Input Of Magmatic Fluids Research Articles

An approach combining chlorine isotopes and hydrochemistry for identification of magmatic fluid-affected hydrothermal systems

Identification of hydrothermal systems affected by magmatic fluid input is relevant for geothermal energy development. Thus, an approach for identifying such systems based on chlorine isotope composition and auxiliary hydrochemical indices was proposed. The approach enabled us to distinguish Rehai, a magmatic hydrothermal area in the Tengchong volcanic region of China, from several other typical non-magmatic hydrothermal areas in Tengchong: Menglian, Shiqiang, and Jietou. The hot springs in the non-Rehai areas featured high δ37Cl values, reflecting an exclusive contribution of chlorine from hostrock leaching. By contrast, some hot springs with much lower δ37Cl values occurred in Rehai, all of which were nearly fully equilibrated waters with respect to reservoir hostrocks, rich in chloride and bromide. These springs should have the closest relationship with the fluids released from the underlying magma chamber. Notably, the potential reservoir processes that geothermal waters may undergo after receiving an input of magmatic fluids can further alter their chlorine isotope compositions to various degrees. Therefore, a deep understanding of these processes would help improve the use of the proposed approach based primarily on chlorine isotopes. Regarding Rehai, all the neutral to alkaline hot springs with δ37Cl values ranging from −0.44 ‰ to 0.31 ‰ evolved from a parent geothermal fluid that received an input of magmatic fluids—via adiabatic cooling, conductive cooling, mixing with low-temperature shallow groundwaters, or a combination of some of these processes. By contrast, the acidic Rehai hot springs with the highest δ37Cl values (up to 1.12 ‰) were essentially locally perched groundwaters heated by geothermal steam separated from underlying geothermal fluids upon their adiabatic cooling; thus, they were much less to do with the parent geothermal fluid than the neutral to alkaline springs were. Hence, only the neutral to alkaline Rehai hot springs not strongly affected by the mixing of shallow groundwaters and adiabatic cooling can reflect the chlorine isotope characteristics of intrusive magmatic fluids. Overall, chlorine isotope composition in combination with hydrochemistry has proven to be a robust tool for identifying a magmatic fluid-affected hydrothermal system.

A conceptual model for the Tufiño-Chiles-Cerro Negro (TCCN) geothermal system (Ecuador-Colombia): New insights into geothermal exploration from chemical and isotopic composition of hydrothermal fluids

The Tufiño-Chiles-Cerro Negro (TCCN), located on the border between Colombia and Ecuador, is one of the most promising geothermal systems in the Andean Northern Volcanic Zone, presenting an estimated geothermal potential of 130–138 MWe associated with two reservoirs located at 500–1000 m, and > 1500 m of depth, respectively. In this work, we present a geochemical conceptual model for TCCN to explore the physicochemical conditions acting on the reservoir, based on a comprehensive dataset of water, gas geochemistry, and stable isotopes of the hydrothermal manifestations of the geothermal system, integrated with previously published data. Four new water samples and two bubbling gas samples were collected from the TCCN area during fieldwork in January 2015. Waters from TCCN, which show temperatures up to 54.7 °C, are near-neutral to slightly acidic and have variable TDS values (687 to 3095 mg/L) that generally increase at increasing temperature. All the samples fall far from the chloride-mature waters field, being characterised by SO42− or HCO3−dominated composition, suggesting steam-heated and peripheral water characteristics. The 3He/4He ratios (R/Ra) of gas samples range between 2.4 and 4.6, indicating an important mantle He contribution diluted by the addition of radiogenic and atmospheric 4He. Based on their CO2/3He and δ13C–CO2 ratios, gas samples show a carbon origin prevailing from subducted limestone, likely connected to the presence of the subducting Carnegie ridge. A mixing between a thermogenic and biogenic origin for methane is suggested, consistent with the dominant inputs from microbial methanogenesis and thermal maturation of sedimentary organic matter in the TCCN system. The high δ34S-H2S values of gas (+6.7 ‰ and 10.7 ‰ vs. V-CDT) and δ34S-SO4 of water (+19.2 ‰ and + 19.3 ‰ vs. V-CDT) samples coupled with the high values of S/3He indicate that the sulphur signature could be mostly controlled by the incorporation of subducted sulphate. Gas geothermometric estimations in the CO2-H2 system indicate a temperature between 245 and 250 °C for the geothermal reservoir. The gas samples appear to be in disequilibrium concerning the redox conditions related to the FeO/FeO1.5 buffer typical for hydrothermal systems, implying that these samples represent immature vapours. This could be explained by the input of magmatic fluids into the roots of the TCCN geothermal systems, possibly related to the seismic crisis linked to the unrest of the volcanic complex at the end of 2014.

