Silica-enthalpy Mixing Models Research Articles

Multi-isotopes (H, O, Sr, and Li) and element geochemistry constrain the formation of Kongchutso helium-rich geothermal field in western Tibet, China

The Kongchutso geothermal field hosts helium-rich hot springs with helium contents (up to 1.54 vol.%) higher than industrial grade of He resources (0.05 – 0.1 vol.%). However, the geochemical signature of helium-rich geothermal fluids, reservoir feature, and their genesis remain poorly understood. Here, we present element geochemistry (major and trace elements) and multi-isotopes (H, O, Sr, and Li) compositions of hot and cold springs, lake, calcareous sinters, and rocks from Kongchutso to determine the formation mechanism of helium-rich geothermal system. The major elements studies of water samples show that most helium-rich hot spring waters belong to the Na-HCO3-Cl type, possibly affected by water-rock interaction in the deep and mixing of geothermal fluids with groundwater in the shallow. Based on chalcedony geothermometers and silica enthalpy mixing model, the reservoir temperature has been calculated to be 106 – 118 °C. The H and O isotopic compositions of water samples manifest that the Kongchutso geothermal system was recharged by snow-melt water and meteoric precipitation from the surrounding mountains at an elevation of 5366 m. The widespread Cretaceous granites in the region featured by high enrichment of uranium (up to 34.15 ppm) and thorium (up to 37.49 ppm) are considered as helium and heat sources, and also are reservoir rocks. The interpretation is supported by consistent signatures of rare earth elements and Sr-Li isotopes for waters, calcareous sinters, and granites. Therefore, we suggest that the helium was initially generated via radioactive decay by U-Th in granites and then dissolved in the deep circulated and heated meteoric waters, and finally ascended to surface through the main fault systems to form the helium-rich hot springs at Kongchutso. Some other hydrothermal manifestations, such as calcareous sinters, formed accompanying the fluids migrated to the surface. Based on these new findings, we further proposed that high radioactive granite, together with tectonic system and surface hydrothermal manifestations can be used as indicators for exploration of helium-rich geothermal resources in Tibet and elsewhere in the world.

Hydrogeochemical Study of Hot Springs along the Tingri—Nyima Rift: Relationship between Fluids and Earthquakes

Studying the hydrogeochemical characteristics of hot springs provides essential geochemical information for monitoring earthquake precursors and understanding the relationship between fluids, fractures, and earthquakes. This paper investigates the hydrogeochemical characteristics of hot springs along the Tingri–Nyima Rift (TNR) in southern Tibet, a seismically active zone at the collision front of the Indian and Asian-European plates. The major elements, hydrogen, and oxygen isotopes of seven thermal springs were analyzed from July 2019 to September 2021. The findings indicate that Mount Everest’s meteoric water, which has a recharge elevation of roughly 7.5–8.4 km, is the main source of recharge for the hot springs. The water samples have two main hydrochemical types: HCO3-Na and Cl-Na. The temperature of the geothermal reservoir is between 46.5 and 225.4 °C, while the circulation depth is between 1.2 and 5.0 km based on silica-enthalpy mixing models and traditional geothermometers. Furthermore, continuous measurements of major anions and cations at the Yundong Spring (T06) near Mount Everest reveal short-term (8 days) seismic precursor anomalies of hydrochemical compositions before an ML4.7 earthquake 64.36 km away from T06. Our study suggests that seismicity in the northern section of the TNR is controlled by both hydrothermal activity and tectonic activity, while seismicity in the southern section is mainly influenced by tectonic activity. In addition to magnitude and distance from the epicenter, geological forces from deep, large fissures also affect how hot springs react to seismic occurrences. A fluid circulation model is established in order to explain the process of groundwater circulation migration. The continuous hydrochemical monitoring of hot springs near Everest is critical for studying the coupling between hot springs, fractures, and earthquakes, as well as monitoring information on earthquake precursory anomalies near Everest.

