The Qaidam Basin is of significant importance in the context of potash salt resources, serving as the primary reservoir and the key production base for potash fertilizer in China. Recent geological explorations have identified a novel type of potassium-bearing sand and sandstone pore brine deposit in the western foreland alluvial fan of the basin. This newly discovered deposit is believed to contain significant potassium chloride resources, with the brine displaying prominent attributes of high sodium and chlorine levels, and a mineralization model resembling that of “inherited halite rock”. The Mahai Basin, a small lake basin located as a secondary basin and distinct from the Kunteyi Basin on the northern margin of Qaidam, is characterized by various types of brines (intercrystalline brines, confined brines, sand-gravel brines, anticlinal structure brines, etc.). The present study is an attempt to draw attention to the following main conclusions, which are based on a comparative analysis of the major and trace ion content, salinity, and hydrogen and oxygen isotope composition of river water and various brines in the Mahai Basin: Firstly, the average K⁺ content and TDS value of the sand-gravel brines in the Mahai Basin are 2.16 g/L and 254.5 g/L, respectively, indicating a notably high ratio of Na⁺+ Cl^-/TDS (0.94). These brines are classified as Na-Cl type according to the hydrochemical classification system. Secondly, while the potassium content and TDS value of the sand-gravel brines in the Mahai Basin are lower than those in the surface brines, intercrystalline brines, and confined brines of Mahai Salt Lake, they are close to the minimum industrial mining grade of brine, suggesting a certain potential for resource development. The spatial distribution of K+ in sand-gravel brines, intercrystalline brines, and confined brines closely aligns with potassium-containing evaporites. It is suggested that the salt solute and potassium in sand-gravel brines originate from the recharge of intercrystalline brines and confined brines; Furthermore, the utilisation of the K-B-Li equivalent map, in conjunction with the analysis of the varying B content in diverse water sources, serves to substantiate the assertion that the sand-gravel brines are formed through the combined influence of the Yuqia River water and intercrystalline brine/confined brine on the formation process of the sand-gravel brine. Finally, the δD-δ¹⁸O values of the sand-gravel brines in the Mahai Basin range from -56.9‰ to -17.6‰ and -5.70‰ to 6.00‰ for hydrogen and oxygen isotopes, respectively. These values exhibit characteristics analogous to those observed in confined and intercrystalline brines, which are distributed uniformly on both sides of the local evaporation line. Furthermore, an increase in the ²H-¹⁸O is accompanied by an increase in the concentrations of B and Li. These findings indicate that the sand-gravel brines have undergone evaporation and concentration. Consequently, the sand-gravel brines have been inheriting solutes from the intercrystalline and confined brines over a protracted period, including ions such as K⁺, Na⁺, and Cl^-, leading to their similar H-O isotopic signatures.