The origin of hypersaline liquid in the quaternary kakkonda granite, sampled from well WD-1a, kakkonda geothermal system, Japan

Hypersaline metal-rich liquid (ca. 40 wt% total chloride species) was obtained from a depth of 3708 m in the Kakkonda geothermal system. Sampling of well WD-1a was conducted by reverse circulation after a standing time of about 196 hours (with temperature recovering to >500°C). Tritium content and the relationship between δD and δ¹⁸O showed that the river water that was circulated in the well had mixed with an isotopically heavy fluid during the standing time. Phase separation occurred during temperature recovery, concentrating the hypersaline liquid in the bottom of the well. This original hypersaline liquid has a salinity of about 55 wt% NaCl eq., consisting of Na–Fe–K–Mn–Ca chloride, rich in Zn and Pb but poor in Cu, Au and Ag. The fluid originates from the Kakkonda granite and mixed with circulating water from the well in a zone of fine fractures induced by thermal stress during drilling.     

Chemical variations in hydrothermal minerals of the Los Humeros geothermal system, Mexico

The Los Humeros geothermal system is composed of more than 2200 m of Quaternary altered volcanic rocks and an underlying Cretaceous sedimentary sequence. The low salinity of the fluids discharged at present (Na⁺ and Cl⁻ concentrations <500 ppm), and the excess steam, indicate that the reservoir contains a mixture of steam and dilute groundwater. Water-rock equilibrium is not attained. Hydrothermal minerals are present in veinlets, vugs, and replacing primary minerals. Three mineral zones are recognized: 1) a shallow argillic zone (<400 m depth), 2) a propylitic zone (ranging between 500 and 1800 m) and 3) a skarn zone (>1800 m). Petrographic examination of cuttings from five wells and temperature data indicate at least two stages of hydrothermal activity. Temperature is the main factor that affects the chemical composition of chlorite, epidote and biotite. Fe²⁺ and Al^IV increase in chlorite with temperature [from 1.4 formula position unit (fpu) to 2.8, and from 0.7 to 2.4 fpu, respectively]. The pistacite content of epidote varies from 18 to 33 mol% in high-temperature regions (>270 °C) and from 13 to 26 mol% in low-temperature regions (<250 °C). Biotite displays a slight increase in Al^IV contents (1.55–2.8) and octahedral occupancy (5.93–6.0 fpu) with temperature. Whole rock composition and variations in oxygen fugacity conditions are factors that also affect the concentrations of Fe, Al and Mg in the octahedral sites of chlorite, epidote, biotite and amphiboles. Chemical variations observed in alteration minerals at different depths in the Colapso Central-Xalapazco region could be used as indicators of relict physico-chemical conditions in the reservoir, before the present economic exploitation.     

Revised hypocenter solutions for microearthquakes in the kakkonda geothermal field, japan

The hypocenters of microearthquakes in the Kakkonda geothermal field have been relocated along the Kakkonda River using a new velocity structure model. Compared to the solution used by the previous velocity model, the depth of the hypocenters is shallower in the relocation. The microearthquakes occurred in a highly fractured region, as suggested by geological and petrological studies based on well data, but did not occur along major tectonic folds and faults. An alternative hydraulic condition might be necessary to trigger the earthquakes. Seismicity in 1995 was lower than in 1988 in Kakkonda. The decrease in the number of events is possibly due to the decrease in the amount of reinjection fluid or the change in the characteristics of the geothermal reservoir.The number of microearthquakes decreases rapidly at 1–2 km below sea-level. Probability density of seismic energy distribution is utilized to indicate the active seismic regions. The model also shows that a contour map of the lower boundary of the high seismic energy region corresponds to the occurrence of cordierite, which was produced by heat from the neo-granitic pluton body, implying that the occurrence of microearthquakes in the Kakkonda geothermal field is controlled by the neo-granitic rocks at depth. The top of the granite can be imaged, using the probability density of seismic energy distribution.     