Hydrogeochemical origin and circulation of spring waters along the Karakorum fault, Western Tibetan Plateau: Implications for interaction between hydrosphere and lithosphere

Geochemical investigation on the origin and circulation of geothermal water is crucial for better understanding the interaction between hydrosphere and lithosphere. Previous studies on the Himalayan geothermal belt mainly distributed in the central and eastern Tibetan Plateau. In this study, water samples (8 hot springs and 1 cold spring) from the Karakorum fault (KKF) zone of western Tibetan Plateau were analyzed for the hydrogeochemical characteristics and isotopic compositions. Three types of spring water along the KKF were classified on basis of ionic concentration and Sr isotopic composition: type A water (HCO3–Mg or Ca), type B water (HCO3–Na) and type C water (Cl–Na). Type A water is originated from the infiltration of meteoric water and the dissolution of silicate/evaporite. Type B water is mainly leached from the metamorphic and granitoid rocks. Type C water is formed by the dissolution of chlorides and sulphates. δD and δ18O isotopes indicate that geothermal fluid along the fault zone was mainly recharged by local precipitation. Moreover, reservoir temperatures of 144.2–208.6°C were estimated by the silica–enthalpy mixing model, and the thermal waters have a relatively deep circulation depth (≥ 7.0 km). Meanwhile, the thermal waters are characterized by extremely high Li, B, Fe and As concentrations and earthquakes frequently happened in the vicinity, suggesting that the KKF is a deep and active fault, which also indicates that the thermal fluids are strongly associated with seismicity. Therefore, thermal fluid can potentially be used as continuous monitoring sites for earthquake forecasting.

Temperature Estimation of a Deep Geothermal Reservoir Based on Multiple Methods: A Case Study in Southeastern China

Estimating deep geothermal reservoir temperatures is an essential mission of geothermal exploration and development. The thermal reservoir temperature estimated directly using geothermometry without comparative analysis is often far from the actual temperature. In this paper, taking the typical geothermal systems in the Xiamen Island–Zhangzhou area of southeastern China as an example, different methods such as a water–rock equilibrium analysis, SiO2 geothermometer, multi-mineral equilibrium diagram, and silica-enthalpy mixing model are used to make a quantitative and qualitative analysis of the chemical equilibrium of minerals and fluids in the geothermal system. Finally, the applicability of different methods was compared and analyzed, and the geothermal reservoir temperature was estimated using the appropriate method. The results show that the calculated results of the Si-enthalpy mixing model of a typical geothermal system in southeastern China are significantly high. At the same time, the SiO2 geothermometer (without vapor loss), which is closest to the results of the multi-mineral equilibrium diagram, was chosen as the geothermal reservoir temperature of the geothermal system in the study area. This study can provide a reference for the future selection of methods of deep geothermal reservoir temperature estimation in similar areas.

Hydrogeochemical characteristics and evaluation of the geothermal fluids in the Gazlıgöl geothermal field (Afyonkarahisar), Western Anatolia, Turkey

The Gazlıgöl geothermal field located in 20 km northern of Afyonkarahisar in Western Anatolia region of Turkey is one of the most important geothermal areas in the Afyon-Akşehir Graben on the basis of potential. The present study has been performed to investigate the hydrogeochemical and isotopic characteristics and evaluate the origin, reservoir temperature and processes controlling the chemical composition of thermal waters of the Gazlıgöl geothermal field. Discharge temperature, electrical conductivity and pH of the thermal waters vary from 58.5 to 74 °C, 3910 µS/cm to 4050 µS/cm and, 7.05 to 7.54, respectively. The Gazlıgöl thermal waters are chemically of Na-HCO3 type, with high salinity, while the cold waters have mainly Ca-Mg-HCO3, Ca-Na-HCO3 and Ca-Mg-HCO3-SO4 types, with low salinity. Geochemical processes controlling chemical composition of the thermal waters are mainly water-rock interactions including dissolution and/or weathering of mostly silicates and partially carbonates and ion-exchange reactions. Besides, the bacterial sulfate reduction is another main process leading to the depletion of SO4 in the thermal water. Higher contents of some minor elements in the thermal waters, such as F, B, Li, As and Sr, are probably derived from enhanced water-rock interaction, and the minor elements can be regarded as indicator of residence times and flow paths.Geothermometer applications including quartz geothermometers and SiO2-K2/Mg, mineral saturation state and silica enthalpy mixing models provide the most reliable estimations of reservoir temperature in the varying of 100-150 °C for the Gazlıgöl thermal waters. Accordingly, the Gazlıgöl geothermal system can be classified as a low-moderate geothermal resource. The isotope data (δ18O, δ2H and 3H) of the thermal waters point to their deep circulating meteoric origin, and let estimating of recharge elevation varying from 1300 to 1400 m.a.s.l. These elevations suggest the mountainous region in the north-northeastern of the study area as the recharge area of the geothermal system. Long-term circulation of meteoric waters within the aquifer is confirmed by low tritium ratios (< 1 TU) of the thermal waters although the fluids do not achieve thermodynamic equilibrium. Based on the all gathering data, a conceptual model of the functioning of geothermal system has been constructed. Accordingly, geothermal fluid is heating at deep zones due to the high geothermal gradient caused by active fault systems and heated waters rise toward the shallow levels through faults and fractures. During moving to the surface, they undergo conductive cooling by mixing with the cold groundwater at shallow depths and during contacting with the colder rocks.

