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 delta Li-7 value in sink MHP ranges from 26.4 parts per thousand to 32.8 parts per thousand, which reflects the mixing of Ca-high water with high delta Li-7 value (36.0 parts per thousand) and river water with low delta Li-7 value (22.2 parts per thousand), 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.