Magmatic Heat Source Research Articles

Hydrochemical Characteristics and Formation Mechanism of Geothermal Fluids in Zuogong County, Southeastern Tibet

Zuogong County is located in the southeast of Tibet, which is rich in hot spring geothermal resources, but its development and utilization degree are low, and the genetic mechanism of the geothermal system is not clear. Hydrogeochemical characteristics of geothermal water are of great significance in elucidating the genesis and evolution of geothermal systems, as well as the sustainable development and utilization of geothermal resources. The hydrogeochemical characteristics and genesis of the geothermal water in Zuogong County were investigated using hydrogeochemical analysis, a stable isotope (δD, δ18O) approach, and an inverse simulation model for water–rock reactions using the PHREEQC. The results indicated that the Zuogong geothermal system is a deep circulation heating type without a magmatic heat source. The chemical types present in the geothermal water from the Zuogong area are HCO3 and HCO3·SO4, and the main cations are Na+ and Ca2+. The groundwater is replenished by atmospheric precipitation and glacier meltwater. The salt content of geothermal water mainly comes from the interaction between water and surrounding rocks during the deep circulation process. The reservoir temperature of geothermal water in Zuogong is 120–176 °C before mixing with non-geothermal water and drops to 62–98 °C after mixing with 58 to 79% of non-geothermal water. According to the proposed conceptual model, geothermal water mainly produces water–rock interaction with aluminosilicate minerals in the deep formation, while in shallow areas it interacts mainly with sulfate minerals. These findings contribute to a better understanding of the geothermal system in Zuogong County, Tibet.

Constraining the properties of the heat sources of high-temperature hydrogeothermal systems: Evidence from the lithium concentrations of geothermal waters

The properties of the heat source determine the exploitation potential and scientific research value of a high-temperature hydrothermal geothermal system. Existing hydrogeochemical and geophysical methods for discriminating heat sources have noticeable shortcomings, and an economic and universally applicable discrimination method is needed. In this study, three groups of typical geothermal waters from magmatic and non-magmatic geothermal systems in China were taken as the research objects. The Li concentrations of these geothermal waters were analyzed and the differences in their Li enrichment mechanisms were explored, while taking the cooling process of the geothermal water and the water–rock interactions into consideration. Finally, a more economical and universal new method for determining whether a hydrothermal geothermal system has a magmatic heat source based on the Li concentration was developed. The results show that: (1) the main ions in the magmatic geothermal waters were Na, HCO3, and Cl, but for the main ions in the non-magmatic geothermal waters were Na, Ca, and SO4. (2) Generally, the Li concentrations of the magmatic geothermal water samples were significantly higher than those of the non-magmatic geothermal water samples and most of the geothermal waters with a magmatic heat source were characterized by Li concentration of ≥5 mg/L and reservoir temperatures of ≥150 °C, while the geothermal waters with a non-magmatic heat source did not have this characteristic. (3) The magmatic geothermal waters mainly underwent the conductive and adiabatic cooling processes and also experienced cold-water mixing, whereas the non-magmatic geothermal waters mainly experienced cold-water mixing and conductive cooling. (4) Li-bearing mica was the main source of the Li from the host rocks and the amount of Li that removed by the adsorption of octahedral silicate minerals was greater in the magmatic geothermal systems than in the non-magmatic geothermal systems. (5) The additional amount of Li and heat provided by magmatic fluids were the key factors causing the Li enrichment, and the water–rock interactions between the geothermal waters and the host rocks were not sufficient to make the Li concentration of the geothermal waters reach 5 mg/L. (6) The Li concentration can be used universally to indicate the properties of the heat source of hydrothermal high-temperature geothermal system. In this study, a universal method for discriminating the heat sources of hydrothermal high-temperature geothermal systems based on the Li concentration was developed. Our findings provide a theoretical basis and effective method for evaluating types of geothermal resource and are of great significance for understanding the enrichment of lithium in geothermal water.

