Hydrothermal Eruptions Research Articles

Dynamics of shallow hydrothermal eruptions: new insights from Vulcano’s Breccia di Commenda eruption

Understanding the dynamics and effects of hydrothermal eruptions is crucial to the hazard assessment in both volcanic and geothermal areas. Eruptions from hydrothermal centres may occur associated with magmatic phases, but also as isolated events without magmatic input, with the most recent examples being those of Te Maari (Tongariro, New Zealand) in 2012 and Ontake (Japan) in 2014. The most recent caldera of the Island of Vulcano (southern Italy) hosts in its centre the La Fossa cone, active since 5.5 ka and now characterised by continuous fumarolic degassing. In historical times, La Fossa cone has experienced several hydrothermal eruptions, with the most violent event being the Breccia di Commenda eruption that occurred during the thirteenth century ad. Based on analysis of 170 stratigraphic logs, we show that the Breccia di Commenda eruption occurred in three main phases. After an opening, low-intensity ash emission phase (phase 1), the eruption energy climaxed during phase 2, when a series of violent explosions produced an asymmetric shower of ballistic blocks and the contemporaneous emplacement of highly dispersed, lithic-rich, blast-like pyroclastic density currents (PDCs). The tephra units emplaced during phase 2, ranging in volume from 0.2 to 2.7 × 105 m3, were covered in turn by thin ash fall deposits (phase 3). The dynamics of the most violent and intense stage of the eruption (phase 2) was investigated by numerical simulations. A three-dimensional numerical model was applied, describing the eruptive mixture as a Eulerian–Eulerian, two-phase, non-equilibrium gas-particle fluid (plus a one-way coupled Lagrangian ballistic block fraction). At the initial simulation time, a mass of about 109 kg, with initial overpressure above 10 MPa, and a temperature of 250 °C, was suddenly ejected from a 200-m-long, eastward inclined, NNE–SSW trending fissure. The mass release formed blast-like PDCs on both sides of the fissure and launched ballistic blocks eastwards. Field investigations and numerical simulations confirm that hydrothermal explosions at La Fossa cone include intense ballistic fallout of blocks, emission of PDCs potentially travelling beyond the La Fossa caldera and significant ash fallout. The hazard associated with both ballistic impact and PDC ingress, as associated with hydrothermal eruption, is significantly larger with respect to that associated with Vulcanian-type events of La Fossa.

Integrating airborne hyperspectral imagery and LiDAR for volcano mapping and monitoring through image classification

Optical and laser remote sensing provide resources for monitoring volcanic activity and surface hydrothermal alteration. In particular, multispectral and hyperspectral imaging can be used for detecting lithologies and mineral alterations on the surface of actively degassing volcanoes. This paper proposes a novel workflow to integrate existing optical and laser remote sensing data for geological mapping after the 2012 Te Maari eruptions (Tongariro Volcanic Complex, New Zealand). The image classification is based on layer-stacking of image features (optical and textural) generated from high-resolution airborne hyperspectral imagery, Light Detection and Ranging data (LiDAR) derived terrain models, and aerial photography. The images were classified using a Random Forest algorithm where input images were added from multiple sensors. Maximum image classification accuracy (overall accuracy = 85%) was achieved by adding textural information (e.g. mean, homogeneity and entropy) to the hyperspectral and LiDAR data. This workflow returned a total surface alteration area of ∼0.4 km2 at Te Maari, which was confirmed by field work, lab-spectroscopy and backscatter electron imaging. Hydrothermal alteration on volcanoes forms precipitation crusts on the surface that can mislead image classification. Therefore, we also applied spectral matching algorithms to discriminate between fresh, crust altered, and completely altered volcanic rocks. This workflow confidently recognized areas with only surface alteration, establishing a new tool for mapping structurally controlled hydrothermal alteration, evolving debris flow and hydrothermal eruption hazards. We show that data fusion of remotely sensed data can be automated to map volcanoes and significantly benefit the understanding of volcanic processes and their hazards.

