Acoculco Geothermal System Research Articles

Soil CO2 fluxes measured in the Acoculco Geothermal System, Mexico: Baseline emissions from a long-term prospection programme

The Acoculco Caldera Complex is considered a promissory hidden high-temperature geothermal system in Mexico. To support the geothermal prospection of this anomalous area, a comprehensive programme of soil CO2 flux measurements was performed. A long-term measurement programme was conducted to determine the baseline of natural soil CO2 effluxes. Significant efforts were devoted both to measuring the CO2 fluxes between 2015 and 2022 and interpreting their origin. Eighteen soil gas surveys of CO2 were carried out by using the accumulation chamber method. >1200 diffuse CO2 fluxes were measured in six different areas of the Acoculco Caldera. Two areas (Los Azufres and Alcaparrosa) exhibited cold degassing sites, acid-sulphate springs, and gas bubbling in surface water bodies. The soil CO2 fluxes ranged from 1 to 26,000 g m−2 d−1, whereas lower fluxes <29 g m−2 d−1 were determined as the degassing baseline. A total CO2 output of 492 t d−1 km−1 was estimated using an integrated SGS-GSA approach, where the highest total soil CO2 fluxes were obtained for Alcaparrosa (299 t d−1 km−2) compared with Los Azufres (164 t d−1 km−2), and Surroundings (29 t d−1 km−2). Such results agree well with those values measured in other worldwide volcanic and active geothermal ecosystems. The range of CO2 isotopic composition values from −28.83 ‰ to −3.11 ‰, together with their statistical distribution, suggests multiple CO2 production sources feeding soil degassing. The combined interpretation of flux and isotopic data allowed us to identify two distinct gas sources: endogenous and biogenic. The present study highlights the importance of using soil CO2 monitoring to determine baseline emissions at the early exploration stage of geothermal systems.

Overlapping events in the Acoculco geothermal system, Puebla, Mexico

The Acoculco geothermal system records two major hydrothermal events (prograde skarn overlapped by retrograde/”epithermal” to geothermal stages) in association with pre-calderic to calderic events in the eponymous complex, which belongs to the Trans-Mexican Volcanic Belt. The prograde skarn is associated with a hornblende granite (hereby dated at 1.01 ± 0.01 Ma; U-Pb zircon age), whose apex is found at a ∼1600 m depth, and later andesite dykes and sills. Mineralizing fluids in this stage have salinities of fluid inclusions that range between 9.5 wt.% NaCl equiv. and 33 wt.% NaCl and temperatures of homogenization (Th) up to 470 °C that record various process of mineral formation, such as boiling, depressurization, conductive cooling, isothermal mixing, and dilution. The retrograde stage is represented by early “fossil” brines with salinities 1.4 and 7.3 wt.% NaCl equiv. and Th up to 303 °C, and present-day gasothermal fluids that attain stabilized temperatures up to ∼300 °C at a depth of almost 2000 m. Th of early brines are consistently higher than stabilized temperatures at most of the depths that were surveyed, the isotherms that correspond to 250° and 200 °C dropped ca. 500 m from the early retrograde to the gasothermal stage. These features support the hypothesis that the Acoculco geothermal system has entered its senile stage. Despite that, upon a calculated geothermal gradient at ∼150 °C/km for both Th of the early retrograde stage and stabilized temperatures in the EAC-1 well, it is not foreseen a rapid waning of the thermal anomaly and its ultimate cause.Besides the early Ca-silicate assemblages that constitute the skarn, the architecture of alteration assemblages also records the overlapping of shallow on deep hypogene assemblages. The deep hypogene assemblages respond to alkaline to moderately acidic fluids that correspond to the early retrograde (or “epithermal”) stage: propylitic, phyllic and argillic (kaolinite + nontronite + montmorillonite), from bottom to top. Contrastingly, the shallow hypogene assemblages respond to dominantly acidic fluids, and consist of advanced argillic assemblages that may grade upwards into argillic (kaolinite + illite-smectite) or silicic (vuggy or massive to brecciated silica). The shallow hypogene character of advanced argillic assemblages, as due to steam-heated ground environments (as opposed to deep hypogene or magmatic-hydrothermal assemblages), is deduced from (1) the overall tabular shape of their distribution, (2) the mineralogical association and characteristics of rhombohedral alunite within it, and (3) its relationship with overlying silicic alteration that, in turn, do not possess likely characteristics of sinters. Shallow hypogene advanced argillic assemblages are associated with the present-day gasothermal activity due to sealing that was possibly promoted during the early retrograde/”epithermal” activity.

