<p>In the Mahai Basin, Qaidam Basin, China, the origin and evolution of spring water, shallow intergranular brines, and conglomerate-hosted pore brine were investigated using elemental geochemistry alongside δD–δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and δ⁷Li isotopes. Solute compositions are primarily controlled by evaporite leaching. Stable isotopes (δD–δ¹⁸O) indicate that while springs experience limited evaporation, both brines undergo significant evaporative concentration. A Bayesian mixing model based on ⁸⁷Sr/⁸⁶Sr reveals that shallow intergranular brines mainly derives from alluvial fan groundwater (71.9%) and the Yuqia River (27.4%). Conversely, conglomerate-hosted pore brine originates from a complex mixture of Niulangzhinv Lake water (40.8%), alluvial fan groundwater (25.2%), Yuqia River water (17.3%), and deep lake water (16.7%), highlighting the involvement of deep-seated fluids. Furthermore, δ⁷Li systematics show progressive increases in water–rock interaction and fluid residence time along the hydrological flow path. Preferential incorporation of ⁶Li into solid phases (e.g., clay adsorption) results in elevated δ⁷Li in the residual brine, particularly within the pore brine. We propose a comprehensive genetic model: atmospheric precipitation and glacial meltwater recharge the system via river seepage and alluvial groundwater. Shallow evaporation forms the intergranular brine, a portion of which subsequently migrates deeper. Through prolonged water–rock interaction and mixing with deep-seated fluids, this migrates to form the conglomerate-hosted pore brine. Ultimately, this study demonstrates that Sr isotopes effectively constrain fluid mixing proportions while Li isotopes trace water–rock interaction intensity, providing a process-based framework for understanding recharge, mixing, water–rock interaction, and Li redistribution in arid continental brine systems.</p>