Playa sediments of the Didwana Lake,Rajasthan: A new environment for surficial-type uranium mineralisation in India |
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Authors: | K Ramesh Kumar Dheeraj Pande Arpan Misra L K Nanda |
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Institution: | (1) Department of Geology, Anna University, Chennai, 600 025, India;(2) Central Arid Zone Research Institute, Jodhpur, 342 003, India;(3) Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA |
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Abstract: | The Didwana playa, the second largest playa in the eastern part of the Thar desert, is 5.6 km long and 2.4 km wide and supports
commercial salt production. The thickness of lake sediment package is reported to be 20 m and comprises fine grained clays
and silts, with abundant calcite, gypsum, and halite, associated with hypersaline water. Isolated hills of graphitic phyllite
and quartzite are seen on the western side of the lake. During the course of investigations for uranium in surficial environment
of semi-arid terrain of Rajasthan, ground water sampling defined a NE-SW trending uranium halo encompassing the Didwana playa.
Subsequent sampling of unlined dug wells, up to water table in central part of the playa, indicated uranium values up to 190
ppm and 2072 ppb in lake sediments and brine respectively. These values are of the order of 21 ppm and 192 ppb towards the
southwestern periphery of the lake. The average uranium content, as inferred from 12 samples in the central part of the lake,
is around 60 ppm over a thickness of 5 m. It appears that the uranium is loosely bonded to the sediments in amorphous form
and is, hence, easily leachable. Samples of brine (n=10), from both the central and southwestern portions of the lake, analysed
high (1,67,500–3,00,000 mg/l) TDS, HCO3− (1128–8395 mg/l), and SO4 (30,536–88,000 mg/l). These are of alkaline (pH: 7.2–9.3) and reducing (Eh: −200 to −340 mV) nature. Under these Eh-pH conditions
below the groundwater table, and for such uranium bearing groundwater, precipitation of primary uranium is expected. It is,
therefore, modelled that uranium in lake sediment package above water table is concentrated by evaporation process and by
chemical reduction below the water table. |
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