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1.
Fluid inclusion studies have been used to derive a model for fluid evolution in the Hohi geothermal area, Japan. Six types of fluid inclusions are found in quartz obtained from the drill core of DW-5 hole. They are: (I) primary liquid-rich with evidence of boiling; (II) primary liquid-rich without evidence of boiling; (III) primary vapor-rich (assumed to have been formed by boiling); (IV) secondary liquid-rich with evidence of boiling; (V) secondary liquid-rich without evidence of boiling; (VI) secondary vapor-rich (assumed to have been formed by boiling). Homogenization temperatures (Th) range between 196 and 347°C and the final melting point of ice (Tm) between −0.2 and −4.3°C. The CO2 content was estimated semiquantitatively to be between 0 and 0.39 wt. % based on the bubble behavior on crushing. NaCl equivalent solid solute salinity of fluid inclusions was determined as being between 0 and 6.8 wt. % after minor correction for CO2 content.Fluid inclusions in quartz provide a record of geothermal activity of early boiling and later cooling. The CO2 contents and homogenization temperatures of fluid inclusions with evidence of boiling generally increase with depth; these changes, and NaCl equivalent solid solute salinity of the fluid can be explained by an adiabatic boiling model for a CO2-bearing low-salinity fluid. Some high-salinity inclusions without CO2 are presumed to have formed by a local boiling process due to a temperature increase or a pressure decrease. The liquid-rich primary and secondary inclusions without evidence of boiling formed during the cooling process. The salinity and CO2 content of these inclusions are lower than those in the boiling fluid at the early stage, probably as a result of admixture with groundwater.  相似文献   

2.
Microthermometric analyses of fluid inclusions on a suite of hydrothermally altered gabbros recovered just south of the eastern intersection of the Kane Fracture Zone and the Mid-Atlantic Ridge, record the highest homogenization temperatures yet reported for mid-ocean ridge hydrothermal systems. Fluid salinities in the high temperature inclusions are more than ten times that of seawater. Multiple generations of fluid inclusions entrapped along healed microfractures exhibit three distinct temperature-compositional groups. We interpret these populations as having been trapped during three separate fracturing events.The earliest episode of brittle failure in the gabbros is represented by coplanar, conjugate vapor-dominated and brine-dominated fluid inclusion arrays in primary apatite. Vapor-dominated inclusions exhibit apparent homogenization temperatures of 400°C and contain equivalent salinities of 1–2 wt.% NaCl. These inclusions are interspersed with liquid-dominated, sulfide-bearing inclusions containing salinities of 50 wt.% NaCl equivalent. These high salinity inclusions remain unhomogenized at temperatures greater than 700°C.Compositional and phase relationships of the fluid inclusions may be accounted for by two-phase separation of a fluid under 1000–1200 bars pressure. These pressures require that fluid entrapment occurred under a significant lithostatic component and indicate a minimum entrapmentdepth of 2 km below the axial valley floor. This depth corresponds to a minimum tectonic uplift of 3 km, in order to emplace the samples at the 3100 m recovery depth. The microfracture networks within magmatic apatites represent fluid flow paths for either highly modified, deeply penetrating seawater or a late stage magmatic aqueous fluid. The inclusions may have formed close to the brittle-ductile transition zone adjacent to an active magma chamber.Following collapse of the high temperature front, lower temperature fluids of definite seawater origin circulated through the open fracture networks, pervasively altering portions of the gabbros. This stage is represented by low-to-moderate (1–7 wt.% NaCl equivalent) salinity inclusions in plagioclase, apatite, epidote, and augite, which homogenize at temperatures of approximately 200–300°C and 400°C. Formation of hydrous mineral assemblages, under greenschist to lower amphibolite facies conditions, resulted in sealing of the vein system and may have resulted in modification of seawater salinities by as much as a factor of two. During or following these later stages of hydrothermal activity the gabbros were emplaced high on the axial walls by differential uplift attending formation of the flanking mountains.  相似文献   

