A geochemical traverse across the Eastern Carpathians (Romania): constraints on the origin and evolution of the mineral water and gas discharges

The inner sector of the Eastern Carpathians displays a large number of Na–HCO₃, CO₂-rich, meteoric-originated cold springs (soda springs) and bore wells, as well as dry mofettes. They border the southern part of the Pliocene–Quaternary Calimani–Gurghiu–Harghita (CGH) calc-alkaline volcanic chain. Both volcanic rocks and CO₂-rich emissions are situated between the eastern part of the Transylvanian Basin and the main east Carpathian Range, where active compression tectonics caused diapiric intrusions of Miocene halite deposits and associated saline, CO₂-rich waters along active faults. The regional patterns of the distribution of CO₂ in spring waters (as calculated pCO₂) and the distribution pattern of the ³He/⁴He ratio in the free gas phases (up to 4.5 R_m/R_a) show their maximum values in coincidence with both the maximum heat-flow measurements and the more recent volcanic edifices. Moving towards the eastern external foredeep areas, where oil fields and associated brines are present, natural gas emissions become CH₄-dominated. Such a change in the composition of gas emissions at surface is also recorded by the ³He/⁴He ratios that, in this area, assume ‘typical’ crustal values (R_m/R_a=0.02).In spite of the fact that thermal springs are rare in the Harghita volcanic area and that equilibrium temperature estimates based on geothermometric techniques on gas and liquid phases at surface do not suggest the presence of shallow active hydrothermal systems, a large circulation of fluids (gases) is likely triggered by the presence of mantle magmas stored inside the crust. If total ³He comes from the mantle or from the degassing of magmas stored in the crust, CO₂ might be associated to both volcanic degassing and thermometamorphism of recently subducted limestones.     

Rare earth elements,neodymium and strontium isotopic systematics in mineral waters: evidence from the Massif Central,France

Rare Earth Elements (REEs), and Sr and Nd isotope distributions, have been studied in mineralized waters from the Massif Central (France). The CO₂-rich springs are characterized by a neutral pH (6–7) associated with total dissolved solids (TDS) from 1 to 7 g l⁻¹. The waters result from the mixing of very mineralized water pools, thought to have equilibrated at a temperature of around 200°C with superficial waters. These two mineral water pools evidenced by Sr isotopes and dissolved REEs could reflect 2 different stages of water–rock interaction and an equilibrium with different mineral assemblages.The concentrations of individual dissolved REEs and total dissolved REEs (ΣREE), in the mineral waters examined, vary over several orders of magnitude but are not dependent on the main parameters of the waters (TDS, T°C, pH, Total Organic C). The dissolved REE concentrations presented as upper continental crust normalized patterns show HREE enrichment in most of the samples. The time evolution of REE patterns does not show significant fluctuations except in 1 borehole, located in the Limagne d’Allier area, which was sampled on 16 occasions over an 18 month period. Ten samples are HREE-enriched, whereas 6 samples show flat patterns.The aqueous speciation of REEs shows that CO²⁻₃ complexes dominate (>80%) over the free metal, F⁻, SO²⁻₄ and HCO⁻₃ complexes. The detailed speciation demonstrates that the fractionation of REEs (i.e. the HREE enrichment) in CO₂-rich and pH neutral fluids is due essentially to the predominance of the CO²⁻₃ complexes.The Sr isotopic composition of the mineral waters in the Massif Central shows different mixing processes; in the Cézallier area at least 3 end-member water types exist. The most dilute end-member is likely to originate as poorly mineralized waters with minimal groundwater circulation. Two other mineralized end-members are identified, although the link between the geographical location of spring outflow and the mixing proportion between the 2 end-members is not systematic. The range in ϵ_Nd(0) for mineralized waters in the Massif Central correlates well with that of the known parent rocks except for 4 springs. One way to explain the ϵ_Nd(0) in these instances is a contribution from drainage of volcanic rocks. The isotopic systematics help to constrain the hydrogeological models for this area.     

