Stable chlorine isotopes in saline springs from the Nangqen basin, Qinghai-Tibet Plateau: Brine genesis and evolution

Chlorine isotopes can be used to study the evolution of different fluids, sources and the causes of various related deposits. In this study, Cl concentrations and chlorine isotope (delta Cl-37, IAEA ISL-354 NaCl standard) values were determined for brine samples from Nangqen basin, located on the southern boundary of the Qinghai-Tibet plateau to study the source and the processes of these saline springs. The results demonstrated that the saline springs are distributed around a fault or fault zone, with a high average salinity of 228.30g/l and flow rates ranging from 1.7 to 0.01l/s. Na+ and Cl- are the predominant cations and anions, respectively, accounting for more than 90% of the total. The delta Cl-37 values range from -1.55 parts per thousand to +0.97 parts per thousand, and the Cl/Br ratios are from 1739 to 175,260. Coupled with the previous H, O and B isotope compositions (delta D, VSMOW2 standard, ranges from -100.91 parts per thousand to -132.98 parts per thousand, delta O-18, VSMOW2 standard, from -12.88 parts per thousand to -16.05 parts per thousand and delta B-11, NIST 951 standard, from +3.55 parts per thousand to +29.59 parts per thousand), it can be interpreted that the saline springs are mainly the result of the dissolution of halite hosted in mudstone and volcanic country rocks.