Magmatic fluid input controlling the geochemical and isotopic characteristics of geothermal waters along the Yadong-Gulu rift, southern Tibetan Plateau

Geothermal resources, regarded as sustainable green energy, are attracting increasing attention. The southern Tibetan Plateau possesses the most abundant intermediate-high geothermal resources in China and has enormous potential for geothermal exploration. Geothermal waters from the Yadong-Gulu Rift (YGR), which is considered as the most active geothermal belt of the plateau, are studied based on hydrochemical and isotopic characteristics to understand the spatial variations of fluid origin, evolution, and circulation mechanism as well as heat sources. The hydrochemical characteristics of cold springs and surface waters are consistently characterized by Ca-HCO3 dominated (Ca-HCO3 and Ca-HCO3·SO4) waters while those of geothermal springs present extensive variability with Na-Cl dominated (Na-Cl·HCO3, Na-HCO3·Cl, Na·Ca-Cl·HCO3) and Na-HCO3 waters. The main geochemical processes controlling the geothermal springs are combinations of silicate weathering, carbonate dissolution, and cation exchange. The characteristics of trace alkali metals and B indicate they undergone common/similar deep process and the potential input of magmatic degassing. The δD and δ18O values suggest that cold springs and surface waters are recharged by meteoric water, and geothermal springs are replenished by meteoric water, snow-melt, and magmatic fluid. The mass balance model reveal that dominant carbon sources for geothermal waters can be deeply derived CO2 (i.e., magmatic source and metamorphic source), contributing from 77% to 99%. Based on empirical chemical geothermometers, Si-Enthalpy mixing model, and geothermometrical modelling, the calculated reservoir temperatures are highly variable (137-312 °C). The circulation depths of geothermal waters are 2.45-6.85 km. A conceptual model is proposed to reveal the deep groundwater circulation.

Tracing the Source of Hydrothermal Fluid in Ophiolite-Related Volcanogenic Massive Sulfide Deposits: A Case Study from the Italian Northern Apennines

The Italian Northern Apennines contain several Fe-Cu-Zn-bearing, Cyprus-type volcanogenic massive sulfide (VMS) deposits, which significantly contribute to the Cu resources of Italy. The massive sulfide lenses and related stockwork mineralizations are hosted by several levels (from basalt to serpentinite) of the unmetamorphosed ophiolitic series; therefore, this region offers perfect locations to study the ore-forming hydrothermal system in detail. A combination of fluid inclusion microthermometry, Raman spectroscopy, electron probe analyses (chlorite thermometry) and stable and noble gas isotope geochemistry was used to determine the fluid source of the VMS system at Bargone, Boccassuolo, Campegli, Casali–Monte Loreto, Corchia, Reppia and Vigonzano. This question of the fluid source is the focus of modern VMS research worldwide, as it has a direct influence on the metal content of the deposit. The obtained temperature and compositional data are both in the typical range of VMS systems and basically suggest evolved seawater origin for the mineralizing fluid. Modification of seawater was most commonly due to fluid–rock interaction processes, which happened during long-lasting circulation in the crust. The role of a small amount of magmatic fluid input was traced only at the lower block of Boccassuolo, which may be responsible for its higher ore grade. This fluid origin model is evidenced by O, H and C stable isotopic as well as He, Ne and Ar noble gas isotopic values.

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.

Origin and evolution of W mineralization in the Tongshanling Cu–polymetallic ore field, South China: Constraints from scheelite microstructure, geochemistry, and Nd–O isotope evidence

Multistage, skarn-type W mineralization has recently been discovered in the Tongshanling Cu–polymetallic ore field, South China. This contribution reports new results from cathodoluminescence, trace element, and Nd–O isotopic analyses of scheelite samples from Tongshanling, designed to better constrain the W mineralization process and origin of associated metals and metallogenic fluids. Three generations of scheelite were identified that can be attributed to three paragenetic phases: (i) an early prograde skarn stage dominated by massive garnet and diopside intergrown with fine-grained scheelite, (ii) an intermediate retrograde skarn stage characterized by an assemblage of abundant scheelite, medium-grained chlorite and epidote, and (iii) a late sulfide-quartz vein stage typified by coarse-grained, subhedral scheelite grains that are typically intergrown with chalcopyrite, sphalerite, and galena. Our data show that although WO3 and CaO concentrations are similar in scheelite of all three stages and rare earth element (REE) patterns vary, the ∑REE + Y values of scheelite markedly and progressively decrease from the prograde skarn (mean = 901 ppm) to retrograde skarn (70.1 ppm) to the sulfide-quartz vein (34.1 ppm) stages. This signature is consistent with either of the following scenarios: The coprecipitation of REE-enriched minerals (e.g., garnet and scheelite), a change in fluid oxygen fugacity change and/or minor distortions in crystal texture. The δ18OH2O values of fluids linked to the prograde skarn and sulfide-quartz vein stages (5.02‰–6.19‰) are consistent with a magmatic fluid input. The δ18OH2O values of fluids associated with the retrograde skarn stage (2.85‰–3.45‰), on the other hand, are indicative of fluid mixing, possibly reflecting dilution of magmatic fluids by meteoric waters. Interestingly, scheelite samples of the sulfide-quartz vein stage returned low εNd(t) values (−7.46 to − 7.07) and moderate two-stage Nd model ages (T2DM = 1526 –1557 Ma) that are identical to the whole rock Nd isotope composition of an adjacent Jurassic I-type granodiorite porphyry pluton. Overall, the results of our study indicate a close genetic relationship between this pluton and the W mineralization, a scenario that is at odds with the common association between I-type granodiorites and Cu–Pb–Zn mineralization in South China.

