High-temperature Hydrothermal Systems Research Articles

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

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

Distinct response of deep and shallow geothermal reservoir temperature following earthquakes in the Kangding Fault zone hydrothermal system

Study regionAs part of the active Xianshuihe Fault system, the Kangding geothermal field is a representative high-temperature hydrothermal system in the southeastern Tibetan Plateau, China. The strong tectonic and hydrothermal activity serves as a unique location to study the interaction between earthquakes and geothermal fluids. Study focusWe have collected water samples taken from thermal springs and boreholes in the geothermal field to identify fluid composition and reservoir temperature of the deep granite and shallow metamorphic rock using geochemical analysis and the Integrated Multicomponent Geothermometry (IMG) method. Two typical thermal springs that offer long-term hydrochemical data were selected to investigate the effect of earthquakes on the shallow and deep reservoir temperatures. New hydrological insights for the regionThe deep reservoir temperature in the Kangding geothermal field is in the range 222 ℃ to 256 ℃, while the shallow reservoir temperature ranges from 128 ℃ to 171 ℃. Both thermal springs show changes in reservoir temperature following the 2008 Wenchuan, the 2013 Lushan, and the 2014 Kangding earthquakes. The temperature changes are similar in the deep reservoir of the two springs but different in the case of the shallow reservoir. We propose that the changes in reservoir temperature, steam fraction, and dilution factor indicate that the deep reservoir is more sensitive to earthquake-induced permeability changes than the shallow reservoir.

Fractionation of iron and chromium isotopes in hydrothermal plumes from the northern Mid-Atlantic Ridge

Hydrothermal venting impacts the global-scale biogeochemical cycles of many trace metals and their isotopes. Processes in hydrothermal plumes regulate the dispersal of vent-derived metals and may vary in response to differences in the geologic setting of vent fields and/or the geochemistry of the overlying ocean water. Here we present results of analyses of dissolved Fe and Cr concentrations, and dissolved Fe isotope (δ56Fe) and Cr isotope (δ53Cr) distributions, in seawater samples collected from above TAG and Rainbow vent sites on the Mid-Atlantic Ridge during the GEOTRACES GA13 cruise. We show that profiles of dissolved Fe and Cr isotopes through the near-field hydrothermal plumes are the mirror image of each other. Oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides account for the low δ56Fe values of dissolved Fe, as low as −1.83 ‰ at TAG and −6.94 ‰ at Rainbow. Plume samples with low δ56Fe values are associated with elevated δ53Cr values of dissolved Cr compared to background seawater (by up to +0.14 ‰ and +0.69 ‰ at TAG and Rainbow, respectively), while particulate Cr is characterised by relatively low δ53Cr values (−1.02 to −1.22 ‰). This striking result suggests that seawater Cr(VI) is reduced to Cr(III) and precipitates on the surface of Fe(III) particles within the hydrothermal plume. Reduction of Cr(VI) and scavenging of Cr(III) by plume Fe-(oxyhydr)oxide particles mean that high-temperature hydrothermal systems are likely a net sink for seawater Cr at Rainbow (and also at TAG). As the removal flux of Cr is related to the flux of hydrothermal Fe(II) and the rate of Fe(II) oxidation in the hydrothermal plume, it may (i) vary across vent sites at a global scale and (ii) change over glacial-interglacial cycles.

Unraveling the Genesis of the Geothermal System at the Northeastern Edge of the Pamir Plateau

High-temperature geothermal systems hold promise for sustainable and environmentally friendly power generation. However, China’s geothermal power capacity significantly underutilizes its abundant resources. This study focuses on the geothermal potential of the Pamir Plateau, particularly its northeastern edge, where complex tectonic forces converge. We aim to unveil the mechanisms driving the emergence of high-temperature geothermal reservoirs in this unique geological setting. Hydrogeochemical analysis reveals diverse profiles in geothermal water, primarily derived from atmospheric precipitation. Estimation of reservoir temperatures and simulation of geotherms unveil distinct geothermal systems. Kongur exhibits a medium–low-temperature hydrothermal system and Tashkurgan demonstrates high-temperature hydrothermal system characteristics, while the Pamir’s northeastern edge hints at a potential high-temperature dry geothermal system where there might not be a fault. These findings have important implications for sustainable energy development and future geothermal exploration.

