Neoproterozoic Rare Element Pegmatites from Gitarama and Gatumba Areas, Rwanda: Understanding Their Nb-Ta and Sn Mineralisation

Abstract

The aim of this work was to study the petrography, geochemistry of the pegmatites, their relationship to the mineralisation in Gitarama and Gatumba areas, and current processes that occurred after the primary emplaced neoproterozoic rare element pegmatites. Previous works on pegmatites were geochemistry and geological maps which are not enough for focused exploration and mine planning. Therefore, geological, petrographic, geochemical studies of neoproterozoic rare element pegmatites of Gatumba and Gitarama areas in relation to their mineralisation were carried out. The samples were analysed for mineral assemblages by petrographic light microscope; major elements by ICP AES; trace and rare earth elements by ICP MS. Petrographic studies revealed the mineral assemblages included quartz, microcline, biotite and major muscovites, which implied that there was the process of muscovitisation occurred after the primary emplacement of pegmatites. The results of geochemical analysis revealed that the silica content (in wt%) ranges from 59.5 - 80.5 with an average of 67.13 (in wt%) for the weathered pegmatite in Gatumba area ,and high percentages of SiO 2 (in wt%) range 73.9 - 75.0 with an average of 73.15 (in wt%) for fresh pegmatite in Gitarama area. The pegmatites from Gatumba area were altered and much enriched in Rb (227 - 3460 ppm), Cs (2.59 - 24.7 ppm), Ta (2.6 - 268 ppm), Li (40 - 9224 ppm), W (240 - 10,000 ppm), Nb (13 - 517 ppm), Sn (24 - 8870 ppm). Their enrichment is commonly used as a marker of a magmatic-hydrothermal alteration. Conversely, the pegmatites from Gitarama area showed the low to moderate concentrations in Rb (321 - 337 ppm), Cs (5.47 - 5.62 ppm), Ta (1.3 - 1.6 ppm), Li (~20 ppm), W (5540 - 6410 ppm), Nb (3.9 - 4.3 ppm), Sn (28 - 44 ppm). The variation plot of ratios: Al 2 O 3 /(Na 2 O + K 2 O) versus Al 2 O 3 /(CaO + Na 2 O + K 2 O) for the pegmatites from study areas are higher than one (A/NK vs. A/CNK > 1) indicating peraluminous, the other samples of pegmatites indicated metaluminous (A/NK > 1 and A/CNK < 1). REE abundance from whole rock analysis of pegmatites of Gatumba and Gitarama areas is low to moderate with the &Sigma; REE varying between 12.1 - 72.78 ppm and 45 - 54.37 ppm respectively, signifying low to medium form of enrichment. The pegmatite from Gatumba and Gitarama areas showed the K/Rb ratios ranging from 15.74 to 80.26 and from 190.41 to 199.39 respectively. As the pegmatite samples show K/Rb ratios less than 100 are commonly accepted for mineralization, therefore the pegmatites from Gatumba area were found mineralised, conversely to the pegmatite samples from Gitarama area, which were found barren.