Silica Geothermometers Research Articles

Hydrogeochemical and isotopic characteristics of the Ilica geothermal system (Erzurum, Turkey)

In this paper, the hydrochemical isotopic characteristics of samples collected from geothermal springs in the Ilica geothermal field, Eastern Anatolia of Turkey, are examined and described. Low-temperature geothermal system of Ilica (Erzurum, Turkey) located along the Eastern Anatolian fault zone was investigated for hydrogeochemical and isotopic characteristics. The study of ionic and isotopic contents shows that the thermal water of Ilica is mainly, locally fed by groundwater, which changes chemically and isotopically during its circulation within the major fault zone reaching depths. The thermal spring has a temperature of 29–39 °C, with electrical conductivity ranging from 4,000 to 7,510 µS/cm and the thermal water is of Na–HCO3–Cl water type. The chemical geothermometers applied in the Ilica geothermal waters yielded a maximum reservoir temperature of 142 °C according to the silica geothermometers. The thermal waters are undersaturated with respect to gypsum, anhydrite and halite, and oversaturated with respect to dolomite. The dolomite mineral possibly caused scaling when obtaining the thermal waters in the study area. According to the enthalpy chloride-mixing model, cold water to the thermal water-mixing ratio is changing between 69.8 and 75 %. The δ18O–δ2H compositions obviously indicate meteoric origin of the waters. Thermal water springs derived from continental precipitation falling on to higher elevations in the study area. The δ13C ratio for dissolved inorganic carbonate in the waters lies between 4.63 and 6.48 ‰. In low-temperature waters carbon is considered as originating from volcanic (mantle) CO2.

Evaluation of Temperature and Mixing Process of Water in Deep and Shallow Aquifers in the Southwestern Tunisia: Case of Djerid Region

This study addresses the hydrogeochemistry of thermal and cold waters circulating in the mineralised area of the Djerid basin (South-western of Tunisia) that aimed basically at understanding the mixing processes influencing their chemical compositions. Temperature intervals are 38–75 and 24.5–26.8 °C for thermal water and cold water, respectively. Two distinct hydrogeological systems supply water either for irrigation or for drinking; they are: (1) the Continental Intercalaire geothermal aquifer (CI), and (2) the Complex Terminal aquifer (CT). Geological, geophysical, hydrogeological, hydrochemical methods are applied to reliably analyze and understand the operating model of the aquifer systems, to determine the hydrogeological, the geochemical behaviours, and the possible inter-aquifers water transfer in south western of Tunisia. The reservoir temperature is estimated to be between 60 and 104 °C according to calculations using silica geothermometers and computation of saturation indexes for different solid phases. Based on chemical and thermal data, it is hypothesized that: (1) mixing rate, which occurs between the ascending deep geothermal water and shallow cold water, is estimated from the enthalpy and chloride methods to be about 65 and 73 % respectively, (2) Mixing models can explain the temperature of the geothermal fluid component.

Hydrogeological and mixing process of waters in deep aquifers in arid regions: south east Tunisia

This study addresses the hydrogeochemistry of thermal and cold waters from south east Tunisia. Temperature intervals are 38.5–68 °C and 22–27.8 °C for thermal water and cold water, respectively. Three distinct hydrogeological systems supply water either for irrigation or for drinking; they are: (1) the Continental Intercalaire geothermal aquifer (CI), (2) the Turonian aquifer and (3) the Senonian aquifer. A synthetic study including hydrochemical, hydrogeological and geothermal approaches have been applied in order to evaluate the inter-aquifers water transfer in south east of Tunisia. By using silica geothermometers and saturation indices for different solid phases, estimated thermal reservoir temperature varies between 52 and 87 °C and between 75 and 110 °C, respectively. Based on chemical and thermal data, mixing, which occurs between the ascending deep geothermal water and shallow cold water, is about 57 % cold water.