High-temperature measurements in well WD-1A and the thermal structure of the kakkonda geothermal system, Japan

The New Energy and Industrial Technology Development Organization (NEDO) drilled well WD-1a between 1994 and 1995 in the Kakkonda geothermal field as part of their Deep Seated Geothermal Resources Survey project. High-temperature measurements were carried out in WD-1a. Logging temperatures above 414°C were confirmed at 3600 m and 3690 m depth after 82 h standing time. Simple Horner extrapolations based on observed temperatures up to 82 h after shut-in suggested a temperature of about 500°C at 3500 m depth. Temperatures between 500°C and 510°C were also confirmed at 3720 m depth after 129–159 h standing time, using calibrated melting .tablets. These are the highest temperatures measured in a geothermal well. These results suggest a thermal structure consisting of three layers. Layer one is a shallow permeable zone of the reservoir, at less than 1500 m depth, at 230°C to 260°C. The second layer is a deep zone of the reservoir, which is less permeable and has a temperature of 350°C to 360°C from 1500 m to about 3100 m depth. The third layer is a zone of heat conduction. The transition between the hydrothermal-convection zone and the deeper heat-conduction zone is at 3100 m depth in well WD-1a.     

Interpretation of magnetic anomalies over the Waimangu geothermal area, Taupo volcanic zone, New Zealand

The study area is located on the eastern side of the Taupo volcanic zone in central North Island of New Zealand. It lies a few kilometres to the southwest of Mt Tarawera, the site of the biggest New Zealand volcanic eruption in historical times (the June 1886 Tarawera eruption). The study area includes the Waimangu geothermal field and a small part of Waiotapu and Waikite fields. The extensive surface thermal expressions (boiling springs, hot lakes, craters, and sinter terraces) occurring at the Waimangu field were all formed following the 1886 Tarawera eruption. Another large area of less intense thermal manifestations (thermal ground and hydrothermally altered rocks) exists about 5 km southwest of Waimangu, extending towards the Waiotapu field in the south. In 1993 an aeromagnetic survey was conducted over the study area at an average altitude of about 350 m above the ground. The results show a subdued negative residual anomaly (about −100 nT) over the Waimangu field, which can be interpreted by near-surface hydrothermal demagnetisation of rhyolitic host rocks. The lateral distribution of the demagnetised rocks is much greater than the thermal area of Waimangu, and is consistent with the extent of low resistivity rocks across the study area. The magnetic interpretation also shows that two-high standing dacite domes situated about 5 and 7 km to the southwest of Waimangu have been affected by hydrothermal demagnetisation. There are negative residual anomalies outside the low resistivity zone that could be associated with reversely magnetised rocks (age >0.78 Ma). A strong positive residual anomaly (up to 450 nT) occurs to the east of the Waimangu field. Results from 3-D magnetic interpretation indicate some alternative models for this positive anomaly: (1) southwest–northeast trending, vertical basalt dykes (magnetisation 10 A/m), tops between −0.1 and −0.65 km RL (reduced LEVEL=relative to sea level), (2) a thick ( 1 km) sequence of rhyolites (magnetisation 2.5 A/m) extending from the surface down to about −0.8 km RL, and (3) a rather thin (0.35 km) sequence of rhyolites (from surface to sea level) underlain by basalt bodies similar to those of model (1).     

Geochemical characteristics of the Yufuin outflow plume, Beppu hydrothermal system, Japan

Thermal water at Yufuin (Kyushu Island, Japan) is tapped through about 820 shallow wells and used mainly for hot-spring bathing purposes. Chemical and isotopic data for fluids from wells and fumaroles in Yufuin and Beppu indicate that the thermal activity at Yufuin represents a dilute, westward-flowing hydrothermal outflow plume from the Beppu hydrothermal system. Two other (eastward-flowing) outflow plumes have long been recognized at Beppu, but the Yufuin outflow plume is first recognized here. The Yufuin outflow plume is apparently a mixture of two end-member fluids: (1) deep high-temperature (250–300°C) fluid from the Beppu system having high chloride concentration (1400–1600 mg/L) and a δ¹⁸O value near −6.0%, and (2) meteoric water having low chloride concentration (≤7 mg/L) and a δ¹⁸O value near −9.2%.A permeable conduit for the vertical and lateral transport of deep fluid from the Beppu system is provided by the Yufuin Fault zone, which extends westward from the southern flank of Mt. Tsurumi volcano to the town of Yufuin. Stable isotope ratios and chloride concentrations for shallow groundwaters near the eastern end of the fault, at an elevation near 700 m, are consistent with those required for the low-chloride meteoric end-member of the Yufuin thermal waters. Recharge of this meteoric water, as well as mixing with the Beppu deep fluid, may occur along the Yufuin Fault. Enthalpy-chloride relations indicate additional conductive heating of the Yufuin waters, in the amount of 350–500 kJ/kg beyond that which can be accounted for by mixing between Beppu deep fluid and meteoric water. This could be a result of conductive heating with convection to a depth of 1–2 km. Estimates of the magnitude of the heat source for the Beppu hydrothermal system should take into account the heat being discharged at Yufuin.     

