Giant gas-rich hydrothermal systems and their role in the generation of vapor-dominated geothermal fields and ore mineralization

This paper characterizes certain unique geological structures on the earth, viz., giant gas-rich hydrothermal systems with major vapor-dominated geothermal fields that are generated beneath them during the present phase of evolution. A review of the relevant literature and materials of our own research are used to show that such systems are formed in zones of deep-seated faults at junctions of oceanic and continental plates, in structures of volcanic island arcs, and in areas of crustal tectono-magmatic activity. The systems extend throughout the crustal thickness and possess enormous geothermal and ore potentials. It was found that in these systems the ascending high-temperature gas-water fluid, as well as all types of mixed waters, and new mineral compounds in the hypergenesis zone of geothermal anomalies, all take part in the transport, accumulation, and rearrangement of complex compounds of many metals (Fe, Al, Ti, Au, Ag, Hg, As, Sb, and others). It was inferred that gas-rich hydrothermal systems and the vapor-dominated geothermal fields that are formed beneath them reflect the conditions for the generation of mesothermal and epithermal gold and complex ores and of Au-Ag-Cu-Mo porphyric deposits.     

Coupled Process Models of Fluid Flow and Heat Transfer in Hydrothermal Systems in Three Dimensions

Geothermal fields and hydrothermal mineral deposits are manifestations of the interaction between heat transfer and fluid flow in the Earth’s crust. Understanding the factors that drive fluid flow is essential for managing geothermal energy production and for understanding the genesis of hydrothermal mineral systems. We provide an overview of fluid flow drivers with a focus on flow driven by heat and hydraulic head. We show how numerical simulations can be used to compare the effect of different flow drivers on hydrothermal mineralisation. We explore the concepts of laminar flow in porous media (Darcy’s law) and the non-dimensional Rayleigh number (Ra) for free thermal convection in the context of fluid flow in hydrothermal systems in three dimensions. We compare models of free thermal convection to hydraulic head driven flow in relation to hydrothermal copper mineralisation at Mount Isa, Australia. Free thermal convection occurs if the permeability of the fault system results in Ra above the critical threshold, whereas a vertical head gradient results in an upward flow field.     

Magmatic Systems and the Conditions for Hydrothermal Circulation at Depth in the Klyuchevskoi Volcanic Cluster as Inferred from Observations of Local Seismicity and Thermo-Hydrodynamic Simulation

An analysis of local seismicity within the Klyuchevskoi Volcanic Cluster and Shiveluch Volcano for the period 2000–2017 revealed a sequence of plane-oriented earthquake clusters that are interpreted here as the emplacement of dikes and sills (magmatic fracking). The geometry of magma bodies reflects the geomechanical conditions in volcanic plumbing systems and at the bases of the volcanoes. Magmatic fracking within active magmatic plumbing systems results in the formation of permeable reservoirs whose vertical extent can reach 35 km (Klyuchevskoi) and can be as wide as 15 km across (Shiveluch), depending on the geomechanical condition of the host rocks. These reservoirs will be the arena of subsequent hydrothermal circulation, producing geothermal and ore fields, as well as hydrocarbon fields. TOUGH2-EOS1sc simulation tools were used to estimate the conditions for the formation of hydrothermal reservoirs at temperatures below 1200°С and pressures below 1000 bars.     

Coupled Models of Brittle-plastic Deformation and Fluid Flow: Approaches,Methods, and Application to Mesoproterozoic Mineralisation at Mount Isa,Australia

Rock deformation has an important effect on the spatial distribution and temporal evolution of permeability in the Earth’s crust. Hydromechanical coupling is of fundamental significance to natural fluid–rock interaction in porous and fractured hydrothermal systems, and in the assessment and production of hydrocarbon resources and geothermal energy. Shearing and fracturing of rocks can lead to the creation or destruction of permeability when fractures or faults form, or when existing structures are reactivated. Changes in stress orientation or fluid pressure can drive rock failure and create dilating fault zones that have the potential to focus fluid flow, or to breach seals above overpressured fluid compartments. Here, numerical models of deformation and fluid flow related to Mesoproterozoic copper mineralisation at Mount Isa, Australia, are presented that show how changes in deformation geometry in multiply deformed geological architectures relate to changes in dilation patterns, fluid pathways and flow geometry. Coupled numerical simulations of deformation and fluid flow can be useful tools to better understand structural control on fluid flow in hydrothermal mineral systems.     

