Hydrothermal Fluid Circulation Research Articles

Numerical Modeling of Hydrothermal System Circulation Beneath Asal Rift, Republic of Djibouti

Asal rift is an aerial rift segment resulting from the westward propagation of the Aden ridge into the Afar Depression. Geothermal manifestations such as hot springs and fumaroles, fault creep, conductivity anomaly, and high geothermal gradient were observed both at the surface and in the subsurface. Despite many scientific works conducted in Asal to understand the rifting mechanisms, the hydrothermal fluid circulation still needs to be evaluated since it is based on simplified conceptual models. To further contribute and progress toward a quantitative evaluation of fluid circulation, a 2D numerical model perpendicular to the rift axis was developed with the objective of better understanding the role of subsurface anisotropy in fluid flow and heat transfer in the Asal rift. Numerical modeling of multiphase flow and heat transfer was carried out with an equivalent porous medium intersected by fault zones having greater permeability. Horizontal anisotropic permeability and magmatic fluid release were taken into account with different simulation scenarios. The results indicate that fault zones act as recharge/discharge areas depending on their location, permeability, and number. Simulations considering horizontal anisotropic permeability allowed the reproduction of the thermal state observed in geothermal wells with the expected general pattern of fluid circulation in the Asal rift. Comparing our result with a recent study made with a 2D numerical modeling parallel to the rift axis, we suggest the presence of a saddle point where fluid flow is both to the northeast and to the southwest direction of the rift. Moreover, magmatic fluid release assumed in two simulation scenarios showed to have an impact on the hydrological behavior of fault zones and facilitate the development of super-critical flow at the center of the rift.

Fluid-assisted metasomatic processes on planetary bodies: Evidence from vestan lithologies

The mechanisms responsible for the formation of Fe-rich, metasomatic features in eucrites and their debris in howardites are yet controversial. In this study, secondary phases found in polymict eucrite NWA (Northwest Africa) 11911, howardite NWA 1664, monomict eucrite NWA 8675 and unbrecciated basaltic eucrite NWA 13269 were investigated to elucidate their origin and history of formation. Among them, NWA 11911 is the only rock that displays widespread Fe-enrichment near fractures in unequilibrated and slightly equilibrated basaltic clasts and pyroxene fragments. Fayalitic olivine veinlets with various textures, thicknesses and compositions (Fa64-82) are detected in unequilibrated pyroxenes in NWA 11911, NWA 1664 and NWA 8675. Neighbouring Fe-enriched pyroxene is also Al-depleted compared to magmatic pyroxene (Al2O3 = ∼0.3 wt%). NWA 13269 contains Ca-rich plagioclase (An97-98) veinlets associated with Cr-spinel, but apparently no Fe-enrichment in pyroxene nor ferroan olivine despite a careful search. Olivine veinlets have been also detected in equilibrated pyroxene in NWA 11911. Our observations support the fluid-assisted metasomatism scenario that involves the circulation of hydrothermal fluids throughout the fractures of eucritic material.

Joint interpretation of gravity and airborne magnetic data along the Calama-Olacapato-Toro fault system (Central Puna, NW Argentina): Structural and geothermal significance

Here, we present new gravity and airborne magnetic geophysical data from a preliminary exploration along the western sector of the NW-striking Calama-Olacapato-Toro fault system. This area is located in the back-arc region of the Central Andes (NW, Argentina) and is considered as a high potential geothermal area within the Central Puna Energy Hub.The gravity and magnetic modelling was carried out with the aim of delineating geological structures that control the geothermal manifestations and estimate the geothermal field extent. Gravity data were separated into regional and residual components. Then, a 2-D gravity and magnetic modelling was carried out through the 2-D GM-SYS - Oasis Montaj software in order to build an inversion model that adjusts the residual gravimetric and magnetic anomalies observed on the surface. The resultant model was also fitted taking into consideration previous electrical resistivity studies, surface manifestations, geology, and geological structures. Bouguer anomalies are negative in the range of -411 to -304 mGal, while magnetic anomalies have a range of − 127 to 15 nT, in the central part of the study area. Hot springs correlate with low residual Bouguer and magnetic anomalies located on the Calama-Olacapato-Toro lineament, and are related to the alteration zone and the main damage zone. Gravity and magnetic inversion show low density and susceptibility contrast of rocks associated with volcano-sedimentary rocks filling a volcano-tectonic depression, limited by the Antuco and Pompeya N-S thrust faults, westward and eastward respectively. The low density and susceptibility contrast can be related to a combination of high temperatures and steam versus liquid-filled pores and fractures. Hence, this geological conceptual model based on the 2-D forward modelling of geophysical data, allows better knowledge and comprehension of hydrothermal fluids circulation in the middle of the Central Puna, in northwest Argentina.

