Fomation mechanism of fault-controlled circulation geothermal spring: evidence from tectonics, thermal model, and isotopes

Fomation mechanism of fault-controlled circulation geothermal spring: evidence from tectonics, thermal model, and isotopes | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Fomation mechanism of fault-controlled circulation geothermal spring: evidence from tectonics, thermal model, and isotopes Wenzhen Yuan, Shiyu Zhi, Shoujun Sun, Guanhua Zhu, Yi Zhang, Xinran Guo, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5583602/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The effective utilization of geothermal energy helps reduce carbon emissions and mitigate climate change. Understanding the formation mechanism of geothermal systems is a prerequisite for effective exploitation. The formation of geothermal systems is influenced by various factors such as tectonic characteristics, heat sources, and water circulation, making it necessary to comprehensively analysis its mechanism by integrating these factors. Tangquan ,a natural geothermal spring in Zhangjiakou City, China, has great potential in geothermal exploitation. However, previous studies have offered incomplete explanations of its genesis. Therefore, this study investigated the stratigraphic properties and structural distribution of the study area through geophysical exploration surveys, mapped the distribution of the low-temperature field using geothermal geological modeling methods and inferred that the heat source in the study area is radioactive; and through isotope analysis methods, clarified that the water source in the study area is atmospheric precipitation. By integrating these information, the formation mechanism of the Tangquan, a deep circulation type geothermal system controlled by faults, is revealed. These results can guide the further development and utilization of Tangquan, while also enhancing the understanding of fault-controlled and deep circulation type geothermal systems. Zhangjiakou Geothermal spring Geothermal genesis Geophysical exploration Yanshanian faults Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Energy is essential for economic growth and social development. Currently, fossil energy dominates global energy consumption, but it is finite (Heinberg and Fridley, 2010 ; Yuan et al., 2022 ) and causes significant environmental problems during its production, transportation, and use (Fenger, 2009 ; Prideaux et al., 2013 ). In particular, the greenhouse gas from fossil energy combustion has become one of the most concerned issues of this century (Fenger, 2009 ). To address the over-reliance on fossil energy, a shift towards clean energy is necessary. Geothermal energy is a leading renewable energy resource with high potential (Dotsika, 2015 ). Achieving sustainable development and utilization of geothermal resources is a key challenge for the global geothermal industry today (Gongke, 2014 ). The formation mechanism of geothermal resources determines the potential and mode of its sustainable utilization, so it is very important to accurately analyze the genesis of geothermal resources. Research on the classification of regional geothermal systems in China has been well established (Moxiang et al., 1990 ). The components of a geothermal system include the heat source, permeable strata, and fluid. The fluid usually refers to groundwater, which transfers heat through flow in limited space, making the utilization of geothermal energy feasible (Arnórsson et al., 2007 ). According to the existing analysis of the genetic types, the causes of thermal reservoir usually involves the structural control of water and heat, heat sources, geothermal water sources, hot water circulation paths and so on (Xu Butai, 1999 ). In the geothermal system, the heat source plays a crucial role as it is the energy source of the geothermal system, providing heat for the whole geothermal system, which promotes the heating and circulation of the geothermal fluid, and then realizes the transfer and utilization of heat (Ji, 1996 ). The common types of heat source in geothermal system include magmatic heat source, which is abnormal by the surrounding rocks by magma; the radioactive heat source depends on the decay of radioactive elements in the rock; the tectonic movement heat source causes rock deformation and fracture caused by the crustal movement such as plate collision extrusion, and the mechanical energy is converted into heat energy. The convection heat source transfers the deep heat to the shallow part of the crust, providing energy support for geothermal activities near the middle ocean ridge (Xu Butai, 1999 ). Water sources act as heat transfer media, absorbing heat from heat sources and circulating it underground to bring it to the surface, and they also participate in water-rock interactions that affect the properties of the geothermal reservoir (Ji, 1996 ). Common types of water sources include atmospheric precipitation, primordial water trapped in rock pores during the formation of the Earth, and magmatic water from the volatilization of magma. Common research methods include hydrogeological survey methods and geochemical analysis methods using isotopes.The fluid circulation types of geothermal systems include deep circulation types that go thousands of meters underground and shallow circulation types that are a few hundred meters below the surface (Na et al., 2021 ), capable of transferring heat and exchanging substances for the geothermal system (Pola et al., 2020 ); when faults are present, they can provide channels for fluid circulation (Przybycin et al., 2017 ), thereby altering the source of groundwater and the circulation path (Craig, 1961b ), and common identification methods include geochemical analysis and geophysical exploration. In summary, tectonic control, geothermal system types, and groundwater circulation paths all play an important role in the formation of geothermal fields and can influence each other (Hu et al., 2022 ). With the progress of research and practice, the analysis of thermal reservoir formation has gradually shifted from a single aspect to a comprehensive integration of multifaceted information. It is an inevitable trend to analyze the causes of thermal reservoir from a systematic perspective. Zhangjiakou City, located in the northwest of Hebei Province, is one of the new energy bases planned by the state. Zhangjiakou area has good geothermal geological conditions and is rich in geothermal resources. More than 20 geothermal fields have been found. Chicheng geothermal anomaly area, which is located in the southeast of Zhangjiakou City and adjacent to Beijing, is one of the typical geothermal fields. As the most representative geothermal resource area in Chicheng area, Tangquan has found 9 hot spring spots in recent years and has great geothermal potential. Understanding the geothermal genesis of Tangquan is of great significance to the understanding and utilization of geothermal resources in Chicheng and even the whole city of Zhangjiakou (Yuan et al., 2022 ). Although preliminary development and utilization has been carried out in Tangquan area, a professional geothermal geological survey has not been carried out, and the cause of formation of the hot spring is not completely clear. Previous work has not characterized complex structures enough to determine the mechanical properties of faults (Hongquan et al., 2020 ), and the depth and period of groundwater circulation in this area are not clear, the analysis of heat source and water source of thermal reservoir is still inconclusive. At present, the research in this area mainly stays in the characterization and interpretation of individual aspects, lacking a overall analysis from the point of view of geothermal system, so it is necessary to analyze the cause of formation of Tangquan geothermal comprehensively from many aspects, such as structure, heat source, water source and so on. This paper aims to elucidate the genetic mechanism of Tangquan geothermal spring through the comprehensive analysis of tectonism, heat source distribution, and water cycle. The characteristics of the thermal reservoir structure, heat conduction structure, and potential heat sources were analyzed using geophysical methods. The occurrence and heat conduction of geothermal resources were analyzed by geological and geothermal model. The isotope method was used to identify the source and circulation pattern of groundwater. The characteristics of geological structure, heat source, and water source were integrated to systematically analyze the the formation mechanism of Tangquan geothermal field, revealing the convective conduction heat accumulation mechanism of fault-deep circulation geothermal system in the study area. This study enhances the understanding of fault-deep circulation geothermal systems, provides a theoretical basis for the development and utilization of geothermal resources in Tangquan area, and offers a reference for the study of geothermal field genesis in similar areas. 2. Materials and Methods 2.1 Overview of the Study Area The study area (Tangquan Geothermal Anomaly Area) is located approximately 7.5km west of Chicheng County in Hebei Province, China, within a low to mid-elevation mountainous region. It extends 5.5km in a northeast direction and 4.6km in a northwest direction, covering an area of 25km²(Fig. 1 ). The topography is high in the north and low in the south, surrounded by mountains on three sides, with steep peaks in the north, east and west, and alluvial valleys in the south, which belongs to the Piedmont hilly landform with an elevation of 110 ~ 190 meters (Gongke, 2014 ). The long-term tectonic evolution of Tangquan area led to the extremely complex structural characteristics in the region. Under the action of Cenozoic tension and strike-slip stress, a large number of large Piedmont faults were developed, and a basin-mountain coupling structure blocked by mountain system in adjacent basins was formed (Shuang, 2021 ). There are about 4 springs with water temperature ≥ 25 ℃ through geothermal survey by predecessors, the water temperature is 37.5 ~ 59 ℃, the highest can reach 60 ℃, and the area of geothermal field with water temperature ≥ 25 ℃ is delineated 0.061km 2 . The geothermal field is controlled by Shangyi-Pingquan deep fault and Dahenan-Chicheng deep fault. The fractures are developed, the rocks are broken, and the hot water rises into a spring through the fracture channel, which emerges from the Yanshanian giant porphyry granite. Water temperature of the main spring (Tangquan) is 60 ℃ and the flow rate is 75.60m 3 /h. There are also some hot springs nearby, such as stomach Spring, Pingquan, Eye Spring and so on, because Tangquan has the largest flow and the highest exposed position, it usually refers to Chicheng Hot Spring (Chunhui, 2006 ). The total area of the spring domain is about 5km 2 , and the spring water spills in the valleys that are less than 1km 2 in the form of group springs, and the total amount of spring water is about 35L/s. Tangquan is the intersection of the deep fault (Shangyi-Pingquan deep fault) and the NE-trending Dahenan-Chicheng deep fault in the southern margin of the Earth's axis in Inner Mongolia. The secondary fault develops and forms a fault fracture zone, which provides a favorable space and channel for the storage and migration of geothermal water. The magmatic rocks in this area are mainly Yanshanian porphyry granite, in addition to granite dike, diorite dike and so on. According to Li Quan's study, the location of Chicheng hot spring is covered with porphyry coarse-grained granite, which is distributed along the Shangyi-Pingquan deep fault, belonging to the early Yanshanian period and intrusive in the shape of rock plants. The geology of this area is Archean metamorphic rocks and unconformable contact with Middle Jurassic strata (Quan, 1994 ). The contact fracture zone between Yanshanian porphyry granite and surrounding rock is a good channel and storage space for geothermal fluid. The whole intrusive rock mass is cut by Shangyi-Pingquan deep fault with an exposed area of about 35km 2 . The radioactive heat of porphyry granite is an important heat source in this area. The temperature of Tangquan spring is not only affected by the nature of the fault, but also closely related to the dip angle of the fault, when the dip angle is large, the depth of fracture cutting is also relatively large, the path of deep water circulation is deeper and longer, its influence by magmatic activity or geothermal gradient is greater, hence its water temperature is also relatively high; On the contrary, it's the opposite. Tangquan belongs to the Bahe hydrogeological region, with the west side being the Qingshui River hydrogeological region. The terrain is generally higher in the northwest and lower in the southeast, with surface elevations ranging from 750 to 2100 meters. The landforms are mainly composed of igneous rocks, metamorphic rocks, and Quaternary Upper Pleistocene to Holocene alluvial and diluvial deposits. The types of groundwater include loose rock pore water, bedrock fissure water, and clastic rock pore-fissure water. The source of underground water supply is from the northern part of Baihe River, Tangquan Reservoir and atmospheric precipitation in the north (Lele, 2016 ). 