Variation of Mineral, Chemical, and Rare Earth Element Composition in Size Fractions of Clay-rich Sediments from the Kajjansi and Ntawo Clay Deposits, Central Uganda

Abstract

Abstract We analyzed the mineralogical and chemical (including rare earth element [REE]) composition of the >63, 32–63, and <32 μm grain-size fractions of clay-rich sediments from Kajjansi and Ntawo valleys, central Uganda. This was done to understand the changes that occur during the formation of clays. All fractions are dominated by kaolinite and quartz, with minor smectite, chlorite, and feldspar. The whole-rock chemistry shows that the <63 μm fractions are enriched in TiO 2, Al 2O 3, and Fe 2O 3, whereas the >63 μm fraction is enriched in SiO 2 when compared to average Upper Continental Crust (UCC). The trace element contents of the <63 μm fractions are similar to those of the bulk samples, whereas the <63 μm fraction shows lower contents. The chondrite-normalized REE distribution patterns are remarkably similar for all grain size fractions, with a general enrichment of the LREEs and a negative Eu anomaly. The >63 μm fractions have the lowest REE abundances, no LREE-HREE fractionation, but show a small negative Eu anomaly. The <63 μm fractions are, in general, enriched in the LREE, and have a slight positive Ce anomaly (for same samples) and negative Eu-anomalies, similar to the bulk samples. The chondrite normalized La/Yb and ratios are highest in the <63 μm fractions, whereas the chondrite normalized Gd/Yb ratios are highest in the >63 μm fraction. The Eu/Eu*, Th/Sc, and Rb/Sr ratios of the <63 μm fractions are most similar to bulk sample values, thus, this size fraction dominates the composition of the bulk sediment. The REE are found to reside predominantly in the <32 μm fraction.