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Fluid Geochemistry of Fault Zone Hydrothermal System in the Yidun-Litang Area, Eastern Tibetan Plateau Geothermal Belt
The geochemical and geothermal characteristics of hydrothermal systems in an area are useful information to appropriately evaluate the geothermal potential. In this paper, we investigated the chemical and isotopic composition of thermal water in an underexploited geothermal belt, Yidun-Litang area, in eastern Tibetan Plateau. 24 hot spring samples from the Yidun and Litang area were collected and analyzed. The water chemical types of the hot springs are mainly Na-HCO3-type water. Water-rock interaction and cation exchange and mixture are the dominant hydrogeochemical processes in the hydrothermal evolution. The significant shift of D and 18O isotopes from the GMWL indicates that these springs have undergone subsurface boiling before rising to the surface. Different ratios of Cl to other conservation species can be found for the springs in Litang and Yidun areas, suggesting the different heat sources of the two hydrothermal systems. The reservoir temperature in the Yidun area is around 230°C while the reservoir temperature in the Litang area is around 200°C. Both hydrothermal systems are recharged by the meteoric water and are heated by the different deep, thermally and topographically driven convection heat along faults and undergoing subsurface boiling before going back to the surface.
As one source of clean and renewable energy, geothermal energy has been developed worldwide. Investigating and evaluating the potential of geothermal fields is an important process prior to geothermal exploitation. Hydrogeochemical and isotopic information can be effective for identifying the source of heat, circulation process, and hydrochemical evolution [1–5]. Understanding the hydrochemical characteristics and geochemical evolution of thermal waters will provide useful information in protecting and developing these resources . As an important part of the eastern Tibetan plateau geothermal belt, fumaroles, geysers, and boiling springs with many other hot springs occurred in the Yidun-Litang area. Due to the high altitude, complex terrain, and low density of population, geothermal energy in this area has not been well exploited. The only utilization of this geothermal energy is for bathing in some areas . Although some basic physical and chemical properties of the hot springs have been investigated by a few studies [8, 9], little has been done in detail describing the geochemical characteristic and reservoir temperature of these geothermal fields. Such studies, however, are important to understanding the genesis and evolution of the geothermal field and also providing insight views for evaluating the geothermal potential.
On the other hand, hot springs occur most commonly at the fault zones and their evolution depends on the interaction between heat sources, circulating fluids, and permeable pathways , and they tend to occur in high-permeability areas of fault zone and have a close relationship with seismic activities [11, 12]. Several previous studies also have documented that the chemical components in the hot springs are influenced by tectonic activity, especially by earthquakes [13–15]. Therefore, hot springs in the fault zone provide a unique opportunity for studying the interaction between tectonic activities and the hydrological systems. In addition to the hydrothermal activity, the Yidun-Litang area has a high level of seismic activity, as demonstrated by the Litang Ms 5.1 earthquake on September 23, 2016 , and several large (7.0 < M < 7.5) earthquakes since 1700 .
In this study, we collected water samples from 24 hot springs located in the Yidun-Litang area, most of which were taken from locations coinciding with mapped fault zones. Hydrochemical evolution, groundwater circulation, and the reservoir temperature of these springs are discussed. The study will provide useful information for appropriately evaluating geothermal potential in this area and the thermal evolution process of the eastern Tibetan Plateau as well.
2. Geological Setting
The study is located between the Songpan-Ganzi fold belt and the Qiangtang Block, with the Ganzi-Litang suture to its east and the Jinsa suture to its west, with an average elevation of 3000~4000 m . The area is covered by widespread Triassic limestone, with multistage magmatic activity. Early magmatic rocks are primarily diorites, and late magmatic rocks are primarily granites. The main eruption activity was from the Variscan to Indosinian, while intrusions occurred from the Yanshanian to Himalayan. The influence of magmatic activity on the present geothermal activity is primarily related to age and scale, which means that less heat is retained by the older intruded or ejected magma and magma intrusions will dissipate heat faster with samller volume. Generally, residual heat caused by some small-scale magmatic activity prior to the Quaternary has disappeared. The Triassic is the most widespread strata in this area (Figure 1); Devonian, Permian, and Cenozoic strata are sporadic. Limestone, sandstone, granite, siltstone, slate, and phyllite are the major lithologies that are exposed in the study area.