Conductivity distribution beneath Socompa volcano, northwestern Argentina, from 3-D magnetotelluric characterization

The Socompa volcano is one of the Holocene volcanoes of the Central Volcanic Zone of the Andes. Given that it records effusive activity after 5910 YBP and that recent studies reveal unrest associated with the uplift of its edifice, it is considered an active volcano. With the aim of explaining new features that provide information about the nature of its state, here we present a geoelectric structures model of the subsurface of Socompa volcano obtained with the magnetotelluric method. One of the main features defined by the 3-D model is a body with an electrical resistivity value lower than 10 Ωm between 2 and 7 km depth and located below the volcanic edifice. This body, interpreted as the magmatic heat source of the volcano, is connected to a shallow and thick high conductive layer that covers the surrounding area. Our results highlight a possible shallow magma reservoir below the Socompa volcano and an associated -active or not- hydrothermal system.

Geochemical and H–O–Sr–B isotope signatures of Yangyi geothermal fields: implications for the evolution of thermal fluids in fracture-controlled type geothermal system, Tibet, China

High-temperature hydrothermal systems are mainly distributed in the north–south graben systems of southern Tibet as an important part of the Mediterranean–Tethys Himalayan geothermal belt in mainland China. As the largest unit capacity and second stable operating geothermal power station in China, Yangyi is the fracture-controlled type geothermal field in the center of Yadong–Gulu Graben. In this paper, hydrogeological and hydrochemical characteristics, isotope composition (δD and δ18O, 87Sr/86Sr and δ11B) of borehole water, hot springs, and surface river samples were analyzed. From the conservative elements (such as Cl− and Li+) and δD and δ18O values, the geothermal water of the Yangyi high-temperature geothermal field is estimated to be of meteoric origin with the contributions of chemical components of the magmatic fluid, which is provided by partially molten granite as a shallow magmatic heat source. According to logging data, the geothermal gradient and terrestrial heat flow value of the Yangyi high-temperature geothermal field are 6.48 ℃/100 m and 158.37 mW m−2, respectively. Combining the hydrothermal tracer experiment, 87Sr/86Sr and δ11B ratios obtained with gradually decreasing reservoir temperatures from the Bujiemu stream geothermal zone to Qialagai stream geothermal zone, we suggested the deep geothermal waters were mixed with local cold groundwater and then flow northeastward, forming the shallow reservoir within the crushed zone and intersect spot of faults in the Himalayan granitoid. Furthermore, in the process of ascent, the geothermal water is enriched in K+, Na+, and HCO3− during the interaction with underlying Himalayan granitoid and pyroclastic rocks that occur as wall rocks. The detailed description and extensive discussion are of great significance for the further exploitation and utilization of north–south trending geothermal belts in Tibet.

Cold water injection near the magmatic heat source can enhance production from high-enthalpy geothermal fields

Although existing deep geothermal wells have confirmed that shallow magmatic intrusions heat circulating hydrothermal fluids to supercritical temperatures, successfully utilizing such fluids for power generation has proven challenging. As an alternative, using deep geothermal wells drilled into the vicinity of intrusions for injection rather than production has the potential to enhance heat extraction from the intrusion, while simultaneously providing fluid pressure support to the overlying geothermal system. However, the dynamic response of the heat source and overlying geothermal system to deep injection has not previously been investigated using numerical modelling. Here, we present numerical models showing that injection in the near-vicinity of a magmatic heat source increases the production potential of shallower geothermal wells. By locally cooling the hot rock near an intrusion, deep injection increases permeability and the rate of heat transfer across the brittle–ductile transition. The injected fluids ascend in upflow zones above the heat source and provide pressure support to shallower geothermal wells, without significantly affecting the temperature and enthalpy of produced fluids. We investigate a variety of scenarios involving only a single deep injector and a single shallower producer. Our models suggest that deep injection could increase the production potential of shallower geothermal wells by up to 10 percent after 30 years of well operation and by 20 percent after theoretical 100 years, with the magnitude of the impact increasing with continued injection over longer timescales.