Evidence of a contact metamorphic aureole with high-temperature metasomatism in the deepest part of the active geothermal field of Larderello,Italy

The deep part (1.5–4.5 km) of the Larderello geothermal field (Tuscany, Italy) consists of rocks that were metamorphosed by the Hercynian and Alpine orogenies, and which were thermally metamorphosed in the same place during the emplacement of granite intrusions of 3.8–1.0 Ma age. These rocks are potential deep-seated reservoirs and could be the target of future exploitation. The petrographic, geochemical and fluid inclusion data indicate that thermally metamorphosed phyllite, micaschist, gneiss, amphibolite and carbonates underwent a recrystallisation at temperatures of 425–670 °C, under a lithostatic pressure regime of 95-130 MPa. Li–Na-rich fluids of magmatic origin, and aqueous-carbonic fluids with varying proportions of H₂O and CO₂ that formed during the contact metamorphism, were present during this stage. The fluids present during the contact metamorphic event were responsible for a widespread B-metasomatism and local F-metasomatism. In some cases, high-temperature metamorphism of graphitic schists can control the composition of the aqueous-carbonic fluids. A late-stage, lower temperature hydrothermal activity was responsible for both the propylitic and sericitic alterations and for the replacement of the high-temperature mineral assemblage. Stable isotope (δ¹⁸O and δD) data on the thermally metamorphosed rocks and granites indicate that these rocks underwent depletion in the heavy isotopes. Magma degassing and dehydration metamorphic reactions can explain the isotopic values of these rocks. Late-hydrothermal fluid of meteoric origin may also have contributed to the depletion of the heavy isotopes from the rocks. Under contact metamorphism conditions the rocks were plastic and impermeable. A transition from plastic to brittle conditions occurred as a consequence of the cooling of the system at depths of <4 km. The brittle-plastic transition at Larderello now occurs at a depth of 4.5–5 km, where present-day temperatures are in the range 400–450 °C. Deep-seated fluids probably occur in this zone, as suggested by the geophysical data.     

Hydrothermal alteration in the Aluto-Langano geothermal field, Ethiopia

The hydrothermal mineral assemblages found in eight wells (with a depth range of 1320–2500 m) of the active geothermal field of Aluto-Langano (Ethiopia) indicate a complex evolution of water-rock interaction processes. The zone of upflow is characterized by high temperatures (up to 335°C) and the presence of a propylitic alteration (epidote, calcite, quartz and chlorite, as major phases) coexisting with calcite and clay minerals. The zone of lateral outflow is characterized by mixing of deep and shallow waters and the occurrence of a calcite-clay alteration that overprints a previous propylitic assemblage. Clay minerals have a mushroom-shaped zonal distribution consistent with the present thermal structure of the field. Microprobe analyses have been carried out on chlorite and illite in order to apply several geothermometers. Most of the chlorite is iron-rich chlorite. It is found that the temperatures calculated from the chlorite geothermometer (159–292°C) after Cathelineau and Nieva [Contrib. Mineral. Petrol. 91, 235–244 (1985)] are in good agreement with in-hole measured temperatures (155–300°C). In the upflow zone, temperatures calculated from this geothermometer (217–292°C), together with fluid inclusion data of Valori et al. [Eur. J. Mineral. 4, 907–919 (1992)], and computed saturation indices of alteration minerals, indicate thermal stability or slight heating. On the other hand, evidence of a significant cooling process (up to 171°C) in the outflow zone is provided by the comparison between fluid inclusion homogenization temperature (240–326°C) and in-hole temperature (155–250°C). The apparent salinities (0.8–2.3 wt% NaCl eq.) of the fluid inclusions are generally higher than the salinity of the present reservoir fluid (0.29–0.36 wt% NaCl eq.). Clay minerals (illite, smectite, Ill/S mixed layers, vermiculite and chloritic intergrades) generally occur at temperatures consistent with their stability fields.     

