Hydrothermal Fluid Circulation Research Articles

Geochemical constraints on reaction temperature and pressure and heat- and mass-transfer efficiency at Rainbow Hydrothermal Field

Seafloor vent fluids hosted by oceanic core complexes (OCCs) are thought to represent circulation of seawater-derived hydrothermal fluid along deeply penetrating, low-angle detachment faults. However, estimation of the source temperatures and pressures of such fluids has been limited because geochemical methods typically require vent fluid silica concentrations to be buffered by quartz, a condition not often met at oceanic core complexes. Here, we extend the calculation of the Si-Cl geothermobarometer to enable predictions of fluid Si concentrations in equilibrium with any Si-buffering mineral assemblage, rather than being restricted to quartz. We apply this method to Rainbow Hydrothermal Field vent fluid compositions and find that they are consistent with buffering by a plagioclase+talc+chlorite+tremolite mineral assemblage at conditions ranging from (430 °C, 359 bar) to (470 °C, 468 bar), which correspond to depths of 1.3–2.4 km below the seafloor. These estimates agree well with the locations of seismically imaged magma chambers within the Rainbow Massif. Additionally, calculations of fluxibility and Fe solubility support this relatively shallow origin for Rainbow vent fluids and imply relatively efficient heat and Fe extraction from the seafloor. We estimate that only 24–30 % of the heat content and almost no Fe is lost during upflow of Rainbow vent fluids.Compared to other OCC-hosted seafloor vents, the source region of Rainbow vent fluids is anomalously shallow, an observation consistent with geological interpretations of the Rainbow Massif. Vent fluids at Rainbow Hydrothermal Field have exhibited apparently stable and greater-than-seawater salinity for over two decades. We interpret these vent fluids as vapors derived from a higher-salinity source fluid that developed over multiple cycles of magma injection and phase-separation inherent to the formation of oceanic crust along slow-spreading, non-volcanic segments of oceanic spreading centers. Alternatively, higher-salinity source fluids could be derived from mineral hydration reactions associated with serpentinization of ultramafic rocks. The occurrence of greater-than-seawater salinity vent fluids is thus predicted to be a common feature of OCC-hosted vent fields, as indicated by several known examples, including the TAG, Kairei, and Edmond vent fields.

Geology of the Tabakoto gold deposit, Kédougou-Kéniéba Inlier, West African Craton, Mali

The Tabakoto gold deposit is part of the highly endowed west-Malian gold belt, which hosts several world-class deposits. This deposit, located in the Paleoproterozoic Kédougou-Kéniéba Inlier (KKI) of the West African Craton (WAC), is contained in Birimian metasedimentary rocks of the Kofi series that are intruded by magmatic dykes. The metasedimentary rocks are characterized by a S0/1 foliation delineated by the alternation of metagreywacke and meta-argillite. This foliation is affected by upright folds with N-S trending axial planes marked by a S2 schistosity and cross-cut by conjugate steep-dipping dextral NE-SW and sinistral NW-SE trending brittle faults. These structures record regional-scale E-W shortening, first associated with crustal thickening and then with N-S stretching, evolving from ductile to brittle deformation. The presence of the pyrrhotite-loellingite assemblage and the presence of scheelite is consistent with temperatures of upper greenshist facies (~ 430°C). Gold mineralization is found in sheared smoky quartz-pyrite veins in subvertical shear zones localized along the S2 schistosity and in S-shaped quartz-carbonate veins located in the NE-SW and NW-SE trending faults. The latter veins are marked by sodic alteration followed by carbonate (i.e., dolomite-ankerite ± calcite ± siderite) and phyllic (i.e., chlorite-muscovite-sericite) alterations. In the quartz-pyrite veins, gold occurs as inclusions in pyrite, arsenopyrite, and pyrrhotite, whereas in quartz-carbonate shear veins, gold is present in fractures cross-cutting pyrite and arsenopyrite and at the contact between pyrite and arsenopyrite or arsenopyrite and pyrrhotite. These features indicate that gold mineralization in the Tabakoto deposit results from hydrothermal fluid circulation during the Eburnean orogeny under ductile to brittle conditions, similar to other orogenic-type deposits in the WAC.

