Active Geothermal Field Research Articles

熱水性鉱床に産する緑泥石の化学組成と緑泥石地質温度計の適用性

Compositional and structural properties of clay minerals from hydrothermal systems show highly diverse and systematic variations related to their localities. The chemical compositions of chlorite, one of the most common clay minerals, are thought to have a close relationship with its formational environments such as geological conditions, temperature, chemistry of hydrothermal solution, etc. In this study two chlorite geothermometers were considered by using the chemical composition of chlorites from some hydrothermal ore deposits in Japan. The two chlorite geothermometers are the Cathelineau's thermometer giving an empirical temperature scale of chlorite formation in an active geothermal field, and the Walshe's thermometer giving a thermodynamic temperature scale based on exchange reactions of chlorite solid solutions. On the basis of a comparison of fluid inclusion geothermometry, it is concluded that the two geothermometers can be applied to different compositional types of hydrothermal chlorite. The Cathelineau's thermometer fits well for Mg-chlorite, and the Walshe's thermometer fits for Fe-chlorite, indicating that both two chlorite geothermometers can be used as exploration indicators for geothermal reservoirs and hydrothermal ore deposits. In addition, it must be emphasized that the impurities in the chlorite should be examined in applying the chlorite geothermometers to chlorites.

X-Ray Diffraction Analysis of Hydrothermal Minerals from the Los Azufres Geothermal System, Mexico

Los Azufres is an active geothermal field located in the middle of the Mexican Volcanic Belt (MVB), a vast Miocene-Recent, E-W-oriented volcanic province spanning central Mexico. Three wells—Az-5, Az-28, and Az-31—from this geothermal field were selected to investigate the distribution, alteration sequence, and thermal stability of hydrothermal minerals. Maximum in situ measured temperatures are 280°C at 1493 m, 265°C at 1700 m, and 288°C at 1300 m depths in Az-5, Az-28, and Az-31, respectively. The host rocks in these wells are dominantly andesite followed by dacite, rhyolite, and basalt. Rock cuttings from different depths were analyzed for clay and non-clay minerals by X-ray Diffraction (XRD) methods. Hydrothermal quartz, calcite, and pyrite, as well as other alteration mineral phases (e.g., chabazite and chlorite) that are difficult to identify by traditional petrography were identified and their abundances semi-quantitatively estimated by XRD. We show that these mineral data present a better perception of distribution trends of hydrothermal minerals in geothermal wells than the qualitative mineral identifications generally used for this purpose. Homogenization temperatures measured in fluid inclusions of hydrothermal minerals, in situ measured temperatures in the wells, and K+/H+ vs. Mg2+/(H+)2 activity diagrams for the chemical characteristics of the present geothermal fluids were used to define the thermal regime and the resultant stability conditions of the clay minerals. Smectite, illite, and chlorite are present in the <2 mu;m size fraction. Gradual variations in relative abundances of clay minerals range from smectite dominant at shallow well depths to a combination of smectite, illite, and chlorite at intermediate depths, and to illite and chlorite in the deepest levels. Excellent crystallinity and lack of mixed-layered clay minerals support a model involving a discontinuous change from smectite to chlorite and/or illite, rather than that involving continuous mixed-layering of smectite-illite and/or smectite-chlorite. Mineralogical and fluid inclusion data suggest that mineral distribution trends documented for the Los Azufres geothermal system reflect the prevailing thermal regime. The mineral parageneses of the Los Azufres geothermal field are broadly comparable with those reported in other geothermal systems of the world.

Monitoring and mapping of hydrogen sulphide emissions across an active geothermal field: Rotorua, New Zealand

Hydrogen sulphide (H 2S) is one of a number of gaseous species associated with geothermal activity in the Taupo Volcanic Zone (TVZ), New Zealand. The city of Rotorua is located within Rotorua Caldera in the TVZ and is one of the few urban areas in the world where a large population (>60,000 people) is frequently exposed to geothermal emissions. In order to evaluate the health hazard from long-term exposure to H 2S being emitted from the Rotorua geothermal field, a passive sampler has been developed to measure concentrations of H 2S at many locations across the city simultaneously. In contrast to other passive or pump-based samplers, the sampler is inexpensive, easily mass-manufactured, and involves the reaction of H 2S with silver halide contained in treated photographic paper. H 2S-exposed paper shows a distinct colour change from white to dark brown as H 2S concentrations increase and is sensitive to concentrations between ≪30 and around 1000 ppb. Rotorua city can be divided into three regions—an area of low H 2S concentration in the west, a ‘corridor’ of high concentrations running north–south through the city centre where H 2S is being emitted, and an area of medium concentration to the east which is influenced by the prevailing wind direction, creating a plume from the central corridor. The data give new insight into the subsurface routes of degassing in the Rotorua geothermal field, by showing the surface expression of the main upflow zone and the direction of the conjectured faulting below.