2.2 Structural features According to the analysis of geothermal genesis, structures play an important role in Tangquan geothermal system, but not all faults or folds have good geothermal significance. Referring to the previous geothermal data and the results of this geological survey, the possible heat conduction structures are further analyzed by magnetotelluric, CSAMT and two-dimensional seismic methods, and the main structures in Tangquan area are further explained (Peng et al., 2019 ). 2.2.1 Shangyi-Pingquan Deep Fault (F1) The main and branch faults of the Shangyi-Pingquan deep fault are the most significant structural faults and also the main heat-conducting structures in the working area (Yongping, 2019 ). According to the previous data and this investigation, the Shangyi-Pingquan deep fault is a thrust fault structure that began to exist at the end of Archean and experienced multi-stage activity, its main active period is in Luliang, Hercynian, Yanshan and other stages, there is little activity after the Yanshanian period. 2.2.2 Tangquan Fault (F2) F2 fault is a secondary fault of Shangyi-Pingquan deep fault, which passes through two working areas of Prince Chongli and Xiaozhangjiakou in Chicheng, and is controlled by survey lines MT10, CMT10, MT11 and CMT11. The overall strike of the fault is close to the east-west, partial to the north-west, mainly dipping to the north, dipping south at the positions of MT11 and CMT11, with a dip angle of more than 55 °, which belongs to a thrust fault with a drop of more than 600m. 2.2.3 Dahenan-Chicheng Deep Fault (F8) Located in the east of Zhangjiakou City, it belongs to the category of NNE trending tectonic magmatic rocks from Dahenan to Dahaituo, with a length of about 170km(Fig. 2 ). The Yanshanian period led to large-scale magmatic activity, resulting in the intrusion of the Dahenan complex and Dahaituo granite, with a spatial long axis of NNW direction and a fracture bandwidth of about 100m. The fault zone is composed of clastic rocks and mylonites, which is linearly distributed and is still active recently. The predecessors believed that the fault was of a grand scale and deeply cut through the Moho surface, controlled magmatic intrusion and eruption in the middle and late Mesozoic, was a deep fault connecting mantle and crustal magma chamber, and shallow multi-stage active new structure, which had the function of heat and water conduction. In this field investigation, it is found that there is an obvious fault triangle on both sides of the fault zone, cutting the Quaternary, and the fault is tensional and shear and should have water conductivity. 2.2.4 Other Faults F53, F54, F55 and F56, controlled by MT11 and CMT11, are NW-trending normal faults intersecting Tangquan fault (F2) near Chicheng Tangquan(Fig. 3 ). They are all newly inferred faults. Among them, the F53 fault inclines to the southwest, the dip angle is more than 70°, and the maximum drop is more than 600m, which is mainly developed in the early Yanshan porphyry granite; The F54 fault dips more than 70° and the maximum drop is more than 800m, which mainly occurs in the early Yanshanian porphyry granite. F55 dips more than 70° and the maximum drop is more than 600m, mainly developed in the early Yanshanian porphyry granite; The F56 fault inclines to the northeast, the dip angle is more than 70°, and the maximum drop is more than 400m, which is mainly developed in the Middle Archean Chongli Group. 2.3 Thermal Reservoir Model 2.3.1 Data Processing The construction of Tangquan geothermal geological model is mainly based on the 3D geological structure of the study area, using the 2019 geophysical results of the research area to control and establish a more precise 3D geological structure model. The geophysical work in this area in 2019 mainly includes MT and CSAMT data. Since the two datasets largely overlap, the MT point number is used as the virtual borehole number (see Fig. 1 for the plane location). Combining two kinds of geophysical results to extract borehole data to form a control profile, so as to generate a more fine 3D geological structure model of the whole study area. The statistics of virtual boreholes extracted from geophysical data amount to 156 points. 2.3.2 Construction of Geological Structure Model The control profile of the solid model is formed by using the virtual borehole data. Determine the stratification interface and code of each stratum of the drill hole, import GMS to complete the joint profile construction, and finally complete the construction of the solid model. The data range of the geological model are as follows: long 5.5km in north-east direction, wide 4.6km in north-west direction, area 25km2, vertical to -2000m elevation. The accuracy of stratigraphic division is that the model strata are divided into Archean (Ar), rock mass (Yanshanian porphyry granite) and Quaternary (Q) from old to new. The Quaternary sand and gravel is mainly distributed in the channel, and the maximum depth is not more than 50m, which is a bad caprock. 2.3.3 Construction of Geothermal Geological Model The EOS1 module is selected in this flow simulation, it’s the most basic module of TOUGH, which can simulate the characteristics of single-phase or two-phase flow, and the changes of groundwater pressure and temperature are taken into account in the case of single-phase. Spatial discretization: the model is divided according to the scope and geological characteristics of the study area, and the regular cuboid is used to construct the one-dimensional cylindrical main grid in the numerical model, the grid spacing in X direction and Y direction is 50m × 50m and divided into 10 layers in Z direction. Boundary condition treatment: the external boundary of the model is obtained according to the actual hydrogeological conditions; The internal boundary is mainly the fault structural zone, and the equivalent substitution method is used to control the internal boundary in this simulation. Initial condition parameters: the model is mainly composed of granite, Archean gneiss and faults. Regional geothermal conduction mainly depends on faults and fracture zones. In the process of numerical simulation, faults and fracture zones are equivalent to strata with good permeability. Sandstone is used for reference. The main hydrogeological parameters of the model are shown in the Table 1 . Reference temperature: 9.6 ℃; Density of water: regarded as constant, take 1026.8kg/m 3 ; Specific heat capacity of water: regarded as constant, take 1kcal/kg·℃. Table 1 Statistical table of main parameters of geothermal geological model in Tangquan area Rock density g/cm 3 Porosity % Permeation rate md Specific heat capacity kJ/(kg•K) Thermal conductivity W/(m•K) Granite body 2.63–3.3 0.9 0.008 0.794 2.333 Archean gneiss 2.5–2.8 1.2 0.002 0.739 2.227 Fracture zone 2.6 3 2 0.2 3 2.4 Identification of Groundwater Recharge Sources The hydrogen and oxygen isotope method is a technique for studying the origin and formation of groundwater. The steam pressure of water composed of hydrogen stable isotope tritium ( 2 H) and oxygen ( 18 O) is lower than that of ordinary water. In the process of evaporation and condensation, heavy isotopes are enriched in the liquid phase and depleted in the vapor phase (Xufei et al., 2024 ). Therefore, the contents of hydrogen and oxygen heavy isotopes in groundwater are different in different water cycle processes. According to Graig standard precipitation curve, different recharge sources of groundwater can be judged by different slope and intercept on δ 2 H-δ 18 O diagram (Craig, 1961a ). 3. Results 3.1 Results of Structural Investigation 3.1.1 Shangyi-Pingquan Deep Fault (F1) Through our research group's hydrogeological survey and field exploration, the overall strike is close to EW (250°~ 280°), most of the areas dip north or northeast, and the dip angle is more than 60°, which is a thrust fault formed by compression. The northern part of the fault (upper wall) is Hongqiyingzi Group, which is composed of quartz schist, biotite plagioclase gneiss, hornblende granulite, granite gneiss and so on. The southern part (lower part) of the fault is the Jiangouhe formation of Chongli Group, which is mainly composed of garnet hornblende gneiss, plagioclase amphibolite and amphibolite. To the south of Zhenningbao, there are Jurassic Middle Lower Garden formation and Archean granitic gneisses. The main fault and branch fault of Shangyi-Pingquan deep fault is the most important fault structure in the working area. Through the physical detection method of magnetotellurics, we discovered that there is a significant difference in water content on both sides of the fault, forming a closed or semi-closed thermal storage space. Since the Cenozoic, the shallow fissures have been filled with carbonatite, sericite, chlorite, ferric oxide and siliceous. The lithology of the surrounding rock on both sides of the fault is gneiss and magmatic rock, which is rich in water and forms a closed or semi-closed thermal reservoir space in the fault, which has the characteristics of tension in the multi-stage activity, the depth and scale of the influence are relatively large, it can pass through the deep heat source, so it is also the main heat conduction structure in this area. 3.1.2 Tangquan Fault (F2) The overall strike of the fault is close to the east-west, partial to the north-west, mainly dipping to the north, dipping south at the positions of MT11 and CMT11, with a dip angle of more than 55 °, which belongs to a thrust fault with a drop of more than 600m. All the cracks in the fracture zone can form a good thermal reservoir space. The fracture zone is mainly composed of flattened conglomerate, cataclastic rock, porphyry, tectonic schist, mylonite and so on, and the fractures are developed, which provide conditions for the formation of thermal reservoirs. The following picture (Fig. 4 ) shows the analysis of the profile obtained by the resistivity inversion of the MT7 line position by MT magnetotelluric method. By observing changes in electric and magnetic fields at different frequencies, the resistivity distribution of subsurface geological bodies was detected, thereby understanding the structure of the subsurface geological bodies and inferring the presence of faults, folds, and other structural features. 