Ca-high water recharge and mixing constrain on evolution and K enrichment of brine deposits in the evaporite basin: Case and analogue study in the Qaidam Basin, Qinghai-Tibet Plateau

A widely accepted consensus suggests that the presence of calcium-rich (Ca-high) water can significantly influence or alter the evolutionary trajectory of brine in evaporitic basins. However, a crucial scientific question regarding how Ca-high water affects brine evolution and its relationship with potassium (K) enrichment in continental and marine basins remains unclear. Currently, there is a relative lack of substantial evidence to establish connections between parent solutions or paleoseawater and the formation of large K deposits. The K-rich area of Mahai playa (MHP), located in the northern Qaidam Basin, western China, serves as a natural experimental site for studying the aforementioned unsolved problems, owing to the continuous replenishment of existing Ca-high water and normal regional river water. This study presents a detailed analysis of Li-H-O isotopic compositions and hydrogeochemical parameters for river waters, Ca-high waters, and intercrystalline brines in MHP. The results reveal several key findings: (1) the spatial distribution and formation of two hydrochemical types (Ca-Cl-type and Cl-SO4-type) and the presence of K-Li-Sr-Ca elements in MHP are constrained by the recharge and mixing of river water and Ca-high water. The fundamental distinction in resource elemental composition between these two recharge end-members stems from their own unique formation and evolution processes. (2) The distribution of δ7Li value in sink MHP ranges from 26.4‰ to 32.8‰, which reflects the mixing of Ca-high water with high δ7Li value (36.0‰) and river water with low δ7Li value (22.2‰), rather than isotope fractionation. Based on this observation, this study has formulated equations to provide rough estimates of the contributions of these two sources to K+ concentrations in sink MHP. These mixing ratios and their corresponding brine evolution types are roughly consistent with the classical mixing model. (3) Abnormal K+ enrichment (>5g/L) occurred in the mixing zone of Ca-high water and river water in MHP. Both evaporation experiments and field samples from Qarhan Salt Lake demonstrate that potash minerals within Ca-Cl-type brines can reach saturation and precipitate (TDS=350 g/L, K= 21 g/L) early in the process. In contrast, Cl-SO4-type brines are more conducive to the formation of brine potash deposits due to their higher critical precipitation point for potash salts (TDS=400 g/L, K =25 g/L). (4) Ca-high waters emerges as a pivotal factor in the formation of both continental and marine potash deposits.