Spatial variations in gas and stable isotope compositions of thermal fluids around Lake Van: Implications for crust–mantle dynamics in eastern Turkey

We investigate the helium (3He/4He) and carbon (δ13C) isotope compositions and relative abundance ratios (CO2/3He) of gas samples together with the stable isotope compositions of dissolved carbon and sulfur and the oxygen and hydrogen isotopic compositions of the associated water phase from a number of geothermal fields located around Lake Van in eastern Anatolia, Turkey. The mantle-derived helium component, which is likely transferred to the crust beneath eastern Turkey by recent magmatism, is found to constitute up to 96% (e.g. Nemrut Caldera) of the total He content in fluids. As regards the spatial distribution of He, samples collected from areas of Pliocene–Quaternary volcanics are characterized by a wide and generally higher range of R/RA ratios (0.93 to 7.76 RA) compared to those of non-volcanic regions (0.85 to 1.0 RA). CO2/3He ratios vary over a wide range (2.4×105–3.8×1013) but are mostly higher than that of the nominal upper mantle (~2×109). Oxygen–hydrogen isotope values of the waters are conformable with the Global Meteoric Water Line and indicate a local meteoric origin. Sulfate in waters is most probably derived from dissolution of marine carbonates and terrestrial evaporite units. Temperatures calculated by SO4–H2O isotope geothermometry lie between 40 and 199°C, and are in poor agreement with reservoir temperatures estimated from silica geothermometers. Discordant temperatures may be due to either the relatively slow rate of isotopic equilibrium between water and sulfate or mixing of geothermal water with sulfate-bearing shallow waters which may modify the δ18O value. The δ13C (CO2) values of gas samples are consistently lower than those of their water counterparts, consistent with loss of CO2 from waters by degassing. Mixing between mantle and various crustal C-sources appears to be the main control on the C-isotope composition. The principal origin of CO2 in all samples is crustal lithologies, mainly limestone (~85 to 98% of the total carbon inventory): thus, the crustal carbon flux is at least 10 times that from the mantle. There is a broad correlation between high 3He/4He values and thinner crust in the western part of the Lake Van region, where several historically-active volcanoes are located. This observation indicates that localized volcanic and magmatic activity exerts the primary control on the balance between mantle and crustally-derived volatiles in the region.

Shallow submarine and subaerial, low-enthalpy hydrothermal manifestations in Punta Banda, Baja California, Mexico: Geophysical and geochemical characterization

The low-enthalpy geothermal system at Punta Banda (NW Baja California Peninsula, Mexico) has been studied because it might provide heat to future desalination plants in the city of Ensenada. Utilization of subaerial, intertidal and submarine hot springs is evaluated based on geochemical and geophysical data. The results of the geochemical studies show that the geothermal fluids have a major meteoric water component because seawater is not present at the subaerial springs and hot wells. The highest estimated reservoir temperature (140 °C) calculated using a silica geothermometer corresponds to the Agua Caliente intertidal manifestation, a promising area also identified by geophysics. Geothermometric calculations applied to the computed composition of the thermal end member yield a reservoir temperature of 137 °C. Cl/B ratios indicate that the thermal fluids discharged by the intertidal vents and subaerial springs are similar, but they differ from those of submarine vents. Geoelectrical models depict an anomalous conductive trend from the La Jolla well to the Agua Caliente manifestation, suggesting the presence of a fault that allows upflow of hot water from depth. Lastly, integration of geochemical and geophysical data identified the best site for future exploration drilling at Punta Banda.

