Fluid Gas Composition during Hydrothermal Alteration of Komatiites in the Kostomuksha Greenstone Structure (Western Karelia, Russia)

In 2001–2002, two ground collapses occurred in the island of Nisyros (Aegean Sea, Greece), which formed a 600 m long and up to 5 m wide fissure in the vegetated central part of the Lakki Plain caldera. The fissure was alternatively ascribed to tensional stress release and hydrothermal alteration. In this study, we present original data of diffuse CO2 soil fluxes, soil temperatures, mineralogical and chemical composition of the caldera-filling deposits exposed on the fissure walls, and chemical and isotopic composition of interstitial soil gases collected from: the bottom of the fissure, the adjacent vegetated areas, the hydrothermal craters, and selected sites outside the caldera. The occurrence of intense hydrothermal alteration was shown by both mineralogical and chemical analyses of the fissure walls material. Typical mineral assemblage and enrichments in incompatible elements related to advanced argillic alteration, characterizing steam-heated hydrothermal environments, were recognized. Although the low-permeable sediment cover in the Lakki Plain concealed the underneath hydrothermal gas flow, preventing anomalous soil temperatures and CO2 fluxes, the chemical and isotopic composition of the interstitial soil gases revealed an active hydrothermal fluids circulation below the collapsed area, likely controlled by buried structural lineaments. Hydrothermal alteration can then be invoked as the most likely trigger mechanism for the 2001–2002 collapse event.

Real-time drill mud gas logging at the USDP-4 drilling, Unzen volcano, Japan

Bottled natural mineral waters from an andesitic aquifer in Slovenia are enriched in magnesium (1.1 g/l), sulphate (2.2 g/l) and dissolved inorganic carbon (204 g/l). We analysed major ions, trace elements, tritium activity, 14C, δ18OH2O, δ2HH2O, δ13CDIC, gas composition and noble gases in six wells. In addition, 87Sr//86Sr, δ34SSO4 and δ11B were analysed here for the first time. Stable isotopes with δ18O = −11.97 to −10.30‰ and δ2H = −77.3 to −63.8 confirm meteoric origin. CO2 degassing is evident at three wells, causing the oxygen shift of about −1.3‰. Tritium activity was detectable only in the shallowest well, where the freshwater component was dated to the 1960s. δ13CDIC in five waters is −1.78 to + 1.33‰, typical of carbonate dissolution. Radiocarbon is low, 1.03–5.16 pMC. Chemical correction with bicarbonate concentration and δ13C correction methods gave best mean residence times, slightly longer than previously published. Sulphate has δ34S 26.6–28.9‰ and δ18O 8.9–11.1‰ due to dissolution of evaporites in carbonate rocks. Boron at concentrations of 1.2–6.1 mg/l has two origins: δ11B = 11.3–16.4‰ from hydrothermal alteration and δ11B = 26.6–31.7‰ from carbonate dissolution. Strontium at concentrations of 0.5–22.0 mg/l has 87Sr//86Sr, indicating three sources: 0.7106 for Miocene clastic rocks, 0.7082 for Triassic carbonates and 0.7070 for Lower Oligocene andesitic rocks. CO2 represents the majority of the dissolved (> 98.84 vol%) and separated gas (> 95.23 vol%). Methane is only found in two wells with a max. of 0.30 vol%. All waters show excess helium and 16–97% of mantle-derived helium. Since all show subsurface degassing, the paleo-infiltration temperature could not be calculated.

Fluid inclusion gas compositions from an active magmatic–hydrothermal system: a case study of The Geysers geothermal field, USA