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

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

Three-dimensional audio magnetotelluric imaging of the Yangyi geothermal field in Tibet, China

The Yangyi geothermal field in southern Tibet has enormous potential for geothermal energy development. It is the high-temperature geothermal field at the highest elevation in the world, and the second largest high-temperature geothermal field in China. The total planned installed capacity of the geothermal power station is 32 MW. At present, no detailed three-dimensional (3D) geophysical survey has been carried out in the Yangyi geothermal field, and in-depth research is urgently needed. The main goal of our geophysical exploration is to study the geothermal structure of this region. In this study, the audio magnetotelluric (AMT) data of 85 stations were collected in the Yangyi geothermal field. The average distance between the stations was about 500 m, and the dimensions of the exploration area were 6 × 7 km. First, we analyzed the magnetotelluric data using the phase tensor method to qualitatively estimate the dimensionality of the subsurface resistivity structure. Then, by performing 3D inversion of 85 AMT data with relatively high quality, the 3D resistivity model from the surface to a depth of 2 km was determined, the high-temperature hydrothermal system under the Yangyi geothermal field was displayed for the first time. In addition, the main characteristics of the geothermal system were identified, and the genetic mechanism of the geothermal system was discussed based on the geological information and borehole data.

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

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

Water-rock exchange of Sr isotopes evaluated through a reactive transport model: Application to the El Chichón hydrothermal system

The strontium isotope ratio (87Sr/86Sr) is a natural tracer commonly used to determine water sources, water flow trajectories, and mixing relationships. In high-temperature hydrothermal systems, the low solubility of retrograde minerals such as calcite and anhydrite may significantly reduce the dissolved Sr concentration. Further contribution of Sr along the flow path from isotopically distinct sources will alter and potentially mask the original Sr isotopic signature of the fluid, limiting the potential of 87Sr/86Sr data for reconstructing fluid flow patterns. This study investigates the tracing potential of 87Sr/86Sr in terms of fluid circulation within the El Chichón volcano hydrothermal system. Although limestone aquifers are evidenced in the region, the 87Sr/86Sr signature of most thermal fluids from the volcano does not indicate any major Sr contribution from such lithologies, but rather reveals that Sr is inherited from volcanic rocks. An isotope-enabled reactive transport model is performed to investigate under which conditions a connection between the limestone and volcanic aquifers is compatible with chemical and Sr isotopic compositions of the thermal springs. Results indicate that the 87Sr/86Sr signature of a sedimentary fluid with moderate Sr concentration can be completely modified into a magmatic signature after interacting with Sr-rich primary minerals, such as plagioclases, within short period of times (<1 year). Primary minerals release magmatic Sr to the fluid while secondary minerals (such as calcite) progressively precipitate and remove the initial sedimentary Sr. The El Chichón hydrothermal system hence represents a case where the 87Sr/86Sr of most springs does not keep any memory of the circulation through the sedimentary basement, as magmatic Sr dominates the isotopic signature. A certain analogy exists with hydrothermal fluids vented at oceanic spreading centers, whose 87Sr/86Sr signature departs from seawater and reflects Sr inherited from water-basalt interaction.

Mercury (Hg) geochemistry of mid-ocean ridge sediments on the Central Indian Ridge: Chemical forms and isotopic composition

To geochemically characterize mercury (Hg) in the deep-sea ridge environment, the total concentration, chemical forms (sequential leaching extraction), and isotopic compositions of Hg in surface sediments from the middle portion of the Central Indian Ridge were investigated. Highly elevated Hg concentrations (up to 13,000 ng/g) in sediments near the hydrothermal vent are associated with intense hydrothermal activity driven by serpentinization. The hydrothermal impact on these sediments is also evident in the REECN fractionation pattern with a remarkably strong positive europium (Eu) anomaly. Most volcanic and hydrothermal Hg in the study area is preferentially precipitated with sulfides; in the hydrothermal vent area, however, scavenging by Fe-Mn hydroxides is another significant removal pathway of Hg. Thus, such precipitation and production of sulfides and hydroxides are a major cause of local enrichment of Hg around the mid-ocean ridge. Most sediments show limited or no mass-independent fractionation (Δ199Hg = +0.02 ± 0.21‰, 2σ, n = 15), indicating that syngenetic magmatic or mantle-derived materials are the dominant Hg source. However, the large variation in mass-dependent fractionation was observed, especially in the vent-distal sediments (δ202Hg = −1.10 ± 0.80‰, 2σ, n = 11), which occurred mainly during the formation of the sulfides and may be associated with preferential precipitation of lighter isotopes. Our study demonstrates that an off-axis high-temperature hydrothermal system driven by exothermic serpentinization of ultramafic mantle rocks may serve as a significant Hg source and provides further insights into grasping the behavior of hydrothermal and volcanogenic Hg in active deep-sea ridge systems.