InSAR analysis for detecting the route of hydrothermal fluid to the surface during the 2015 phreatic eruption of Hakone Volcano, Japan

Although the 2015 Hakone Volcano eruption was a small-scale phreatic eruption with a discharged mass of only about 100 tons, interferometric synthetic aperture radar successfully detected surface deformations related to the eruption. Inversion model of the underground hydrothermal system based on measured ground displacements by ALOS-2/PALSAR-2 images showed that a crack opened at an elevation of about 530–830 m, probably at the time of the eruption. A geomorphological analysis detected several old NW–SE trending fissures, and the open crack was located just beneath one of the fissures. Thus, the crack that opened during the 2015 eruption could have been a preexisting crack that formed during a more voluminous hydrothermal eruption. In addition, the inversion model implies that a sill deflation occurred at an elevation of about 225 m, probably at the time of the eruption. The deflation of sill-like body represents a preexisting hydrothermal reservoir at an elevation of 100–400 m, which intruded fluid in the open crack prior to eruption. The volume changes of the open crack and the sill were calculated to be 1.14 × 105 m3 (inflation) and 0.49 × 105 m3 (deflation), respectively. A very local swelling (about 200 m in diameter) was also detected at the eruption center 2 months before the eruption. The local swelling, whose rate in satellite line-of-sight was 0.7–0.9 cm/day during May 2015 and declined in June, had been monitored until the time of the eruption, when its uplift halted. This was modeled as a point pressure source at an elevation of about 900 m (at a depth of about 80–90 m from the ground surface) and is considered to be a minor hydrothermal reservoir just beneath the fumarolic field. Our analysis shows that the northernmost tip of the open crack reached within 200 m of the surface. Thus, it is reasonable to assume that the hydrothermal fluid in the open crack found a way to the surface and formed the eruption.Graphical abstractXXXX

A conceptual geochemical model of the geothermal system at Surprise Valley, CA

Characterizing the geothermal system at Surprise Valley (SV), northeastern California, is important for determining the sustainability of the energy resource, and mitigating hazards associated with hydrothermal eruptions that last occurred in 1951. Previous geochemical studies of the area attempted to reconcile different hot spring compositions on the western and eastern sides of the valley using scenarios of dilution, equilibration at low temperatures, surface evaporation, and differences in rock type along flow paths. These models were primarily supported using classical geothermometry methods, and generally assumed that fluids in the Lake City mud volcano area on the western side of the valley best reflect the composition of a deep geothermal fluid. In this contribution, we address controls on hot spring compositions using a different suite of geochemical tools, including optimized multicomponent geochemistry (GeoT) models, hot spring fluid major and trace element measurements, mineralogical observations, and stable isotope measurements of hot spring fluids and precipitated carbonates. We synthesize the results into a conceptual geochemical model of the Surprise Valley geothermal system, and show that high-temperature (quartz, Na/K, Na/K/Ca) classical geothermometers fail to predict maximum subsurface temperatures because fluids re-equilibrated at progressively lower temperatures during outflow, including in the Lake City area. We propose a model where hot spring fluids originate as a mixture between a deep thermal brine and modern meteoric fluids, with a seasonally variable mixing ratio. The deep brine has deuterium values at least 3 to 4‰ lighter than any known groundwater or high-elevation snow previously measured in and adjacent to SV, suggesting it was recharged during the Pleistocene when meteoric fluids had lower deuterium values. The deuterium values and compositional characteristics of the deep brine have only been identified in thermal springs and groundwater samples collected in proximity to structures that transmit thermal fluids, suggesting the brine may be thermal in nature. On the western side of the valley at the Lake City mud volcano, the deep brine-meteoric water mixture subsequently boils in the shallow subsurface, precipitates calcite, and re-equilibrates at about 130 °C. On the eastern side of the valley, meteoric fluid mixes to a greater extent with the deep brine, cools conductively without boiling, and the composition is modified as dissolved elements are sequestered by secondary minerals that form along the cooling and outflow path at temperatures <130 °C. Re-equilibration of geothermal fluids at lower temperatures during outflow explains why subsurface temperature estimates based on classical geothermometry methods are highly variable, and fail to agree with temperature estimates based on dissolved sulfate-oxygen isotopes and results of classical and multicomponent geothermometry applied to reconstructed deep well fluids. The proposed model is compatible with the idea suggested by others that thermal fluids on the western and eastern side of the valley have a common source, and supports the hypothesis that low temperature re-equilibration during west to east flow is the major control on hot spring fluid compositions, rather than dilution, evaporation, or differences in rock type.