Geochemistry and origin of listwaenites within the Northern Branch of Neo-Tethyan ophiolites, Turkey

The study presents geological field observations, petrographic characteristics and bulk rock geochemical data of listwaenitic occurrences related to Jurassic–Cretaceous IAESZ ophiolites of the northern part of Turkey. Listwaenites of the Jurassic–Cretaceous Eldivan ophiolite and the Cretaceous Dağküplü ophiolite occur as mineralised or non-mineralised blocks and/or veins associated with serpentinised peridotites and ophiolitic mélanges along major thrust zones within the Izmir–Ankara–Erzincan Suture Zone (IAESZ) of the Northern Branch of the Neo-Tethys, Turkey. The silica–carbonate listwaenites (Type I) consist mainly of dolomite and minor calcite with quartz and minor phyllosilicates (chlorite, mica), serpentine and hematite. They are formed by circulation of meteoric hydrothermal fluids, along major décollement faults and thrust faults during the emplacement of serpentinised peridotites. The Kaymaz listwaenites (Type II) are composed mainly of fine-grained microcrystalline quartz with negligible dolomite, calcite, magnesite, serpentine and Fe-hydroxides. They display hard resistant masses, and are adjacent to the Eocene granitic bodies. Listwaenites are gentically related to the serpentinised peridotites, while the mineralisation appears to be related to the granite-related hydrothermal fluids.

Ore forming and reworking processes in the Xitieshan Pb–Zn deposit, Qinghai Province, China: Constraints from in situ trace-element and S isotope compositions of sulfides

Although volcanogenic massive sulfide (VMS) deposits always form in extensional settings, many have been reworked by metamorphism, deformation, and the circulation of hydrothermal fluids during subsequent accretionary events. The genesis of VMS deposits has been well documented, however, the mechanisms that rework the mineralization have been relatively poorly constrained. An understanding of these mechanisms, though, is critical to the development of robust mineral deposit models as they are capable of remobilizing ore-forming metals. The Xitieshan Pb–Zn deposit, located in Qinghai Province, western China, is the third largest Pb–Zn deposit in China, with a total Pb–Zn metal reserve of 6.4 million tons. The occurrence of laminated and non-laminated orebodies in the Xitieshan deposit make it an ideal candidate to i) constrain the effects of metamorphic and hydrothermal processes on remobilization of metals in these base-metal systems, and ii) assess the genetic nature of this deposit. This is accomplished here by integrating new S isotope data and trace-element chemistry of sulfides from laminated ores with previous information on regional and deposit geology, and deformation characteristics preserved by the mineralized system. The S isotope composition of pyrite and sphalerite grains from laminated ores are consistently positive and range from 2.08 ‰ to 5.81 ‰, indicative of mixing of reduced sulfur from three distinct sources — volcanic rocks, seawater and thermochemical reduced sulfate, which is typical of VMS deposits globally. Greigite (Fe3S4) grains in laminated ores exhibit oscillatory zonation with respect to Cu, Co, Mn, Sb, and Tl concentrations; such compositional variations indicate that at least some of the primary VMS mineralization characteristics have been preserved despite subsequent reworking by metamorphism, deformation, and hydrothermal circulation. Combined with the results of previous studies, a two-stage genetic model is proposed for the formation and modification of the Xitieshan Pb–Zn deposit — i) initial formation of the primary Pb–Zn mineralization with minor of Cu, and ii) subsequent metamorphism and hydrothermal reworking of the deposit, which did not completely destroy the textural and chemical characteristics of the primary mineralization.

