Abstract:
Talc is formed from diverse geological processes including hydrothermal alteration and contact or regional metamorphism of mafic, ultramafic rocks and dolomitic carbonates. Previous studies on the genesis of talc deposits in Nigeria have been speculative due to inadequate compositional data leading to misclassification of the talc genesis. Therefore, this study was aimed at investigating the geology, tectonic setting and composition of rocks around Wonu, Ibadan-Apomu area, southwestern Nigeria.
Geological mapping on a scale of 1:25,000 was carried out around Wonu, Ibadan-Apomu area. Representative samples (105) of the rocks were collected from outcrops and subjected to petrographic analysis. Polished sections were prepared from the rocks and were subjected to electron probe micro-analysis to determine the mineral chemistry. Scanning electron microscope-energy dispersive spectrometry was used to determine the morphology and fluid components. Additional mineralogical study was carried out using X-ray diffraction. Major, trace and rare-earth elements of the rocks were obtained using X-ray fluorescence, and inductively coupled plasma-mass spectroscopy. Geochemical discrimination diagrams were used to interprete the data obtained.
The lithologic units mapped were migmatites, talc schist, granites, amphibolites and lherzolites. These have been intruded variably by the pegmatites. The migmatites and the granitic intrusive are composed of quartz, oligoclase, albite, K-feldspar, ferro-hornblende, and biotite. The amphibolite is fine-grained, composed of enstatite, augite, anthophyllite, actinolite, magnesio-hornblende, magnesio-anthophyllite, titanium-K-alumino-anthophyllite, potassium-gedrite, potassium-alumino-anthophyllite, cummingtonite, tremolite, actinolite, biotite, bytownite, oligoclase, andesine, albite, K-feldspar, perthites, ilmenite, magnetite and garnet. The lherzolite and talc are composed of forsterite and fayalite, orthopyroxenes, clinopyroxenes, magnesio-hornblende, tchermakitic-hornblende, magnesio-gedrite, anthophyllite and bytownite. Alteration minerals include serpentine, amesite, talc, Cr-bearing clinochlore, ferritchromite, graphite and carbonate. Ilmenite, garnet, spinel, chromite, pyrite and magnetite are the opaque minerals present. Olivine-spinel geothermometry of 700-750ºC is consistent with medium to high amphibolite regional metamorphism prior to talc formation. Alteration of olivines, pyroxenes and amphiboles to talc, chlinochlore; and spinel to ferritchchromite, depicted on the Cr/(Cr+Al) vs Fe2+/(Fe+Mg) and Al2O3 vs Cr2O3 diagrams showed retrograde amphibolites-greenschist facies metasomatism at 500-550ºC. This led to the formation of asbestiform amesite, anthophyllite, actinolite and tremolite. The Al2O3-Fe2O3+TiO2-MgO discrimination diagram suggests tholeiitic protolith for the amphibolite and komatitic protolith for the lherzolite. The lherzolites have flat rare-earth element patterns typical of komatiites and dunites. The MgO+FeO+CaO vs SiO2+Al2O3 diagram indicated ultramafic-hosted talc with variable purity.
The talc deposits around Wonu, Ibadan-Apomu area is derived from metasomatic alteration of the amphibolites and the lherzolites. The tectonic setting is an island arc basaltic material deposited near supra-subduction-zone.
