The of Agbaja Ironstone deposits was investigated with a view of providing comprehensive data on the geology, petrographic characteristics and mineralogical composition. The investigation involved field description, thin section analysis and X-ray diffraction for phase identification. The Agbaja Ironstones comprises of oolitic, pisolitic and argillaceous (concretionary) materials which ranges in colour from red to brown and black with yellowish to red, brown and dark grey matrix and some with whitish materials in some of the rocks. The pisoliths occur mostly close to the surface overlying the ooliths and the rocks entirely lack sedimentary structures.Petrographically, the ironstone is well-sorted, characterized by loosely packed spherical ooids within a kaolinitic matrix with the major mineral assemblage of the concentric laminae being kaolinite, kaolinite-goethite and hematite ooids. While the pisolitic ironstones occur as poorly sorted, loosely packed and fragmented sub spherical pisoids and peloids of goethite and hematite within dominantly geotithe and kaolinite matrix. XRD diffraction pattern for the samples portrays remarkable reflection for the principal components identified under the microscope. These includes; Kaolinite , goethite, hematite and as major phases while minor phases such as quartz, illite and micas, illmenite, rutile, zircon, tourmaline and psilomelane siderite and sepiolite occurs as detrital inclusions and as secondary or alteration derivatives. The result of these studies supports a continental origin for the ironstones since benthic foraminifera, skeletal algae and fossils in general are absent. Also, the absence of any calcareous ooids and oncoids from the surrounding rocks indicates that the ferruginous ooids and oncoids are plausibly primary depositional grains forming kaolinite (at the expense of detrital feldspars) as the precursor for the ooids and pisoids.
Key words: Petrography, X-Ray Diffraction, ooids, pisoids, kaolinite.
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