Fractionation of chlorine isotope in salt mineral sequences and application: Research on sedimentary stage of ancient salt rock deposit in Tarim Basin and western Qaidam Basin

The brine must have evaporated to reach the late stage of sylvinite precipitation and must have continued to concentrate the salts for a long time. Then, it is possible to form potash deposit. Hence, this paper, examines the isotopic composition of different sequences of halite, sylvite and carnallite minerals for man made chemical solutions and salt lake brine evapo-concentrated precipitations, ancient potash deposits in an attempt to examine whether chlorine isotopic ratios can be used to study potash deposits and then be utilized to prospect for new potash deposits in the Tarim Basin and western Qaidam Basin. The results show that δ~(37)Cl values of the precipitates or salt deposits decreased systematically during the main phase of halite crystallization. Generally, the sequence of δ~(37) Cl values in salt rock is early precipitated halite (>0‰) >late precipitated halite (<0‰) carnallite (<-1.0‰). Hence, δ~(37) Cl in salts can be used as an indicator of evaporation stage. Based on this, we conclude that the concentration of ancient brines formed ancient salt rocks is much more concentrated in Tarim-Shache subbasin than that in Tarim-Kuche subbasin. Particularly, the δ~(37)Cl values of salt rock in Kashi depression of western Shache subbasin is greatly negative (<-1.00‰) , which may predicate that the ancient brine has been concentrated to the terminal stage of salt deposit or even later stage of sylvinite deposit. Then it is possible to find sylvite bearing deposits around the western region of the Shache subbasin. But to the western Qaidam Basin, all the 8 Cl values of Tertiary salt rock deposit samples from Youdunzi, Nanyishan, Shizigou and Tulingou depressions show obviously positive, which indicate that they are early halite deposits and the primary brine do not evapo-concentrate to reach the late stage of potash bearing minerals precipitation. Therefore, there maybe have no much more hope to find potash deposit in Tertiary salt rock deposit of western Qaidam Basin. The late stage of highly evapo-concentrated brines possibly moved to the south area.