Gas Geothermometers Research Articles

Conceptual modelling on water-rock reaction and genesis of high pH fluids in a typical granitoid geothermal reservoir: A case from Indus-Tsangpo Suture Zone, India

Two novel thermal springs are investigated along ITSZ of north-west Himalayas in Demchok geothermal belt. The area consists of deep-seated faults in LGB; despite being situated in ITSZ, fluid chemistry of these low-temperature springs differs significantly, specifically in terms of low TDS (161–168 mg/l) and high alkalinity, with neighbouring springs in Puga and Chumathang (∼2100 mg/l). Thermal waters are mixed type (Na–Cl–HCO3–SO4) with elemental composition influenced through silicate rock weathering and partial carbonate dissolution. Variable temperature speciation (25 °C–200 °C) indicates that in reservoir fluid, Na+ precedes over other cations, while sulfate and carbonate complexes being prominent for Mg and Ca, respectively. Aquifer boiling modelling suggests calcite scaling and silica mineral equilibration in reservoir. Environmental stable isotopes (δ18O and δD) suggest high-altitude recharge from Jamlung La (6156 m), with altitude-effect of 0.43 ‰ isotopic depletion per 100 m elevation in altitude. As only silica minerals equilibrate with thermal waters, silica, and gas geothermometers give most conservative estimate of reservoir temperature (125°−135 °C). The steep topography enables extensive lateral flow of hot fluids in outflow zone, leading to high mixing and a high water-rock ratio, which contribute to lower TDS. Conceptual modelling reveals that geothermal system is a low-enthalpic hydrothermal system controlled by joints and permeable fractures, having deep fluid circulation of meteoric waters of ∼1275 m at sub-surface, with anomalous geothermal gradient and steam migration as reservoir heat sources. These fluids closely resemble springs of Guerrero state, Mexico, exhibiting similarities in low-TDS, high-pH, and high concentration of dissolved silica.

Geochemistry of thermal springs and associated gases along the Strymon River Valley (Bulgaria and Greece)

This paper presents and discusses the water and gas geochemistry of a large number of thermal springs occurring along the N-S trending Strymon Valley, from its source, near Sofia (Bulgaria), to the Aegean Sea (Greece). In Bulgaria springs have markedly alkaline pH, relatively low Total Dissolved Solids and prevalent Na-HCO3 to Na-Cl(SO4) in composition while the associated gas phase is mostly N2-dominated. When moving to the Greek sector, the thermal springs, Ca(Mg)-HCO3 to Na-HCO3, become less alkaline and more saline whereas the associated gas phase is CO2-dominated. The abrupt geochemical change in the Greek sector is caused by a variation in the thickness and nature of the sediments filling the Strymon Valley, the latter being characterized by a relevant amount of Neogene marine material. Such changes occur south of an important E-W lineament named Middle Mesta, south of which marble formations extensively crop out and are likely occurring below the sedimentary succession. The presence of these carbonate sequences embedded in the Neogene sediments is explaining the CO2-rich gases associated to the Greek springs. Water isotopes indicate a meteoric origin for the studied waters. From a geothermometric point of view, solute (previous studies) and gas (this work) geothermometers suggest that no high enthalpy systems occur in the Bulgaria and northern sector of Greece with estimated temperatures <120 °C. Consequently, these thermal springs can be regarded as tectonically-derived along the many fault systems that border the Strymon Valley. The convective circuits are thus originated from rainfall in the crystalline massifs that border the valley, i.e. the Serbo-Macedonian to the west and the Rhodope to the east.

Seawater hydrothermal system in the middle of the Kuril Arc: Yankich Island, Ushishir Archipelago

Ushishir volcano is located in the middle of the Kuril Arc. The Ushishir crater, a closed bay connected with the ocean by a narrow and shallow strait, is characterized by a strong hydrothermal activity. Boiling springs, hot pools, fumaroles and shallow submarine vents are manifestations of a magmatic-seawater hydrothermal system that discharges a boiling solution that is generally similar in chemical and isotopic composition to the seafloor hydrothermal fluids at different tectonic settings. The main features of the Ushishir fluids are: (1) water has close to zero δD and a large oxygen isotopic shift (6 to 9 ‰); (2) 3He/4He = 7.3Ra, very low CH4 and N2/Ar > 200; (3) high boron concentration (∼70 ppm); (4) a significant uptake of Ca and Sr from the host rock and Ca/Sr ratio higher than that for seawater with 87Sr/86Sr 0.7037–0.7039, a bit higher than that of the rock value (0.7031). The measured onshore discharge of boiling water is ∼5 kg/s; however, a large plume of the discolored seawater is released from the outer submarine slope of the volcano indicating a much higher total mass and heat output. We compare the seafloor hydrothermal fluids with boiling water of the coastal Ushishir springs and estimate the effective water/rock ratio using our data on oxygen and Sr isotopes and some trace elements. We also discuss the correctness of the application of solute and gas geothermometers for both seafloor and coastal seawater hydrothermal systems.