Gas geochemistry for the Los Azufres (Michoacán) geothermal reservoir, México

Gas data of the Los Azufres geothermal field were analyzed using a method based on equilibrium of the Fischer- Tropsch (FT) reaction: CH4 + 2H2O = 4H2 +CO2 and on the combined pyrite-hematite-magnetite (HSH2) reactions: 5/4 H2 +3/2 FeS2 +3/4 Fe2O3 + 7/4 H2O = 3 H2S +Fe3O4 in order to estimate reservoir temperature and excess steam. The solution of equilibrium equations produces a grid (FT-HSH2). This method is suitable for reservoirs with relatively high H2S but low H2 and NH3 concentrations in the fluid as is the case of the Los Azufres well discharges. Reservoir temperature and reservoir excess steam values were estimated for initial and present conditions in representative wells of the field to study the evolution of fluids, because of exploitation and waste fluids reinjection. This method was very useful in estimating reservoir temperatures in vapor wells, while in two-phase wells it was found that as the well produces a smaller fraction of water, the reservoir temperature estimation agrees qualitatively with results from cationic or silica geothermometers. For liquid-dominated wells the reservoir temperature estimations agree with temperatures obtained from the well simulator WELFLO. This indicates that FT-HSH2 results provide the temperature of the fluid entering the well where the last equilibrium occurs. Results show a decrease in reservoir temperatures in the southern zone of the field where intensive reinjection takes place. With exploitation, it was also noted that the deep liquid phase in the reservoir is changing to two-phase increasing the reservoir steam fraction and the non-condensable gases in well discharges.

Evaluation of groundwater hydrochemical characteristics and mixing behavior in the Daying and Qicun geothermal systems, Xinzhou Basin

This paper examines groundwater hydrochemical characteristics during mixing between thermal and non-thermal groundwater in low-to-medium temperature geothermal fields. A case study is made of Daying and Qicun geothermal fields in the Xinzhou basin of Shanxi province, China. The two geothermal fields have similar flow patterns, with recharge sourced from precipitation in mountain areas heated through a deep cycle, before flowing into overlying Quaternary porous aquifers via fractures. Hydrochemical features of 60 ground- and surface water samples were examined in the context of hydrogeologic information. The average temperatures of the deep geothermal reservoirs are estimated to be 125 °C in Daying field, and 159 °C in Qicun field, based on Na–K–Mg geothermometry, while slightly lower estimates are obtained using silica geothermometers. Hydrochemical features of thermal water are distinct from cold water. Thermal groundwater is mainly Cl·SO 4–Na type, with high TDS, while non-thermal groundwater is mostly HCO 3–Ca·Mg and HCO 3–Ca type in the Daying and Qicun regions, respectively. Hydrogeochemical processes are characterized by analyzing ion ratios in various waters. Higher contents of some minor elements in thermal waters, such as F, Si, B and Sr, are probably derived from extended water–rock interaction, and these elements can be regarded as indicators of flow paths and residence times. Mixing ratios between cold and thermal waters were estimated with Cl, Na, and B concentrations, using a mass balance approach. Mixing between ascending thermal waters and overlying cold waters is extensive. The proportion of water in the Quaternary aquifer derived from a deep thermal source is lower in Daying geothermal field than in Qicun field (5.3–7.3% vs. 6.3–49.3%). Mixing between thermal and non-thermal groundwater has been accelerated by groundwater exploitation practices and is enhanced near faults. Shallow groundwater composition has also been affected by irrigation with low-temperature thermal water.

Characterization of a Pleistocene thermal spring in Mozambique

A hydrogeological study was conducted with the objective to investigate the only currently known hot spring of Sofala Province in Mozambique with respect to the origin of the water, the discharge, and its chemical composition. Field investigations comprised a general land use survey, mapping of sediment and water temperatures, discharge measurements and on-site water chemistry as well as sampling for further chemical analyses and groundwater dating. Thermal water discharge occurs along a 100 m long NE–SW zone with water temperatures ranging from 42 to 64.5°C. The thermal water is a low-mineralized sodium-chloride-sulfate water enriched in phosphate, fluorine and nickel. The silica geothermometer, the silica concentration of 43 mg/kg and the ratios of Br/Cl and I/Cl of 2.5 × 10–3, suggest that the thermal water stems from approximately 5,000 m depth and had a long residence time with silicate rocks. This points towards Gorongosa Mountain as the water source area. 14C dating suggests a groundwater age of 11,000 years.

