Tectonique et minéralisations aurifères dans les formations birimiennes de Frandi-Boboti, boutonnière de Kédougou-Kéniéba, Sénégal

The lithological setting of the study area consists of lower Birimian sedimentary rocks (B1), cross-cut both by a volcanic and subvolcanic complex (B2) and a plutonic intrusive complex. Late to post-Birimian silico-clastic sedimentary and volcanic rocks took place later. These Birimian rocks are affected by the Eburnean polyphased tectonics composed of the following phases: (1) D1 compression that affected the lower sedimentary formations B1; (2) D2 composed of three stages: D2a compression, D2b sinistral transpression and D2c thrusting; (3) D3 dextral transtension. An anti-clockwise rotation of the main stress direction is produced between D1 and D3 phases. These Eburnean tectonic phases are separated by extensional tectonic events associated with volcanic flows and sedimentary deposits. Three gold mineralization phases were identified. An ante-D1 primary phase, located in the sediments of the lower unit, is often disseminated and stratiform. The second phase, in relationship with D2 tectono-magmatic event, produces a silico-carbonated and sulphurized stockwerk, located in tension gashes, V2 veins and shear zones. The third phase would be associated to the late D3 tectono-magmatic event, with a paragenesis of quartz and pyrite, concentrated in the V3 veins. Mineralization is more frequent in the ~N20° oriented silicocarbonated veins and shear zones, suggesting that a post-Birimian tectono-magmatic event would have probably played an important role in the remobilisation of gold along these structures.