Oxygen isotope evidence for input of magmatic fluids and precipitation of Au-Ag-tellurides in an otherwise ordinary adularia-sericite epithermal system in NE China

Abstract Tellurium-rich (Te) adularia-sericite epithermal Au-Ag deposits are an important current and future source of precious and critical metals. However, the source and evolution of ore-forming fluids in these deposits are masked by traditional bulk analysis of quartz oxygen isotope ratios that homogenize fine-scale textures and growth zones. To advance understanding of the source of Te and precious metals, herein, we use petrographic and cathodoluminescence (CL) images of such textures and growth zones to guide high spatial resolution secondary ion mass spectroscopy (SIMS) oxygen isotope analyses (10 µm spot) and spatially correlated fluid inclusion microthermometric measurements on successive quartz bands in contemporary Te-rich and Te-poor adularia-sericite (-quartz) epithermal Au-Ag vein deposits in northeastern China. The results show that large positive oxygen isotope shifts from –7.1 to +7.7‰ in quartz rims are followed by precipitation of Au-Ag telluride minerals in the Te-rich deposit, whereas small oxygen isotope shifts of only 4‰ (–2.2 to +1.6‰) were detected in quartz associated with Au-Ag minerals in the Te-poor deposits. Moreover, fluid-inclusion homogenization temperatures are higher in comb quartz rims (avg. 266.4 to 277.5 °C) followed by Au-Ag telluride minerals than in previous stages (~250 °C) in the Te-rich deposit. The Te-poor deposit has a consistent temperature (~245 °C) in quartz that pre- and postdates Au-Ag minerals. Together, the coupled increase in oxygen isotope ratios and homogenization temperatures followed by precipitation of Au-Ag tellurides strongly supports that inputs of magmatic fluid containing Au, Ag, and Te into barren meteoric water-dominated flow systems are critical to the formation of Te-rich adularia-sericite epithermal Au-Ag deposits. In contrast, Te-poor adularia-sericite epithermal Au-Ag deposits show little or no oxygen isotope or fluid-inclusion evidence for inputs of magmatic fluid.

The Westwood Deposit, Southern Abitibi Greenstone Belt, Canada: An Archean Au-Rich Polymetallic Magmatic-Hydrothermal System—Part II. Hydrothermal Alteration, Mineralization, and Geologic Model

