Mineralogical and chemical composition and distribution of rare earth elements in clay-rich sediments from central Uganda

Clay-rich sediments from the Kajjansi, Kitiko, Kitetika, and Ntawo valleys in central Uganda were analyzed for mineralogical and chemical composition, including the rare earth element (REE) contents. The valleys are filled with Quaternary to Recent alluvial and lacustrine sands, silt, and gravels, which formed from the bed rock metasediments of the Buganda-Toro System and from granitoid rocks that include rocks of the basement. The sediments are dominated by kaolinite and quartz, and minor phases include smectite, chlorite, and illite/muscovite. Whole rock chemistry shows that sediment samples rich in SiO2 have low Al, Fe, Sc and Cr contents. The high chemical index of alteration (CIA) values (87 to 96), chemical index of weathering (CIW) values around 98 and low contents of the alkali and alkali earth elements of the clay-rich sediments suggest a relatively more intense weathering source area. Barium, Rb, Ca, and Mg were probably flushed out by water during sedimentation. The chondrite-normalized REE patterns of the clay-rich sediments show LREE enrichments and a negative Eu anomaly. The high chondritenormalized La/Yb ratios, and Gd/Yb ratios lower than 2.0, indicate that the sediments are enriched in the LREEs and are similar to typical post-Archean shales, such as Post-Archean Australian Shale (PAAS). The mineralogical composition, REE contents, and elemental ratios in these sediments suggest a provenance from mainly felsic rocks, with only minor contributions from basic sources. The basic sediments were most likely derived from metasedimentary rocks, such as muscovite-biotite schists, which are characteristic of the Buganda-Toro System rocks, whereas the felsic sediments are derivatives of granitoid rocks of the basement. The most significant geochemical finding is that despite intense weathering, which has affected most elements, the REE, Th, and Sc remain immobile.