Post-obduction listwaenite genesis in the Oman Mountains inferred from structural analysis and U-Pb carbonate dating

Listwaenite, a distinctive rock formed by carbonation of peridotite, is important for understanding carbon fluxes and storage in the deep Earth. In northern Oman, this lithology occurs near/at the base of the Semail Ophiolite and has been proposed to have formed in the mantle wedge during Late Cretaceous obduction and ophiolite emplacement. Listwaenite occurs as tabular sheets associated with post-obductional extensional faults. Specifically, listwaenite formed in (1) extensional duplexes bound by shallowly-dipping normal faults, (2) moderately- to steeply-dipping extensional faults, and (3) layers that overlie rocks of the metamorphic sole and unmetamorphosed platform carbonates. Two dolomite veins cutting listwaenite yield near-identical LA-ICP-MS U-Pb ages of 60.3 ± 15.4 and 55.1 ± 4.7 Ma (2 standard error). Thus, listwaenite formed prior to or is coeval with the ∼60-55 Ma veins. One carbonate listwaenite sample yields a LA-ICP-MS U-Pb age of 64.31 ± 6.28 Ma. Further six listwaenite samples yield imprecise ages of ∼33-3 Ma. Thus, listwaenite is interpreted to have formed during at least two post-obductional deformational events in the Oman Mountains. Hydrothermal circulation of carbon-rich fluids along upper crustal extensional faults facilitated listwaenite formation. Our results indicate that listwaenite formed during post-obductional extension, challenging models of listwaenite genesis in the mantle wedge during obduction.

Fluid inclusions and C–H–O–S–Pb isotope systematics of the Senj Mo–Cu deposit, Alborz magmatic belt, northern Iran: implications for fluid evolution and regional mineralization

AbstractThe Middle Eocene Senj Mo–Cu deposit (1.3 Mt at 1.5 wt % Cu and 0.2 wt % Mo) is located in the central Alborz magmatic belt, northern Iran. Mineralization is characterized by multistage veins, which are hosted in volcanic and volcano-sedimentary strata of the Karaj Formation (c. 41–45 Ma), genetically associated with the Senj Mafic Sill (c. 40 Ma). Four generations of veins are documented based on mineral assemblages and cross-cutting relationships, sequentially: quartz–biotite–chalcopyrite (QBC veins), quartz–molybdenum (QM veins), quartz–pyrite (QP veins) and late calcite (LC veins). Three types of fluid inclusions (FIs) are present in multistage veins: liquid-rich two-phase (L-type), vapour-rich two-phase (V-type) and solid-bearing multi-phase (S-type) inclusions. FIs in the QBC and QM veins are predominantly V-type and S-type, together with minor L-type, whereas the QP and LC vein minerals only contain L-type fluid inclusions. The homogenization temperatures of FIs from QBC to LC veins vary from 320 to 425 °C, 308 to 390 °C, 214 to 288 °C and 145 to 243 °C, and their salinities range from 5.3 to 49.0, 6.2 to 39.7, 5.7 to 13.0, and 2.1 to 7.2 wt % NaCl equiv., respectively. The H–O–C isotope compositions favour a dominantly magmatic origin for the hydrothermal fluids, which were gradually diluted by meteoric waters. The S–Pb isotope data of sulphide minerals indicate that the sulphur and metals were sourced from a deep-seated magmatic reservoir. The Senj Mafic Sill and associated Mo–Cu mineralization was developed in a back-arc extension regime, which faults and crustal fracture systems served as conduits for hydrothermal fluid circulation and Mo-rich veining.

Geology and Controls on Gold Enrichment at the Horne 5 Deposit and Implications for the Architecture of the Gold-Rich Horne Volcanogenic Massive Sulfide Complex, Abitibi Greenstone Belt, Canada

