Origin and Evolution of Deep K-Rich Confined Brine in Mahai Basin, Qinghai–Tibet Plateau

Mahai Basin (MH), located in the northern Qaidam Basin (QB), possesses abundant K-rich brine resources. The investigation on the origin of deep K-rich confined brine and the variations in K–Mg elements corresponding to the evolution in MH shed light on the significance of assessment and utilization of brine deposits. This study presents multiple isotopes (δ¹⁸O–δD, ⁸⁷Sr/⁸⁶Sr) and hydrochemical characteristics for various waters (including river water, surface brine, intercrystalline brine, confined brine and anticlinal brine) in the MH. Our findings corroborate that: (1) confined brine exhibits relatively high K⁺ (average value of 6.88 g/L) and low Ca²⁺–Sr²⁺ concentrations, compared to anticlinal brine, and its chemical composition resembles the evolution of Yuqia River in Ca–SO₄–HCO₃ diagram, suggesting that contemporary river water is the primary source of confined brine. (2) The δ¹⁸O–δD values of confined brine in MH ranged from − 17.80 to − 27.40‰ and 1.50 to 2.40‰, respectively, and fall on the right field of the local evaporation line, indicating successive evaporation and concentration processes. (3) The ⁸⁷Sr/⁸⁶Sr ratios (0.71142–0.71145) of confined brine fall between river water (0.71150–0.71183) and anticlinal brine (0.71135), combining with river water and confined brine which exhibit low Sr content, and further confirming the origin of confined brine is a mixture by river and anticlinal brine and much river recharge budget. (4) Considering the evolution of sedimentary facies (Dezongmahai Lake area as an example) and the gradual increase in K and Mg contents in MH, the enrichment of K and Mg exhibits a certain correlation with the evolution of MH. Notably, the brine in the northeast of the basin displays the highest levels of K and Mg, indicating that this region serves as the ultimate depositional center.