Abstract The Westwood deposit, located in the Archean Doyon-Bousquet-LaRonde mining camp in the southern Archean Abitibi greenstone belt, contains 4.5 Moz (140 metric t) of gold. The deposit is hosted in the 2699–2695 Ma submarine, tholeiitic to calc-alkaline volcanic, volcaniclastic, and intrusive rocks of the Bousquet Formation. The deposit is located near the synvolcanic (ca. 2699–2696 Ma) Mooshla Intrusive Complex that hosts the Doyon epizonal intrusion-related Au ± Cu deposit, whereas several Au-rich volcanogenic massive sulfide (VMS) deposits are present east of the Westwood deposit. The Westwood deposit consists of stratigraphically stacked, contrasting, and overprinting mineralization styles that share analogies with both the intrusion-related and VMS deposits of the camp. The ore zones form three distinct, slightly discordant to stratabound corridors that are, from north (base) to south (top), the Zone 2 Extension, the North Corridor, and the Westwood Corridor. Syn- to late-main regional deformation and upper greenschist to lower amphibolite facies regional metamorphism affect the ore zones, alteration assemblages, and host rocks. The Zone 2 Extension consists of Au ± Cu sulfide (pyrite-chalcopyrite)-quartz veins and zones of disseminated to semimassive sulfides. The ore zones are spatially associated with a series of calc-alkaline felsic sills and dikes that crosscut the mafic to intermediate, tholeiitic to transitional, lower Bousquet Formation volcanic rocks. The metamorphosed proximal alteration consists of muscovite-quartz-pyrite ± gypsum-andalusite-kyanite-pyrophyllite argillic to advanced argillic-style tabular envelope that is up to a few tens of meters thick. The North Corridor consists of auriferous semimassive to massive sulfide veins, zones of sulfide stringers, and disseminated sulfides that are hosted in intermediate volcaniclastic rocks at the base of the upper Bousquet Formation. The Westwood Corridor consists of semimassive to massive sulfide lenses, veins, zones of sulfide stringers, and disseminated sulfides that are located higher in the stratigraphic sequence, at or near the contact between calc-alkaline dacite domes and overlying calc-alkaline rhyodacite of the upper Bousquet Formation. A large, semiconformable distal alteration zone that encompasses the North Corridor is present in the footwall and vicinity of the Westwood Corridor. This metamorphosed alteration zone consists of an assemblage of biotite-Mn garnet-chlorite-carbonate ± muscovite-albite. A proximal muscovite-quartz-chlorite-pyrite argillic-style alteration assemblage is associated with both corridors. The Zone 2 Extension ore zones and associated alteration are considered synvolcanic based on crosscutting relationships and U-Pb geochronology and are interpreted as being the distal expression of an epizonal magmatic-hydrothermal system that is centered on the upper part of the synvolcanic Mooshla Intrusive Complex. The North and Westwood corridors consist of bimodal-felsic Au-rich VMS-type mineralization and alteration produced by the convective circulation of modified seawater that included a magmatic contribution from the coeval epizonal Zone 2 Extension magmatic-hydrothermal system. The Westwood Au deposit represents one of the very few documented examples of an Archean magmatic-hydrothermal system—or at least of such systems formed in a subaqueous environment. The study of the Westwood deposit resulted in a better understanding of the critical role of magmatic fluid input toward the formation of Archean epizonal intrusion-related Au ± Cu and seafloor/subseafloor Au-rich VMS-type mineralization.

Intrusion-Associated Gold Systems and Multistage Metallogenic Processes in the Neoarchean Abitibi Greenstone Belt

In gold-endowed greenstone belts, ore bodies generally correspond to orogenic gold systems (OGS) formed during the main deformation stage that led to craton stabilization (syntectonic period). Most OGS deposits postdate and locally overprint magmatic-hydrothermal systems, such as Au-Cu porphyry that mostly formed during the main magmatic stage (synvolcanic period) and polymetallic intrusion-related gold systems (IRGS) of the syntectonic period. Porphyries are associated with tonalite-dominated and sanukitoid plutons, whereas most IRGS are related to alkaline magmatism. As reviewed here, most intrusion-associated mineralization in the Abitibi greenstone belt is the result of complex and local multistage metallogenic processes. A new classification is proposed that includes (1) OGS and OGS-like deposits dominated by metamorphic and magmatic fluids, respectively; (2) porphyry and IRGS that may contain gold remobilized during subsequent deformation episodes; (3) porphyry and IRGS that are overprinted by OGS. Both OGS and OGS-like deposits are associated with crustal-scale faults and display similar gold-deposition mechanisms. The main difference is that magmatic fluid input may increase the oxidation state and CO2 content of the mineralizing fluid for OGS-like deposits, while OGS are characterized by the circulation of reduced metamorphic fluids. For porphyry and IRGS, mineralizing fluids and metals have a magmatic origin. Porphyries are defined as base metal and gold-bearing deposits associated with large-volume intrusions, while IRGS are gold deposits that may display a polymetallic signature and that can be associated with small-volume syntectonic intrusions. Some porphyry, such as the Côté Gold deposit, demonstrate that magmatic systems can generate economically significant gold mineralization. In addition, many deposits display evidence of multistage processes and correspond to gold-bearing or gold-barren magmatic-hydrothermal systems overprinted by OGS or by gold-barren metamorphic fluids. In most cases, the source of gold remains debated. Whether magmatic activity was essential or marginal for fertilizing the upper crust during the Neoarchean remains a major topic for future research, and petrogenetic investigations may be paramount for distinguishing gold-endowed from barren greenstone belts.

Magmatic volatiles episodically flush oceanic hydrothermal systems as recorded by zoned epidote

Circulation of seawater at oceanic spreading centers extracts heat, drives rock alteration, and transports leached metals to shallower levels of the crust, where they may precipitate and form ore deposits. Crystallization of the lower crust, may exsolve and introduce magmatic volatiles into the seawater-dominant system. However, the role of magmatic volatiles added to the hydrothermal system, including pathways of these fluids are lesser known. Here we present coupled in-situ strontium isotope and rare earth element data of distinct domains in epidote, a common hydrothermal mineral throughout the Troodos ophiolite, to track magmatic fluid input and flow. Epidote crystal domains characterize three distinct strontium isotope-rare earth element signatures—suggesting sequential growth from magma-derived fluids (0.704, negative europium anomalies), rock-buffered fluids (0.7055, positive europium anomalies) and seawater-derived fluids (0.7065, negative cerium anomalies). Epidote records episodic fluxing of magmatic fluids from plagiogranites, through epidosites in the upflow zone and into metal ore deposits.