Assessment of possibility of finding geothermal energy resources around Mount Meru in northern Tanzania using classical solute geothermometry

This study uses classical solute geothermometry analysis to estimate the temperature and circulation depth of the geothermal reservoir around Mount Meru. Five springs affected by magmatic gases from a deep magmatic heat source are investigated; these include two hydrothermal springs. Results from silica geothermometers show that estimated reservoir temperatures range from 70 to 110 °C; this corresponds to a low-temperature hydrothermal system. Since Mount Meru is an active volcano, one would expect a high-temperature hydrothermal system; the low temperatures are ascribed to the mixing of hydrothermal and cold recharge waters. The geothermal reservoir is estimated to be about 1.3–1.4 km deep. Thus, the study shows that a low-temperature geothermal energy resource can be found on the eastern flank of Mount Meru.

Experimental constraints on the stability and oscillation of water vapor film—a precursor for phreatomagmatic and explosive submarine eruptions

Pre-mixing of magma and external water plays a key role in driving explosive phreatomagmatic and submarine volcanic eruptions. A thin film of water vapor forms at the magma–water interface as soon as hot magma comes in direct contact with the cold water (Leidenfrost effect). The presence of a stable vapor film drives efficient mixing and mingling between magma and water, as well as magma and wet and water-saturated sediments. Such mixing occurs before explosive molten fuel–coolant type interactions. Using high-temperature laboratory experiments, we investigate the effect of magma and water temperatures on the stability of vapor film, which has not been performed systematically for a magmatic heat source. The experiments were performed with re-melted volcanic rock material, from which spherically-shaped rock samples were produced. These samples were heated to 1,110°C and then submerged in a water pool with a constant temperature (3–93°C). The experiments were recorded on video, and, synchronously, sample and water temperatures were measured using thermocouples. The time-dependent thickness of the vapor film was measured from the video material. The vapor film tends to oscillate with time on the order of 102 Hz. We find that the vertical collapse rates of vapor films along the sample–water interfaces are 13.7 mm s−1 and 4.2 mm s−1 for water temperatures of 3.0°C and 65°C, respectively. For a given initial sample temperature, the thickness and stability time scales decrease with decreasing water temperature, which has implications for the efficiency of pre-mixing required for explosive eruptions. Using thermodynamics and previously measured material parameters, it is shown that a sudden collapse of the vapor film can start brittle fragmentation of the melt and thus serves as the starting point of thermohydraulic explosions.

Heat, noble gases and CO2 sources in geothermal fields of Mexico

The Trans-Mexican Volcanic Belt (TMVB) and the Gulf Extensional Province (Baja California) are the regions in Mexico with the highest heat flow and where the main geothermal energy resources are currently exploited. Here, previously published and newly collected heat and volatile species (He, Ar and CO2) data from 132 fluid samples of four geothermal areas belonging to TMVB (Los Azufres, Los Humeros, Acoculco and Cerritos Colorados) and two in Baja California (Cerro Prieto and Las Tres Vírgenes) were analyzed. The goal was to identify heat sources, understand how heat is transported into the geothermal reservoirs and how the multiple fluid sources (meteoric, magmatic and crustal) in the reservoir might impact the heat in each field. Distinct R/Ra ratios (where R is the measured 3He/4He ratio and Ra is the atmospheric ratio) are observed in the TMVB and Baja California geothermal fields, with those in the TMVB displaying significantly higher R/Ra values (7.14–7.27) than those in Baja California (1.21–6.62), indicating a higher contribution of mantle helium from possibly younger, active magmatic heat sources. Lower R/Ra values in Baja California might reflect local tectonic features, where reservoirs are primarily affected by crustal contributions from an old, subducted fossil slab related to the Farallon Plate. The analysis of heat to 3He ratios (or Q/3He) together with information provided by 4He/36Ar ratios suggest that heat and volatiles are transferred into the hydrothermal system by advection. The magma bodies beneath the geothermal areas of interest are sufficiently voluminous to provide aqueous fluids by exsolution to sustain this advective flow. The release of He and Ar from magma, during its degassing, appears to be controlled by their diffusivities, fractionating the pristine mantle 4He/36Ar ratio toward higher values, while advection transfers volatiles more rapidly than heat, fractionating the pristine mantle Q/3He ratios toward lower values. The Q/3He vs. 4He/3He relationship shows that boiling and dilution by meteoric water led to further fractionation of heat and volatiles, particularly in Los Azufres and Los Humeros fields, while Cerro Prieto has Q/3He ratios close to those at mid-ocean ridges or fractionated by circulation of low-heat, 4He-rich fossil waters. Distinct sources of heat and volatiles are also reflected in the CO2 content, the major dry gas component in these geothermal fields. Main sources of CO2 are mantle carbon (1%-51%) and limestone (34%–100%) from local sources and the subducting slab, although expected fractionation of the C isotopic composition and CO2/3He during magma ascent and degassing introduce uncertainties on the precise estimation of each source contributing C to the studied geothermal reservoirs. Yet, contribution of carbon from sediments might be minor (0%–35%), and mainly found in Cerro Prieto and Las Tres Virgenes fields located in Baja California, which are affected by the release of volatiles from the subducted Farallon plate and presence of connate waters in the reservoir of Cerro Prieto.