Advanced exploration of rare metal mineralization through integrated remote sensing and geophysical analysis of structurally-controlled hydrothermal alterations

Fusing multi-source (remote sensing and geophysical) data and diverse approaches validation in targeting hydrothermal alteration and structural anomalies enhances the potential for accurately detecting and characterizing mineralization zones. Sentinel 2 data and ASTER were processed for lithological and hydrothermal alteration mapping in the rare metal-rich Umm Naggat area (Egypt). Different image processing techniques were implemented, including false color composites, minimum noise fraction, band rationing, band math, mineral indices, relative absorption band depth, and constrained energy minimization. The rare metal-bearing Umm Naggat younger granite (NYG) pluton was lithologically discriminated and intra-differentiated to mafic-rich biotite granites, mafic-poor alkali feldspar granites, and albitized granites. Extensive hydrothermal alterations, such as albitization, ferrugination, propylitization, argillization, and phyllitization, overprint the NYG pluton. Normalized standard deviation, automatic lineament extractions, and trend analysis highlighted the key structural directions (NW, NNW, NNE, and NE) and distinguished the NYG pluton as a moderate to high structural density zone. The high structural density and intensive alteration zones are spatially associated and more localized within the NYG pluton than the surrounding rocks. Spatial overlay analysis confirmed that the hydrothermal alterations and fluid circulation systems are structurally-controlled. Furthermore, the hydrothermal alteration mapping and structural analysis outcomes were verified by combining fieldwork, slab polishing, petrographic investigations, and mineral chemistry through semi-quantitative scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and quantitative electron probe microanalysis (EPMA) analysis. As a result, the hydrothermal genesis of rare metal-bearing minerals (Nb-rutile, Nb-ilmenite, and columbite) close to or incorporated within alteration minerals (chlorite, muscovite, and hematite) is confirmed from the alteration zones (propylitic, phyllic, and ferruginated). In addition, biotite muscovitization and chloritization significantly contribute to the secondary rare metal enrichment. The current study emphasizes the extensive distribution of secondary rare metal-bearing minerals within the entire NYG pluton (not only limited to the northern albitized granite as depicted by previous studies), which might shed light on these hydrothermally-altered younger granites as a new potential source for Nb and Ta in Egypt.

Noble gases in shocked igneous rocks from the 380 Ma-old Siljan impact structure (Sweden): A search for paleo-atmospheric signatures

Finding direct records of the ancient Earth's atmosphere is key to understand the evolution of the entire planet. Nevertheless, such records are scarce, and only a few geological samples with preserved paleo-atmospheric signatures are available today. In this study, we analyzed noble gases contained in shocked granitic and volcanic rocks collected across the 380 Ma-old Siljan impact structure (in Sweden) to investigate the potential of shocked quartz crystals with planar deformation features (PDFs), often decorated by fluid inclusions, to preserve atmospheric signatures.Our results reveal that most of the investigated samples show relative elemental abundances of noble gases falling between those of air and meteoric water values. Atmospheric gases are present in the samples but excesses in radiogenic, nucleogenic, and fissiogenic noble gas isotopes highlight interactions of fluids with crustal rocks during the impact-generated hydrothermal circulation of fluids after the impact. Such fluid circulation event(s) could also have remobilized older fluids originally present in fluid inclusions preserved in the mineral lattices. Furthermore, results obtained on the two pure quartz fractions highlight a stronger atmospheric signature correlated with the presence of PDFs. The atmospheric component detected in this study could represent either a paleo-atmospheric signature trapped shortly after the Siljan impact event, or it may result from modern atmospheric contamination. Future studies of noble gases contained within shocked quartz crystals from other impact structures will unveil if such samples present advantages compared to other paleo-atmospheric proxies.

