Pétrogenèse, structurogenèse et potentialités métallogéniques des formations précambriennes de Ngoura-Colomines: Est-Cameroun

Abstract

The present work concerns petrogenesis, structurogenesis and metallogenic potentialities of the Precambrian formations of Ngoura-Colomines. It essentially aims to determine the geodynamic context and the genetic model of the auriferous granites of the sector and thus to contribute to a more global understanding of the tectonic evolution of the North Equatorial Pan African Fold Belt (NEPFB) and the gold mineralizations associated with the pan-African shear zones. This region is made up of plutonic and metamorphic rocks. Plutonic rocks are mainly syn-tectonic granitoids that is granites, granodiorites, adamellites and tonalites, metamorphic rocks being biotite and amphibole gneisses, amphibolites and mylonites. The mineralogical assemblages Hbl + Pl + Qtz + Kfs + Bt ± Chl in amphibolites and Qtz + Kfs + Pl + Hbl + Bt ± Spn in biotite and amphibole gneisses show that the paroxysm of metamorphism is up to amphibolite facies. The plutonic rocks have granular to porphyritic textures with a mineralogical assemblage mainly made up of Qtz + Kfs + Pl + Bt. The structural analysis allowed to highlight a deformation chronology with four (04) phases that is D1 of compressive tectonics, D2 of transcurrent tectonics in transpression, D3 of tectonic superposition and the later D4 called brittle tectonics. In addition, the structural approach allowed the identification of a dextrial ductile to fragile shear zone at regional scale generally oriented NE-SW. These structural elements highlight the evolution of the deformation from the magmatic phase to the post-magmatic phase in shear context. The plutonic rocks of Ngoura-Colomines are peraluminous type-I and belong to both calc-alkaline hyperpotassic series and shoshonitic series. Their chemical characters are simillar to those of the hyperpotassic calc-alkaline series of the orogenic domains. The rare earth spectra normalized with respect to chondrite are split overall, with positive anomalies in Ce and negative in Eu. The metamorphic rocks of Ngoura-Colomines have an igneous origin with basalts as probable protolith. Their insignificant Cr/Th ratios as well as the weak Th/La ratios (0.08-0.11), are characteristic of volcanics rocks. The rare earth spectra of these rocks are flat and very little fractionated, with an enrichment almost in the same proportions in LREE and HREE and negative anomalies in Ce. Several zones with proven gold potential generally associated with hydrothermal alteration of iron oxide/hydroxide minerals, clay minerals and silicification have been identified in the Ngoura-Colomines sector. These areas have a close spatial relationship with known mining areas and gold occurrences. These are the Ngoura-Colomines prospect and the Yangamo-Ndatanga and Taparé-Tapondo prospects, which are completely new exploration targets. Gold mineralization in the Ngoura-Colomines sector has a double origin, sometimes disseminated in the granite massifs, sometimes intimately linked to the dominant quartz, sulphide and calcite vein systems. The ductile-fragile shear zone oriented NNE-SSW to E-W which crosses the study area controls the said mineralization along meso-fractures and quartz veins. Petrological, structural, geochemical and metallogenic data show that the Ngoura- Colomines shear zone corresponds to a tectonic zone of oblique continent-continent collision involving the Congo craton/Sao Francisco Craton and the Saharian East Block, the associated gold mineralization being intimately linked to the genesis of granitoids and to the final stages of deformation. These results are in agreement with the previous hypothesis formulated for the geodynamics of the central domain of the North Equatorial Pan African Fold Belt (NEPFB) in Cameroon.