Cation migration in hydrothermal clays: The problem of mineralization criteria in gas-hydrothermal fluids of hydrothermal fields in Southern Kamchatka

Based on a comprehensive study of the hydrothermal clay layer that occurs in geothermal fields, the conditions of formation of cation composition in argillitized rocks are discussed. Under the influence of gas-water fluids and pore solutions, micro- and nano-mineral mixtures are formed in hydrothermal clays; these mixtures include crystalline, amorphous, and transitional mineral phases. A considerable role in their composition belongs to cations of several metals (Fe, Al, Ti, Na, Mg, Ca, K, Mn, and Ba), as well as Si, C, N, S, and volatiles (F^? and Cl^?). The sources of cations and other elements are unaltered host rocks, newly formed hydrothermal-metasomatic rocks, hydrothermal clays, salt deposits, siliceous, carbonate, and other sediments, as well as deep fluids. In the structures of geothermal anomalies and deposits the ??hydrothermal metasomatic rock??gas-water fluid??newly formed mineral chemical compounds?? united system is formed. Each of the elements of this system takes part in the transportation, accumulation, and redistribution of metals. This approach to studies of the geochemistry of present-day geothermal systems may serve as a foundation for developing criteria for the presence of mineralization in metasomatites, gas-hydrothermal fluids, and new mineral associations.     

Hydrothermal alteration in the Reykjanes geothermal system: Insights from Iceland deep drilling program well RN-17

The Reykjanes geothermal system is a seawater-recharged hydrothermal system that appears to be analogous to seafloor hydrothermal systems in terms of host rock type and low water/rock alteration. The similarities make the Reykjanes system a useful proxy for seafloor vents. At some time during the Pleistocene, the system was dominated by meteoric water recharge, and fluid composition at Reykjanes has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions. The purpose of this study is to characterize secondary mineralization, degree of metasomatic alteration, and bulk composition of cuttings from well RN-17 from the Reykjanes geothermal system. The basaltic host rock includes hyaloclastite, breccia, tuff, extrusive basalt, diabase, as well as a marine sedimentary sequence. The progressive hydrothermal alteration sequence observed with increasing depth results from reaction of geothermal fluids with the basaltic host rock. An assemblage of greenschist facies alteration minerals, including actinolite, prehnite, epidote and garnet, occurs at depths as shallow as 350 m; these minerals are commonly found in Icelandic geothermal systems at temperatures above 250 °C (Bird and Spieler, 2004). This requires hydrostatic pressures that exceed the present-day depth to boiling point curve, and therefore must record alteration at higher fluid pressures, perhaps as a result of Pleistocene glaciation. Major, minor, and trace element profiles of the cuttings indicate transitional MORB to OIB composition with limited metasomatic shifts in easily mobilized elements. Changes in MgO, K₂O and loss on ignition indicate that metasomatism is strongly correlated with protolith properties. The textures of alteration minerals reveal alteration style to be strongly dependent on protolith as well. Hyaloclastites are intensely altered with calc-silicate alteration assemblages comprising calcic hydrothermal plagioclase, grandite garnet, prehnite, epidote, hydrothermal clinopyroxene, and titanite. In contrast, crystalline basalts and intrusive rocks display a range in alteration intensity from essentially unaltered to pervasive and nearly complete albitization of igneous feldspar and uralitization of clinopyroxene. Hydrothermal anorthite (An92–An98) occurs in veins in the most altered basalt cuttings and is significantly more calcic than igneous feldspar (An48–An79). Amphibole compositions change from actinolite to hornblende at depth. Hydrothermal clinopyroxene, which occurs in veins, has greater variation in Fe content and is systematically more calcic than igneous pyroxene and also lacks uralitic textures. Solid solutions of prehnite, epidote, and garnet indicate evolving equilibria with respect to aluminum and ferric iron.     

Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation

The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by extensive volcanism and by high rates of magma production. Associated with this volcanism are numerous high-temperature (> 250 °C) geothermal systems through which the natural heat output of 4200 ± 500 MW is channelled. Outside the geothermal fields the heat flow is negligible. The average heat flux from the central 6000 km² of the TVZ, which contains most of the geothermal fields, is 700 mW/m³. This heat flux appears to be more concentrated along the eastern margin of the TVZ.Schlumberger resistivity measurements (AB/2 of 500 m and 1000 m) have identified 17 distinct geothermal fields with natural heat outputs greater than 20 MW. An additional six, low-heat-output geothermal fields also occur, and may represent formerly more active systems now in decline. Two extinct fields have also been identified. The average spacing between fields is 10–15 km. The distribution of geothermal fields does not appear to be directly associated with individual volcanic features except for the geothermal system that occurs within Lake Taupo and which occupies the vent of the 1800 yr.B.P. Taupo eruption. The positions of the geothermal fields do not appear to have varied for at least the last 200,000 years. These data are consistent with a model of large-scale convection occurring throughout the TVZ, in which the geothermal fields represent the upper portion of the rising, high-temperature, convective plumes. The majority of the recharge to the convection system is provided by the downward movement of cold meteoric water between the fields which suppresses the heat flow in these regions.Gravity measurements indicate that to a depth of about 2.5 km the upper layers of the TVZ consist of low-density pyroclastic infill. A seismic refraction interface with velocity change from 3.2 km/s to 5.5 km/s occurs at a similar depth. The cross-sectional area of the convection plumes (identified electrically) appears to increase at depths of 1–2 km, consistent with a decrease in permeability at the depth at which the velocity and density increase.The seismicity is dominated by swarm activity which accounts for about half of all earthquakes and is highly variable in both space and time. The small number of seismic events (and swarms) that have well determined depths show a cut off of seismicity at depths of 7–9 km. The depth of the transition from brittle to ductile behaviour of the rocks is identified with the transition from a regime where heat is transported by (hydrothermal) convection and pore pressures are near-hydrostatic to a regime where heat transport is dominantly conductive and pore pressures are lithostatic. Within the convective region, temperatures are moderated by the circulation of water so that the depth of the transition from convective to conductive heat transfer can be linked to the bottom of the seismogenic zone. Rocks must become ductile within about 1 km of the bottom of the overlying convective zone.Seismic refraction studies suggest that the crust beneath the TVZ is highly thinned with a seismic velocity of about 7.5 km/ s, typical of the upper mantle, occurring at depth of 15 km. Seismological studies indicate the upper mantle is highly attenuating beneath the TVZ. Conductive heat transfer between the bottom of the convective system, at about 8 km, and the base of the material with crustal velocities, at 15 km, is not able to provide all the heat that is discharged at the surface. Repeated intrusion from the mantle may provide the additional heat transport required.     

Kaipohan: An apparently nonthermal manifestation of hydrothermal systems in the Philippines

Hydrothermal systems in regions of high relief may not have obvious thermal features indicative of their central upflow zone at depth. Investigation of such areas for geothermal energy production in the Philippines has, however, encountered relatively large ( > 10,000 m²) areas of volumetrically significant but diffuse, cold-gas emission and associated intense argillic alteration. These features are likely to be best developed above, or close to, hydrothermal upflow zones. The identification of such features is important for interpreting the hydrology of active geothermal systems in the course of exploration for power development, or when interpreting the paleohydrology of fossil geothermal systems which host epithermal mineral deposits. Such zones of argillic alteration are likely to be barren of precious metals or other elements (except mercury) which are commonly used as pathfinders for precious-metal deposits, but are indicative of subsurface processes significant in ore genesis.It is proposed that the term “kaipohan” be used for such features. Their existence depends on the presence of either a low-permeability geologic formation or where a suitable relationship exists between the hydrology of the deep geothermal reservoir and that of the shallow groundwater system. Such a situation occurs in steep terrain, where much of the area supplying meteoric recharge to the geothermal system is at a lower elevation than the area overlying the centre of the system, and where there is a high annual rainfall. These conditions are typical of andesitic stratovolcanoes in tropical or subtropical island arcs.     