Effects of Seawater Recharge on the Formation of Geothermal Resources in Coastal Areas and Their Mechanisms: A Case Study of Xiamen City, Fujian Province, China

The southeast coastal areas of China have abundant geothermal resources. Most especially, seawater-recharged geothermal systems in the coastal areas have large quantiles of recharge but suffer water salinization and low water temperature. Moreover, the geothermal water development in these areas may induce seawater intrusion. Understanding the genetic patterns of geothermal resources is significant for rational exploration and protection. This study analyzed the hydrochemical and environmental isotopic characteristics of geothermal water, groundwater, and surface water samples collected in the area with geothermal resources in Xiamen Province in the southeast coastal areas of China. Based on this, the recharge of geothermal water circulation and the genetic patterns of geothermal resources were revealed. The results of this study indicate that the geothermal water in mountainous areas and piedmonts in Xiamen is mainly recharged by rainfall infiltration. In contrast, the geothermal water in coastal areas in Xiamen is recharged by seawater mixing to different extents, as indicated by hydrochemical types, isotopic characteristics, and the C1-/Br- ratio of geothermal water. As revealed by the calculation results using the Cl−mixing model, 10 of 13 geothermal fields in Xiamen are recharged by seawater mixing, with a mixing ratio of up to 73.20% in the Pubian geothermal field. After being recharged by rainfall in the low mountainous areas, geothermal water migrates toward deep parts along NW-trending faults. Then, it converges with regional NE-trending deep faults to absorb heat conducted from deep parts to form deep geothermal reservoirs. The deep geothermal reservoirs were estimated to be 185–225°C using the silica-enthalpy mixing model. The geothermal water is mixed with cold water or seawater while rising along faults. The temperature of shallow geothermal reservoirs was estimated to be 71–145°C using SiO2geothermometers.

Hydrogeochemistry of Hot Springs and the 2018 Mojiang M 5.9 Earthquake-Related Chemical Changes in the Simao Basin, China

The Simao Basin is characterized by strong tectonic activity and frequent seismicity. This study investigated the hydrochemical characteristics of 21 thermal springs in the Simao Basin from 2018 to 2020. In this study period, the 2018 Mojiang M5.9 earthquake caused several hydrochemical changes. The results indicate that the Simao Basin contained saline spring waters, HCO3−-rich spring waters, and SO42−-rich spring waters. In the study area, the water chemistry types were controlled by stratum lithology. Saline springs flowed through red beds and dissolved large amounts of halite, which is a rich source of Cl−and Na+ ions. In the hot spring waters, Ca2+ (Mg2+) and HCO3− were mainly derived from the dissolution of carbonate minerals, gypsum, and anhydrite of Triassic rocks. The higher SO42- content in the hot spring waters was caused by the pyrite present in Ailaoshan metamorphic rocks. The reservoir temperatures (121–289 °C) in the Simao Basin were estimated by the silica-enthalpy mixing model equation and the silica-enthalpy diagram. The hot springs had higher reservoir temperatures (&amp;gt;250 °C) and were mainly located at the edges of the basin. Metamorphic rocks exposed in the region had low permeabilities and these springs was close to nearby deep faults that provided deep heat. In most springs, the concentrations of Ca2+ and HCO3− ions increased obviously before the 2018 Mojiang M5.9 earthquake; however, the concentrations of these ions decreased after the earthquake. The hydrogeochemical variations might be attributed to the vigorous water-rock interactions and the mixing of secondary fluids. The entry of cold shallow groundwater caused changes in the reservoir temperatures of some spring samples.