The magmatic plumbing system of the Acoculco volcanic complex (Mexico) revealed by ambient noise tomography

The GEMex project is a Mexican-European cooperation in geothermal energy focusing on enhancing the Acoculco geothermal system. The volcanic system belongs to the northeastern part of the Trans-Mexican Volcanic Belt. In this study, we present ambient noise seismic tomographies of the anisotropic shear wave velocities and reveal the magmatic plumbing system. The tomographies result from the joint inversion of the multi-mode Rayleigh and Love surface wave velocities and Horizontal to Vertical Spectral Ratios (HVSR or H/V). We also discuss the advantage of this technique compared to traditional ambient noise tomography methods and underline the importance of considering multiple surface wave modes to correctly reveal velocity anomalies buried a few kilometers in depth. A reservoir is identified at about 5 km depth at the northeast outside of the caldera. This reservoir is linked to the south caldera surface via a conduit that follows the caldera's eastern border, mainly the Chignahuapan graben. An intermediate reservoir is also observed at about 2.5 km depth, just below the three most recent monogenetic volcanoes. Results agree well with geological studies, the caldera collapse, its temporal evolution, and other geophysical studies recently reported.

Mineralogical and geochemical changes due to hydrothermal alteration of the volcanic rocks at Acoculco geothermal system, Mexico

Most of the volcanic rocks in the Acoculco geothermal system (AGS), near to the surface manifestations, are intensively altered. Thirty‐four surface volcanic rocks and five rocks from outcrop sections are considered in this work. Silica polymorphs (quartz, cristobalite and tridymite) are the dominant hydrothermal minerals in almost all the rock samples. The following other minerals are present in minor concentrations: alkali‐feldspars (sanidine potassic, anorthoclase and microcline), plagioclase feldspars (albite and anorthite), clay minerals (illite, kaolinite and montmorillonite), anhydrite, buddingtonite, titanoaugite, magnetite and anatase. This type of mineral assemblage of the rocks indicates an intensive silicic and argillic alteration. Comparison of the elemental concentrations of the surface acid rocks of the AGS with that of the precursor rock has indicated alteration‐induced changes in (a) chemical mobility in the concentrations of major element oxides, with an increase in the concentrations of SiO2, and TiO2, loss in those of Al2O3, Fe2O3, MnO, MgO, CaO, Na2O and K2O, and no statistically significant changes in those of P2O5, (b) no significant changes in the concentrations of LREE and MREE (except Dy and Ho), however, less but significant loss in the concentrations of Dy and Ho, and HREE, and (c) there is an enrichment in the concentrations of Nd, Sr, Sc, V, Nb, Ag, Sn, Pb, Tl, Ta, U and Th; and depletion in Be, Co, Zn, Ga, Ge, Rb, Y, Zr, Mo, Cs, Ba and Hf compared to those of the precursor rock. This type of distribution of the elements may indicate (a) an intense silicic alteration; and (b) REEs might have been leached from the original rocks because of hydrothermal alteration processes, but only LREEs, and MREEs (except Dy and Ho) can be redistributed and accommodated in the hydrothermal minerals, providing them with somewhat inert (immobile) behaviour. The mass change calculations have indicated that MnO and Sr have experienced the greatest and smallest mass changes, respectively. The identification of outliers in chemical data has revealed the presence of two distinctive groups of rocks. Ten out of the total 27 acid volcanic rocks with several elements as outlier data are negligibly to moderately altered (Group‐1), and the remaining 17 acid rocks without outlier data are intensively altered rocks (Group‐2).

Eruptive chronology of the Acoculco caldera complex – A resurgent caldera in the eastern Trans-Mexican Volcanic Belt (México)