A geochemical and geophysical approach to derive a conceptual circulation model of CO2-rich mineral waters: A case study of Vilarelho da Raia, northern Portugal

The Vilarelho da Raia-Chaves region, located in northern Portugal adjacent to the Spanish border, is characterized by both hot and cold CO₂-rich mineral waters issuing from springs and drilled wells. The present paper updates the conceptual circulation model of the Vilarelho da Raia cold CO₂-rich mineral waters. Vilarelho da Raia mineral waters, dominated by Na and HCO₃ ions, have formed mainly by interaction with CO₂ of deep-seated mantle origin. The δ ¹⁸O, δ ²H and ³H values indicate that these waters are the result of meteoric waters infiltrating into Larouco Mountain, NW of Vilarelho da Raia, circulating at shallow depths in granitic rocks and moving into Vilarelho da Raia area. The conceptual geochemical and geophysical circulation model indicates that the hot and cold CO₂-rich mineral waters of Chaves (76 °C) and Vilarelho da Raia (17 °C) should be considered manifestations of similar but not the same geohydrological systems. Electronic Publication     

Deep groundwater in the crystalline basement of the Black Forest region

Two major types of groundwater can be readily distinguished in the Variscian crystalline basement of the Black Forest in S–W Germany. Saline thermal water utilized in spas has its origin in 3–4 km deep reservoirs and developed its composition by 3 component mixing of surface freshwater, saltwater (of ultimately marine origin) and a water–rock reaction component. In contrast to the thermal water, CO₂-rich mineral water, tapped and bottled from many wells in the Black Forest, has low salinities but a TDS distribution similar to that of thermal water. It developed its chemical composition entirely by reaction of CO₂-rich water with the gneissic or granitic aquifer rock matrix. Particularly important is the contribution of various plagioclase dissolution and weathering reactions that may, at some locations, involve precipitation and dissolution of secondary calcite. Sodium/Ca ratios of water and of rock forming plagioclase in the basement rocks suggests that plagioclase weathering is strongly incongruent. Calcium is released to the water, whereas Na remains fixed to the albite feldspar component.The major element composition of 192 water samples used in this study also indicates a clear vertical stratification of the type of water chemistry; Ca–HCO₃ near the surface, Na–Ca–HCO₃–SO₄ at intermediate depth and Na–Ca–Cl at great depth.The mean permeability of Black Forest granite is about K=10⁻⁶ m/s; it is significantly lower in gneisses (gneiss: mean K=5×10⁻⁸ m/s) leading to focused flow through granite. Highly permeable fracture and fault zones, particularly in granite, are utilized by high-TDS saline deep groundwater as ascent channels and flow paths. Although spatially closely associated, the topography driven upwelling system of saline deep water and the near-surface flow system of CO₂-rich mineral waters are hydraulically and chemically unconnected.     

Geochemistry of the high-PCO2 waters in Logudoro,Sardinia, Italy

A geochemical study of the high-PCO₂ waters in Logudoro, northern Sardinia, was carried out starting from regional hydrogeochemical prospecting for geothermal energy, based on the major dissolved components and some minor elements. This preliminary investigation led to the identification of five different lithologies marking the different aquifers. The high-PCO₂ waters can be divided into the less saline (TDS < 1g/l) with high tritium unit (T.U.) values and the more saline ones (TDS= 1–3.5 g/l) with T.U. values close to zero. The water-rock interaction process affecting the major components is shown to be the result of interaction between CO₂-rich waters and aluminosilicates; the process takes place at different degrees depending on the depth at which CO₂ interacts with different aquifers while migrating upward from the mantle. Consideration of the SO₄/Cl and F/Cl ratios in the solution allowed the deep ciruits of S. Martino and Abbarghente in the Oligo-Miocene volcanic rocks and S. Lucia in the carbonate-schistose-granitic basement of the Goceano Mountains to be located.     