Mercury in soils of the Long Valley, California, geothermal system

An evaluation of the Hg distribution in soils of the Long Valley, California, geothermal area, was made. A₁-horizon soil samples were collected utilizing a grid system from the resurgent dome area and the Long Valley area. In addition, samples were collected in five traverses across three fault systems and four traverses across east-west-oriented gullies to measure the importance of aspect. Additional samples were collected in an analysis of variance design to evaluate natural variability in soil composition with sampling interval distance.The primary objectives of this study were to further evaluate the applicability of anomalously high Hg concentration in soils to exploration for geothermal systems and the importance of secondary controls on Hg concentration in soils above geothermal systems.Statistical analysis indicates that there are two populations of Hg concentrations in soils; one affected by geothermal activity and the other unaffected. Samples from the resurgent dome are shown to be statistically different from the samples collected in Long Valley proper with respect to Hg, organic carbon, and pH. This suggests that secondary influences may be important in controlling Hg distribution in soils.Organic carbon in soils is shown by stepwise multiple regression to be the most important secondary parameter controlling Hg concentration. For the most part, the secondary controls of Hg are overwhelmed in an area of prominent geothermal activity. Some faults exhibit prominent anomalies in total Hg concentration and others do not, indicating the possibility of low levels of hydrothermal activity or effective sealing of faults to gas leakage.The analysis of variance results indicate that there is a regional trend in Hg concentration across the resurgent dome. Soils can be sampled for Hg by utilizing a grid of about 0.4 km spacing. However, some local irregularities appear in this pattern and anomalous areas should be prospected at intervals of 100 m or less.     

Fluid circulation in a complex volcano-tectonic setting,inferred from self-potential and soil CO2 flux surveys: The Santa María–Cerro Quemado–Zunil volcanoes and Xela caldera (Northwestern Guatemala)

The region encompassing Santa María, Cerro Quemado, and Zunil volcanoes, close to Quetzaltenango, the second largest city of Guatemala, is volcanically and tectonically complex. In addition, the huge Xela caldera, about 20 km in diameter, crosses this area and links up to the important Zunil fault zone located between the three volcanoes. Two highly active geothermal sites, named Zunil-I and Zunil-II, are also located between these three volcanic edifices at the southeastern boundary of Xela caldera. In order to determine the permeability variations and the main structural discontinuities within this complex volcano-tectonic setting, self-potential and soil CO₂ flux measurements have been coupled, with a step of 20 m, along a 16.880 km-long profile crossing the entire area. Two shallow hydrothermal systems, with maximum lateral extensions of 1.5 km in diameter, are indicated by positive self-potential/elevation gradients below Santa María and Cerro Quemado volcanoes. Such small hydrothermal systems cannot explain the intense geothermal manifestations at Zunil-I and Zunil-II. Another minor hydrothermal system is indicated by self-potential measurements on the flank of Santa María along the edge of the Xela caldera. CO₂ flux measurements display slight variations inside the caldera and decreasing values crossing outside the caldera boundary. We hypothesize the presence of a magmatic body, inside the southeastern border of Xela caldera, to explain the deeper and more intense hydrothermal system manifested by the Zunil-I and the Zunil-II geothermal fields. This magmatic system may be independent from Santa María and Cerro Quemado volcanoes. Alternatively, the hypothesized Xela magmatic system could have a common magmatic origin with the Cerro Quemado dome complex, consistent with previous findings on regional gas emissions. Sectors bordering the Cerro Quemado dome complex also have high amplitude minima-short wavelength anomalies in self-potential, interpreted as preferential rain water infiltration along faults of major permeability, probably related with the most recent stages of Cerro Quemado dome growth.     

Geothermal data analysis at the high-temperature hydrothermal area in Western Sichuan