Chemical and isotopic composition of geothermal discharges from the Puyehue-Cordón Caulle area (40.5°S), Southern Chile

The Puyehue-Cordón Caulle area (40.5°S) hosts one of the largest active geothermal systems of Southern Chile, comprising two main thermal foci, Cordón Caulle and Puyehue. Cordón Caulle is a NW-trending volcanic depression dominated by fumaroles at the top (∼1500 m) and boiling springs at the northwest end (∼1000 m). In the latter, the alkaline-bicarbonate composition of the springs with low Mg (<0.06 mg/l) relative to the local meteoric waters (∼5 mg/l), low chloride (<60 mg/l), high silica (up to 400 mg/l) and δ 18 O –δD values close to the Global Meteoric Water Line (GMWL), in combination with the large outflow (100 l/s), suggest the existence of a secondary steam-heated aquifer overlying a main vapor-dominated system at Cordón Caulle. Subsurface temperatures of the secondary aquifer are estimated to be about 170–180 °C (corrected silica geothermometers). The Puyehue thermal area, on the other hand, includes Mg-rich hot springs discharging along stream valleys, with maximum temperatures of 65 °C and a δ 18 O –δD signature resembling the local meteoric composition, which suggests that the surface manifestations contain a reservoir component that is strongly diluted by meteoric waters. Topographic/hydrologic and chemical characteristics suggest that Cordón Caulle and Puyehue represent two separate upflows.

Hydrogeochemical and hydrogeological investigations of thermal waters in the Emet area (Kütahya, Turkey)

Metamorphic rocks host the majority of the thermal waters of the Emet area. Only Dereli springs are hosted by non-metamorphic carbonates and ophiolitic rocks. The carbonated rocks of the lower parts of the Neogene sequence are also secondary reservoir rocks. The measured surface temperatures of thermal waters are between 33 and 54 °C. Most of the thermal waters are characterized as Ca–Mg–SO 4–HCO 3 type although there are a few Ca–Na–HCO 3, Na–Ca–SO 4 and Ca–Mg–HCO 3 waters. Calcium concentrations in the thermal waters are 89–354 mg/kg. High SO 4 contents of the thermal waters (up to 1309 mg/kg) are related to rocks and minerals in the Red Unit below the Emet borate deposits. Although the SO 4 concentrations are high and SO 4 is the major anion, gypsum and anhydrite are undersaturated for all of the thermal waters indicating that dissolution of SO 4 is still taking place in the reservoir. Thermal waters are oversaturated at outlet conditions with respect to calcite, chalcedony, dolomite and quartz. According to the activity diagrams thermal waters are likely to form illite as an alteration product in the reservoir and Ca and Mg contents are controlled by exchange with smectite. Reservoir temperatures obtained by silica geothermometers and assessments of the saturation states of minerals are more appropriate for Emet geothermal waters. Assessments of the various geothermometers suggest that reservoir temperature is around 75–87 °C.

Reply to comments of T. Yalcin and F. Suner (Environmental Geology 39(5): 433–436) – "More on the Jelenia Góra-Cieplice thermal waters"