Conditions Of The Present-Day Hydrothermal Metamorphism Of Volcanic Rocks

Hydrothermal metamorphism in volcanic areas causes intense leaching of wall rocks with the development of secondary quartzites and rocks rich in alunite, dickite, and sericite. Three zones of different chemical activity, because of differences in P-T conditions, are evident: 1) zone of liquid hydrotherms (300m – 800m); 2) vaporization zone (10 m – 300m); and 3) condensation zone. Propylitization is common at greatest depth. In the vaporization zone, cooling and increase in pH lead to potassic metasomatism and development of zeolites and orthoclase. Argillization occurs at the surface. The change in the hot waters from acid solutions at depth to alkaline solutions at shallow depths is caused largely by vaporization of dissolved gases such as CO₂ and H₂S at lower pressures. Analyses of the deep circulating thermal waters in the Pauzhetka trough and of altered rocks in the discharge area are given as well as analyses of thermal waters and altered rocks in active volcanoes. Analyses of altered rocks from Wairakei are also compared.- -J. A. Redden.     

Hydrogeochemistry and stable isotopes of thermal springs: earthquake-related chemical changes along Belice Fault (Western Sicily)

Three geothermal systems, Montevago, Castellammare-Alcamo and Sciacca, are located along the main seismogenetic structures in Western Sicily. Concentrations of dissolved species including the gases CO₂, N₂, He and the results of stable isotope measurements δ¹⁸O, δD and δ¹³C_TDIC in water samples collected from six thermal springs and 28 cold discharges were used to characterise their feeder aquifers and to reveal the relationships between water chemistry and regional seismicity. The Sciacca thermal springs differ chemically and isotopically from those of Montevago and the Castellammare-Alcamo areas. The inferred deep end-members of the thermal waters of Montevago and Castellammare-Alcamo are almost similar, suggesting that both systems are fed by carbonate waters and selenite waters. A slight contribution (1–3%) of seawater, during groundwater ascent it is also present. The Sciacca thermal springs are fed by a deep reservoir comprising a mixture of 50% carbonate water and 50% seawater. During ascent towards the surface, these waters interact with NaCl-rich evaporite layers. By combination of published and present data significant temporal variations of temperature and some chemical parameters in the thermal waters of Western Sicily have been recorded. These variations were mostly between 1966 and 1969. Although the data are discontinuous it is still possible to reveal a direct link between physico–chemical changes in the Acqua Pia and Terme Selinuntine springs and the 1968 Belice Valley earthquake. Within the studied springs, two kinds of geochemical behaviour have been recognised. The chemistry of the Montevago thermal springs was permanently changed in response to changes in the groundwater system. Water temperature and dissolved SO₄, Cl, Na, and TDS showed minimum values before the earthquake and maximum values after the event. Almost constant values substantially higher than before, were recorded after the seismic event. Conversely, the temporal variations observed in the waters of the Terme Selinuntine spring, from 1965 to 1991, exhibit a transient increase most probably caused by a temporary contribution of deep CO₂-rich fluids caused by the strain release during the 1968 earthquake.     

Magma-derived gas influx and water-rock interactions in the volcanic aquifer of Mt. Vesuvius, Italy

We report in this paper a systematic investigation of the chemical and isotopic composition of groundwaters flowing in the volcanic aquifer of Mt. Vesuvius during its current phase of dormancy, including the first data on dissolved helium isotope composition and tritium content. The relevant results on dissolved He and C presented in this paper reveal that an extensive interaction between rising magmatic volatiles and groundwaters currently takes place at Vesuvius.Vesuvius groundwaters are dilute (mean TDS ∼ 2800 mg/L) hypothermal fluids ( mean T = 17.7°C) with a prevalent alkaline-bicarbonate composition. Calcium-bicarbonate groundwaters normally occur on the surrounding Campanian Plain, likely recharged from the Apennines. δD and δ¹⁸O data evidence an essentially meteoric origin of Vesuvius groundwaters, the contribution from either Tyrrhenian seawater or ¹⁸O-enriched thermal water appearing to be small or negligible. However, the dissolution of CO₂-rich gases at depth promotes acid alteration and isochemical leaching of the permeable volcanic rocks, which explains the generally low pH and high total carbon content of waters. Attainment of chemical equilibrium between the rock and the weathering solutions is prevented by commonly low temperature (10 to 28°C) and acid-reducing conditions.The chemical and isotope (C and He) composition of dissolved gases highlights the magmatic origin of the gas phase feeding the aquifer. We show that although the pristine magmatic composition may vary upon gas ascent because of either dilution by a soil-atmospheric component or fractionation processes during interaction with the aquifer, both ¹³C/¹²C and ³He/⁴He measurements indicate the contribution of a magmatic component with a δ¹³C ∼ 0‰ and R/R_a of ∼2.7, which is consistent with data from Vesuvius fumaroles and phenocryst melt inclusions in olivine phenocrysts.A main control of tectonics on gas ascent is revealed by data presented in this paper. For example, two areas of high CO₂ release and enhanced rock leaching are recognized on the western (Torre del Greco) and southwestern (Torre Annunziata-Pompeii) flanks of Vesuvius, where important NE-SW and NW-SE tectonic structures are recognized. In contrast, waters flowing through the northern sector of the volcano are generally colder, less saline, and CO₂ depleted, despite in some cases containing significant concentrations of magma-derived helium. The remarkable differences among the various sectors of the volcano are reconciled in a geochemical interpretative model, which is consistent with recent structural and geophysical evidences on the structure of Somma-Vesuvius volcanic complex.     

