The Genesis of the Shallow Intercrystalline Brine in the Qaidam Dezong Mahai Basin as Indicated by Hydrochemistry and Hydrogen, Oxygen and Strontium Isotope Compositions

The potential resources of potassium and lithium in the underground brine of the Qaidam Mahai Basin are considerable. However, the research on the mineralization regularity of lithium-rich clay layer brine lacks a theoretical basis. This article takes the shallow intercrystalline brine of Dezong Mahai as the research object and employs hydrochemical and isotope geochemical methods to discuss the salinization and mineralization characteristics, evolution patterns, and the sources of brine-forming fluids of the underground brine. The shallow intercrystalline brine deposits in the study area belong to alkaline brines with high salinity, and the hydrochemical type is mainly the magnesium sulfate subtype. The contents of K⁺ and Li⁺ are relatively high, attaining the minimum industrial grade and the boundary grade respectively, and thus possess mining value. The Hydrochemical analysis reveals that the water chemical composition of the shallow intergranular brine in the study area is dominated by evaporation and crystallisation. Under the effect of evaporation and crystallisation, minerals such as gypsum, rock salt, mirabilite, glauberite, sylvite, carnallite and bischofite are precipitated. Under the water-rock interaction, carbonate rock precipitates are generated, evaporite and silicate rock minerals are dissolved, and the main ion sources of the brine are enriched. The hydrogen and oxygen isotope analysis indicates that the shallow intergranular brine in the study area has a meteoric water origin, with a recharge elevation of approximately 3440 m, and is replenished by the meteoric water and snowmelt water from the Seshten Mountain and Nanbaxian Mountain. The strontium isotope analysis demonstrates that the material source of the shallow intergranular brine in the study area is multi-sourced, and there are certain supplies of rock salt leachate water, deep fluid, and water from the Yuqia River. There exists a close relationship between the genesis of the shallow intercrystalline brine in the study area and factors such as atmospheric precipitation, infiltration of surface water, admixture of deep fluids, evaporation and concentration, and water-rock interaction.