Abstract The Archean Horne 5 deposit, located in the Rouyn-Noranda district in the southern Abitibi greenstone belt, Canada, contains a total resource of 172.4 t Au (5.6 Moz) from 112.7 Mt of ore grading at 1.53 g/t Au. The deposit is part of the Au-rich Horne volcanogenic massive sulfide (VMS) complex that also includes the past-producing Horne mine (i.e., the Upper and Lower H zones plus small subsidiary lenses) that yielded 325.4 t Au (10.5 Moz Au) from 53.7 Mt of ore grading at 6.06 g/t Au. Combined, the Horne mine and Horne 5 deposit contain ~500 t Au (16 Moz), making them the world’s single largest accumulation of VMS-related Au. The Horne 5 deposit consists of stacked lenses of massive to semimassive sulfides alternating with extensive zones of disseminated and stringer sulfides. The mineralization is hosted within thick accumulations of steeply dipping dacitic to rhyodacitic volcaniclastic units of transitional to calc-alkaline magmatic affinity. Dacitic-rhyodacitic synvolcanic units (lobes, sills, and/or domes) intrude the host succession, which is also crosscut by a series of post-ore mafic and younger intermediate to felsic feldspar ± quartz porphyry dikes. A broad and diffuse halo of distal sericite-chlorite-epidote alteration extends outboard of intensely sericite-altered zones proximal to the sulfide lenses. Gold is interpreted to be synvolcanic on the basis of Au-rich massive sulfide clasts in the volcaniclastic units, the presence of preserved Au-rich primary pyrite, Au zones limited to the sulfide envelope, crosscutting deformed but unaltered and barren dikes, and the absence of typical syndeformation, orogenic-style alteration and mineralization despite overprinting high-strain corridors and faults. Gold is spatially associated with pyrite, sphalerite, and chalcopyrite, and its distribution is largely controlled by the higher porosity and permeability of the volcaniclastic host rocks, which are interpreted to have facilitated hydrothermal fluid circulation in the subseafloor environment. Synvolcanic intrusions and fine-grained tuffs overlying auriferous zones also influenced the distribution of the mineralization by acting as cap rocks to ascending fluids. Evidence suggests that Au enrichment at the Horne 5 deposit is due to efficient transport and precipitation of Au in the subseafloor environment, a favorable geodynamic setting (transitional to calc-alkaline magmatism over thick crust), and possible input of magmatic fluids as suggested by high Te and Cu in the mineralization. Minor and very local remobilization of metals occurred in response to regional deformation and associated greenschist facies metamorphism. The detailed study of the Horne 5 deposit geology and a review of the available information on the Horne mine and recent 3-D modeling indicate that the Horne 5 deposit may have formed higher in the stratigraphy than the Upper and Lower H orebodies of the former Horne mine, which originally formed a single lens. Therefore, the Horne Au-rich VMS complex originally formed as a stacked system in which the Horne 5 deposit was deposited above the Upper and Lower H zones and not in a distal or lateral position as previously proposed, indicating that a robust hydrothermal system was responsible for the formation of the world’s largest Au-rich VMS complex.

Polymetallic mineralization prospectivity modelling using multi-geospatial data in logistic regression: The Diapiric Zone, Northeastern Algeria

Prospecting and exploring minerals present major challenges in tectonically complex regions for sustainable development as in Northeastern Algeria. This area is promising for its mineral potential, especially the metallogenic province ‘The Diapiric Zone’. This study concerns mapping and predicting potential polymetallic mineralization locations by integration of remote sensing, gravity, and magnetic datasets. Several enhancement and processing methods have been applied on Landsat8_OLI and ASTER_1T remote sensed data to reduce uncertainty for achieving the best detection of hydrothermal alteration zones and lithological mapping. Furthermore, the Centre for Exploration Targeting grid analysis technique, the contact occurrence density and entropy orientation tools were employed on ground-gravity and aeromagnetic data to understand and visualize the pathways for hydrothermal fluids circulation of mineral deposits. The polymetallic mineralization prospective areas were produced using a logistic regression model on the resulting multifactor. High zones of lead-zinc cover most the area that has been confirmed by field investigation.

