COMPOSITION OF RARE EARTH AND TRACE ELEMENTS IN SOSAN AKOKO RESIDUAL CLAY, SOUTHWEST NIGERIA: EVIDENCE OF WEATHERING PROCESSES AND PROVENANCE

Abstract

This study presents a comprehensive geochemical and mineralogical analysis of residual clay from Sosan Akoko, located within the Precambrian basement complex of Southwestern Nigeria, to elucidate its weathering history and provenance. Fifteen representative clay samples were collected from three excavated pits and were analyzed using X-ray diffractometry (XRD) and inductively coupled plasma-mass spectrometry (ICP-MS). The mineralogical analysis identified kaolinite (1.4%) and nontronite (2.77%) as the principal clay minerals. The non-clay mineral fraction is dominated by quartz, with minor occurrences of orthoclase, labradorite, and albite. The geochemical data reveal high mean values for the Chemical Index of Alteration (CIA) at 79.43% and the Chemical Index of Weathering (CIW) at 90.84%. These high index values, coupled with the depletion of alkali and alkaline earth elements, signify an intense degree of chemical weathering in the source area. The provenance of the parent material was investigated using elemental ratios and discriminant diagrams. Plots of TiO_2 versus Al_2O_3 suggest that the clay was derived predominantly from acidic igneous rocks. This is further substantiated by the high concentrations of certain trace elements, such as Rubidium (Rb), which ranges from 113.4 to 138.2 ppm, and Strontium (Sr), ranging from 218.3 to 229.2 ppm, indicating active chemical weathering processes. The chondrite-normalized Rare Earth Element (REE) patterns show an enrichment in Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Elements (HREE), with a significant negative Europium (Eu) anomaly (average 0.83%). This signature is characteristic of felsic source rocks. Ternary diagrams, including the La-Th-Sc plot, corroborate these findings, with samples plotting firmly in the felsic source region. The collective evidence from mineralogy, major and trace element geochemistry, and REE distributions strongly indicates that the Sosan Akoko residual clay originated from the in-situ chemical weathering of felsic, likely granitic, parent rocks under intense weathering conditions, although the presence of some primary minerals suggests that the weathering process has not reached complete maturity.