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

摘要

We analyzed the mineralogical and chemical (including rare earth element [REE]) composition of the > 63, 32-63, and < 32 mum 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 kalinite and quartz, with minor smectite, chlorite, and feldspar. The whole-rock chemistry shows that the < 63 mum fractions are enriched in TiO_2, Al_2O_3,and Fe_2O_3, whereas the > 63 mum fraction is enriched in SiO_2 when compared to average Upper Continental Crust (UCC). The trace element contents of the < 63 mum fractions are similar to those of the bulk samples, whereas the > 63 mum fractions 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 mum fractions have the lowest REE abundances, no LREE-HREE fractionation, but show a small negative Eu anomaly. The < 63 mum 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 mum fractions, whereas the chondrite normalized Gd/Yb ratios are highest in the > 63 mum fraction. The Eu/Eu, Th/Sc, and Rb/Sr ratios of the < 63 mum 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 mum fraction.