Hydrogeochemistry and geothermometry of geothermal waters from the Pearl River Delta region, South China

The Pearl River Delta (PRD) region, located in the southern edge of South China Foldbelt, is one of the best known geothermal areas in south China. This study focuses on the hydrogeochemical features and reservoir temperatures of the geothermal waters in the PRD region, which can provide useful information and overall assessments on the geothermal potential. 24 geothermal well waters and 8 hot spring waters were collected from the inland and coastal areas of the PRD region. Hydrochemical and isotopic analysis (δ18O, δ2H, 3H), empirical chemical geothermometer, mineral saturation induce, and silica-enthalpy mixing model have been applied in the PRD thermal waters. The hydrochemical facies of inland geothermal waters are of the bicarbonate-dominated and sulfate-dominated type, and the coastal waters are of chloride-dominated type. Chemical compositions of the geothermal waters reveal strong fluid-rock interactions, the common ion effect, mixture between seawater and thermal waters in coastal waters, and mixing process between cold groundwater and thermal waters in inland waters. Hydrochemical results and enriched heavy isotopes suggest that coastal waters are recharged by local precipitation and seawater. δ18O and δ2H compositions of the inland waters plot along the Local Meteoric Water Line (LMWL), indicating the recharge of local precipitation. The activities of tritium verify that inland and coastal waters have modern meteoric components. Calculations of chalcedony geothermometers, mineral saturation states, and silica-enthalpy models provide the most reliable estimations of reservoir temperature: 90-154 °C for inland thermal waters, 104-156 °C for coastal thermal waters, likely indicating low-moderate temperature geothermal resources in the PRD region.

Geochemical characteristics and genesis of the high-temperature geothermal systems in the north section of the Sanjiang Orogenic belt in southeast Tibetan Plateau

A large number of thermal springs are widely distributed along the deep suture zones and faults in the north section of the Sanjiang Orogenic belt in southeast Tibetan Plateau, connecting the two high-temperature hydrothermal areas in southern Tibet and western Sichuan. Whether the genesis of all geothermal water in the north section of the Sanjiang Orogenic belt is consistent is still an unanswered question. In order to fill the research gap of the high temperature geothermal system in this region, in this study, we classified the geothermal springs into three geothermal fields (Jinsha, Lancang, and Nujiang geothermal fields) and attempted to find clues by comparing the hydrochemical and isotopic characteristics of geothermal fluids in the three geothermal fields. The δ2H and δ18O composition of geothermal springs suggest that all thermal springs are replenished by the meteoric water. Low content of trace elements and low degree of oxygen isotope shift indicate that the partial melting of the crust at a depth of about 20 km mainly provide heat for all thermal springs in the geothermal fields through conduction. But due to the difference of tectonic conditions, there are two formation patterns of geothermal waters in the study area, one was controlled by regional faults, the other was controlled by the folded complex. The geothermal waters controlled by regional faults were heated after deep circulation, and the reservoir temperatures calculated by the Silica-enthalpy mixing model are 175–200 °C and 200–225 °C, respectively. But the geothermal waters controlled by the folded complex were heated after shallow circulation, and the reservoir temperature is only 100–125 °C. Upon rising to the surface, the geothermal waters mixed with cold groundwater to different degrees. The dissolution of Na/K-silicates and carbonate rocks controlled the formation of Na-HCO3 and Ca·Mg-HCO3 type water, respectively. But dissolution of halite and gypsum (or anhydrite) in some areas can lead to further diversity of hydrochemical species, resulting in the formation of Na-Cl, Na-HCO3·Cl, Ca·Mg-SO4 and Na-HCO3.SO4 type waters.

Formation mechanism and mixing behavior of Nanyang thermal spring, Xingshan County of Hubei Province, central China

This study elucidates the hydrochemical characteristics and formation mechanisms of the Nanyang thermal spring, situated in a typical low-medium temperature geothermal field in central China. Based on isotopic and hydrochemistry analyses, the thermal groundwater was found to originate from precipitation via the Shennongjia group of mountains. In addition, hydrochemical features of the thermal groundwater derived from the regional flow system show it is generally of Na-Cl type, with high values of total dissolved solids and minor elements (Sr, F). This is different from the shallow cold water of the local flow system which is characterized by Ca-HCO3 or Ca·Mg-HCO3 type. The Na-K-Mg geothermometer indicates that none of the thermal groundwater reached water–rock equilibrium during pumping, while the mixing model shows that Nanyang thermal spring comprises approximately 84.9% shallow cold groundwater and 15.1% deep thermal groundwater. Furthermore, the reservoir temperature was evaluated by chemical geothermometry and validated by fluid-mineral equilibria calculations. The results show that quartz geothermometers, along with a silica-enthalpy mixing model, can provide a consistent reservoir temperature of 136.4–145.0 °C (average 141.8 °C) considering mixing behavior. Finally, a conceptual model of the formation housing the thermal spring was developed to highlight the origin-source pathway. The conceptual model shows that the thermal groundwater originates from long-distance flow of precipitation recharged via a deep-seated fault and fracture zone (mainly the Jiuchong Fault) from the Shennongjia Mountains, and then flows upwards to the surface, mixing with the local shallow cold groundwater as it encounters an impermeable shale layer.