The effect of granite fracture network on silica-enriched groundwater formation and geothermometers in low-temperature hydrothermal system

Silica-enriched mineral groundwater has important commercial and health value and is most common in volcanic areas and high-temperature geothermal areas. But we found abundant silica-enriched mineral groundwater (dissolved SiO2 up to 51.57 mg/L) exposed from the granite fracture network in Suining, South China, where is no volcanoes and high-temperature hydrothermal system. Pumping test and hydrogeochemistry were employed to investigate the formation mechanism of silica-enriched mineral groundwater in granite fracture network. It is HCO3-Ca type and high content of dissolved silica, formed warm groundwater on the F86-1 fault and recharged by meteoric water with a close distance of 3.90 km. Pumping test results indicate that the hydraulic conductivity decreases from top to bottom in the granite fracture network (in order of 2.27 × 10−1 m/d, 5.42 × 10−2 m/d, 7.04 × 10−4 m/d), and they are much smaller than that of the F86-1 fault zone (0.751 m/d). The flow framework constructed by water chemistry suggest the groundwater has experienced the maximum temperature of 68–75 °C and reached the maximum depth of 1.93 km. And the average seepage velocity is about 2.8 × 10−3 m/d and 1.5 × 10−3 m/d in the horizontal and vertical directions, respectively. Water-rock interaction confirms that the granite fracture network provides long-term and sufficient water-rock interaction and dominantly contributes the formation of silica-enriched mineral groundwater because of the weatherability of granite. But the temperature of SiO2-geothermometer will be slightly underestimated for the deep warm groundwater continuously mixing with the shallow groundwater in the granite fracture network.

Characterization of Geochemistry in Hydrothermal Sediments From the Newly Discovered Onnuri Vent Field in the Middle Region of the Central Indian Ridge

The recently discovered Onnuri hydrothermal vent field (OVF) is a typical off-axis ultramafic-hosted vent system, located on the summit of the dome-like ocean core complex (OCC) at a distance of ∼12 km from the ridge axis along the middle region of the Central Indian Ridge (CIR). The plume chemistry with high methane anomaly was consistent with the precursor of hydrothermal activity; however, the fundamental characteristic of the OVF system, such as the hydrothermal circulation process and source of heat, remains poorly understood. Here, we focus on the geochemical features of surface sediments and minerals collected at and around the OVF region in order to better understand this venting system. The results reveal that the OVF sediments are typified by remarkably high concentrations of Fe, Si, Ba, Cu, and Zn, derived from hydrothermal fluid and S and Mg from seawater; depleted C-S isotope compositions; and abundant hydrothermally precipitated minerals (i.e., Fe–Mn hydroxides, sulfide and sulfate minerals, and opal silica). Notably, the occurrence of pure talc and barite bears witness to strong hydrothermal activity in the OVF, and their sulfur and strontium isotope geochemistry agree with extensive mixing of the unmodified seawater with high-temperature fluid derived from the gabbroic rock within the ultramafic-dominated ridge segment. The findings reveal that the OVF is a representative example of an off-axis, high-temperature hydrothermal circulation system, possibly driven by the exothermic serpentinization of exposed peridotites. Given the widespread distribution of OCC with detachment faults, furthermore, the OVF may be the most common type of hydrothermal activity in the CIR, although the paucity of data precludes generalizing this result. This study provides important information contributing to our understanding of the ultramafic-hosted hydrothermal vent system with a non-magmatic heat source along mid-ocean ridges.

Habitability of Martian Noachian Hydrothermal Systems as Constrained by a Terrestrial Analog on the Colorado Plateau

Abstract Volcanic features and impact craters are ubiquitous features on Mars, and hydrothermal systems associated with the production of these features should have been abundant in Mars’s early history. These hydrothermal systems represent potentially habitable environments and are therefore a high priority for continued investigations of the Martian crust. Here we present a Mars analog study where basaltic magma intruded water-bearing sediments to produce a high-temperature (as high as ∼700°C) hydrothermal system, which we use to constrain the potential habitability of similar systems on Mars via mineralogy and geochemistry including S, C, and O isotopic systematics. Our analog site suggests evidence for a habitable environment once the system cooled below 120°C and the potential presence of microbial activity based on the combination of dolomite and C-isotopic systems in the same sample. These findings highlight the importance of future missions to investigate the interface of sediments with magmas and/or late-stage impact melts where microbial life may have taken hold when temperature conditions allowed.

