International Journal of Earth Sciences Knowledge and Applications
Yazarlar: Daniel Imariabe OMORUYI, Ebo Gabriel IMEOKPARIA
Konular:Yerbilimleri, Ortak Disiplinler
Anahtar Kelimeler:Pegmatite,Rare metals mineralization,Trace elements,Niobium,Tantalum
Özet: Five pegmatite dykes in suburbs of Lokoja area, central Nigeria are studied using field, petrographical and geochemical data to characterize and determine the rare-metals (Ta-Nb) mineralization attributes. The pegmatites which occur as near-vertical, large veins and dykes are hosted by migmatitic biotite gneiss. The pegmatites consist of quartz, feldspar, muscovite with rare biotite, occasional beryl and some minor/accessory apatite and zircon. Composite whole-rock samples were analysed for major, trace and rare earth elements using X-Ray Fluorescence Spectrometer (XRF) and Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The results reveal the pegmatites are considerably siliceous with SiO2 values of 69.00% to 74.60%. Al2O3 values are relatively moderate (14.80% to 19.60%). Na2O and K2O abundances are low with ranges of 2.32% – 5.93%, 1.22% – 11.00% respectively. MgO abundances are generally low (0.03 – 0.07%). Fe203 values are also low; ranging from 0.31% to 1.16%. Alumina Saturation Index (A/CNK) computation with values ranging from 1.40 to 3.06 indicates the pegmatites are peraluminous. Trace and rare-earth elements results show Bakunba location pegmatite is mineralised (Nb: 571.86ppm, Ta: 180ppm) and compare favourably with other mineralised pegmatites in Nigeria; while Ganaja-2 location (Nb: 100.37ppm, Ta: 26.6ppm) are moderately mineralised. Nb-Ta values of Shagari location (33.14ppm; 2.7ppm), Ganaja-1 location (12.6ppm; 3.6ppm) and Crusher location (0.91ppm; 0.2ppm) show they are barren. Various variation plot diagrams, such as Kb/Rb vs Cs, K/Rb vs Rb, also show mineralization status of these pegmatite bodies as compared with some local and foreign pegmatites. Plots of Zr vs SiO2, Rb vs Y+Nb and Rb vs Sr indicate the pegmatites are magmatic, from within plate granites and in crustal thickness of more than 30 km during the time of emplacement.