The central part of the Karagwe-Ankole Belt (KAB) in the Great Lakes area of Central Africa is rich in various rare and strategic elements. The area north of Kalehe and Idjwi island within the Sud-Kivu Province, in the eastern part of the Democratic Republic of the Congo (DRC), contains Nb, Ta, Sn and W mineralization hosted in pegmatites and quartz veins that are related to leucogranite intrusions. In this study, two granite-pegmatite fields were studied: the Bugarula field on the northern part of Idjwi island, and the Kalehe field, along the west coast of Lake Kivu, with a view to determining the co-genetic link between different pegmatites and leucogranites in these regions. Based on the mineralogical assemblages of the pegmatites, five zones were identified, occurring with some spatial overlap: 1: albite pegmatites, 2: two-mica pegmatites, 3: muscovite pegmatites, 4: beryl-bearing Nb-Ta-Sn mineralized pegmatites, 5: tourmaline-bearing Nb-Ta-Sn mineralized pegmatites. The use of petrogenetic parameters for the alkaline elements K, Rb, and Cs, which are involved in the Rayleigh fractionation model, has shown that different leucogranites are found directly associated with the pegmatites, making it difficult to identify the exact parental granite. However, the leucogranites in these areas have an average composition of 3.6 wt.% K2O, 8 ppm Cs, and 300 ppm Rb. The mineralized pegmatites (4 and 5) show high concentrations of Nb, Ta and Sn. Muscovite compositions were analysed from each zone, and used to calculate fractionation trends using a Rayleigh-type modelling starting from the average leucogranite composition. In the Idjwi granite-pegmatite field, the degree of fractionation with respect to the initial composition of the leucogranite varies from 20% to 99%, respectively for the least fractionated and most evolved Nb-Ta-Sn mineralized pegmatites. In the Kalehe area, the fractionation values also range from 22% up to 99% for the least fractionated and most evolved (mineralized) pegmatites, respectively. The geochemical characteristics of the muscovite in these pegmatites are examined to guide Nb-Ta, Sn, and W exploration in these zones. This high in degrees of fractionation associated with the enrichment in economically interesting elements is comparable as reported for other pegmatite fields in the Karagwe-Ankole Belt.