3D deep geoelectrical exploration in the Larderello geothermal sites (Italy)

The paper describes a new experimental deep electrical resistivity acquisition (down to 1600 m) for exploring deep and shallow geothermal systems. The test site is located in the Larderello geothermal area, the oldest geothermal field in the world under exploitation for power production. In this area, many data have been acquired in the frame of previous exploration projects but nowadays several critical issues are still matter of debate: permeability distribution, depth and volume of the magmatic heat source, supercritical fluid condition at depth, and the occurrence of low resistivity anomalies in a dry-steam crystalline and carbonate reservoir. In order to develop new methods for contributing to the hydrothermal reservoir issues, an experimental high resolution 3D Surface-Hole Deep Electrical Resistivity Tomography (SH-DERT) was designed and the Venelle2 well in the Larderello geothermal site, hosted in the crystalline units, was used for the experiment. The design of the in-hole experiment and the results of the deep geoelectrical survey are hereby presented. SH-DERT was properly designed to face extreme conditions at depth characterizing the geothermal well. It provided a 3D resistivity distribution. Transmitting and receiving electrodes were distributed on a large surface (6 km2) and in the Venelle2 well (down to 1600 m). The in-hole electrical cable was equipped to be able to operate in very high temperature conditions. The experiment represents a challenge and an opportunity for the applied geophysics in geothermal areas, where a lowest resistivity is highlighted in a zone above the reservoir and the resistivity of the reservoir is higher. Moreover, the relationship between temperature, clay alteration and resistivity can define a challenger to enable better prediction of reservoir temperature distribution from resistivity measurements. It is a potential improvement of the reservoir knowledge and a useful success for exploration drilling.

Magnetic anomaly patterns and volcano-tectonic features associated with geothermal prospect areas in the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift

The Ziway-Shala Lakes Basin is a structural depression within the Main Ethiopian rift that is associated with Cenozoic volcanism, faulting and sedimentation. The Aluto-Langano volcano complex contains a geothermal field within this basin that is currently being exploited for geothermal power production. Recent magnetotelluric analyzes did not reveal a heat source beneath the Aluto-Langano geothermal field but found an electrical conductor that may be the magmatic heat source near the Silti Debre Zeyet Fault Zone (SDFZ) northwest of the Aluto- Langano geothermal field. To investigate the subsurface structure of the Ziway-Shala Lakes Basin and especially the Aluto-Langano geothermal field, a ground based magnetic survey was performed. Volcano-tectonic features were identified using a differentially reduced to the pole total magnetic field anomaly and a variety of map transformations including upward continuation, first vertical derivatives and tilt derivatives, to map the regional and residual magnetic fields. To identify subsurface magnetic susceptibility sources between the SDFZ and the Aluto-Shala geothermal field, a power spectral analysis, 2D-Werner Deconvolution, and 2D forward modeling were applied. The integrated interpretation of the maps and models, indicates that magnetic maxima occurred over the SDFZ and the Wonji Fault Zone and the minima over the Aluto-Langano geothermal field. The magnetic maxima may be caused by shallow (< 5 km) igneous intrusions that are related to deeper magma movement. The magnetic minima may be caused by warm geothermal waters causing thermal alteration of the magnetic minerals under the Aluto-Langano volcanic complex. The residual and derivative magnetic anomaly maps suggest the existence of lineations that may transfer geothermal fluids suggested by the magnetotelluric models from the SDFZ to the Aluto-Langano geothermal field. The upward movement of the fluids is evident by the magnetic minimum in the Aluto-Langano geothermal field and agrees with a recent earthquake analysis. Additionally, a majority of the magnetic lineaments are oriented parallel to pre-existing Mesozoic structures related to the opening of the Ogaden Basin.

