Genetic Characterisation of Ironstone Deposits from the Agbaja Formation Southern Bida Basin Central Nigeria: Evidence From Rare Earth Elements

The oolithic and pisolithic Agbaja ironstone deposit of the Campano- Maastrichtian southern Bida Basin around Lokoja districts is one of the most studied ironstone formations within the Nigerian Inland basins where controversies still surrounds the origin of the ironstone deposits. Though analyses of Rare Earth Element (REE+Y) distributions in chemical sedimentary rocks has been proved to offer an opportunity to gain new insights into likely ancient palaeo-environmental conditions based on the distinct differences in the rare earth element geochemistry between different types of water and their respective precipitates, but this has not been properly utilized in unraveling the mystery surrounding the origin of this deposit. The present study therefore, is an attempt to provide clues to further understand the genetic characterization of these ores from geochemical data based on the rare earth elements. Geological field mapping was carried out in the study area and representative samples from different rock exposures and outcrops were collected. Seventeen (17) representative samples of the various rock types collected from the field were selected for geochemical analysis. The results reveal a total REE contents of the oolithic and pisolithic ironstone samples range from 189.2 to 471.3ppm with an average of 332.56ppm, with all of the samples having positive Gd anomalies and characterized by the unique absence of Ce anomaly (Ce/Ce*=0.951.01), a slight positive Eu anomaly (Eu/Eu*=0.991.12) with slight enrichment in Light Rare Earth Element (LREEs) and depletion in Heavy Rare Earth Element HREEs except in sample BH1 which shows gradual increase from LREE to HREE (La-Lu). Absence of negative Ce anomaly and the slight positive Ce anomaly in the oolithic and pisolithic ironstone samples is attributed to enrichment of Ce relative to other REE3+ through mobilization of Ce as Ce3+ under reducing condition during early diagenesis and re-precipitation at the oxidized front while the LREE may be attributed to contribution of enriched particulate during chemical precipitation and recrystallization of iron and during kaolinitization. Also, the absence of Ce fractionation suggests that detrital sedimentation dominates over a hydrothermal contribution. Similarity, the positive Eu anomaly suggests that the samples are sourced from continental source. The absence of significant correlations or weak correlations between the Ce/Ce* and REE, and between Ce/Ce* and Eu/Eu*, suggests a slight effect of diagenetic processes on the REE concentrations. The lower Y/Ho ratios rules out marine input and points towards the influence of terrigenous materials. Thus, the combinations of LREE depletion along with low Y/Ho ratios below those of seawater suggest records of continental signatures which are significantly masked by detrital components. With all the above inferences, Fe oxides and oxyhydroxides and phosphates minerals which have a high affinity for REE in diagenetic fluid along with kaolinite are the possible sources of the REE contamination. Thus, the change in REE abundance results from changes in various diagenetic or progressive alterations of REE contents with increasing geological time, scavenging of REE on iron oxides and oxyhroxides and affinity of the REE to Kaolinite. It also reflects the existence of REE-rich minerals such as monazite and apatite. Thus, the above evidence from the REE studies have refuted marine or hydrothermal sources for these deposits and points towards detrital sources from fluvial input into the basin as material for the formation of the precursor for the ironstone.