Hydrogeochemistry of geothermal waters in eastern Turkey: Geochemical and isotopic constraints on water-rock interaction

The hydrogeochemistry of geothermal waters from 31 geothermal fields in eastern Turkey is investigated with regard to major ion compositions, stable (δ18O-δ2H-δ34S) and tritium (3H) isotope systematics. Discharge temperature of studied waters varies from 24 to 65 °C. Four different geochemical processes were found to control the major ion concentrations of waters which include dissolution of carbonates, fluid-mineral interaction, oxidation of sulfur-bearing minerals, and chloride enrichment. The northern, central and southern provinces are represented by different local meteoric water lines (LMWL) with deuterium excess of 15.0, 13.9 and 16.5‰ V-SMOW, respectively. The stable isotope values of the thermal waters are close to LMWLs and indicate a meteoric origin. The enrichment in oxygen isotope composition (0.6 to 7.7‰ V-SMOW) in some thermal waters resulted from water–rock interaction process and 18O-exchange process between CO2 and H2O. The δ34S and δ18O contents in dissolved sulfate cover a wide range from 6.2 to 32‰ V-CDT and from −2.5 to 14.8‰ V-SMOW, respectively, indicating that the sulfate isotope systematics of the majority of waters is governed by dissolution of terrestrial sulfate and marine evaporites. The reservoir temperatures estimated by chemical and isotopic geothermometers of KMg (27–127 °C), silica (29–179 °C) and 18OSO4-H2O (51–196 °C), and by the silica-enthalpy mixing model (130 to 235 °C) yielded inconsistent results. The geological factors (e.g., relatively thick crust, low surface heat flux, absence of ideal cover units) in eastern Turkey have resulted in the development of low- or moderate-temperature geothermal systems.

Hydrochemistry and geothermometry of geothermal water in the central Guanzhong Basin, China: a case study in Xi’an

The Xi’an geothermal field is a typical medium–low-temperature group of geothermal reservoirs in the Guanzhong Basin. Three distinct groundwater systems occur in this field: the cold groundwater (CG) system (quaternary cold-water reservoir with a depth of less than 300 m); the shallow geothermal water (SGW) system (the Quaternary, Zhangjiapo Group, and Lantian-Bahe Group thermal reservoirs with depths ranging between 300 and 2000 m); the deep geothermal water (DGW) system (the Gaoling Group, Bailuyuan Group, and Proterozoic crystalline rock fissure thermal reservoirs, with depths of more than 2000 m). The chemical composition of the CG system consists of HCO3–Ca and HCO3–Ca–Mg water types with a low Total Dissolved Solids’ (TDS) content. The chemical composition of the SGW system consists of SO4–Na and SO4–HCO3–Na water types that have resulted from the partial mixing with water from the CG system through fault structures. Calculations suggest that about 39.4–42.7% of the water in the SGW system was derived from the CG system. The chemical composition of the DGW system consists of SO4–Na and SO4–Cl–Na water types with elevated TDS, F−, SiO2, and high ion concentrations due to the long residence time of water in this largely closed system. The use of geothermometers, multiple mineral equilibrium calculations, and the silica-enthalpy mixing model suggests that temperatures in the SGW and DGW systems lie in the range of 40–85 °C and 94.7–135 °C, respectively. The chalcedony geothermometer is considered to provide the most reliable temperature estimates of the shallow reservoir, whereas the quartzb (maximum steam loss 100 °C) and Na–K geothermometers are considered to provide the most reliable estimates for the deep thermal reservoir. The results of a Q-cluster classification of the water agree well with the field classification of the sampled waters, and the associations of the principal variables obtained by the R-cluster method are consistent with the results obtained by the conventional hydrogeochemical assessment of the samples.

Better constraining silica-enthalpy mixing models in a setting of two separate (karst and non-karst) dilution regimes

It would be possible to elucidate an ambiguity associated with the “silica-enthalpy mixing models” geothermometric technique, provided that an independent estimate was obtained concerning the enthalpy and the dissolved SiO2 content of the hot end-member involved in a mixture between two liquid-phase groundwaters. This would allow one to ascertain whether or not, prior to mixing with the cold parent-water, the hot end-member had undergone a boiling process. The specificities of the Băile Herculane geothermal system (South Carpathians, Romania) provides an opportunity to solve this uncertainty, in contrast to the majority of the so-far investigated natural settings. Advantage has been taken, in this respect, of the fact that a hot brackish Na-Ca-Cl-type parent-water was found to be involved at this site in two separate mixing settings: one which involved only a diluting fluid that was, itself, chloride-rich, and another one which additionally involved, as a third end-member, karst freshwater. By exploiting the two distinct, and converging, linear mixing trends (obtained from the conservatively-behaving natural tracers Cl− and Na+, as well as reasonably tight correlations between the chloride concentration and the contents of both heat and SiO2), the concerned hot brackish end-member contents of heat (enthalpy) and of conservative solutes were quite accurately assessed. This result enabled independent validation of the “no steam loss” hypothesis, out of various alternative scenarios proposed by the “silica-enthalpy mixing models” technique.