Characterization of an exhumed high-temperature hydrothermal system and its application for deep geothermal exploration: An example from Terre-de-Haut Island (Guadeloupe archipelago, Lesser Antilles volcanic arc)

We present integrated structural, petrological, mineralogical and geochemical investigations conducted on an exhumed hydrothermal system identified in Terre-de-Haut Island (Guadeloupe archipelago). This work demonstrates, for the first time, the occurrence of ductile shear zones, marked by the development of spaced schistosity planes generated by pressure-solution processes. We unravel first that the development of spaced schistosity is coeval with high-temperature hydrothermalism (approximately 350 °C) and second that this hydrothermal alteration occurred at a depth of less than 2 km. We highlight, at shallow crustal depths, the record of both vertical and lateral fluid pathways in the studied volcanic arc. Finally, combining this new dataset with the large number of published surface data obtained in the active geothermal system of Bouillante, we propose a conceptual scheme for the studied fossil reservoir to document the roots of magmatic arc-related active geothermal systems and to introduce new perspectives for deep geothermal exploration.

Three-dimensional resistivity structure in the hydrothermal system beneath Ganzi Basin, eastern margin of Tibetan Plateau

The Ganzi geothermal system located in the eastern margin of Tibetan Plateau, is found to distribute the strongly active surface manifestations, such as boiling springs, and fumaroles. The resistivity structure of high-temperature hydrothermal system beneath Ganzi Basin has been obtained from the three-dimensional (3-D) magnetotelluric inversion of 103 broadband MT stations in period range from 320 Hz to 1000s. The MT stations have an average spacing of 500 m and completely cover the Ganzi Basin. The detailed 3-D resistivity structure which the authors found has been described characteristics of the shallow and deep geothermal reservoirs as well as to correlate with the structural features present in the Ganzi Basin. The shallow geothermal reservoir is mainly composed of the sedimentary layer, with a depth of 300−800 m, while the deep geothermal reservoir mainly consists of the fault fracture zone with a depth of 1−5 km. The porosity of the geothermal reservoirs is in the range 5–10 %, estimated based on the resistivity model. The Ganzi fault provides the pathway for transport heat and the circulation of geothermal fluids. The possible heat source for the Ganzi geothermal system has been attributed to tectonic deformation heating in the crust, radiogenic heat production derived from the decay of U, Th and K, and heat from the mantle.

Experimental partitioning of osmium between pyrite and fluid: Constraints on the mid-ocean ridge hydrothermal flux of osmium to seawater

This study presents experimental, geochemical modeling, and field data focusing on the partitioning of osmium (Os) between pyrite and fluid upon precipitation at conditions representative of mid-ocean ridge hydrothermal environments. Dissolved Os partitions strongly into pyrite upon precipitation at experimental conditions, 350 °C and 50 MPa, with a representative relative Os/Fe partition coefficient, DPyrite-FluidOs/Fe, between 10 and 15. Integrating the experimentally determined DPyrite-FluidOs/Fe into a geochemical model indicates that a significant amount of Os is retained within the subseafloor due to sulfide precipitation induced by mixing of conductively heated seawater with pristine high temperature hydrothermal fluids that are enriched in dissolved metals, such as Os and Fe. Comparison with existing Os concentration and isotopic data of hydrothermal fluids and sulfide minerals from a wide range of hydrothermal systems with the experimental and modeling constraints suggest that modern high temperature hydrothermal systems are a minor source of unradiogenic Os to the modern global ocean dissolved Os budget due to the majority of Os being sequestered into sulfide minerals formed within and on the seafloor.

