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

Abstract

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. Ca²⁺ 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^44/40Ca in nine deep brines ranged from 0.59 ‰ to 0.86 ‰, with an average 0.71 ‰. Values of insoluble-phase δ^44/40Ca 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^44/40Ca 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^44/40Ca data of deep brine in the QB have been reported. It appears that δ^44/40Ca is a good tracer for determining the deep brine origin and the key controlling factors in Ca accumulation processes.