Geochemical Appraisal of the Mamu Shales Exposed around Igodor in the Benin Flank of the Anambra Basin, Nigeria

Abstract

The Auchi area of Edo state which lies within the Benin flank of the Anambra Basin host shaly sediment exposures that have been classified by  previous researches as units of the Mamu Formation. This study evaluated samples of this sediment from Igodor near Auchi for its geochemical and  mineralogical properties, and intereted its, depositional environment and geotectonic setting. In order to achieve this, field studies were  carried out and representative samples obtained for Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) analyses of major oxides, trace and rare earth elements. X-Ray Diffraction analysis was also carried out to determine the mineralogical composition. Some of the minerals determined were Aragonite and galena. Binary plots, triplots and elemental ratio plots including SiO₂/, Al₂O₃, Th/Sc, Th/Co and La/Sc , Th-Sc-Zr, and the abundance of Cr, Ni were employed to determine the provenance. The concentration of detrital indicators such as SiO₂, Al₂O₃ and TiO₂, with averages of 51.95, 25.34 and 1.39 respectively, indicate high detrital influx into the Benin Flank of the Anambra basin. SiO₂/Al2O₃ ratios of 1.80-2.20, indicate that the shales were made up of pure kaolinite. The Ni and Cr abundance indicated a mafic and felsic provenance for the sediments, however, Th/Sc, Th/Co and La/Sc ratios show that the provenance was predominantly felsic, while the Th-Sc-Zr triplot shows that the depositional setting was passive.