3.
Homogenization temperatures of individual fluid inclusions from the geothermal test well sites near Los Alamos, New Mexico, systematically change as a function of depth in the cores. Inclusions in samples from depths between 1.5 and 3.0 km have re-equilibrated to thermal gradients higher than the present gradient of 50–60°C/km. The loci of maximum temperatures attained has a slope of about 70°C/km; the deepest sample has cooled to 200°C from a maximum of 230°C. The wide range of salinities (0.0 wt.% equivalent NaCl to more than 25 wt.% equivalent NaCl) observed in each sample indicates a large amount of pervasive fluid circulation had not occurred at the time of re-equilibration of these inclusions. The results are relevant to calculations for the thermal history of the test site.  相似文献   

4.
The Platanares geothermal area in western Honduras consists of more than 100 hot springs that issue from numerous hot-spring groups along the banks or within the streambed of the Quebrada de Agua Caliente (brook of hot water). Evaluation of this geothermal area included drilling a 650-m deep PLTG-1 drill hole which penetrated a surface mantling of stream terrace deposits, about 550 m of Tertiary andesitic lava flows, and Cretaceous to lower Tertiary sedimentary rocks in the lower 90 m of the drill core.Fractures and cavities in the drill core are partly to completely filled by hydrothermal minerals that include quartz, kaolinite, mixed-layer illite-smectite, barite, fluorite, chlorite, calcite, laumontite, biotite, hematite, marcasite, pyrite, arsenopyrite, stibnite, and sphalerite; the most common open-space fillings are calcite and quartz. Biotite from 138.9-m depth, dated at 37.41 Ma by replicate 40Ar/39 Ar analyses using a continuous laser system, is the earliest hydrothermal mineral deposited in the PLTG-1 drill core. This mid-Tertiary age indicates that at least some of the hydrothermal alteration encountered in the PLTG-1 drill core occured in the distant past and is unrelated to the present geothermal system. Furthermore, homogenization temperatures (Th) and melting-point temperatures (Tm) for fluid inclusions in two of the later-formed hydrothermal minerals, calcite and barite, suggest that the temperatures and concentration of dissolved solids of the fluids present at the time these fluid inclusions formed were very different from the present temperatures and fluid chemistry measured in the drill hole.Liquid-rich secondary fluid inclusions in barite and caicite from drill hole PLTG-1 have Th values that range from about 20°C less than the present measured temperature curve at 590.1-m depth to as much as 90°C higher than the temperature curve at 46.75-m depth. Many of the barite Th measurements (ranging between 114° and 265°C) plot above the reference surface boiling-point curve for pure water assuming hydrostatic conditions; however, the absence of evidence for boiling in the fluid inclusions indicates that at the time the minerals formed, the ground surface must have been at least 80 m higher than at present and underwent stream erosion to the current elevation. Near-surface mixed-layer illite-smectite is closely associated with barite and appears to have formed at about the same temperature range (about 120° to 200°C) as the fluid-inclusion Thvalues for barite. Fluid-inclusion Th values for calcite range between about 136° and 213°C. Several of the calcite Th values are significantly lower than the present measured temperature curve. The melting-point temperatures (Tm) of fluid-inclusion ice yield calculated salinities, ranging from near zero to as much as 5.4 wt. % NaCl equivalent, which suggest that much of the barite and calcite precipitated from fluids of significantly greater salinity than the present low salinity Platanares hot-spring water or water produced from the drill hole.  相似文献   

5.
Measurements of the temperature and composition of effluent from vents on the sea floor can be used to deduce the in-situ density of this fluid, which is required for calculations of flow in the chimneys and through their porous walls. This density is, however, not directly relevant when calculating the buoyancy flux in the plume above a smoker. It is the asymptotic buoyancy flux, following extensive dilution with seawater, which is required when estimating the height of rise of plumes in a stably stratified ocean, and when calculating the criterion for reversal of buoyancy due to non-linear mixing effects. The results of mixing calculations show that the effluent from hydrothermal vents on the sea floor will exhibit reversing buoyancy if the ejected fluid has a temperature of 300°C and a salinity greater than 8 wt.% NaCl. If the temperature of the effluent is 200°C the salinity required for reversing buoyancy falls to 5.5 wt.% NaCl. Measurements of temperature and salinities of sea-floor hydrothermal fluid suggest that fluids with the characteristics required to form reversing plumes are ejected at the sea floor. The possibility that reversing plumes may be found has important implications for the formation of massive sulfide deposits.  相似文献   

