Structural constraints of the Birimian lithium pegmatites of Bougouni (Southern Mali, Leo-Man shield)

The Bougouni region is located in the southern Mali, 170 km south-east of Bamako. It is part of the northern edge of the Leo-Man [UdMO1] shield (southern part of the West-African Craton). It is known for its swarm of pegmatites, most investigated as a lithium resource (spodumene pegmatites) and its gold potential. It is made up of metavolcanosedimentary rocks and plutonic complexes of the Birimian (Paleoproterozoic).

The objectives of this study are: 1) to make a review of the main regional structural features, 2) to define the structural control of the pegmatite emplacement. To reach these aims a structural analysis was carried out using the information collected during our field campaigns (2018 and 2019) and data from previous works.

Metavolcanosedimentary rocks had undergone a low grade metamorphism degree. Schistosities in these rocks (n=156 measurements) are distributed along 2 main directions which indicate two deformation phases. The first one, D1, oriented NNW-SSE to N-S. It is ductile, responsible for low grade regional metamorphism (Baratoux et al., 2011) and crustal thickening of the volcano-sedimentary rocks (Wane et al., 2018). It is linked to a compression event oriented E-W to ESE-WNW (Baratoux et al., 2011; Wane et al., 2018). The second one, D2, oriented NNE-SSW to SE-NW, is a ductile-brittle transpressive deformation. It affects metavolcano-sedimentary rocks. It would be contemporaneous with the location of most birimian granitoids (Baratoux et al., 2011; Wane et al., 2018) and it would be responsible for the gold mineralization (McFarlane et al., 2011). The third one, oriented E-W is marked by fracture cleavage and extensional cracks sometimes sigmoidal and generally filled by quartz. It has a brittle-ductile character. The faults have been observed in the field but weren’t measured. Feybesse et al. (2006) showed that they intersect all the lithologies without having any direct link with the different deformation phases.

 

Pegmatites are hosted in both metavolcanosedimentary and granitoids. Lithium bearing pegmatites are characterized by an assemblage containing spodumene (15-65%; main lithium-host mineral), albite (10-55%), microcline (1-10%), quartz (25-50%) and muscovite (2-10%). The accessory minerals are: apatite; garnet; columbo-tantalite, tourmaline, beryl and rutile. Lithium pegmatites are distributed in three directions (n=209): NNE-SSW (minor, ≃10 %?), ENE-WSW to E-W and ESE-WNW to SE-NW. Most of them are characterized by a steep dip (≃90°). The dyke-host unit contacts are generally sharp and brittle. These results suggest that the emplacement of the lithium bearing pegmatites of Bougouni even of all the Birimian pegmatites (Example of Issia pegmatite, côte d'ivoire Allou, 2005) could be related to the brittle deformation phase D3. This phase is also thought to be linked with the gold mineralization (McFarlane et al., 2011).

 

Ref: Allou, 2005, thèse à l’université de Chicoutimi ; Baratoux et al., 2011, Precamb. Res. 191, 18–45 ; Feybesse et al., 2006, Carte et Notice explicative de la Carte du Birimien du Mali ; McFarlane et al., 2011, Econ. Geol. 106, 727-750; Wane et al., 2018, Precamb. Res. 305, 444-478

Key words: pegmatite, mineralization, lithium, spodumene, granitoid, Leo-Man shiel, Mali, West African Craton