HYDROGEOLOGICAL CONCEPTUAL MODEL OF 'AFH' GEOTHERMAL FIELD AT SALAHUTU REGENCY, MALUKU PROVINCE USING WATER AND GAS GEOCHEMISTRY METHODS

The research area is a geothermal field located in the Tulehu area, Salahutu District, Maluku Province. The purpose of this research is to interpret the conceptual model of the geothermal system with the data that has been obtained. The first method is water geochemistry which includes ion balance, water type, water source, water geothermometer and water geoindicator. Theremore, the isotopic geochemical method is applied, which is to determine the source of the fluid. Then using gas geochemical methods including gas sources, gas geothermometers, and gas geoindicators. Based on the descriptive provisions, the research area is included in the low relief liquid dominated system. Based on the heat source, the research area is included in seawater. The research area is a medium temperature geothermal system with reservoir temperatures ranging from 185˚C to 240˚C with reservoir temperature parameters. Based on geological and geochemical interpretations, the upflow area in the study area is in the middle of the study area in the Hatuasa regionor between Mt. Salahutu and Mt. Erikawang, while the outflow area is in the eastern part of the study area, or towards the coastal line in Tulehu region

Geochemical evidence for the nonexistence of supercritical geothermal fluids at the Yangbajing geothermal field, southern Tibet

Exploring and exploiting high-temperature (even supercritical) geothermal resources are significant to meet energy demands and reduce carbon emissions. The Yangbajing geothermal field is the most exploited in China, with the currently highest temperature (329.8 °C) measured in a geothermal well. However, whether there are supercritical geothermal fluids beneath the deep parts of this geothermal field is under controversy. In this paper, the water isotope, chemical compositions, and C–He isotopes of gas samples were collected and analyzed. The geothermal water originated from the mixing of meteoric water and magmatic water (25%). The sources of CO2 in the geothermal field were dominated by the thermogenic degassing of carbonates and metasediments in the crust while the radioactive decay of U and Th in granite is the dominated source of He. The temperatures of three different reservoirs are 150 ± 15 °C, 250 ± 10 °C, and ∼320 °C (with a depth of ∼8 km), respectively. These were obtained using dissolved gas, soil CO2 flux, and noble gas geothermometers. Unlike other supercritical geothermal fields worldwide with larger, shallower, basaltic magma chambers, the Yangbajing geothermal field has a deep-seated, small-scale, granitic magma chamber. Thus, its geological conditions are not conducive for gestating supercritical fluids. These results are of great significance for exploring and developing high-temperature (even ultra-high-temperature) geothermal resources in China.

GaS_GeoT: A computer program for an effective use of newly improved gas geothermometers in predicting reliable geothermal reservoir temperatures

A geochemometric study based on a multi-criteria decision analysis was applied, for the first time, for the optimal evaluation and selection of artificial neural networks, and the prediction of geothermal reservoir temperatures. Eight new gas geothermometers (GasG1 to GasG8) were derived from this study. For an effective and practical application of these geothermometers, a new computer program GaS_GeoT was developed. The prediction efficiency of the new geothermometers was compared with temperature estimates inferred from twenty-five existing geothermometers using gas-phase compositions of fluids from liquid- (LIQDR) and vapour-dominated (VAPDR) reservoirs. After applying evaluation statistical metrics (DIFF%, RMSE, MAE, MAPE, and the Theil's U test) to the temperature estimates obtained by using all the geothermometers, the following inferences were accomplished: (1) the new eight gas geothermometers (GasG1 to GasG8) provided reliable and systematic temperature estimates with performance wise occupying the first eight positions for LIQDR; (2) the GasG3 and GasG1 geothermometers exhibited consistency as the best predictor models by occupying the first two positions over all the geothermometers for VAPDR; (3) the GasG3 geothermometer exhibited a wider applicability, and a better prediction efficiency over all geothermometers in terms of a large number of samples used (up to 96% and 85% for LIQDR and VAPDR, respectively), and showed the smallest differences between predicted and measured temperatures in VAPDR and LIQDR; and lastly (4) for the VAPDR, the existing geothermometers ND84c, A98c, and ND98b sometimes showed a better prediction than some of the new gas geothermometers, except for GasG3 and GasG1. These results indicate that the new gas geothermometers may have the potential to become one of the most preferred tools for the estimation of the reservoir temperatures in geothermal systems.