The Acoculco caldera complex (ACC) is located in the eastern part of the Trans-Mexican Volcanic Belt in the northern part of the State of Puebla. The complex sits at the intersection of two regional fault systems with NE-SW and NW-SE orientations. The ACC was built atop Cretaceous limestones, the Zacatán basaltic plateau of unknown age, early Miocene domes (~12.7–10.98 Ma), and Pliocene lava domes (~3.9–3 Ma). Detailed field mapping and stratigraphy studies complemented by 40Ar/39Ar and 14C dating allowed the division of the ACC volcanic succession into 30 volcanic units. Based on the new results and previous studies, the ACC eruptive chronology was grouped in four eruptive phases: syn-caldera, early post-caldera, late post-caldera, and extra-caldera. Inception of the ACC volcanism began around 2.7 Ma with the dispersion of an andesitic ignimbrite followed by the collapse of the magma chamber roof as attested by the presence of a lithic breccia in isolated parts of the caldera rim. The collapse produced a 18 × 16 km caldera depression which was partly filled by the ignimbrite (total volume of ~127 km3) followed by the establishment of an intracaldera lake of unknown total extension. Early post-caldera collapse activity (2.6–2.1 Ma) was restricted within the caldera producing 27 km3 of lava flows and domes dominantly of basaltic trachyandesite to basaltic composition. Late post-caldera collapse activity (2.0-<0.016 Ma) migrated dominantly to the caldera rim and periphery emplacing 90 km3 of magma as rhyolitic domes, lava flows, scoria cones, and two younger ignimbrites. The 1.2 Ma Encimadas ignimbrite (26 km3) was vented through the eastern margin of the caldera and dispersed to the northeast, and the 0.6–0.8 Ma Tecoloquillo ignimbrite and dome (11 km3) erupted from the southwestern margin of the caldera. The most recent eruption of this phase was vented close to the southeastern caldera rim producing the Cuatzitzingo (<16,710 ± 50 years BP) scoria cone. Extra-caldera activity (2.4–0.19 Ma) of the Apan–Tezontepec volcanic field produced scoria cones and lava flows of basaltic trachyandesite to basaltic andesite composition that are interbedded with the products of the caldera complex.Aeromagnetic data further constrains the edge of the caldera rim and is consistent with the presence of at least four intrusive bodies at depths of >1 km hosted in the Cretaceous limestones. These bodies might represent a series of horizontal mafic intrusions located at different depths that provide the energy that maintains the Acoculco geothermal system active.

Water-rock interactions in the Acoculco geothermal system, eastern Mexico: Insights from paragenesis and elemental mass-balance

Geochemical mass balances, supported by mineral characterization and whole-rock elemental analyses, were used to decipher the alteration paragenesis and to understand the elemental mobility related to the interaction of hydrothermal fluids with volcanic rocks (ignimbrites and lavas ranging from rhyolite to dacite) from the Acoculco geothermal system, in the eastern portion of the Trans-Mexican Volcanic Belt. Three extensive events of hydrothermal alteration were distinguished, from earliest to latest: (1) selective silicification and argillization, (2) hydraulic brecciation by silica-rich fluid, and (3) acid-sulfate (vuggy and argillic advanced) alteration. According to the Gresens-Grant mass-balance method, the altered volcanic rocks of Acoculco are generally enriched in Ti and Si and depleted in the rest of major elements; this chemical behavior is striking in the case of vuggy and silicic altered rocks. Small changes in some element concentrations calculated by the Gresens-Grant method may be due to chemical heterogeneity between the rock chosen to be the reference (“fresh rocks”) and the protolith of the altered ones. The chondrite-normalized REE gave different patterns for distinct types of alteration; vuggy and silicified rocks present low LREE and high HREE in opposition to the advanced argillic alteration. Boiling, primarily related to the last alteration event, may have happened at variable depths (825, 325 and 102 m below present surface); it produced changes fluid composition, resulting in acidic pH surface alterations.

The Acoculco Caldera Complex magmas: Genesis, evolution and relation with the Acoculco geothermal system

The Acoculco Caldera Complex (ACC) is located in the eastern part of the Trans-Mexican Volcanic Belt, México. The ACC was formed 2.7 Ma, and since then, volcanic activity has persisted until 0.06 Ma, through the emission of domes, cinder cones, fissure lava flows and two ignimbrite eruptions dated at 1.2 and 0.65 Ma. The pervasive hydrothermal alteration in the central part of the ACC has motivated considerable geothermal exploration, however, none of it assessed the relation between the magmatic heat supply and the evolution of magmas, including their reservoir depth.Magmatic processes were investigated with petrography, major oxides-trace-element geochemistry, mineral chemistry and isotopic analysis. In addition, we performed a series of hydrothermal experiments in order to constrain the storage depth for the magma tapped 1.2 Ma during a reactivation of the caldera. We investigated the origin of magmas tapped during the eruptive history of the ACC, and the magmatic processes (magma mixing, assimilation and crystallization) that modified these magmas.After the caldera collapse 2.7 Ma, the local stress field was probably modified and allowed the ascent of peralkaline magmas through new plumbing systems. Such magmas mixed with calc-alkaline magmas and formed the post-caldera volcanism. Influx of new magma is considered as the probable heat supply that maintains the active magmatic system, whereas assimilation has been negligible, but yields insights about the depth at which processes have occurred. Our work suggests that there are either shallow storage zones (200–500 bars) where magmas crystallize and either erupt or stall and cool, or there are intrusive magmas unrelated to the ACC suite that have accumulated nearby and heat the reservoir magmas.

