Compositional trends and rare metal (Ta-Nb) mineralization potential of pegmatite and associated lithologies of Igbeti area, Southwestern Nigeria

摘要

The increase in global demand for rare metal Ta-Nb deposits has created the need for renewed search for economically viable deposits. This study therefore, involves petrographic and geochemical evaluation of near flat lying pegmatite veins intruding gneisses, schists and granitic rocks around Igbeti area, southwestern Nigeria. This is with a view to determine their compositional characteristics and rare metal Ta-Nb potential. A total of 24 samples comprising whole rock pegmatites and mineral extracts of albite, mica and tourmaline were analysed for major, trace and rare earth elements using Inductively Coupled Plasma-Mass Spectrometry technique (ICP-MS). From the results, the whole rock pegmatite is considerably siliceous, but with noticeable depletion of silica in the tourmaline extract. Average Fe_2O_3, MgO and MnO values are low in all the samples. Trace element analysis also reveal Ta-Nb enrichment in all the samples (>246 mu g/g) with highest values in the tourmaline phase and lowest values in the whole rock pegmatite. Similarly, Cs values are highest in the tourmaline extracts, but Ti, Ba and Zr values are generally low. All samples analysed reveal that the pegmatites are magmatic with evidence of late metasomatic alterations with appreciable degree of fractionation and mineralization as indicated in the low K/Cs (0.04) Th/U (0.08) and K/Rb (0.001) values. Also, from plots of Ta/(Ta+Nb)/Mn/(Mn+Fe), and other bulk geochemical features, the pegmatite is complex, of the rare element class and belongs to the Lithium, Ceasium Tantalum (LCT) petrogenetic family. Furthermore, variation plots involving K, Rb, Ta, Ga, Cs, and W show enhanced Ta-Nb mineralization potential comparable to world class Tanco and Noumas deposits. Compositional trends within the various mineral phases analysed indicate preffered enrichment in the tourmaline, mica, and albite phases in that order respectively.