6.
The Shizishan copper-gold deposit at Tongling, Anhui Province consists of two magmato-hydrothermal mineralization types: the crypto-explosive breccia type and the skarn type. At least four episodes of boiling occurred to the ore-forming fluids in this deposit. The first episode took place in accompany with the formation of the crypto-explosive breccias. The melt-fluid inclusions giving temperatures above 600℃ and salinities higher than 42% NaCl equiv represent a residual magma related to this episode. The second episode occurred during skarnization, giving fluid temperatures of 422℃-472℃, averaging 458℃, and salinities of 10.2%-45.1% NaCl equiv. The third episode corresponds to the main mineralization stage, i.e., the quartz-sulphide stage. Fluid temperatures of this episode vary in a range of 337℃-439℃ with an average of 390℃, and salinities in a range of 3%-30% NaCl equiv. The forth episode happened at the waning stage of mineralization, giving fluid temperatures below 350℃ with an average of 265℃ and salinities of 2.1%-40.4% NaCl equiv.  相似文献   

7.
The Latera field (Vulsini volcanic complex, Latium, Italy) is one of the geothermal areas of the peri-Tyrrhenian belt along which a regional, high thermal anomaly has been detected. So far nine deep wells have been drilled within the Latera caldera and four of them have been productive. The geothermal reservoir is located within the fractured carbonatic rocks of the Tuscan nappe; the overlying volcanic units, sealed by hydrothermal minerals (mainly calcite and anhydrite), act as an impervious cover.The fluid produced by the wells comes from a deep aquifer (about 1000–1500 m depth) which at present is not connected with the shallow aquifer in the volcanoclastic units. Fluid temperatures range between 200 and 230°C; in-hole temperatures as high as 343°C at 2775 m depth have been measured in dry wells.The study of the newly formed mineral assemblages from both volcanic and sedimentary units as sampled from the geothermal wells can be used to reconstruct the thermal evolution of the geothermal field. The intrusion of a syenitic melt, up to a depth of about 2000 m, dated 0.86 Ma, represents the major thermal event for the units in the area and is assumed to represent the first step in the geothermal evolution of the Latera system.The above mentioned newly formed mineral assemblages can be divided into three groups: (a) “contact-metasomatic”: calcite, anhydrite, diopsidic pyroxene, grossularitic garnet, phlogopite, wollastonite or monticellite; (b) “high-temperature hydrothermal”: calcite, anhydrite, K-feldspar, vesuvianite, melanitic garnet, tourmaline, amphibole, epidote, sulphides; (c) “low-temperature hydrothermal”: calcite, anhydrite, K-feldspar, clay minerals, sulphides. Group (a) minerals are now relics. Part of (b) and all of (c) group are still in equilibrium with the existing conditions in different parts of the geothermal system.Thermodynamic calculations on the observed mineral assemblages permitted estimates of the P, T conditions and gas fugacities.  相似文献   

8.
Thermal waters hosted by Menderes metamorphic rocks emerge along fault lineaments in the Simav geothermal area. Thermal springs and drilled wells are located in the Eynal, Çitgöl and Na a locations, which are part of the Simav geothermal field. Studies were carried out to obtain the main chemical and physical characteristics of thermal waters. These waters are used for heating of residences and greenhouses and for balneological purposes. Bottom temperatures of the drilled wells reach 163°C with total dissolved solids around 2225 mg/kg. Surface temperatures of thermal springs vary between 51°C and 90°C. All the thermal waters belong to Na–HCO3–SO4 facies. The cold groundwaters are Ca–Mg–HCO3 type. Dissolution of host rock and ion-exchange reactions in the reservoir of the geothermal system shift the Ca–Mg–HCO3 type cold groundwaters to the Na–HCO3–SO4 type thermal waters. Thermal waters are oversaturated at discharge temperatures for aragonite, calcite, quartz, chalcedony, magnesite and dolomite minerals giving rise to a carbonate-rich scale. Gypsum and anhydrite minerals are undersaturated with all of the thermal waters. Boiling during ascent of the thermal fluids produces steam and liquid waters resulting in an increase of the concentrations of the constituents in discharge waters. Steam fraction, y, of the thermal waters of which temperatures are above 100°C is between 0.075 and 0.119. Reservoir pH is much lower than pH measured in the liquid phase separated at atmospheric conditions, since the latter experienced heavy loss of acid gases, mainly CO2. Assessment of the various empirical chemical geothermometers and geochemical modelling suggest that reservoir temperatures vary between 175°C and 200°C.  相似文献   

