Tracing a Multiphase Fluid Flow Path in the Changbaishan Volcanic Geothermal System (Northeast China) by B-Sr Isotope Decoupling

B-Sr isotopes and selected trace elements were used to assess the multiphase fluid flow path in the Changbaishan volcanic two-phase geothermal system (Jilin, Northeast China). The Ca/Sr ratio can distinguish volcanic and sedimentary rock reservoirs, while clastic and carbonate rocks need to be combined with the Sr/Na ratio. The lithology of the reservoir can be further ensured through contrastive water and rock 87 Sr/ 86 Sr ratios. Additionally, with caution in determining chemical composition of different endmembers, the B-Sr isotope model is useful in tracing the steam-heated/condensate water (group 1, which can be further divided into Group 1–1 and Group 1–2, according to the reservoir lithology), karst water (group 2) and residual water (group 3) flow paths. From the geochemistry and isotopes in the Changbaishan geothermal system, we infer that (1) basalt, clastic and carbonate rocks are the three main reservoirs and (2) the geochemical character of thermal waters is the result of four fluid-participating endmembers: surface water (TC), near-surface water (EM-2), residual sea water (EM-4) and vapor separated from deep sea water (EM-5). In addition, based on the B-Sr isotope model, we also established a simple conceptual model: group 1–1 represents mixing between EM-5 and TC after fluid-basalt interaction; group 1–2 indicates mixing EM-2 with EM-5 and experienced a Mesozoic granite leaching process; and group 3 is the result of EM-2 mixing with EM-4 after interaction with carbonates. The formation of Group 2 is mainly related to karst water, which is directly affected by hot magmatic volatile inputs.