The western Sichuan hydrothermal area is located at the northeastern margin of the eastern syntaxis of the Qinghai-Tibet Plateau, which is also the eastern end of the Mediterranean-Himalayan geothermal activity zone. There are 248 warm or hot springs in this area, and 11 have temperatures beyond the local boiling temperature. Most of these hot springs are distributed along the Jinshajiang, Dege-Xiangcheng, Ganzi-Litang, and Xianshuihe faults, forming a NW-SE hydrothermal belt. A geothermal analysis of this high-temperature hydrothermal area is an important basis for understanding the deep geodynamic process of the eastern syntaxis of the Qinghai-Tibet Plateau. In addition, this study offers an a priori view to utilize geothermal resources, which is important in both scientific research and application. We use gravity, magnetic, seismic, and helium isotope data to analyze the crust-mantle heat flow ratio and deep geothermal structure. The results show that the background terrestrial heat flow descends from southwest to northeast. The crustal heat ratio is not more than 60%. The high temperature hydrothermal active is related to crustal dynamics processes. Along the Batang-Litang-Kangding line, the Moho depth increases eastward, which is consistent with the changing Qc/Qm(crustal/mantle heat flow) ratio trend. The geoid in the hydrothermal zone is 4–6 km higher than the surroundings, forming a local "platform". The NW-SE striking local tensile stress zone and uplift structure in the upper and middle crust corresponds with the surface hydrothermal active zone. There is an average Curie Point Depth(CPD) of 19.5–22.5 km in Batang, Litang, and Kangding. The local shear-wave(S-wave) velocity is relatively low in the middle and lower crust. The S-wave shows a low velocity trap(Vs3.2 km s.1) at 15–30 km, which is considered a high-temperature partial melting magma, the crustal source of the hydrothermal active zone. We conclude that the hydrothermal system in this area can be divided into Batang-type and Kangding-type, both of which rely on a crustal heating cycle of atmospheric precipitation and surface water along the fracture zone. The heat is derived from the middle and lower crust: groundwater penetrates the deep faults bringing geothermal energy back to the surface and forming high-temperature springs.     

Modelling Feedback of Chemical Reactions on Flow Fields in Hydrothermal Systems

Coupled reactive transport models of hydrothermal systems provide new insights and deeper understanding of the processes occurring due to fluid flow, heat transfer, solute transport, and chemical reactions. Basic concepts of species transport (diffusion, dispersion, and advection) and chemical precipitation and dissolution reactions are discussed, and five end-member types of reactive transport environments are introduced. One of these reactive transport environments, named ‘reactions within thermal gradients’, is used to demonstrate how free thermal convection can lead to redeposition of minerals and, due to the feedback of reaction on the flow field, a change of the convection pattern. The direct consequence of changing the flow field is a significant variation of the temperature distribution within the modelled area. With the example it is shown how reactive transport simulation can be applied for the detailed study of fossil and recent hydrothermal systems.     

Time-space variation of volcano-seismic events at La Fossa (Vulcano, Aeolian Islands, Italy): new insights into seismic sources in a hydrothermal system

We investigated the relationship between volcano-seismic events, recorded at La Fossa crater of Vulcano (Aeolian Islands, Italy) during 2004-2006, and the dynamics of the hydrothermal system. During the period of study, three episodes of increasing numbers of volcano-seismic events took place at the same time as geothermal and geochemical anomalies were observed. These geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids. Three classes of seismic events (long period, high frequency and monochromatic events), characterised by different spectral content and various similarity of the waveforms, have been recognised. These events, clustered mainly below La Fossa crater area at depths of 0.5–1.1 km b.s.l., were space-distributed according to the classes. Based on their features, we can infer that such events at Vulcano are related to two different source mechanisms: (1) fracturing processes of rocks and (2) resonance of cracks (or conduits) filled with hydrothermal fluid. In the light of these source mechanisms, the increase in the number of events, at the same time as geochemical and geothermal anomalies were observed, was interpreted as the result of an increasing magmatic component of the hydrothermal fluids, implying an increase of their flux. Indeed, such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids. Increased pore pressures gave rise to fracturing processes, while the increased fluid flux favoured resonance and vibration processes in cracks and conduits. Finally, a gradual temporal variation of the waveform of the hybrid events (one of the subclasses of long period events) was observed, likely caused by heating and drying of the hydrothermal system.     

3-D Magnetotelluric Exploration of Tenerife Geothermal System (Canary Islands,Spain)

The resistivity structure of the Tenerife geothermal system has been determined by the 3-D inversion of data from different magnetotelluric surveys. In this paper, the ocean and topography effects on the magnetotelluric data were investigated by constructing a 3-D conceptual geoelectrical model of the island. The study showed that these effects should be taken into account in order to obtain a reliable subsurface model of the island. Data from 148 sites were used during three-dimensional inversion. The most interesting feature in the final geoelectrical model of the geothermal system is a low resistivity structure (<10 Ωm) above the resistive core of the system. The low resistivity structure has been interpreted as a hydrothermal clay alteration cap typically generated in the conventional geothermal systems. The resistivity model has been correlated with a recent seismic velocity model, showing that a low resistivity structure surrounds an area with high P wave velocity and medium–high resistivity. This medium–high resistivity area can be associated with a slowly solidified magma and, therefore, with a hotter part of the geothermal system.     