Yalcin and Suner (2001) made important and pertinent remarks concerning the content of the paper by Dowgiallo (2000). The necessity for some clarifications is obvious. It is, of course, true that K in the formula (a) on page 436 should be within square brackets. This printing error, however, did not influence the computational results, as K was in this case given in mol/dm. The observation of the above-mentioned authors concerning the silica (chalcedony) geothermometer (Arnorsson and others 1983) is not quite clear. After recalculation, the results appear the same as in the discussed paper, except the C-1 samples from depths of 1,790 and 2,002 m, where the values should be rounded. The differences, however, are less than 1 C, and not 3 C. For geothermometric temperature estimates, such differences do not play any important role. As far as the silica concentrations are concerned, analytical errors suggested by Yalcin and Suner (2001) regarding some samples from the C-1 borehole (depths 1,605, 1,700, 2,002, and perhaps 1,790 m) cannot be excluded. Samples were taken during drilling and they did not always immediately reach the chemical laboratory. However, results (expressed in ppm) from another drilled well (C-2) and from spring no. 5 providing thermal water do not arouse doubts but serve as a sound basis for temperature estimation. Pang Zhong-he (1992) states that the chalcedony geothermometer applied to low-temperature granitic geothermal areas in southeast China (thus, to conditions analogous to the Cieplice area) is one of the most suitable for this type of geothermal systems. The pH values of the Cieplice undiluted thermal waters measured in the field do not exceed 8.2 (Leśniak and Nowak 1993). However, pH in samples taken from borehole C-1 during drilling was measured only in the laboratory with a certain delay. Obtained pH values vary between 8.4 and 8.9, and are presumably higher than if pH were measured directly at the outflow. It has to be emphasized that the dependence of silica solubility on pH shown on the graph by Krauskopf (1956) concerns low-temperature solutions. In more recent papers dealing with silica geothermometers (e.g., Rimstidt and Barnes 1980; Arnorsson and others 1983), both temperature and pH as well as the contribution of H3SiO 4 to alkalinity are taken into account. Yalcin and Suner (2001) correctly point out that waterminerals equilibria in reservoirs are the basis of chemical geothermometry. However, partial equilibria shown by the water may also be used as an indicator of temperature at depth. It can be shown that thermal waters at Cieplice have reached partial (although far from full) equilibrium with rock-forming minerals containing K, Na and Mg (Fig. 1), and that calculated temperatures are in most cases realistic and close to those observed in well C-1.

Chemical equilibria of thermal waters for the application of geothermometers from the Guanzhong basin, China

In this study, representative samples from thermal wells and springs were chemically analyzed and geothermometers were used to calculate the deep temperatures of geothermal reservoirs on the basis of water–mineral equilibrium. In some cases, however, the chemical components are not in equilibrium with the minerals in the reservoir. Therefore, log( Q/ K) diagrams are used to study the chemical equilibrium for the minerals that are likely to participate. The Na–K–Mg triangular diagram is also applied to evaluate the equilibrium of water with reservoir rocks. Standard curves at the reference temperatures are prepared to reveal which type of silica geothermometer is appropriate for the specified condition. This study shows that water samples from geothermal wells W9 and W12 are in equilibrium with the selective minerals, and chalcedony may control the fluid–silica equilibrium. It is estimated that there is an exploitable low-temperature reservoir with possible temperatures of 80–90°C in the Guanzhong basin.

Calculated heat flow for the Ensenada region, Baja California, Mexico

Due to the scarcity of heat flow data in the region of Ensenada, Baja California (Mexico), the present study calculated them using the silica geothermometer technique. The average heat flow value determined was 123.33 ± 35.57 mW m–2 (2.95 ± 0.85 HFU). Four zones within the study area, related to the main fault systems of the region, were found to have high hydrothermal activity. The potential use of geothermal resources in the area is feasible as it is considered a low geothermal system with water temperature values, above the static water level, that vary from 30ºC to 108ºC.