The Seven Sisters Hydrothermal System: First Record of Shallow Hybrid Mineralization Hosted in Mafic Volcaniclasts on the Arctic Mid-Ocean Ridge

We document the discovery of an active, shallow, seafloor hydrothermal system (known as the Seven Sisters Vent Field) hosted in mafic volcaniclasts at a mid-ocean ridge setting. The vent field is located at the southern part of the Arctic mid-ocean ridge where it lies on top of a flat-topped volcano at ~130 m depth. Up to 200 °C phase-separating fluids vent from summit depressions in the volcano, and from pinnacle-like edifices on top of large hydrothermal mounds. The hydrothermal mineralization at Seven Sisters manifests as a replacement of mafic volcaniclasts, as direct intraclast precipitation from the hydrothermal fluid, and as elemental sulfur deposition within orifices. Barite is ubiquitous, and is sequentially replaced by pyrite, which is the first sulfide to form, followed by Zn-Cu-Pb-Ag bearing sulfides, sulfosalts, and silica. The mineralized rocks at Seven Sisters contain highly anomalous concentrations of ‘epithermal suite’ elements such as Tl, As, Sb and Hg, with secondary alteration assemblages including silica and dickite. Vent fluids have a pH of ~5 and are Ba and metal depleted. Relatively high dissolved Si (~7.6 mmol/L Si) combined with low (0.2–0.4) Fe/Mn suggest high-temperature reactions at ~150 bar. A δ13C value of −5.4‰ in CO2 dominated fluids denotes magmatic degassing from a relatively undegassed reservoir. Furthermore, low CH4 and H2 (<0.026 mmol/kg and <0.009 mmol/kg, respectively) and 3He/4He of ~8.3 R/Racorr support a MORB-like, sediment-free fluid signature from an upper mantle source. Sulfide and secondary alteration mineralogy, fluid and gas chemistry, as well as δ34S and 87Sr/86Sr values in barite and pyrite indicate that mineralization at Seven Sisters is sustained by the input of magmatic fluids with minimal seawater contribution. 226Ra/Ba radiometric dating of the barite suggests that this hydrothermal system has been active for at least 4670 ± 60 yr.

Environmental impacts of geothermal waters with extremely high boron concentrations: Insight from a case study in Tibet, China

Boron is a potentially toxic element in the environment and commonly enriched in high-temperature geothermal waters. In this study, seven typical boron-rich geothermal areas in Tibet (China) were selected as the study areas for comprehensively investigating the geochemistry of geothermal boron and its environmental impact. The geothermal waters were classified into four types: acid low-boron waters, acid high-boron waters, neutral/alkaline low-boron waters, and neutral/alkaline high-boron waters. The enrichment of boron in the neutral/alkaline high-boron geothermal waters is attributed to the input of magmatic fluids, whereas the host rock leaching is the primary boron source for the neutral/alkaline low-boron geothermal waters. The acid low-boron waters are essentially locally-perched groundwaters heated by geothermal vapors separated from flashed deep geothermal fluids, and thereby their boron comes mainly from the limited leaching of the host rocks of the shallow aquifers. As for the acid high-boron waters, they could be formed via a mixing between acid low-boron waters and neutral/alkaline geothermal waters, with the latter as the primary boron source. The boron species in these geothermal waters comprise mainly H3BO3, B(OH)4−, polyborate ions and fluoroborate complexes, all of which are affected by the pH, temperature, and F and B concentrations of the waters. When the geothermal waters flow out of the hot spring vents, the rapid changes in unstable parameters, like pH and water temperature, will lead to the conversion of B(OH)4− and polyborate ions to H3BO3. The discharge of the boron-rich geothermal waters has contaminated the surface waters and sediments in and around the investigated geothermal areas. In turn, the boron sorbed onto/into the hot spring sediments is liable to be released either in short-term if it is originally exchangeable boron or in long-term if carbonates-, Fe-Mn oxides-, or organic matters/sulfides-combined boron. This study enhances the understanding of geothermal boron speciation and its environmental impact, and provides insights for rational exploitation of hydrothermal resource without inducing geothermal boron pollution.

