Géochimie des muscovites comme indicateur du fractionnement des pegmatites de la région de Kabarore-Mparamirundi (nord-ouest du Burundi, Afrique centrale) [Geochemical signature of muscovites as pathfinder for fractionation of pegmatites in the Kabarore-Mparamirundi area (northwestern Burundi, Central Africa)]

Abstract

The Kabarore-Mparamirundi area hosts numerous pegmatites spatiotemporally related to leucogranites dated at 986 ± 10 Ma in Karagwe-Ankole belt. The deposits are intensively exploited for columbite-tantalite and cassiterite. Alkali metals in muscovite (Rb 370–7590 ppm, Cs 8–1470 ppm) are modeled by Rayleigh fractional crystallization from a parental leucogranitic composition (K 4.1 wt%, Rb 321 ppm and Cs 9 ppm). The power law declining behavior of the ratio K/Rb versus Cs indicates the Rayleigh fractional crystallization as the main process of differentiation of the various pegmatite facies. Moreover, the continuous trend from granite to the most evolved, exploited pegmatites demonstrates a co-genetic link among them. The fractionation model shows that unmined and abandoned pegmatites are less fractionated (less than 94% of fractionation) while mined pegmatites are highly fractionated and constitute fractionated products of more than 94% of the initial leucogranite composition. The Rb, Cs, Ta, Sn and Li elements in muscovite can be used as a valuable tool in the exploration of fertile and sterile pegmatites in this area.