Late Triassic to present-day tectono-thermal history of the coastal part of the Asturian basin: Implications for hydrocarbon exploration and for the North-Iberian margin evolution

The Asturian Basin, situated on the North Iberian margin, is a Permian-Mesozoic extensional basin developed over a Variscan Palaeozoic basement, subsequently inverted during the Cenozoic Alpine orogeny. The compilation of cartographic, structural, biostratigraphic and vitrinite reflectance data collected in a Late Triassic-Jurassic succession, excellently exposed along the Cantabrian Sea coast, has allowed us to gain insight into the Mesozoic and Cenozoic tectono-thermal history of this basin. The oldest recorded event occurred in Middle-Late Jurassic times and consisted of uplift, development of a large syncline, and normal faulting accompanied by circulation of hydrothermal fluids along the faults. The basin emersion caused an angular unconformity, together with a change in the sedimentary environment that passed from marine to continental. A second event, whose age was probably Late Jurassic-Early Cretaceous, was responsible for extensional reactivation of previous normal faults accompanied by a weaker fault-related hydrothermal episode. In Cenozoic (Eocene-Miocene) times, as a consequence of the Alpine orogeny some of the previous faults were reactivated as reverse and/or strike-slip faults, and folds developed. Here we analyse which structures were active during each event, their relationship with the hydrothermal fluids, the control they exerted on the hydrocarbon generation, and how the large-scale models proposed for this portion of the North Iberian margin fit the onshore field observations, in particular regarding the structural position of the Asturian Basin within the Iberian Plate Mesozoic rifts and the role of the Ventaniella Fault.

Sulfur and metal mobilization during the life cycle of an oceanic core complex: Implications for seafloor massive sulfide deposits formation at slow and ultra-slow spreading ridges

Seafloor massive sulfide (SMS) deposits at slow and ultra-slow spreading ridges are often spatially related to, or hosted in oceanic core complexes (OCCs). The specific oceanic crust architecture, magmatism, hydrothermal fluid circulation and lithologies at OCCs, however, imply different S and metal (e.g. Cu, Zn, Co, Ni) fluxes relative to well-structured oceanic crust at-fast spreading ridges and which are not yet fully constrained. The study of S and metal distribution in the ODP Hole 735B deep drill core from the Atlantis bank allows to understand these fluxes along detachment faults and to better constrain the source zone of S and metals for OCC-related SMS deposits. Significant depletion of S, Cu, Zn and Ni are observed within the upper 250 m of the drill core where intense deformation and hydrothermal fluid circulation occurred. During the complex tectono-magmatic-hydrothermal evolution of the Atlantis Bank, four important stages are recognized for S and metal mobilization: 1) magmatic stratification leading to a higher proportion of sulfide-rich and S, Cu, Zn and Co fertile oxide gabbros in the root zone of the Atlantis Bank detachment, 2) high temperature ductile deformation leading to magmatic sulfide reworking and onset of sulfide leaching with limited metal mobilization, 3) extensive sulfide leaching and metal mobilization during amphibolite to greenschist facies metasomatism and, 4) late stage secondary sulfide precipitation and S enrichment during low temperature fluid circulation. Mass balance calculations from the source zones of the Atlantis Bank detachment highlights that metal mobilization during hydrothermal alteration of gabbroic rocks along detachment faults can fully account for the formation of OCC-related SMS deposits at slow and ultraslow spreading ridges. The Atlantis Bank detachment system, however, is gabbroic-dominated and represent the magmatic end-member of OCCs and further work is necessary for understanding metal fluxes in ultramafic-dominated detachment systems.