Salt loaded heat pipes: steady-state operation and related heat and mass transport

Fluids in the deep-seated zones (3.5–4.5 km) of active geothermal zones are known to have increased salinity and acidity that can enhance interaction with surrounding porous rocks. A possible mechanism for brine generation is the separation of the rising magmatic fluid into a gas-like and a liquid-like component. This work illustrates the main features of this mechanism by investigating the conditions for heat pipe convection of natural brines in hydrothermal systems. The well-established heat pipe regime for convection of two-phase pure water (vapor–liquid) in a porous column is extended to the case of boiling brines. In particular, the NaCl–H 2O system is used to model the 1-D reactive flow with dissolution-precipitation in geothermal reservoirs. The quasi steady-state equations of the conservation of matter, Darcy’s law for the gas and liquid phases, and the heat balance equation have been examined while neglecting the temporal variation of porosity. A semi-analytical procedure is used to solve these equations for a two-phase fluid in equilibrium with a solid salt. The solution is in the form of the dependence of liquid volume fraction as a function of temperature for different heat fluxes. The solution is separated into two isolated regions by the temperature T=596°C, at the maximum fluid pressure for three-phase (H–L–V) equilibrium. In the case of unsaturated two-phase flow at the reference permeability of porous rocks (3·10 −16 m 2), the maximum heat flux that can be transferred through the porous column via convection is analytically estimated to be 4.3 W/m 2. This is close to the corresponding value for the three-phase case that is numerically calculated to be 6 W/m 2. Due to dissolution (partial leaching of oxide components by acid condensates) and precipitation of salt at the boiling front, heat transfer in a heat pipe in soluble media occurs in a direction opposite to the associated mass transfer. This can cause deep hydrothermal karsting that is manifested as surface subsidence at rates of about several cm/yr as observed in some active geothermal fields.

Deformation of the Long Valley Caldera, California: inferences from measurements from 1988 to 2001

Two periods of volcanic unrest occurred between 1989 and 1998 in the Long Valley Caldera, eastern California. Numerous earthquakes were recorded, and these periods of unrest were documented with high-precision geodetic measurements. The first round of unrest started rapidly in late 1989 and slowly decreased in rate through the early 1990s. For this interval there are both leveling and two-color electronic distance meter (EDM) measurements. The second round of unrest started slowly in mid-1997, climaxed in late 1997, and rapidly returned to quiescence by mid-1998. Deformation was recorded by both the two-color EDM and continuous GPS. Both episodes require inflation at 6–7 km beneath the resurgent dome, and both episodes had roughly 0.1 m extension across the resurgent dome. In addition, the data presented here suggest that there is a deeper, 10–20 km, inflation source beneath the south moat of the caldera. For both episodes, the better-resolved inflation beneath the resurgent dome is a near-vertical, prolate spheroid rather than an isotropic source, which suggests that magma came up through vertical cracks. However, the modeling suggests that the location changed with the depth from 6.0 to 6.7 km for the later episode. In contrast to the earlier episode, the 1997–1998 episode has additional deformation in the south moat, where the simplest model is that of a right-lateral slip on a steeply dipping plane that is defined by the location of earthquakes in the south moat. Models of the time-dependent behavior suggest that slip on this fault occurred from late November through December 1997, corresponding to the time of greatest moment release by the earthquake swarm in the south moat. Confounding the interpretation of these data is an active geothermal field near the center of the EDM network and adjacent to the south moat and resurgent dome. Additional modeling of leveling and EDM data within the geothermal field during a period of low rate of inflation of the dome suggests some methods of adjustments to the EDM data during the inflation episodes.