Heat generation effects from shear friction along Xianshui river strike-slip fault in western Sichuan, China

The Xianshui river fault zone is located in the western Sichuan Plateau, on the eastern edge of the Himalayan Geothermal Belt; numerous hot springs are exposed at the surface and distributed along the fault zone, forming an area of high-temperature hydrothermal activity. The genetic mechanisms of these high-temperature geothermal systems is currently unclear, which poses an urgent scientific problem in the field of geothermal research This study uses numerical simulation methods based on geological, hydrological, geothermal, and geophysical data to study the effects of shear frictional heat on high-temperature hydrothermal activity in the research area under the conditions of different slip velocities, parameters of thermal materials, and strain rates of the Xianshui river left-slip fault zone. The results show that shear frictional heat is the main contributor of the local thermal anomalies in the high-temperature hydrothermal system of the Xianshui river fault zone. The local high temperature generated by shear friction causes the crust to partially melt at a depth of 20–25 km; this provides the heat source for the striped geothermal system, which is controlled by the Xianshui river fault zone. In addition, the surface heat flow generated by shear friction was measured at 10 Ma, along a two-dimensional section across the Xianshui river fault in an E–W direction, and it was found to be approximately 5–50 mW/m2. Furthermore, the nonlinear relationship between thermal conductivity, thermal capacity, shear stress, and temperature gives the shear frictional heat generation process "positive" and "negative" feedback effects, which maintain the dynamic equilibrium.

In Situ Electrochemical Study of the Growth Kinetics of Passive Film on TC11 Alloy in Sulfate Solution at 300 °C/10 MPa.

TC11 alloy is a promising structural material, and has a wide range of applications in many corrosive and high temperature hydrothermal systems. The passive film has an important influence on its electrochemical behavior. In this study, in-situ electrochemical methods (that is, open circuit potential (OCP), linear polarization (LP) and electrochemical impedance spectroscopy (EIS)) were used to monitor the long period electrochemical behavior of TC11 alloy in 0.01 M Na2SO4 solution at 300 °C/10 MPa. The growth kinetics of the passive film was mainly studied. The correlation between the evolution of the electrochemical behavior and the growth of the oxide film was discussed. The results showed that although the OCP gradually stabilized after twenty thousand seconds, henceforth the polarization resistance (Rp) was still increasing due to the thickening of the passive film. An equivalent circuit was proposed to fit the EIS experimental data, leading to determination of film capacitance and film resistance. Besides, the electrochemical data was interpreted in terms of the point defect model (PDM). The EIS results are consistent with the Rp results.

Magma Fracking Beneath Active Volcanoes Based on Seismic Data and Hydrothermal Activity Observations

Active volcanoes are associated with microearthquake (MEQ) hypocenters that form plane-oriented cluster distributions. These are faults delineating a magma injection system of dykes and sills. The Frac-Digger program was used to track fracking faults in the Kamchatka active volcanic belt and fore-arc region of Russia. In the case of magma laterally injected from volcanoes into adjacent structures, high-temperature hydrothermal systems arise, for example at Mutnovsky and Koryaksky volcanoes. Thermal features adjacent to these active volcanoes respond to magma injection events by degassing CO2 and by transient temperature changes. Geysers created by CO2-gaslift activity in silicic volcanism areas also flag magma and CO2 recharge and redistributions, for example at the Uzon-Geyserny, Kamchatka, Russia and Yellowstone, USA magma hydrothermal systems. Seismogenic faults in the Kamchatka fore-arc region are indicators of geofluid fracking; those faults can be traced down to 250 km depth, which is within the subduction slab below primary magma sources.

Rhenium in Molybdenite: a Database Approach to Identifying Geochemical Controls on the Distribution of a Critical Element

