Geochemical Characteristics and Origin of Nuanquanzi Geothermal Water in Yudaokou,Chengde,Hebei,North China

Study on the Nuanquanzi geothermal field in the Yanshan uplift is of great significance for understanding the origin of geothermal fluid in the intracontinental orogenic belt of the fault depression basin margin in North China.The geochemical characteristics and formation mechanism of the Nuanquanzi geothermal system were elucidated by classical hydrogeochemical analysis,multi-isotopes approach(δD,δ^(18)O,δ^(13)C,δ^(87)Sr/^(86)Sr),14CAMSdating,and integrated geophysical prospecting of surface-soil radon gas measurement and CSAMT inversion.The results show that the Nuanquanzi geothermal field is a medium-low temperature convection-fault semi-enclosed geothermal system.The hydrochemical type of thermal water is primarily HCO_(3)-Na,and rich in soluble SiO_(2),F^(-)and Cl^(-).The geothermal water primarily originated from the recharging meteoric water with a maximum circulation depth of 2400-3200 m,but affected by the mixing of endogenous sedimentary water.The reservoir temperature calculated by Na-K and quartz geothermometer of the Nuanquanzi geothermal system was determined to be 73.39-92.87℃.The conduction-cooling and shallow cold-water mixing processes occurred during the parent geothermal fluid ascent to surface,and the proportion of cold-water mixing during circulation was approximately 88.3%to 92.2%.The high-anomaly radon zones matched well to the low apparentresistance areas and hiding faults,indicating that the Nuanquanzi geothermal field was dominated by a graben basin restricted by multiple faults.