Genesis of geothermal water in the hinterland of Guanzhong Basin, China: Insight from hydrochemical and isotopic analysis

The Guanzhong Basin, located in Shaanxi Province, China, has abundant geothermal resources in its hinterland. Focusing on typical thermal reservoir structural units, namely the Xianli fault-step (XL) and Xi'an depression (XA), this study reveals the genesis of geothermal water by combining hydrochemistry and environmental isotopes. The geothermal waters of XL uniformly correspond to the Na-Cl type with high contents of major ions, while the geothermal water of XA has complex hydrochemical types. The quartz geothermometer (with maximum steam loss) and Na-K-Ca geothermometer are recommended for XL and the quartz geothermometer (with no and maximum steam loss) and Na-K geothermometer for XA. Their temperature ranges are 99-159℃ and 93-146℃, respectively. The circulation depth of geofluids in XL and XA range from 2579 to 4766 m and from 2541 to 4330 m, with mean apparent ages of 21543 and 20345 yr B.P., respectively. The geothermal water in XL originates from a mixture of sedimentary water and ancient infiltration water, and its formation mode was the vertical heat conduction with cooling process. In contrast, the geothermal water in XA originates from the dual process of cooling and mixing, with the process being dominated by ancient infiltration water and supplementation by modern infiltration water.