The layered model of the Copahue geothermal reservoir, Argentina

An exhaustive review of the available data related to the reservoir of the volcano-hosted Copahue geothermal system is presented. Reservoir interpretation to date suggests a vapor-dominated nature. However, the review provided reveals the limitations of this model, regarding the transition from shallow argillic to deep propylitic alteration zones, gas geothermometers calibrated to near-neutral pH environment, production tests, and low recharge of the feed zone, among the other features. We propose that a layered model with a shallow vapor zone above a fluid pressurized reservoir better accounts for the known features of the geothermal reservoir. Temperature profiles define the 800-m-thick upper thermal boundary that includes the impermeable clay cap. An argillic alteration layer (smectite + chlorite + illite + kaolinite) with low electric resistivity response, consistent with the presence of smectite, composes the clay cap. A shallow steam cap develops immediately below the impermeable layer. This vapor zone has 200–215 °C, low seismic activity, and high electrical resistivity, and is highlighted by the isothermal segment of the well logs. The bottom part of the reservoir is subjected to controversy because of the lack of direct measurements. The deep liquid-dominated reservoir has a temperature of ~ 280 °C, develops below 1500 m depth under near-neutral pH condition, and probably has a pervasive propylitic alteration, and wairakite + laumontite + chlorite + epidote + prehnite + actinolite is the likely mineral assemblage. Pressure collapse might trigger the development of the shallow steam cap during deflation–inflation volcanic cycles or Pleistocene-to-recent tectonic deformation. This work points out the main weak points of the previous conceptual model, providing an alternative one based on the first comprehensive compilation of data of the Copahue reservoir. Further research, including a deep exploration drilling stage, is required to establish the physicochemical reservoir state precisely and to validate the proposed layered model.

Major geochemical characteristics of geothermal brines from the Upper Rhine Graben granitic basement with constraints on temperature and circulation

This paper is the first to present the main geochemical characteristics of the native brines collected from all the geothermal wells penetrating the granite basement underlying the sedimentary cover, in the Upper Rhine Graben. These deep wells (from 2580 to 5000m) were only drilled in four geothermal sites (Soultz-sous-Forêts and Rittershoffen in France; Landau and Insheim in Germany). The Na–Cl geothermal brine samples collected from the granite returned TDS values ranging from 99 to 107g/l with pH values close to 5, along with Cl and Br concentrations and δD, δ18O and δ34S values that indicate a multiple origin with mixing between primary brine formed by advanced evaporation of seawater (probably until the stage of halite precipitation) and dilute meteoric water, plus contributions from halite dissolution following successive marine transgression-regression cycles from the Triassic to the Oligocene. Chemical, isotopic and gas geothermometers indicate concordant reservoir temperatures close to 225±25°C for all the fluids, even though the maximum temperature so far measured on site is 200°C. An exhaustive literature review has indicated that only the geothermal brine from the deep Cronenbourg well (2870m) ending in the Buntsandstein has similar chemical and isotopic compositions (apart from Br and Ca) to the fluids from the granite, with an identical estimation of reservoir temperature from geothermometry. Geothermal brine from the deep Bruchsal well (2540m), drilled down to the junction of the Buntsandstein and the Saxo-Thuringian formations, has a higher TDS value (120–125g/l) with its chemical and isotopic compositions giving a lower estimation of reservoir temperature (190±25°C). By contrast, geothermal brine from the Bühl well (2655m) ending in the Buntsandstein has an even higher TDS value of about 201g/l and a lower temperature-at-depth estimation of 110±25°C, close to the temperature measured on site (115°C). The above results indicate that the geothermal fluids collected from the granite probably originate from Triassic sedimentary formations located at great depth (≥4km) with temperatures close to 225±25°C in the centre of the Rhine Graben, but that their different TDS and Cl/Br values reflect the presence of several distinct geothermal reservoirs. Many discrepancies due to high-temperature water-rock interactions are revealed on comparing the chemical and isotopic compositions of the hot brines with those of cooler brines from Bühl and the Landau Eocene–Oligocene oilfield wells, among others. The hottest brines are much enriched in K, Ca, SiO2, Li, Rb, Cs, As, Sr, Ba, Mn, Nd, U and in metals such as Zn, Pb, Cu, Co, Cd, Sb, but are depleted mainly in Mg, SO4 and B and have much lower isotopic Li and B signatures. The He isotopic signatures of the gases associated with these fluids (R/Ratm. = 0.128 at Bruchsal and 0.252 at Insheim) confirm that the thermal anomalies are mainly crustal and not mantle-derived (1.46% and 2.88% of mantellic He, respectively, for the two sites). Thus it is concluded that the thermal anomalies are associated mainly with the convective circulation of hot fluids along probable NE–SW faults between the graben's deep sedimentary centre and the fractured granite basement at its edges. Moreover, the western part of the Upper Rhine Graben (the Landau, Insheim, Soultz, Rittershoffen and Cronenbourg sites) seems to be hotter than the eastern part (Bruchsal and Bühl). According to the U-Th isotope system, the minimum transit time of these deep geothermal brines would be about 1000years.