Crustal and uppermost mantle structure of Cape Verde from ambient noise tomography

SUMMARY We present a seismic ambient noise tomography of the Cape Verde archipelago, located in the Atlantic Ocean, approximately 600 km west of Senegal. We used 38 seismic broad-band stations that continuously recorded for 10 months, in order to construct the first 3-D model of Sv-wave velocities for the crust and uppermost mantle beneath the Cape Verde region. We started by computing phase cross-correlations for vertical component recordings using all possible inter-island station pairs. Next, a time–frequency phase-weighted stack was applied to obtain robust Rayleigh-wave group-velocity dispersion curves in the period band between 10 and 24 s. Group-velocity maps at different periods are obtained by inverting the dispersion curves. We then inverted the group-velocity maps to obtain the 3-D shear wave velocity structure of the crust and uppermost mantle beneath Cape Verde. The final 3-D model extends from 8 km down to 23 km and has a lateral resolution of about 50 km. The crust in the southwestern sector, encompassing Fogo, presents lower S-wave velocities that may be caused by the presence of melt pockets and/or hydrothermal fluids circulation. The uppermost mantle beneath the northwestern sector is characterized by higher S-wave velocities in agreement with previous results obtained from Ps and Sp receiver functions. Those high-velocity anomalies can reflect non-altered crust or remnants of magma chambers or solidified basaltic intrusions, which fed the volcanism in these islands. Our maps revealed the presence of crustal underplating across the entire archipelago, yet stronger beneath the groups Santo Antão—São Vicente—São Nicolau and Fogo—Santiago—Maio.

Near-bottom survey of hydrothermal sulfide on an extremely magmatic accretion segment along an ultraslow-spreading ridge

An extremely magmatic accretion could exist along an ultraslow-spreading ridge that has been approved and surveyed, but less attention has been paid to its associated hydrothermal system, as it is supposed to be similar to those of fast/medium-spreading ridges. Here using an autonomous underwater vehicle and a human-occupied vehicle, a systematic investigation was conducted on the hydrothermal system of an extremely magmatic accretion segment along the Southwest Indian Ridge (50°28′ E). A collapsed chimney, microbial mat, and diffused fluid were observed in the old Duanqiao hydrothermal field (DQF), which was earlier a weakly active hydrothermal area. Notably, evident hydrothermal anomalies, including oxidation–reduction potential (ORP), turbidity, and methane, were detected above the new DQF, which lies southwest of the old DQF. Based on this investigation, we concluded that ORP is one of the most effective parameters available to identify vent location by analyzing each type of detected hydrothermal anomaly and the near-bottom magnetic inversion can image the suspected hydrothermal fluid circulation pathways. In general, we believe that the new DQF is controlled by a magmatic chamber different from those found on fast/medium-spreading ridges, and could be a large hydrothermal system.

Constraining the spectral behavior of the clay-bearing outcrops in Oxia Planum, the landing site for ExoMars “Rosalind Franklin” rover

Oxia Planum (335.5°E, 18.2°N) is selected as the landing site for ExoMars rover mission (ESA/Roscosmos), where the “Rosalind Franklin” rover is scheduled to land in the decade. The region reveals several extensive clay-bearing outcrops recently exhumed, where biosignatures are possibly preserved. The objectives of the mission are to search for organics and investigate traces of past or extant life on Mars. Preliminary surveys of these outcrops show infrared absorptions typical of Fe,Mg-rich clays in the 1.0–2.6 μm range (1.4, 1.9, 2.3 and 2.4 μm) and an additional absorption at 2.5 μm implying a possible mixture with other mineral phase(s). Here we provide a detailed description of absorptions of the clay-rich materials detected in Oxia Planum, and map their strength and distribution throughout the region using hyperspectral data gathered by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard NASA's Mars Reconnaissance Orbiter (MRO) mission. Our analysis suggests that the Fe,Mg-rich clays identified in Oxia Planum mainly correspond to either Fe-bearing saponites (e.g., Griffithite) or vermiculite ores (i.e., vermiculite associated with a hydrobiotite component). Conversely, large clay-bearing outcrops found in the catchment area (337°E, 16.7°N) are rather consistent with nontronites in association with Al-rich clays and kaolins, in agreement with previous identification in the Mawrth Vallis – west Arabia Terra province. Presence of Fe,Ca-rich carbonates is recognized with the absorption near 2.53 μm and the observation of a broad peak in the 3–4 μm range, supporting their co-occurrence with the clays in Oxia Planum and its catchment area. Although we favor a pedogenesis alteration for the clays found in the catchment area, the origin of those studied in Oxia's basin remains enigmatic, where alternative scenarios could be either lacustrine and deltaic sedimentation, groundwater circulation, or even hydrothermal fluid circulation. Future in-situ measurements by “Rosalind Franklin” rover will indubitably provide new insights on the mineralogical diversity seen in the region and their origins.

