Hydrothermal Fluid Circulation Research Articles

Gold Deposits Related to the Island Arc Formations and Ophiolitic Complexes of Eastern Cuba: A Review

Several gold deposits in the eastern region of Cuba are genetically related to the island arc- and the ophiolitic complex formation. These have been studied and exploited since the time of the Spanish colonization in the mid-sixteenth century. These deposits belong to the Aguas Claras-Guajabales mineral field in the Holguín Province (Cuba) and lie in an elongated zone approximately 15 km in length. The object of this work is to make a methodical, detailed, and chronological review of the geological and mining work carried out in this region, as well as highlight the degree of the previously achieved studies. To realize this, an extensive bibliographic review of all available data, including published reports and articles, as well as unpublished material, was carried out. Moreover, ore mineralogy and petrography were reviewed by thin section analyses from samples from these deposits by petrographic and scanning electron microscopy. The results obtained from this study highlight that the gold mineralization in that area is closely linked to metasomatic processes produced by the circulation of hydrothermal fluids that affected the different volcanic and ultramafic rocks. This study shows that the highest gold contents observed are controlled by the contacts between the different host lithologies with high rheological contrasts. The presence of different alteration styles such as serpentinization, listvenitization, rodingitization, and propylitization have played a primary role in the deposition of gold during mineralization processes. This work could be a very useful exploration guide for future research in this region, as it provides a useful and practical compilation of the characteristics of the mineralization and alteration styles, as well as a precise indication of the spatial position, thicknesses, and contents of the gold-rich horizons.

A marine controlled-source electromagnetic application using towed and seafloor-based receivers capable of mapping seafloor and embedded massive sulfides

Deep-sea massive sulfide deposits formed by hydrothermal fluid circulation are potential metal resources. They can exist not only as mound manifestations on the seafloor (seafloor massive sulfides) but also as embedded anomalies buried beneath the seafloor (embedded massive sulfides). The distribution of embedded massive sulfides is largely unknown, despite their expected high economic value. Recent drilling surveys have revealed a complex model suggesting embedded massive sulfides coexist beneath seafloor massive sulfides. In the coexisting case, geophysical methods are required to distinguish and map seafloor and embedded massive sulfides for accurate resource estimation. Marine controlled-source electromagnetic (CSEM) methods are useful for mapping massive sulfides because they exhibit higher electrical conductivity compared with the surrounding host rock. However, CSEM applications capable of distinguishing and mapping the massive sulfides are lacking. We use a towed electric dipole transmitter with two types of receivers: stationary ocean-bottom electric (OBE) and short-offset towed receivers. This combination uses differences in sensitivity: the towed receiver data are sensitive to seafloor massive sulfides, and the stationary OBE receiver data are sensitive to embedded massive sulfides. Our synthetic data example demonstrates that the combined inversion of towed and OBE data can recover resistivities and positions of the massive sulfides more accurately than existing inversion methods using individual applications. We perform the combined inversion of measured CSEM data obtained from the middle Okinawa Trough. The inversion models demonstrate that a combined inversion can map the location and shape of embedded massive sulfides identified during drilling more accurately than the inversion of individual data sets.

Geochemical characteristics of hot springs in active fault zones within the northern Sichuan-Yunnan block: Geochemical evidence for tectonic activity

The geochemistry of the fluids is spatially closely related to faults and earthquakes that have been detected in the northern Sichuan-Yunnan (SC-YN) block. However, the processes and factors that control hydrothermal fluid circulation on a regional scale remain challenging. The geochemical circulation of the hot springs integrated with geological and geophysical data revealed the processes and controlling factors of the relationship between fluid circulation and tectonic activity. Our results showed that the hydrochemical type of the hot springs at the intersection of the Yushu-Ganzi fault (YS-GZF), Xianshuihe fault (XSHF) and Yunongxi fault (YNXF), Jinshajiang fault (JSJF) and Batang fault (BTF) as well as Ganzi-Litang fault (GZ-LTF) is composed of Na-HCO3 and Na-HCO3·Cl. It demonstrated higher reservoir temperature and circulation depth (5 − 7 km). The helium isotope (0.22 − 2.36 Ra) of the gas indicated that the mantle helium composition was up to 5 − 30 %. These hot springs may undergo water–rock-gas exchange reactions with deep fluids. Considering geological and geophysical background as well as previous studies, the geochemical characteristics of the hot spring at the intersections of faults in the north of the SC-YN block are thought to be controlled by the deep-driven upwelling lower crustal flows, mixing with shallow hot spring water and the temporal and spatial heterogeneity of the high strain fields, slip rates, and permeability of the fault intersections. Whereas, the transport of fluids in hot springs of fault may increase pore fluid pressure, and reduce the effective stress at the intersection of the fault zones, which can promote fault sliding and provoke seismic events as well as fault movements. It could result in the enhancement of regional tectonic activity. This study provides an important basis for seismic prediction with hot spring hydrochemistry stations.