Carbon-bearing gas geothermometers for volcanic-hydrothermal systems

The genetic relationship between carbon-bearing species (CO, CO2, CH4, C2H6, C3H8, C2H4 and C3H6) was investigated in volcanic-hydrothermal gases emitted from Nisyros (Greece), Vesuvio, La Solfatara (Campi Flegrei) and Pantelleria (all Italy).Apparent carbon isotopic temperatures of the CH4–CO2 system are ~360°C at Nisyros, 420–460°C at Vesuvio, ~450°C at La Solfatara and ~540°C at Pantelleria. These temperatures are confirmed by measured propene/propane and H2/H2O concentration ratios. CH4 and CO2 equilibrate in the single liquid phase prior to the onset of boiling, whereas propene and propane attain equilibrium in the saturated water vapor phase. Boiling in these high-enthalpy hydrothermal systems might occur isothermally. Once vapor has been extracted from the parental liquid, CO/CO2 responds most sensitively to the temperature gradient encountered by the ascending gases.Our results imply that the CH4–CO2 isotopic geothermometer can provide reliable information about temperatures of deep hydrothermal liquids associated with volcanism. Propene/propane and H2/H2O concentration ratios should be measured along with the carbon isotopic composition of CO2 and CH4 to provide independent constraints on the geological significance of the apparent carbon isotopic temperatures.

Gas chemistry and nitrogen isotope compositions of cold mantle gases from Rungwe Volcanic Province, southern Tanzania

We report the first complete bulk gas chemistry and nitrogen isotope data for geothermal volatiles from the Rungwe Volcanic Province, located in the western branch of the East African Rift north of Lake Malawi. Temperatures of springs and gas emissions at Rungwe vary from 13°C to 65°C with the highest temperatures observed at the springs in the northern and southern lowlands. The vigorously degassing cold CO2 vents and springs have temperatures between 13°C and 36°C and are located at higher elevation than the hot springs. The gas compositions are ~99% CO2, 0.0008 to 0.0078mmol/mol H2, 0.0004 to 0.062mmol/mol He, 0.08 to 0.77mmol/mol Ar, 3.1 to 28.5mmol/mol N2, 0.4 to 3.73mmol/mol O2, <0.002 to 1.541mmol/mol CH4, <0.001 to 0.009mmol/mol CO, and are poor in H2S (0.045 to 0.201mmol/mol). The CO2 flux at a local gas collection plant is estimated to be 1.6×105 mol/year. Gas geothermometry indicates a range of equilibration temperatures from >250°C (from CO2–Ar) to ~60°C (from H2–Ar), which is interpreted to reflect deep equilibration with hot saline fluids and shallow re-equilibration of kinetically fast gas geothermometers with cold meteoric recharge from the highlands. N2–He–Ar systematics show that the gases fall on a well-defined mixing line between upper mantle or sub-continental lithospheric mantle and air saturated water endmembers. Details of an improved method for analyzing nitrogen isotope compositions in gas samples are presented. Nitrogen isotope compositions (δ15N values) range between +2‰ and −5.9‰, overlapping with the upper mantle range, with only one sample location displaying δ15N values greater than air (0‰). The results emphasize the importance of the East African Rift as a potential, but poorly constrained, contributor of sub-continental lithospheric mantle volatiles to the Earth's surface even in regions that are currently volcanically dormant, but are seismically active.