Grain-size-reducing- and mass-gaining processes in different hydrothermal fault rocks

AbstractInteractions of hydrothermal and deformation processes in active fault zones are important in light of seismic and geothermal activities. This study investigates different tectonites of the Grimselpass breccia fault, a hydrothermally active strike-slip structure in the Central Alps (Switzerland). We combine microstructural and geochemical investigations to decipher the interaction of grain-size reduction and associated dilation (volume gain) during frictional deformation with geochemical processes such as mass gain via precipitation, grain growth and healing. Three types of tectonites were investigated in the fault zone: (1) cockade-bearing tectonites, (2) cataclasites and (3) fault gouges. They differ in terms of microstructure of clasts, mass and volume gain/loss, as well as type and degree of cementation. Clast size distributions show decreasing median values from low-strain cataclasites to high-strain cataclasites. Mass/volume gains are up to 20 %, with samples of smallest clast sizes often showing the highest mass gain. In the case of cockade-bearing tectonites large volume and mass gain occur. In particular, the precipitation of SiO2-rich cements is inferred to (i) reduce permeability and (ii) induce a gain in strength of the respective fault rocks. Location, timing and degree of healing directly control the fault architecture by deactivating fault strands from the circulation of hydrothermal fluids. In this case, new rupturing events are necessary to reactivate locked fault strands. Such feedback cycles of episodic deformation, mass/volume and clogging preserved hydrothermal circulation and deformation over the last 3.4 Ma, representing a process chain for the long-term preservation of orogenic hydrothermal systems.

Surface hydrothermal activity controlled by the active structural system in the self-sealing geothermal field of Acoculco (Mexico)

The active volcanic caldera of Acoculco has been explored in the last few years to develop geothermal energy from non-conventional sources. This caldera located in eastern Mexico has been classified as a Hot Dry Rock geothermal system. Our results bring a new understanding to the interaction between two intra-caldera fault systems in terms of their permeability and degassing processes through self-potential, temperature, and CO2 flux surveys. We demonstrate the absence of hydrothermal fluid circulation with convective activity, and we provide new evidence for the origin of cold degassing. These outcomes may have important implications for future exploitation targets in the area.

Fluid geochemistry and geothermal anomaly along the Yushu-Ganzi-Xianshuihe fault system, eastern Tibetan Plateau: Implications for regional seismic activity

Hydrothermal activity and seismicity are abundant along the Yushu-Ganzi-Xianshuihe fault system (YGXFS) in the eastern Tibetan Plateau. However, the processes and factors that control hydrothermal fluid circulation at a regional scale remain less constrained, and the relationship between geothermal anomaly and seismic activity is poorly known. In this study, we report an integrated dataset of thermal water chemistry (δ18OH2O, δDH2O, and δ13C of dissolved inorganic carbon (DIC)) to identify the origin, circulation path, and reservoir properties of hydrothermal fluids and then reveal the relationship between fluid circulation and seismic activity. The thermal waters are mainly classified as Na-HCO3 type with minor Ca-Na-HCO3 and Na-HCO3-Cl types. The δ18OH2O (−18.7‰ to −14.5‰) and δDH2O (−142‰ to −110‰) values indicate recharge of meteoric waters, which infiltrated along the YGXFS to 2–6 km depths and underwent mixing with deep fluids prior to their discharge at the surface. A mass balance model based on δ13CDIC values (−4.3‰ to 5.9‰) and DIC contents of thermal waters reveals the involvement of CO2-bearing fluids from mantle and thermo-metamorphic decarbonation of crustal marine limestones in tectonic CO2 outgassing from the active fault. The estimated reservoir temperatures (85–336 °C) based on silica geothermometer of the thermal waters decrease from southeast to northwest along the YGXFS. Notably, the epicenters of strong earthquakes (Ms ≥ 6) are mostly located in the transition zone of geothermal anomalies according to spatial distribution of the reservoir temperatures. Combined with geological and geophysical information, we suggest that the heterogeneity of crustal thermal structure and the upwelling of deep fluids may play an essential role in the distribution of thermal springs and the high frequency of earthquakes along the YGXFS.

