The Belahourou granite pluton (Djibo greenstone belt, Burkina Faso): Emplacement mechanism and implication for gold mineralization along a shear zone

Abstract

The Belahourou granite pluton (BG pluton) is a magmatic body that belongs to the Djibo greenstone belt, north Burkina Faso. Calc-alkaline in composition and 2132 ± 4 Ma in age (U-Pb zircon), this pluton is bordered by the curved Belahourou-Souma (BS) sinistral shear-zone, NNW-trending in the northern half of the pluton and WNW-trending in the southern part. In order to study the emplacement mechanism relationships with the BS shear-zone and the gold-deposits, the pluton and its host-rocks have been subjected to standard and modern mapping techniques (airborne geophysics, satellite imagery, anisotropy of magnetic susceptibility - AMS - and microstructural observations), in addition to field observations. The pluton, 100 km2 in area, 16 km in length and 7 km in width, is surrounded by metamorphic formations, turbiditic clastic to the west and volcanic material along the southern and eastern sides. To the north the pluton is in contact with granodioritic and gabbroic rocks. Pluton emplacement was likely instigated by opening of a tension gash associated with NW-directed, sinistral shearing along the curved BSS. The overall NW-trending magnetic lineations inside the pluton support this interpretation. The microstructures indicate that shearing continued during pluton cooling, explaining the inside-pluton magmatic to orthogneissic transition, and after cooling, explaining the mylonitic microstructures against the BS shear-zone. A 3D conceptual representation integrates the available data and concludes that the pluton is late-tectonic with respect to the regional D2 deformation event. Once emplaced, the pluton may have provided low pressure zones, at lithological contacts and/or inside brittle fractures in the host formations, concentrating the mineralizing fluids that were pressurized during pluton cooling, and that helped to precipitate the ore.