3.1.3 Dahenan-Chicheng Deep Fault (F8) From the results of the MT and CSAMT exploration lines, the northern line shows that the dip angle of the fault is steep and the cutting depth is large, while the fault anomaly above the 1km in the shallow part of the southern line is not obvious. The fracture inclination angle shown by the M9 line is slow, which may be due to the small angle between the fracture and the line. Therefore, the cutting depth of the fault in the north of the working area is larger than that in the south. The fault distance in the Quaternary is not large, indicating that the activity is weakened, mainly cutting the Archean and its deep strata. The measured results of each line are shown in Fig. 5 , the peak value of soil radon content appears at the intersection of L2, L4 and L5 and fault F8, the peak value of L3, L7 and L8 appears in the upper wall of fault F8, and the south side is about 50m. It shows that the fault distance of the Dahenan-Chicheng deep fault (F8) is deep and the water conductivity is strong. 3.1.4 Other Faults The impact depth and width of faults F53, F54, F55, and F56 are significant. The test results from magnetotelluric methods suggest that they are located within the same fault zone and have similar fault properties, the rock in the fracture zone is relatively broken, the north-south faults are connected with the east-west faults, and the fractures are developed, which provides favorable conditions for the replenishment, migration and storage of geothermal fluids. The surface water converges to the deep part of the EW-trending deep fault through SN and NW-trending faults. In the process of deep cycle, the geothermal fluid is formed under the influence of magmatic activity and geothermal gradient. 3.2 Results of Thermal Reservoir Model 3.2.1 Geological Structure Model Figure 6 shows the geological structure map of Tangquan area stripped of Quaternary. The magmatic rocks cover the whole area, mainly Yanshanian porphyry granite, granite dikes and diorite dikes, which are distributed along Shangyi-Pingquan deep faults, belonging to the early Yanshanian period and intruded in the shape of rock plants. The basement of this area is Archean metamorphic rocks and has unconformable contact with the Middle Jurassic strata. The contact fracture zone between Yanshanian porphyry granite and surrounding rock is a good channel and storage space for geothermal fluid. The whole intrusive rock mass is cut by Shangyi-Pingquan deep fault. As shown in Fig. 6 , the profile CC' is located on the west side of the Tangquan geothermal model area, with a strike of 125°, and the end coordinates are C:390983.6911m, 4530511.886m; C':394425.6815m, 4528100.942m. The section passes through fault F2, which is a reverse fault with a strike of EW/NW and a dip angle of about 55° and a fault distance of about 600m. The F58 fault is a normal fault, trending NE, dipping SE, with a displacement of 1km and a dip angle of 70°. Except for the Archean exposed between 300-800m, the other parts are covered by granite in the Archean, and the thickness varies greatly, about 0-1500m. The DD' section is located on the south side of the Tangquan area, with a strike of 64°. The endpoint coordinates are D:391938.3522m, 4527933.287m; D': 394888.0121m, 4532362.287m. The strikes of Tangquan faults F53, F54, and F56 are NW, with F53 dipping SW, F54 dipping NE, and a dip angle of 70 degrees. They are normal faults with a displacement of 400-800m. Chicheng County fault F55 strikes NW, dips NE, dip angle is 70°, positive fault property, fault distance is 600m. All the granite bodies are exposed in this section, and the thickness of the rock mass varies from deep to shallow to deep from west to east, with a thickness of 400-1600m, and the granite covers the Archean strata. 3.2.2 Geothermal Geological Model Figure 7 shows the result map of geothermal geological simulation in Tangquan area, in which the arrow indicates the direction of flow movement and the color code indicates the temperature change. It can be seen from the figure that the pressure in Tangquan area tends to decrease gradually from deep to shallow, and there is an obvious arrow convergence in the fault and its periphery. The simulation results can objectively reflect the characteristics of pressure change in Tangquan area. From the temperature map, it can be seen that the heat source mainly originates from the upwelling of deep terrestrial heat flow, which diffuses towards the shallow parts and gradually decreases in temperature. Near the faults, there is a significant convergence of temperature, indicating that the Tangquan area mainly relies on faults for water and heat conduction. 3.3 Distribution Characteristics of Water Temperature in Tangquan Area The geothermal field with anomalies greater than 25°C is as shown in Fig. 8 , encompassing Tangquan and its surrounding areas. Its location on the map is indicated by the square in Fig. 3 , with a northeast-southwest length of approximately 500m, a southwest width of 200m, a northeast width of about 60m, and an approximate triangular distribution, covering an area of 0.061km 2 . The temperature of spring water is closely related to the dip angle of the fracture; When the dip angle is larger, the depth of the fracture cutting is relatively greater, the pathway for deep water circulation is deeper and longer, and thus it is more influenced by magmatic activity or geothermal gradients, hence the water temperature tends to be relatively higher. As shown in the isotherm distribution in the figure: Through the isotherms, it can be seen that the temperature in the southwest is high, with dense isotherms. The total spring water temperature of Tangquan is the highest, with a recorded maximum temperature of 60°C, and the measured temperature is 57.9°C. 3.4 Heat Source in Tangquan Area The average measured heat flux in Chinese mainland area is 62.6 ± 24.2 mW/m 2 , which is controlled by the structural pattern between depression and uplift. The geodetic heat flow in the Beijing-Tianjin-Hebei Plain also has the banded distribution characteristics of low-high-low-high from west to east, and the average value is close to 62.6mW/m 2 . The geodetic heat flux in Xiongxian, Gu'an and Bazhou is 79.1–90 mW/m 2 , which is relatively high, and the geodetic heat flux in the area is relatively low (WANG Gui-ling, 2017). The data indicates that the range of the existing 11 heat flow measurements is 25.5 ~ 61.0mW/m², which falls within a low heat flow background area, representing the region with the lowest heat flow on the Chinese mainland. It shows that the tectonic activity is weak and there is no magmatic heat source, the main heat sources should be mantle heat flow and crustal radioactive element decay heat. The magmatic rocks in this area are mainly Yanshanian porphyry granite, and the radioactive heat of porphyry granite is an important heat source in this area. The geothermal gradient in the area is affected by groundwater movement, generally in shallow strata (such as Quaternary and bedrock weathering zone, generally no more than 70m), due to groundwater disturbance, the geothermal gradient is low, generally 0.85–0.89 ℃ / 100m. However, the geothermal gradient is higher in gneiss and magmatic rock areas with large area distribution, which is generally between 1.4 ℃ and 2.81 ℃ / 100m, and the local anomaly near the thermal conduction fault can reach 4.5 ℃ / 100m. The depth of the constant temperate zone is 30m to 40m, with an average of 35m, and the temperature is about 7.5 ℃. The main activity time of magma in this area is Mesozoic, and when it is strong, it is in the middle and late Mesozoic. The magmatic rock geological body formed in this period has the characteristics of large scale, wide distribution, complex and diverse morphology, remarkable mineralization and so on. It mainly shows Batholith, rock plant, dyke, rock branch, lava quilt, rock sheet, rock cone, and the scale is different. Magmatic intrusions and volcanic eruptions are controlled by the NE-NNE-trending Dahenan-Chicheng deep fault (F8). However, because the magmatic rocks were formed for a long time (earlier than the Quaternary), the heat of the magmatic rocks has been lost and does not constitute an attached heating source. The magmatic rocks in this area are mainly Yanshanian porphyry granite, and the radioactive heat of porphyry granite is an important heat source in this area. 3.5 Water Supply Source in Tangquan Area From the deuterium-oxygen relationship diagram of the water in the area (Fig. 9 ) (Vuille et al., 2005 ), it can be seen that the isotopes of geothermal water, shallow groundwater and surface water all fall near the atmospheric waterline, indicating that there are significant differences in deuterium and oxygen composition (Craig, 1961b ). The deuterium and oxygen isotopes of geothermal water are the most depleted, indicating that the recharge elevation of geothermal water is higher. The slight enrichment of isotopes than precipitation may be due to the slight "oxygen drift" caused by the exchange of oxygen isotopes between water and rocks caused by water-rock interaction. The shallow groundwater sample also falls near the waterline, but its isotope is richer than that of geothermal water, indicating that its recharge elevation is lower, which is mainly supplied by local precipitation (Gibson et al., 2005 ). There is a partial overlap between the deuterium and oxygen isotopes of shallow groundwater and geothermal water, indicating that the geothermal water is mixed with shallow groundwater when it enters the shallow aquifer, and the isotope of surface water is the most enriched, obviously experiencing different degrees of evaporation. Therefore, the main source of hot water in the interior of the area is the atmospheric precipitation in the surrounding mountains where the dew point of the hot spring or the geothermal field is higher. 4. Discussion Genesis of Tangquan Geothermal Field The geothermal genetic mechanism map of Tangquan area (Fig. 10 ) is summarized based on the results, combined with historical data. The hot spring appears in the Yanshanian granite, it is inferred that the heat source is the residual heat of magmatic rock or radioactive heat (El-Mageed et al., 2013 ). The study area is surrounded by Yanshanian granitic porphyry bodies, granite dikes, and diorite dikes. These formations are mostly fleshy red, mainly composed of quartz and syenite, with little biotite and light gray weathering surface. The spring emerges in granite fissures and partially covers Quaternary deposits. The high-temperature field in this area is associated with the regional tectonic background. Mantle heat flow can be considered the primary factor contributing to the higher thermal background in this area (Siegel et al., 2014 ). Tangquan is located at the confluence of the NE-trending Dahenan-Chicheng deep fault and the deep faults (Shangyi-Pingquan deep fault, Fengning Longhua deep fault) in the southern margin of Inner Mongolia axis. Secondary faults are developed (F6, F58 in Fig. 10 , F6, F58 and Shangyi-Pingquan deep faults are EW-trending faults with similar properties) (Yuan et al., 2022 ), fracture zones are formed, providing favorable space and channels for the storage and migration of geothermal water. The fracture zones of F1 and F8 extend to depths of approximately 25km, located respectively in the western and northeastern parts of the research area, with widths exceeding 100m and 500m, respectively. The fracture bandwidth is generally significant in both depth and scale of influence. The fracture zone mainly consists of flattened conglomerate, cataclastic rock, porphyry, tectonic schist, mylonite, and so on, with well-developed fractures. The surrounding rocks on both sides of the fault are gneiss and magmatic rocks with low permeability, thus forming a closed or semi-closed thermal reservoir space in the fault. The tensional fracture zones trending NW, NE and