北京南口—孙河断裂带水热系统特征与成因分析

中低温对流型地热资源在华北地区广泛分布,是一种清洁的替代能源.与活动断裂带相关的水热型地热资源是中低温地热系统的重要组成部分.本文基于高精度重力测量、微动测深及钻孔温度测量等数据,从热源、通道、储层和盖层四个方面探讨了南口—孙河断裂带水热系统特征.低重力异常揭示的燕山期花岗二长岩、闪长岩岩体范围为23.8 km~2和14.3 km~2,放射性测井数据计算得到其生热率均值为3.14μW·m~(-3),侏罗系火山岩生热率均值为1.65μW·m~(-3),隐伏岩体和火山岩均难以构成地热系统的附加热源.重力异常显示南口—孙河断裂带宽度约500~800 m,断裂带切割蓟县系雾迷山组白云岩热储层.钻井温度曲线显示断裂带内水热活动强烈,说明该断裂带是导水、导热的重要通道.断裂带南西侧马池口一带第四系松散层与侏罗系火山岩形成了热储盖层,微动测深显示火山岩最大厚度约1500 m.综上源、通、储、盖四个要素分析,该地热系统为热传导一对流复合型,来自京西北山区的大气降水经远距离径流深循环吸收地层热量后沿南口—孙河断裂上移到达裂隙发育的白云岩地层中形成热水.总之,沿南口—孙河断裂带具备了良好的地热地质条件,可达到规模开采的条件.     

Timing of volcanic, plutonic and geothermal activity at Ngatamariki, New Zealand

Four ⁴⁰Ar/³⁹Ar dates on mineral separates from fresh and hydrothermally altered volcanic and plutonic rocks from the Ngatamariki geothermal field indicate that andesitic volcanism took place in the eastern portion of the Taupo Volcanic Zone (TVZ) prior to 1.2 Ma and probably considerably earlier. These data significantly extend the onset and duration of andesitic volcanism in the east-central TVZ over previous estimates. Intrusive activity is represented at Ngatamariki by a dioritic pluton, the only such pluton yet recognized in the entire TVZ. Hornblende from the pluton yields a crystallization age of near 550 ka. Hydrothermal alteration spatially associated with the pluton produced sericite of a similar age. Overlying and postdating the most intense hydrothermal alteration zone is the Whakamaru Ignimbrite (or its equivalent) which was emplaced at 330 ka. Two distinct geothermal systems may have been active at nearly the same site from 550 ka to present. The most intense activity occurred before 330 ka and was associated with emplacement of the Ngatamariki diorite. This was followed by the less intense system that is currently active. The geothermal regime at Ngatamariki has, therefore, probably been active intermittently for at least 550 ka.     

Geothermal studies in China

Geothermal studies have been conducted in China continuously since the end of the 1950's with renewed activity since 1970. Three areas of research are defined: (1) fundamental theoretical research on geothermics, including subsurface temperatures, terrestrial heat flow and geothermal modeling; (2) exploration for geothermal resources and exploitation of geothermal energy; and (3) geothermal studies in mines.Regional geothermal studies have been conducted recently in North China and more than 2000 values of subsurface temperature have been obtained. Temperatures at a depth of 300 m generally range from 20 to 25°C with geothermal gradients from 20 to 40°C/km. These values are regarded as an average for the region with anomalies related to geological factors.To date, 22 reliable heat flow data from 17 sites have been obtained in North China and the data have been categorized according to fault block tectonics. The average heat flow value at 16 sites in the north is 1.3 HFU, varying from 0.7 to 1.8 HFU. It is apparent that the North China fault block is characterized by a relatively high heat flow with wide variations in magnitude compared to the mean value for similar tectonic units in other parts of the world. It is suggested that although the North China fault block can be traced back to the Archaean, the tectonic activity has been strengthening since the Mesozoic resulting in so-called “reactivation of platform” with large-scale faulting and magmatism.Geothermal resources in China are extensive; more than 2000 hot springs have been found and there are other manifestations including geysers, hydrothermal explosions, hydrothermal steam, fumaroles, high-temperature fountains, boiling springs, pools of boiling mud, etc. In addition, there are many Meso-Cenozoic sedimentary basins with widespread aquifers containing geothermal water resources in abundance. The extensive exploration and exploitation of these geothermal resources began early in the 1970's. Since then several experimental power stations using thermal water have been set up in Fengshun (Fungshun),     