Stable isotope and fluid inclusion evidence for the origin of the Brandberg West area Sn–W vein deposits, NW Namibia

The Brandberg West region of NW Namibia is dominated by poly-deformed turbidites and carbonate rocks of the Neoproterozoic Damara Supergoup, which have been regionally metamorphosed to greenschist facies and thermally metamorphosed up to mid-amphibolite facies by Neoproterozoic granite plutons. The meta-sedimentary rocks host Damaran-age hydrothermal quartz vein-hosted Sn–W mineralization at Brandberg West and numerous nearby smaller deposits. Fluid inclusion microthermometric studies of the vein quartz suggests that the ore-forming fluids at the Brandberg West mine were CO₂-bearing aqueous fluids represented by the NaCl–CaCl₂–H₂O–CO₂ system with moderate salinity (mean=8.6 wt% NaCl_equivalent).Temperatures determined using oxygen isotope thermometry are 415–521°C (quartz–muscovite), 392–447°C (quartz–cassiterite), and 444–490°C (quartz–hematite). At Brandberg West, the oxygen isotope ratios of quartz veins and siliciclastic host rocks in the mineralized area are lower than those in the rocks and veins of the surrounding areas suggesting that pervasive fluid–rock interaction occurred during mineralization. The O- and H-isotope data of quartz–muscovite veins and fluid inclusions indicate that the ore fluids were dominantly of magmatic origin, implying that mineralization occurred above a shallow granite pluton. Simple mass balance calculations suggest water/rock ratios of 1.88 (closed system) and 1.01 (open system). The CO₂ component of the fluid inclusions had similar δ ¹³C to the carbonate rocks intercalated with the turbidites. It is most likely that mineralization at Brandberg West was caused by a combination of an impermeable marble barrier and interaction of the fluids with the marble. The minor deposits in the area have quartz veins with higher δ ¹⁸O values, which is consistent with these deposits being similar geological environments exposed at higher erosion levels.     

Chemical and isotopic characteristics of fluids along the Cameroon Volcanic Line, Cameroon

Results of the chemical and isotopic analysis of the water and gases discharged from volcanic crater lakes and soda springs located along the Cameroon Volcanic Line were used to characterize and infer their genetic relationships. Variations in the solute compositions of the waters indicate the dominant influence of silicate hydrolysis. Na⁺ (40–95%) constitutes the major cation in the springs while Fe²⁺ + Mg²⁺ (70%) dominate in the CO₂-rich lakes. The principal anion is HCO₃ (>90%), except in the coastal springs where Cl-predominates. Lakes Nyos and Monoun have FeMgCaHCO₃⁻ type signatures; the soda springs are essentially NaHCO₃⁻ type, while all other lakes show similar ionic compositions to dilute surface waters. Dissolved gases show essentially CO₂ (>90%), with small amounts of Ar and N₂, while CH₄ constitutes the principal component in the non-gassy lakes. Active volcanic gases are generally absent, except in the Lobe spring with detectable H₂S. Stable isotope ratio evidence indicates that the bicarbonate waters are essentially of meteoric origin. CO₂ (δ¹³C = −2 to −8%₀ and He (³He/⁴He = 1 to 5.6R_a) infer a mantle contribution to the total CO₂. CH₄ has a biogenic source, while Ar and N₂ are essentially atmospheric in origin, but mixing is quite common.     