SN, which intersect the thermal structure of the east-west guide, collect the scattered water around it and direct it into the deep fracture zones of the deep fault. Groundwater accumulates in the deep parts of the fracture zone, where it is heated by deep geothermal convective conduction heat accumulation of magmatic rocks, forming geothermal water storage (El-Mageed et al., 2013 ). The undulating topography with steep mountains and developed valleys of this sparsely vegetated area makes the precipitation easy to form surface runoff down the slope and converge into Qingshui River and Baihe River in the valleys. This condition is not conducive to the direct infiltration and recharge of groundwater. However, the exposed bedrock surface and structural conditions with developed fracture structure and broken rock in mountainous areas are favorable factors for precipitation infiltration. The results of isotopes in Tangquan geothermal fluid show that (Mohammadi et al., 2010 ) δ 2 H accounts for 86‰ and δ 18 O accounts for 12‰ while δ 2 H-δ 18 O coordinates fall near the standard precipitation line, which indicates that atmospheric precipitation is the main source of local recharge (Shepherd, 2011 ). The spring age calculated using radium radon method is 12 years, indicating that the circulation of geothermal water in this area is fast (Xufei et al., 2024 ). It can be therefore inferred that the atmospheric precipitation in this area mainly infiltrates downward along the fault-related water diversion channels. Therefore, the vertical hydraulic connectivity of groundwater in the region is not obvious. The recharge of groundwater is primarily from the lateral recharge through secondary faults while the vertical infiltration is limited. The fate of these recharge water is the deep thermal reservoir, where the geothermal fluid are formed are stored and heated. The main fault F2 and the secondary fault F54 in the area collect dispersed water and channel it into the fracture zone along the fault (Fig. 10 ). The flow absorbs the convective conduction heat accumulation of the surrounding rock under the regional geothermal gradient (Ilani et al., 2006 ) during its movement in the fault channel. In the closed or semi-closed space formed by the weakly permeable upper rock layer, the increase of temperature enhances the pressure of hot water and steam, thus making it possible to surge up through cracks (such as F6 and F58 in Fig. 10 ) (Lambrakis et al., 2013 ). The rising hot water and the infiltrating cold water circulate repeatedly due to their density differences and the convention subsequently formed. When the resistance of the open fissures is low, the water rises out of the surface and forms hot springs (Yongping, 2019 ). In summary, the formation of geothermal system is controlled by neotectonic faults, in which NNE and NEE faults serve as the main thermal conduction faults, and NNW and NWW faults facilitate water conduction. The main source of hot water in this area is the atmospheric precipitation, which penetrate into the thermal reservoir through fracture zone (Craig, 1963 ). During the migration process, the hot water is heated by normal earth heat flow blanket conduction and heat accumulation. The geothermal field forms at the confluence of the two groups of main faults. This confluence of neotectonic faults located in the groundwater discharge area is the priority target of geothermal resources exploration. The formation mechanism of the Tangquan geothermal spring is multifaceted. The surface source is dominated by normal earth heat flow blanket conduction and heat accumulation. The deep thermal conduction fault and the contact zone between intrusive rock mass and soluble rock accept convective conduction heat accumulation; in addition, Tangquan's deep circulation flow system is different from the general free convection hydrothermal system caused by temperature difference, which belongs to forced convection system. Because the strata in this area are mainly rock masses with poor permeability, water is mainly guided by fissures and fracture zones, and the geothermal water circulation system is formed under the action of topographic height difference and corresponding water head difference (Delvaux et al., 2010 ; Lelli et al., 2021 ). Faults are developed in this area, and some faults can reach the depth of crystalline basement, which can be used as a good channel for rapid heat transfer in the lower part, and the discharge or recharge of groundwater can cause high or low regional geotemperature field. After the infiltration of atmospheric precipitation, it seeps along the water diversion fault, and receives conduction-convection heat accumulation on the way after entering the deep circulation runoff, the temperature increases, and the hot water rises to the surface discharge area along the fault zone, forming hot springs. 5. Conclusions Through geophysical exploration methods such as MT and CSAMT, we effectively detected the stratigraphic properties as well as faults, folds, and other structural features in the Tangquan research area; by conducting geothermal measurements and constructing a geothermal geological model in the Tangquan area, we identified the heat source of the Tangquan geothermal anomaly zone; isotopic methods were used to ascertain the water source of the Tangquan area; finally, a comprehensive analysis of the heat source, water source, and structural aspects of the Tangquan area revealed the geothermal genesis of the region. Based on the analysis of the cause of formation of thermal reservoir in Tangquan area, the convective conduction heat accumulation mechanism of deep circulation flow system controlled by fault water conduction and thermal conductivity is revealed, which provides a reference for the further development and utilization of geothermal resources in Tangquan and even the vast geothermal anomaly areas, it provides a new and more comprehensive understanding of the cause of geothermal water. Abbreviations MT Magnetotelluric Sounding CSAMT Contrelled-Source Audio-frequency Magnetotelluric Declarations Acknowledgements We thank the anonymous reviewers for their constructive criticism and useful suggestions, which helped to improve the manuscript. Author contributions W.Y.,Y.Z.,D.W. conceived the research, prepared the data, prepared the fgures, and wrote the paper. W.Y. conducted the formal analysis. S.Z., S.S., W.Y., G.Z., Y.Z., X.G., B.Y. and G.W. analyzed the results and drafted the manuscript. All authors read and approved the final manuscript. Funding This work was jointly supported by the National Science Foundation of China (grant numbers: 42102289), Geological Survey Project Foundation (grant numbers: DD20243504), Shandong Provincial Natural Science Foundation (grant numbers: ZR2020QD122, ZR2022QD060). Availability of data and materials The data supporting the fndings of this study are available from the corresponding author upon reasonable request. Consent for publication Not applicable. Competing interests The authors declare that they have no competing interests. References Arnórsson, S., Stefánsson, A. and Bjarnason, J.n.O.r. 2007. Fluid-fluid interactions in geothermal systems. Reviews in Mineralogy and Geochemistry 65(1), 259-312. Chunhui, L. (2006) Hydrochemical and Isotopic Study of Miao Hot Spring, Chicheng Hot Spring, and Tangzi Temple Hot Spring. 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Tangquan\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-5583602/v1/5d727f0443a0d8f8aa4dceb7.png"},{"id":71863803,"identity":"9c9e2606-2b1b-4c4a-9a5a-a881256b0fb4","added_by":"auto","created_at":"2024-12-19 09:21:11","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":177244,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eIsoline map of hot water temperature in Tangquan geothermal anomaly area\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-5583602/v1/58b9ae97e6a17fb450094182.png"},{"id":72617206,"identity":"ec013baf-fa6e-438e-a56b-0e107c1e5a03","added_by":"auto","created_at":"2024-12-30 11:37:00","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":168051,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDeuterium-oxygen relationship diagram of water in Zhangjiakou area\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-5583602/v1/95b9528a20487ce782de58d8.png"},{"id":71865052,"identity":"58619fcd-9928-4298-b339-2df9725f1c61","added_by":"auto","created_at":"2024-12-19 09:29:11","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1069642,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMap of geothermal genetic mechanism in Tangquan area\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eF2: Tangquan fault; F54: Tangquan fault;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eF6: Haojiagou fault; F58: Yujiagou fault\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-5583602/v1/34d844848c3f79c0e6d3b6db.png"},{"id":77369959,"identity":"05a7f0ba-ad09-44a4-a083-5379e86d4011","added_by":"auto","created_at":"2025-02-28 00:01:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":7913741,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5583602/v1/8403224e-478f-4191-abb7-6d5e19e68d39.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fomation mechanism of fault-controlled circulation geothermal spring: evidence from tectonics, thermal model, and isotopes","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eEnergy is essential for economic growth and social development. Currently, fossil energy dominates global energy consumption, but it is finite (Heinberg and Fridley, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Yuan et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) and causes significant environmental problems during its production, transportation, and use (Fenger, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Prideaux et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). In particular, the greenhouse gas from fossil energy combustion has become one of the most concerned issues of this century (Fenger, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). To address the over-reliance on fossil energy, a shift towards clean energy is necessary. Geothermal energy is a leading renewable energy resource with high potential (Dotsika, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Achieving sustainable development and utilization of geothermal resources is a key challenge for the global geothermal industry today (Gongke, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The formation mechanism of geothermal resources determines the potential and mode of its sustainable utilization, so it is very important to accurately analyze the genesis of geothermal resources.