Impact and implications of hydrothermal fluids migration in the Frolov hydrocarbon province in West Siberia

Both the prolific source rocks of the Bazhenov Formation and the reservoir-prone units of the Abalak Formation cover most of the West Siberian region. Hydrothermal fluids circulation has been suggested to affect hydrocarbon potential and reservoir properties at some localities of this vast hydrocarbon province. However, integrated studies are currently lacking. Here we present an extensive multidisciplinary study combining regional studies with core logging properties and sample analyses (i.e. geochemistry, petrography, organic geochemistry) to reveal processes related to the hydrothermal activity that affected both formations. We also focus on the factors and implications for secondary unconventional reservoir formation within the deposits from the Kamennaya summit of the Frolov oil and gas region. Three boreholes corresponding to type sections located in different parts of the study area (elevated, transitional and deep) were investigated. The elevated and transitional parts of the basin are adjacent to the weakened contact zones of a granite massif. These tectonic discontinuities acted as conductors for hydrothermal fluids circulation on the area. Lithological and stable isotope studies revealed the presence of intensive hydrothermal secondary alterations in various types in rocks, including massive metasomatic carbonatization, sulfatization, sulfide mineralization and leaching processes. This hydrothermal fluid activity resulted in reservoir formation within siliceous radiolarite layers, while in the deep part of the basin potential reservoirs were completely healed. Organic matter maturity varies in different parts of the basin. We identify several mechanisms related to the various hydrothermal processes that influenced rocks composition, structure and organic matter maturity: (1) A weakened zone located on the periphery of the granite massif in the basin basement is characterized by extensive fault damage zones facilitating the migration of high temperature fluids along deposits during post-sedimentation stages; (2) The thickness, mineral composition of the rocks overlying the basement control fault attenuation and fluid workflow; (3) The fluids composition, their acidity/alkalinity, prompted various changes in rock structure, especially in secondary porosity formation. The identification of various hydrothermal processes and their influence on pore space allowed to determine the approach for the spatial distribution prediction of unconventional reservoirs in the studied formations, and subsequently improve the efficiency and effectivity of the exploration process not only at the Kamennaya summit, but also on the other similar West Siberian basin areas.

Volcano tectonic setting of the Salares Norte Au–Ag epithermal belt, central Andes of northern Chile

The Salares Norte (SN) Au–Ag belt is here defined as an Upper Miocene-Pliocene metallogenic area, extending for nearly 200 km, located NE of the northern end of the Upper Oligocene-Lower Miocene Maricunga Belt in the central Andes of northern Chile. The belt is located just east of the main reverse NS fault systems which are part of the Sierra Castillo, Potrerillos Mine and Claudio Gay Cordillera faults, limited to the north and south by the NW-striking oblique to the arc Imilac and Wheelwright-Incahuasi regional faults systems.The epithermal deposits within the SN belt are intimately associated with the occurrence of Middle Miocene volcaniclastic basins and Middle to Upper Miocene phreatomagmatic vents and their related high-sulfidation hydrothermal systems. The volcanostratigraphy of the SN epithermal belt comprises volcaniclastic sequences, subvolcanic and volcanic rocks ranging between the Lower Miocene and the Pliocene. High-angle NS reverse faults involving Paleozoic and lower Cenozoic units form the western-southwestern limit of the belt. Folded and faulted Middle Miocene volcaniclastic and volcanic sequences form the main host rock of at least 4 phreatomagmatic events that generated maar-diatreme fields and discrete maar vents, with their related subvolcanic facies, within the widespread Upper Cenozoic volcanic arc, ranging in age between ca. 15 and ca. 4 Ma. These four phreatomagmatic events are spatially and temporarily associated to 4 main high-sulfidation hydrothermal events which have altered both the phreatomagmatic rocks and the Cenozoic country rock of the explosive activity. These hydrothermal systems have formed dozens of discrete hydrothermally altered areas where argillic, advance argillic, silicification and steam-heated alteration are common. Most of the maar fields and the hydrothermal alteration systems are emplaced within a strike-slip tectonic regime with NE-trending shortening which is accommodated in the NW-striking sinistral-normal faults and NE-striking extension structures. These fault systems crosscutting interaction favor the hydrothermal fluid circulation and migration, as well as mineral precipitation on structural and/or primary lithological traps and on NE-striking tension cracks.In summary, the SN epithermal belt is an almost 200 km long and 60 km wide area, mainly NNE-oriented, where epithermal deposits have formed during the Upper Miocene and Pliocene (ca. 13-4 Ma), emplaced within Middle to Upper Miocene volcanic and volcaniclastic sequences associated to small volcanotectonic basins oriented in NS, NE-SW and NW-SE directions, in which most of the high-sulfidation systems are associated to discrete or repetitive structurally-controlled Upper Miocene-Pliocene phreatomagmatic vents and maar fields.