Reaction path modelling in the As–S system: a case study for geothermal As transport

Geochemical speciation and reaction path modelling with the Geochemists Workbench (GWB) software was used to investigate zonal As sulphide mineral precipitation and As transport in an active geothermal field, the Uzon Caldera, Kamchatka. A new compilation and critical review of published experimental and theoretical thermodynamic data for As phases was used to modify and update a SUPCRT92 database with important missing phases. The equilibrium constants for these As phases were then added or modified in the current GWB database. Speciation calculations predict that the sampled fluids are undersaturated with respect to As phases and aqueous As is dominantly transported as the complex H 3AsO 3(aq) and to a lesser extent the As sulphide complexes As 2S 3(aq), HAs 2S 4 − and As 2S 4 2−. Modelling the changes in concentration of dissolved As between the samples (0.2–8.6 mg/kg) indicates a strong dependence on redox (log O 2(g) from −53 to −60) and temperature (95–65 °C), and illustrates the importance of mixing between the hydrothermal fluid and an oxygenated fluid. Reaction path models that follow the cooling of a H 2S (aq) dominated, As enriched (15 mg/kg) fluids from 125 to 25 °C, with sliding redox from log O 2(g) −55 to −60, predict the mineral paragenesis: (native As+pyrite)−(realgar+pyrite)−(orpiment+pyrite)−(pyrite). This mineral sequence closely resembles the natural layering observed in the Uzon Caldera. Although field measurements contain a lower reduced S concentration than the model, the error margin in the measured sulphide inherent from in situ oxidation during sampling is enough to account for the discrepancy. Despite assumptions in fluid parameters and modelling approaches, as well as deficiencies in the thermodynamic data and an equilibrium approach, this study has shown that acceptable and useful analogues for natural As-rich systems can be developed.

火山性テクトニック陥没帯における地熱系の特徴：日本とニュージーランド

地熱系の分布における構造支配について研究するために，日本の九重-別府地溝帯(KBG)とニュージーランドのタウポ火山帯(TVZ)を比較検討した．その結果，その違いと類似性が明らかとなった．地熱地域の分布は，両者で大きく異なり，KBG では中央部を斜交する若い火山フロントの直上ないしはその背後に限定されるのに対して，TVZでは地溝帯全体に広く分布している．最も若い火山活動に関しても両者で異なり，KBGでは安山岩質の成層火山を形成する火山活動であるのに対して，TVZではカルデラ形成を伴う流紋岩質火山活動である．従って，現在の地熱地域の熱源としては，KBGでは安山岩質マグマと固結したプルトンが想定されるのに対して，若いTVZでは浅所貫入した流紋岩質のプルトンと岩脈群が想定される．このような両者における熱源と地熱分布の違いは，異なる地熱地域特性を生じさせている．TVZ浅部(深度500-1500 m)では，沸騰温度およびそれに近い流体(250-300°C)が中生界の上位に重なる第四紀火山岩-堆積岩よりなる多孔質帯水層に賦存している．一方，KBGでは，より深い深度(1000-2000 m)に，より低い温度(220°C)の流体が賦存している．KBGでは第四紀安山岩質マグマ近傍域がTVZより深く位置し，天水は深くまで浸透して，地温勾配の低い熱伝導域の上位の破砕された火山岩類の中で熱せられている．このように，火山性テクトニック陥没帯で地熱流体から放出される熱は，マグマ近傍域の深度と天水の浸透する深度とに大きく規制されている．

Spatial characteristics of groundwater temperature in the Ishikari Lowland, Hokkaido, northern Japan: analytical and numerical applications

In porous sediments of the Ishikari Lowland, there is a gradual increase in the background geothermal gradient from the Ishikari River (3–4 °C 100 m–1) to the southwest highland area (10 °C 100 m–1). However, the geothermal gradient at shallow depths differs in detail from the background distribution. In spite of convective heat-flow loss generally associated with groundwater flow, heat flow remains high (100 mW m–2) in the recharge area in the southwestern part of the Ishikari basin, which is part of an active geothermal field. In the northeastern part of the lowland, heat flow locally reaches 140 mW m–2, probably due to upward water flow from the deep geothermal field. Between the two areas the heat flow is much lower. To examine the role of hydraulic flow in the distortion of the isotherms in this area, thermal gradient vs. temperature analyses were made, and they helped to define the major components of the groundwater-flow system of the region. Two-dimensional simulation modeling aided in understanding not only the cause of horizontal heat-flow variations in this field but also the contrast between thermal properties of shallow and deep groundwater reservoirs.