\u003c/p\u003e \u003cp\u003eResearch on the classification of regional geothermal systems in China has been well established (Moxiang et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). The components of a geothermal system include the heat source, permeable strata, and fluid. The fluid usually refers to groundwater, which transfers heat through flow in limited space, making the utilization of geothermal energy feasible (Arn\u0026oacute;rsson et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). According to the existing analysis of the genetic types, the causes of thermal reservoir usually involves the structural control of water and heat, heat sources, geothermal water sources, hot water circulation paths and so on (Xu Butai, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). In the geothermal system, the heat source plays a crucial role as it is the energy source of the geothermal system, providing heat for the whole geothermal system, which promotes the heating and circulation of the geothermal fluid, and then realizes the transfer and utilization of heat (Ji, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). The common types of heat source in geothermal system include magmatic heat source, which is abnormal by the surrounding rocks by magma; the radioactive heat source depends on the decay of radioactive elements in the rock; the tectonic movement heat source causes rock deformation and fracture caused by the crustal movement such as plate collision extrusion, and the mechanical energy is converted into heat energy. The convection heat source transfers the deep heat to the shallow part of the crust, providing energy support for geothermal activities near the middle ocean ridge (Xu Butai, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Water sources act as heat transfer media, absorbing heat from heat sources and circulating it underground to bring it to the surface, and they also participate in water-rock interactions that affect the properties of the geothermal reservoir (Ji, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). Common types of water sources include atmospheric precipitation, primordial water trapped in rock pores during the formation of the Earth, and magmatic water from the volatilization of magma. Common research methods include hydrogeological survey methods and geochemical analysis methods using isotopes.The fluid circulation types of geothermal systems include deep circulation types that go thousands of meters underground and shallow circulation types that are a few hundred meters below the surface (Na et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), capable of transferring heat and exchanging substances for the geothermal system (Pola et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2020\u003c/span\u003e); when faults are present, they can provide channels for fluid circulation (Przybycin et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), thereby altering the source of groundwater and the circulation path (Craig, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1961b\u003c/span\u003e), and common identification methods include geochemical analysis and geophysical exploration. In summary, tectonic control, geothermal system types, and groundwater circulation paths all play an important role in the formation of geothermal fields and can influence each other (Hu et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). With the progress of research and practice, the analysis of thermal reservoir formation has gradually shifted from a single aspect to a comprehensive integration of multifaceted information. It is an inevitable trend to analyze the causes of thermal reservoir from a systematic perspective.\u003c/p\u003e \u003cp\u003eZhangjiakou City, located in the northwest of Hebei Province, is one of the new energy bases planned by the state. Zhangjiakou area has good geothermal geological conditions and is rich in geothermal resources. More than 20 geothermal fields have been found. Chicheng geothermal anomaly area, which is located in the southeast of Zhangjiakou City and adjacent to Beijing, is one of the typical geothermal fields. As the most representative geothermal resource area in Chicheng area, Tangquan has found 9 hot spring spots in recent years and has great geothermal potential. Understanding the geothermal genesis of Tangquan is of great significance to the understanding and utilization of geothermal resources in Chicheng and even the whole city of Zhangjiakou (Yuan et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Although preliminary development and utilization has been carried out in Tangquan area, a professional geothermal geological survey has not been carried out, and the cause of formation of the hot spring is not completely clear. Previous work has not characterized complex structures enough to determine the mechanical properties of faults (Hongquan et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), and the depth and period of groundwater circulation in this area are not clear, the analysis of heat source and water source of thermal reservoir is still inconclusive. At present, the research in this area mainly stays in the characterization and interpretation of individual aspects, lacking a overall analysis from the point of view of geothermal system, so it is necessary to analyze the cause of formation of Tangquan geothermal comprehensively from many aspects, such as structure, heat source, water source and so on.\u003c/p\u003e \u003cp\u003eThis paper aims to elucidate the genetic mechanism of Tangquan geothermal spring through the comprehensive analysis of tectonism, heat source distribution, and water cycle. The characteristics of the thermal reservoir structure, heat conduction structure, and potential heat sources were analyzed using geophysical methods. The occurrence and heat conduction of geothermal resources were analyzed by geological and geothermal model. The isotope method was used to identify the source and circulation pattern of groundwater. The characteristics of geological structure, heat source, and water source were integrated to systematically analyze the the formation mechanism of Tangquan geothermal field, revealing the convective conduction heat accumulation mechanism of fault-deep circulation geothermal system in the study area. This study enhances the understanding of fault-deep circulation geothermal systems, provides a theoretical basis for the development and utilization of geothermal resources in Tangquan area, and offers a reference for the study of geothermal field genesis in similar areas.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Overview of the Study Area\u003c/h2\u003e \u003cp\u003eThe study area (Tangquan Geothermal Anomaly Area) is located approximately 7.5km west of Chicheng County in Hebei Province, China, within a low to mid-elevation mountainous region. It extends 5.5km in a northeast direction and 4.6km in a northwest direction, covering an area of 25km\u0026sup2;(Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The topography is high in the north and low in the south, surrounded by mountains on three sides, with steep peaks in the north, east and west, and alluvial valleys in the south, which belongs to the Piedmont hilly landform with an elevation of 110\u0026thinsp;~\u0026thinsp;190 meters (Gongke, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe long-term tectonic evolution of Tangquan area led to the extremely complex structural characteristics in the region. Under the action of Cenozoic tension and strike-slip stress, a large number of large Piedmont faults were developed, and a basin-mountain coupling structure blocked by mountain system in adjacent basins was formed (Shuang, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). There are about 4 springs with water temperature\u0026thinsp;\u0026ge;\u0026thinsp;25 ℃ through geothermal survey by predecessors, the water temperature is 37.5\u0026thinsp;~\u0026thinsp;59 ℃, the highest can reach 60 ℃, and the area of geothermal field with water temperature\u0026thinsp;\u0026ge;\u0026thinsp;25 ℃ is delineated 0.061km\u003csup\u003e2\u003c/sup\u003e. The geothermal field is controlled by Shangyi-Pingquan deep fault and Dahenan-Chicheng deep fault. The fractures are developed, the rocks are broken, and the hot water rises into a spring through the fracture channel, which emerges from the Yanshanian giant porphyry granite. Water temperature of the main spring (Tangquan) is 60 ℃ and the flow rate is 75.60m\u003csup\u003e3\u003c/sup\u003e/h. There are also some hot springs nearby, such as stomach Spring, Pingquan, Eye Spring and so on, because Tangquan has the largest flow and the highest exposed position, it usually refers to Chicheng Hot Spring (Chunhui, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The total area of the spring domain is about 5km\u003csup\u003e2\u003c/sup\u003e, and the spring water spills in the valleys that are less than 1km\u003csup\u003e2\u003c/sup\u003e in the form of group springs, and the total amount of spring water is about 35L/s.\u003c/p\u003e \u003cp\u003eTangquan is the intersection of the deep fault (Shangyi-Pingquan deep fault) and the NE-trending Dahenan-Chicheng deep fault in the southern margin of the Earth's axis in Inner Mongolia. The secondary fault develops and forms a fault fracture zone, which provides a favorable space and channel for the storage and migration of geothermal water. The magmatic rocks in this area are mainly Yanshanian porphyry granite, in addition to granite dike, diorite dike and so on. According to Li Quan's study, the location of Chicheng hot spring is covered with porphyry coarse-grained granite, which is distributed along the Shangyi-Pingquan deep fault, belonging to the early Yanshanian period and intrusive in the shape of rock plants. The geology of this area is Archean metamorphic rocks and unconformable contact with Middle Jurassic strata (Quan, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1994\u003c/span\u003e). The contact fracture zone between Yanshanian porphyry granite and surrounding rock is a good channel and storage space for geothermal fluid. The whole intrusive rock mass is cut by Shangyi-Pingquan deep fault with an exposed area of about 35km\u003csup\u003e2\u003c/sup\u003e. The radioactive heat of porphyry granite is an important heat source in this area.\u003c/p\u003e \u003cp\u003eThe temperature of Tangquan spring is not only affected by the nature of the fault, but also closely related to the dip angle of the fault, when the dip angle is large, the depth of fracture cutting is also relatively large, the path of deep water circulation is deeper and longer, its influence by magmatic activity or geothermal gradient is greater, hence its water temperature is also relatively high; On the contrary, it's the opposite. Tangquan belongs to the Bahe hydrogeological region, with the west side being the Qingshui River hydrogeological region. The terrain is generally higher in the northwest and lower in the southeast, with surface elevations ranging from 750 to 2100 meters. The landforms are mainly composed of igneous rocks, metamorphic rocks, and Quaternary Upper Pleistocene to Holocene alluvial and diluvial deposits. The types of groundwater include loose rock pore water, bedrock fissure water, and clastic rock pore-fissure water. The source of underground water supply is from the northern part of Baihe River, Tangquan Reservoir and atmospheric precipitation in the north (Lele, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Structural features\u003c/h2\u003e \u003cp\u003eAccording to the analysis of geothermal genesis, structures play an important role in Tangquan geothermal system, but not all faults or folds have good geothermal significance. Referring to the previous geothermal data and the results of this geological survey, the possible heat conduction structures are further analyzed by magnetotelluric, CSAMT and two-dimensional seismic methods, and the main structures in Tangquan area are further explained (Peng et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1 Shangyi-Pingquan Deep Fault (F1)\u003c/h2\u003e \u003cp\u003eThe main and branch faults of the Shangyi-Pingquan deep fault are the most significant structural faults and also the main heat-conducting structures in the working area (Yongping, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). According to the previous data and this investigation, the Shangyi-Pingquan deep fault is a thrust fault structure that began to exist at the end of Archean and experienced multi-stage activity, its main active period is in Luliang, Hercynian, Yanshan and other stages, there is little activity after the Yanshanian period.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2 Tangquan Fault (F2)\u003c/h2\u003e \u003cp\u003eF2 fault is a secondary fault of Shangyi-Pingquan deep fault, which passes through two working areas of Prince Chongli and Xiaozhangjiakou in Chicheng, and is controlled by survey lines MT10, CMT10, MT11 and CMT11. The overall strike of the fault is close to the east-west, partial to the north-west, mainly dipping to the north, dipping south at the positions of MT11 and CMT11, with a dip angle of more than 55 \u0026deg;, which belongs to a thrust fault with a drop of more than 600m.