Geochemical Characterization of Shale Deposits Around Afikpo Area, Southeastern Nigeria

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Abstract The mineralogical and geochemical composition of shale sourced from Mamu and Nkporo Formations, around Afikpo area in south-eastern Nigeria, were investigated to evaluate the source rock composition/provenance, infer paleo-redox conditions, sedimentary cycle, maturity and the intensity of source area weathering. Dominant clay mineral in both formations is kaolinite. Plots of Al2O3 vs. SiO2, TiO2 vs. Al2O3, Th/Sc*100 vs. Zr/Sc, La – Th – Sc and Cr/V vs. Y/Ni with observed light rare-earth elements (LREE) enrichment and negative Eu anomalies (Eu/Eu*: 0.04 – 0.08 for Nkporo and 0.06 – 0.08 for Mamu Formation samples) indicates largely felsic sources provenance, supported by presence of kaolinitic mineral. The redox-sensitive trace element values (Mo, U, and V) suggest a marginal marine (oxic) environment for Mamu Formation shale but marine (oxic to anoxic) setting for Nkporo Formation, this was supported by U/Th and Ni/Co values as their ratios suggest that the depositional basin experienced oxic to anoxic depositional conditions. The characteristic moderate HREE depletion pattern with low GdN/YbN ratios for the investigated Afikpo shale indicates non- monazite accumulation in the samples. The sediments are mature according to values of ICV, ratios of Th/Sc and Zr/Sc. Plots of CIA vs. SiO2, ICV vs. CIA with Al2O3-(CaO + Na2O)-K2O signpost an intense weathering in the source area. There is insignificant differences between the weathering and sediment maturity for the shale sediments of both formations from the same sedimentary basins, however, there exist disparity in their source rock/provenance and redox settings justifying the different formations.

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