Major hydrogeochemical processes controlling the composition of geothermal waters in the Kangding geothermal field, western Sichuan Province

The Kangding geothermal field, located in western China, is a high-temperature geothermal system potentially rich in geothermal resources. In this study, we employed various hydrogeochemical methods to gain more insight into the heat source and cooling processes involved in forming various thermal springs in the geothermal field. The study showed that though the majority of the samples analyzed were immature in terms of mineral-aqueous equilibria, coupling classical geothermometers with the FixAl method enabled more reliable reservoir temperature estimations. A deduction from the silica-carbonate, the chloride-enthalpy, and the silica-enthalpy mixing models indicated that the parent geothermal fluid exists beneath this study area. The thermal waters discharged from the Kangding geothermal field originate in the same deep reservoir; and the parent geothermal fluid has a temperature of about 260 °C, with a Cl− concentration of 1056 mg/L. Isotope (δD and δ18O) studies confirmed the magmatic heat effect on the parent geothermal fluid. Also, though all the thermal spring waters in the field are derived from the parent fluid, they undergo different cooling processes during ascent to the earth's surface. However, the thermal spring waters in both the Yulingong and Erdaoqiao geothermal sites were mainly formed by the mixing of the parent geothermal fluid and infiltrating groundwater. The thermal spring waters in the Erdaoqiao also had the highest levels of Ca2+, Mg2+ and TIC (total inorganic carbon) due to the presence of carbonate in its geologic stratum. The thermal springs at Zhonggu formed as a result of the “CO2 condensate”, consisting of snow-melt water and meteoric water, mixing with the deep parent geothermal fluid. We attribute the absence of acid springs in the Kangding geothermal setting to the deep-seated magma chamber and a relatively small concentration of H2S in the deep thermal waters in the area.

Hydrochemical Characteristics and Evolution of Geothermal Fluids in the Chabu High-Temperature Geothermal System, Southern Tibet

This study defines reasonable reservoir temperatures and cooling processes of subsurface geothermal fluids in the Chabu high-temperature geothermal system. This system lies in the south-central part of the Shenzha-Xietongmen hydrothermal active belt and develops an extensive sinter platform with various and intense hydrothermal manifestations. All the geothermal spring samples collected systematically from the sinter platform are divided into three groups by cluster analysis of major elements. Samples of group 1 and group 3 are distributed in the central part and northern periphery of the sinter platform, respectively, while samples of group 2 are scattered in the transitional zone between groups 1 and 3. The hydrochemical characteristics show that the geothermal waters of the research area have generally mixed with shallow cooler waters in reservoirs. The reasonable reservoir temperatures and the mixing processes of the subsurface geothermal fluids could be speculated by combining the hydrochemical characteristics of geothermal springs, calculated results of the chemical geothermometers, and silica-enthalpy mixing models. Contour maps are applied to measured emerging temperatures, mass flow rates, total dissolved solids of spring samples, and reasonable subsurface temperatures. They indicate that the major cooling processes of the subsurface geothermal fluids gradually transform from adiabatic boiling to conduction from the central part to the peripheral belt. The geothermal reservoir temperatures also show an increasing trend. The point with the highest reservoir temperature (256°C) appears in the east-central part of the research area, which might be the main up-flow zone. The cooling processes of the subsurface geothermal fluids in the research area can be shown on an enthalpy-chloride plot. The deep parent fluid for the Chabu geothermal field has a Cl− concentration of 290 mg/L and an enthalpy of 1550 J/g (with a water temperature of 369°C).