Hazard assessment of volcanic ballistic impacts at Mt Chihshin, Tatun Volcano Group, northern Taiwan

This study investigates the hazard posed by Volcanic Ballistic Projectiles (VBPs) in the area surrounding Mt Chihshin, Tatun Volcano Group, northern Taiwan. Based on the volcano’s current evolutionary stage, we consider two types of volcanic activity during which VBPs can be generated, namely hydrothermal and vulcanian eruptions. Hydrothermal eruptions may occur after a sudden decompression of water in the hydrothermal system of the volcano, typically due to mass removal processes, while vulcanian eruptions are caused by solidified magma that plugs the eruptive vent and gets blasted when this caprock is no longer able to withstand the pressure in the volcanic conduit. Initial velocities of ejected VBPs were estimated for each type of activity based on physical models and inserted as initial conditions to the equations that describe their motion. A hydrothermal eruption is assumed to occur at the NW flank of Mt Chihshin near the Hsiaoyiokeng fumarole, which is a place prone to flank instability, while a vulcanian eruption is assumed to originate from a central vent at the peak of Mt Chihshin. Modeling results suggest that the radii of the areas impacted by VBPs vary between 0.1 and 1.1 km for a hydrothermal eruption, while they become 1.4–5.1 km for a vulcanian eruption. Within these areas, roads, hiking trails, and public buildings lie within the impact areas; therefore, VBPs may potentially cause damage, injury, and even casualties.

The hydrothermal evolution of the Kawerau geothermal system, New Zealand

Hydrothermal alteration zoning and processes provide insights into the evolution of heat source(s) and fluid compositions associated with geothermal systems. Traditional petrological techniques, combined with hydrothermal alteration studies, stable isotope analyses and geochronology can resolve the nature of the fluids involved in hydrothermal processes and their changes through time. We report here new findings along with previous unpublished works on alteration patterns, fluid inclusion measurements and stable isotope data to provide insights into the thermal and chemical evolution of the Kawerau geothermal system, New Zealand. These data indicate the presence of two hydrothermal events that can be coupled with chronological data. The earlier period of hydrothermal activity was initiated at ~400 ka, with the heat driving the hydrothermal system inferred to be from the magmatic system that gave rise to rhyolite lavas and sills of the Caxton Formation. Isotopic data fingerprint fluids attributed to this event as meteoric, indicating that the magma primarily served as a heat source driving fluid circulation, and was not releasing magmatic fluids in sufficient quantity to affect the rock mineralogy and thus inferred fluid compositions. The modern Kawerau system was initiated at ~16 ka with hydrothermal eruptions linked to shallow intrusion of magma at the onset of activity that gave rise to the Putauaki andesite cone. Likely associated with this later event was a pulse of magmatic CO2, resulting in large-scale deposition of hydrothermal calcite enriched in 18O. Meteoric water-dominated fluids subsequently overwhelmed the magmatic fluids associated with this 18O-rich signature, and both the fluid inclusion microthermometry and stable isotope data reflect a change to the present-day fluid chemistry of low salinity, meteoric-dominated waters.

Furthering the investigation of eruption styles through quantitative shape analyses of volcanic ash particles

Volcanic ash morphology has been quantitatively investigated for various aims such as studying the settling velocity of ash for modelling purposes and understanding the fragmentation processes at the origin of explosive eruptions. In an attempt to investigate the usefulness of ash morphometry for monitoring purposes, we analyzed the shape of volcanic ash particles through a combination of (1) traditional shape descriptors such as solidity, convexity, axial ratio and form factor and (2) fractal analysis using the Euclidean Distance transform (EDT) method. We compare ash samples from the hydrothermal eruptions of Iwodake (Japan) in 2013, Tangkuban Perahu (Indonesia) in 2013 and Marapi (Sumatra, Indonesia) in 2015, the dome explosions of Merapi (Java, Indonesia) in 2013, the Vulcanian eruptions of Merapi in 2010 and Tavurvur (Rabaul, Papaua New Guinea) in 2014, and the Plinian eruption of Kelud (Indonesia) in 2014. Particle size and shape measurements were acquired from a Particle Size Analyzer with a microscope camera attached to the instrument. Clear differences between dense/blocky particles from hydrothermal or dome explosions and vesicular particles produced by the fragmentation of gas-bearing molten magma are well highlighted by conventional shape descriptors and the fractal method. In addition, subtle differences between dense/blocky particles produced by hydrothermal explosions, dome explosions, or quench granulation during phreatomagmatic eruptions can be evidenced with the fractal method. The combination of shape descriptors and fractal analysis is therefore potentially able to distinguish between juvenile and non-juvenile magma, which is of importance for eruption monitoring.

