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Morphological characterization and quantification of gold particles by optical image analysis (OIA) and by compositional analysis of microprobes using scanning electron microscope and electron microprobe analysis techniques were carried out on gold grains from the Minvoul area (Archean greenstone belt in Gabon). Large grains of almost pure gold were found throughout a weathering profile, which consisted of saprolite, mottled clay zone, iron duricrust, pisolitic gravels and yellow latosol. In the deeper horizons, gold was dissolved as shown by corrosion features on the surface of particles with average sizes of 2.6Φ and 2.35Φ in the saprolite and mottled clay zones, respectively. The occurrence of secondary gold in the duricrust was indicated by the larger size of the nuggets (average, 1.8Φ), the high fineness (> 995 in average) and the close textural relationship between gold and neoformed iron oxyhydroxides. The uppermost horizons composed of yellow latosol and pisolitic gravels were interpreted as transported materials based on their size distribution (average 0.85Φ and 1.34Φ), sorting and shape parameters. The best morphological parameter to describe the whole weathering profile was found to be the perimeter/area ratio. The highest ratios were recorded in the saprolite (average 0.192 μm−1), and decreased towards the surface (average 0.057 μm−1). The combination of the OIA technique and the microchemical analysis of gold grains allowed us to define specific morphological and compositional characteristics of the gold particles for each horizon. Both methods proved to be of great utility to understand gold concentration, dissolution and dispersion processes in supergene environments.

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.