Evaporite mineralogy and geochemical evolution of the Sambhar Salt Lake, Rajasthan, India

摘要

The Sambhar Lake is the largest playa within the Thar desert of western India. A detailed mineralogical investigation was carried out with bed rock and soil samples collected from the catchment area of the lake and with two deep cores obtained from the lake bed. The clastic fraction of the lake sediment consists of quartz, alkali feldspar, mica, chlorite, amphibole and weathering products such as kaolinite and goethite. The non-clastic evaporite fraction is dominated by calcite and halite. There is a break in evaporite mineralogy at a depth of around 5 m. For example, gypsum is the major sulfate mineral below this depth while in shallower horizons, its place is taken by an assemblage of thenardite, kieserite and polyhalite. Using the principle of chemical divides, such variations in mineralogy have been explained in terms of a change in brine chemistry from K-Na-Ca-Mg-SO_4-Cl to K-Na-CO_3-SO_4-Cl type. It is also suggested that at an earlier stage, the Sambhar Lake brine underwent evaporation under the condition of Ca>alkalinity whereas in more recent times, the evaporite mineralogy has developed with alkalinity>Ca. Dolomitisation of calcite and formation of Mg-clay helped Mg-removal. Presence of K-bearing evaporites in the core sediments suggests that the evaporation of brine exceeded the halite saturation stage. ~(14)C ages from one of the cores indicate that the geochemical evolution of the lake spanned a period of more than 30 ka. This may have important paleoclimatic implications.