Shear zone-hosted hypogene high-grade iron deposit: The Cuité case study

The Cuité iron ore deposit consists of high-grade schistose lenses (>60 wt% Fe) hosted in a tectonic sliver of iron formation associated to mica schist (Serra da Serpentina Group). It is located at the southeastern border of the São Francisco Craton, enclosed by sheared Sthaterian granitoids (Borrachudos Suite), Neoarchean ortogneiss (Guanhães Complex), quartz and pegmatite veins. The iron formation encloses schistose to massive high-grade ore lenses, with a pervasively, continuous to anastomosed schistosity defined by millimetric elongated hematite plates and quartz grains enclosing hematite granoblastic domains. Kenomagnetite/maghemite and martite pseudomorphs (after magnetite) overgrow the foliation and intergrow with granoblastic hematite. Three high-grade iron ore types are identified, according to the fabric and mineralogy of the main iron oxides: lamellar-granular hematitic (LGO), magnetitic (MO), and granular hematitic ore (GO). Intrusive pegmatite bodies related to the swarms of the Eastern Brazilian Province (late Ediacaran to early Cambrian) and to post-collisional Cambrian vents of the Brasiliano Orogeny caused extensive hydrothermal alteration in the country rocks, encompassing recrystallization of the detrital zircon grains from the associated psammopelitic units, which yielded Cambrian ages, formation of kaolinite-rich lenses, and generation of granoblastic ore bodies. The association of intense stretching along foliation and circulation of hydrothermal fluids from pegmatite intrusion is responsible for the ore formation.

Sulfur isotopes in Archaean crustal reservoirs constrain the transport and deposition mechanisms of nickel-sulfides in komatiites

AbstractAssimilation and prolonged suspension of crust-derived sulfide liquid in komatiites are essential to form Ni-rich mineralisation. Evaluating the spatial relationship between komatiite-hosted Ni mineralisation and crustal S sources may thus provide insights into mechanisms of transport, metal enrichment and deposition of assimilated sulfide liquid. This study applied facies analysis and S isotopes to sulfides in Ni-mineralised komatiites and stratigraphically underlying bimodal volcanic-volcaniclastic and sedimentary rocks, which formed during rifting in the Agnew-Wiluna Greenstone Belt, Western Australia. The results revealed a lateral variation from rift-distal sedimentary sulfides, through sulfidic BIF, to rift-proximal VMS-style sulfides, the latter of which was predominantly assimilated by komatiites. Both crustal and komatiite-hosted sulfides were overprinted by granite-related skarn alteration during later basin inversion. Spatial S isotopes correlation revealed that Ni mineralisation in komatiites predominantly formed < 5 km from their crustal S sources, excluding long lateral transport as the main metal enrichment mechanism. Rather, metal enrichment likely happened through multiple cycles of sulfide entrapment and entrainment in lava flow vortices that formed in the wake of topographic steps represented by syn-rift faults. These faults were the main loci for pre-existing crustal weaknesses, hydrothermal fluid circulation, and VMS-style sulfide deposition, which were subsequently utilised by komatiites for enhanced thermo-mechanical erosion and crustal sulfide assimilation. This study shows that proximity to the syn-rift faults was the dominant control on the formation of komatiite-hosted Ni–sulfide mineralisation, regardless of substrate lithology. The S isotope signatures of crustal sulfides may be used as a proxy to identify syn-rift faults in highly deformed terranes.