Mechanisms of salinization in a middle Eocene lake in the Tanggu area of the Huanghua Depression

The Sha-3-5 Submember of the Shahejie Formation in the Tanggu area of the Huanghua Depression contains analcime-bearing dolomite, indicating salinization of an ancient lake during deposition. Solar evaporation and hydrothermal salinization have both been proposed as ways to generate these saline conditions. Based on a comprehensive analysis of core data, thin sections, spore and pollen data, elemental geochemistry, pyrolysis results, and vitrinite reflectance, we assessed the mechanisms driving salinization in light of provenance evolution, geothermal evolution, paleoclimate, mineral responses to evaporation, and the relationship between fluid temperature and salinity. The vertical profiles of ΣREE, Eu/*Eu, and Tmax exhibit little variation, and the maximum paleotemperatures attained by organic matter are comparable to present-day drill hole temperature. No major changes in provenance or paleotemperature and no petrological fabrics that would indicate a hydrothermal input are observed in the studied section. However, evidence for a semi-humid to semi-arid climate, primary evaporitic textures and structures, and a positive relationship between fluid temperature and salinity all support evaporation as the primary mechanism driving rising salinity. Solar evaporation, rather than hydrothermal eruption, appears to have been the primary factor driving lake salinization during middle Eocene deposition of the dolomitic Sha-3-5 Submember.

Key travertine tectofacies for neotectonics and palaeoseismicity reconstruction: effects of hydrothermal overpressured fluid injection

Travertine deposits have long been considered as powerful tools for investigating neotectonics and reconstructing palaeoseismic events. We document, for the first time, the effects of overpressured hydrothermal fluids injected within travertine deposits. We also describe tectofacies interpreted as a consequence of coseismic events. Calcite veins, banded or massive, associated with hydrofracture and fluid-escape features, promoted hydrothermal eruptions and lithoclast formation in latest Quaternary travertine exposed in two quarries near Rapolano Terme (Northern Apennines, Italy). The isotopic composition of the calcite veins confirms the hydrothermal origin of the parent fluids and their rapid ascent, as indicated by the estimated palaeo-temperatures (43–50°C). Integration of U–Th ages obtained for the calcite veins with palaeoseismic evidence from a local archaeological site built at the top of one of the analysed travertine deposits suggests that hydrofracture and fluid-escape structures were associated with five main seismic events that occurred from the latest Pleistocene to the fourth century AD. In sum, the travertine tectofacies have a key role in better constraining the seismotectonic setting of a region and thus offer a powerful tool for the evaluation of seismic hazard for areas characterized by low seismicity and travertine deposition.

Evolution of a small hydrothermal eruption episode through a mud pool of varying depth and rheology, White Island, NZ

White Island volcano, New Zealand was a host to multiple hydrothermal eruptive episodes within a mud-sulphur pool in 2013. Although hydrothermal activity is common at White Island, past events have largely gone undescribed in favour of the larger phreatomagmatic and magmatic eruptions. Here, we detail the first and longest hydrothermal episode of 2013, lasting from 15 January to 7 February using video and photo analysis from tour operators and staff responsible for monitoring the volcano. Differences in the dominant bubble burst style across this episode led to the classification of four distinct eruption regimes: (1) multiple irregular bursts on the pool surface, (2) larger distinct symmetric hemispheres with starbursts and/or followed by mud heaves, (3) no initial pool surface deformation but a vertical steam jet followed by a sometimes large directed mud heave and (4) no lake and continuous pulsating dry ash and block venting. The progression through these regimes is associated with a lowering lake level and a concomitantly increasing viscosity of the pool, which initially comprises a low viscosity muddy water, and partially evaporates to yield a shallow layer of high viscosity mud that ends with the complete drying up of the mud pool. Formation of primary mud hemispheres or gas jets is followed by heaves or secondary upheaval events. The heights of these heaves are used as a measure of explosivity. Heights increase from ∼8 m during regime 1 on 15 January to ∼102 m during regime 3 on 28 January. Venting of dry mud during regime 4 developed on 29 January before a regression back to regime 1 took place on 7 February as the pool re-established. Through observations of the shapes of ejected mud clots, we propose that the increasing explosivity of higher number regimes is primarily due to increasing slug bubble lengths teamed with increasing mud pool viscosity. We attribute a lesser control to the decreasing depth of the pool during its progressive desiccation, which may in turn influence the bubble burst depth. Occasionally, visible yellowing of the steam/gas plume led us to suggest that elemental sulphur may also be present in the conduit and may also play a role in regulating bubble release dynamics. Although, evidence for magmatic/phreatomagmatic eruptions was present during eruptions later in 2013, we found no evidence for juvenile magma in the January-February eruption episode described here. However, we concur with other investigators that magma was probably intruded to shallow levels and may have driven heat and gas flux. Our explanation for the correlation of pool depth, mud viscosity and eruption regime is based on a conceptual model in which a pool is perched above a two phase hydrothermal system and is sensitive to changes in the heat and gas flux from shallow magma. The variable release of gas and thermal perturbations in the course of the January-February eruptive episode impacted the pool level, the water to sediment ratio in the pool, and thus its viscosity, and in turn modulated the eruption regime. The varying degree of explosivity throughout this episode calls for a new consideration of pool properties in assessing eruption hazards at this frequently visited volcano. We additionally emphasise that ballistic hazards from small eruptions exist coupled with a range of seismic signals and that the hazard was greatest during infrasound tremor.

