Geochemical Characterization and Mineralogical Assemblage of Burhi-Dihing and Noa-Dihing—The Largest South Bank Tributaries of the Brahmaputra River

Abstract

Mineralogical studies are valuable in understanding past weathering regimes induced by changing climatic conditions. Information about the bedrock lithology, weathering regimes, erosion and sedimentation rates are fundamental issues for better understanding of the river catchment behaviours. In this context, therefore, major ions, trace metals and clay mineral compositions of the sediment and water of Noa-Dihing and Burhi-Dihing rivers, the southbank tributaries of the Brahmaputra river, have been examined. Chemical index of alteration (CIA) values of both Noa-Dihing and Burhi-Dihing rivers suggests the prevalence of moderate chemical weathering events and formation of minerals such as muscovite, illite and smectite in the river catchment. Metal distribution in both rivers shows highly toxic metals, comprised of 32% of total mineral composition, and are less abundant with respect to moderately toxic metals (37%) except Zr, which comprises 37% and 32% respectively of the total mineral compositions. Comparison of sediment chemistry with composition of source rocks and average Upper Continental Crust (UCC) suggests higher depletion of CaO, Na₂O, P₂O₅, Al₂O₃, SiO₂, MgO and MnO signifying the dominance of mafic mineral phases. Loss of these metal oxides from the bed rocks during weathering and/or less abundance of clay in bed sediments compared to that in UCC may also be attributed to depleted values of these minerals. This is further substantiated by grain size analysis i.e. more abundance of sandy silt as compared to clay minerals in the overbank and channel sediments of Noa-Dihing and Burhi-Dihing rivers. Chemical Index of Alteration (CIA) varies significantly from 64.49 to 81.21, indicating large spatial variability in the intensity of chemical weathering of upper Brahmaputra basin. Multivariate analysis suggests that natural weathering processes of alkaline earth metals can be associated with release of trace metals in a riverine system and releasing mechanism of transition metal and their oxides are same and that of alkali and alkaline earth metal are similar.