Temperature evaluation of the Bugok geothermal system, South Korea

Using a variety of chemical geothermometers and statistical analysis, we estimate the temperature of a possible deeper geothermal reservoir at Bugok, Southern Korea. Shallow thermal aquifers (down to about 400 m depth) are under exploitation in this area; the temperatures (up to 78 °C) of the produced fluids are the highest found in South Korea. Based on hydrochemical data and occurrence, the groundwaters at Bugok can be classified under three groups: Na-SO₄ thermal groundwaters (CTGW) occurring in the central (about 0.24 km²) part of the area; Ca-HCO₃ cold groundwater (SCGW) found in shallow peripheral parts of the CTGW; the intermediate-type groundwater (STGW). The CTGW type is typical of the Bugok thermal waters; they have the highest discharge temperatures and contain very high concentrations of Na (75.1–101.0 mg/L), K (2.9–6.9 mg/L) and SiO₂ (62.0–84.5 mg/L) and are rich in sulfates.The major ion composition of the CTGW suggests that these waters are in partial equilibrium with rocks at depth. The application of various alkali-ion geothermometers yields temperature estimates in the 88–198 °C range for the thermal reservoir. Multiple-mineral equilibrium calculations indicate a similar but narrower temperature range (from about 100 to 155 °C). These estimates for CTGW are significantly higher than the measured discharge temperatures. Considering the heat losses occurring during the ascent of the waters, one can infer the presence of a deeper (around 1.8 km) thermal reservoir in the Bugok area that could be developed for district heating or other direct applications of geothermal heat.     

Geochemical approach to the Bou Hadjar hydrothermal system (NE Algeria)

The Bou Hadjar low-temperature hydrothermal system is located in northeast Algeria. The four main thermal springs that are the subject of the study emerge ith temperatures between 32 and 60°C from allochthonous formations. The reservoir temperature has been estimated from chemical compositions by utilizing simultaneously the silica, gas and sulfate-water oxygen isotope geothermometers, fluid-mineral equilibrium calculations, and a mixing model. According to these thermometric methods, the most probable subsurface temperature is in the range 75–106°C. The mixing model suggests a temperature of 125°C for the parent water.     

Fluid flow processes in the BEPPU geothermal system, Japan

The Beppu geothermal system is centred beneath the late Quaternary volcanoes of Tsurumi and Garandake at the northern end of the Ryukyu volcanic arc. The deep fluid has a temperature of at least 250–300°C, and an inferred chloride concentration of 1400–1600 mg/kg. Apart from fumarolic areas near the summits of the two volcanoes, most thermal activity occurs at low elevation along the two main outflow paths towards the coast. The hot spring waters of downtown Beppu have originated from outflow along the Asamigawa Fault, with their chemistry indicating predominantly dilution of the deep fluid by groundwater. The second outflow zone towards the hot spring area of downtown Kamegawa coincides with a ridge of lavas. Here boiling, steam loss, and subsequent mixing with steam-heated groundwaters have significantly modified both the deep fluid and host rocks. The area of the geothermal system above 200°C is at least 15 km² at sea level, and the total natural heat output is inferred to be at least 250 MW. Most of this heat output occurs as subsurface hot water outflows towards the coast due to the 1300 m of topographic relief across the system.     

Secondary mineral growth in fractures in the Miravalles geothermal system, Costa Rica

A mineralogical, fluid-chemical, and theoretical study of hydrothermal alteration in veins from drillcore from the Miravalles Geothermal Field, Costa Rica has revealed a complex history of mineral-fluid reaction which may be used to characterise changes in temperature and fluid composition with time. Mineralogical and mineral-chemical data are consistent with hydrothermal alteration in the temperature range 200°–270°C, with deeper portions of the system having undergone temperatures in excess of 300°C. Thermodynamic calculations suggest that the observed alteration assemblage is not in equilibrium with current well fluids, unless estimates of reservoir pH are incorrect. Fe-Al zoning of prehnite and epidote in veins is consistent with rapid, isothermal fluctuations in fluid composition at current reservoir temperatures, and may be due to changes in volatile content of the fluid due to tectonic activity.     

The Iceland Deep Drilling Project: a search for deep unconventional geothermal resources