Tidal triggering of seismic swarm associated with hydrothermal circulation at Blanco ridge transform fault zone, Northeast Pacific

Seismicity associated with hydrothermal systems (e.g., submarine volcanoes, mid-oceanic ridges, oceanic transform faults, etc.) share a complex relationship with the tidal forcing and induced fluid flow process under different tectonic settings. The hydrothermal circulation drives the deformation at the brittle-ductile transition zone within a permeable brittle crust. Although the tidal loading amplitudes are too small to generate a brittle deformation, the incremental pressure exerted by the tidal loading can modulate the flow of hydrothermal fluid circulation and trigger the critically stressed faults or fracture zones. We present a compelling case of tidal modulation in seismicity along the Blanco Ridge Transform Fault Zone (BRTFZ), in the northeast Pacific. The strong diurnal and fortnightly periodicity has been observed in the deeper seismic swarm (7–15 km), whereas the shallow seismic swarm (0–7 km) does not exhibit any such tidal periodicity. The dominance of diurnal and fortnightly periodicity in the deeper seismic swarm is explained by the high amplitude tidal cycles providing additional stress on the fluid circulation at the crust-mantle boundary. Moreover, our robust statistical correlation of seismicity with tidal stress and resonance destabilization model under rate-and-state friction formalism suggests that the fault segments are conditionally unstable and more sensitive to periodic tidal stress perturbation.

Evolution of fluid redox in a fault zone of the Pic de Port Vieux thrust in the Pyrenees Axial Zone (Spain)

Abstract. In mountain ranges, crustal-scale faults localize multiple episodes of deformation. It is therefore common to observe current or past geothermal systems along these structures. Understanding the fluid circulation channelized in fault zones is essential to characterize the thermochemical evolution of associated hydrothermal systems. We present a study of a palaeo-system of the Pic de Port Vieux thrust fault. This fault is a second-order thrust associated with the Gavarnie thrust in the Axial Zone of the Pyrenees. The study focused on phyllosilicates which permit the constraint of the evolution of temperature and redox of fluids at the scale of the fault system. Combined X-ray absorption near-edge structure (XANES) spectroscopy and electron probe microanalysis (EPMA) on synkinematic chlorite, closely linked to microstructural observations, were performed in both the core and damage zones of the fault zone. Regardless of the microstructural position, chlorite from the damage zone contains iron and magnesium (Fetotal / (Fetotal + Mg) about 0.4), with Fe3+ accounting for about 30 % of the total iron. Chlorite in the core zone is enriched in total iron, but individual Fe3+/Fetotal ratios range from 15 % to 40 %, depending on the microstructural position of the grain. Homogeneous temperature conditions about 280–290 °C have been obtained by chlorite thermometry. A scenario is proposed for the evolution of fluid–rock interaction conditions at the scale of the fault zone. It involves the circulation of a single hydrothermal fluid with homogeneous temperature but several redox properties. A highly reducing fluid evolves due to redox reactions involving progressive dissolution of hematite, accompanied by crystallization of Fe2+-rich and Fe3+-rich chlorite in the core zone. This study shows the importance of determining the redox state of iron in chlorite to calculate their temperature of formations and to consider the fluid evolution at the scale of a fault.

Integrated geochemical and microbiological assessments of Astroni lakes reveals Campi Flegrei unrest signatures

Astroni volcano in the Campi Flegrei caldera (southern Italy) is a 2 km wide, densely vegetated tuff ring and hosting several volcanic structures, including tuff cones, scoriae cones, lava domes, and three small lakes. Geochemical data of waters and dissolved gases from the lakes, coupled with microbiological analyses on lake water and sediments, were used to shed light on the possible relationship between the lakes and the hydrothermal fluid circulation system as suggested by previous geophysical surveys. Water chemistry was dominated by solutes, mainly Na+ and HCO3−, deriving from fluids and CO2-rich gases typically found in discharges located at the periphery of hydrothermal-volcanic systems. Lago Grande (LG) lake showed an anoxic hypolimnion with abundant non-atmospheric dissolved gases, consisting of biogenic CH4 and CO2, the latter having a twofold origin, biogenic and hydrothermal. The occurrence of anaerobic methanotrophs coupled with the lack of hydrogenotrophic methanogenic archaea along the whole vertical profile of LG suggested that CH4 was mostly produced from degradation of abundant terrestrial organic matter within the deep lake sediments, and then consumed during its diffusion through the lake. Notwithstanding, the output rate of CH4 from LG surface was anomalously high relative to those commonly measured in lakes. Carbon dioxide from the hydrothermal source and produced by CH4 oxidation was partially fixed in the lake via the acetyl-CoA pathway. Accordingly, the CO2 fluxes from the LG surface were relatively low, in the range of those measured in volcanic lakes dominated by biogenic CO2. The dependence of the chemistry of the Astroni lakes on inputs from the Campi Flegrei hydrothermal system, besides on biogeochemical processes, offers a possible explanation for the anomalous increase of the LG water level occurred in the last years, which was not consistent with the recorded local rainfall but likely caused by an increasing hydraulic pressure related to the enhanced hydrothermal activity recorded at Campi Flegrei in the last decades. According to this hypothesis, the future evolution of the current volcanic unrest may govern the fate of the lake water level with important implications for the functioning of the precious Astroni ecosystem.