The edifice of Mount Rainier, an active stratovolcano, has episodically collapsed leading to major debris flows. The largest debris flows are related to argillically altered rock which leave areas of the edifice prone to failure. The argillic alteration results from the neutralization of acidic magmatic gases that condense in a meteoric water hydrothermal system fed by the melting of a thick mantle of glacial ice. Two craters atop a 2000-year-old cone on the summit of the volcano contain the world's largest volcanic ice-cave system. In the spring of 1997 two active fumaroles (T=62°C) in the caves were sampled for stable isotopic, gas, and geochemical studies.Stable isotope data on fumarole condensates show significant excess deuterium with calculated δD and δ18O values (−234 and −33.2‰, respectively) for the vapor that are consistent with an origin as secondary steam from a shallow water table which has been heated by underlying magmatic–hydrothermal steam. Between 1982 and 1997, δD of the fumarole vapor may have decreased by 30‰.The compositions of fumarole gases vary in time and space but typically consist of air components slightly modified by their solubilities in water and additions of CO2 and CH4. The elevated CO2 contents (δ13CCO2=−11.8±0.7‰), with spikes of over 10,000ppm, require the episodic addition of magmatic components into the underlying hydrothermal system. Although only traces of H2S were detected in the fumaroles, most notably in a sample which had an air δ13CCO2 signature (−8.8‰), incrustations around a dormant vent containing small amounts of acid sulfate minerals (natroalunite, minamiite, and woodhouseite) indicate higher H2S (or possibly SO2) concentrations in past fumarolic gases.Condensate samples from fumaroles are very dilute, slightly acidic, and enriched in elements observed in the much higher temperature fumaroles at Mount St. Helens (K and Na up to the ppm level; metals such as Al, Pb, Zn Fe and Mn up to the ppb level and volatiles such as Cl, S, and F up to the ppb level).The data indicate that the hydrothermal system in the edifice at Mount Rainier consists of meteoric water reservoirs, which receive gas and steam from an underlying magmatic system. At present the magmatic system is largely flooded by the meteoric water system. However, magmatic components have episodically vented at the surface as witnessed by the mineralogy of incrustations around inactive vents and gas compositions in the active fumaroles. The composition of fumarole gases during magmatic degassing is distinct and, if sustained, could be lethal. The extent to which hydrothermal alteration is currently occurring at depth, and its possible influence on future edifice collapse, may be determined with the aid of on site analyses of fumarole gases and seismic monitoring in the ice caves.

The Tuwu deposit is one of the largest porphyry copper deposits in the Eastern Tianshan, Northwest China. Ore bodies of the deposit are associated with the tonalite porphyry, which intruded the Early Carboniferous Qi´eshan Group. The hydrothermal ore‐forming processes include the porphyry mineralization episode with potassic, chlorite‐sericite/albite, phyllic, and propylitic alteration and the overprinting mineralization episode. These hydrothermal alteration episodes were studied on the basis of mass transfer and element mobility. The mass balance data and isocon diagrams reveal that Cu is highly associated with the chlorite‐sericite (albite) stage and followed by phyllic and potassic stages. On the basis of electron probe analysis, the SO3 contents of the igneous apatite from the tonalite porphyries range from 0.016 to 0.288 wt.%, which are higher than those from the diorite porphyries (SO3 = 0.088–0.179 wt.%). Similarly, the apatite saturation temperature (861–926°C) and magmatic S content (mean = 52.16 ppm) from the tonalite porphyries are also higher that from the diorite porphyries (T = 813–877°C; S = 28.25 ppm). These results suggest that the S content of the apatite is related to the temperature and S content of the melt. In the hydrothermal alteration zones, the F favours partitioning into apatite under lower temperature, the F contents of hydrothermal apatite increase from the potassic zones to the propylitic zones accompanying with the decrease of temperature. Cl element is much more sensitive to the pH, pressure, and fluid composition than F element. The Cl contents of the apatite increase from the potassic zones (0.18–0.46 wt.%) to the chorite‐sericite (albite) zones (0.28–0.58 wt.%), resulting from the decrease of fluid pH and pressure. The lower Cl contents of apatite (0.07–0.48 wt.%) in the phyllic zones were due to the lower Cl content of hydrothermal fluids. The highest Cl contents (0.09–0.69 wt.%) of apatite in the propylitic zones were caused by the decrease of pressure. The δ18Owater values of quartz in chlorite‐sericite (albite) zones and phyllic zones range from −0.8‰ to −0.4‰ and from −5.3‰ to 4.8‰, and the δD values vary from −57‰ to −47‰ and from −52‰ to −44‰, respectively. Gaseous compositions of the fluid inclusions in quartz are dominated by H2O with minor CO2, and the liquid compositions mainly consist of Na+ and Cl−. These data imply that the ore‐forming fluids belong to the H2O‐NaCl system and evolved from magmatic water to meteoric water in origin.

