Geological setting of gold mineralisation of the Minvoul greenstone belt (Ntem complex, northern Gabon)

The geology of vast portions of the northwestern margins of the Congo craton at the border between Gabon and Cameroon remains poorly known. The Minvoul area corresponds to a NE-SW to ENE-WSW-trending band of Archean supracrustal rocks interpreted as containing greenstone belt rock formations. The area forms part of the Ayina unit, which represents the most eastern unit of the Ntem complex. This paper characterises petrographically these rock formations for the first time. The Minvoul greenstone belt is emplaced in the Archean charnockitic gneiss basement and consits of two series of supracrustal formations metamorphosed under epi to mesozonal conditions. The first is composed of amphibolites and metagabbros that could represent metamorphosed mafic volcanics and igneous intrusives respectively, together with schists and itabirites. The other comprises fine to medium-grain biotitic quartzo-feldspatic gneisses that probably represents either meta-sediments or felsic meta-volcanics. K-feldspar granitoids intrude the earlier assemblages. The deep weathering and the thick vegetation, typical of equatorial latitudes, have hampered a detailed structural study of the region but locally, the effects of complex folding, faulting and shearing are observed at both macro and microscopic scales. Earlier authors suggest that the Eburnean orogeny has played a major role in the deformation of these sequences and is responsible for a steeply-dipping E-W foliation which is displaced by NE-SW shear zones. Tectonothermal banding of amphibolites and gabbros, as well as extensive micro-shear structures and dynamic recrystallization are observed in thin sections. Artisanal alluvial gold mining activity in the rivers draining the Minvoul greenstone belt has been observed. Also, gold anomalies have been reported in geochemical samples taken from soils, laterite and weathered bedrock. These results give credence to the possibility that these supracrustal formations host gold mineralisation. Although, the mode of occurrence of gold in bedrock is not completely understood, geological, petrographical and geochemical evidences given in this paper indicate that the primary mineralisation corresponds to free gold related to quartz veins. This is typical in Archean metallotects. The present work enhances the knowledge of this remote part of the Congo craton and offers a geological integration of the Ayina unit in its geological context. The large volume of highly prospective supracrustal rocks revealed in this paper encourage further gold and other exploration, particularly iron, over the entire Ayina unit.