9.
To improve understanding of Ca isotope transport during water-rock interaction on the continents, we measured dissolved δ44Ca values along a 236 km flow path in the Madison aquifer, South Dakota, where fluids have chemically evolved according to dolomite and anhydrite dissolution, calcite precipitation, and Ca-for-Na ion-exchange over a timescale spanning ~ 15 kyr. We used a reactive transport model employing rate data constrained from major ion mass-balances to evaluate the extent to which calcite precipitation and ion-exchange fractionate Ca isotopes. Elevated δ44Ca values during the initial and final stages of water transport possibly result from calcite precipitation under supersaturated conditions and Ca-for-Na ion-exchange, respectively. However, for the bulk of the flow path, δ44Ca values evolve by mixing between anhydrite and dolomite dissolution, with no fractionation during calcite precipitation under saturated conditions. We attribute the absence of Ca isotope fractionation to the long timescale of water-rock interaction and slow rate of calcite precipitation, which have enabled fluids to chemically and isotopically equilibrate with calcite. We therefore conclude that the equilibrium Ca isotope fractionation factor between calcite and water (Δcal–w) is very close to zero. To the extent that the Madison aquifer typifies other groundwater systems where calcite slowly precipitates from solutions at or near chemical equilibrium, this study suggests that groundwater contributions to δ44Ca variability on the continents can be modeled according to simple mixing theory without invoking isotope discrimination.  相似文献   

10.
Results of drilling, logging, and testing of three exploration core holes, combined with results of geologic and hydrogeochemical investigations, have been used to present a reservoir model of the Platanares geothermal system, Honduras. Geothermal fluids circulate at depths ≥ 1.5 km in a region of active tectonism devoid of Quaternary volcanism. Large, artesian water entries of 160 to 165°C geothermal fluid in two core holes at 625 to 644 m and 460 to 635 m depth have maximum flow rates of roughly 355 and 560 l/min, respectively, which are equivalent to power outputs of about 3.1 and 5.1 MW(thermal). Dilute, alkali-chloride reservoir fluids (TDS ≤ 1200 mg/kg) are produced from fractured Miocene andesite and Cretaceous to Eocene redbeds that are hydrothermally altered. Fracture permeabillity in producing horizons is locally greater than 1500 and bulk porosity is ≤ 6%. A simple, fracture-dominated, volume-impedance model assuming turbulent flow indicates that the calculated reservoir storage capacity of each flowing hole is approximately 9.7 × 106 l/(kg cm−2), Tritium data indicate a mean residence time of 450 yr for water in the reservoir. Multiplying the natural fluid discharge rate by the mean residence time gives an estimated water volume of the Platanares system of ≥ 0.78 km3. Downward continuation of a 139°C/km “conductive” gradient at a depth of 400 m in a third core hole implies that the depth to a 225°C source reservoir (predicted from chemical geothermometers) is at least 1.5 km. Uranium-thorium disequilibrium ages on calcite veins at the surface and in the core holes indicate that the present Platanares hydrothermal system has been active for the last 0.25 m.y.  相似文献   

