Petrographic, geochemical and structural characteristics of gold-bearing metasedimentary rocks from the Atacora structural unit, Northwestern Bénin Republic

Abstract

The metasedimentary rocks of the Atacora structural unit around Natitingou in Benin are composed of quartzites, quartz muscovite schist and mica schist. Being auriferous, the petrographic, geochemical and structural features of these rocks were investigated to ascertain the source of gold and its relation to structural deformations. Quartz, muscovite and opaque minerals constitute the dominant minerals in the quartz muscovite schist. The quartzites and mica schists are predominantly composed of quartz, plagioclase, microcline, muscovite and chlorite in varying proportions, with biotite, garnet and epidote in mica schist. The rocks have quartzose sedimentary composition and plot within the passive margin setting on major element discrimination diagrams. The Pan-African ductile and brittle deformation phases produced predominantly folds, strike-slip faults, dextral shear plane and fractures within the area. Gold-bearing veins in the host rocks occur within and near the N-S to NE-SW trending microfolds and the major brittle E-W and NE-SW faults. Gold mineralization is hosted within the NE-trending shear zones and NE-SW trending asymmetric folds which are attributed to transpressional D2 phase of the Pan-African orogeny. Pathfinder elements for gold mineralization in the Atacora are W–Cu–As–Mo ± Sn. This may suggest interaction between host rocks and mineralizing fluids, probably from deformation and metamorphic processes during the late stages of the Pan-African Orogenic event. However, fluid inclusion and stable isotopic studies are required to constrain the origin of ore fluids.