Differential enrichment in B-Li and isotopic spatial variation in deep brines of the northern Tibetan Plateau: Dominance of Cenozoic depocenter shift and orogenic geothermal systems

The Indo–Asian collision has generated numerous anticlinal and folding structures in the central–western Qaidam Basin (QB), northern Tibetan Plateau (TP), hosting deep brine resources enriched in K, Li, B, Rb, and Cs. These brine resources exhibit Li–B concentrations far exceeding China’s minimum industrial mining grade (Li+: 49.5 mg/L; B3+: 310.5 mg/L), underscoring their significant potential for development and utilization in both the near and future. The genesis of Li and B enrichment in deep brines from individual anticline reservoirs has been well-documented, however, key scientific questions regarding their spatial distribution, primary sources, and enrichment mechanisms across the entire QB remain unresolved. This study presents detailed analyses of H–O–Li–B isotopic compositions and hydrogeochemical parameters for deep brines, salt lake brines and river waters in the QB. Results reveal spatial heterogeneity in Li–B contents and δ11B–δ7Li values of deep brines in the QB, subdividing the central-western region into four zones (Zone I–IV) based on Paleocene-Eocene and Pleistocene-Holocene depocenter positions. Marginal QB regions (Lenghu, Eboliang, Gasi, Kunbei anticlines) show low B–Li enrichment (B: 1.14–370.90 mg/L; Li: 0.06–54.5 mg/L), while the western area (Nanyishan, Xiaoliangshan, Dafengshan, and other anticlines) exhibits high enrichment (B: 43.50–1301.10 mg/L; Li: 5.94–237.50 mg/L). Enrichment patterns are controlled by recharge sources, evaporation, lake basin migration, and clay mineral adsorption. Deep brines and Quaternary salt lake brines share similar B–Li recharge patterns and enrichment mechanisms, primarily involving geothermal waters as the main recharge source and evaporation as an enrichment process. Geochemical data indicate that deep brines exhibit high Na+ and Cl− concentrations in Piper diagrams, along with significantly positive δ18O values in δD–δ18O diagrams, suggesting strong water–rock interactions or magmatic fluid contributions, particularly in Dafengshan, Xiaoliangshan, and Nanyishan anticlines. The Li–B concentrations in deep brines show a decreasing trend from west to central areas, while δ11B values increase progressively (+14.58 ‰ in Zone II, +24.16 ‰ in Zone III, +31.67 ‰ in Zone IV). In contrast, δ7Li values display an inconsistent pattern, likely influenced by mixing with shallow salt lake brines in Yahu and Jianshishan anticlines (Zone III). On the whole, the observed stepwise increase in δ11B–δ7Li values in deep brines, attributed to clay mineral adsorption, aligns with the gradual migration of Paleogene–Neogene depocenters driven by surrounding orogenic belt uplift. These findings highlight spatial differentiation in B–Li enrichment, isotope heterogeneity, and metallogenic mechanisms within QB deep brines.