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.2.3 Dahenan-Chicheng Deep Fault (F8)\u003c/h2\u003e \u003cp\u003eLocated in the east of Zhangjiakou City, it belongs to the category of NNE trending tectonic magmatic rocks from Dahenan to Dahaituo, with a length of about 170km(Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The Yanshanian period led to large-scale magmatic activity, resulting in the intrusion of the Dahenan complex and Dahaituo granite, with a spatial long axis of NNW direction and a fracture bandwidth of about 100m. The fault zone is composed of clastic rocks and mylonites, which is linearly distributed and is still active recently.\u003c/p\u003e \u003cp\u003eThe predecessors believed that the fault was of a grand scale and deeply cut through the Moho surface, controlled magmatic intrusion and eruption in the middle and late Mesozoic, was a deep fault connecting mantle and crustal magma chamber, and shallow multi-stage active new structure, which had the function of heat and water conduction. In this field investigation, it is found that there is an obvious fault triangle on both sides of the fault zone, cutting the Quaternary, and the fault is tensional and shear and should have water conductivity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.2.4 Other Faults\u003c/h2\u003e \u003cp\u003eF53, F54, F55 and F56, controlled by MT11 and CMT11, are NW-trending normal faults intersecting Tangquan fault (F2) near Chicheng Tangquan(Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). They are all newly inferred faults. Among them, the F53 fault inclines to the southwest, the dip angle is more than 70\u0026deg;, and the maximum drop is more than 600m, which is mainly developed in the early Yanshan porphyry granite; The F54 fault dips more than 70\u0026deg; and the maximum drop is more than 800m, which mainly occurs in the early Yanshanian porphyry granite. F55 dips more than 70\u0026deg; and the maximum drop is more than 600m, mainly developed in the early Yanshanian porphyry granite; The F56 fault inclines to the northeast, the dip angle is more than 70\u0026deg;, and the maximum drop is more than 400m, which is mainly developed in the Middle Archean Chongli Group.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Thermal Reservoir Model\u003c/h2\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e2.3.1 Data Processing\u003c/h2\u003e \u003cp\u003eThe construction of Tangquan geothermal geological model is mainly based on the 3D geological structure of the study area, using the 2019 geophysical results of the research area to control and establish a more precise 3D geological structure model. The geophysical work in this area in 2019 mainly includes MT and CSAMT data. Since the two datasets largely overlap, the MT point number is used as the virtual borehole number (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e for the plane location). Combining two kinds of geophysical results to extract borehole data to form a control profile, so as to generate a more fine 3D geological structure model of the whole study area. The statistics of virtual boreholes extracted from geophysical data amount to 156 points.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e2.3.2 Construction of Geological Structure Model\u003c/h2\u003e \u003cp\u003eThe control profile of the solid model is formed by using the virtual borehole data. Determine the stratification interface and code of each stratum of the drill hole, import GMS to complete the joint profile construction, and finally complete the construction of the solid model. The data range of the geological model are as follows: long 5.5km in north-east direction, wide 4.6km in north-west direction, area 25km2, vertical to -2000m elevation. The accuracy of stratigraphic division is that the model strata are divided into Archean (Ar), rock mass (Yanshanian porphyry granite) and Quaternary (Q) from old to new. The Quaternary sand and gravel is mainly distributed in the channel, and the maximum depth is not more than 50m, which is a bad caprock.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e2.3.3 Construction of Geothermal Geological Model\u003c/h2\u003e \u003cp\u003eThe EOS1 module is selected in this flow simulation, it\u0026rsquo;s the most basic module of TOUGH, which can simulate the characteristics of single-phase or two-phase flow, and the changes of groundwater pressure and temperature are taken into account in the case of single-phase.\u003c/p\u003e \u003cp\u003eSpatial discretization: the model is divided according to the scope and geological characteristics of the study area, and the regular cuboid is used to construct the one-dimensional cylindrical main grid in the numerical model, the grid spacing in X direction and Y direction is 50m \u0026times; 50m and divided into 10 layers in Z direction.\u003c/p\u003e \u003cp\u003eBoundary condition treatment: the external boundary of the model is obtained according to the actual hydrogeological conditions; The internal boundary is mainly the fault structural zone, and the equivalent substitution method is used to control the internal boundary in this simulation.\u003c/p\u003e \u003cp\u003eInitial condition parameters: the model is mainly composed of granite, Archean gneiss and faults. Regional geothermal conduction mainly depends on faults and fracture zones. In the process of numerical simulation, faults and fracture zones are equivalent to strata with good permeability. Sandstone is used for reference. The main hydrogeological parameters of the model are shown in the Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eReference temperature: 9.6 ℃;\u003c/p\u003e \u003cp\u003eDensity of water: regarded as constant, take 1026.8kg/m\u003csup\u003e3\u003c/sup\u003e;\u003c/p\u003e \u003cp\u003eSpecific heat capacity of water: regarded as constant, take 1kcal/kg\u0026middot;℃.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eStatistical table of main parameters of geothermal geological model in Tangquan area\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRock density\u003c/p\u003e \u003cp\u003eg/cm\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePorosity\u003c/p\u003e \u003cp\u003e%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePermeation rate\u003c/p\u003e \u003cp\u003emd\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSpecific heat capacity\u003c/p\u003e \u003cp\u003ekJ/(kg\u0026bull;K)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eThermal conductivity\u003c/p\u003e \u003cp\u003eW/(m\u0026bull;K)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGranite body\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.63\u0026ndash;3.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.008\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.794\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.333\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eArchean gneiss\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.5\u0026ndash;2.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.002\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.739\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.227\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFracture zone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Identification of Groundwater Recharge Sources\u003c/h2\u003e \u003cp\u003eThe hydrogen and oxygen isotope method is a technique for studying the origin and formation of groundwater. The steam pressure of water composed of hydrogen stable isotope tritium (\u003csup\u003e2\u003c/sup\u003eH) and oxygen (\u003csup\u003e18\u003c/sup\u003eO) is lower than that of ordinary water. In the process of evaporation and condensation, heavy isotopes are enriched in the liquid phase and depleted in the vapor phase (Xufei et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Therefore, the contents of hydrogen and oxygen heavy isotopes in groundwater are different in different water cycle processes. According to Graig standard precipitation curve, different recharge sources of groundwater can be judged by different slope and intercept on δ\u003csup\u003e2\u003c/sup\u003eH-δ\u003csup\u003e18\u003c/sup\u003eO diagram (Craig, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1961a\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Results of Structural Investigation\u003c/h2\u003e \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1 Shangyi-Pingquan Deep Fault (F1)\u003c/h2\u003e \u003cp\u003eThrough our research group's hydrogeological survey and field exploration, the overall strike is close to EW (250\u0026deg;~ 280\u0026deg;), most of the areas dip north or northeast, and the dip angle is more than 60\u0026deg;, which is a thrust fault formed by compression. The northern part of the fault (upper wall) is Hongqiyingzi Group, which is composed of quartz schist, biotite plagioclase gneiss, hornblende granulite, granite gneiss and so on. The southern part (lower part) of the fault is the Jiangouhe formation of Chongli Group, which is mainly composed of garnet hornblende gneiss, plagioclase amphibolite and amphibolite. To the south of Zhenningbao, there are Jurassic Middle Lower Garden formation and Archean granitic gneisses.\u003c/p\u003e \u003cp\u003eThe main fault and branch fault of Shangyi-Pingquan deep fault is the most important fault structure in the working area. Through the physical detection method of magnetotellurics, we discovered that there is a significant difference in water content on both sides of the fault, forming a closed or semi-closed thermal storage space. Since the Cenozoic, the shallow fissures have been filled with carbonatite, sericite, chlorite, ferric oxide and siliceous. The lithology of the surrounding rock on both sides of the fault is gneiss and magmatic rock, which is rich in water and forms a closed or semi-closed thermal reservoir space in the fault, which has the characteristics of tension in the multi-stage activity, the depth and scale of the influence are relatively large, it can pass through the deep heat source, so it is also the main heat conduction structure in this area.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2 Tangquan Fault (F2)\u003c/h2\u003e \u003cp\u003eThe overall strike of the fault is close to the east-west, partial to the north-west, mainly dipping to the north, dipping south at the positions of MT11 and CMT11, with a dip angle of more than 55 \u0026deg;, which belongs to a thrust fault with a drop of more than 600m. All the cracks in the fracture zone can form a good thermal reservoir space. The fracture zone is mainly composed of flattened conglomerate, cataclastic rock, porphyry, tectonic schist, mylonite and so on, and the fractures are developed, which provide conditions for the formation of thermal reservoirs. The following picture (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) shows the analysis of the profile obtained by the resistivity inversion of the MT7 line position by MT magnetotelluric method. By observing changes in electric and magnetic fields at different frequencies, the resistivity distribution of subsurface geological bodies was detected, thereby understanding the structure of the subsurface geological bodies and inferring the presence of faults, folds, and other structural features.