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

• 作者: Anthony Temidayo Bolarinwa Gabriel Oladapo Adeyemi Jean de Dieu Ndikumana
• 来源期刊: 地质学期刊(英文)
• 学科: 地球科学
• 关键词: PEGMATITES PETROGRAPHY GEOCHEMISTRY Gitarama Gatumba
• 年，卷（期）: 2019,（13）
• 页码范围: 1069-1083
• 页数: 15页
• 分类号: P59

摘要

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 SiO2 (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: Al2O3/(Na2O + K2O) versus Al2O3/(CaO + Na2O + K2O) 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 ΣREE