Fluid geochemistry and geothermometry applications of the Kangding high-temperature geothermal system in eastern Himalayas

High-temperature geothermal systems hold an enormous capacity for generating geothermal energy. The Kangding area is a typical high-temperature geothermal field in the Himalayan Geothermal Belt. Hydrogeochemical, gas geochemical and isotopic investigations were performed to identify and qualify the main hydrogeochemical processes affecting thermal water composition, including mixing and degassing, and then to estimate a reliable reservoir temperature. Nine water samples and four geothermal gas samples were collected and analysed for chemical and isotopic components. The results demonstrate the alkaline deep geothermal water is the mixtures of approximately 75% snow-melt water and 25% magmatic water. It is enriched in Na, K, F, Li and other trace elements, indicating the granite reservoir nature. The shallow geothermal water is the mixtures of approximately 30% upward flow of deep geothermal water and 70% meteoric cold water. High concentrations of Ca, Mg and HCO3 indicate the limestone reservoir nature. There is no remarkable oxygen isotope shift in the geothermal water since the rapid circulation is difficult to trigger off strong water-rock interaction. CO2 is the predominant geothermal gas, accounting for more than 97% of total gases in volume percentage. The concentration of CO2 degassing ranged from 0.4 mol L−1 to 0.8 mol L−1 via geothermometrical modelling. As a result, the geothermal water pH increased from 6.0 to 9.0, and approximately 36% of the total SiO2 re-precipitate. The sources of CO2 are the metamorphism of limestone and magmatic degassing based on the composition of carbon isotope. The appropriate geothermometers of Na-K and Na-Li yield reservoir temperature of 280 °C. The geothermometrical modelling, developed to eliminate the effects of CO2 degassing, yields temperature of 250 °C. The silica-enthalpy mixing model yields temperature of 270 °C with no steam separation before mixing.

Estimation of deepwater temperature and hydrogeochemistry of springs in the Takab geothermal field, West Azerbaijan, Iran.

Chemical analyses of water samples from 19 hot and cold springs are used to characterize Takab geothermal field, west of Iran. The springs are divided into two main groups based on temperature, host rock, total dissolved solids (TDS), and major and minor elements. TDS, electrical conductivity (EC), Cl(-), and SO4 (2-) concentrations of hot springs are all higher than in cold springs. Higher TDS in hot springs probably reflect longer circulation and residence time. The high Si, B, and Sr contents in thermal waters are probably the result of extended water-rock interaction and reflect flow paths and residence time. Binary, ternary, and Giggenbach diagrams were used to understand the deeper mixing conditions and locations of springs in the model system. It is believed that the springs are heated either by mixing of deep geothermal fluid with cold groundwater or low conductive heat flow. Mixing ratios are evaluated using Cl, Na, and B concentrations and a mass balance approach. Calculated quartz and chalcedony geothermometer give lower reservoir temperatures than cation geothermometers. The silica-enthalpy mixing model predicts a subsurface reservoir temperature between 62 and 90°C. The δ(18)O and δD (δ(2)H) are used to trace and determine the origin and movement of water. Both hot and cold waters plot close to the local meteoric line, indicating local meteoric origin.

The main chemical properties of hot and cold mineral waters in Bayankhongor, Mongolia

In the current study, hot and cold mineral springs and sub mineral waters in the Bayankhongor province were examined for their chemical characteristics and identified cold mineral waters classification according to mineral water classification of Mongolia. The hot spring waters belong to Na+-HCO3- and Na+-SO42- types. The cold mineral spring of Lkham belongs to Ca2+-HCO3- type. All sub mineral waters are generally located in the two areas (northern part or mountain forest area and the southern part or Gobi desert area). TDS concentrations of cold springs of the southern part in the study area were higher than northern part’s cold springs. The total dissolved silica content of cold spring was ranged from 4.5mg/L to 26 mg/L which did not correspond to requirements of mineral water standard of Mongolia. Thus, these cold springs are belonging to sub mineral water classification. The sub mineral waters were characterized into four types such as a Ca2+-SO42-, Na+-SO42-, Na+-HCO3 and Ca2+ - HCO3 by their chemical composition in the study area. The values for the quartz, chalcedony geothermometer and the Na/K geothermometer were quite different. The silica-enthalpy mixing model predicts a subsurface reservoir temperature between 124 and 197°C and most of the hot waters have been probably mixed with cold water. The result shows that an averaged value of calculated temperature ranges from 77°C to 119°C which indicates that studied area has low temperature geothermal resources. DOI: http://doi.dx.org/10.5564/mjc.v15i0.324 Mongolian Journal of Chemistry 15 (41), 2014, p56-62

Geothermal activities in the Main Ethiopian Rift: Hydrogeochemical characterization of geothermal waters and geothermometry applications (Dofan-Fantale, Gergede-Sodere, Aluto-Langano)