Magnetite geochemistry as a proxy for metallogenic processes: A study on sulfide-mineralized mafic–ultramafic intrusions peripheral to the Kunene Complex in Angola and Namibia

AbstractTrace element concentrations in magnetite are dictated by the petrogenetic environment and by the physico-chemical conditions during magmatic, hydrothermal, or sedimentary processes. This makes magnetite chemistry a useful tool in the exploration of ore-forming processes. We describe magnetite compositions from Ni-Cu-(PGE)-sulfide mineralized rocks from seven mafic–ultramafic intrusions peripheral to the Mesoproterozoic AMCG (anorthosite-mangerite-charnockite-granite) suite of the Kunene Complex of Angola and Namibia to investigate metallogenic processes through the geochemical characterization of Fe-oxides, which were analyzed in-situ via Electron Probe Microanalysis (EPMA), and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). We identified magmatic magnetite, segregated from both a silicate liquid and an immiscible sulfide liquid. Elements like Cr, Co and V suggest that the sulfide-related magnetite segregated from a relatively primitive Fe-rich monosulfide solid solution (MSS). Secondary Cr-rich magnetite appears in intrusions with abundant chromite or Cr-spinel. Two types of hydrothermal magnetite were identified, related to the pervasive replacement of sulfides and a late-stage, low-T fluid circulation event. Magnetite replacing sulfides is associated with serpentinized ultramafic rocks and is preferentially observed in the intrusions with the highest base and precious metal tenors. The high concentration of Ni, Co, Cu, Pd, As and Sb in these grains is corroborated by the identification of micron-size PGE mineral inclusions. We infer that serpentinization during hydrothermal fluid circulation was accompanied by desulphurization of sulfides with metal remobilization and reconcentration to generate magnetite carrying Pd microinclusions. We suggest that the highly serpentinized ultramafic rocks in the Kunene Complex region may become a possible target for economic Ni-Cu-(PGE) mineralization.

Transtension, brittle-ductile shear zones and hydrothermal ore deposits: Towards quantitative structural and kinematic models

Transtension is a fundamental process for the development of hydrothermal ore deposits, since it allows the required extension and dilatancy for hydrothermal fluid circulation and resulting ore mineralization. Transtension operates at multiple scales and, therefore, is not only relevant for deposits linked with extensional tectonics but also for those related to contraction/transpression. In this context, brittle-ductile shear zones represent a first-order metallotect that control the emplacement of mineralization. The distribution of ore minerals is largely influenced by bulk strain, kinematics and strain fabrics of these mineralized structures. The modified strain triangle is proposed as a simple tool to characterize strain conditions of transtension-related ore deposits associated with brittle-ductile shear zones, providing valuable information for metallogenetic models. In addition, a structural classification of ore deposits is proposed, based on the discussion of key deposit types and case studies. The main advantage of this approach is that it mainly relies on quantitative structural data and, therefore, is extremely useful for exploration. Furthermore, this classification avoids a priori assumptions on the tectonic setting, which can subsequently be inferred based on further regional evidence.

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

Assimilation 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.