Mineralogy and geochemistry of the evaporitic crust from the hypersaline Sambhar Lake playa, thar desert, India

Abstract

The Sambhar Lake in Rajasthan, India, is the largest playa in the Thar desert located in the gaps of the Aravalli mountain range. Fed by two major streams, namely the Mendha flowing NE-SW and the Rupangarth flowing SW-NE, the Sambhar Lake carries water only during the monsoonal months. This paper reports the zonal distribution of evaporitic and clastic minerals of the playa surface and attempts to follow the evaporation path of the Sambhar brine. The zonal distribution of clastic minerals in the evaporitic crust of the Sambhar reflects two distinct environments of deposition; the eastern part of the lake shows enrichment of Corg (organic carbon) and light minerals (clay minerals) along with all the oxides and trace elements derived from the clay minerals and the western part of the playa is enriched in heavier minerals such as amphibole and the minerals forming from hydrolysates. The mineralogical associations are correlated with the geochemical data and indicate their provenance from (ultra) mafic igneous or metamorphic rocks. The evaporite minerals present in the playa sediments clearly manifest the general trend of evaporation. The evaporite mineralogy and geochemistry shows four distinct zones as per the solubility of the occurring minerals. The minerals such as halite and calcite occur in all zones of the playa. The outermost zone is characterised by dolomite, CaMg(CO3)2, followed by areas characteristic for complex salts including K and Mg e.g. by carnallite, KMgCl3.6H2O and polyhalite, Ca2MgK2(SO4)4. 2H2O. The last stage of evaporation caused the precipitation of sylvite, KCl. The concentration of cations Na+, Ca+2, Mg+2, K+, and anions HCO3-1, SO4-2, Cl-1 controlled the succession of carbonates, sulfates and then chlorides. In case of the Sambhar Lake, the remarkable presence of K in the evaporites distinguishes it from salt playas occurring in other parts of the world.