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 H 2O and CO 2 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 ( δ 18O 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.

Mineralogical and Chemical Studies of Volcanic-RelatedArgillaceous Industrial Minerals of the Central American Cordillera(Western El Salvador)

The western provinces of El Salvador, which are covered almost exclusively by igneous rocks, offer favorable conditions to study the entire suite of volcanic-related, nonmetallic deposits involving accumulations of zeolite-feldspar, smectite, kaolinite, and diatomite. Formation of nonmetallic deposits, which is still going on in the active geothermal field near Ahuachapan, began during the late Pliocene. This age is based on micropaleontological data obtained during the study of lacustrine sediments laid down in volcanogenic depressions. Judging by their regional distribution and geological setting, the deposits may be subdivided into regional deposits (T <100°C) and structurally controlled deposits (T = 140°–250°C). Both types of volcanic deposits can be further subdivided into various stages:(1) zeolite-feldspar stage, (2) smectite stage, (3) quartz-kaolinite stage, (4) kaolinite-cristobalite stage, and (5) kaolinite stage. Stages (4) and (5) are characterized by the presence of alunite-, woodhouseite-, and crandallite-group minerals and some minerals of the aluminite-alunogen group. Within the deposits, gradual changes (progressive vs. regressive trends) may be attributed to various alteration processes. The progressive trends are correlative with the differentiation of the mineralizing fluids throughout hydrothermal activity and supergene alteration. The regressive trends are caused by hot-spring diagenesis. Knowledge of the minerostratigraphy in a geothermal field and of the intracrystalline processes within minerals such as siliceous compounds and aluminum-phosphate-sulfate minerals may help to answer the question of whether hypogene or supergene processes have triggered nonmetallic mineralization. The major and trace element compositions (Ag, As, Be, Co, Cr, Cs, Cu, Ge, Hf, Hg, Mo, Nb, Ni, Pb, Rb, Sb, Se, Sr, Tl, Th, U, V, Y, Zn, Zr, and REE) for several argillaceous deposits are presented in this paper. Cross plots using the couples Ba + Sr vs. Ce + Y + La, Ti + Fe vs. Cr + Nb, and Zr vs. Ti may help discriminate hypogene and supergene kaolin concentrations in these epithermal argillaceous deposits. The nonmetallic deposits represent a subtype of high-sulfidation-type deposits . The Salvadorian deposits formed distally to the arc-trench gap and belong to the most recent stages of the Cenozoic Ando-Caribbean mineralizing processes.

Relationship Between Illite Crystallinity and Temperature in Active Geothermal Systems of New Zealand

Illite crystallinity (IC), widely used to evaluate variations in metamorphic conditions, primarily depends on temperature ( e.g., Kisch, 1983; Frey, 1987). IC is determined by measuring the half-peak-width of the 10-A illite peak on oriented mineral aggregate preparations of the <2-μm size fractions (Kubler, 1967, 1968), and is expressed in °Δ2𝛉. Over the past three decades, major advances were made in the study and understanding of illite crystallinity. The effects of sample preparation (Krumm and Buggisch, 1991), instrumental conditions and interlaboratory standardization and calibration (Kisch, 1990, 1991), and measured precision and reproducibility (Robinson et al., 1990; Warr and Rice, 1994), represent some of these advances in technique. Although methods more advanced than the IC method for sample characterization have been developed recently, such as the integral peak width method (Drits et al., 1997) and the Warren-Averbach method ( e.g., Eberl et al., 1998), the IC method still remains a most suitable monitor of very low-grade metamorphism of clastic sedimentary rocks. However, the factors that precisely control crystallinity are not known. Active geothermal systems are especially fruitful places to study the genesis of clay minerals since these environments are natural laboratories where fluid/rock interactions occur under measurable conditions. Illite is abundant and occurs at temperatures > 160°C in active geothermal fields of New Zealand. Host rocks are mainly andesitic to rhyolitic lavas and pyroclastics. In this study, we correlate crystallinity values of illites in cores recovered from two wells, drilled into two active geothermal systems, with the measured drillhole temperatures. Hence, we can evaluate factors that may influence the crystallinity of illites. The Taupo Volcanic Zone is one of the most active volcanic regions on earth; the area has experienced volcanic and tectonic activity for at least one million years (Wilson, 1993). Because of this activity, geothermal systems occur …