11.
Masanori  Kurosawa  Satoshi  Ishii  Kimikazu  Sasa 《Island Arc》2010,19(1):40-59
Fluid inclusions in quartz from miarolitic cavities, pegmatites, and quartz veins in Miocene biotite-granite plutons, Kofu, Japan, were analyzed by particle-induced X-ray emission to examine chemistries and behaviors of granite-derived fluids in island-arc granite. Most inclusions are aqueous two-phase inclusions, and halite-bearing polyphase inclusions are also observed in quartz veins in the upper part of the plutons. From element contents of fluid inclusions in the miarolitic cavities, the original fluid released from the granite plutons during solidification is inferred to have concentrations of Mn, Fe, Cu, Zn, Ge, Br, Rb, Pb, and Ba of several tens to hundreds of parts per million by weight (ppm) and a salinity of about 10 wt% NaCl equivalent. We estimated the formation conditions of the fluid to have been at about 1.3–1.9 kb and 530–600°C on the basis of the homogenization temperatures of the inclusions and the solidification conditions of the plutons. The polyphase inclusions probably originated from hypersaline fluid by boiling of part of the released fluid during its ascent in the plutons. The polyphase inclusions contain several hundreds to tens of thousands of ppm of Fe and Mn, and tens to several hundreds of ppm of Cu, Zn, Br, Rb, and Pb. The salinities are about 35 wt% NaCl equivalent. Compositional variations in two-phase inclusions from the miarolitic cavities and quartz veins are primarily explained by mineral precipitation with dilution by surface water exerting a secondary influence. Thus, chemistries and behaviors of the granite-derived fluids in the plutons can be explained by mineral precipitation, boiling, and dilution of the originally released fluid.  相似文献   

12.
Thermal springs associated with normal faults in Utah have been analyzed for major cations and anions, and oxygen and hydrogen isotopes. Springs with measured temperatures averaging greater than 40°C are characterized by Na + K- and SO4 + Cl-rich waters containing 103 to 104 mg/l of dissolved solids. Lower temperature springs, averaging less than 40°C, are more enriched in Ca + Mg relative to Na + K. Chemical variations monitored through time in selected thermal springs are probably produced by mixing with non-thermal waters. During the summer months at times of maximum flow, selected hot springs exhibit their highest temperatures and maximum enrichments in most chemical constituents.Cation ratios and silica concentrations remain relatively constant through time for selected Utah thermal springs assuring the applicability of the geothermometer calculations regardless of the time of year. Geothermometer calculations utilizing either the quartz (no steam loss), chalcedony or Mg-corrected Na/K/Ca methods indicate that most thermal springs in Utah associated with normal faults have subsurface temperatures in the range of 25 to less than 120°C. This temperature range suggests fluid circulation is restricted to depths less than about three kilometers assuming an average thermal gradient of about 40°C/km.Thermodynamic calculations suggest that most thermal springs are oversaturated with respect to calcite, quartz, pyrophyllite, (Fe, Mg)-montmorillonite, microcline and hematite, and undersaturated with respect to anhydrite, gypsum, fluorite and anorthite. Chalcedony and cristobalite appear to be the only phases consistently at or near saturation in most waters. Theoretical evaluation of mixing on mineral saturation trends indicates that anhydrite and calcite become increasingly more undersaturated as cold, dilute groundwater mixes with a hot (150°C), NaCl-rich fluid. The evolution of these thermal waters issuing from faults appears to be one involving the dissolution of silicates such as feldspars and micas by CO2-enriched groundwaters that become more reactive with increasing temperature and/or time. Solution compositions plotted on mineral equilibrium diagrams trend from product phases such as kaolinite or montmorillonite toward reactant phases dominated by alkali feldspars.Isotopic compositions indicate that these springs are of local surface origin, either meteoric (low TDS, < 5000 mg/l) or connate ground water (high TDS, > 5000 mg/l). Deviations from the meteoric water line are the result of rock-water isotopic exchange, mixing or evaporation. Fluid source regions and residence times of selected thermal spring systems (Red Hill, Thermo) have been evaluated through the use of a σ D-contour map of central and western Utah. Ages for waters in these areas range from about 13 years to over 500 years. These estimates are comparable to those made for low-temperature hydrothermal systems in Iceland.  相似文献   