Ethiopia holds an enormous capacity to generate geothermal energy in the volcano-tectonically active zones of the East African Rift System. In this study, we investigate the potential of three geothermal prospect areas in the Main Ethiopian Rift (Dofan-Fantale, Gergede-Sodere, Aluto-Langano). We examine existing and new data from a water sampling campaign, both of which are evaluated in terms of geochemistry and applicability for the estimation of geothermal subsurface temperatures. Several solute geothermometers, Cl–SO4–HCO3 and Na–K–Mg ternary diagrams as well as silica-enthalpy and chloride-enthalpy mixing models were applied to the prevailing alkaline and Na–HCO3 or Na–Cl–HCO3 dominated waters. Mixing was most pronounced in the individual subregions of Dofan-Fantale and Gergede-Sodere, yet these areas still indicate high heat-generating capabilities. The applied enthalpy-chloride mixing model suggests a subsurface temperature of 190±20°C for these hot springs. This temperature estimate is in good agreement with those obtained from the Na–K and Na–K–Ca geothermometers (185±20°C) for both geothermal areas. Additionally, for Gergede-Sodere it agreed well with the silica-enthalpy mixing model hot spring water results with the assumption that steam loss occurs before mixing (170±20°C). Furthermore, the enthalpy-chloride mixing model refers to reservoir temperatures between 300°C and 370°C for the Aluto-Langano geothermal fluids, which are in the same magnitude than the ones obtained by the silica-enthalpy mixing model for hot springs, if no steam loss occurs before mixing (270±30°C). In addition, they are comparable with few known data collected in drilled wells in the Aluto-Langano geothermal field (231–282°C for LA-4 and LA-8 as well as 315–363°C for LA-3 and LA-6).

Chemical and stable isotopic models for boundary creek warm springs, southwestern Yellowstone National Park, Wyoming

Abstract Thermal springs of the Boundary Creek hydrothermal system in the southwestern part of Yellowstone Park outside the caldera boundary vary in chemical and isotopic composition, and temperature. The diversity may be accounted for by a combination of processes including boiling of a deep thermal water, mixing of the deep thermal water with cool meteoric water and/or with condensed steam or steam-heated meteoric water, and chemical reactions with surrounding rocks. Dissolved-silica, Na + , K + and Ca 2+ contents of the thermal springs could result from a thermal fluid with a temperature of 200 ± 20°C. Chloride-enthalpy and silica-enthalpy mixing models suggest mixing of 230°C, 220 mg/l Cl − thermal water with cool, low-Cl − components. A 350 to 390°C component with Cl − ≥ 300 mg/l is possibly present in thermal springs inside the caldera but is not required to fit observed spring chemical and isotopic compositions. Irreversible mass transfer models in which a low-temperature water reacts with volcanic glass as it percolates downward and warms, can account for observed pH and dissolved-silica, K + , Na + , Ca 2+ and Mg 2+ concentrations, but produces insufficient Cl − or F − for measured concentrations in the warm springs. The ratio of a Na / a H , and Cl − are best accounted for in mixing models. The water-rock interaction model fits compositions of acid-sulfate waters observed at Summit Lake and of low-Cl − waters involved in mixing. The cold waters collected from southwestern Yellowstone Park have δD values ranging from −118 to −145 per mil and δ 18 O values of −15.9 to −19.4 per mil. Two samples from nearby Island Park have δD values of −112 and −114 per mil and δ 18 O values of −15.1 and −15.3 per mil. All samples of thermal water plot significantly to the right of the meteoric water line. The low Cl − and variable δD values of the thermal waters indicate isotopic compositions are derived by extensive dilution with cold meteoric water and by steam separation on ascent to the surface. Many of the hot springs with higher δD values may contain in addition a significant amount of high-D, low-Cl − , acid-sulfate or steam-heated meteoric water. Mixing models, Cl − content and isotopic compositions of thermal springs suggest that 30% or less of a deep thermal component is present. For example, the highest-temperature springs from Three Rivers, Silver Scarf and Upper Boundary Creek thermal areas contain up to 70% cool meteoric water and 30% hot water components, springs at Summit Lake and Middle Boundary Creek spring 57 are acid-sulfate or steam-heated meteoric water; springs 27 and 48 from Middle Boundary Creek and 49 from Mountain Ash contain in excess of 50% acid-sulfate water; and Three Rivers spring 46 and Phillips could result from mixing hot water with 55% cool meteoric water followed by mixing of acid-sulfate water. Extensive dilution by cool meteoric water increases the uncertainties in quantity and nature of the deep meteoric, thermal component.