The Iceland Deep Drilling Project (IDDP) is a long-term program to improve the economics of geothermal energy by producing supercritical hydrous fluids from drillable depths. Producing supercritical fluids will require the drilling of wells and the sampling of fluids and rocks to depths of 3.5–5 km, and at temperatures of 450–600 °C. The IDDP plans to drill and test a series of such deep boreholes in the Krafla, Nesjavellir and Reykjanes geothermal fields in Iceland. Beneath these three developed high-temperature systems frequent seismic activity continues below 5 km, indicating that, even at supercritical temperatures, the rocks are brittle and therefore likely to be permeable, even where the temperature is assumed to exceed 550–650 °C. Temperature gradients are greater and fluid salinities smaller at Nesjavellir and Krafla than at Reykjanes. However, an active drilling program is underway at Reykjanes to expand the existing generating capacity and the field operator has offered to make available one of a number of 2.5 km deep wells to be the first to be deepened to 5 km by the IDDP. In addition to its potential economic significance, drilling deep at this location, on the landward extension of the Mid-Atlantic Ridge, is of great interest to the international science community. This paper examines the prospect of producing geothermal fluids from deep wells drilled into a reservoir at supercritical temperatures and pressures. Since fluids drawn from a depth of 4000–5000 m may prove to be chemically hostile, the wellbore and casing must be protected while the fluid properties are being evaluated. This will be achieved by extracting the fluids through a narrow retrievable liner called the “pipe”. Modelling indicates that if the wellhead enthalpy is to exceed that of conventionally produced geothermal steam, the reservoir temperature must be higher than 450 °C. A deep well producing 0.67 m³/s steam (2400 m³/h) from a reservoir with a temperature significantly above 450 °C could, under favourable conditions, yield enough high-enthalpy steam to generate 40–50 MW of electric power. This exceeds by an order of magnitude the power typically obtained from a conventional geothermal well in Iceland. The aim of the IDDP is to determine whether utilization of heat from such an unconventional geothermal resource at supercritical conditions will lead to increased productivity of wells at a competitive cost. If the IDDP is an economic success, this same approach could be applied in other high-temperature volcanic geothermal systems elsewhere, an important step in enhancing the geothermal industry worldwide.     

Diffusion measurements using the shear cell technique: Investigation of the role of Marangoni convection by pre-flight experiments on the ground and during the Foton M2 mission

An experimental investigation on the disturbing influence of the solutal Marangoni convection during diffusion measurements is presented. We used the systems Sn–Bi, Pb–Ag and Sn–In with decreasing differences in surface tension. For all systems we measured surface tension and – under varying free surface conditions – diffusion coefficients. We succeeded in measuring diffusion coefficients on the ground under nearly non-free surface conditions. The results are compared with the results of the FOTON M2-satellite mission with similar μg-experiments. We show that temperatures from around 400 °C to about 800 °C and the degree of free surfaces influences strongly the measured value for the diffusion coefficient.     

Volcano–tectonic structures, gravity and helium in geothermal areas of Tuscany and Latium (Vulsini volcanic district), Italy

Since the early 1980s, geological and structural mapping, gravity, and helium soil–gas studies have been performed in the eastern sector of the Vulsini Volcanic District (Roman Magmatic Province) in an attempt to locate potential geothermal reservoirs. This area is characterised by an anomalous geothermal gradient of >100°C/km, and by widespread hydrothermal mineralization, thermal springs, high gas fluxes, and fossil and current travertine deposits. The results of these surveys indicate the existence of a number of fault systems, with N–S and E–W structures that appear to be superimposed on older NW–SE and NE–SW features. Comparison of the results of the various studies also reveals differences in permeability and potential reservoir structures at depth.     

Hydrogeochemical exploration of geothermal prospects in the Tecuamburro Volcano region, Guatemala

Chemical and isotopic analyses of thermal and nonthermal waters and of gases from springs and fumaroles are used to evaluate the geothermal potential of the Tecuamburro Volcano region, Guatemala. Chemically distinct geothermal surface manifestations generally occur in separate hydrogeologic areas within this 400 km² region: low-pressure fumaroles with temperatures near local boiling occur at 1470 m elevation in a sulfur mine near the summit of Tecuamburro Volcano; non-boiling acid-sulfate hot springs and mud pots are restricted to the Laguna Ixpaco area, about 5 km NNW of the sulfur mine and 350–400 m lower in elevation; steam-heated and thermal-meteoric waters are found on the flanks of Tecuamburro Volcano and several kilometers to the north in the andesitic highland, where the Infernitos fumarole (97°C at 1180 m) is the primary feature; neutral-chloride hot springs discharge along Rio Los Esclavos, principally near Colmenares at 490 m elevation, about 8–10 km SE of Infernitos. Maximum geothermometer temperatures calculated from Colmenares neutral-chloride spring compositions are 180°C, whereas maximum subsurface temperatures based on Laguna Ixpaco gas compositions are 310°C. An exploration core hole drilled to a depth of 808 m about 0.3 km south of Laguna Ixpaco had a bottom-hole temperature of 238°C but did not produce sufficient fluids to confirm or chemically characterize a geothermal reservoir. Hydrogeochemical data combined with regional geologic interpretations indicate that there are probably two hydrothermal-convection systems, which are separated by a major NW-trending structural boundary, the Ixpaco fault. One system with reservoir temperatures near 300°C lies beneath Tecuamburro Volcano and consists of a large vapor zone that feeds steam to the Laguna Ixpaco area, with underlying hot water that flows laterally to feed a small group of warm, chloriderich springs SE of Tecuamburro Volcano. The other system is located beneath the Infernitos area in the andesitic highland and consists of a lower-temperature (150–190°C) reservoir with a large natural discharge that feeds the Colmenares hot springs.     