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e \u003ch2\u003e3.1.3 Dahenan-Chicheng Deep Fault (F8)\u003c/h2\u003e \u003cp\u003eFrom the results of the MT and CSAMT exploration lines, the northern line shows that the dip angle of the fault is steep and the cutting depth is large, while the fault anomaly above the 1km in the shallow part of the southern line is not obvious. The fracture inclination angle shown by the M9 line is slow, which may be due to the small angle between the fracture and the line. Therefore, the cutting depth of the fault in the north of the working area is larger than that in the south. The fault distance in the Quaternary is not large, indicating that the activity is weakened, mainly cutting the Archean and its deep strata.\u003c/p\u003e \u003cp\u003eThe measured results of each line are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the peak value of soil radon content appears at the intersection of L2, L4 and L5 and fault F8, the peak value of L3, L7 and L8 appears in the upper wall of fault F8, and the south side is about 50m. It shows that the fault distance of the Dahenan-Chicheng deep fault (F8) is deep and the water conductivity is strong.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003e3.1.4 Other Faults\u003c/h2\u003e \u003cp\u003eThe impact depth and width of faults F53, F54, F55, and F56 are significant. The test results from magnetotelluric methods suggest that they are located within the same fault zone and have similar fault properties, the rock in the fracture zone is relatively broken, the north-south faults are connected with the east-west faults, and the fractures are developed, which provides favorable conditions for the replenishment, migration and storage of geothermal fluids. The surface water converges to the deep part of the EW-trending deep fault through SN and NW-trending faults. In the process of deep cycle, the geothermal fluid is formed under the influence of magmatic activity and geothermal gradient.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Results of Thermal Reservoir Model\u003c/h2\u003e \u003cdiv id=\"Sec21\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Geological Structure Model\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the geological structure map of Tangquan area stripped of Quaternary. The magmatic rocks cover the whole area, mainly Yanshanian porphyry granite, granite dikes and diorite dikes, which are distributed along Shangyi-Pingquan deep faults, belonging to the early Yanshanian period and intruded in the shape of rock plants. The basement of this area is Archean metamorphic rocks and has unconformable contact with the Middle Jurassic strata. The contact fracture zone between Yanshanian porphyry granite and surrounding rock is a good channel and storage space for geothermal fluid. The whole intrusive rock mass is cut by Shangyi-Pingquan deep fault.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the profile CC' is located on the west side of the Tangquan geothermal model area, with a strike of 125\u0026deg;, and the end coordinates are C:390983.6911m, 4530511.886m; C':394425.6815m, 4528100.942m. The section passes through fault F2, which is a reverse fault with a strike of EW/NW and a dip angle of about 55\u0026deg; and a fault distance of about 600m. The F58 fault is a normal fault, trending NE, dipping SE, with a displacement of 1km and a dip angle of 70\u0026deg;. Except for the Archean exposed between 300-800m, the other parts are covered by granite in the Archean, and the thickness varies greatly, about 0-1500m.\u003c/p\u003e \u003cp\u003eThe DD' section is located on the south side of the Tangquan area, with a strike of 64\u0026deg;. The endpoint coordinates are D:391938.3522m, 4527933.287m; D': 394888.0121m, 4532362.287m. The strikes of Tangquan faults F53, F54, and F56 are NW, with F53 dipping SW, F54 dipping NE, and a dip angle of 70 degrees. They are normal faults with a displacement of 400-800m. Chicheng County fault F55 strikes NW, dips NE, dip angle is 70\u0026deg;, positive fault property, fault distance is 600m. All the granite bodies are exposed in this section, and the thickness of the rock mass varies from deep to shallow to deep from west to east, with a thickness of 400-1600m, and the granite covers the Archean strata.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section3\"\u003e \u003ch2\u003e3.2.2 Geothermal Geological Model\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the result map of geothermal geological simulation in Tangquan area, in which the arrow indicates the direction of flow movement and the color code indicates the temperature change. It can be seen from the figure that the pressure in Tangquan area tends to decrease gradually from deep to shallow, and there is an obvious arrow convergence in the fault and its periphery. The simulation results can objectively reflect the characteristics of pressure change in Tangquan area.\u003c/p\u003e \u003cp\u003eFrom the temperature map, it can be seen that the heat source mainly originates from the upwelling of deep terrestrial heat flow, which diffuses towards the shallow parts and gradually decreases in temperature. Near the faults, there is a significant convergence of temperature, indicating that the Tangquan area mainly relies on faults for water and heat conduction.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec23\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Distribution Characteristics of Water Temperature in Tangquan Area\u003c/h2\u003e \u003cp\u003eThe geothermal field with anomalies greater than 25\u0026deg;C is as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, encompassing Tangquan and its surrounding areas. Its location on the map is indicated by the square in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, with a northeast-southwest length of approximately 500m, a southwest width of 200m, a northeast width of about 60m, and an approximate triangular distribution, covering an area of 0.061km\u003csup\u003e2\u003c/sup\u003e. The temperature of spring water is closely related to the dip angle of the fracture; When the dip angle is larger, the depth of the fracture cutting is relatively greater, the pathway for deep water circulation is deeper and longer, and thus it is more influenced by magmatic activity or geothermal gradients, hence the water temperature tends to be relatively higher. As shown in the isotherm distribution in the figure: Through the isotherms, it can be seen that the temperature in the southwest is high, with dense isotherms. The total spring water temperature of Tangquan is the highest, with a recorded maximum temperature of 60\u0026deg;C, and the measured temperature is 57.9\u0026deg;C.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec24\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Heat Source in Tangquan Area\u003c/h2\u003e \u003cp\u003eThe average measured heat flux in Chinese mainland area is 62.6\u0026thinsp;\u0026plusmn;\u0026thinsp;24.2 mW/m\u003csup\u003e2\u003c/sup\u003e, which is controlled by the structural pattern between depression and uplift. The geodetic heat flow in the Beijing-Tianjin-Hebei Plain also has the banded distribution characteristics of low-high-low-high from west to east, and the average value is close to 62.6mW/m\u003csup\u003e2\u003c/sup\u003e. The geodetic heat flux in Xiongxian, Gu'an and Bazhou is 79.1\u0026ndash;90 mW/m\u003csup\u003e2\u003c/sup\u003e, which is relatively high, and the geodetic heat flux in the area is relatively low (WANG Gui-ling, 2017). The data indicates that the range of the existing 11 heat flow measurements is 25.5\u0026thinsp;~\u0026thinsp;61.0mW/m\u0026sup2;, which falls within a low heat flow background area, representing the region with the lowest heat flow on the Chinese mainland. It shows that the tectonic activity is weak and there is no magmatic heat source, the main heat sources should be mantle heat flow and crustal radioactive element decay heat. The magmatic rocks in this area are mainly Yanshanian porphyry granite, and the radioactive heat of porphyry granite is an important heat source in this area.\u003c/p\u003e \u003cp\u003eThe geothermal gradient in the area is affected by groundwater movement, generally in shallow strata (such as Quaternary and bedrock weathering zone, generally no more than 70m), due to groundwater disturbance, the geothermal gradient is low, generally 0.85\u0026ndash;0.89 ℃ / 100m. However, the geothermal gradient is higher in gneiss and magmatic rock areas with large area distribution, which is generally between 1.4 ℃ and 2.81 ℃ / 100m, and the local anomaly near the thermal conduction fault can reach 4.5 ℃ / 100m. The depth of the constant temperate zone is 30m to 40m, with an average of 35m, and the temperature is about 7.5 ℃.\u003c/p\u003e \u003cp\u003eThe main activity time of magma in this area is Mesozoic, and when it is strong, it is in the middle and late Mesozoic. The magmatic rock geological body formed in this period has the characteristics of large scale, wide distribution, complex and diverse morphology, remarkable mineralization and so on. It mainly shows Batholith, rock plant, dyke, rock branch, lava quilt, rock sheet, rock cone, and the scale is different. Magmatic intrusions and volcanic eruptions are controlled by the NE-NNE-trending Dahenan-Chicheng deep fault (F8). However, because the magmatic rocks were formed for a long time (earlier than the Quaternary), the heat of the magmatic rocks has been lost and does not constitute an attached heating source. The magmatic rocks in this area are mainly Yanshanian porphyry granite, and the radioactive heat of porphyry granite is an important heat source in this area.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec25\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Water Supply Source in Tangquan Area\u003c/h2\u003e \u003cp\u003eFrom the deuterium-oxygen relationship diagram of the water in the area (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e) (Vuille et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2005\u003c/span\u003e), it can be seen that the isotopes of geothermal water, shallow groundwater and surface water all fall near the atmospheric waterline, indicating that there are significant differences in deuterium and oxygen composition (Craig, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1961b\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe deuterium and oxygen isotopes of geothermal water are the most depleted, indicating that the recharge elevation of geothermal water is higher. The slight enrichment of isotopes than precipitation may be due to the slight \"oxygen drift\" caused by the exchange of oxygen isotopes between water and rocks caused by water-rock interaction. The shallow groundwater sample also falls near the waterline, but its isotope is richer than that of geothermal water, indicating that its recharge elevation is lower, which is mainly supplied by local precipitation (Gibson et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). There is a partial overlap between the deuterium and oxygen isotopes of shallow groundwater and geothermal water, indicating that the geothermal water is mixed with shallow groundwater when it enters the shallow aquifer, and the isotope of surface water is the most enriched, obviously experiencing different degrees of evaporation. Therefore, the main source of hot water in the interior of the area is the atmospheric precipitation in the surrounding mountains where the dew point of the hot spring or the geothermal field is higher.