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The exploration of novel geothermal systems, particularly those promising for electrical power generation, plays a fundamental role in incorporating new renewable sources into the energy matrix. Geothermal systems associated with volcanic calderas are considered ideal targets for exploration. This study focuses on the geochemical features of fluids from the Cerro Galán hydrothermal system, which is hosted within a major resurgent caldera with >3.5 Myr of magmatic evolution situated on the Southern Puna (Central Volcanic Zone of the Andes, NW Argentina). The main aim is constructing the first geochemical conceptual model and provide information on the geothermal potential of this interesting resource. The main hydrothermal reservoir consists of a Na–Cl aquifer with estimated temperatures up to 187 °C at depth. This reservoir is likely hosted within the fractured pre-caldera basement rocks, mainly including Miocene-Pliocene volcanic rocks and Proterozoic-Cambrian igneous and metamorphic rocks. The confinement of the deep reservoir is attributed to the deposits of the Toconquis Group and Cueva Negra Ignimbrite, along with the basal section of the Cerro Galán Ignimbrite, which exhibit low permeability due to hydrothermal alteration. The presence of a phreatic explosion crater near one of the hot spring-rich areas is likely indicating past over-pressurization of the hydrothermal aquifer, resulting from efficient sealing. Furthermore, the absence of anomalous soil CO2 flux values on the top of the reservoir, except where the thermal spring discharges are located, can be explained by an effective cap-rock layer. Deep circulation of meteoric water, enriched with atmospheric gases, receives inputs of magmatic fluids (∼11% of primordial helium), leading to the development of the hydrothermal Na–Cl aquifer. However, this deep fluid contribution might be underestimated due to significant crustal assimilation (up to 50%) involved in the magma genesis of the Cerro Galán Vol

By hosting significant amounts of extra-atmospheric dissolved gases, including geogenic CO2 and CH4, volcanic lakes provide relevant ecosystem services through the key role the aquatic microbial community in mediating freshwater carbon fluxes. In view of elucidating the mechanisms governing the microbial spatial distribution and the possible implications for ecosystem functioning, we compared the hydrogeochemical features and the microbial community structure of two adjacent stratified volcanic lakes (Lake Grande - LG and Lake Piccolo - LP). Water chemistry, gases and their isotopic composition were coupled with microbial pigment profiling, cell counting, and phylogenetic analyses. LP showed transparent waters with low concentrations of chlorophyll-a and the occurrence of phycoerytrin-rich cyanobacteria. LG was relatively more eutrophic with a higher occurrence of diatoms and phycocyanine-rich cyanobacteria. Considering the higher concentrations of CO2 and CH4 in bottom waters, the oligotrophic LP was likely a more efficient sink of geogenic CO2 in comparison to the adjacent eutrophic LG. The prokaryotic community was dominated by the mixothrophic hgcI clade (family Sporichthyaceae) in the LG surface waters, while in LP this taxon was dominant down to -15 m. Moreover, in LP, the bottom dark waters harbored a unique strictly anaerobic bacterial assemblage associated with methanogenic Archaea (i.e. Methanomicrobiales), resulting in a high biogenic methane concentration. Water layering and light penetration were confirmed as major factors affecting the microbial distribution patterns. The observed differences in the geochemical and trophic conditions reflected the structure of the aquatic microbial community, with direct consequences on the dynamics of dissolved greenhouse gases.

This paper presents new chemical and isotopic data on gases from deep oil and gas fields, bubbling gases, dissolved gases in groundwaters and dry seeps of the Southern Po River Basin (Emilia-Romagna, Italy), aiming to (i) characterize and differentiate the various types of deep natural gases; (ii) identify the source(s) of methane and light hydrocarbons in shallow aquifers and surface gas-rich emissions; (iii) propose a conceptual model of natural fluid migration pathways in the sedimentary prism of the Southern Po River Basin. Based on the isotopic composition of CH4 and C2–C4 n-alkanes, CH4/(C2H6+C3H8) ratio, relative proportion of the C7 hydrocarbons and relative concentration of cyclic compounds with respect to the total cyclic abundance, three main deep reservoirs of hydrocarbons are identified in the subsurface of the Southern Po River Basin: (1) microbial gas hosted in Pliocene-Pleistocene marine sediments, (2) thermogenic gas hosted in Miocene deposits and (3) thermogenic gas produced in Triassic carbonates. Helium isotopes of these deep fluids indicate an almost pure crustal origin (Rc/Ra values = 0.014–0.04), with negligible contributions from mantle-derived helium. A variable contribution of atmosphere-derived fluids is highlighted by low 4He/20Ne (down to 5.42) and 40Ar/36Ar (≤319.5) values. Comparison of chemical and isotopic signatures of deep and surficial hydrocarbon occurrences suggests that methane in shallow groundwaters or gas seeps is sourced by microbial gas migrating upward from deep Plio-Pleistocene reservoirs, with no detectable contributions of Triassic or Miocene thermogenic hydrocarbons. At shallow depths (roughly around 20–50 m.b.g.l.), Plio-Pleistocene microbial methane appears to be mainly stored in anoxic aquifers. However, where CH4 further migrates upwards and reaches aerobic environments (e.g., aquifers or soils), it readily undergoes a process of exothermic microbial oxidation mediated by methanotrophic bacteria. Where the structu

Fluid primary source(s) and chemical-physical processes controlling water and gas chemistry of thermal springs from Eastern Cordillera, sub-Andean Ranges and Santa Bárbara (Jujuy Province, northern Argentina) were investigated to provide information for a preliminary evaluation of the geothermal potential in these areas. Thermal manifestations in Eastern Cordillera (Reyes) and part of those in the western sector of sub-Andean Range (Aguas Calientes) are fed by shallow aquifers, interacting with Quaternary- Neogene rocks and the upper portion of Pliocene-Miocene formations (Orán Group), whereas the meteoric water recharge area is located at >2500 m a.s.l., corresponding to Chañi hill. Differently, El Jordán thermal spring in the sub-Andean Range is fed by a hydrothermal aquifer hosted within highly porous and fractured formations of the Salta Group (Yacoraite Formation) and recharged by meteoric water from Sierra de Calilegua (∼1500 m a.s.l.). The latter is the recharge area of the La Quinta geothermal waters as well, but these have been fed at higher altitudes (>2500 m a.s.l.) in the range. The hydrothermal reservoir feeding the other thermal springs from the Santa Barbara system (Caimancito, El Palmar, and Siete Aguas) is recharged by meteoric water from Zapla Ranges and Santa Barbara Hill at [removed]16,000 mg/L) Na+-Cl- La Quinta thermal springs are produced by interaction with the evaporite deposits of Salta Group, including halite, whereas the chemistry of El Palmar, El Jordán and Caimancito thermal springs, showing a Na+-SO4 2-(Cl−) composition, depends on mixing with shallower SO4 2--rich waters interacting with gypsum deposits of Anta Formation. Dissolved and bubbling gases from all the investigated provinces are related to CO2- and CH4-rich crustal fluids produced by both thermogenic processes occurring within the hydrothermal systems and microbial activity at relatively low depth, with low to negligible mantle contribution, as indicated by the 3