FIRST ATTEMPT ON THE STRUCTURAL AND LITHOSTRATIGRAPHIC CONTROL OF SN-NB-TA-W AND AU MINERALIZATION IN THE KARAGWE-ANKOLE BELT (NE BURUNDI, EAST AFRICA)

The Karagwe-Ankole belt (KAB) across Burundi (East Africa) which consists to Meso-Protorezoic multi-layered sedimentary rocks, is characterized by two structurally distinct domains, namely the Western (WD) and the Eastern (ED) Domains. The WD is composed of deformed Meso-Proterozoic metasedimentary rocks metamorphosed in the greenschits to the amphibolite facies and intruded by numerous S-type granitoid. WD is underlain by Paleo-Proterozoic gneisses and migmatites, whereas the ED is of comparable stratigraphy but with a Archean craton basement. Sn-Nb-Ta Mineralization is associated with 986 Ma leucocratic muscovite granites, which precipitates in the form pegmatites highly enriched in incompatible elements and volatiles, crosscut by W-bearing quartz veins sometimes associated with Sn. Au mineralization associated with shear zones and occurring as quartz veins (generally grey in color) and ferruginous breccia can be associated with the same Neo-Proterozoic granites or with metavolcanic rocks enriched in pyrites.

However studies focusing on the relationship between mineralization, deformation and lithostratigraphy are sparse. Here we present a compilation of the most recent geological data gathered by a consortium of exploration companies operating in the NE of Burundi since April 2009. We particularly present models of the structural and lithological controls from four examples of Sn-Nb-Ta-W and Au deposits:

(1) The Sn-Nb-Ta mineralization hosted in the Murehe quartzitic ring formation, which corresponds to meter wide sheeted pegmatite vein system trending N60 and crosscutting S-type granitoids and highly weathered meta-sedimentary rocks in relationship with the 980Ma leucogranites.

(2) The Masaka Au-deposit consists to a faulted and overturned anticline with two main generations of Au-bearing veins: the massive N-S white quartz vein which formed during the top to the SE thrust movement, and the East-West grey quartz veining system crosscutting the white quartz vein.

(3) The Baziro gold mineralization, characterized by a series of contemporaneous saddle-reef and en-echelon veins developed along fold limbs and hinges formed during the main shortening phase of KAB, and overprinted by a N-S compression.

(4) The Kabuyenge deposit corresponds to intensively sheared concordant “saddle-reef” gold-bearing grey quartz vein system.