Risk of hydrothermal eruption in the course of development of high-grade geothermal groundwater reservoirs

Risk of hydrothermal eruption in the course of development of high-grade geothermal groundwater reservoirs

Archive of bacterial community in anhydrite crystals from a deep-sea basin provides evidence of past oil-spilling in a benthic environment in the Red Sea

Abstract. In deep-sea sediment, the microbes present in anhydrite crystals are potential markers of the past environment. In the Atlantis II Deep, anhydrite veins were produced by mild mixture of calcium-rich hydrothermal solutions and sulfate in the bottom water, which had probably preserved microbial inhabitants in the past seafloor of the Red Sea. In this study, this hypothesis was tested by analyzing the metagenome of an anhydrite crystal sample from the Atlantis II Deep. The estimated age of the anhydrite layer was between 750 and 770 years, which might span the event of hydrothermal eruption into the benthic floor. The 16S/18S rRNA genes in the metagenome were assigned to bacteria, archaea, fungi and even invertebrate species. The dominant species in the crystals was an oil-degrading Alcanivorax borkumensis bacterium, which was not detected in the adjacent sediment layer. Fluorescence microscopy using 16S rRNA and marker gene probes revealed intact cells of the Alcanivorax bacterium in the crystals. A draft genome of A. borkumensis was binned from the metagenome. It contained all functional genes for alkane utilization and the reduction of nitrogen oxides. Moreover, the metagenomes of the anhydrites and control sediment contained aromatic degradation pathways, which were mostly derived from Ochrobactrum sp. Altogether, these results indicate an oxic, oil-spilling benthic environment in the Atlantis II basin of the Red Sea in approximately the 14th century. The original microbial inhabitants probably underwent a dramatic selection process via drastic environmental changes following the formation of an overlying anoxic brine pool in the basin due to hydrothermal activities.

40Ar/39Ar dating and thermal modeling of adularia to constrain the timing of hydrothermal activity in magmatic settings

Abstract Hydrothermal systems develop via interspersed thermal events over tens to hundreds of thousands of years. The timing of how these systems evolve is commonly established via application of geochronology to a variety of phases and/or the indirect correlation of dated stratigraphy. Here we report 40Ar/39Ar results from adularia extracted from a single mineralized fracture in the late Quaternary Tauhara geothermal system of New Zealand. By utilizing both the age and Ar diffusion properties, we demonstrate how adularia can provide reliable temporal and thermal constraints on the evolution of geologically youthful and active geothermal systems. Our results indicate that adularia formation occurred after 30 ka (mean age 15 ± 10 ka), possibly resulting from subsurface fracturing induced by a 25.4 ka hydrothermal eruption. Simulation of transient heat effects upon Ar retention in adularia with respect to the thermal history of the Tauhara geothermal system confirm that this age is consistent with the time of adularia crystallization. Overall, 40Ar/39Ar dating on geologically young hydrothermal adularia with respect to its thermal history may be used to assess the timing and potential events (e.g., eruptions and fracturing) related to hydrothermal system evolution.