Investigation of the Extent of the Geothermal Resource South of MT. Longonot Volcano Using Gravity and Magnetics, Kenya

Geothermal energy is increasingly becoming an important source of electricity generation in Kenya. The country has prominent Geodynamic features such as the Rift Valley which gives rise to high temperature magmatic heat sources and hydrothermal reservoirs which can be exploited to produce geothermal energy for power generation and direct heating. Identifying and delineating this resource can be done using combined appropriate Geological, Geochemical and Geophysical methods. The main objective of geothermal exploration is to obtain from surface studies adequate information about the promising features of a prospective geothermal system and therefore reducing uncertainties before embarking on costly and risky drilling. The main objective of this research was to investigate the extent of the geothermal resource to the South of Longonot Volcano using gravity and magnetic data that was availed by Kenya Electricity Generating Company. Â Surfer 11 software was used to process and analyse the data, presence of bodies with high density contrast and high susceptibility contrast were revealed. The research identified three potential wells in the study area.

Gas geochemistry of geothermal fluids from the Hatchobaru geothermal field, Japan

We analyzed the chemical and isotopic composition of the geothermal fluids from the Hatchobaru geothermal field in the Kyushu Island, Japan. Fluid chemistry showed similarity with that reported in earlier studies. Chemical geothermometry provided an estimated reservoir temperature of 250–300 °C. Helium and carbon isotopic ratios of the steam is likely to reflect the signature of the magmatic heat source. The apparent equilibrium temperature for carbon isotope exchange between CO2 and CH4 was 375–430 °C. The sulfur isotope data were in accordance with the idea that an acidic Cl-SO4-type fluid was modified from a neutral Cl-type fluid.

Imaging the heat source of the Kangding high-temperature geothermal system on the Xianshuihe fault by magnetotelluric survey

The Kangding geothermal field along the Xianshuihe fault is a typical high-temperature geothermal system with an enormous capacity for geothermal power generation. The Xianshuihe fault is a major active sinistral strike-slip fault in eastern Tibet Plateau, where the fault splits into three parallel branches near Kangding. We collected data from 17 magnetotelluric stations in the Kangding geothermal system. After data processing, the dimensionality of the subsurface structure was qualitatively estimated using the phase tensor. The electrical resistivity model inverted with ModEM shows the Kangding geothermal system from the surface to a depth of 20 km, which correlates closely with the near-surface geology. The synthetic tests confirmed that the main resistivity anomalies identified in the electrical resistivity model were robust. There are several significant low resistivity anomalies of less than 10 Ωm beneath the Kangding geothermal system. The cap rock and liquid-dominated reservoir can be interpreted as shallow conductive zones. The conductors beneath the Kangding geothermal field at a depth of 10 km are interpreted as an underlying zone of partial melt, which provides fluids and heat for the geothermal system. The partial melt fractions are in the range of 4–19%, confirming that there is a magmatic heat source beneath the Kangding geothermal system.

Damming Induced Natural Attenuation of Hydrothermal Waters by Runoff Freshwater Dilution and Sediment Biogeochemical Transformations (Sochagota Lake, Colombia)