Black shale LA-ICP-MS Rb-Sr and monazite SIMS U-Pb geochronology from the Cryogenian successions in the northern Yangtze Block

The late Cryogenian (possibly Marinoan-aged) sedimentary successions from the Shennongjia area in the northern margin of the Yangtze Block host important biological evidence for multicellular eukaryotes that have survived the Neoproterozoic glaciation. However, absolute constraints on the depositional age of these fossiliferous successions are lacking. Herein, we present direct dating of black shale and diagenetic monazite, which yield an Rb-Sr isochron age of 630 ± 27 Ma and a lower intercept U-Pb age of 626.4 ± 56.8 Ma from these fossiliferous successions. These dates are consistent with deposition during the Marinoan glaciation. A Marinoan-aged deposition is also demonstrated by the initial 87Sr/86Sr ratio (0.7084) of the ca. 630 Ma black shale sample, which is consistent with the initial 87Sr/86Sr ratio of contemporaneous seawater. In addition, petrographic observations coupled with REE distribution patterns and Th/Ce-Th and LREE-Y2O3 discrimination plots, facilitate identification of detrital and hydrothermal monazite. The former yields U-Pb dates that are older than the youngest detrital zircon peak at ca. 824 Ma and may have been recycled from igneous precursors. The later, in contrast, yields dates of 585.0 ± 95.1 Ma and 197.3 ± 31.2 Ma and likely records post-depositional hydrothermal events. Further evidence of an old hydrothermal event is recorded in the black shales, which yield Rb-Sr isochron dates of 554 ± 27 Ma and 545 ± 17 Ma that overlap the monazite date. Hydrothermal fluid circulation is associated with precipitation of minerals such as pyrite, sphalerite, and calcite, and potentially has genetic links with the large-scale Bingdongshan Pb-Zn ore deposit around the Shennongjia area, which has an Rb-Sr date of 571 ± 86 Ma.

Hydrogeochemical and geothermal features of thermal springs along the Ganzi-Yushu fault, eastern Tibetan Plateau and its geological implications

Important fault zone processes can be discerned from the hydrothermal fluid circulation associated with active seismic sources. This study focused on the geochemical and isotopic features of the thermal waters along the Ganzi-Yushu fault (GZYSF) located in the eastern Tibetan Plateau. 22 spring samples were collected to characterize the chemical compositions and calculate the geothermal reservoir temperatures. The hydrothermal waters are mainly recharged by meteoric water based on the δ18O and δ2H values. Immature waters with a Na + -HCO3- or Ca2+(Mg2+)-HCO3- composition were recognized in the study area, combined with the poor correlation between K+, Na+, Mg2+, HCO3− and Cl− suggesting a shallow hydrothermal fluid source. The 87Sr/86Sr ratios and high concentrations of B (from 20.3 to 9445 μg/L) and Li (from 3.04 to 2380 μg/L) show a dissolution of silicate minerals containing calcium and magnesium during fluid circulation. The geothermometry in the silicon-enthalpy system suggested equilibrium temperatures up to 146 °C, whereas most are low temperatures. The shallow geothermal fields along GZYSF show that most large earthquakes (M＞5) were distributed in the transitional zone between high-value area and low-value area of the shallow geothermal field, suggesting a potential genetic link between geothermal field structure and earthquake nucleation along active faults. In conclusion, we proposed a conceptual model to portray the fluid circulation process. Such work can provide insights into the potential geothermal resource in Tibetan Plateau, and also investigate the processes controlling the fluid chemistry and the correlation with occurrence of earthquakes in the region.

Development characteristic and main controlling factors of the Ordovician karst caves in the Keping area, Tarim Basin

