Major and trace elements distribution in stream sediments of the lesser ZAB river at northeastern Iraq : implications to weathering and transportation

Abstract EN

Fifteen samples of stream sediments were collected from the Lesser Zab River (LZR), which represent one of three major tributaries of the Tigris River at northeastern Iraq.

The sediment samples were subjected to laboratory studies including chemical analyses (using polarized energy dispersive XRF) for major (SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, P2O5, TiO2 and MnO, in weight %) and trace (Ba, Co, Cr, Cu, Ni, Pb, Rb, Sr and Zn in ppm) elements.

These results have been used to characterize chemical weathering and transportation in the catchments area, to assess relative mobility of elements during weathering and to understand trace element associations.

Correlation coefficients being significant for most of the major elements among themselves which suggests that concentrations of these elements are significantly controlled by Ferro manganese oxideshydroxides, clay and carbonate mineral abundances during weathering and transported of Lesser Zab river sediments.

Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW) and Index of Compositional Variation (ICV) for LZR sediments averages at (46%, 50% and 2.25 respectively), indicating that weathering in the basin is of moderate intensity.

Comparison of the chemistry of sediments with the composition of source rocks and average Upper Continental Crust (UCC) suggests significant loss of most major elements from source rocks during weathering, the degree of loss being more for Na2O and K2O.

Available results seem to indicate that Sr and Ba are effectively more mobile than other metals, which is thought to be a combined effect of higher solubility of Sr and Ba.

Trace elements show higher concentrations in the river sediments and significant positive correlation with themselves (exception Cr) and with Al2O3, Fe2O3 and MnO.

These observations suggest that trace element concentrations are also controlled mainly by clay mineral abundances and those of Fe-Mn oxides.

Cr shows significant positive correlation with SiO2 which indicates to diffusion of Cr from chromite margin toward the silicate matrix during alteration of chromites and metamorphism, resulted in the development of ferrit-chromite and chlorite.

Observed higher Al2O3 concentrations and its strong association with trace elements in LZR sediments would indicate to preponderance of clay minerals over K-bearing minerals such as K-feldspars and micas.

Hence, the sediments of the LZR are mainly enriched in clays in comparison with rock forming minerals.

Enrichment factor and geo-accumulation index calculated for trace elements in LZR sediments suggest that they are mainly of natural origin and that anthropogenic activities exert little influence on their abundances.