13.
Silica chimneys were discovered in 1985 at 86°W in the rift valley of the Galapagos Spreading Center at 2600 m depth (“Cauliflower Garden”). The inactive chimneys lack any sulfides and consist almost entirely of amorphous silica (up to 96 wt.% SiO2, opal-A); Fe and Mn oxides are minor constituents. Oxygen isotope data show that formation of the silica chimneys took place at temperatures between 32°C (+29.9‰ δ18O) and 42°C (+27.8‰ δ18O).Th/Udating reveals a maximum age of 1440 ± 300y. Amorphous silica solubility relations indicate that the silica chimneys were formed by conductive cooling of pure hydrothermal fluids or by conductive cooling of a fluid/seawater mixture. Assuming equilibrium with quartz at 500 bars, initial fluid temperatures of more than 175°C (i.e., a concentration of > 182 ppm SiO2) were required to achieve sufficient supersaturation for the deposition of amorphous silica at 40°C and 260 bars. If the silica chimneys originate from the same or a similar fluid as higher-temperature ( < 300°C) sulfide-silica precipitates found nearby (i.e., 2.5 km away), then subsurface deposition of sulfides may have occurred.  相似文献   

14.
A dacitic magma (64.5 wt.% SiO2), a mixture of phenocryst-rich rhyodacite and an aphyric mafic magma, was erupted during the recent 1991–1995 Mount Unzen eruptive cycle. The experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas, and the nature of the mixing process. The preeruption rhyodacitic magma was at a temperature of 790±20°C according to Fe–Ti oxide phenocryst cores, and at a depth of 6 to 7 km (160 MPa) according to Al-in-hornblende geobarometry. The mafic magma that mixed with the rhyodacite is found as andesitic (54 to 62 wt.% SiO2) enclaves in the erupted magma and was essentially aphyric when intruded. Phase equilibria indicate that an aphyric andesite at 160 MPa is >1030°C (H2O-saturated) and possibly as high as 1130°C (2 wt.% H2O). The composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt.% SiO2. Using these compositions and temperatures, the temperature of the Unzen magma after mixing is estimated to be at least 850° to 870°C. The groundmass Fe–Ti oxide microphenocrysts and those in pargasite-bearing reaction zones around biotite phenocrysts both give 890±20°C temperatures; the oxide–oxide contacts give temperatures of 910±20°C. The 900±30°C postmixing temperatures are consistent with phase-equilibria experiments which show that the magma was not above 930°C at 160 MPa. Our Fe–Ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption, which is a shorter time than is calculated using available diffusion data. There is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma.  相似文献   

15.
The Quaternary Takidani Granodiorite (Japan Alps) is analogous to the type of deep-seated (3–5 km deep) intrusive-hosted fracture network system that might support (supercritical) hot dry/wet rock (HDR/HWR) energy extraction. The I-type Takidani Granodiorite comprises: porphyritic granodiorite, porphyritic granite, biotite-hornblende granodiorite, hornblende-biotite granodiorite, biotite-hornblende granite and biotite granite facies; the intrusion has a reverse chemical zonation, characterized by >70 wt% SiO2 at its inferred margin and <67 wt% SiO2 at the core. Fluid inclusion evidence indicates that fractured Takidani Granodiorite at one time hosted a liquid-dominated, convective hydrothermal system, with <380°C, low-salinity reservoir fluids at hydrostatic (mesothermal) pressure conditions. ‘Healed’ microfractures also trapped >600°C, hypersaline (35 wt% NaCleq) fluids of magmatic origin, with inferred minimum pressures of formation being 600–750 bar, which corresponds to fluid entrapment at 2.4–3.0 km depth. Al-in-hornblende geobarometry indicates that hornblende crystallization occurred at about 1.45 Ma (7.7–9.4 km depth) in the (marginal) eastern Takidani Granodiorite, but later (at 1.25 Ma) and shallower (6.5–7.0 km) near the core of the intrusion. The average rate of uplift across the Takidani Granodiorite from the time of hornblende crystallization has been 5.1–5.9 mm/yr (although uplift was about 7.5 mm/yr prior to 1.2 Ma), which is faster than average uplift rates in the Japan Alps (3 mm/yr during the last 2 million years). A temperature–depth–time window, when the Takidani Granodiorite had potential to host an HDR system, would have been when the internal temperature of the intrusive was cooling from 500°C to 400°C. Taking into account the initial (7.5 mm/yr) rate of uplift and effects of erosion, an optimal temperature–time–depth window is proposed: for 500°C at 1.54–1.57 Ma and 5.2±0.9 km (drilling) depth; and 400°C at 1.36–1.38 Ma and 3.3±0.8 km (drilling) depth, which is within the capabilities of modern drilling technologies, and similar to measured temperature–depth profiles in other active hydrothermal systems (e.g. at Kakkonda, Japan).  相似文献   