This research endeavors to characterize the primary factors that influence the formation of Ordovician karst caves in the Keping area of China. A 3D digital model of the cave structure and fracture sets was generated using an Unmanned Aerial Vehicle (UAV). The characterization of fracture and cavity development involved the examination of thin sections, fluid inclusion testing, and the analysis of C and O isotopes. Key parameters controlling karst development were identified through the application of multiple linear regressions and statistical analysis. The Ordovician limestone karst cave exhibited four distinct fracture sets. Set 1 consisted of partially filled fractures with a sub-horizontal orientation and a striking direction of SEE, interpreted to have formed during the Middle-Late Caledonian orogeny. Set 2 comprised inclined tensile-shear fractures with a striking direction of NEE, likely formed during the Early Hercynian orogeny. Set 3 included fully filled conjugate shear fractures with variable orientations, which developed during the Indo-Yanshanian orogeny. Set 4 comprised high-angle shear fractures with striking directions of NNE 20–40° and NEE 60–80°, formed during the Himalayan orogeny. Two stages of cave filling deposition were identified. Stage I coincided with the Middle-Late Caledonian Set 1 fractures and can be attributed to the circulation of freshwater fluid. Stage II occurred concurrently with the Early Hercynian Set 2 fractures and can be attributed to deep hydrothermal fluid circulation. The karst caves are interconnected and aligned along a fault zone. The Ordovician limestone possesses high permeability, which facilitates karst development. The lithologies in the Aksu area play a crucial role in cavity formation and dissolution. The development of cavities is influenced by the combined patterns of the fracture system, with larger fault and fracture zones resulting in larger cave sizes. As one moves away from the fault zone, limestone dissolution decreases, resulting in less pronounced karst development.

Hydrothermal fluid circulation within the restless structural frame of Hasandağ volcanic system (Central Anatolia, Türkiye) inferred from Self-potential, CO2, and temperature measurements

Hasandağ is an active composite volcanic system located in the Central Anatolian Volcanic Province (CAVP). A comprehensive field survey was conducted to reveal the hydrothermal activity within the Hasandağ volcanic system, with particular emphasis on Self-Potential (SP) mapping. The Hasandağ system is characterised by the presence of twin stratovolcanoes that arise from pre-existing partially buried calderas. In conjunction with the SP measurements, carbon dioxide concentration (CO2) and surface temperature (ST) measurements were taken. The temperature measurements were elaborated with thermal infrared remote (TIR) sensing methods. The SP results, with a peak-to-peak magnitude of 3.8 Volts, revealed the presence of a significant hydrothermal input in both of the twin cones, characterised by a typical W-shaped anomaly commonly observed in active volcanoes. The highest SP signal coincides with the active fumarole zone at the western flank of G. Hasandağ summit, where we measured ∼70 °C vapour temperatures and relatively high CO2 output (>100,000 ppm). The SP/elevation (Ce) gradient assisted in revealing the hydrothermal and hydrogeological zonation, providing substantial evidence of the relationship between hydrothermal circulation and the structural frame of the volcano. Integrated evaluation of SP, CO2, and temperature data revealed the structural control on the hydrothermal and hydrogeological circulation, with the contribution of Hasandağ, Karacaören, and Karkın Faults as major structural components. The partially buried rim of the Hasandağ caldera was revealed through the detection of a ‘double Ce-low’ signal, providing valuable insights on the dimensions of the structure and its role in the hydrogeological cycle of the system. The extent and magnitude of the hydrothermal activity observed, the Holocene eruptive history of the volcano, and its proximity to populated areas denote the requirement of further geophysical and geochronological research and the necessity of hazard assessment and instrumental monitoring.

Temporal constraints on hydrothermal circulation associated with strike-slip faulting in the Permian Maokou carbonates, central Sichuan Basin (SW China)

Despite the ubiquity of hydrothermal fluid circulation in the crust, its timing is difficult to precisely determine using traditional thermometric dating due to strongly disturbed geothermal fields. We present a case study from the mid-Permian hydrocarbon-bearing dolostone reservoirs, situated proximally to strike-slip faults in the central Sichuan Basin (China), to highlight the utility of carbonate in-situ U-Pb geochronology in delineating the timing of hydrothermal fluid flow. Matrix and void-filling cement dolomite phases were identified in the porous Maokou Formation carbonates. Predominantly manifesting as saddle dolomites along fractures, these cements yielded statistically homogeneous U-Pb ages (242.5 ± 3.1 Ma, 238.9 ± 8.2 Ma, 244 ± 15 Ma, and 239.2 ± 6.5 Ma), slightly postdating the established stratigraphic timeline (ca. 273 to 259 Ma). The narrow temporal span suggests early emplacement following a short burial, compatible with the petrographic observation of burial stylolites crosscutting saddle dolomites. Integrating shallow emplacement depths inferred from a comparison between the dolomite U-Pb ages and a published burial history, with fluid-inclusion microthermometry, elucidates the hydrothermal system that was once present in this field. The pervasive occurrence of hydrofracturing, along with dolomite geochemical indicators represented by bell-shaped (REE + Y) PAAS patterns and elevated Y/Ho ratios, provide additional evidence for the hydrothermal fluid injection and resulting dolomitization. The dolomites' positive Eu anomalies and the parent fluids’ 18O enrichment indicate a source from deeper basinal pore waters that have interacted significantly with ambient rocks. The overlying Triassic evaporites may have provided Mg-rich brines, mixing into the circulating hot fluids responsible for hydrothermal dolomitization. Our dolomite U-Pb ages coincide with the seismically estimated timing of transtensional strike-slip fault activation. This temporal correspondence, combined with spatial associations and lateral slickensides along fracture planes, suggests that the hydrothermal fluid movement is highly relevant to the active strike-slip faults. This observation reveals a previously unrecognized early-middle Triassic, structurally controlled hydrothermal activity in the region. We propose that this episode of hydrothermal circulation was driven by extensional tectonism in response to the spreading of the Proto-Tethys Ocean. This finding is not in line with the earlier research that considered late Permian Emeishan volcanism to have been the main trigger for hydrothermal fluid influx. The long-standing view regarding hydrothermal dolomitization in this region may need to be re-evaluated.