The volcanic area of the Paipa system (Boyacá, Colombia) contains a magmatic heat source and deep fractures that help the flow of hot and highly mineralized waters, which are further combined with cold superficial inputs. This mixed water recharges the Salitre River and downstream feeding Sochagota Lake. The incoming water can contribute to substantial increases in hydrothermal SO42−-Na water in the water of the Salitre River basin area, raising the salinity. An additional hydrogeochemical process occurs in the mix with cold Fe-rich water from alluvial and surficial aquifers. This salinized Fe-rich water feeds the Sochagota Lake, although the impact of freshwaters from rain on the hydrochemistry of the Sochagota Lake is significant. A series of hydrogeochemical, biogeochemical, and mineralogical processes occur inside the lake. The aim of this work was to study the influence of damming in the Sochagota Lake, which acts as a natural attenuation of contaminants such as high concentrations of metals and salty elements coming from the Salitre River. Damming in the Sochagota Lake is considered to be an effective strategy for attenuating highly mineralized waters. The concentrations of dissolved elements were attenuated significantly. Dilution by rainfall runoff and precipitation of iron sulfides mediated by sulfate-reducing bacteria in deposits rich in organic material were the main processes involved in the attenuation of concentrations of SO42−, Fe, As Cu, and Co in the lake water. Furthermore, the K-consuming illitization processes occurring in the sediments could favor the decrease in K and Al.

High temperature generation and equilibration of methane in terrestrial geothermal systems: Evidence from clumped isotopologues

Fluids emanating from geothermal areas contain trace quantities of methane and other simple hydrocarbons. These hydrocarbons are thought to derive from thermal cracking of organic matter dissolved in circulating meteoric or seawater or found in pre-existing organic-rich sedimentary rocks, but an abiotic origin has also been proposed. We measured the relative abundances of four CH4 isotopologues (12CH4, 13CH4, 12CH3D, and 13CH3D) in hydrothermal gases discharged by steam vents and geothermal wells from Iceland and Nisyros island (Greece) in order to investigate the origin of methane. Measured methane samples yielded consistently low Δ13CH3D values (13CH3D abundance relative to stochastic) of 0.82–1.77‰, which correspond to high apparent temperatures of isotopologue equilibrium (TΔ13CH3D = 278–490 °C). Hydrothermal well fluids from the Krafla and Námafjall geothermal fields in Iceland yielded the lowest Δ13CH3D values, and thus the highest Δ13CH3D-based temperatures averaging 438-45+55 °C. Those samples also show the most pronounced departures in δDCH4 and δ13CCH4 values expected for isotopic equilibrium with respect to δDH2O and δ13CCO2. In contrast, CH4 samples from natural steam vents in other Iceland locations and in Nisyros have slightly higher Δ13CH3D values (with TΔ13CH3D=351-35+42 °C) and have δDCH4 and δ13CCH4 values that are consistent with those expected for isotopic equilibrium with both H2O and CO2. The short fluid residence times (1–50 years) in systems that are exploited for geothermal energy, such as Krafla and Námafjall, combined with the proximity of a hot magma chamber, favor the preservation of kinetic signals. The initial disequilibrium δDCH4 and δ13CCH4 values are consistent with a thermogenic origin from immature organics dissolved in hydrothermally heated groundwater, but an abiotic origin cannot be excluded. The high apparent Δ13CH3D-based temperatures at Krafla and Námafjall could therefore represent nonequilibrium signals associated with either pyrolysis or abiotic generation of CH4 in a superheated vapor or supercritical water phase (>374 °C), considered to exist in the roots of the system above the magmatic heat source. Isotopologue equilibration calculations demonstrate that under such conditions (e.g. > 400 °C) kinetic signals would be erased in days to months, implying rapid migration and quenching of CH4 into the overlying subcritical (<300 °C) hydrothermal reservoir fluids. In systems with longer fluid residence times such as Nisyros, equilibrium isotopologue distributions at temperatures of ~ 350 °C are consistent with long fluid residence times on the order of > 100 years. Our calculations further reveal that CO2–CH4 isotopic equilibration requires unreasonably long fluid residence times, suggesting that any apparent 13C equilibrium may be coincidental.

