Mineralogy and geochemistry of the sediments in rivers along the east coast of India: Inferences on weathering and provenance

Abstract

The clay fraction of the sediments in 15 rivers along the east coast of India showed The clay fraction of sediments in the lower reaches of 15 rivers along the east coast of India showed high kaolinite followed by illite and smectite for the rivers dominantly draining the Archaean–Precambrian Terrain (APT), high smectite followed by illite and kaolinite for those draining the Deccan Trap Volcanic Terrain (DVT) and, high illite followed by kaolinite, smectite or chlorite for those draining through Mixed-Lithology Terrain (MLT). The CIA (Chemical Index of Alteration) and PIA (Plagioclase Index of Alteration) values, depletion of Ca, K, Na and Sr and enrichment of Al, Fe and Ti, high Rb/Sr and Th/U ratios of sediments relative to the Upper Continental Crust indicate moderate to intense chemical weathering on source rocks. The average composition of clays exhibits slight enrichment of Fe, Mg, Sc, V, Co, Cr and Ni and depletion of Nb, Zr, Hf, Y and Ta relative to the Post-Archaean average Australian Shale. The Cu, Zn and Pb contents were in the range ‘significantly polluted to moderately polluted’ in APT- and DVT-sediments. The plots of TiO2 vs. Fe2O3+MgO, Th/Sc vs. Sc and La–Th–Sc showed sediment composition intermediate between granite and basalt, while the plots of TiO2 vs. Zr, Th vs. Sc and V–Ni–Th indicate intermediate provenance between mafic and felsic sources. The mineralogy of the sediments indicates mixed sources, but their chemical composition is affected by weathering and the provenance is intermediate between mafic and felsic sources.