Environmental and diagenetic implications for the rare earth element geochemistry of sediments of the Niger Delta

The REE (rare earth element) contents of the bulk, heavy mineral fraction and various size fractions (0.250-0.037 mm) of recent sediments of the Niger Delta vary considerably. ΣREE varies from 26 to 5140 ppm in the bulk samples, 54–77,400, in the heavy mineral fraction, while the 0.125 mm size is the most enriched. All samples are enriched in La, the lower and the intermediate REE (compared to the average shale). There is a correlation of REE with mineralogy.

The central delta sediments, with an average SiO₂ content of 82% and kaolinite as the dominant clay, have a low REE content. The heavy minerals contribute a large fraction of the REE content. The pH and Eh conditions favour the leaching of REE. The eastern delta, with high smectite content and an average SiO₂ of 58%, has the highest ΣREE and pronounced negative Eu and Ce anomalies. The higher REE content in eastern and coastal delta sediments is attributed to the basic conditions, the organic matter and REE exchange between sea water and the smectitic clays.