Geochemistry and mineralogy of the Campanian Sandstone of Lokoja-Basange Formation, Middle Niger Basin (Lokoja sub-basin), Nigeria: Implications for provenance, weathering, tectonic setting and paleo-redox condition

Twenty six road-cut sandstone samples from two lithological sections of Lokoja-Basange Formation in Middle Niger Basin (Lokoja sub-basin) situated by the side of Auchi-Igarra road (07o 07.201'N, 006o 13.011'E) were investigated using integrated mineralogical, geochemical and pore water chemistry studies. The medium to coarse grained sandstone bodies are poorly sorted suggestive of deposition in a low energy setting, probably in a shelf or floodplain. The observed variations in the sandstone colourations are attributed to the nature of the cementing materials. Based on the mineralogical composition, two specific geochemical intervals were established; the first interval revealed quartz and kaolinite as major crystalline minerals with traces of hematite. The second geochemical interval showed quartz and kaolinite as the major crystalline minerals with minor quantities of grossite and halloysite. The geochemical datasets obtained revealed mature, lithic arenites including sub-greywacke and protoquartzites. The inverse correlation between redox potential (Eh) and electrical conductivity (EC), total dissolved solids (TDS) and Mg (at 0.05 significant levels) suggest well oxygenated environment of deposition. The high chemical index of alteration (CIA), plagioclase index of alteration (PIA), and chemical index of weathering (CIW) indices revealed high detrital input dominated by intense chemical weathering. This process eventually led to the formation of clay minerals by hydration and leaching of all major cations, such as Ca +2 , K + , and Na + , present in feldspar minerals. The average mineralogical index of alteration (MIA) values are indicative of intense to extreme weathering of mineralogical component of the detrital materials from the source areas. The studied sandstones samples are plotted in the field of the active and passive continental margin settings. The mean ratio of Ti/Zr also corroborates active and passive continental margin settings. The higher ratios of La/Y and La/Th and corresponding lower ratios of La/Co and Th/Co indicates felsic source rock. Moreover, the lower ratios of Ba/Sr, Cr/Zr, Ti/Zr and higher ratio of Zr/Y probably suggest felsic source rock. Based on the previously established thresholds, the low Cu/Zn ratios in the studied sandstone samples suggest deposition under oxidizing conditions. Keywords: Lokoja-Basange Formation, Middle Niger Basin, Mineralogy, Geochemistry, Pore water chemistry, provenance, tectonic setting, weathering, Redox proxy.