Petrological and Geochemical Evaluation of Carbonatite in Obu Opkela, Edo State

Abstract

This study presents an integrated geological, petrographic, and geochemical investigation of carbonate-rich rocks within the Obu–Okpella area of the Nigerian Basement Complex with the aim of determining their origin, petrogenesis, and economic significance. Detailed geological mapping reveals that the study area is underlain by migmatite–gneiss complexes intruded by schists, quartzites, calc-silicate gneisses, and carbonate-rich units occurring as lenses, bands, and discontinuous bodies structurally controlled by regional deformation. Although these carbonate units exhibit field relationships comparable to metasedimentary marbles, further investigations were undertaken to ascertain their true nature. Petrographic analysis shows that the carbonate-rich rocks are characterized by inequigranular and locally magmatic calcite textures, abundant accessory apatite, Fe–Ti oxides, and occasional silicate phases, features inconsistent with sedimentary marble but diagnostic of carbonatitic rocks. Major oxide geochemistry indicates very high CaO and variable MgO contents, extremely low SiO2, and elevated P2O5 and alkali concentrations. Trace element data reveal significant enrichment in incompatible elements such as Sr, Ba, Nb, Th, and light rare earth elements, with primitive mantle–normalized spider plots displaying pronounced LILE and HFSE enrichment patterns typical of carbonatite magmatism. Geochemical discrimination diagrams and elemental variation plots suggest a mantle-derived, within-plate alkaline affinity, consistent with carbonatite emplacement in an extensional or rift-related tectonic setting. The apparent stratigraphic association of the carbonate bodies with schists and quartzites is therefore interpreted as the result of tectonic emplacement, folding, and transposition during Precambrian deformation rather than primary sedimentary deposition. The results of this study confirm that the investigated carbonate-rich units are carbonatites rather than marbles. Their geochemical signatures and mineralogical characteristics indicate significant economic potential, particularly for Ca-rich industrial applications and possible enrichment in Nb and rare earth elements. This study demonstrates the importance of integrated petrographic and geochemical approaches in distinguishing carbonatites from marble in complex Precambrian basement terrains.

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