Fault systems are well developed in this area with frequent earthquakes. The Litang fault zone and the Batang fault zone are the two major fault zones in the area (Figure 1). The Batang fault is also a dextral fault, which strikes N30°E, dipping northwest with a steeply dipping angle, with a total length of about 200 km. The Litang fault zone comprises four subfaults: Cuopu fault, Maoya fault, Litang fault, and Dewu fault. All of the four subfaults are dominated by a left-lateral strike-slip with a reverse dip-slip component on different segments . Three large earthquakes with magnitude > 7.0 had occurred since 1700, and small to moderate seismic events occurred frequently in this area . Also, fault scarps, triangular facets, beheaded channels, and shutter ridges are widely observed in the Litang fault zone. Hot springs with temperature ranges from 25 to 89°C are distributed along the Litang fault zone and the intersection area of the Litang and Batang fault zones (Figure 1). Hot springs are mainly concentrated in Chaluo, Qukailong, and Maoya areas, especially high-temperature springs with temperature higher than 80°C which are found in Chaluo and Qukailong geothermal fields.
3. Sampling and Analysis
24 water samples were collected from hot springs during July 2016 in the Yidun-Litang area: 14 samples from the Chaluo area, 6 samples from the Maoya area, and 4 samples from the Qukailong area (Table 1). Temperature, pH, ORP, DO, and EC were measured on-site by a Clean M30 pen-type tester. At each site, we collect two 250 mL water samples for water chemical analysis and 50 mL for stable isotope analysis. Also, we collect 17 samples for tritium analysis, each with 500 mL volume, and stored in polyethylene bottles (Table 1). All the water chemical samples were filtered through 0.45 μm membranes on-site. For cation analysis, reagent-grade HNO3 with a molar concentration up to 14 M was added to the samples at each site to bring the pH below 1. The major ions were analyzed by means of ion chromatography (Dionex-900). Stable isotopes of oxygen-18 and deuterium were analyzed with the liquid-water isotope analyzer LGR, with accuracy of 0.2‰ for δ18O and 0.3‰ for δD. The CO32− and HCO3− concentrations were measured by a potentiometric titrator. The SO42− and Cl− concentrations were determined on an unacidified sample by ion chromatography. The Ca2+, Mg2+, Na+, and K+ concentrations were analyzed by ICP-AES, the other metal elements by ICP-MS within 2 weeks after sampling. All of the chemical analyses were completed at the Sinomine Rock & Mineral Analysis Co. Ltd. The hydrochemistry of all water samples is summarized in Table 1. The tritium analysis of the samples was measured by an ultra-low background liquid scintillation mass spectrometer at the Key Laboratory of Groundwater Sciences and Engineering, Ministry of Land and Resources. The result of all the water samples is listed in Table 1.
4.1. General Hydrogeochemistry
The water chemistry of the 24 hot springs is presented with a Piper diagram, where HCO3-Na-type water dominated in these springs (Figure 2). Only number 6 and number 7 springs show Na-SO4-HCO3 type. The pH ranges from 6.6 to 9.8 with TDS ranging from 0.29 mg/L to 1.9 mg/L. The springs that show a higher TDS (>1.0 g/L) are located in the Maoya and Qukailong geothermal fields, whereas other springs in the Chaluo geothermal field exhibit low TDS (<1.0 g/L). High-temperature springs (T > 80°C) can be found in Chaluo and Qukailong geothermal fields. From the Stiff plot (Figure 1), we can find that Na+ is the predominant cation, ranging from 127.8 to 758.0 mg/L. The HCO3− is the predominant anion, ranging from 176.47 to 2123.0 mg/L; Cl− and SO42− are the second highest anions ranging from 8.79 to 74 and 5.04 to 208.4 mg/L, respectively. Li, B, Rb, and Cs can also be found from dozens to thousands of μg/L. HCO3− concentrations are high in the Litang area and low in Chaluo geothermal fields (Figure 1).
4.2. Isotope Composition
The stable oxygen isotope composition of the hot spring water samples ranges from −20.74‰ to −17.25‰ and deuterium from −161.99‰ to −148.91‰ (Table 1). All the hot springs (except for the number 10 spring) are falling below and away from the GMWL, and the springs in the Litang area show more negative δD and δ18O than those in the Yidun area (Figure 3). For the surface water collected from nearby, the δ18O ranges from −18.52‰ to −15.52‰, with δD ranging from −139.08‰ to −123.78‰.