The Aghia Kyriaki, Melos survey: prospecting for the elusive earths in the Roman period in the Aegean

The industrial minerals (IM) of antiquity, the ‘earths’ of the Classical sources, have for long eluded the archaeological record because of the limited material evidence they leave behind in the course of their extraction and processing. The extensive references in Classical and medieval literature and travellers' accounts have never previously been followed up by detailed surveys and by scientific analysis. In the course of April–May 1998 archaeological, topographic and geological surveys were carried out at the Roman site of Aghia Kyriaki, on the island of Melos, with the long term aim of putting the elusive ‘earths’ on the archaeological map of ancient industries. The present article outlines the work so far and highlights the complex factors that may contribute to the identification and characterisation of industrial minerals working in the absence of pyrotechnological waste. Furthermore, since IM processing requires a sustainable low temperature energy source, the hypothesis is put forward that Aghia Kyriaki may have been built on agriculturally inhospitable terrain mainly in order to take advantage of the area's still active geothermal field. If proven, this may constitute evidence for the first use of a ‘soft’ and environmentally friendly energy source during the Roman period in the Aegean.

A U–Th calcite isochron age from an active geothermal field in New Zealand

We report here the first U–Th disequilibrium age for a hydrothermal mineral from an active geothermal system in New Zealand. Vein calcite recovered from a depth of 389 m in Well Thm-1 at the Tauhara geothermal field has an age of 99±44 ka BP. This age was determined using a leachate–leachate isochron technique on four silicate containing sub-samples of calcite from a single vein. Although the error on this isochron age is considerable, it is significantly younger than the earlier estimated age of ∼200 ka BP for the onset of activity at the Tauhara system and probably records the date of brecciation and veining, which may be associated with volcanic activity at the adjacent dacitic Tauhara Volcanic Complex. These results demonstrate that hydrothermal vein calcite can now be dated directly, and opens the way for more detailed studies of the evolution of the New Zealand geothermal systems.

Hydrothermal Origin of Smectite in Volcanic Ash

AbstractSmectite and fine-grained quartz were separated from 6 volcanic ash samples collected in Japan from Shinmoe-dake Volcano, southern Kyushu, and Mt. Usu Volcano, southern Hokkaido. Oxygen isotope ratios of smectite in the volcanic ash range from +6.9 to +12.9 per mill (‰), which are comparable to the values of smectite (δ18O = +6.5 and +9.4‰) from currently active geothermal fields. Evaluation of the oxygen isotope data from smectite (δ18O = +6.9 and +8.4‰) and quartz (δ18O = +6.6 to +11.2‰) in volcanic ash erupted from Shinmoe-dake in 1959 indicates a hydrothermal origin at elevated temperatures (150 to 270 °C) from waters that have calculated 8180 values (8180 = -3 to +5%o) that are enriched relative to local meteoric water (δ18O = -7 to -8‰). This precludes an authigenic formation of the smectites under ambient temperatures in crater lakes and/or somma-atrios which had been previously proposed as a plausible mechanism. A peculiar clay flow was extruded on the somma-atrio of Mt. Usu Volcano. The isotopic composition of the clay (&lt;0.5 μm, δ18O = + 12.2‰) and evidence from geophysical exploration over volcanic vents of Shinmoe-dake support the concept that extensive hydrothermal alteration is taking place within volcanic vents.

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.