Mixing of magmatic CO2 into volcano groundwater flow at Aso volcano assessed combining carbon and water stable isotopes

To understand deep groundwater flow systems and their interaction with CO₂ emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ¹³C_DIC) and DIC concentrations of the sampled waters show that magma-derived CO₂ mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ¹³C_DIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ¹⁸O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ¹³C_DIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO₂ is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO₂ mixes into deeper groundwater flowing nearer the magma conduit or chamber.     

Mercury deposits of western California: an overview

Summary Mercury deposits in western California are near a thrust fault that separates two groups of Mesozoic rocks. The Franciscan Assemblage, a metamorphosed melange with serpentine and graywacke, is structurally overlain by the Great Valley Sequence, a sedimentary series resting on oceanic crust. These Mesozoic rocks are partly covered by volcanic and sedimentary rocks of Cenozoic age. Cinnabar with silica minerals, dolomite, native mercury, and bituminous matter occurs around the fractured margins of serpentine bodies and around hot springs that emanate from the Franciscan Assemblage. Fluid inclusions and hot springs suggest that cinnabar precipitated from CO₂-H₂O fluids with <2 wt% chlorine at T<250 °C. Prograde metamorphism of Mesozoic sediments expulsed mercury-bearing fluids that migrated up serpentine-related fractures and exhaled onto the surface.     

Input of Sr/Sr ratios and Sr geochemical signatures to update knowledge on thermal and mineral waters flow paths in fractured rocks (N-Portugal)

Strontium isotopes and other geochemical signatures are used to determine the relationships between CO₂-rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic-290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have ⁸⁷Sr/⁸⁶Sr isotopic ratios between 0.716713 and 0.728035. ⁸⁷Sr/⁸⁶Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO₂-rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have ⁸⁷Sr/⁸⁶Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO₂-rich thermal and mineral waters and the granitic rocks. The mean ⁸⁷Sr/⁸⁶Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations.     

松辽盆地含CO_2火山岩气藏的形成和分布

松辽盆地特有的深部构造背景和裂谷演化特征,造成盆地内含CO_2火山岩气藏的形成和富集。松辽裂谷盆地中新生代火山岩浆活动发育,总体上具有多期喷发、分布广泛和储集条件良好的特点。火山活动以中心式喷发为主,主要发育中基性-酸性火山岩,发育流纹岩、凝灰岩等多种岩石类型,爆发相和溢流相2种火山岩相。中生代火山岩在盆地内分布广泛,营域组构成深层有利储层,新生代火山岩在盆地外围出露较多,而在盆内出露较少。盆地高含量的二氧化碳为无机幔源成因,由青山口期和新生代幔源岩浆脱气形成。含CO_2火山岩气藏的形成主要受深部构造背景、深大断裂和中新生代火山岩控制。已发现含CO_2火山岩气藏主要分布于古中央隆起带及其两侧断陷的营城组火山岩中,具有点状、带状分布,局部富集的特点。根据主控因素分析,预测了5个CO_2富集区带。     

Carbon dioxide degassing and thermal energy release in the Monte Amiata volcanic-geothermal area (Italy)

