Geochemistry, Classification Characteristics of Pegmatites From Ijero-Ekiti, Ekiti-State, Southwest Nigeria. (Published)
Geochemical analysis of ten (10) representative pegmatite samples reveal high silica (SiO2) 66-78 wt % and alumina (Al2O3) 10.08-19.57 wt % contents. The alumina (Al2O3) is greater than the alkali Na2O + K2O +CaO in all the rock samples by values ranging between 1.43 and 4.90 wt % implying that the rocks are peraluminous. MgO (0.02-1.60wt%), CaO (-0.01-1.39wt %) and Fe2O3 T (0.77–6-99 wt %) have low contents. Rocks that are characterised by low Mg, Ca, and Fe as well as low ratio of Na2O/K2O are termed “fertile” and peraluminous. A plot of Al2O3/(CaO + Na2O + K2O) versus Al2O3/Na2O + K2O shows and confirms the peraluminous character of the pegmatite from Ijero while the molecular Al2O3/Cao+Na2O+K2O versus SiO2 shows the plottings on the S-type granitoids . Rocks that are fertile and peraluminous are rich in albite (NaAlSi3O8), potassium feldspar (KAlSi3O8) and quartz (SiO2). The abundance of feldspar and mica are geochemical indicators for Sn-Ta mineralisation. Industrial minerals (Kaolin, feldspar and gemstones (tourmaline, beryl and topaz)) occur in abundance considered significant and exploitable. The crossplots of MgO, Fe2O3, Na2O Al2O3 and TiO2 against SiO2.respectively indicate negative correlation that suggest non association with SiO2. However, some oxides like Fe2O3, Na2O Al2O3 trend negatively downward but cluster along the trend showing their relative abundance with silica . The plotting of samples in both the tholeiitic and calc-alkaline fields shows that the magma from which the pegmatite was formed was not totally restricted in occurrence only to subduction-related environment but also from the oceanic crust of calc-alkaline environments. Samples plottings on both the igneous, sedimentary and metasedimentary origins as seen in Na2O/Al2O3 versus K2O/Al2O3 support the derivation of materials that made up the rock from mixed sources. Further classification using various parameters show that pegmatite samples plot on the granodiorite, syenogranite, alkali granite and quartz syenite as well as granitic groups, all having the attributes of granites, and supporting the granitic character of pegmatites .
Keywords: Geochemistry, S-type granitoid, continental/oceanic., mixed sources, peraluminous