Chemical thermodynamics of silica: a critique on its geothermometer

The chemical thermodynamic concepts used in the calculation of solubility data of silica (quartz) are presented taking into account the PVT characteristics of water. The temperature-dependence trends between the thermodynamically calculated and the experimental quartz solubility data are very similar, but the values are widely different at high temperatures. The experimental solubility, especially along the saturation curve at high temperature and thermodynamic data for silica need to be reevaluated in order to use silica chemistry to understand geological processes. There could exist a wide range of values for silica solubility at a specified temperature, depending upon the amount of water in the reaction vessel. Thus the silica contents in geothermal fluid, in general, cannot be used as a geothermometer to estimate the reservoir temperature. The derivation of a silica geothermometer needs an extra assumption about the total amount of water in the system. The solubility data for the two extreme cases, i.e. when the vessel (bomb) is completely filled with water and when there is just enough water to make the total specific volume equal to the critical volume of water at room temperature (25°C), are considered here. These lie on the two respective straight lines of log (SiO 2(ppm)) against temperature (K). The equations for the two straight lines are log (SiO 2 (ppm)) = 0.0179 T (K)−4.3214 and log SiO 2 (ppm) = 0.0088 T (K)−1.6513, respectively. In the case of the well M-19A at Cerro Prieto, the silica concentration in the reservoir liquid is higher than the experimental solubility, but is lower than the calculated solubility value.

A basic program for calculating subsurface water temperatures using chemical geothermometers-implication to geothermal reservoir estimation

GEOTHERM is a computer program written in BASIC language to estimate geothermal reservoir temperature using the well-known chemical gèothermometers. The empirical equations used in the program were obtained from the literature. Three different chemical geothermometers are included in the program: Na-K, Na-K-Ca, and silica geothermometers; this gives the user the opportunity not only to select the most reliable geothermometer in estimating subsurface temperature, but also to select the type of geothermometer according to available data. A sample input file of geothermal waters obtained from Iceland has been tested, so as to show the applicability and usefulness of the program.

Thermal waters along the Konocti Bay fault zone, Lake County, California: a re-evaluation

The Konocti Bay fault zone (KBFZ), initially regarded by some as a promising target for liquid-dominated geothermal systems, has been a disappointment. At least five exploratory wells were drilled in the vicinity of the KBFZ, but none were successful. Although the Na-K-Ca and Na-Li geothermometers indicate that the thermal waters discharging in the vicinity of Howard and Seigler Springs may have equilibrated at temperatures greater than 200°C, the spring temperatures and fluid discharges are low. Most thermal waters along the KBFZ contain >100 mg/l Mg. High concentrations of dissolved magnesium are usually indicative of relatively cool hydrothermal systems. Dissolution of serpentine at shallow depths may contribute dissolved silica and magnesium to rising thermal waters. Most thermal waters are saturated with respect to amorphous silica at the measured spring temperature. Silica geothermometers and mixing models are useless because the dissolved silica concentration is not controlled by the solubility of either quartz or chalcedony. Cation geothermometry indicates the possibility of a high-temperature fluid (> 200°C) only in the vicinity of Howard and Seigler Springs. However, even if the fluid temperature is as high as that indicated by the geothermometers, the permeability may be low. Deuterium and oxygen-18 values of the thermal waters indicate that they recharged locally and became enriched in oxygen-18 by exchange with rock. Diluting meteoric water and the thermal water appear to have the same deuterium value. Lack of tritium in the diluted spring waters suggest that the diluting water is old.

Experimental water-rock interaction: silica geothermometers in the temperature range 100-350°C

The reliability of silica geothermometers for different natural mineral assemblages and fluid compositions has been experimentally assessed in the P, T range 185-435 bar, 100-350°C using the Barnes hydrothermal kinetic apparatus. A series of metasediments (two phyllosilicate-rich schists and one dolomitic carbonate rock) of the Latium-Tuscany geothermal area (central Italy) and fluids of pure and sea-water compositions were used at starting materials. Experiments with the carbonate rock indicate that silica geothermometry is poorly applicable, probably because of the finite character of silica in the starting material. Experiments with feldspar-free, phyllosilicate-rich schists indicate that both the rock chemistry (mainly the redox ratio) and the fluid composition affect silica solubility at any given P-T. Oxidized assemblages in general favour higher silica solubilities. The effect of rock composition is greatly diminished when saline fluids are used. The observed solubility-temperature relationships are assessed against commonly used experimental and empirical geothermometers, giving a best approximation to the chalcedony geothermometer, followed by the quartz - no steam loss geothermometer. Three new silica geothermometers were calibrated : two for oxidized and reduced silicate assemblages and pure water fluids (EST-o and EST-r, respectively) and one for a variety of silicate assemblages interacting with saline (marine) fluids (EST-m). These geothermometers are effective for the whole experimentally investigated range (100-350°C). The new geothermometers have been tested for various geothermal areas and found to give accurate results. Combined equations allow a reliable chemical geothermometry over the temperature range 25-350°C, provided the redox conditions of the reservoir and the composition of the initial fluid are approximately known

