Formation of deep brine in the Qaidam Basin, north Qinghai-Xizang Plateau: Constraints from Ca isotopes and geochemistry

Pioneeringly applying the Gibbs and Gaillardet groundwater analysis method to the deep highly mineralized brine in Qaidam Basin and obtaining reliable and scientific understandings. • Important discovery. • For the first time, Ca isotopes directly confirm that dolomitization process plays an important role in the enrichment of Ca in deep brine of Qaidam Basin and the transformation of water to Ca-Cl type. • It is found that the initial water of the deep Ca-Cl brine in Qaidam Basin is not completely derived from atmospheric precipitation. Although it is deep water, it still undergoes a certain degree of evaporation and concentration process. This is a supplement to previous studies. • From the perspective of Ca isotopes, an explanation is given for the calcium source and formation mechanism of the deep calcium-rich brine in Qaidam Basin, further confirming its close relationship with the deep hydrothermal system. • Innovative method. • This is the first time that the δ 44/42 Ca data of deep brine in the QB have been reported. It appears that δ 44/42 Ca is a good tracer for determining the deep brine origin and the key controlling factors of Ca accumulation processes. Expanded the application of Ca isotopes in the geological field. Rigorous analysis results show that the Ca isotope data of deep highly mineralized brine in Qaidam Basin obtained by MC-ICP-MS is scientific and this testing method is feasible. Ca-rich deep brines in Qaidam Basin (QB) are an important source of lithium, potassium and rubidium. But the initial source of water and mecahnisms of geochemical evolution through interaction with rock hydrothermal systems is still unclear. Ca 2+ is the dominant elements in deep brines, the sources and enrichment processes are crucial for the correct interpretation of the formation mechanism of deep brines. This brine is an ideal carrier for researching the fractionation mechanism and tracing significance of Ca isotopes in high-salinity deep groundwater, and it is expected to expand the application of Ca isotopes. Ca isotopes and hydrochemistry were studied in deep paleo-brine from the Yahu, Jianshishan, Jianshan, Eboliang and Lenghu anticlinal structures of the Yiliping and Lenghu areas in the middle and north parts of QB respectively. The values of soluble-phase δ 44/40 Ca in nine deep brines ranged from 0.59 ‰ to 0.86 ‰, with an average 0.71 ‰. Values of insoluble-phase δ 44/40 Ca in six deep brines were 0.80 ‰, and were consistently higher than those of the soluble phase in the same sample because light Ca isotopes diffuse more easily compared to heavy Ca isotope. δ 44/40 Ca values of deep brine are lower than those of salt lake water and inter-brine, but close to those of volcanic rocks, carbonates and precipitation. These results indicate that the deep water with significant Ca-enrichment is associated with local hydrothermal activity, including dolomitization and partial calcite dissolution, and the initial water mainly originates from atmospheric precipitation. Hydrochemical analysis shows that leaching evaporite and evaporation process still play a key role in the formation of deep brine. In this study, it is the first time that the δ 44/40 Ca data of deep brine in the QB have been reported. It appears that δ 44/40 Ca is a good tracer for determining the deep brine origin and the key controlling factors in Ca accumulation processes.