Constraints of major ions and arsenic on the geological genesis of geothermal water: Insight from a comparison between Xiong'an and Yangbajain, two hydrothermal systems in China

Hydrochemical similarities are shared by the geothermal waters from Xiong'an, a typical non-magmatic hydrothermal system, and Yangbajain, a typical magmatic hydrothermal system. Both systems are primarily discharging chloride- and sodium-rich geothermal waters with similar TDS values. However, the hydrochemical investigations made in this study show that the chloride in the Xiong'an geothermal waters originates mainly from the mixing of deep saline waters, while that in the Yangbajain geothermal waters is basically of magmatic origin, like many other magmatic hydrothermal systems hosted by felsic rocks worldwide. In contrast to chloride, the concentrations of arsenic in the geothermal waters from Xiong'an and Yangbajain differ greatly. The higher arsenic concentrations of the Yangbajain geothermal waters are attributable to the much greater reservoir temperatures in Yangbajain, and possibly, the input of magmatic fluids to the Yangbajain reservoirs. The contents of arsenic in reservoir rocks, instead, don't seem to be an important factor controlling the geothermal arsenic concentrations. Leaching of arsenic from arsenic-rich sandstone or carbonate rocks at relatively low Xiong'an reservoir temperatures should be much less than that from arsenic-poor granitic rocks at higher Yangbajain reservoir temperatures. The results obtained in this study indicate that arsenic coupled with other hydrochemical constituents can be used as a useful tool to identify the geological geneses of geothermal waters.

Reconstruction of the fossil hydrothermal system at Lake City caldera, Colorado, U.S.A.: Constraints for caldera-hosted geothermal systems

Reconstruction of the physiochemical characteristics of fossil hydrothermal systems can help guide exploration for modern geothermal or mineral resources in similar settings. The 22.9 Ma Lake City caldera in Colorado, U.S.A., is well-exposed and contains an exhumed fossil shallow hydrothermal system. In this study, alteration mineralogy, vein textures and fluid inclusions are used to characterise the temperature and composition, spatiotemporal variability, and structural controls of the hydrothermal system. At paleo-depth equivalent between 1 and 2 km, the hydrothermal system was dominantly moderate temperature (up to ~290 °C), low salinity (<3% NaCl equivalent) and neutral to weakly acidic pH. There is evidence for boiling in veins throughout the exposed depth range; however, boiling textures are most common at the highest elevations of the system (~1150 m paleo-depth), and in structurally controlled fluid conduits. Quartz-illite alteration assemblages in the centre of the caldera indicate slightly more fluid-dominated conditions compared to those that formed propylitic alteration on the caldera margin. These alteration types reflect contrasting fluid pathways; a more pervasively fractured and faulted resurgent dome in the caldera centre, compared to fewer, larger conduits in the basement granite at the caldera margin. Based on the lack of high-temperature (<350 °C), hypersaline (>20 wt% NaCleq) fluid inclusions within the caldera centre, we interpret that the resurgent syenite intrusions provided little magmatic fluid input and had cooled significantly by the time the hydrothermal system had established. In contrast, in the eastern portion of the caldera, late distinct magma batches provided high temperature (up to ~540 °C) and hypersaline (up to ~65% NaCl eq.) magmatic fluid input above intrusions. Our conceptual hydrothermal model emphasizes the importance of discontinuity intersections in facilitating permeability in caldera settings. We also recognize the contrasting hydrothermal manifestations of a waning, degassed magma batch “left-over” from a caldera forming eruption, compared to fresh, volatile-rich magma.

Hazard Scenarios Related to Submarine Volcanic-Hydrothermal Activity and Advanced Monitoring Strategies: A Study Case from the Panarea Volcanic Group (Aeolian Islands, Italy)

Geohazards associated to submarine hydrothermal systems still represent a tricky enigma to face and solve for the scientific community. The poor knowledge of a submarine environment, the rare and scarce monitoring activities, and the expensive and sometimes complicated logistics are the main problems to deal with. The submarine low-energy explosion, which occurred last November 3, 2002, off the volcanic island of Panarea, highlighted the absence of any hazard scenario to be used to manage the volcanic crisis. The “unrest” of the volcanic activity was triggered by a sudden input of deep magmatic fluids, which caused boiling water at the sea surface with a massive CO2 release besides changes in the fluids’ geochemistry. That event dramatically pushed scientists to develop new methods to monitor the seafloor venting activity. Coupling the information from geochemical investigations and data collected during the unrest of volcanic activity, we were able to (a) develop theoretical models to gain a better insight on the submarine hydrothermal system and its relationships with the local volcanic and tectonic structures and (b) to develop a preliminary submarine volcanic hazard assessment connected to the Panarea system (Aeolian Islands). In order to mitigate the potential submarine volcanic hazard, three different scenarios are described here: (1) ordinary hydrothermal venting, (2) gas burst, and (3) volcanic eruption. The experience carried out at Panarea demonstrates that the best way to face any submarine volcanic-hydrothermal hazard is to improve the collection of data in near real-time mode by multidisciplinary seafloor observatories and to combine it with periodical sampling activity.

