Cartographie morphonéotectonique et pétro-structurale de la ceinture de roches vertes de Kopongo (sw-Cameroun) : évolution géodynamique et styles géotectoniques

Abstract

The Kopongo area belongs to the Nyong Complex, which represents the North-Western margin of the Congo craton in Southern Cameroon. The present study aims to propose a geodynamic evolution model of the Kopongo region. This landscape illustrates an asymmetrical and elongated relief following the NNE-SSW direction. Morphometric parameters showed that this region has been rejuvenated due to the interaction of several factors such as climatic conditions, lithological variability and neotectonic activities. The spatial heterogeneity of the relative tectonic activity (RTA) enabled to subdivide this region into zones of high (14.89%), moderate (72.71%) and low (12.33%) tectonic activity. The Kopongo region shows polyphase tectonic, metamorphic and metalliferous evolution with four phases of deformation DnMn-Dn+3Mn+3 and metalliferous Zn-Zn+3. DnMnZn deformation is a horizontal general flattening. Dn+1Mn+1Zn+1 event evolves in two episodes: a horizontal general flattening /pur shear and simple shear. Dn+2Mn+2Zn+2 deformation shows simple shear/general constriction while Dn+3Mn+3Zn+3 event exhibited a transtension. Mineralization at Kopongo is related to Fn+1 and Fn+2 folds (S, M and Z-shaped) disturbed by regional lithoclases. The Kopongo region is composed of iron formations and gneissic host rocks, with varied textures and structures. Geochemical analyzes have shown that the iron formations are composed of Fe2O3 and SiO2, mass >98% suggesting chemical inputs of silica and iron. Low concentrations of Al2O3, TiO2, K2O, Na2O, MgO and CaO, coupled with a strong positive correlation between MgO and CaO (r=0.96) are indicative of negligible terrigenous influx, in marine environment. From these geochemical data, detrital iron, magnetite-quartzite, quartz hematite breccia, magnetite and enriched magnetite mineralization types are observed, with Fe2O3 contents (29.03-87.16%), SiO2 (3.02-55.5%), Al2O3 (0.48-10.20%), phosphorus of about 0.15% and 0.48% for sulfur. The results of this thesis: (1) highlight the geomorphological, lithological, structural, tectonic activity and mineralization complexity; (2) show that this region exhibits magnetite-pyroxene iron formations; (3) make a structural correlation of this greenstone belt to those observed on the Congo craton, in Australia, and in Brazil; and (4) show that the Kopongo iron formations are of the modified Algoma type and acceptable for metal production.