Gas discharges from four remote volcanoes in northern Chile (Putana, Olca, Irruputuncu and Alitar): a geochemical survey

We analyzed gas samples collected from fumaroles and bubbling pools at Irruputuncu, Putana, Olca and Alitar volcanoes located in the central Andes volcanic zone (northern Chile). The Irruputuncu and Putana fumarolic discharges showed outlet temperatures ranging from 83 ˚C to 240 ˚C and from 82 ˚C to 88 ˚C, respectively. The chemical and isotopic ( 3 He/ 4 He, d 13 C-CO 2 , d 18 O-H 2 O and dD-H 2 O) compositions of these discharges were similar to medium-to-high temperature volcanic gases from other active volcanoes in this sector of the Andean volcanic chain (e.g. Lascar volcano). Inorganic and organic gas geothermometers for the H 2 O-CO 2 -CO-H 2 , CO 2 -CH 4 and C 2 -C 3 alkenes-alkanes systems indicated equilibrium temperatures that exceed 500 ˚C at the gas sources. These relatively high temperatures are in agreement with the presence of relevantly high concentrations of magmatic gas emissions, including SO 2 . Olca and Alitar volcano fluid chemistries indicated lower amounts of magmatic-derived gas species, while both the helium and the water isotopic compositions suggested significant fractions of shallow, crustal/meteoric-originated fluids. These indicate contributions from a hydrothermal environment with temperatures <400 ˚C. The geochemical and isotopic features derived from the present study show that the Irruputuncu, Putana, Olca and Alitar volcanoes should be considered as active and thus warrant periodic geochemical monitoring to determine the evolution of these systems and their potential hazards.

New gas geothermometers using CO2/H2 and CH4/H2 ratios

New gas geothermometers using CO2/H2 and CH4/H2 ratios

Discovery of active hydrothermal venting in Lake Taupo, New Zealand

The Horomatangi geothermal system of Lake Taupo, New Zealand, is a sub-lacustrine equivalent of subaerial geothermal activity nearby in the Taupo Volcanic Zone (TVZ). The setting of this system is rare within the TVZ as it is directly associated with an individual volcanic feature, that of the 1.8 ka Taupo eruption vent. Two distinct hydrothermal vent areas, named Te Hoata and Te Pupu, have been discovered during dives with the submersible Jago. Venting of gases was seen at both sites and hot water (up to 45°C) discharges at the Te Pupu site. Dilute water samples have concentrations of SO4, Cl, Na and SiO2 above ambient lake water values. Gas samples have compositions similar to other TVZ geothermal systems. Gas geothermometers indicate the existence of a high-temperature hydrothermal environment beneath the lake with reservoir temperatures in excess of 300°C. Chimney structures were found at the Te Pupu site. They are up to 30 cm tall and mineralized by an ‘epithermal’ suite of elements, including S, Hg, As, Sb and Tl. The walls of the chimneys are largely composed of diatoms and strands of silicified filamentous bacteria embedded in an amorphous silica groundmass. Bacterial mats are commonly associated with the gas vents and also occur at two hot springs. Close to the vents, commonly perched on top of dead chimneys and/or exposed outcrops, are dense assemblages of what are probably a new species of sponge of the genus Heterorotula. The sponges host a notably diversified, associated invertebrate fauna and represent a previously unseen biomass on the lake floor. The sponges appear to have bored into the mineralized chimneys.

Hydrothermal hydrocarbon gases: 2, Application in the East African Rift System

Hydrocarbon gases are ubiquitous in the hydrothermal systems of the East African Rift System (EARS), though often found at very low concentrations in the ‘volcanic’ eastern branch as compared to the ‘sedimentary’ western branch. Study of the chemical and isotopic compositions of these hydrocarbons from sites in Kenya, Ethiopia, Djibouti and Uganda reveals considerable homogeneity over hundreds of km of the various rift units. Consideration of C and He isotopic evidence points to a predominantly crustal thermogenic origin for the hydrocarbons, there being no evidence of mantle inputs in either the MORB or ‘hotspot’ sectors of the EARS. Temperature information from geothermal wells has been utilised to investigate the relationship between reservoir temperatures and ratios of CH 4 to C 2H 6. The general C 1/C 2 geothermometric relationship proposed in Part 1 of this study holds reasonably well, and is shown to give results equal to or better than the ‘inorganic’ gas geothermometers presently in use, both in the wellfields and undeveloped high-enthalpy geothermal areas. Results from low-enthalpy hot spring systems are less well correlated with apparent deep temperatures, but consistent with data from similar systems elsewhere in the world.