Boron geochemistry of the geothermal waters from two typical hydrothermal systems in Southern Tibet (China): Daggyai and Quzhuomu

The Tibetan plateau is characterized by a widely and intensively distributed geothermal activity and an extreme enrichment of boron in some geothermal waters. Twenty-five hot spring samples were collected from the Daggyai and Quzhuomu geothermal areas in Tibet to determine their boron concentrations and isotope composition. The boron concentrations and δ11B values for the neutral geothermal waters in Daggyai range from 72.5 to 106.9 mg/L and −16.4‰ to −12.3‰ while the Daggyai acid waters have much lower boron concentrations (1.1–1.2 mg/L) and slightly higher δ11B values (−12.9‰). The hot spring samples from the Quzhuomu geothermal area have boron concentrations varying from 21.9 to 44.6 mg/L and δ11B values from −11.3‰ to −7.1‰. The hydrochemical and B isotopic evidence imply that the boron in Daggyai geothermal waters is contributed by both magmatic fluid input and host rock leaching. In contrast, marine carbonates and granites, especially the former, are confirmed as the major boron sources for the Quzhuomu geothermal waters. In addition, there also exist some secondary processes affecting the boron isotope fractionation in geothermal waters, e.g. water-vapor separation and absorption or incorporation of B into hydrothermally altered minerals. Finally, based on the analysis of B geochemistry in these two typical geothermal areas in Southern Tibet with different heat sources and reservoir lithology, the mechanisms for the accumulation of boron in the Daggyai and Quzhuomu geothermal waters are suggested. This study is promising to provide insight into the sources of boron in the geothermal waters in other Tibetan geothermal areas.

Metal fluxes during magmatic degassing in the oceanic crust: sulfide mineralisation at ODP site 786B, Izu-Bonin forearc

Volcanogenic massive sulfide deposits are enriched in metals that are either derived from hydrothermal alteration of the basement rocks or supplied by exsolution of metal-rich volatiles during magmatic differentiation. The extent to which each process contributes to metal enrichment in these deposits varies between different tectonic settings. Ocean Drilling Program Hole 786B recovered > 800 m of upper oceanic crust from a supra-subduction zone setting and includes a 30-m-thick mineralised zone. In situ S isotopic compositions of pyrite decrease from 5.9 ± 2.9‰ in the upper mineralised zone down to − 3.3 ± 2.1‰ in the extensively altered central mineralisation zone, potentially indicating strong magmatic fluid input in this area. Whole rock data and in situ trace element analyses in sulfide minerals show enrichment of Ag, As, Au, Bi, Mo, S, Se, Sb and Te in the mineralised zone. Evaluation of metal behaviour during magmatic differentiation and primary metal fertility of basement rocks suggests that degassing melt is the main source for the high Au, Se and S enrichment observed in the mineralised zone. Magmatic volatile exsolution occurred late during the magmatic differentiation (~ 2 wt.% MgO), concomitant with oxide crystallisation and metal depletion in the melt. Comparison of Ocean Drilling Program Hole 786B with volcanogenic massive sulfide deposits hosted by boninitic volcanic successions, such as in the Semail ophiolite, the Newfoundland Appalachians and the Flin Flon Belt, suggests that magmatic fluid exsolution could be a common mechanism for Au enrichment in bimodal mafic volcanogenic massive sulfide deposits.

Tourmaline boron and strontium isotope systematics reveal magmatic fluid pulses and external fluid influx in a giant iron oxide-apatite (IOA) deposit

Tourmaline is a common boron-bearing mineral in hydrothermal system and has been widely used as a mineral probe to reconstruct geological processes because of its broad range in composition and resistance to metasomatic alteration. The origin of Kiruna-type iron oxide-apatite (IOA) deposits, commonly linked to andesitic subvolcanic or volcanic rocks, is highly controversial. Constraints on the evolution of these mineralizing systems are needed to advance understanding of the ore-forming process. In this study, we apply in situ elemental and combined B-Sr isotopic analyses of tourmaline to elucidate the nature and evolution of the subsurface hydrothermal system associated with IOA mineralization in the giant Taocun deposit, eastern China. Taocun is hosted at the top of a diorite intrusion and exhibits three stages of hydrothermal alteration that contain tourmaline: pre-ore Na alteration (Tur I), syn-ore magnetite formation and associated Ca-Fe alteration (Tur II), and post-ore Ca-Mg alteration with sulfide veins (Tur III). Compositional data for each stage of tourmaline plot along the “oxy-dravite”–povondraite join, which is indicative of precipitation from relatively oxidizing fluids. Ranges of Sr-isotopic compositions in Tur I (0.7065–0.7078) and Tur II (0.7068–0.7076) are identical to those of the igneous host rocks, indicating precipitation from magmatic-hydrothermal fluids. The range of B-isotopic compositions in Tur I (δ 11 B values of −6.3‰ to −1.2‰) is also consistent with a magmatic source. Higher δ 11 B values (−2.4‰ to 5.4‰) obtained from Tur II are mainly ascribed to Rayleigh fractionation in the magmatic-hydrothermal system as tourmaline precipitated. Post-ore Tur III has a wide range of mostly lower B-isotopic compositions (−8.5‰ to 0.8‰) that record another pulse of magmatic fluid input. This interpretation is supported by the enrichment of Na, Li, Be, W, Sn, V, and Ti in Tur III, relative to Tur I and II. However, the higher Sr-isotope composition (0.7076–0.7086) of Tur III and available O-isotope composition (−7‰ to 3.5‰) of fluids of this stage record the infiltration of meteoric ground water from adjacent sedimentary country rocks. The results suggest that the Taocun IOA deposit formed in a magmatic-hydrothermal system characterized by two (or more) pulses of magmatic fluid discharge from subvolcanic diorite intrusions, followed by the influx of external ground water as the system waned. This study highlights the utility of tourmaline as a robust geochemical and isotopic monitor of ore-forming processes in such systems.

