Abstract: Based on a large number of chemical composition and environmental isotope (δD, δ¹⁸O, ¹⁴C) test data of geothermal fluid, this paper analyzes the chemical characteristics and distribution rules of the convective and conductive geothermal fluid in the uplifted mountainous area, Gansu Province, China. The environmental isotope distribution characteristics and formation ages of the geothermal fluid are then then analyzed. Results show that the geothermal fluid in the convective geothermal field in the uplifted mountainous area is mainly exposed in the form of fracture rising spring, which is distributed in the west Qinling and Qilian orogenic belt. The source of supply is the infiltration of local and surrounding atmospheric precipitation. The formation age of the geothermal fluid is generally less than 5 000- 30 000 a and good water quality is observed. Hence, the geothermal fluid belongs to the“open” geothermal system. The geothermal fluid in the conductive geothermal field in the subsidence basin is mainly exposed by tube well exploitation. Exploitation wells with a buried depth of less than 1 600 m are supplied by the infiltration of local and surrounding atmospheric precipitation. Here, the formation ages of the geothermal fluid are generally less than 30 000 a and the water quality is relatively good. Hence, this belongs to the“semi-open semi- closed”geothermal system. The geothermal open exhibits a buried depth of 1 600-2 600 m. The production wells are mainly the“ancient water”formed gradually in the geological historical period with a complex hydrochemical type and with poor water quality. This then belongs to the“closed”geothermal system with the formation ages of geothermal fluid between 30 000-50 000 a. Generally speaking, the lithology of the thermal reservoir, burial depth, retention time, and circulation alternation of groundwater in the rock layer obviously control the chemical type, environmental isotope, and formation ages of geothermal fluid. Based on the geothermal occurrence law, if the buried depth of the thermal reservoir is deeper, the geological conditions are more closed, and δD and δ18O are more enriched, indicating that the formation age of the fluid is older.

Key words: thermal fluid, hydrochemistry, environmental isotope, formation age, geothermal system