Effects of lithological and evolutionary processes on geochemical changes of Shahrokht-Yazdan Playa brines (east of Iran-west of Afghanistan)

Abstract

The Shahrokht-Yazdan Playa is a sedimentary inland basin in the east of Iran and west of Afghanistan. It covers an area of 212.5 km2, lying 135 km northeast of Qayen at a border region known as Shahrokht. Some parts of the playa have stretched into Afghanistan (Yazdan area). In this playa, water level is controlled by the amount of rainfall, evaporation and groundwater recharge. To study the brine’s origin and evolution, some 95 water samples with specified intervals of 1 km were taken. The geochemistry of the brines was undertaken using the ICP-MS method and X-ray diffraction analysis on the sediments, the ionic concentrations of brine and mineralogical composition of sediments were identified. Major cations, as per their abundance, are sodium, calcium, magnesium and potassium and anions are chloride, sulphate and hydrogen carbonate, respectively. The results showed that the sediments of the lake’s bed were composed of chemical (evaporite) and clastic minerals (quartz, feldspar, muscovite and some clay minerals). Dominant evaporite minerals in sediments are gypsum, halite, calcite and dolomite while the minor mineral is clinochlore. Careful examination of surface samples indicates that carbonate and sulphate minerals deposited in the margins and minerals such as halite deposited as highly soluble in the centre of the playa. Chemical analyses of inflow water indicate that water has a molar ratio of $$ {\mathrm{Ca}}^{2+}>{\mathrm{CO}}_3^{2-} $$ , Cl−+ $$ {\mathrm{SO}}_4^{2-}>{\mathrm{HCO}}_3^{-},\kern0.5em {\mathrm{HCO}}_3^{-}\ll {\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+} $$ and is following the same second geochemical path (II) as that of Saline Valley and Dead Valley brine lakes on the brine evolution flow diagram. The inflow water to the basin is of the Na+-(Ca2+)-(Mg2+) $$ {\mathrm{SO}}_4^{2-} $$ -Cl−-( $$ {\mathrm{CO}}_3^{2-} $$ ) type which changes into Na+-Cl−- $$ {\mathrm{SO}}_4^{2-} $$ type during the geochemical evolution and deposition of evaporate minerals. In order to investigate the origin of major ions in the brine, the groundwater of the surrounding desert, which is the only supplier of ions to this area, was studied. The relationships between groundwater and regional brine were investigated. The study of the chemical changes of the brines in relation to the sedimentation of evaporite minerals indicates marked evolution of the brines in Shahrokht-Yazdan Playa to in so far as the formation of chloride minerals and type brines Na+-Cl−- $$ {\mathrm{SO}}_4^{2-} $$ occurred. The study of correlational relations with the help of SPSS software also shows the formation of carbonate and sulphate minerals in this playa and the saturation of brine from chloride.