Molybdenite is the world’s principal source of rhenium (Re), a critical element in multiple high-tech applications. However, the Re contents in molybdenite vary by orders of magnitude on scales ranging from single grains to whole deposits. In order to better understand the systematics of this variation and what geochemical factors control molybdenite Re concentration, and hence overall Re resources, we examine global patterns in molybdenite Re contents through a compilation of > 3000 measurements of Re in molybdenite from > 700 mainly ore-bearing moderate- to high-temperature hydrothermal systems of different types. Our results are similar to but expand on those of earlier studies. Rhenium concentration in molybdenite has a lognormal distribution and varies systematically with type of geologic system, intrusive lithology, and Mo grade. The lowest-Re molybdenite occurs in greisens (geometric mean 1 ppm ± a multiplicative standard deviation of 9), quartz vein-hosted W-Sn deposits (2 ± 5 ppm), unmineralized granites and granodiorites (12 ± 8 ppm), intrusion-related deposits (24 ± 8 ppm), and porphyry W-Sn deposits (16 ± 11 ppm). Rhenium is most enriched in molybdenites from volcanic sublimates (23,800 ± 5 ppm), with skarn Fe and Au (560 ± 5 ppm and 540 ± 3 ppm respectively) and porphyry Cu and Cu-Au deposits next (470 ± 4 and 430 ± 7 ppm respectively). Among porphyries, skarns, and quartz vein-hosted deposits, Re is most highly concentrated in molybdenites from Cu and Au systems and its concentration decreases systematically through Cu-Mo, Mo, Sn, and W deposits. In nearly all cases, molybdenites from systems associated with intermediate igneous rocks contain more Re than molybdenites from systems of the same type with more felsic rock associations. The disparity between Re contents of molybdenite in felsic and intermediate systems is largest for porphyries, quartz vein-hosted, and skarn deposits and is near zero for subeconomic or barren granite and granodiorite Mo systems; felsic intrusion-related deposits have slightly higher molybdenite Re than their equivalents associated with intermediate intrusions. In most systems, molybdenite Re content does not correlate with metal grade, but may have an inverse correlation with Au grade in intrusion-related deposits (based on a small number of data points) and does exhibit a strong inverse correlation with deposit Mo grade. Dilution of Re through larger amounts (higher deposit grades) of molybdenite explains about 40% of this correlation, but the relative enrichment of Re in molybdenite from low-Mo deposits must also reflect some selective enrichment of Re/Mo in porphyry Cu systems compared to porphyry Mo systems. We found no evidence for secular increase or other systematic temporal variation in molybdenite Re content. The data regarding the use of molybdenite Re content as a proxy for mantle influence are ambiguous. Nearly all observed empirical correlations can be traced back to differences in redox state and sulfide concentration, the two geochemical factors identified here and by previous experimental work as the controlling influences on Re mobility under hydrothermal conditions. Hydrothermal systems with reducing conditions (W- and Sn-rich) tend to have low molybdenite Re even though compiled whole-rock data indicate that their source rocks have as much or more Re as those of more oxidized systems (e.g., Cu-rich). Vapor-phase exsolution, crustal assimilation, and mixing with external fluids may all enrich molybdenite Re concentrations in individual deposits and deposit types, but their extent and importance in overall hydrothermal concentration of Re is uncertain. Thus, it appears that the available molybdenite Re resource in an ore deposit largely depends on how the deposit’s redox and sulfidation conditions have varied over time and space during the timespan of hydrothermal activity. Oxidized, high-sulfide conditions tend to concentrate Re in molybdenite, whereas reducing conditions tend to leave Re dispersed at low concentrations in the bulk rock.

O, H, and Sr isotope evidence for origin and mixing processes of the Gudui geothermal system, Himalayas, China

The Gudui geothermal field records the highest temperature at equivalent borehole depths among the mainland hydrothermal systems in mainland China. Located about 150 ​km southeast of Lhasa City, the capital of Tibet, the Gudui geothermal field belongs to the Sangri–Cuona rift belt, also known as the Sangri–Cuona geothermal belt, and is representative of the non-volcanic geothermal systems in the Himalayas. In this study, oxygen-18 and deuterium isotope compositions as well as 87Sr/86Sr ratios of water samples collected from the Gudui geothermal field were characterized to understand the origin and mixing processes of the geothermal fluids at Gudui. Hydrogen and oxygen isotope plots show both, deep and shallow reservoirs in the Gudui geothermal field. Deep geothermal fluids are the mixing product of magmatic and infiltrating snow-melt water. Calculations show that the magma fluid component of the deep geothermal fluids account for about 21.10%–24.04%; magma fluids may also be a contributing source of lithium. The linear relationship of the 87Sr/86Sr isotopic ratio versus the 1/Sr plot indicates that shallow geothermal fluids form from the mixing of deep geothermal fluids with cold groundwater. Using a binary mixing model with deep geothermal fluid and cold groundwater as two end-members, the mixing ratios of the latter in most surface hot springs samples were calculated to be between 5% and 10%. Combined with basic geological characteristics, hydrogen and oxygen isotope characteristics, strontium concentration, 87Sr/86Sr ratios, and the binary mixing model, we infer the 6th-Class Reservoirs Evolution Conceptual Model (6-CRECM) for the Gudui geothermal system. This model represents an idealized summary of the characteristics of the Gudui geothermal field based on our comprehensive understanding of the origin and mixing processes of the geothermal fluid in Gudui. This study may aid in identifying the geothermal and geochemical origin of the Gudui high-temperature hydrothermal systems in remote Tibet of China, whose potential for geothermal development and utilization is enormous and untapped.