Geochemical, diagenetic and depositional characteristics of the Maastrichtian Agbaja Formation ironstone exposed at Enegbaki, southern Bida Basin, Nigeria

Abstract

This work examines the Maastrichtian Agbaja Formation ironstones at Enegbaki, southern Bida Basin, Nigeria. The ironstone deposit has attracted significant exploration and resource evaluation efforts but its physical characteristics and paleogeographic significance remain uninvestigated. Lithostratigraphic features of the ironstones reveal association with claystone and shale. Twelve ironstone samples were selected for thin-section and polished-section microscopy, XRD, XRF, and ICPMS. Microscopy reveals that the coated grains are either rounded to spherical, partly elongated and the occurrence of whole and broken Fe-coated grains suggest an environment which is relatively agitated, involving multiple episodes of growth and reworking. Mineralogical analysis reveals the presence of amorphous components, goethite, magnetite, siderite, chamosite, maghemite and hematite in the order of abundance. Fe 2 O 3, SiO 2 and Al 2 O 3 constitute more than 80 % of the oxides in the samples and the Rare Earth Elements range from 129 to 249 ppm. The mineral assemblage of the ironstones indicate that the clay precursors were derived from deep chemical weathering, transported and deposited during periods of reduced clastic sediment influx in shallow to full marine environments. Two depositional facies of the ironstones are recognized; the reduced facies at the lower part comprising chamosite, siderite and magnetite and the oxidized facies are at the upper part consisting of goethite, maghemite and hematite. The main diagenetic features are the early diagenetic chamosite and siderite formation, followed by replacement of kaolinitic clays by goethite and dehydration of goethite to form hematite at the middle to late-stage diagenesis. The ironstone is characterized by intermediate Nd concentration (13.4 – 38.9 ppm), negative Ce and positive Y anomalies suggesting it is of replacement diagenetic origin and this is supported by the presence of preserved clay precursors in the ironstone. The study reveals occurrence of chamosite and siderite in the investigated ironstones indicating that extreme Eh-pH conditions in full marine environment were recorded in the Bida Basin during the Upper Cretaceous period. The presence of chamositic oolitic ironstone and reducing condition in the lower part of the ironstones in study area offers probably evidence of K/T boundary. • Investigated ironstone occurs as oolitic and pisolitic facies in association with siltstone and shale. • Clay precursors were derived from chemical weathering and deposited in a shallow marine environment. • Ironstones were formed in an agitated water and involving multiple episodes of growth and reworking. • Paleoenvironmental conditions of the ironstone range from dysoxic anoxic. • The main diagenetic imprints exhibited by the ironstones are chamosite and siderite.