Etude pétrographique et géochimique des formations géologiques de Moloundou (Sud-Est Cameroun) et des minéralisations ferrifères associées

Abstract

The Moloundou area is located in the Eastern region of Cameroon and belongs to the the lower Dja series which rest partially unto the west of the Ayna unit of the Congo craton.From the petrography point of view, this area is mainly composed of sedimentary to very low grade metamorphic rocks made up of iron formations and their host rocks (cherts and pelites). The latter have heterogranular structure while the iron formations have either massive fractured structure (hematite-goethite ore); either beehive-like structure (hematite- kenomagnetite and goethite ore) ; or polygonal structure (hematite-kenomagnetite and limonite ore) ; and finally pisolitic structure (hematite-limonite and magnetite relics ore). In terms of deformation, the main structures recorded in the area are primary syn- sedimentary features such as bedding and folds. Also, triple junction mineral points are observed in cherts, suggesting low grade metamorphic imprints. Geochemical composition has revealed that chert and pelite correspond to quartz- arenite and arkose respectively. Quartz-arenites are mature and unaltered (average composition of SiO2 = 98.32% ; LOI = 0.05), while arkoses with relatively high CaO and MgO contents, are immature and have been subjected to meteoric alteration (CIA = 65.32) resulting in alkaline mobility. Arkoses mainly derived from the dismantling of a felsic crust, contaminated by mafic inputs, and deposited in a passive continental margin setting. Iron formations display high Fe2O3 (96.22) and very low SiO (1) contents. The average of native iron (Fe) content is 67.35, characteristic of high-grade ore or DSO (Direct Shipping Ore ; > 65 Fe). The geochemical features of the Moloundou iron ore suggest that it originates from a mixed hypogene-supergene enrichment of the BIF (Banded Iron Formation) protore.Geochemical correlations reveal that the average composition of the Moloundou arkoses is closed to that of the Proterozoic shales, but differ from the Yaoundé metashale; The Moloundou iron ore, as well as Mbalam and Mamelles (Kribi) iron ore, derived from BIF protore. However, they differ in their enrichment process (mixed hypogene-supergene at Moloundou and supergene enrichment model for Mbalam and Mamelles iron ore).