Gamma-ray spectrometry of the Araguainha impact structure, Brazil: Additional insights into element mobilization due to hydrothermal alteration.

We present the analysis of airborne and ground gamma-ray spectrometry signatures of the Araguainha impact structure, located in central Brazil, the largest impact structure in South America with ~ 40 km diameter. The airborne data are total gamma-ray counts per second collected along flight lines spaced 1 km apart. The ground gamma-ray data are concentrations of potassium, uranium, and thorium isotopes calculated from radiations measured in three individual channels. The objectives are to distinguish lithologies within the structure, which have naturally distinctive radiogenic signatures, and identify potential post-impact hydrothermal alteration zones, as indicated by high K concentrations. Based on results obtained by numerical modeling of the crater formation, we infer the locations of potential occurrences of target rocks that may have undergone hydrothermal alteration as a result of the impact. The deviations from the background potassium concentration show significant anomalous K values at the center and in the northwestern crater rim, where high concentrations of U are also observed. The numerical model shows that ideal temperature conditions for hydrothermal fluid circulation were attained right after pos-impact gravitational stabilization.

Structural analysis and fluid geochemistry as tools to assess the potential of the Tocomar geothermal system, Central Puna (Argentina)

The Argentinean Andean region hosts a vast geothermal resource clustered by active magmatic and tectonic activity. One of the most studied geothermal areas is the Tuzgle-Tocomar geothermal system in Central Puna (NW Argentina). However, despite the existence of several studies since the 1970′s highlighting the geothermal potential of the area, only highly schematic and dissimilar conceptual models have been proposed for the Tocomar Geothermal System. This study presents new detailed geological-structural and hydrogeochemical data, together with in-situ permeability analysis of fault zones and Raman spectroscopic characterization of hydrothermal-alteration minerals. The electrical generating capacity has also been evaluated using the volumetric method and a stochastic approach. A new comprehensive conceptual model of the studied area was constructed highlighting the role of the Calama-Olacapato-El Toro (COT) fault-system in the circulation of hydrothermal fluids. The reservoir of the Tocomar geothermal system has a Na+-Clˉ(HCO3)ˉ composition and an estimated temperature of ∼235 °C. Such a reservoir is hosted in fractured Ordovician rocks and controlled by the COT-like Chorrillos transpressive fault at 1000–1500 m depth b.g.l. The water isotopic data and hydrological features indicate a regional recharge beyond the Tocomar sub-basin boundaries (>5000 m a.s.l.). Additionally, the main hydrothermal reservoir receives inputs of magmatic fluids from the degassing of the intra-crustal rhyolitic magma chamber of the Tocomar volcanic center. The Monte Carlo simulations suggest that the Tocomar geothermal system has a probable power production capacity above 1.23 MWe (P90), 6.18 MWe (P50) and 11.67 MWe (P10) at different confidence levels. All calculations were biased towards minimum values, thus a tighter definition of the resource size and fracture porosity would significantly impact on the predictions. Notwithstanding, the strategic position of the Tocomar geothermal field encourages move forward towards more in-depth exploration phases.

SEM-EDS imaging of multi-stage cleat minerals formation in the South Walker Creek Coals, Bowen Basin

The South Walker Creek coals are moderately to highly developed fracture systems. These fractures can be recognized from the top to the bottom of the seam, with both open and closed apertures. They are filled with several different minerals, occurs either as single-phase infillings (monomineralic), or as mixed mineral deposits (polymineralic). This paper presents scanning electron microscope (SEM) analysis of mineralogical association within coals fractures or cleats, as well as their origin and phase of development. SEM-EDS analysis indicates that the polymineralic cleats seem to be more common in the coal seam compared to monomineralic cleats. The polymineralic cleat infillings are composed of clay minerals, carbonates, and other minerals, such rutile and apatite, with minor occurrences of diaspore. This polymineralic association within the single cleat spaces represents a multi-phase minerals formation, as a result of different stages of epigenetic activity. The hydrothermal fluid circulation are responsible for this epigenetic process, they are re-opened previous minerals filled cleats and transformed pre-existing mineral within the cleats, and/or to remobilization earlier minerals, which then precipitated in the cleat as epigenetic minerals.