Experimental estimates of the energy budget of hydrothermal eruptions; application to 2012 Upper Te Maari eruption, New Zealand

Sudden hydrothermal eruptions occur in many volcanic settings and may include high-energy explosive phases. Ballistics launched by such events, together with ash plumes and pyroclastic density currents, generate deadly proximal hazards. The violence of hydrothermal eruptions (or explosive power) depends on the energy available within the driving-fluids (gas or liquid), which also influences the explosive mechanisms, volumes, durations, and products of these eruptions. Experimental studies in addition to analytical modeling were used here to elucidate the fragmentation mechanism and aspects of energy balance within hydrothermal eruptions. We present results from a detailed study of recent event that occurred on the 6th of August 2012 at Upper Te Maari within the Tongariro volcanic complex (New Zealand). The eruption was triggered by a landslide from this area, which set off a rapid stepwise decompression of the hydrothermal system. Explosive blasts were directed both westward and eastward of the collapsed area, with a vertical ash plume sourced from an adjacent existing crater. All explosions ejected blocks on ballistic trajectories, hundreds of which impacted New Zealand's most popular hiking trail and a mountain lodge, 1.4 km from the explosion locus. We have employed rocks representative of the eruption source area to perform rapid decompression experiments under controlled laboratory conditions that mimic hydrothermal explosions under controlled laboratory conditions. An experimental apparatus for 34 by 70 mm cylindrical samples was built to reduce the influence of large lithic enclaves (up to 30 mm in diameter) within the rock. The experiments were conducted in a temperature range of 250 °C–300 °C and applied pressure between 4 MPa and 6.5 MPa, which span the range of expected conditions below the Te Maari crater. Within this range we tested rapid decompression of pre-saturated samples from both liquid-dominated conditions and the vapor-dominated field. Further, we tested dry samples at the same pressure and temperature conditions. Results showed that host rock lithology and state of the interstitial fluid was a major influence on the fragmentation and ejection processes, as well as the energy partitioning. Clasts were ejected with velocities of up to 160 m/s as recorded by high-speed camera. In addition to rare large clasts (analogous to ballistics), a large amount of fine and very fine (<63 μm) ash was produced in all experiments. The efficiency of transformation of the total explosive energy into fragmentation energy was estimated between 10 to 15%, depending on the host rock lithology, while less than 0.1% of this was converted into kinetic energy. Our results suggest that liquid-to-vapor (flashing) expansion provides an order of magnitude higher energy release than steam expansion, which best explains the dynamics of the westward (and most energetic) directed blast at Te Maari. Considering the steam flashing as the primary energy source, the experiments suggested that a minimum explosive energy of 7×1010 to 2×1012 J was involved in the Te Maari blast.Experimental studies under controlled conditions, compared closely to a field example are thus highly useful in providing new insights into the energy release and hazards associated with eruptions in hydrothermal areas.

Discovery, Geology, and Origin of the Fruta del Norte Epithermal Gold-Silver Deposit, Southeastern Ecuador

Fruta del Norte is a completely concealed and extraordinarily well preserved epithermal gold-silver deposit located in the remote Cordillera del Condor mountain ranges of southeastern Ecuador. Currently defined resources in a single, steep, relatively small body, displaying exceptional grade continuity, contain 9.81 million ounces (Moz) of gold and 15.0 Moz of silver, with an indicated resource grade of 9.59 g/t Au and 12.9 g/t Ag. The deposit was discovered in early 2006 during a greenfields program conducted by Aurelian Resources, a Canadian junior explorer. Discovery resulted from systematic drill testing of a conceptual geologic model, which predicted that auriferous veins would be present in andesitic volcanic rocks inferred to underlie a steep silicified rib cutting a fluvial conglomerate sequence. The rib is highly anomalous in arsenic, antimony, and mercury but contains low-order gold values. The second drill test—the discovery hole—intersected >230 m of ore-grade gold-silver mineralization beneath ~200 m of the barren conglomerate cover. The host andesitic volcanic rocks, crosscutting feldspar porphyry, and associated phreatic breccias are assigned to the Piuntza unit of the Santiago Formation, which, along with the Zamora batholith and a series of porphyry copper stocks, was generated in a continental margin magmatic arc during the Middle to Late Jurassic. The deposit is located near the northeastern extremity of the ~10-km-long, Suarez pull-apart basin where it is linked to the steep West and Central faults, part of the regionally extensive Las Penas strike-slip fault zone. The pull-apart basin was progressively filled by fluviatile conglomerate, dacitic ignimbrite flows, finer grained siliciclastic sedimentary rocks, and, finally, andesite flows, all assigned to the Chapiza Formation. The Fruta del Norte deposit consists of a 1.3-km-long and up to >300-m-wide vein-stockwork associated with quartz-illite-pyrite alteration. The deposit comprises two principal vein types, one in the south dominated by quartz, manganoan carbonates, and abundant base metal sulfides and the other in the north dominated by manganese- and base metal-poor quartz and calcite. Adularia is a minor main-stage gangue mineral in each. Both types are abruptly transitional upward and westward to a third ore type marked by intense silicification and chalcedony veining, with disseminated and veinlet marcasite (±pyrite). The uppermost part of this silicic zone is sulfide deficient, probably the result of a short-lived supergene oxidation event prior to initial conglomerate deposition. The deposit is notable for the widespread occurrence of fine to coarse visible gold, which gives rise to bonanza grades and is closely associated with quartz, chalcedony, carbonate, and sulfide gangue. The sulfide-free, silicic zone is overlain by an extensive silica sinter horizon, which may either directly overlie the Piuntza volcanic rocks and/or occur as interbeds in the basal 20 m of the conglomerate, which are invariably silicified and marcasite bearing. Otherwise, the conglomerate above the orebody lacks silicification, with one critical exception: the steeply inclined zone that is exposed as the silicified rib that led to discovery. The sinter horizon, containing localized mud-pool deposits and hydrothermal eruption breccias, is in unusual proximity to the underlying gold-silver orebody. The northern and southern parts of the Fruta del Norte deposit possess characteristics that are usually considered typical of low and intermediate sulfidation epithermal deposits, respectively; they may have required two discrete mineralizing fluids, both of which are suspected to have ascended via the West and Central faults. The upward change to the silicified zone is attributed to cooling of the ascendant fluid on approach to the paleosurface sinter. Progressive burial of the deposit beneath the Suarez basin fill may have contributed to the suppression and eventual extinction of the epithermal system, with gold mineralization of the conglomerate being inhibited by rheologic and chemical factors. Formation of the Fruta del Norte deposit and complete filling of the Suarez basin took place in >3 to 6 m.y. during the Late Jurassic ( 157–154 Ma).

