Geochemistry and Mineralogy of Clays from Ewohimi, Northern Niger Delta Basin, Nigeria

Abstract

Neogene age clay samples of the Ogwashi-Asaba Formation at Ewohimi, were analysed for geochemical and mineralogical properties, using XRF, ICP-AES and XRD in order to investigate their compositional characteristics of major and trace elements. With integrated geochemical and mineralogical approach, the composition, provenance, weathering, paleo – redox and the tectonic setting under which they were formed was defined. Mineralogical analysis indicate subordinate amounts of quartz and kaolinite as the only observable clay minerals. Major oxides content of SiO₂ (56.03 – 72.84%) and Al₂O₃ (13.63 – 22.92%) constitute over 84% of the bulk chemical compositions, indicating source rocks rich in quartz and silica-rich. Low occurrences of Na₂O, MgO and K₂O indicate the extent of weathering and transport, thus, suggest a continental granitic origin, possibly from the Oban Massif, east of the Anambra Basin. Although notable disparities exist in the SiO₂ and Al₂O₃ contents of the clays in Ewohimi; the River Odu clay is more siliceous and less aluminous than the others whereas, the Asaboro clay is more aluminous than others confirming a high degree of weathering activities. Redox-sensitive trace element ratios (Ni/Co, V/Cr and V/V+Ni) indicate a predominantly oxic environments of deposition. The dominance of detrital kaolinite, the enrichment of chemically immobile elements (SiO₂, Al₂O₃ and TiO₂) and high values of Chemical Index of Alteration (CIA) suggest intensive chemical weathering of the source rocks under tropical to subtropical humid climatic conditions.