Mineralogy and geochemistry of beach sediments associated with the Precambrian crystalline rocks (Vijayan Complex) of Sri Lanka; perspective for heavy minerals

Abstract

The southeastern landmass of Sri Lanka is laid on the Vijayan Complex (VC) rocks, a Precambrian basement characterized by specific geological features since the mineralized contact zone of the Highland–Vijayan boundary is nearby. However, the coastal sedimentation of such a basement is rarely interpreted. Hence, the mineralogy and geochemistry of surface sediments exposed along 45 km long southeast (SE) coast were studied to find the compositions and to interpret the terrestrial compart. Studied beach faces expose as sandy siliciclastic sediments with scatter depositions of black and red colored heavy sediments, as 10–15 cm beds, within 1–3 km from deltaic environments. X-ray diffraction analysis of such heavy placers indicates the presence of predominant ilmenite, almandine garnet, zircon, monazite, and ferrihydrite. The sediments from other areas are rich in lighter minerals of quartz, albite, and magnesium-calcite representing the average beach sediment mineralogy of the SE coast. Upper continental crust normalized X-ray fluorescence data show an enriched tendency of Zr (9.8) > V > Nb > Th > TiO2 (6.2) > Cr (2.5) > MnO > Fe2O3 > As > Y > Zn and depleted tendency of Pb (0.8) > CaO > P2O5 > Sr > Sc > Cu (0.1) indicating the leachability of depleted elements and strong recycling processes of High Field Strength Elements (HFSE). Hierarchical cluster analysis and Pearson correlation matrixes indicate the presence of four geochemically related sample clusters. These analyses further showed that cluster 1 relates to heavy sediment deposits with higher terrigenous influences while cluster 4 relates to siliciclastic sediments with higher marine influences. Hence, geochemical anomalies of clusters 1 to 4 show decreasing abundances for HFSE and opposite tendencies for CaO, Sr, I, Br, and F confirming the mineralogical results. Higher terrigenous influenced sediment samples are evident for the presence of solid phases of titanium carriers (As–V–TiO2), phosphorus–HFSE carriers (Pb–Th–P2O5–Y–Cl–Nb–Zr), authigenic carbonates (Sr–Br–CaO) and Fe–Mn coatings (Zn–Cr–MnO–Fe2O3-Cu). Compatibility variable ratios of Th/Sc, Zr/Sc, TiO2/Zr, Cr/Th, and V/Th respectively show 15.16, 364.38, 20.85, 2.95, and 9.73 values for these coastal sediments. Based on the published international and VC average lithological data, the present study concluded that the primary sources for the sedimentation have felsic signatures showing relations to granitic gneisses and granitic intrusive rocks of the VC. Apart from the terrestrial provenances, the coastal area has been affected by lower secondary influences of anthropogenic, marine inorganic, and organic matter.