Mineralogical study on volcanic ash of the eruption on September 27, 2014 at Ontake volcano, central Japan: correlation with porphyry copper systems

The volcanic ash of the eruption on September 27, 2014 at Ontake volcano consists mostly of altered rock fragments. The ash contains partly altered volcanic rock fragments consisting of primary igneous minerals (plagioclase, orthopyroxene, titanomagnetite, and feldspars) and volcanic glass accompanied by alteration minerals to some extents, and contains no juvenile fragments. These features indicate that the eruption was a non-juvenile hydrothermal eruption that was derived from the hydrothermal system developed under the crater. The major minerals derived from hydrothermal alteration zones are silica mineral, kaolin-group mineral, smectite, pyrophyllite, muscovite, alunite, anhydrite, gypsum, pyrite, K-feldspar, albite, and rutile. Minor chlorite, biotite, and garnet are accompanied. Five types of alteration mineral associations are identified from observations on individual ash particles: silica–pyrite, silica–pyrite ± alunite ± kaolin, silica–pyrophyllite–pyrite, silica–muscovite ± chlorite, and silica–K-feldspar ± albite ± garnet ± biotite. The associations indicate development of advanced argillic, sericite, and potassic alteration zones under the crater. Occurrence of anhydrite veinlet and the set of alteration zones indicate hydrothermal alteration zones similar to late-stage porphyry copper systems. Comparing the mineral associations with the geologic model of the late-stage porphyry copper systems, the source depths of mineral associations are estimated to range from near surface to >2 km. The depths of advanced argillic alteration, sericite, and potassic zones are 0 to ~2, ~1.5 to ~2, and >2 km, respectively.

Hydrothermal eruption and porphyry cupper system

Hydrothermal eruption and porphyry cupper system

Dissolved gases in hydrothermal (phreatic) and geyser eruptions at Yellowstone National Park, USA

Multiphase and multicomponent fluid flow in the shallow continental crust plays a significant role in a variety of processes over a broad range of temperatures and pressures. The presence of dissolved gases in aqueous fluids reduces the liquid stability field toward lower temperatures and enhances the explosivity potential with respect to pure water. Therefore, in areas where magma is actively degassing into a hydrothermal system, gas-rich aqueous fluids can exert a major control on geothermal energy production, can be propellants in hazardous hydrothermal (phreatic) eruptions, and can modulate the dynamics of geyser eruptions. We collected pressurized samples of thermal water that preserved dissolved gases in conjunction with precise temperature measurements with depth in research well Y-7 (maximum depth of 70.1 m; casing to 31 m) and five thermal pools (maximum depth of 11.3 m) in the Upper Geyser Basin of Yellowstone National Park, USA. Based on the dissolved gas concentrations, we demonstrate that CO 2 mainly derived from magma and N 2 from air-saturated meteoric water reduce the near-surface saturation temperature, consistent with some previous observations in geyser conduits. Thermodynamic calculations suggest that the dissolved CO 2 and N 2 modulate the dynamics of geyser eruptions and are likely triggers of hydrothermal eruptions when recharged into shallow reservoirs at high concentrations. Therefore, monitoring changes in gas emission rate and composition in areas with neutral and alkaline chlorine thermal features could provide important information on the natural resources (geysers) and hazards (eruptions) in these areas.