Coseismic Frictional Heating With Concomitant Hydrothermal Fluid Circulation Revealed by Rock Magnetic Properties of Fault Rocks From the Rupture of the 2008 Wenchuan Earthquake, China

AbstractCoseismic frictional heating and associated hydrothermal fluid circulation play an essential role in the dynamic weakening of seismic faults. However, temperature rise induced by frictional heating during an earthquake is still difficult to constrain. Magnetic properties of fault rocks convey abundant information on faulting processes. In this study, detailed rock magnetic measurements in combination with electron microscopic observations are conducted on fault rocks and protoliths from the Shaba outcrop (Beichuan County) on the Yingxiu‐Beichuan Fault ruptured during the 2008 Wenchuan Mw 7.9 earthquake. Results show that protoliths and the majority of fault rocks are dominated by paramagnetic pyrite and/or Fe‐bearing clay minerals; in contrast, the presence of pyrrhotite and goethite is confined to the fault gouges just next to the principal slip surface. Pyrrhotite is a product of pyrite alteration at high temperatures (>500°C) induced by seismic frictional heating during earthquake slip. Meanwhile, the goethite implies the presence of coseismic hot fluids within the fault zone. All these observations strongly suggest the occurrence of thermal pressurization as a plausible mechanism of coseismic fault weakening during the Wenchuan earthquake.

Extremely deuterium depleted methane revealed in high-temperature volcanic gases

Active volcanoes often discharge hot (T ≫ 100 °C) magmatic gases whose original composition has been modified through partial interaction with an externally fed hydrothermal system. The study of methane (CH4) in these volcanic discharges may provide useful information on the interplay between deep magmatic gases and shallow circulation of hydrothermal fluids. However, the origin of CH4 in high-temperature volcanic gases and the factors exerting control on its abundance and stable isotope composition are still largely unknown. Here, we present the abundances and stable isotopic composition of CH4 in hot (99–387 °C) volcanic gases from the La Fossa volcanic crater of Vulcano Island (Southern Italy).Our investigation revealed low (<1.5 μmol/mol) CH4 concentrations and an extraordinarily large variability in CH4 stable isotopic composition, with δ13C and δ2H values being positively correlated and varying from −35 to −9.2 ‰ and −670 to −102 ‰, respectively. Notably, CH4 isotopes measured at Vulcano almost encompasses the global-scale variability observed in natural fluids, with δ2H values ≤ −500 ‰ being the first ever reported in nature. Gases showing extremely negative δ13C-CH4 and δ2H-CH4 values systematically display higher CH4 abundances.We propose two possible scenarios in order to explain the observed huge variation in δ13C and δ2H: (1) mixing of 13C- and 2H-depleted CH4 with 13C- and 2H-enriched CH4 of thermogenic origin formed under hydrothermal conditions; (2) post-genetic removal and isotopic alteration of 13C- and 2H-depleted CH4 occurring during the ascent of volcanic gases. Comparing our dataset with available isotopic data from naturally occurring and artificially produced CH4, a thermogenic origin for the isotopically light CH4 seems unlikely. We postulate that the 13C- and 2H-depleted CH4 may have formed via kinetically-controlled abiotic synthesis through CO (or CO2) hydrogenation reactions in the hot ascending gas phase, possibly at temperatures intermediate between those typical of magmatic and hydrothermal conditions. Further investigations of methane in high-temperature volcanic gases are necessary to test this hypothesis.

