<p>Recently discovered deep sand-gravel brine (SGB) deposits in the pediment alluvial fan zone of Mahai Playa (MHP), northern Qaidam Basin (QB) have been regarded as a new potential brine resource. The differential spatial distribution and enrichment mechanisms of resource elements (K, Li, B) in the sand-gravel porous brine aquifers as well as the metallogenic linkage between deep SGB and salt lake brine remain inadequately understood. In this study, we systematically investigated the horizontal-vertical distribution and solute source of the borehole SGB from the MHP through employing geochemical comparisons, isotopic tracing, and mass balance calculations. Results reveal several key findings: (1) the SGB in the MHP exhibited Na-Cl type. The total dissolved solids (TDS) ranged 240–310 g/L, and Na⁺ and Cl⁻ was at stable levels and contributed over 90% of TDS; (2) K-Ca-Mg concentrations increased from west to east, and K and TDS accumulated synchronously with depth; (3) boron (B) and lithium (Li) concentrations displayed positive correlation (<i>R</i>²=0.73) with spatially higher concentrations in eastern regions; (4) based on B³⁺-δ¹¹B geochemical characteristics, the MHP constituted a regional B-rich lake system with a distinct “source-transport-sink” spatial distribution pattern, validating the multi-source recharge metallogenic model for SGB formation. This study provides comprehensive evidence of the multi-source recharges (salt lake brines with high δ¹¹B values as a necessary source) for SGB formation in the MHP through B isotope mass balance calculations, and offered important theoretical guidance for efficient exploration and development of deep brine resources in the QB.</p>