Application of chemical geothermometers to some Tunisian hot springs

Forty-five Tunisian hot springs are studied as geothermal indicators. Several chemical geothermometers are applied in this sedimentary geological environment. Hot springs are selected among 70 existing within the country. Selection is based on complete water analyses with a good ionic balance. Measured temperature ranges from 21 to 73°C and discharge rates range from 0.1 to 40 l/s. Studied springs show mainly chloride type waters. Geothermometers applied are: silica, Na/K, NaKCa, NaKCaMg, Na/Li and Mg/Li. Temperatures estimated by those geothermometers are plotted against the measured values to evaluate the applicability of the geothermometers used. The Na/K and Na/Li geothermometers appear to give unreliable results, as water composition seems to be greatly affected by interaction with the evaporitic and dolomitic rocks that are ubiquitous in Tunisia. The NaKCaMg, Mg/Li and silica geothermometers seem to give plausible values, emphasizing the role played by Mg and its importance for Tunisian hot waters.

Subsurface temperatures and surface heat flow in the Michigan Basin and their relationships to regional subsurface fluid movement

Linear regression of 405 bottomhole temperature (BHT) measurements vs. associated depths from Michigan's Lower Peninsula results in the following equation relating BHT and depth: BHT(°C) = 14.5 + 0.0192 × depth(m) Temperature residuals, defined as (BHT measured)—(BHT calculated), were determined for each of the 405 BHT's. Areas of positive temperature residuals correspond to areas of regional groundwater discharge (determined from maps of equipotential surface) while areas of negative temperature residuals correspond to areas of regional groundwater recharge. These relationships are observed in the principal aquifers in rocks of Devonian and Ordovician age and in a portion of the principal aquifer in rocks of Silurian age. There is a similar correspondence between high surface heat flow (determined using the silica geothermometer) and regional groundwater discharge areas and low surface heat flow and regional groundwater recharge areas. Post-Jurassic depositional and tectonic histories suggest that the observed coupling of subsurface temperature and groundwater flow systems may have persisted since Jurassic time. Thus the higher subsurface palaeotemperatures (and palaeogeothermal gradients) indicated by recent studies most likely pre-date the Jurassic.

Hot spring activity in the Dakongbeng geothermal area, China

The Dakongbeng geothermal area, whose hot springs reach a temperature of up to 96°C, has been considered one of the potential high-temperature hydrothermal systems in south-west China. The concentration of dominant cations and anions indicates an NaHCO 3 type of thermal water, whose major constituents in decreasing order are: Na>K>Ca>Mg, HCO 3>SiO 2>Cl>SO 4. On the basis of the silica geothermometer, cation geothermometers, gas geothermometer and activity diagram, the reservoir temperature is estimated at about 200°C. All the thermal waters have originated from meteoric water of a higher altitude that circulated as ground water at considerable depth along faults. The stability of their contents of Cl, SiO 2, δD, δ 18O and of the Cl B , Na Li ratios suggests that the main heat loss process is through steam loss. The geochemistry of the initial liquid has been estimated by single and continuous steam loss. On the basis of its geologic and geographic setting, the Dakongbeng geothermal area appears to belong to the Himalayan geothermal belt and is thus regarded as an area of interest for further study.