Diffuse soil CO2 emissions at rift volcanoes: Structural controls and total budget of the Olkaria Volcanic Complex (Kenya) case study

Continental rift systems are first-order emitters of deep-sourced CO2 but their impact on the global budget is largely unknown. For instance, the estimate of the total amount of deep-sourced CO2 released by the East African Rift (EAR) is largely undocumented due to the small number of direct measurements available and the impracticability of remote sensing methods induced by atmospheric noise. This study explores the degassing budget and the structural control of soil CO2 emissions at an active rift volcano aiming to implement the database of available direct measurements of EAR's volcanoes. We investigated soil CO2 degassing at the Olkaria Volcanic Complex, a large, multicentred, geothermally exploited volcanic complex in the southern portion of the Kenyan Rift. A total of 1158 soil CO2 flux measurements were collected on fumarolic fields or crosscutting unaltered major faults. The contribution of multiple sources of CO2 (biogenic background vs magmatic/hydrothermal) was tested using both graphical statistical analysis and the carbon isotopic signature (13C-CO2). Soil CO2 fluxes reached up to ∼5500 gm-2d-1. The emission of deep-sourced CO2 coupled with the structurally controlled hydrothermal fluid circulation, sums up to ∼280 tCO2 released per day by the entire complex. Finally, we provide an estimate of the total budget for soil CO2 of the rift, summing up the contributions of all volcanoes in the EAR.

Estudios de tomografía de ruido sísmico para la exploración geotérmica en Canarias

Ambient Noise Tomography is a geophysical exploration technique that has proven to be highly efficient for studies at different scales and for multiple purposes, such as geothermal exploration. In this article, we introduce this technique by reviewing its various steps. Additionally, we present some examples of applications from studies conducted in the Canary Islands (sp ecifically in Tenerife, Gran Canaria, and La Palma) for geothermal exploration purposes.The study realized in Gran Canaria reveals a series of low-velocity zones in the southern and eastern parts of the island, which could be linked to convective cells. In Tenerife, a low-velocity zone has been observed, potentially associated with a superficial clay cap that could facilitate the ascent of gases to the surface. Finally, the study carried out in La Palma highlights the existence of two low-velocity zones in the southern part of the island, possibly related to hydrothermally altered clay zones, indicating a circulation of hydrothermal fluids.

Litho-bio-stratigraphy model and palaeoenvironments of the Babouri-Figuil Basin (NE Cameroon): A lacustrine/fluvial/deltaic setting associated to the Benue Trough (Nigeria)

This paper focuses on the lithostratigraphy and the palaeoenvironmental evolutions of the Babouri-Figuil Basin depositional system. Facies analysis, palynology and organic geochemistry were investigated to propose a 2D sedimentary model of the basin. The study reveals that, this basin is made up of four main facies (sandstones, siltstone, claystone/shale and marls) resting on the granite-gneissic basement. The general trend of the beds is E-W; they dip to the S (SW or SE) and NE, with angles ranging between 4 and 30°. The sedimentary analysis indicates an arid to semi-arid climate. The pollen assemblage strongly suggests a Cenomanian-Turonian age for the sedimentary interval which contains the black shale formations. The presence of the mixture of marine and terrestrial organic matter (type II, I and III) in the same interval indicates a possible marine shallow-water deposit in the basin. The deposits were also influenced by volcanic events and hydrothermal fluid circulation during diagenesis, which are the source of carbonate enrichment in the sediments. The analysis of the sedimentary bodies, the encountered palynology assemblage and the 2D reconstruction show that the sedimentary formations of the Babouri-Figuil Basin were deposited in a lacustrine/fluvial/deltaic sedimentary environment.

Characterizing the supra- and subsolidus processes that generated the Current PGE-Cu-Ni deposit, Thunder Bay North Intrusive Complex, Canada: insights from trace elements and multiple S isotopes of sulfides.

The online version contains supplementary material available at 10.1007/s00126-023-01193-9.