Volcanostratigraphic Approach and its Implication for Geothermal Evaluation in Talang Volcano West Sumatra

The desk studies are a fundamental preliminary step in any geologic investigation of volcanic regions. These studies are very important approaches used in the reconnaissance survey stage for the exploration of volcanic geothermal systems. Volcanostratigraphic study has been done in Talang volcano and nearby regions, where the two topographic maps on the scale of 1:100000 and 1:50000 were utilized by drawing the patterns of drainage, ridges and flows without intersecting these flows. The first scale employed to distinguish eruption product units based on their respective eruptive centers (Crown), as well as a map on 1:50000 scale to find detailed parasitic distributed unit (s) product such as (Hummock). The challenge based on this approach is to distinguish different volcanic products based on their origins without ground truthing. Thus, the correlation with existing published geological map was employed. One volcanic unit were identified (Talang Brigade) which consists of three eruption centers (The Talang Bawah, Talang Batino and Talang Jantan Crowns) and three Hummocks with different ages based on age of eruption. The volcanostatigraphic units, is bounded by a large Talang Brigade caldera with a diameter more than 2 km. Based on analyses of the dimension, maturity, stress regimes, and an estimation of the thermal resource base of its magmatic heat source, it was concluded that Talang volcano as a geothermal potential that deserves further detailed studies before embarking to development phase. For verifying desk work studies, the detailed volcanostratigraphic mapping is recommended to understand the physical- and chemical properties of the volcanic rocks and provide the history of volcanism and validation of the potentially geothermal boundaries.

Interplay of magmatic and diapiric environments in the Djebel El Hamra Pb-Zn-Hg ore district, northern Tunisia

The Djebel El Hamra Pb-Zn-Ba-Sr (Hg) deposits in northern Tunisia are hosted in a post-nappe anticline with a core of a Triassic evaporite diapir affected by the NE–SW-trending Ghardimaou-Cap Serrat lineament. Three stages of mineralization occurred in the Triassic dolostone: stages I and II caused alternating deposition of sulfate (Ba, Sr) and sulfide (sphalerite, galena) minerals; stage III formed late-stage calcite-marcasite-cinnabar. Zebra textures record the syntectonic transition from compression to extension in the Late Tortonian-Messinian interval. Two fluid end-members were involved in sulfate deposition: one low-salinity (L1, ~3 wt% eq. NaCl) fluid, probably from a meteoric origin, and a Na-Ca-Cl brine (L2, ~22 wt% eq. NaCl) solution which originated from the Triassic diapiric source. A third end-member fluid (L3) with long residence time in the basement was also involved in the Pb-Zn deposition. The δ34S values from a cluster of sulfates around +16‰, show a Triassic evaporate source. The sulfur in sphalerite resulted from bacterial sulfate reduction (BSR); however, crystallization in a closed system resulted in a range of δ34S between +1.6 and +26.5‰. The δ34S values in galena (−28.4 to +8.2‰) are consistent with a BSR and thermochemical sulfate reduction (TSR) origin of the sulfur. Secondary ion mass spectrometry (SIMS) lead isotope data in galena (207Pb/204Pb: 15.595 to 16.193, 206Pb/204Pb: 18.673 to 18.939, 208Pb/204Pb: 38.330 to 40.572) point to local contributions (sedimentary and Cenozoic magmatic rocks) to the main source from the Precambrian basement. Ore deposition occurred at a depth of about 2 km at temperatures between 80 and 250 °C. A shallow magmatic heat source was the cause of these thermal fluctuations.

The Alpehue geyser field, Sollipulli Volcano, Chile

Natural geysers on Earth are uncommon and most of them are clustered in large basins that share common characteristics, specifically, a magmatic heat source, meteoric water supply, and a set of fractures and/or porous rocks that allow circulation, accumulation and periodic release of fluids. Most active geysers and sinter deposits in the Andes occur in the El Tatio geyser field (~10 km2) in the Chilean Altiplano. Few of these features are reported in the southern volcanic zone. In this study, we characterized a small geyser field (with at least 4 geysers in ~0.2 km2) called Alpehue located in southern Chile. This field is found in a steep canyon on a flank of Sollipulli Volcano. Besides geysers, the Alpehue field hosts several types of surface hydrothermal manifestations including perpetual spouters, sub-boiling hot springs, mud pools and sinter deposits. The different features occur in distinct geological units and are distributed at different topographic elevations. Geysers are restricted exclusively to a single lithological unit of highly hydrothermally altered breccia. The geochemical and δD-δ18O isotopic compositions of thermal waters sampled from the geysers and perpetual spouters share a dominantly meteoric signature, and fluid-rock equilibration was inferred to have occurred in a deep reservoir >200 °C. This results in chloride-alkaline high silica waters that precipitate silica sinter at the surface. Radiocarbon ages of organic material contained within the sinter are up to at least 7.37 ± 0.02 ka CalBP. Micro-terraces (<3 cm) are the dominant texture of the sinter deposits with features that are controlled by the high topographic gradient. Our observations at Alpehue suggest that the high incision of the river enables the discharge of boiling water (rich in silica) at the surface. Precipitation of sinter at the surface, coupled with the high permeability of the breccia, creates a configuration that enables geysers to form and sustain episodic activity.