The quaternary volcanic complex of Mount Amiata is located in southern Tuscany (Italy) and represents the most recent manifestation of the Tuscan Magmatic Province. The region is characterised by a large thermal anomaly and by the presence of numerous CO₂-rich gas emissions and geothermal features, mainly located at the periphery of the volcanic complex. Two geothermal systems are located, at increasing depths, in the carbonate and metamorphic formations beneath the volcanic complex. The shallow volcanic aquifer is separated from the deep geothermal systems by a low permeability unit (Ligurian Unit). A measured CO₂ discharge through soils of 1.8 × 10⁹ mol a⁻¹ shows that large amounts of CO₂ move from the deep reservoir to the surface. A large range in δ¹³C_TDIC (−21.07 to +3.65) characterises the waters circulating in the aquifers of the region and the mass and isotopic balance of TDIC allows distinguishing a discharge of 0.3 × 10⁹ mol a⁻¹ of deeply sourced CO₂ in spring waters. The total natural CO₂ discharge (2.1 × 10⁹ mol a⁻¹) is slightly less than minimum CO₂ output estimated by an indirect method (2.8 × 10⁹ mol a⁻¹), but present-day release of 5.8 × 10⁹ mol a⁻¹ CO₂ from deep geothermal wells may have reduced natural CO₂ discharge. The heat transported by groundwater, computed considering the increase in temperature from the infiltration area to the discharge from springs, is of the same order of magnitude, or higher, than the regional conductive heat flow (>200 mW m⁻²) and reaches extremely high values (up to 2700 mW m⁻²) in the north-eastern part of the study area. Heat transfer occurs mainly by conductive heating in the volcanic aquifer and by uprising gas and vapor along fault zones and in those areas where low permeability cover is lacking. The comparison of CO₂ flux, heat flow and geological setting shows that near surface geology and hydrogeological setting play a central role in determining CO₂ degassing and heat transfer patterns.     

Lithium isotopes in island arc geothermal systems: Guadeloupe, Martinique (French West Indies) and experimental approach

We report lithium (Li) isotopic measurements in seawater-derived waters that were discharged from geothermal wells, thermal springs, and sub-marine springs located in volcanic island arc areas in Guadeloupe (the Bouillante geothermal field) and Martinique (Lamentin plain and the Diamant areas). While Li isotopic signatures of the geothermal fluids collected from deep reservoirs were found to be homogeneous for a given site, the δ⁷Li signatures for each of these reservoirs were significantly different. The first low temperature (25-250 °C) experiments of Li isotope exchange during seawater/basalt interaction confirmed that Li isotopic exchange is strongly temperature dependent, as previously inferred from natural studies. Li isotopic fractionation ranged from +19.4‰ (Δ_{solution-solid}) at 25 °C to +6.7‰ at 250 °C. These experiments demonstrated the importance of Li isotopic fractionation during the formation of Li-bearing secondary minerals and allowed us to determine the following empirical relationship between isotopic fractionation and temperature: Δ_{solution-solid} = 7847/T − 8.093. Application of experimental results and literature data to the Bouillante area suggested that geothermal water was in equilibrium at 250-260 °C. It likely has a deep and large reservoir located in the upper sheeted dike complex of the oceanic crust, just below the transition zone between andesite volcanic flows and the basaltic dikes. The upper dike section, from which Li is extracted by hydrothermal fluids, was characterized by light Li isotopic values in the rocks, indicating retention of ⁶Li by the altered rocks. For the Lamentin and Diamant areas, the geothermal fluids appeared to be in equilibrium with reservoir volcano-sedimentary rocks at 90-120 °C and 180 °C, respectively. Further evidence for this argument is provided by the fact that only the Na/Li thermometric relationship determined for sedimentary basins yielded temperature values in agreement with those measured or estimated for the reservoir fluids. This suggests the importance of a sedimentary signature in these reservoir rocks. Altogether, this study highlights that the use of Li isotopic systematics is a powerful tool for characterizing the origin of geothermal waters as well as the nature of their reservoir rocks.     

Coal seam gas distribution and hydrodynamics of the Sydney Basin,NSW, Australia

This paper reviews various coal seam gas (CSG) models that have been developed for the Sydney Basin, and provides an alternative interpretation for gas composition layering and deep-seated CO₂ origins. Open file CSG wells, supplemented by mine-scale information, were used to examine trends in gas content and composition at locations from the margin to the centre of the basin. Regionally available hydrochemistry data and interpretations of hydrodynamics were incorporated with conventional petroleum well data on porosity and permeability. The synthesised gas and groundwater model presented in this paper suggests that meteoric water flow under hydrostatic pressure transports methanogenic consortia into the subsurface and that water chemistry evolves during migration from calcium-rich freshwaters in inland recharge areas towards sodium-rich brackish water down-gradient and with depth. Groundwater chemistry changes result in the dissolution and precipitation of minerals as well as affecting the behaviour of dissolved gases such as CO₂. Mixing of carbonate-rich waters with waters of significantly different chemistries at depth causes the liberation of CO₂ gas from the solution that is adsorbed into the coal matrix in hydrodynamically closed terrains. In more open systems, excess CO₂ in the groundwater (carried as bicarbonate) may lead to precipitation of calcite in the host strata. As a result, areas in the central and eastern parts of the basin do not host spatially extensive CO₂ gas accumulations but experience more widespread calcite mineralisation, with gas compositions dominated by hydrocarbons, including wet gases. Basin boundary areas (commonly topographic and/or structural highs) in the northern, western and southern parts of the basin commonly contain CO₂-rich gases at depth. This deep-seated CO₂-rich gas is generally thought to derive from local to continental scale magmatic intrusions, but could also be the product of carbonate dissolution or acetate fermentation.     