Geochemistry of geopressured hydrothermal waters in the Niigata Sedimentary Basin, Japan

Abstract   Geothermal waters in the Niigata Sedimentary Basin, central Japan, are divided into four groups based on their chemical composition (i.e. Na-SO₄-type, Na-SO₄-Cl-type, Na-Cl-type and Na-Cl-HCO₃-type). The Na-SO₄-type geothermal water forms as a consequence of water–rock interaction and generally occurs in the outer part of the basin. The Na-Cl-type geothermal water is further subdivided into the original Na-Cl-type geopressured thermal water and the mixed Na-Cl-type geothermal water, in terms of its geochemical and isotopic composition. The original Na-Cl-type geopressured thermal water originates from a geopressured hydrothermal system containing the altered fossil formation waters that are sealed at depth. It moves up to the upper part of the depositional succession or the ground, and generally does not mix with groundwater that is of meteoric origin. This type of water is cooled by heat conduction. The concentration of Cl^– in this type of thermal water is very similar to that in seawater. The δD and δ¹⁸O values are approximately constant and independent of temperature. The original Na-Cl-type geopressured thermal water is distributed mainly along anticlinal axes in folded Neogene formations. The mixed Na-Cl-type geothermal water is related to the expulsion activity of the geopressured hydrothermal system and occurs mostly along active faults. It is formed by shallow groundwater of meteoric origin being mixed with geopressured hydrothermal water when the geopressured hydrothermal system was expulsed along active faults by paroxysmal tectonic events.     

New evidence relating to the structure of the zone of hydrothermal discharges in the East Pauzhetka thermal field,Kamchatka

This paper is concerned with a discussion of a fundamental problem in geothermal research, that of reaching an understanding of the structure and physical nature of zones of thermal discharges, taking the Pauzhetka hydrothermal system as an example along with the eponymous geothermal field situated at the southern tip of Kamchatka. We combined geological and geophysical results from magnetic surveying, electrical surveying, gravity surveying, soil thermometry, pitting and well drilling in hydrothermal clay, to carry out a detailed study of a large zone of hydrothermal discharges in the structure of this hydrothermal system, that is, the East Pauzhetka thermal field. We identified a system of blocks that control aquifers, mineralization zones at the base of the sequence of hydrothermal clay, steam-charged and water-saturated volumes, and volumes where ascending hydrothermal brines circulate via fissures and pores. We hypothesize the existence of a shallow (a few tens of meters) top of a subintrusive body of an intermediate or basic composition. The intrusion of that body may have given rise to the formation of a tectono-magmatic uplift in the East Pauzhetka thermal field. The resulting geological and geophysical data stimulate multidisciplinary surveys of other areas in the Pauzhetka geothermal field and make their contribution to the solution of a major scientific and applied problem, which is to determine the source of heat for the field.     

Interpretation of gravity data to delineate structural features connected to low-temperature geothermal resources at Northeastern Portugal

A great number of low-temperature geothermal fields occur in Northern-Portugal related to fractured rocks. The most important superficial manifestations of these hydrothermal systems appear in pull-apart tectonic basins and are strongly conditioned by the orientation of the main fault systems in the region. This work presents the interpretation of gravity gradient maps and 3D inversion model produced from a regional gravity survey. The horizontal gradients reveal a complex fault system. The obtained 3D model of density contrast puts into evidence the main fault zone in the region and the depth distribution of the granitic bodies. Their relationship with the hydrothermal systems supports the conceptual models elaborated from hydrochemical and isotopic water analyses. This work emphasizes the importance of the role of the gravity method and analysis to better understand the connection between hydrothermal systems and the fractured rock pattern and surrounding geology.