Geochemical characterisation of hydrothermally altered ignimbrites in active geothermal fields from the central Taupo Volcanic Zone, New Zealand

Three subsurface ignimbrites at the active Orakeikorako geothermal field can be distinguished from one another despite their having been hydrothermally altered. They can also be recognised in drillcores from five other geothermal fields up to 18 km distant from Orakeikorako. “Fingerprinting” of the three Quaternary ignimbrites was attempted by comparing their petrographic, mineralogical and chemical compositions. Analyses of 43 samples shows that a combination of the following parameters can be used to characterise the ignimbrites: Paeroa Ignim. Te Kopia Ignim. Akatarewa Ignim. Zr Yb 44.2–57.8 90.7–107.6 68.3–91.5 Zr Y 4.89–6.35 9.67–11.92 7.55–9.22 Yb Hf 0.48–0.63 0.33–0.39 0.39–0.47 La Lu 59.7–83.9 67.8–78.0 55.3–66.0 Nb Hf 1.61–2.20 1.18–1.63 1.37–1.69 P 20 5 (%) 0.013–0.040 0.060–0.075 0.070–0.15 Ta (ppm) 0.59–0.77 0.39–0.46 0.38–0.60 The Te Kopia and Akatarewa ignimbrites are also characterised by their flatter REE spidergrams, whereas the sanidine-bearing Paeroa Ignimbrite has a distinctive Eu depletion. These patterns are not clear, however, where alteration is most intense.The proportions of primary minerals present in the three ignimbrites are not diagnostic of them, and glass and their Fe-Ti oxides are very unstable in a geothermal environment. Plagioclase, hornblende and biotite were much more stable during hydrothermal alteration but only hornblende has compositions (with respect to Na, A1 iv and Ti) that are sufficiently different to distinguish between the Paeroa and Te Kopia ignimbrites.

Smectite-to-illite conversion in natural hydrothermal systems

Smectite-to-illite (S-to-I) conversion in natural hydrothermal systems including active and fossil geothermal fields can be characterized from the viewpoints of mode of conversion, kinetics, and mechanism and this can be related to the durability of buffer materials of high level radioactive waste. From the viewpoint of conversion, in the active geothermal field which can be used as a case study, the S-to-I conversion occurs in different ranges of present day temperature from drill hole to drill hole, which is related to the thermal history of the field up to two or three discrete illite/smectite (I/S) phases having different percents of illite layers (%I) coexist metastably at intermediate stages of the conversion. This is particularly visible in drill holes where the conversion occurs at high temperatures. The S-to-I conversion from a fossil geothermal field suggests that the greater the thermal gradient, the more the smectite illitization is facilitated. By having the average rate of entire S-to-I conversion, which was calculated from the data of K-Ar and % I in I/S, the activation energy required to complete the S-to-I conversion was estimated to be approximately 30 ± 5 kcal/mol in natural hydrothermal systems, where the S-to-I conversion was assumed as a first order reaction and the pre-exponential factor is the same as Eberl and Hower (1976). The estimated value is close to those deduced previously from laboratory experiments. This suggests that the experiments made by using specific geologic materials and conditions may be applicable to evaluate the kinetics of S-to-I conversion in hydrothermal systems from the viewpoint of an empirical equation. Nevertheless, since the S-to-I conversion in natural hydrothermal systems is in fact controlled by mechanisms of smectite dissolution and recrystallization of illite, it will be necessary in the future to construct the real reaction kinetics based on a proven mechanism.

Water-rock interaction in a zone of lateral flow: A natural example from the active geothermal field and gold-mineralized zone of Beppu (Kyushu Island, Japan)

Beppu is a water-dominated active geothermal system in Kyushu (Japan). Gold mineralization exists in places within altered Pleistocene volcanites in the southernmost part of this field. A shallow 300 m well was drilled in an alluvial fan deposit made up of unsorted volcanic clasts of andesite and dacite. Water and core samples were collected and a temperature log was carried out. In the depth range of 150-230 m and at temperatures of 126-148° C the main authigenic minerals are quartz, calcite, K-feldspar, mixed layers illite/smectite, stilbite and pyrite. The chemical data of the waters indicate that mixing occurs between shallow bicarbonate waters and deep Na-Cl geothermal waters in the outflow zone. The authigenic mineral assemblages found in the well are analogous to those found in the epithermal gold deposits of southern Kyushu. Au concentration data for the hydrothermal springs of Beppu (0.53 μg/kg) indicate that the fluids found in the borehole at 148° C could theoretically deposit gold. It is reasonable to hypothesise that a process of mixing between deep Na-Cl fluids and shallow Ca-HCO3-SO4 fluids were responsible for massive silicate deposition, accompanied in places by Au mineralizations.