Soil and fumarole gases of Mount Etna: geochemistry and relations with volcanic activity

Between 1993 and 1996, gas samples were collected on Mt. Etna volcano from both high- and low-temperature fumaroles and from soils in areas of anomalous degassing located on the flanks of the volcanic edifice. The chemical composition of all the emitted gases is consistent with their magmatic origin. During their transport to the surface, they would undergo shallow mixing processes, mostly with air. Locally, as in the case of a point in the lower southwestern flank of the volcano (P39), very high contents of He and CH 4 indicate that deep gases mainly interact with ground waters and also with a hydrocarbon reservoir. The isotopic composition of carbon in CO 2 from these gases also suggests its primary magmatic origin, with inferred δ 13 C values of `magmatic' CO 2 in the range −2 to −1‰ vs. PDB. Such values, significantly more positive than those measured in MORBs, are in the typical range measured at the other Mediterranean volcanoes (−5 to 0 δ‰). More negative values can be explained by shallow interactions between magmatic and organic CO 2 and/or between the former and cold ( T<120°C) ground waters, whereas more positive values are thought to be mainly the result of interactions between magmatic CO 2 and hot ( T>120°C) ground waters. Both chemical and isotopic gas geothermometers agree in indicating calculated temperatures ranging from 180° to 270°C for the soil gases collected from low temperature fumaroles near the summit and temperatures around 150°C for the cold gases from P39, thus confirming the presence of a hydrothermal aquifer beneath this point. A chemical geothermometer applied on the gases collected from the high temperature fumaroles near the summit craters shows equilibrium temperatures ranging from 385° to 690°C. Lastly, many of the studied parameters showed anomalous variations in time during the studied period. The most important of these concerned the flux of CO 2 through the soil and the concentration of soil H 2, which were closely correlated with strong increases in the volcanic activity of Etna.

Gas geothermometry in selected icelandic geothermal fields with comparative examples from Kenya

Using data from selected geothermal fields in Iceland, several gas geothermometers are applied to calculate reservoir temperatures. Results for well fluids are compared to estimated inflow temperatures. The effects of gases from other sources, e.g. magmatic fluid entering reservoirs, condensation/boiling, loss of components and mixing of different fluids during upflow, are discussed. Those geothermometers that give results reasonably close to the inflow temperatures, and are not constrained by the need to know thermodynamic parameters, are used to estimate subsurface temperatures from fumarole steam composition. It is suggested that geothermometers based on H 2 and H 2S concentrations are relatively effective. The CO 2, H 2S and H 2 geothermometers are also evaluated by thermodynamic simulation using plausible mineral buffers. The CO 2 geothermometer generally gives slightly low temperatures for values in the range 100–200°C. The calibration of the existing H 2S and H 2 geothermometers needs improvement when reservoir temperatures are higher than 220°C. The mineral buffer controlling H 2S in reservoirs seems to be pyrite + magnetite + epidote + prehnite changing into pyrite + pyrrhotite + epidote + prehnite with increasing temperature.

Hydrogeochemistry of the volcanic district in the Tolfa and Sabatini Mountains in central Italy

A geochemical study on 26 thermal springs and 5 samples from thermal wells ( t > 20°C), 19 cold springs, 11 cold gas pools, and 14 stream samples from the Tolfa and Sabatini Mountains volcanic district (about 1300 km 2) has clarified their genesis and evolution. Meteoric waters infiltrate the more permeable exposed rocks (carbonates and/or volcanites) and percolate down into two main aquifers. One is deep and located in Mesozoic anhydritic-carbonate formations, and is the regional geothermal reservoir that feeds springs that emerge at the margins of the carbonate series (either exposed or barely covered); its waters display typical bicarbonate-sulfate alkaline earth composition. The other aquifer(s) is shallower, consisting of waters that circulate primarily in volcanic deposits. The chemical characteristics of these shallow waters are heavily influenced by lithology and by a gas phase, mainly CO 2, originating at depth. Temperature estimates obtained with SiO 2 and gas geothermometers, which seem to be the most reliable under these conditions, indicate the widespread occurrence of deep fluids at temperatures higher than 110°C under the area covered by the Sabatini volcanites. In some areas (Manziana, Trevignano, Nepi, and Cesano) the subsurface waters reach temperatures within the range characteristic of medium to high enthalpy systems (120–160°C).

