Geology, Petrography and Geochemical Evaluation of Basement Rocks In Bakomba–Kabba Junction Area, Sheet 247 Lokoja SW, North Central, Nigeria

Abstract

Geologic mapping provides many types of imformation in the exploration for new mineral deposits, including the lithology, morphology, age relationships and structural information. Field geological mapping of basement rocks around the Bakomba-Kabba junction was carried out on a scale of 1:30,000 to establish the petrogenesis of rocks that underlain the area. The study area covers 30.80 km2 and falls at the eastern flank of the southwestern Basement Complex of Nigeria. Migmatite, banded gneiss, aplite and charnockite rock types were encountered during the mapping exercise. Predominant trends of joints, veins and lineation are indicative of imprints of the Pan-African thermotectonic Orogeny. Five different representative rock samples were subjected to both petrographic and geochemical analyses. Migmatite is composed of 62.12 % quartz, 24.50 % potash-feldspar, 6.60 % biotite, 5.82 % muscovite and 0.96 % opaque. The banded gneiss near Zariagi has 51.83 % quartz, 21.65 % potash-feldspar, 18.90 % biotite, 4.88 % muscovite and 2.74 % myrmekite minerals; whereas, the banded gneiss close to Oyi-Apataoworo has 58.63 % quartz, 21.58 % plagioclase, 6.47 % biotite, 7.19 % muscovite and 2.88 % opaque minerals. The aplite is dominated by 57.40 % potash-feldspar and 23.14 % quartz, 6.10 % biotite, 5.25 % perthite, 5.38 % muscovite, while the charnockite consists of 50.00 % quartz, 25.26% pyroxene, 12.89 % biotite, 8.76 % plagioclase and 3.09 % opaque minerals. The geochemical composition of these rocks indicate 69.90 % SiO2, 12.71 % Al2O3, 4.50 % Fe2O3, 0.48 % MgO, 6.62 % K2O, 3.09 % CaO, 1.32 % Na2O, 0.96 % TiO2, 0.07 % MnO and 0.34 % SO3 on the average. Harker diagram indicated a high fractionation of mafic minerals in the rocks. Other discriminating diagrams revealed that all the rock types have igneous origin and crystallized from magmas that are strongly alkaline. However, the migmatite, aplite, and banded gneiss from Zariagi crystallized from the high-K alkaline magma series; the banded gneiss close to Oyi-Apataoworo originated from medium-K alkaline magma series; while the charnockite evolved from low-K alkaline series. This implied that rocks from the study area were derived from heterogeneous alkaline magma series.