Tritium values of thermal water vary from <1 TU to 2.4 ± 1.2 TU. Tritium concentration can be used to qualitatively determine whether groundwater is modernly recharged or not (Clark and Fritz, 1997). Tritium concentrations below 1 TU indicate that groundwater is at least 50 years old (premodern) and tritium values equal to or greater than 1 TU are considered as modern groundwater. Values of tritium of about 3 TU indicate a residence time of the water of about 30–40 years. The 3H concentration ranging from 1 to 8 TU could be attributed to an admixture of recent water with old groundwater and groundwater having been subjected to radioactive decay [5, 19]. The tritium concentrations are less than 1 in number 1, number 4, number 8, number 2, number 13, number 12, number 21, number 15, and number 11 springs and are larger than 1 in the number 10 spring (Table 1).
5.1. Hydrochemistry Evolution
According to the relationship between (Na + K)-Cl and (Ca + Mg)-(SO4 + HCO3) (Figure 4), the springs in the Qukailong and Maoya geothermal fields are falling on the y = −x line, indicating that cation exchange is an important process that controls hydrochemical features in these two areas. For the springs in the Yidun area, most of them are located a little above the y = −x line, indicating the possible dissolution of albite origin from granite or diorites, together with cation exchange between Ca + Mg and Na, which are the dominant processes in the hydrochemical evolution. The high concentration of HCO3 in hot springs of the Litang area may indicate a strong water-rock interaction with the limestone strata of the hydrothermal system when rising for discharge.
As the most conservative element in geothermal water, Cl is an important diagnostic solute and is frequently used in ratios with other elements in the interpretation of water chemistry. For the springs in the present study, we plot the ratios of Cl/B, Cl/F, Cl/SiO2, Cl/Li, Cl/Rb, and Cl/Cs. A linear relationship could only be found in the relationship of Cl/Cs and Cl/Li for all the hot springs; others show a distinctly different linear relationship for the springs in Yidun and Litang areas (Figure 5). Furthermore, Na, K, HCO3, and SO4 behave differently from Li, Rb, Cs, and B, and no linear relationship can be found with Cl (Figure 6). The ratios of Cl/B, Cl/Li, Cl/Rb, Cl/Cs, and Cl/F are widely used to indicate a common reservoir source for water by the same ratios from different samples . We calculated the ratios of Cl/B, Cl/Li, Cl/Rb, Cl/Cs, Cl/F, and Cl/SiO2 for all of the 24 samples (Table 2). Most of the ratios in the Yidun and Litang areas show distinctly different values (i.e., Cl/B is around 40.12 (exclude anomalously high value) in the Chaluo area and 10.11 in the Litang area; Cl/F is around 2.4 in Chaluo and around 11.25 in the Litang area). Only the ration of Cl/Cs shows similarity for both areas; such similarity may be caused by the similar host rocks during the geothermal fluid rising to the surface as Cs may be incorporated into secondary, alternated mineral during migration to the surface. Thus, we infer that the hot springs in the two areas may derive from different heat sources.
5.2. Trace Element Geochemistry
Lithium (Li), rubidium (Rb), and cesium (Cs) generally act as less reactive and conservative elements in thermal waters and help to identify the existence of a common origin or of common deep processes shaping the composition of the surface water discharges . The relative contents of Li, Rb, and Cs are plotted in the ternary plot (Figure 7). All the samples are falling close to the corner of Cs, indicating that all the thermal water discharges fall in the upflow region (low Li/Cs), but with different concentrations of the two areas. Furthermore, the data points of these hot springs are far removed from the compositional area of crustal rocks, pointing toward the existence of the secondary processes during the upflow process.
Li, as a good tracer for initial deep rocks, is used to evaluate the possible origin of boron and chloride (another two conservative constituents in hot water) . In the Cl-Li-B ternary plot, most of the hot springs fall close to the compositional area of the crustal rocks, indicating that rock leaching is the main contributing factor for boron and lithium concentrations in these thermal waters (Figure 8). And it can be concluded that there is no absorption of magmatic vapour (low B/Cl or high B/Cl steam); thus, these hydrothermal systems have no association with any volcanic/magmatic activity. For the hot springs in Chaluo and Litang areas, although the concentrations of these elements are located in similar locations in Li-Rb-Cs and Cl-Li-B ternary plots, two distinct clusters could be found (Figures 7 and 8), again supporting that the hot springs of the two areas originate from different sources.