Mineralizations related to the Tocota pluton. Southern Colangüil batholith, Cordillera Frontal, San Juan, Argentina

The southern portion of the Colangüil Batholith is formed by units of the Tocota Pluton, of Permian age, which intrude late Carboniferous-Permian (Asselian) sandstones. Sub-volcanic bodies with andesitic composition and tuffs of Cenozoic age, modern layers of piedmont sediments and both alluvial and glacier deposits also crop out. The region exhibits a set of metalliferous deposits (Cu-Au-As) and occasionally with bismuth. These are mostly filonian type. These are spatially associated with granodiorites, granite and sandstone. Their development is interpreted as belonging to the Gondwanan cycle.Within granitic rocks, mineralizations were formed between 139 ºC and about 400 °C, probably after the consolidation of the Tocota pluton and by the upward circulation of hydrothermal fluids, probably mixed with meteoric waters. In the host rock, mineralization occurred in an interval between 200 ºC and 350 °C, through hydrothermal activity detected in areas of phyllic-argillic alteration. The gangue constitutes mainly of quartz, tourmaline and and sometimes potassium feldspar and calcite. The mineralization pattern, in both the pluton and the sandstone, has been controlled by 1) a strong domain of ore filled faults and joints, with ENE and WNW as the more common strikes and NNW subordinated, 2) fault breccias and hydrothermal breccias, and 3) dykes of andesite composition with low metal content. The host rock mineralization is commonly emplaced in joints as well as finely disseminated, related to intense silicification. Such deposits contain hypogene minerals whose isotopic 34S/32S ratios are compatible with a granite-like filiation but other provenances cannot be ruled out.

Pyritic mineralization halo above the Tara Deep Zn-Pb deposit, Navan, Ireland: Evidence for sub-seafloor exhalative hydrothermal processes?

The Tara Deep Zn-Pb deposit, at Navan, Ireland, includes sub-economic pyrite-rich mineralization extending laterally for about 2 × 2 km within the overlying Lower Viséan calc-turbidites, known as the ‘New-Thin Bedded Unit’. Here, we investigate the genesis of this pyritic mineralization and its links to the limestone-hosted Zn-Pb deposit lying 100 m below it.Four mineral assemblages have been identified: 1) in black shales, laminated pyrite comprises thin framboid-rich layers with minor interstitial sphalerite, both showing variable but low δ34S values ranging from −37.4 to 3.3‰; 2) in calcarenite and calcsiltite layers, pyritized fossils are widely distributed and occur chiefly as biodebris replaced by low δ34S pyrite (mean values of −13.2‰); 3) a replacive assemblage occurs as late remobilizations exhibiting both crosscutting and bedding-parallel styles, overprinting the early laminated pyrite. It comprises mostly of marcasite, with minor pyrite, sphalerite, chalcopyrite, galena, stibnite and Co-pentlandite, with high δ34S values up to 24.5‰; and 4) in black shales, bedded sulfide-rich cherts comprising microcrystalline quartz lenses exhibit δ18O mean values of 25.3‰. Cherts are rimmed by dolomite, associated with marcasite, pyrite, sphalerite, chalcopyrite, galena, siegenite and stibnite also with high δ34S values up to 44.2‰. In general terms, pyrite shows a relatively high Co/Ni ratio > 1 and sphalerite Zn/Cd ratios vary from 268 to 364.Textural analysis indicates overlapping of early-diagenetic and multi-phase hydrothermal sulfide mineralization. Development of laminated pyrite and pyritized calcarenites suggests that this mineralization was generated during early diagenesis, close to the seawater-sediment interface in oxygen-poor conditions under the influence of low-temperature hydrothermal fluid circulation. Later hydrothermal cherts and replacive sulfides suggest discharge of relatively warm hydrothermal fluids during early to mid-diagenesis, presumed to be linked to movements of nearby normal faults. Similarities in mineralogy and S isotope compositions suggest genetic links between the sub-economic pyritic mineralization and the underlying Tara Deep deposit, and consequently, that the former represents a geochemical halo with direct applicability in exploration for Zn-Pb deposits.