<p>Vulcano is an active volcanic island located in the south-central sector of the Aeolian Archipelago (Tyrrhenian Sea, Italy). The most recent active edifice is the La Fossa crater located in the center of the island, neighboring the main settlement Vulcano Porto. Its eruptive history is characterized by frequent transitions from phreatomagmatic to minor magmatic activity. The last eruption occurred in 1888–90 with strong phreatic (“Vulcanian”) eruption pulses. During the last decades, it has undergone several periods of volcanic unrest, accompanied by increasing degassing, rising fumarole temperatures, changing gas compositions, or increasing groundwater- and soil temperatures. Major unrest periods were reported in the 1920s, 1940s, and 1990s. Here we report on the ongoing crisis that initiated in September 2021. Rapidly increasing degassing levels and fumarole temperatures, accompanied by seismic activity and surface deformation were detected and monitored by the monitoring network (INGV bulletin reports). The fast evolution and dynamics of the crisis caused authorities to raise the alert level to orange and led to temporary evacuations in Vulcano Porto. We monitored this crisis from the beginning by monthly drone-based optical and thermal infrared overflights. The drone data was processed by using the Structure-from-Motion approach, allowing to generate spatially dense optical and thermal infrared maps. This way we captured the response of the hydrothermal system at the surface in great detail, were able to monitor the spatio-temporal evolution of the high-temperature fumarole field but also associated mean and low-temperature anomalies of diffuse degassing areas. We compared observations to a previous study considering in detail the structure and thermal expression of the La Fossa fumarole field, and the hydrothermal alteration associated (Müller et al., 2021, JVGR). Major aspects of changes observed at the surface during the crisis that could be constrained are (i) an increase of fumarole temperatures, (ii) the development of new fumarole vents, (iii) the evolution of a thermal aureole surrounding the major fumarole field at a distance, and (iv) the formation of a net-shaped thermal anomaly network. Changes are presented on a spatial and temporal scale and highlight the dynamics of degassing systems at the surface with implications for volcanic monitoring and hydrothermal alteration research and suggest that unrest is detectable at fumaroles but also at diffuse degassing zones elsewhere affecting a larger region of the La Fossa cone. </p>

Multi-disciplinary volcanic gas observations, including FTIR, Multi-GAS, and Alkali filter pack, were made at Masaya Volcano, Nicaragua, in January 2018. During the observation period, a lava lake was present, and the majority of the volcanic gases were likely emitted directly from the lava without any hydrothermal alterations. It is expected that the volcanic gas composition reflects the conditions of the magma, exhibiting a high equilibrium temperature. The fractions of the major components showed good consistency with previous studies; however, we found the fraction of the combustible components (such as H2) were lower than expected. The RH [log(H2/H2O)] value was measured to be less than −6, compared to the equilibrium RH value calculated to be around −3 from the iron speciation. The equilibrium calculations suggest oxidation of the volcanic gases by high-temperature mixing with the air once they are emitted from the lava lake, lowering the H2 content of the plume. In contrast to H2, a small amount of CO, another combustible species, was detected and the derived CO2/CO ratio of ca. 1,000 is consistent with the equilibrium magmatic value. This indicates that CO is kinetically inert compared to H2. Our findings suggest that volcanic gases in the Masaya plume do not preserve information from when the gases were originally in equilibrium with the high-temperature surrounding magma.

Noble gas composition of Indian carbonatites (Amba Dongar, Siriwasan): Implications on mantle source compositions and late-stage hydrothermal processes

Fluid inclusions in vein quartz from 10 granitoid-hosted gold deposits and prospects in the Birimian terrane of Ghana, as well as from the Sansu mine (Ashanti shear zone type) and quartz veins in the nonmineralized Princess Town granodiorite, were studied by microthermometry and Raman microspectrometry. Fluid inclusions from the granitoid-hosted gold deposits are dominated by aqueous H2O-CO2-NaCl type 1 and liquid CO2-N2 ± CH4 type 2, with minor (<10%) aqueous H2O-NaCl type 3. Type 1 inclusions show large variations in CO2 phase volume proportions (10–90%) at 25°C and have salinities commonly between 0 and 6 wt percent NaCl equiv. Their bulk densities fall in a major range from 0.62 to 1.11 g/cm3. Type 2 inclusions show no visible H2O phase at room temperature and have bulk densities between 0.30 and 0.92 g/cm3. Type 3 inclusions, containing 10 to 20 vol percent H2O vapor, have salinities of 1 to 8 wt percent NaCl equiv and bulk densities of 0.77 to 1.03 g/cm3. In most cases, the three types of fluid inclusions coexist as groups in individual quartz grains of the samples studied. These fluid inclusions are interpreted to be trapped during phase separation of an originally homogeneous H2O-CO2 fluid, with low salinity (<6 wt % NaCl equiv) and moderate to high density (0.65–0.95 g/cm3). The type 1 fluid inclusions are suggested to be heterogeneous mixtures of the two end members of type 2 and 3 inclusions. Fluid immiscibility is documented by petrographic characteristics and microthermometric results of the three types of inclusions. Trapping temperatures and pressures, estimated from microthermometry of type 3 inclusions, equation of state, and the P-T-X nature of the H2O-CO2-NaCl system, are mainly between 200° and 350°Cand 1 and 3 kbars for the gold deposits. By contrast, fluid inclusions in the Sansu mine at Ashanti mostly comprise the liquid CO2-N2 ± CH4 type 2. The inclusions are considered to represent postentrapment modifications of trapped fluids. In addition, fluid inclusions in barren vein quartz from the Princess Town granodiorite comprise the low-salinity (commonly <6 wt % NaCl equiv) H2O-NaCl type 3. Raman microspectrometry shows that gaseous compositions of fluid inclusions from both granitoid- and shear zone-hosted gold deposits are mainly composed of CO2 (80–95 mol %), with significant amounts of N2 (2–20 mol %) and CH4 (0–10 mol %). The distinct low-salinity H2O-CO2-rich fluids from the granitoid-hosted gold deposits are comparable in composition to those from the major Ashanti and Tarkwaian types of gold deposits in the Birimian terrane of Ghana. These fluids are most likely to be metamorphic in origin and associated with the waning stages of the regional Birimian orogeny. Gold deposition within the Birimian granitoids was related to fluid phase separation and sulfidization of host rocks during hydrothermal alteration and mineralization. The present study, together with previous publications, suggests that fluid inclusions are characterized by H2O-CO2-NaCl and/or CO2-N2 ± CH4 types in mineralized areas, whereas H2O-NaCl fluid compositions dominate in barren areas. Fluid inclusion characteristics may, therefore, be a useful tool for regional gold exploration in the Birimian terrane of Ghana.