16.
Philippine geothermal systems occur in the vicinity of large Holocene calc-alkaline volcanic complexes. Wells drilled in these areas encountered multiple intrusions; the latest dikes are the subsurface manifestations of the youngest heat source. Commonly, at least two hydrothermal regimes are juxtaposed in a single area, with the latest being in equilibrium with the present temperature and chemical regime.Alteration by neutral-pH water is pervasive and abundant. A contact-metamorphic aureole also occurs near intrusives. Alteration due to acid-sulfate fluids is generally confined to permeable structures. Neutral-pH alteration is divided into four zones on the basis of key clay minerals, and two subzones are defined by calc-silicates. These are the smectite (ambient to 180°C), transition (180–230°C), illite (230–320°C) and biotite (270–340°C) zones. Subzones are defined by epidote (250–340°C) and amphibole (280–340°C). The four main zones of acid alteration are: kaolinite (ambient to 120°C), dickite ± kaolinite (120–200°C), dickite ± pyrophyllite (200–250°C), and pyrophyllite ± illite (230–320°C). Where relict high-temperature alteration reaches the surface, the area being drilled is usually the outflow zone of the present system.These hydrothermal mineral assemblages are used: (1) as geothermometers; (2) to assist in determining the depth at which the production casing will be set during drilling; (3) to estimate fluid pH and other chemical parameters; (4) to predict possible corrosion and scaling tendencies of the fluids; (5) as a measure of permeability and possible cold water influx into wells; (6) as a guide to field hydrology; and (7) to estimate roughly the thickness of the eroded overburden.  相似文献   

17.
Detailed geochemistry supported by geologic mapping has been used to investigate Sulphur Springs, an acid-sulfate hot spring system that issues from the western flank of the resurgent dome inside Valles Caldera. The most intense activity occurs at the intersection of faults offsetting caldera-fill deposits and post-caldera rhyolites. Three geothermal wells in the area have encountered pressures <1 MPa and temperatures of 200°C at depths of 600 to 1000 m. Hot spring and fumarole fluids may discharge at boiling temperatures with pH 1.0 and SO4 8000 mg/l. These conditions cause argillic alterations throughout a large area.Non-condensible gases consist of roughly 99% CO2 with minor amounts of H2S, H2, and CH4. Empirical gas geothermometry suggests a deep reservoir temperature of 215 to 280°C. Comparison of 13C and 18O between CaCO3 from well cuttings and CO2 from fumarole steam indicates a fractionation temperature between 200 and 300°C by decarbonation of hydrothermally altered Paleozoic limestone and vein calcite in the reservoir rocks. Tritium concentrations obtained from steam condensed in a mudpot and deep reservoir fluids (Baca #13, 278°C) are 2.1 and 1.0 T.U. respectively, suggesting the steam originates from a reservoir whose water is mostly >50 yrs old. Deuterium contents of fumarole steam, deep reservoir fluid, and local meteoric water are practically identical even though 18O contents range through 4‰, thus, precipitation on the resurgent dome of the caldera could recharge the hydrothermal system by slow percolation. From analysis of D and 18O values between fumarol steam and deep reservoir fluid, steam reaches the surface either (1) by vaporizing relatively shallow groundwater at 200°C or (2) by means of a two-stage boiling process through an intermediate level reservoir at roughly 200°C.Although many characteristics of known vapor-dominated geothermal systems are found at Sulphur Springs, fundamental differences exist in temperature and pressure of our postulated vapor-zone. We propose that the reservoir beneath Sulphur Springs is too small or too poorly confined to sustain a “true” vapor-dominated system and that the Sulphur Springs system may be a “dying” vapor-dominated system that has practically boiled itself dry.  相似文献   

