1 foliation, β1 boudins, and P1 folds. This phase is characterized by extensive general flattening. The second phase is marked not only by regional S1/S2 foliation carrying the stretching lineation, but also by P2 folds with subvertical axial planes of mean E-W direction. The third phase, D3, is marked by P3 folds and shear zones. This phase shows double shortening (maximum E-W to NW-SE and minimum N-S to NESW). The fourth D4 phase is essentially brittle (faults and joints). IFs geochemistry shows that SiO2 and Fe2O3 comprise more than 90% of the rock, typical of chemical sediments. High Si/Al,Fe/Al and Fe/Ti ratios, and positive Eu anomalies, indicate that the chemical elements in the Toko-Nlokeng FF are derived from a hydrothermal source. Their native iron content is low, characteristic of low-grade ore. The geotectonic depositional context highlighted is that of an active continental margin, where the chemical elements precipitated from a mixture of seawater and low-temperature hydrothermal fluid. The positive Eu anomalies (Eu/Eu*PAAS > 1.8) and the association with metavolcanites are compatible with the FF character of the Algoma type. The geodynamic evolution of the Toko-Nlokeng host rock protoliths shows E-MORB, P-MORB, and G-MORB type affinities and arc, back-arc 'B' type affinities in a subduction margin context unrelated to subduction, with minor crustal contamination. The tholeiitic to calc-alkaline and peraluminous affinity of these rocks indicates a mature arc and thickened crust during the Eburnian Orogenesis of the Congo Craton.