Multiple metamorphic buried-hill hydrocarbon reservoirs have been recently discovered in the Bozhong Depression, Bohai Bay Basin. Among them, the buried-hill reservoirs in the BZ13, BZ19 and BZ26 regions have the largest condensate gas reserve in eastern China. The hydrocarbon-bearing reservoir has a large thickness with a maximum of ∼1 km, making the oil-equivalent reserve exceed 5 × 108 tons. This study used petrography, isotopes, gas composition, zircon U–Pb geochronology, X-ray diffraction, electronic probe and hydrothermal dissolution experiments to discuss the formation mechanisms of the metamorphic buried-hill reservoirs. The results indicate that the reservoirs 500 m below the surface of the buried hill have been significantly altered by the mantle-derived CO2. This is supported by the carbon isotope of CO2, He isotope of the helium gas, strong negative excursions in δ18OPDB, positive excursions in δ13CPDB, Co/Ni ratio greater than 1 in pyrite and secondary characteristic minerals such as dawsonite. Hydrothermal dissolution experiments using CO2 result in the precipitation of kaolinite, ankerite and quartz cements, which is consistent with petrographic observation. The mantle-derived CO2 reacts with feldspars, hornblende, biotite and carbonates, creates secondary dissolution pores and significantly improves reservoir quality. Moreover, partial carbonate cements exhibit negative excursions in δ13CPDB and the depletion of light rare earth elements, indicating the influence of organic acids-bearing low-temperature hydrothermal fluids. Therefore, three development models illustrating the formation of metamorphic buried-hill reservoirs were proposed. This study may serve as a reference for the hydrocarbon exploration in metamorphic buried-hill reservoirs.

Techniques based on the variations in composition of water, gas and stable isotopes in the liquid and gas phases of the geothermal fluids have been applied for some time now in the major geothermal fields and are now also used regularly in geothermal exploration. There are numerous processes capable of modifying isotopic composition after infiltration of water from the surface, such as water-rock exchanges, formation of secondary minerals and exchange with the gaseous phase (CO2 and H2S). During ascent to the surface, the two main processes are steam separation and dilution and mixing with shallower waters. This paper also deals with the chemical characteristics of the waters, their classification and the water-rock interaction producing hydrothermal alteration. During exploration the chemical and isotopic geothermometers represent a unique method for investigating the deep system. The choice of geothermometer and interpretation of geothermometric data are two crucial steps in geothermal exploration. Finally, the paper discusses the geochemistry of gas mixtures, especially the origin of the gas species and the main chemical reactions that produce semi-empirical geothermometers and some recent non-empirical geothermometers based on models of a two-phase system in the reservoir. Gas-geothermometers can be developed to calculate the reservoir temperature for natural manifestations.