Magmatic fluid input explaining the geochemical anomaly of very high arsenic in some southern Tibetan geothermal waters

Tibet is characterized by extremely high terrestrial heat flow and widely distributed hydrothermal systems, among which many are discharging geothermal waters with extremely high arsenic concentrations from over 10 mg/L to up to 126 mg/L. The distribution of these high arsenic waters is basically in accordance with the location of the Indian continental subduction zone. A detailed analysis indicates that host rock leaching alone cannot explain the observed arsenic anomaly. Instead, substantial contribution of arsenic from the underlying magma chambers is postulated, which are likely mantle-derived intrusions that have been severely contaminated by the very thick crustal material below southern Tibet, possibly including deep-seated arsenic-rich sedimentary rocks. Alternatively, the partial melts at crustal depths reflected by the detected low-velocity and/or high-conductivity anomalies in southern Tibet, serving as the magmatic heat source of the high‑arsenic hydrothermal systems, may also form indirectly under the high-temperature conditions generated by mantle upwelling or due to crustal thickening. Low sulfide concentrations further promote high dissolved arsenic concentrations with little formation of thioarsenates and no precipitation of limiting arsenic-sulfide phases. In contrast, mantle magmas less affected by crustal contamination, serving as the heat source of many hydrothermal systems in rift zones and hot spots worldwide where the crust is thinner, are releasing magmatic fluids lower in arsenic and higher in mantle sulfur species. Overall, the existence of a magmatic heat source alone does not ensure the formation of geothermal waters with very high arsenic concentrations, and geothermal waters from sedimentary rock-hosted systems are neither necessarily arsenic-rich. The most critical factor controlling the arsenic concentration of geothermal water discharging from a magmatic hydrothermal system is the geological genesis of the magma fluid and its chemical composition.

Tungsten from typical magmatic hydrothermal systems in China and its environmental transport

Tungsten is of extraordinarily high concentrations in the geothermal waters discharging from several representative Tibetan magmatic hydrothermal systems (up to 1103 μg/L), which are also characterized by exceptionally high W/Mo molar ratios (up to 1182). The geochemical origins of the tungsten in these geothermal waters were investigated, with a comparison to those from Rehai, the sole magmatic hydrothermal system in Yunnan, which is another major part of the Yunnan-Sichuan-Tibet Geothermal Province of China. The results show that the lithology of reservoir host rocks is the primary factor controlling the tungsten concentrations of the geothermal waters, although the contribution of magmatic fluid input cannot be ruled out. In this study, the geothermal waters are generally rich in sulfide, and therefore the molybdenum in the reservoir fluids has been substantially precipitated as the form of molybdenite; in contrast, the reservoir fluids are well undersaturated with respect to tungstenite which is much more soluble than molybdenite. Thus the neutral/alkaline hot springs, i.e. the evolved reservoir fluids, have high W/Mo molar ratios as well. In the hot spring sediments, the distribution pattern of tungsten is quite different. The concentrations of tungsten are the highest in the sediments with high iron concentrations collected from the acid hot spring vents and outflow channels. The adsorption of aqueous tungsten onto iron-bearing minerals, like goethite or pyrite, is favorable at acid pH values and thereby responsible for the very high tungsten concentrations of these acid hot spring sediments. The proportions of thiotungstates in total tungsten are quite low for all the hot springs, as indicated by thermodynamic calculations, suggesting that thiolation of tungstate has little impacts on the environmental transport and fate of geothermal tungsten in the investigated hydrothermal areas. This is the first study to report the tungsten geochemistry of hot springs in mainland China.