Elemental enrichment of deep brines in areas of abnormal geothermal gradient of the Qaidam Basin,northern Tibetan Plateau

Abstract

Deep brines in the Qaidam Basin (QB, northern Tibetan Plateau) hold abundant K, Li, B, Br and Sr resources with great exploitation potential. Nevertheless, mechanisms controlling spatial enrichment heterogeneity of these valuable elements remain unclear, restricting efficient brine resource exploration and utilisation. This study systematically integrated hydrogeochemical parameters, multi-isotope (H–O–B–Li–He) signatures and temperature data acquired from 55 deep boreholes to reveal elemental differentiation laws and formation controlling factors of deep brines. Quantitative results show central basin brines yield average concentrations of 0.36–1.12 g/L K (avg = 0.73 g/L), 108–495 mg/L B (avg = 252 mg/L), 27–114 mg/L Li (avg = 90 mg/L), 38–83 mg/L Br (avg = 59 mg/L) and 142–381 mg/L Sr (avg = 245 mg/L), all distinctly lower than those in western brines. Western brine isotopic features (δD–δ¹⁸O patterns) reflect predominant water–rock interaction, while central brines experience combined water–rock reaction and evaporation concentration. Helium isotopes verify crustal fluid origin for central brines and mantle-derived magmatic fluid contribution in the western basin. High geothermal anomalies are exclusively identified in the Nanyishan–Dafengshan and Yahu–Hongsanhan anticlinal zones, which facilitate B and Li migration and accumulation. Lithologic properties at basin margins dominate Li/B ratio variation in central brines, whereas K, Br and Sr distribute evenly and are mainly regulated by lake depocentre migration. Geothermal condition, fluid source and sedimentary evolution jointly shape regional brine elemental discrepancy. Different fluid genesis models are confirmed between western and central basin brines. Distinct from prior research focusing on a single element or partial local area, this work clarifies the coupled control of fluid source, geothermal activity and depositional evolution on multi-element spatial heterogeneity across the whole basin. The quantitative dataset and genetic mechanism understanding offer practical guidance for targeted deep brine prospecting and comprehensive resource development in the QB.