Purpose. To determine the matter composition, formational types and degree of hydrothermal-metasomatic alterations of the ultramafic rocks from the South-Bilozersky massif and substantiate the prospects for the ore potential of geological formations. Methods. To accomplish the tasks set, traditional methods of studying the matter composition of rocks and ores, such as petrographic, mineragraphic, were used, the results of chemical, X-ray diffraction, semi-quantitative spectral and thermal analyzes were interpreted. To determine the prospects of ore-bearing, ore-formation analysis and a comparative geological method were used.The use of a complex of modern methods and the interpretation of the research results made it possible to substantiate the ore-formational types of associated raw materials. Findings. The features of the matter composition, internal structure and ore content of the s South-Bilozersky serpentinite massif are characterized and the genetic relationship with the known differentiated basic-hyperbasite massifs of the Bushveld, Stillwater, Duluth, Big Zimbabwe dyke types is substantiated. The presence of cumulative dunites and peridotites within the South-Bilozersky massif suggests their intrusive origin from a deeper source with chamber differentiation. The prospects for the discovery within the South-Bilozersky massif of a medium-sized deposit of talc-magnesite, easily accessible for development in the conditions of the infrastructure of the iron ore plant, are substantiated. The affiliation of talc-magnesite ore occurrences to the hydrothermal-metasomatic formation of listvenites, which were formed during the active introduction of silica and carbon dioxide into the high-magnesian environment of deformed serpentinites, is substantiated. The originality. For the first time, the dislocation-metamorphic genesis of hydrothermal ore-bearing formations within the South-Bilozersky ultramafic massif was substantiated and the influence of the degree of hydrothermal alterations on the quality and degree of preservation of ore formations was shown. Practical implementation. The prospects for ore occurrences of minerals accompanying iron deposits in the Belozerskaya greenstone structure are substantiated, which will make it possible to implement an integrated approach to the development of the bowels of the traditionally iron ore region.

Seven gas fields have been developed on the Rock Springs uplift in southwestern Wyoming— North Baxter Basin, Middle Baxter Basin, South Baxter Basin, Little Worm Creek, Joyce Creek, Salt Wells, and Nitchie Gulch. Production from these fields has amounted to 214 billion cu ft of gas from the Frontier, Dakota, and Morrison Formations, and the Nugget Sandstone. Minor oil production has been established from the Dakota Formation at Joyce Creek and the Morrison Formation at Little Worm Creek. Most of these fields have prospects of additional gas in untested fault blocks and new zones in the deeper Paleozoic formations. Remaining proved reserves are estimated to be 102 billion cu ft of gas. Hydrothermal alteration possibly may explain areal variations in the composition of gas found in certain formations

Granitoid-hosted gold deposits at Obuasi are located along the eastern margin of the Kumasi basin, about 5 km west of the Ashanti goldfields, and comprise the auriferous Nhyiaso, Ayankyerim, Yamensakrom, and Kunka plutons. These deposits represent a relatively new style of gold mineralization in the Ghanaian Birimian. The mineralization consists of quartz veins/stockworks and pervasive alteration zones within the granitoids in brittle structures. The ore mineral assemblage is mainly composed of pyrite and arsenopyrite, with minor chalcopyrite, sphalerite, and rutile. Hydrothermal alteration minerals are dominated by quartz, sericite (muscovite), sulphides (mainly pyrite, arsenopyrite) and carbonates. Gold is closely associated with the sulphides in both quartz veins and alteration zones. Chemical mass balance calculations show that major and trace element contents in alteration zones were probably buffered by the original granitoid composition. Fluid inclusions in vein quartz are composed of aqueous H2O-CO2±NaCl and gaseous CO2-N2±CH4 types, with minor (<5%) aqueous H2O±NaCl types. The mineralizing fluids typically have salinities <6 wt.% NaCl equivalent with bulk densities in the range 0.65 to 0.95 g/cm3. The petrographic and microthermometric data suggest that the H2O- and CO2-rich fluid inclusions resulted from phase separation of an initial homogeneous fluid and were trapped at around 180 to 300–350°C and 1 to 3 kbar. Gaseous compositions of fluid inclusions, obtained from Raman spectroscopy, are dominated by CO2 (80 to 95 mol%), with significant amounts of N2 (2 to 20 mol%) and CH4 (0 to 10 mol%). Thermodynamic calculations show that the initial homogeneous H2O-CO2-rich fluid contains 50 to 80 mol% H2O at 300–350°C and 2 kbar. Oxygen fugacities ( ƒ O2) of fluids were estimated to be equal to QFM + 1.4–2.2 and coincide with the NNO buffer in the pyrite field, indicating relatively reduced conditions of fluids during ore formation. Gold deposition within the granitoids may be mainly induced by fluid immiscibility and sulphidization of host rocks during alteration. Based on the geological setting, ore geology and geochemistry, and fluid inclusion characteristics, it is suggested that the granitoid-hosted gold deposits at Obuasi are not intrusion-related, but were formed by metamorphic low-salinity H2O-CO2-N2±CH4 fluids related to the waning stages of the regional Birimian orogeny at about 2100 Ma. The host granitoids acted as favourable sites for fluid flow due to their brittle lithological characteristics.