Some simple models of chemical alteration caused by the movement of metamorphic fluids in the deeper parts of the crust

Pore fluids in the crust are generally in chemical equilibrium with the common inorganic rock minerals they contact. The chemistry of the rock-buffered pore fluid is not very sensitive to the particular aluminosilicate minerals contacted. For this reason the temperature at which the fluid equilibrated can be inferred from analyses of fluid composition alone through the application of various geothermometers (e.g., N a K C a , S i O 2 and various gas geothermometers; Henley et al., 1984). Pore fluids may equilibrate at temperatures as low as 50 °C under common crustal conditions. If pore fluids are equilibrated with their host rock minerals in the deeper parts of the crust as the above discussion indicates must be the case under reasonably quiescent conditions, rock alteration will occur only when pore fluids move with respect to physical fields that affect their equilibrium chemistry. The most important physical fields are temperature, pressure, and chlorinity (C1being a non-reactive and therefore unbuffered fluid component that controls fluid pH through charge balance). When pore fluids move through such physical gradients, the chemical flux from the rock minerals to the fluid is easily defined:

Fluid inclusion gas chemistry as a potential minerals exploration tool: Case studies from Creede, CO, Jerritt Canyon, NV, Coeur d'Alene district, ID and MT, southern Alaska mesothermal veins, and mid-continent MVT's

Recent advances in instrumentation now permit quantitative analysis of gas species from individual fluid inclusions. Fluid inclusion gas data can be applied to minerals exploration empirically to establish chemical (gas composition) signatures of the ore fluids, and conceptually through the development of genetic models of ore formation from a framework of integrated geologic, geochemical, and isotopic investigations. Case studies of fluid inclusion gas chemistry from ore deposits representing a spectrum of ore-forming processes and environments are presented to illustrate both the empirical and conceptual approaches. We consider epithermal silver-gold deposits of Creede, Colorado, Carlin-type sediment-hosted disseminated gold deposits of Jerritt Canyon, Nevada, metamorphic silver-base-metal veins of the Coeur d'Alene district, Idaho and Montana, gold-quartz veins in accreted terranes of southern Alaska, and the mid-continent base-metal sulfide deposits of Mississippi Valley-Type (MVT's). Variations in gas chemistry determine the redox state of the ore fluids, provide compositional input for gas geothermometers, characterize ore fluid chemistry (e.g., CH 4CO 2, H 2SSO 2, CO 2/H 2S, organic-rich fluids, gas-rich and gas-poor fluids), identify magmatic, meteoric, metamorphic, shallow and deep basin fluids in ore systems, locate upwelling plumes of magmatic-derived volatiles, zones of boiling and volatile separation, interfaces between contrasting fluids, and important zones of fluid mixing. Present techniques are immediately applicable to exploration programsas empirical studies that monitor fluid inclusion gas threshold concentration levels, presence or absence of certain gases, or changes in gas ratios. We suggest that the greater contribution of fluid inclusion gas analysis is in the integrated and comprehensive chemical dimension that gas data impart to genetic models, and in the exploration concepts based on processes and environments of ore formation derived from these genetic models.

Geochemical study on natural gas and water discharges in the Southern Latium (Italy): circulation, evolution of fluids and geothermal potential in the region

The waters discharged in Southern Latium (south of Rome) are mainly meteoric in origin. Two types of circulation occur in the region: one infiltrates the carbonate sediments outcropping in the area and emerges from fractures along their borders, the other consists of waters that infiltrate and circulate essentially within the Albani volcanic rocks. The first type produces either alkaline earth bicarbonate-sulfate waters or, if they mix with fossil marine waters or interact with recent marine sediments, alkaline-chloride waters (on the Pontina Plain). The chemistry of the waters produced by the second type of circulation (alkaline earth-bicarbonate or alkaline-bicarbonate) is strictly related to their gaseous phases, which consist for the most part of the CO 2 produced at depth. Silica and gas geothermometers, whose results correlate well, indicate that low enthalpy fluids (≤ 80°C) are present throughout the study area, with the exception of the northwestern part where medium enthalpy fluids (150°C) also appear.