Regional geologic setting of epithermal gold deposits, Chile

Epithermal gold deposits in Chile are Cretaceous to Cenozoic in age. The ores and the volcanic rocks with which they are associated were generated at a noncollisional, ocean-continent convergent plate margin. They are related to long, slightly sinuous north-northeast-trending magmatic belts roughly parallel to the Pacific coast. Most of the deposits are associated with eroded volcanic centers and subvolcanic porphyries. North-south variations in the nature, extent, and timing of magmatism control the succession of mineralization events.In the Late Cretaceous, vein districts (e.g., El Bronce) and hydrothermal alteration zones were formed in central Chile. Paleocene-Eocene volcanic rocks constitute a large, diffuse volcanic field in the Antofagasta region (21 degrees -26 degrees S), which contains epithermal precious metal and porphyry copper deposits associated with coeval extrusive rocks (e.g., Faride, San Cristobal, Guanaco, and Cachinal de la Sierra). In the late Eocene-early Oligocene, volcanism diminished and finally ceased in northern Chile, probably as a consequence of highly oblique plate convergence. Oligocene magmatism was restricted to emplacement of porphyries carrying copper deposits along transtensional zones within a strike-slip fault system. One of these deposits (Potrerillos), hosted by sedimentary rocks, developed epithermal gold mineralization in its cupola (El Hueso).The late Oligocene to mid-Miocene magmatic province of north-central Chile (26 degrees -31 degrees S) coincides approximately with the modern, nonvolcanic, fiat slab segment of the Chilean Andes and hosts the largest number of known epithermal developments. Deposits such as El Indio-Tambo, Nevada, La Pepa, Lobo, Marte, and La Coipa are hosted by Oligocene-Miocene volcanics. The deposits are associated with large zones of argillic and advanced argillic alteration, in some places related to shallow-level stocks and flow domes. A younger event of alteration-mineralization is recognized in the eroded cores of middle Miocene composite volcanoes, which cap some of the older altered areas (e.g., Refugio, La Laguna). The main alteration event began at 23 Ma, continued after a major deformation event initiated at 18 to 16 Ma, and ended at 11 to 8 Ma. The precious metal deposits and mineralized zones of the Oligo-Miocene province were exposed by erosion after the cessation or eastward shift of volcanism in the late Miocene, which was probably due to shallowing of the subduction zone between 26 degrees 30 and 33 degrees S.Late Miocene to Holocene magmatism in Chile is present in two segments beneath which the dip of the Benioff zone remains steep: north of 27 degrees S (Central Volcanic zone, CVZ) and south of 33 degrees S (Southern Volcanic zone, SVZ). The Central is characterized by widespread Neogene hydrothermal alteration zones and native sulfur deposits associated with composite volcanoes, calderas, subvolcanic intrusions, and active geothermal fields which are superimposed on the Oligocene-Miocene volcanic belt. These features are believed to represent a very shallowly eroded epithermal environment. In this setting the late Miocene Choquelimpie deposit is located in the exposed core of a volcano.

Local source tomography: applications to Italian areas

ABSTRACTWe describe three study cases in which we used local earthquake and shot travel‐time residuals to investigate the upper crustal structure of three regions in Italy. We inverted for velocity and hypocentral parameters using a damped least‐squares technique making use of parameter (velocity and hypocentre) separation. The three studied regions are in Italy, namely (a) the Vulsinian Volcanic Complex (Latium), where there is an active geothermal field; (b) the Irpinia (Campania–Lucania) region, in the Southern Appennines, site of the strongest earthquake in Italy for at least 65 years (November 1980, Ms= 6.9); (c) the Friuli region, in Northeastern Italy, where another strong earthquake (Ms= 6.5) occurred in 1976. The computed shallow velocity models generally correspond with surface geological structures. For the three studied areas, the main results are, respectively: (a) A low‐velocity anomaly detected in the centre of the Vulsinian Volcanic Complex at a depth of 5–8 km, probably due to anomalous heat flow caused by a partially molten or cooling intrusive body; (b) the identification of a deep (10 km) discontinuity in the crust beneath the Irpinia fault zone, approximately corresponding with the fault extension at depth; (c) the detection of a wedge of high‐velocity, high density material at seismogenic depth (5–10 km) beneath the Friuli region, interpreted as a buried thrust of the metamorphic basement.