Mineralogical and Geochemical Analyses of the Maastrichtian Ironstones in Share-Tsaragi Area, Northern Bida Basin, Nigeria: Implications for Origin of the Ironstones

Abstract

The ironstones in Share-Tsaragi area, occur as capping for the extensive ridge of the Upper Cretaceous sequence in northern part of the Bida Basin, Nigeria. Methods of study include logging, sampling of the ironstones, mineralogical (thin section and X-ray diffraction) and geochemical analyses. Field study shows that the ironstone occurs as capping for the sequence of conglomerate, sandstone, siltstone and claystone. Generally, the ironstones are poorly sorted, massive and sandy to clayey. Goethite and hematite are the main iron minerals identified in the analyzed samples, with values ranging from 18.91 to 50.85% and 1.57 to 47.56% respectively, while quartz with values ranging from 30.25 to 50% (with exception of SH7M having the least value of 1.88%) and kaolinite (6.55 to 21.0%) represents the detrital materials. Geochemical composition presents variable concentrations of major oxides, trace and rare earth elements. The Fe O (26.01-77.62 %, average of42.2 %) constitutes the dominant major oxide, followed by SiO and 2 3 2 Al O with range of values from 9.95 to 46.63 % and 3.15 to 15.13 % respectively. Predominance of iron oxide 2 3 minerals (goethite and hematite) and low concentration of CaO, MgO and MnO (<0.5 %) suggest that iron oxide minerals were diagenetically precipitated as cements and replacement of the kaolinite precursor. Generally, the geochemical and mineralogical compositions of the analyzed samples suggest influence of lateritization in the formation of the ironstones. A genetic model involving transportation of the weathered products such as amorphous iron oxy-hydroxides and quartz, deposition in a non-marine environment and subsequent diagenetic iron enrichment of the detrital precursors is suggested. An enrichment of the analyzed samples in oxides of Al, Si, Na and K support terrigenous sources.