5.3. Origin of the Water by O and H Isotope
Although the hydrogen and oxygen stable isotope data of the hot springs show a meteoric source of groundwater, they do not originate from the local surface water as the stable isotope values are more negative than those of the local surface water (Figure 3) and may be recharged from the high mountains. The recharge location of these hot springs can be roughly estimated by using the isotope altitude effect . Because of the significant 18O shift and water-rock interaction in the hydrothermal systems, we choose deuterium as a trace to estimate the recharge altitude. Taking the altitude effect of −2.6‰/100 m for the deuterium isotope , we obtained the approximate recharge locations of number 5, number 12, number 16, and number 22 spring as 4300 m, 4640 m, 4750 m, and 5200 m, respectively. Thus, the recharge altitude of the hydrothermal system in the Yidun area (number 5, number 12) is lower than that of the Litang area (number 16, number 22). Both of them received recharges from the different high mountains nearby.
Furthermore, a good linear relationship of δD and δ18O can be both found for hot springs in Yidun and Litang areas (with square correlation coefficients of 0.602 and 0.697, resp.). Such good linear relationship is likely caused by subsurface boiling when the high-temperature geothermal waters uprising towards the surface exceed the hydrostatic burden; the slope of such process is commonly slight . Thus, the supply of hot water is primarily dependent on atmospheric precipitation and surface water infiltration along the fault zone; after being heated by the deep heat source, they return to the surface along different subfaults with different geochemical processes. The different isotope shift of springs in the Chaluo and Litang areas indicates the different subsurface properties of the reservoirs in the two areas.
The time that meteoric water spends in the geothermal system being heated, reacting with rocks, stored in the reservoir, and finally discharged at the surface can be estimated from the radioactive isotopes . Although tritium can only be used for waters with short residence time, it is still insightful to estimate the geothermal fluid ages and identify the possible geothermal processes during the geothermal fluid evolution. In our samples, only the number 10 hot spring shows a tritium value larger than 1 (2.4 ± 1.4 TU), indicating that the number 10 spring is mixing with more modern water (shallow cold water) recharge and has a younger age than the other springs. Although no tritium is measured in the number 11 well, a similar low Li/Cl ratio with the number 10 well also indicates the mixing of shallow meteoric water (Table 2).
5.4. Geothermal Characteristic
Subsurface reservoir temperature is one of the most important characteristics in assessing the potential of geothermal resources. Several chemical geothermometers based on the chemical composition of thermal water can be used in calculating the reservoir temperature . A temperature equation for a geothermometer is a temperature equation for a specific equilibrium constant referring to a specific mineral-solution reaction under specific conditions [26, 27]. The silica and cationic geothermometers (Na-K, Na-K-Ca, and K-Mg) are the most commonly applied . Silica-quartz/chalcedony-based geothermometers are widely applied to calculate the temperature of low enthalpy reservoirs . The Na-K geothermometer is mostly applicable to reservoir temperatures above 150°C and may yield erroneous values for low-temperature waters , while the Na-K-Ca geothermometer is more suitable for waters with high Ca contents . Thus, different chemical geothermometers applied to the same geothermal fluid yield appreciably different subsurface temperatures due to the lack of attainment of equilibrium between fluid and hydrothermal minerals or as a result of mixing with shallow groundwater or degassing during upflow , which can have important effects on geothermometry calculations—especially for silica geothermometers.