Undeveloped high-enthalpy geothermal fields of the Taupo Volcanic Zone, New Zealand

In the Taupo Volcanic Zone there are 21 high enthalpy (>225°C) geothermal fields with 7 utilised for power generation. This paper presents an overview of the undeveloped high enthalpy geothermal systems of the region; namely, Rotoma—Tikorangi, Tikitere, Taheke, Rotomahana—Waimangu, Waiotapu (including Waikite), Reporoa, Te Kopia, Orakei Korako, Mangakino, Horomatangi, Tokaanu—Waihi and Tongariro. Most of these geothermal systems are hosted in rocks of rhyolitic composition, although Tokaanu—Waihi and Tongariro are hosted in andesitic rocks. Some of the geothermal systems have extensive surface thermal manifestations (e.g. Waiotapu, 17km2), whereas others have few (e.g. Rotoma—Tikorangi), largely reflecting limited hydrological connectivity to the surface (i.e. impermeable rocks, structures). Uniquely, Horomatangi is fully submerged beneath Lake Taupo. All have liquid-dominated reservoirs of neutral pH alkali chloride waters, except for Tongariro which is inferred to have a vapour-dominated reservoir.Volcanic and tectonic activity, both past and present has impacted the near surface hydrology of geothermal systems in the Taupo Volcanic Zone. For example, the 1886 volcanic eruption of Mount Tarawera resulted in a series of phreatomagmatic and hydrothermal eruptions through the Rotomahana geothermal system, and created new fluid flow pathways and established thermal features at Waimangu Valley. Similarly, tectonic activity with movement along the Paeroa Fault has disturbed the Te Kopia geothermal field by changing the hydrology and resulted in the juxtaposition of acid on neutral alteration minerals in the upthrown fault block. But despite these volcanic and fault interruptions, thermal activity has continued. Several systems show evidence of thermal decline (Mangakino, Orakei Korako, Te Kopia), and the occurrence of altered ground and relict sinter between Te Kopia and Orakei Korako suggest they may at one time have been been hydrologically connected in the near surface.

La Josefina Au–Ag deposit (Patagonia, Argentina): A Jurassic epithermal deposit formed in a hot spring environment

The La Josefina Jurassic epithermal Au–Ag deposit located in Patagonia, Argentina, developed in an extensional setting of a back-arc environment, associated with a widespread Middle–Late Jurassic calc-alkaline volcanism. Block faulting has juxtaposed shallow level features evidenced by hot spring manifestations, hydrothermal eruption breccias and Au-rich veins, which suggest that mineralization in these veins, could extend far below the depths already tested by core drilling. Veins are filled by quartz, chalcedony, opal and minor adularia and barite with massive, comb, cockade, colloform–crustiform bandings and lattice-bladed textures. Ore minerals include electrum, Ag-rich sulfosalts (freibergite), pyrite, galena, sphalerite, chalcopyrite and specular hematite with minor arsenopyrite, marcasite, tetrahedrite and bornite. Four mineralizing stages have been identified, the first two (S1 and S2) are Au and Ag-rich, with temperatures ranging from 225 to 290° and salinities from up to 15wt.% in S1 decreasing to ~1wt.% NaCl in S2. The third stage (S3) displays higher base metal contents at lower temperatures (~200°C). Finally, the last stage (S4) is barren with temperatures lower than 100°C. Veins are surrounded by a proximal alteration halo of quartz+pyrite±adularia±illite followed outwards by illite/smectite interstratified clays and smectites (with less chlorite) to a propylitic zone. Stable isotope values calculated for the fluids show a mostly meteoric origin for mineralization fluids. Such distinct features place the La Josefina deposits in a hot spring environment with evidences of being formed at a proximal position of the Jurassic paleosurface and paleowater level.