Major engineering constructions on top of a high-temperature geothermal system: problems encountered at Tokaanu, New Zealand

The Tokaanu hydroelectric power station, a 2-km long section of the headrace tunnel, and an approximately 0.5-km long segment of the tailrace were sited along the broad, southern resistivity boundary of the active, high-temperature Tokaanu–Waihi geothermal system. Exploration drilling over the tunnel route showed that thermally altered rocks with temperatures up to 42 °C would be found at tunnel level. No toxic gases were detected and excavation of the tunnel went ahead. Temperatures above 99 °C, however, were encountered at shallow depths (<20 m) near the base of the powerhouse and along a section of the tailrace canal. This produced flashing discharges at the wellhead of several test bores. The potential for hydrothermal eruptions, which could be triggered by reducing hydrostatic pressures, was tested by stepwise dewatering of two, max. 10,000 m², artificial ponds. No eruptions occurred and the canal was excavated after completing the tests. A heat balance analysis of the final test is presented. For safety, the bottom of the canal was raised and widened over the hot ground.     

Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging

Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral pH, 215±10 °C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean T_h of 213 °C; 0.2–0.4 wt.% NaCl_eq.). Relict quartz–adularia–illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for “pulses” of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T_h values of 210±40 °C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with T_h values of 201±6 °C. Low-pH fluids in the near-surface setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous “silica residue”.     

Hydrothermal alteration of Hercynian granites,its significance to the evolution of geothermal systems in granitic rocks

We discuss geochemical and isotopic (¹⁸O/¹⁶O, ²H/¹H and ⁸⁷Sr/⁸⁶Sr) data recording the hydrothermal alteration of northern Portuguese Hercynian granites by Na–HCO₃–CO₂-rich mineral waters. Whole-rock samples from drill cores of Vilarelho da Raia granite have δ¹⁸O values in the +11.47 to +10.10‰ range. The lower values correspond to highly fractured granite samples displaying vein and pervasive alteration. In the pervasive alteration stage, which probably results from a convective hydrothermal system set up by the intrusion of the granites, the metamorphic waters are in equilibrium with hydrous minerals. In contrast, the vein alteration of these granitic rocks was caused by water of meteoric origin. The oxygen ratios between water (W) and rock (R), the so-called W/R ratios, obtained for the open system (where the heated water is lost from the system by escape to the surface) range between 0.05 and 0.11, suggesting that the recrystallization of the veins was influenced by a small flux of meteoric water. Stable isotope analyses performed on the cores show that the vein alteration stage relates to post-emplacement tectonic stresses acting on the granite, probably of late Hercynian age. Our results are consistent with the existence of two separate alteration events (pervasive and vein) caused by hydrothermal waters of different isotopic characteristics. The studies presented in this paper should be viewed as a natural analogue that uses the alteration features observed in a “fossil” geothermal system at Vilarelho da Raia to assess possible water–rock reactions presently occurring at depth in granitic rocks of the nearby Chaves area.     

Geothermal resources of Hachijojima

Hachijojima is a gourd-shaped volcanic island in the Pacific Ocean. Nishiyama and Higashiyama volcanoes consist of basalt lava and associated pyroclastic rocks. A promising geothermal resource was found in south Higashiyama, associated with an uplift of Tertiary rocks consisting of mainly andesite lava and related pyroclastic rocks, overlain by Quaternary volcanic rocks. Steep high-temperature (over 250°C) and high-pressure gradients occur in the deeper portion of the system near the Tertiary–Quaternary contact, indicating the presence of a cap rock. The cap rock formed by deposition of hydrothermal minerals. Geothermal fluid ascends from the deeper portions to shallow depths along vertical fractures through the cap rock. These vertical fractures form the geothermal reservoir in the Tertiary formation. Three wells were drilled into these vertical fractures, and approximately 30 t/h of superheated steam was obtained from each well during flow tests. The geothermal fluid is mainly a mixture of seawater and meteoric water in an approximate ratio of 1 to 2, based on chemical analyses, with a portion of volcanic gas included. At present a 3.3 MW_e, geothermal power plant is being constructed here.