The reservoir temperatures estimated by chalcedony, quartz, Na-K-Ca, Na-K-Mg, Na-K, K-Mg, and Na-Li geothermometers are listed in Table 3. All the calculated reservoir temperatures are higher than the discharge temperature. Although the reservoir temperature varies with the different geothermometers, the estimated subsurface reservoir temperatures in the Yidun area (number 1~number 14 spring) are generally higher than that of the Litang area (number 15~number 24 spring) (Table 3). The silica geothermometers produced similar reservoir temperatures with the cation geothermometers in the Chaluo geothermal field (number 1~number 9). The low reservoir temperature in number 10 spring may be caused by the mixing of shallow cold water or other shallow surface processes as it shows local shallow water characteristics. For the other springs, the estimated reservoir temperatures by silica geothermometers and K-Mg geothermometers are lower than by other cation geothermometers. The good linear relationship between Cl and the conservation species indicate the mixing processes and dilution for these hot springs (Figure 5); thus, it is expected that the silica geothermometers and K-Mg geothermometers will show lower reservoir temperature than the other cation geothermometers. The Na-K-Mg ternary diagram shows that these hot springs are located on the isothermal line of 220~240°C and shows a linear trend from full equilibrium to disequilibrium water area, especially for the hot springs in the Yidun area (Figure 9). Hot springs located in the Chaluo geothermal field are mostly located in the partial equilibrium area while other springs are located in the disequilibrium water area (Figure 9) . Thus, the K-Mg geothermometer and silica geothermometer may not be appropriate for the reservoir temperature calculation as both will be largely affected by the dilution processes. The 10/(10 K + Na) versus 10 Mg/(10 Mg + Ca) diagram proposed by Giggenbach  (Figure 10) also shows the deviation of hot springs from the full equilibrium line, and the high Ca + Mg concentration in the Litang area (numbers 15–24) may indicate the faster equilibration of K-Mg and mixing with Ca-Mg-rich shallow waters . This also explains why the reservoir temperatures calculated by the silica and K-Mg geothermometer are much lower than the other cation geothermometer in the Litang area. Thus, the Na-K and Na-Li geothermometers tend to provide more reliable reservoir temperatures since they are less influenced by SiO2 reequilibration and mixing with Ca-Mg-rich shallow water than the other geothermometers in the Litang area.
According to the above discussion, hot springs in the Litang area are located in a nonequilibrium state and may be affected by the mixing (dilution) processes, which is also indicated by the good linear relationship of B/Cl, F/Cl, and SiO2/Cl in Maoya and Qukailong (Figure 6). Here, we use the silica-enthalpy method  to evaluate the temperature of the hot water component before mixing. The temperature of cold water is assumed as 15°C and SiO2 concentration of 10 mg/L; we use the average spring temperature of 50°C and SiO2 of 51 mg/L for the springs in the Maoya geothermal field. The calculated SiO2 concentration in the reservoir is about 200 mg/L, with temperature of 207°C. And a similar result could be obtained for the hot springs in the QKL geothermal field. The mixing ratio in this area is 0.6~0.74 for cold water. Thus, we can roughly get the conclusion that the reservoir temperature in the Yidun area is around 230°C and is around 200°C in the Litang area. Thus, the reservoir temperature also supports that the Yidun area and the Litang area should have different heat sources.
From the chemical and geothermal analysis, we propose a conceptual model for the genesis mechanism of the hydrothermal system in the Yidun and Litang areas. Meteoric precipitation penetrates into the crust, is heated by the different deep, thermally and topographically driven convection along faults, and undergoes subsurface boiling before going back to the surface. The same chemistries in the two areas may be caused by the same host rock in the different fault zones. Although the measured age of the magmatic rock revealed that the magmatic residual heat has disappeared , the residual radioactive decay in rock will continue to heat the water during the circulation .
The geochemical and geothermal characteristics of the fault zone hydrothermal system in the Yidun-Litang area are investigated in this paper. Na-HCO3 water is the dominant water type in the Yidun-Litang area. Water-rock interaction and cation exchange are the dominant hydrogeochemical processes in the hydrothermal evolution. The D and 18O isotopes of all the hot springs show a linear shift from the GMWL, indicating that these springs have undergone subsurface boiling before rising to the surface. Different ratios of Cl to other conservation species can be found for the springs in the Litang and Yidun areas, suggesting the different heat sources of the two hydrothermal systems. The reservoir temperature in the Yidun area is around 230°C while the reservoir temperature in the Litang area is around 200°C. Both hydrothermal systems are recharged by the meteoric water and are heated by the different deep, thermally and topographically driven convection heat along faults and undergoing subsurface boiling before going back to the surface.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors greatly thank Xianhe Yang and Haijiang Niu for their help in the field. This work is supported by the National Natural Science Foundation of China (41602266 and U1602233), the Fok Ying Tung Education Foundation (161014) and partly support from the Engineering Research Center of Geothermal Resources Development Technology and Equipment, Ministry of Education, Jilin University.
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