Geochemical Cycling of Rare Earth Elements in the Basement-Derived Lateritic Profiles of South West Nigeria: Fractionation Mechanisms and Exploration Implications

Rare earth element geochemistry in deeply weathered lateritic profiles developed on basement complex rocks of South West Nigeria is controlled by the interplay of primary mineral dissolution, secondary mineral precipitation, and organic ligand complexation, but the quantitative partitioning of rare earth elements among these processes remains poorly constrained for Nigerian lateritic systems. This study investigates the geochemical cycling and fractionation mechanisms of rare earth elements through complete lateritic weathering profiles developed on granitic and gneissic parent rocks in Oyo and Osun states, South West Nigeria. Complete vertical profiles through saprolite, mottled zone, ferruginous crust, and upper horizon were sampled at centimetre resolution at four localities. Rare earth element concentrations were determined by inductively coupled plasma mass spectrometry after lithium borate fusion dissolution and acid digestion. Sequential extraction experiments quantified the distribution of rare earth elements among exchangeable, carbonate, iron oxyhydroxide, organic matter, and residual fractions. Europium anomaly, cerium anomaly, and chondrite-normalised patterns were computed to identify redox-sensitive fractionation signatures. Thermodynamic speciation modelling using MINTEQ evaluated the dominant aqueous complexation reactions governing mobility. The study develops an original fractionation model applicable to exploration targeting in lateritic terrains and contributes a theoretical framework for predicting heavy rare earth element enrichment in specific profile horizons. Keywords: rare earth elements, laterite, geochemical cycling, fractionation, South West Nigeria.