High heat production of the Late Mesozoic Luanchuan granites from East Qinling Orogen, China: Implications for petrogenesis and mineral exploration

Some of the high heat production granites in the continental crust have been suggested to be linked with I‐, A‐, and partly S‐type granitoids which aided in the formation of associated polymetallic deposits. Here, we investigate the Shibaogou and Laojunshan plutons in the Luanchuan region, East Qinling Orogen, China, and integrate these with published geochemical information to compute the rate of heat production per unit rock mass (RHP value, unit μW/m3). In addition, we also present RHP contour maps of all the Late Mesozoic granitic plutons in the Luanchuan region, with a view to evaluate the link among the high heat production granites, their petrogenesis and mineralization. The RHP values of granitoids in the Luanchuan region vary from 1.7 to 14.44, with the Nannihu pluton showing the highest RHP peak value of 6.04, whereas the Huoshenmiao pluton has the lowest RHP peak value of 2.59. The RHP values have a positive correlation with the degree of magma differentiation, as suggested that the Nannihu pluton is a highly fractionated I‐type granite and the Huoshenmiao pluton is a quartz diorite. This is also consistent with U, Th and K which are incompatible elements that tend to enter the melt during magma evolution. Furthermore, the higher RHP value intrusions (e.g., Nannihu, Daping) show close genetic links with high economic deposits, whilst the plutons with lower RHP values are generally related to small ore deposits. We envisage that the high heat production granites underwent protracted cooling process, which promote the convection and circulation of hydrothermal fluids, so that ore‐forming materials (e.g., Mo, W) can sufficiently accumulate and precipitate as economic mineralization. This also indicates that these granitoids with relatively higher RHP values have a greater metallogenic potential. The RHP contour mapping results show high RHP value domains ranging from 2.68 to 4.82 in the southeast and northeast segments of the Shibaogou pluton, whereas the RHP values of other regions in this pluton vary from 2.14 to 2.68. In contrast, the high RHP value regions mostly occur within the western and eastern parts of the Laojunshan pluton, and range from 4.91 to 6.21. Other regions in the Laojunshan pluton have low RHP values in the range of 2.69–4.91. A few Mo‐W deposits are mostly hosted in the southeastern part of the Shibaogou pluton, implying that these regions with high RHP values might have higher metallogenic potential. The contour mapping of RHP maximum, minimum, average and peak values of all the Mesozoic granitic plutons in the Luanchuan region also show high value zones within and/or at peripheral regions of the Nannihu ore fields and these segments between the Nannihu and Yuku ore fields, which suggest that these zones likely have high potential for Mo‐W exploration.

Mineral chemistry and in-situ U-Pb geochronology of altered pegmatite from the vicinity of the Strashimir Pb-Zn vein deposit

The current study presents new mineralogical, geochemical and geochronological data for a pegmatite body hosted in gneisses and marbles from the vicinity of the Strashimir Pb-Zn vein deposit, Madan ore district, South Bulgaria. The mineral composition of the studied pegmatite is represented by oligoclase–andesine (An10.1–33.2) and albite (An0–7.6), which prevail over K-feldspar (Or87.9–92.4), and quartz. The established accessory minerals are allanite-(Ce), titanite, apatite and zircon. The pegmatite/marble contact is affected by later hydrothermal silicate-carbonate alteration without detected ore mineralization, despite the spatial proximity with the Strashimir Pb-Zn vein deposit. Epidote-group minerals in pegmatite are defined as members of the clinozoisite–epidote series. As a major constituent of the hydrothermal alteration zone, they are manifested in two well-distinguished generations along with chlorite, quartz and carbonates. The calculated temperature of chlorite mineralization yields T° of crystallization in the range of 223–266 °C. As a result of the hydrothermal fluid circulation, the accessory allanite-(Ce) is transformed to REE-rich epidote-clinozoisite, marked by depletion of REE and Fe and enrichment of Si, Al, and Ca. Due to the limited mobility of REE in fluids, after leaching these elements are incorporated in nearby crystallized epidotes. According to the occurrence, mineral association and chemical properties, three titanite populations are distinguished in the pegmatite. Two of them were in-situ dated by the LA-ICP-MS U-Pb method and reveal overlapping ages of 39.9±2.1 Ma and 39.5±2.2 Ma (39.3±1.2 Ma combining all titanite analyses). The ages are interpreted as titanite growth in a pegmatite body related to granitic melts in the Late Alpine high-metamorphic units (Madan Unit) of the Central Rhodopes. Hydrothermal fluids either did not affect the U-Pb isotope system of the titanites or were derived from the same fluid-rich melt.