Geochemistry of Lignite Associated Shales in Ogwashi-Asaba Formation: Implication for Paleoredox, Source-Area Weathering, Provenance and Tectonic Settings

Osokpor, J. and Nkwagu, S.
Keywords: Lignite geochemistry, provenance, alteration, paleoredox, paleoweathering
Tropical Journal of Science and Technology 2020 1(1), 16-34. Published: June 29, 2020


Abstract

Shale samples obtained from the Obomkpa lignite belt were analysed for geochemical attributes such as major, trace and rare earth elements using XRF, ICP-MS and ICP-OES techniques. Paleoweathering and paleoredox conditions, provenance and tectonic settings were evaluated. High CIA (97.79 – 99.00), CIW (99.92 – 99.93), and MIA (95.58 – 98.00) values indicated intense weathering in the source areas and high detrital input dominated by strong chemical weathering, which led to the formation of clay minerals by hydration and leaching of ions such as Ca⁺2, K⁺ and Na⁺ known to be present in the feldspar minerals. Cross plot of CIA vs SiO2 shows that the mineral matter composing the shale samples have been substantially recycled in the course of weathering. Ratios of redox-sensitive trace elements (V/Cr = 0.92 av., Ni/Co = 2.8 av., and Th/U = 4.14 av.) suggests an oxic paleodepositional environment for the sediments. The strongly positive Ce anomaly (Ce/Ce*) (0.83 – 0.96) also affirms an oxic setting. Trace element ratios (La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th and Eu/Eu*) applied for provenance discrimination and with a range of values: 1.31 – 2.89, 0.86 – 1.04, 0.35 – 1.74, 0.11 – 0.14, 7.23 – 9.52, and 0.52 – 1.38 respectively, for the samples, compared to those of sediments derived from mafic and felsic sources. A felsic provenance was inferred for the shales after due comparison. A plot of of Zr/Sc vs. SiO2 suggested substantial reworking of the sediments. Inferences made by integrating various discrimination plots of major element concentrations (SiO2/Al2O3, TiO2, Na2O+CaO, Al2O3 (Na2O+CaO) vs. Fe2O3+MgO), classified the shales as Active Continental Margin (ACM) sediments.