18.
The equilibrium distribution of CO2H2O fluids in synthetic rock samples (principally dunite and quartzite) has been characterized by measurements of the dihedral wetting angle, θ, resulting from 5-day annealing periods at 950–1150°C and 1 GPa. For fluids in equilibrium with polycrystalline quartz, θ varies systematically from 57° for pure H2O to 90° at XCO2 0.9. Similarly, for San Carlos olivine, θ varies from 65° for pure H2O to 90° at XCO2 0.9. The addition of solutes (NaCl, KCl, CaF2, Na2CO3) to H2O causes a major decrease in θ in the quartz/fluid system (to values as low as 40°), but has no effect on fluid wetting in dunite. Reconnaissance experiments on other mono- and polymineralic aggregates indicate universally high wetting angles (θ 60°) in upper mantle assemblages and for CO2 in felsic compositions. For diopside + H2O, θ 80°, with large variation due to crystalline anisotropy. In no case does θ approach 0°, the condition necessary for fluid to be present along all grain boundaries.Because a value of θ greater than 60° precludes the existence of an interconnected fluid phase in a rock, our results have important implications not only for fluid transport but also for the physical properties of the bulk fluid/rock system. Any static fluid present in the upper mantle must exist as isolated pores located primarily at grain corners, and transport can occur only by hydrofracture. In the continental crust, aqueous fluids (especially saline ones) are likely to form an interconnected network along grain edges, thus contributing to high electrical conductivity and allowing the possibility of fluid transport by porous flow or surface energy-driven infiltration.  相似文献   

19.
The Sierra La Primavera, a late Pleistocene rhyolitic caldera complex in Jalisco, México, contains fumaroles and large-discharge 65°C hot springs that are associated with faults related to caldera collapse and to later magma insurgence. The nearly-neutral, sodium bicarbonate, hot springs occur at low elevations at the margins of the complex, whereas the water-rich fumaroles are high and central.The Comisión Federal de Electricidad de México (CFE) has recently drilled two deep holes at the center of the Sierra (PR-1 and Pr-2) and one deep hole at the western margin. Temperatures as high as 285°C were encountered at 1160 m in PR-1, which produced fluids with 820 to 865 mg/kg chloride after flashing to one atmosphere. Nearby, PR-2 encountered temperatures to 307°C at 2000 m and yielded fluids with chloride contents fluctuating between 1100 and 1560 mg/kg after flashing. Neither of the high-temperature wells produced steam in commercial quantities. The well at the western margin of the Sierra produced fluids similar to those from the hot springs. The temperature reached a maximum of 100°C near the surface and decreased to 80°C at 2000 m.Various geothermometers (quartz conductive, Na/K, Na-K-Ca, δ18O(SO4-H2O) and D/H (steam-water) all yield temperatures of 170 ± 20°C when applied to the hot spring waters, suggesting that these spring waters flow from a large shallow reservoir at this temperature. Because the hot springs are much less saline than the fluids recovered in PR-1 and PR-2, the mixed fluid in the shallow reservoir can contain no more than 10–20% deep fluid. This requires that most of the heat is transferred by steam. There is probably a thin vapor-dominated zone in the central part of the Sierra, through which steam and gases are transferred to the overlying shallow reservoir. Fluids from this reservoir cool from 170°C to 65°C by conduction during the 5–7 km of lateral flow to the hot springs.  相似文献   

20.
To test the possible effect of different fluid compositions on some standard geothermometry techniques, experiments were conducted in which a rhyolite from the Presidio Bolson area of West Texas was interacted with fluids of two different compositions (0.1 M NaCl and 0.01 M NaHCO3). The temperature range was 100–500°C, pressure was 1000 bars, water/rock mass ratios were 6:1 and 5:1, and the duration of the experiments ranged from 12 to 130 days.Results showed that the quartz geothermometer worked well in the experimental system up to temperatures of 400°C. The results were not affected by differences in the major anionic species.The Na-K geothermometer gave temperatures an average of 76°C lower than the experimental temperatures, regardless of fluid type. The experimental data from this study agree well with previous experimental work in feldsparquartz systems.The Na-K-Ca geothermometer did not work well for experiments using 0.01 M NaHCO3 but did work well for experiments using 0.1 M NaCl. Benjamin et al. (1983) concluded that the Na-K-Ca geothermometer is based on alteration reactions rather than feldspar exchange; however, no evidence for alteration reactions was observed in this study.  相似文献   

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