Fluid inclusions in late minerals from the paleovalley-type uranium deposits of the West Siberian ore region: Thermochemical characteristics and genetic applications

Comprehensive microthermometric investigations revealed similar temperature ranges (280–120°C) for the formation of late carbonates in the Khokhlovskoe, Semizbai, and Malinovskoe deposits of the West Siberian uranium ore region. A close chemical similarity was definitely established between the solutions of fluid inclusions and thermal nitrogen-methane waters with elevated CO₂ concentrations typical of this region in general. It was noted that such CO₂-rich mineral waters (Yessentuki no. 4 type) are common in the Mesozoic sequences of the Shadrinsk region, where Transuralian uranium deposits occur, and are similar in composition and temperature to the modern CO₂-rich formation waters of the host sequences of the Khokhlovskoe deposit. The mineralogical and geochemical features of newly formed late minerals and uranium ores were considered as the most probable reflection of the exfiltration of such thermal solutions into the host levels. Two late mineral assemblages were distinguished: (1) hematite-calcite and (2) goethite-berthierine and goethite-smectite-chlorite with siderite or goethite-kaolinite-illite with siderite; they occur both in the host sequences and in the underlying basement rocks. The development of the latter assemblage causes a significant change in rock color (bleaching); it is widespread and was observed in all the deposits. It was shown that these altered rocks and uranium ores (especially high-grade) are very similar in mineral and chemical composition to the products of acid leaching and accompanying mineralization, which could be related to low-temperature argillization. It was suggested that exogenic epigenetic processes of ancient soil-bedrock oxidation contributed certainly to the development of uranium mineralization, and the modern character of the uranium ores and their host rocks is related to a large extent to the influence of hydrothermal CO₂-rich solutions related to the neotectonic activation of the region. This resulted in the development of their specific mineral and chemical compositions and corresponding technological characteristics. It seems expedient to estimate the possible contributions of exogenic and endogenic factors to the formation of the uranium mineralization rather than oppose the roles of these processes of different stages.     

Carbonic fluid inclusions in ultrahigh-temperature granitoids from southern India

The granite plutons of Vattamalai (VT), Gangaikondan (GK) and Pathanapuram (PT) intruding granulite facies rocks in southern India were emplaced during the Late Neoproterozoic tectonothermal event. Feldspar thermometry of mesoperthites from the granites yield temperatures of 800–1000?°C indicating high- to ultrahigh-temperature conditions, comparable to similar estimates derived from some of the host granulite facies rocks in the region. This study reports results from a detailed investigation of fluid inclusions in the three granite plutons. Carbonic inclusions characterize the major fluid species in all the cases and their unique abundance in some of these plutons indicates up to 1 wt.% CO₂. In most of the cases, the inclusions show a near-pure CO₂ composition as deduced from melting temperatures which cluster close to $?56.6°$C, and as confirmed by laser Raman spectroscopy. The VT granite preserves the highest density CO₂ fluids among all the three plutons with a density up to 0.912 g?cm^?3 (molar volume of 48.25 cm³?mol^?1). A combination of CO₂ isochores, feldspar thermometry data and dehydration melting curves, and liquidus for water-undersaturated granitic systems clearly bring out a genetic link between these granites and granulitic lower crust. The ultimate origin of the CO₂-rich fluids is linked to sub-lithospheric mantle sources through tectonic processes associated with the assembly of the Gondwana supercontinent. To cite this article: M. Santosh et al., C. R. Geoscience 337 (2005).