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003e\u003cstrong\u003eGenesis of Tangquan Geothermal Field\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe geothermal genetic mechanism map of Tangquan area (Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e) is summarized based on the results, combined with historical data. The hot spring appears in the Yanshanian granite, it is inferred that the heat source is the residual heat of magmatic rock or radioactive heat (El-Mageed et al., \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e). The study area is surrounded by Yanshanian granitic porphyry bodies, granite dikes, and diorite dikes. These formations are mostly fleshy red, mainly composed of quartz and syenite, with little biotite and light gray weathering surface. The spring emerges in granite fissures and partially covers Quaternary deposits. The high-temperature field in this area is associated with the regional tectonic background. Mantle heat flow can be considered the primary factor contributing to the higher thermal background in this area (Siegel et al., \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eTangquan is located at the confluence of the NE-trending Dahenan-Chicheng deep fault and the deep faults (Shangyi-Pingquan deep fault, Fengning Longhua deep fault) in the southern margin of Inner Mongolia axis. Secondary faults are developed (F6, F58 in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e, F6, F58 and Shangyi-Pingquan deep faults are EW-trending faults with similar properties) (Yuan et al., \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e), fracture zones are formed, providing favorable space and channels for the storage and migration of geothermal water. The fracture zones of F1 and F8 extend to depths of approximately 25km, located respectively in the western and northeastern parts of the research area, with widths exceeding 100m and 500m, respectively. The fracture bandwidth is generally significant in both depth and scale of influence. The fracture zone mainly consists of flattened conglomerate, cataclastic rock, porphyry, tectonic schist, mylonite, and so on, with well-developed fractures. The surrounding rocks on both sides of the fault are gneiss and magmatic rocks with low permeability, thus forming a closed or semi-closed thermal reservoir space in the fault. The tensional fracture zones trending NW, NE and SN, which intersect the thermal structure of the east-west guide, collect the scattered water around it and direct it into the deep fracture zones of the deep fault. Groundwater accumulates in the deep parts of the fracture zone, where it is heated by deep geothermal convective conduction heat accumulation of magmatic rocks, forming geothermal water storage (El-Mageed et al., \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eThe undulating topography with steep mountains and developed valleys of this sparsely vegetated area makes the precipitation easy to form surface runoff down the slope and converge into Qingshui River and Baihe River in the valleys. This condition is not conducive to the direct infiltration and recharge of groundwater. However, the exposed bedrock surface and structural conditions with developed fracture structure and broken rock in mountainous areas are favorable factors for precipitation infiltration. The results of isotopes in Tangquan geothermal fluid show that (Mohammadi et al., \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e) \u0026delta;\u003csup\u003e2\u003c/sup\u003eH accounts for 86\u0026permil; and \u0026delta;\u003csup\u003e18\u003c/sup\u003eO accounts for 12\u0026permil; while \u0026delta;\u003csup\u003e2\u003c/sup\u003eH-\u0026delta;\u003csup\u003e18\u003c/sup\u003eO coordinates fall near the standard precipitation line, which indicates that atmospheric precipitation is the main source of local recharge (Shepherd, \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e). The spring age calculated using radium radon method is 12 years, indicating that the circulation of geothermal water in this area is fast (Xufei et al., \u003cspan class=\"CitationRef\"\u003e2024\u003c/span\u003e). It can be therefore inferred that the atmospheric precipitation in this area mainly infiltrates downward along the fault-related water diversion channels.\u003c/p\u003e\n\u003cp\u003eTherefore, the vertical hydraulic connectivity of groundwater in the region is not obvious. The recharge of groundwater is primarily from the lateral recharge through secondary faults while the vertical infiltration is limited. The fate of these recharge water is the deep thermal reservoir, where the geothermal fluid are formed are stored and heated. The main fault F2 and the secondary fault F54 in the area collect dispersed water and channel it into the fracture zone along the fault (Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e). The flow absorbs the convective conduction heat accumulation of the surrounding rock under the regional geothermal gradient (Ilani et al., \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e) during its movement in the fault channel. In the closed or semi-closed space formed by the weakly permeable upper rock layer, the increase of temperature enhances the pressure of hot water and steam, thus making it possible to surge up through cracks (such as F6 and F58 in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e) (Lambrakis et al., \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e). The rising hot water and the infiltrating cold water circulate repeatedly due to their density differences and the convention subsequently formed. When the resistance of the open fissures is low, the water rises out of the surface and forms hot springs (Yongping, \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eIn summary, the formation of geothermal system is controlled by neotectonic faults, in which NNE and NEE faults serve as the main thermal conduction faults, and NNW and NWW faults facilitate water conduction. The main source of hot water in this area is the atmospheric precipitation, which penetrate into the thermal reservoir through fracture zone (Craig, \u003cspan class=\"CitationRef\"\u003e1963\u003c/span\u003e). During the migration process, the hot water is heated by normal earth heat flow blanket conduction and heat accumulation. The geothermal field forms at the confluence of the two groups of main faults. This confluence of neotectonic faults located in the groundwater discharge area is the priority target of geothermal resources exploration.\u003c/p\u003e\n\u003cp\u003eThe formation mechanism of the Tangquan geothermal spring is multifaceted. The surface source is dominated by normal earth heat flow blanket conduction and heat accumulation. The deep thermal conduction fault and the contact zone between intrusive rock mass and soluble rock accept convective conduction heat accumulation; in addition, Tangquan\u0026apos;s deep circulation flow system is different from the general free convection hydrothermal system caused by temperature difference, which belongs to forced convection system. Because the strata in this area are mainly rock masses with poor permeability, water is mainly guided by fissures and fracture zones, and the geothermal water circulation system is formed under the action of topographic height difference and corresponding water head difference (Delvaux et al., \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e; Lelli et al., \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). Faults are developed in this area, and some faults can reach the depth of crystalline basement, which can be used as a good channel for rapid heat transfer in the lower part, and the discharge or recharge of groundwater can cause high or low regional geotemperature field. After the infiltration of atmospheric precipitation, it seeps along the water diversion fault, and receives conduction-convection heat accumulation on the way after entering the deep circulation runoff, the temperature increases, and the hot water rises to the surface discharge area along the fault zone, forming hot springs.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThrough geophysical exploration methods such as MT and CSAMT, we effectively detected the stratigraphic properties as well as faults, folds, and other structural features in the Tangquan research area; by conducting geothermal measurements and constructing a geothermal geological model in the Tangquan area, we identified the heat source of the Tangquan geothermal anomaly zone; isotopic methods were used to ascertain the water source of the Tangquan area; finally, a comprehensive analysis of the heat source, water source, and structural aspects of the Tangquan area revealed the geothermal genesis of the region. Based on the analysis of the cause of formation of thermal reservoir in Tangquan area, the convective conduction heat accumulation mechanism of deep circulation flow system controlled by fault water conduction and thermal conductivity is revealed, which provides a reference for the further development and utilization of geothermal resources in Tangquan and even the vast geothermal anomaly areas, it provides a new and more comprehensive understanding of the cause of geothermal water.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eMT \u0026nbsp; \u0026nbsp; \u0026nbsp; Magnetotelluric Sounding\u003c/p\u003e\n\u003cp\u003eCSAMT \u0026nbsp; \u0026nbsp;Contrelled-Source Audio-frequency Magnetotelluric\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank the anonymous reviewers for their constructive criticism and useful suggestions, which helped to improve the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eW.Y.,Y.Z.,D.W. conceived the research, prepared the data, prepared the fgures, and wrote the paper. W.Y. conducted the formal analysis. S.Z., S.S., W.Y., G.Z., Y.Z., X.G., B.Y. and G.W. analyzed the results and drafted the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was jointly supported by the National Science Foundation of China (grant numbers: 42102289), Geological Survey Project Foundation (grant numbers: DD20243504), Shandong Provincial Natural Science Foundation (grant numbers: ZR2020QD122, ZR2022QD060).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data supporting the fndings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eArn\u0026oacute;rsson, S., Stef\u0026aacute;nsson, A. and Bjarnason, J.n.O.r. 2007. 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Understanding the formation mechanism of geothermal systems is a prerequisite for effective exploitation. The formation of geothermal systems is influenced by various factors such as tectonic characteristics, heat sources, and water circulation, making it necessary to comprehensively analysis its mechanism by integrating these factors. Tangquan ,a natural geothermal spring in Zhangjiakou City, China, has great potential in geothermal exploitation. However, previous studies have offered incomplete explanations of its genesis. Therefore, this study investigated the stratigraphic properties and structural distribution of the study area through geophysical exploration surveys, mapped the distribution of the low-temperature field using geothermal geological modeling methods and inferred that the heat source in the study area is radioactive; and through isotope analysis methods, clarified that the water source in the study area is atmospheric precipitation. By integrating these information, the formation mechanism of the Tangquan, a deep circulation type geothermal system controlled by faults, is revealed. These results can guide the further development and utilization of Tangquan, while also enhancing the understanding of fault-controlled and deep circulation type geothermal systems.\u003c/p\u003e","manuscriptTitle":"Fomation mechanism of fault-controlled circulation geothermal spring: evidence from tectonics, thermal model, and isotopes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-19 09:05:06","doi":"10.21203/rs.3.rs-5583602/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"29c01c78-fbe5-4b92-97ed-d1b816b97090","owner":[],"postedDate":"December 19th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-02-27T23:53:23+00:00","versionOfRecord":[],"versionCreatedAt":"2024-12-19 09:05:06","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5583602","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5583602","identity":"rs-5583602","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}