A detailed geologic mapping of clay deposits in the Patti Formation around the Ahoko area in Kogi State (north-central Nigeria) was carried out on a scale of 1:12,500. The study area lies within the Lokoja Sub-basin of the Bida Basin between longitudes 60 51l and 60 53l E and latitudes 80 18l and 80 19l N covering a total land area of about 6.85km2 of the Koton-Karfi N-E Topographic sheet 227. The study was aimed at determining the provenance, weathering history of the clay deposits using bulk chemical and mineralogical compositions as well as their engineering properties to determine suitability for construction purposes. X-Ray Fluorescence (XRF) method was used for the determination of the bulk elemental composition. The XRF analysis revealed kaolinite minerals as the predominant clay mineral (SiO2 and Al2O3) indicating intensive chemical weathering of aluminium-rich source rocks and exhaustive leaching under a warm, humid condition. Results of geochemical analysis show predominance of SiO2 (27.87 to 80.70 weight %) and Al2O3 (14.00 to 20.04 weight %) which support the kaolinitic nature of the clay and classify the clay as aluminosilicates. The relatively high mean values of the Chemical Index of Alteration (CIA) and Chemical Index of Weathering (CIW) also suggest intensive chemical weathering at the source area. Other relevant geochemical indices, plot of TiO2 against Al2O3 reveal felsic igneous parent rock. The test for engineering properties revealed the clays to be moderately compressible, with liquid limit values ranging from 26.1% to 34.4%, plastic limit ranges from 14.3% to 21.36% and plasticity index ranges from 10.1% to 15.3%. It is therefore recommended that they are generally not good as base course except at low water content and thus should not be used for construction purposes. Properties of these clays make them favorable for use when compared with standards for refactory, ceramics, pottery and paints.
