The Gakara Rare Earth Deposit, Burundi: geology, mineralogy, U-Th-Pb dating
Résumé
One of the world's richest Rare Earth Elements (REE) deposits, Gakara, is in Burundi, near Lake Tanganyika, along the western branch of the East African Rift. It is enclosed in Mesoproterozoic rocks mainly composed of metasedimentary rocks and granitoids formed during the Kibaran orogeny between 1375 and 986 Ma. These rocks are intruded by a network of granitic pegmatites. Previous structures inherited from the Kibaran orogeny may have been reused during the mineralizing event. Hydrothermal fluids circulated along fractures and led to the formation of economic REE concentrations. Primary REE mineralization consists of bastnaesite (LREE-CO 3 F), however, economically exploitable mineralization contains secondary monazite (LREE-PO 4) that formed by late hydrothermal processes. Evidence includes silicification of secondary mineralization in monazite which is associated with idiomorphic quartz crystals. The Gakara REE mineralization share characteristics with other hydrothermal occurrences of these minerals and from carbonatites such as Mountain Pass (California) and Bayan Obo (China). These observations are combined to better define the geodynamic and magmatic context of the alkaline magmatism in the Gakara region, and to prospect for the presence of carbonatite. The U-Th-Pb ages of bastnaesite and monazite (around 600 Ma), belong to the Pan-African cycle, probably in an extensional context.