Geochemistry of geothermal fluids in the Zhangjiakou-Penglai Fault Zone, North China: Implications for structural segmentation

•Four structural segments of the ZPFZ correspond to four different geothermal systems.•The same geothermal system shares the same parent geothermal fluid.•Both waters and gas samples reveal the inflow of both shallow and deep fluids.•Intense fault-fluid interactions occur along four structural segments of the ZPFZ. The Zhangjiakou-Penglai Fault Zone (ZPFZ) is a large-scale WNW-trending deep fault zone across the North China Craton, which is composed of four segments. There are abundant hot springs and geothermal wells in the ZPFZ and vicinity, from which 191 water and gas samples were collected and chemically analyzed with the aim of characterizing spatial variations of geothermal fluids and unraveling the influence of regional and local tectonics on fluid compositions. The investigations indicated that: (1) the geothermal waters were classified into 3 chemical types: HCO3-Ca·Mg, HCO3-Na and SO4·Cl-Na. The waters exhibited spatially significant discrepancies in physical-chemical parameters along four segments of ZPFZ. (2) The waters in different tectonic segments underwent different geochemical processes, including water-rock interactions, mixture of waters from different aquifers, dissolution of gases of different origins from geothermal water, continuous gas-water interactions. (3) Hydrogen and oxygen isotope compositions of the water samples indicated meteoric origin with different geographical effects on geothermal waters. Carbon and helium isotope compositions of the geothermal gas samples indicated that underlying mantle-derived magmas acting as heat source rise along the conduits induced by the crossing of NWN- and NE-trending faults to shallow reservoirs. (4) Intense interplay of fault-fluid relationships has been found along the four segments of ZPFZ. (5) Based on these findings, a conceptual model was established to tentatively simulate the origins and evolution processes of geothermal fluids.