Sources and transport of fluid and heat at the newly-developed Theistareykir Geothermal Field, Iceland

Successful management of geothermal energy requires detailed understanding of physical and chemical conditions within the field prior to exploitation. It is thus crucial to identify fluids involved and their residence times, as well as the heat source, so as to assess the potential of the resource in terms of energy production. To this end, a geochemical study of relatively undisturbed fluids from the newly-developed Theistareykir geothermal field, Northern Volcanic Zone, Iceland was carried out on production wells, fumaroles, and mud pots. Noble gas (He, Ne, Ar, Kr, and Xe) elemental and isotopic abundances and stable isotopes (δ18O and δ2H) were measured to determine the system fluid sources and dynamics as exploitation proceeds. Results of this study, together with previously published data, show that four fluid sources are present: modern and local meteoric water (48.9%); sub-modern meteoric water from regional highlands precipitation (10.6%); pre-Holocene glaciated meteoric water (40.4%) with strongly depleted δ2H values of −127‰, calculated 40K-40Ar* fluid residence times from 57 ± 20 ka to 92 ± 30 ka and a (U/Th)-4He fluid residence times from 96 ± 50 ka to 160 ± 80 ka; and, finally, 3He-rich magmatic fluids. Concomitant enrichment in 18O and radiogenic 4He suggests that some fluids reside a long time in the reservoir, exchanging O and He with reservoir rocks. Maximum estimated helium isotopic ratios, 3He/4He (R), of 11.45 Ra (Ra = atmospheric ratio) show that the magma beneath Theistareykir is a depleted mid-ocean ridge basalt (MORB) mantle (DMM), with less influence (8.7 to 12.7%) of the Icelandic mantle plume source. Calculated heat (Q)/3He ratios plotted vs. R/Ra and 4He/36Ar ratios suggest that convective heat transport dominates the eastern part of the field where the magmatic heat source is located, while in other parts of the field, heat conduction seems to be dominant. Boiling and phase separation exists in the field, as indicated by δ18O values which fall to the left of the Global Meteoric Water Line in a δ18O vs. δ2H plot, but Q/3He ratios indicate that boiling affects only 1–10% of the fluid reservoir. With this obtained knowledge, any subsequent changes in the field conditions during the exploitation phase of Theistareykir can be better understood, helping to sustainably manage the resource.

Radiocarbon Dating of Silica Sinter and Postglacial Hydrothermal Activity in the El Tatio Geyser Field

AbstractThe El Tatio geothermal field in the Chilean Altiplano contains hydrothermal silica sinter deposits overlaying glacial and volcanic units, providing an opportunity to constrain the timing of deglaciation and volcanic activity in an area with sparse absolute chronologies. We obtained 51 new radiocarbon ages and δ13C values on the organic material trapped in these sinter deposits. Based on the δ13C values, we exclude 29 samples for possible contamination with bacterial mats that incorporate old carbon. We infer that hydrothermal activity initiated ~27 ka ago and has been nearly continuous ever since. The ages of the oldest sinter deposits coincide with ages of moraines that stabilized after the most recent deglaciation. Whereas late Pleistocene sinters are broadly distributed in the field, Holocene deposits are found around active hydrothermal features. Although recent volcanism is absent in the vicinity of El Tatio, persistent hydrothermal discharge implies a long‐lived magmatic heat source.