Thermal history of the Acoculco geothermal system, eastern Mexico: Insights from numerical modeling and radiocarbon dating

Acoculco is a geothermal prospective area hosted by a volcanic caldera complex in the eastern Trans-Mexican Volcanic Belt. Surface manifestations are scarce and consist of gas discharges (CO2-rich) and acid-sulfate springs of low temperature, whereas hydrothermal explosive activity is profusely manifested by meter-scale craters and mounds of hydrothermal debris and breccias. Silicic alteration extends for several square kilometers around the zone with gas manifestations and explosive features, affecting surficial volcanic rocks, primarily tuffs and breccias. In the subsurface, an argillic alteration zone (ammonium illite) extends down to a depth of ∼600m, and underneath it a propylitic zone (epidote–calcite–chlorite) occurs down to ∼1000m.Thermal logs from an exploratory borehole (EAC-1, drilled in 1995 down to 1810m) showed a conductive heat transfer regime under high geothermal gradient (∼140°C/1000m). In contrast, the thermal profile established from temperatures of homogenization of fluid inclusions—measured on core samples from the same drill hole—suggests that convection occurred in the past through the upper ~1400m of the geothermal system. A drop in permeability due to the precipitation of alteration minerals would have triggered the cessation of the convective heat transfer regime to give place to a conductive one. With the purpose of determining when the transition of heat transfer regime occurred, we developed a 1D model that simulates the time-depth distribution of temperature. According to our numerical simulations, this transition happened ca. 7000years ago; this date is very recent compared to the lifespan of the geothermal system. In addition, radiocarbon chronology indicates that the hydrothermal explosive activity postdates the end of the convective heat transfer regime, having dated at least three explosive events, at 4867–5295, 1049–1417 and 543–709y cal. BP. Therefore, hydrothermal explosions arise from the self-sealing of the Acoculco geothermal system, involving a natural hazard that could affect future geothermal-power infrastructure.

Combining ammonium mapping and short-wave infrared (SWIR) reflectance spectroscopy to constrain a model of hydrothermal alteration for the Acoculco geothermal zone, Eastern Mexico

The Acoculco geothermal system is hosted by a caldera complex located at the eastern portion of the Trans-Mexican Volcanic Belt. Surface manifestations are scarce and consist of low temperature, bubbling, acid–sulfate springs that are concentrated in two zones separated from each other by ∼1750m. In the northernmost one, there are conspicuous features suggesting recent, explosive, hydrothermal activity. Most of the rocks that crop out are tuffs and breccias that show pervasive hydrothermal alteration. Six SWIR-active minerals have been revealed by reflectance spectroscopy in the surface altered rocks: Opal, kaolinite, alunite, ammoniojarosite, buddingtonite and interstratified illite–smectite; they are indicative of alteration assemblages comparable to those reported in other geothermal systems and in epithermal deposits. Opal is the most widespread alteration mineral and occurs in association with tridimite and anatase. Kaolinite is also widely distributed but has a greater presence in the area with active surface manifestations, which, moreover, is the only area with alunite and ammoniojarosite occurrences. Buddingtonite occurs in the same zones as sulfates. As a complement to the alteration maps, we are proposing a simple method for mapping ammonium anomalies, based on total N analyses in altered rocks. Total N data show a discontinuous distribution within a wide range of variation (0.01–3.28wt.%). All medium and high N values (>0.26wt.%) plot in three clusters, two of which coincide with zones of gas emission.A deep circulating, active hydrothermal system cannot be ruled out in Acoculco although it may be confined to the deep caldera structure by an impermeable cap-rock of silicic and argillic alteration. In this scenario, hydrothermal fluids would reach the surface periodically, when explosive hydrothermal fracturing allowed it. The occurrence of hydrothermal explosion features and of acid–sulfate alteration supports this scenario. Therefore, further exploration by drilling would be advisable in Acoculco.