Contribution of deep material sources to shallow potash formation of the Quaternary Mahai Salt Lake in the Qaidam Basin: Evidence from isotopes and trace elements
Yanjun Zhao, Nan Wang, Lijian Shen, Ruiqin Li, Chenglin Liu, Pengcheng Jiao, Yufei Hu, Xianfu Zhao
Abstract
Deep brines in the Qaidam Basin were explored as replacement resources due to the continuous and intensive exploitation of shallow high-quality solid potash salts and brines. Here, the chemical composition and Sr, Li, H, and O isotopes of Mahai Salt Lake and its surrounding waters, including deep confined brine, brine in the water solution mining block, surface runoff, and fault-related water, were analyzed. The chemical composition, Li isotopes, and H and O isotopes of water from the alluvial fan region, Yuqia River, Dezong Mahai Lake, deep confined brine, and shallow brine in the water solution mining block were generally characterized by increasing degrees of evaporation, concentration, and evolution. The relatively low content of SO42- in the chemical composition and obvious deviation of δ18O value from the evaporation line further indicates that the source of the water from Niulang-Zhinv Lake was influenced by the upwelling water from the fault zone. The Sr isotopes of the deep confined brine and halite at different depths in the ZKE9 borehole indicated that the mined deep confined brine was a mixture of ancient salt lake brine from multiple periods during the evolution of salt lakes. Combined with the brine evaporation balance model of EQL/EVP, this study simulated the evaporation and concentration processes of brine in the Mahai Salt Lake and obtained mixing ratio data on the possible water sources of deep confined brine. Our findings showed that the water sources were river (mainly) and fault-related waters mixed in a ratio greater than 15:1, followed by evaporation and concentration processes until the salting-out stage.