Origin of K-Li-B-rich brines in the Qianjiang depression, Jianghan Basin: Evidence from stable isotopes

Potassium, lithium, and boron resources are anomalously enriched in deep brines of the Qianjiang Depression, Jianghan Basin. This study focuses on the sources and enrichment mechanisms of these elements. Thirteen brine samples from the Qianjiang Depression were collected for hydrochemical analyses and multi-isotope investigations (H-O-Sr-Li-B-K). The results show that the maximum concentrations of K + , Li + and B 3+ in the brines reach 4.32 g/L, 64.04 mg/L, and 456.40 mg/L, respectively. The brines are classified as Cl-Na type with neutral pH values. Major and trace element compositions indicate that halite dissolution and water–rock interactions have significantly contributed to brine evolution. The δD and δ 18 O values of the brines range from −57.7 ‰ to −40.5 ‰ and from −4.2 ‰ to +1.1 ‰, respectively. Their H-O isotopic characteristics indicate a mainly meteoric precursor that was subsequently modified by evaporation, fluid mixing, and water–rock oxygen isotope exchange. The 87 Sr/ 86 Sr ratios range from 0.710076 to 0.710841, comparable to those of sandstone reservoirs, revealing remarkable water–rock interactions between brines and host sandstones. The δ 11 B values vary between +19.63 ‰ and +38.61 ‰, implying a continental source; continental rock weathering and water–rock interactions constitute the primary sources of boron in the deep brines. The δ 7 Li values range from +12.21 ‰ to +19.11 ‰. Lithium is mainly supplied by silicate weathering and water–rock interactions during burial, and subsequent redistribution is driven by clay-mineral reactions, whereas evaporation and deep fluid inputs exert secondary controls. The brines exhibit δ 41 K values ranging from −1.24 ‰ to +0.09 ‰. Comparisons with published rock compositions suggest contributions from silicate dissolution and K-bearing salt dissolution, while adsorption modeling indicates that clay-related fractionation can explain the isotopically light samples. These results highlight the combined influence of K sources and subsequent water–rock interactions on brine K isotope compositions.