Hydrothermal Fluid Circulation Research Articles

Fluid migration recorded by fluid inclusions in crack-sealed quartz veins and sandstone host rock; Cameros Basin, Spain

The role of deep hydrothermal fluid circulation through fractures and their impact on the sandstone host rock is studied in an extensional sedimentary basin (Cameros Basin, Spain) affected by a post-extensional hydrothermal metamorphism. The quartzarenites of the Urbión Group constituted a hydrocarbon carrier affected by very low to low-grade hydrothermal metamorphism during Late Albian to Coniacian. This process generated abundant quartz veins and transformed the quartzarenites into quartzites. This study compares the microthermometry of the fluid inclusion asemblages (FIAs) in the veins and in the quartz grain overgrowths in the quartzites, in order to understand the behavior of the hydrothermal fluids through fractures and their effects in the host rock. Fluid inclusions in the quartz grain overgrowths contain liquid and vapor at room temperature and homogenize to the liquid (Th: 124–265 °C, H2O + NaCl system). Those of quartz veins present both liquid and vapor CO2 and an aqueous liquid phase (room temperature). Final homogenization is to the liquid (Th: 109–282 °C, H2O + NaCl + CO2, mean values of amount-of-substance fractions: 0.92, 0.01, 0.07). Large Th variation within each FIA is common, due to crack and sealing processes and to reequilibration by successive thermal pulses. In contrast, the narrow Th range in each FIA towards the top of the record indicates that these inclusions are probably not reequilibrated. Two growing stages are recognized under SEM-CL in the quartz grain overgrowths, one diagenetic and another hydrothermal, the later with FIAs showing Th similar than the veins. The results can help in the evaluation of the geo-energy resources in sedimentary basins.

The role of magmatic fluids in the ~3.48 Ga Dresser Caldera, Pilbara Craton: New insights from the geochemical investigation of hydrothermal alteration

Hydrothermal fluids played a key role in establishing the environmental conditions in which ancient stromatolites grew within the North Pole Chert of the ~3.48 Ga Dresser Formation (Pilbara Craton, Western Australia). However, there has been uncertainty as to the physicochemical conditions of the hydrothermal system in relation to (i) the distribution of hydrothermal alteration, (ii) the relative contribution of seawater and/or magmatic volatiles to the hydrothermal fluids, and (iii) the origin of some of the major elements mobilized in the hydrothermal fluids.This study examines the hydrothermal alteration of the underlying North Star Basalt in order to better understand the nature of the circulating fluids and to determine the processes responsible for the transport and accumulation of metals and metalloids to the near surface environment. Detailed geological mapping reveals a complex distribution of alteration mineral assemblages that is controlled at all stratigraphic depths by the distance to the major fluid pathways that are now represented by hydrothermal silica veins. With increasing distance from the vein margins, alteration assemblages change from argillic (kaolinite–quartz) to phyllic (illite–quartz), and then to propylitic (chlorite–albite–epidote) and actinolitic (actinolite–albite–chlorite–epidote) at more distal positions. Illite Ar–Ar dating of argillic–altered basalt proximal to major hydrothermal veins immediately below the North Pole Chert confirms a syn–depositional hydrothermal origin of alteration, and demonstrates that the mineralogical and chemical features developed through the circulation of hydrothermal fluids were largely preserved after subsequent thermal overprints at 3.25, 3.06, and 2.29 Ga.The spatial distribution of the alteration mineral assemblages indicates that the fluids circulating in the hydrothermal system were highly acidic (pH < 3) for at least some time during the evolution of the Dresser Caldera. Such highly acidic fluid conditions were likely promoted by the input of magmatic volatile phases, such as HCl, SO2, H2S, and F. Bulk geochemical analyses of altered basalts reveal that large amount of metals, including Fe, Mg, Ni, and Zn, were leached from the North Star Basalt during hydrothermal alteration and delivered to the surface. Furthermore, our data indicate that K and Ba were introduced into the hydrothermal system from external reservoir(s). Although the contribution of K–rich seawater cannot be completely discounted, we argue that the bulk of K and Ba was sourced from an underlying magma chamber undergoing fractional crystallization of a melt with TTG–like composition.

The significance of galena Pb model ages and the formation of large Pb-Zn sedimentary deposits

In an attempt to clarify the significance of Pb model ages in Pb-Zn sedimentary deposits, we report high-precision Pb isotopic compositions for 64 galenas and 52 K-feldspars, the former from ores and the latter separated from granites. All samples are from Spain and the French Pyrenees. Lead from galena ores is of unequivocal continental origin. With few exceptions, Pb model ages systematically exceed emplacement ages by up to 400 Ma, a gap which is well outside the uncertainties of ~ 30 Ma assigned to the model. The histogram of the new high-precision Pb isotope data shows prominent peaks of galena Pb model ages at 94 ± 38 Ma and 392 ± 39 Ma. When the data are consolidated with literature data and examined in 3-dimensional Pb isotope space, cluster analysis identifies five groups. The model ages of the peaks occur, in order of decreasing peak intensity, at 395 ± 40 (Middle Devonian), 90 ± 34 Ma (Middle Cretaceous), and 613 ± 42 Ma (Neoproterozoic), with two minor peaks at 185 + 26 Ma (Jurassic) and 313 ± 41 (Upper Carboniferous). To a large extent, the model ages centered around these peaks correspond to distinct localities. The ages of the peaks do not coincide with any of the Betic, Variscan, or Pan-African tectonic events, which are the main tectonic episodes that shaped Iberian geology, but instead match well-known global oceanic anoxic events. It is argued that surges of metals weathered from continental surfaces scorched during anoxic events accumulated and combined in anoxic water masses with unoxidized marine sulfide released by submarine hydrothermal activity to precipitate the primary Pb-Zn stock. Frozen Pb isotope compositions require that galenas from black shales are the source of the final ores. The sulfides were later remobilized by large-scale convective circulation of basinal and hydrothermal fluids. The peaks of K-feldspar Pb model ages are distinct from those of galenas and do not correlate with magmatic emplacement ages. It is suggested that they instead reflect local circulation in Paleozoic sediments surrounding individual plutons. While Pb isotopes can be used as a regional provenance tool, such an approach requires that the data are considered in a fully 3-dimensional space.

Active and fossil hydrothermal zones of the Apacheta volcano: Insights for the Cerro Pabellón hidden geothermal system (Northern Chile)

Sulphate, clay, silica and oxide-hydroxide minerals were identified by means of fieldwork, XRD and SEM, both in the active and fossil zones of the Cerro Apacheta volcano (Cerro Pabellón geothermal field, northern Chile). Mineral assemblages and their spatial distribution in the active zone allow defining Superficial advanced argillic alteration facies associated with Argillic alteration facies, consistent with an acid-sulphate steam-heated environment. Poor crystallized kaolinite and cristobalite (opal-C), alunite in the steaming ground, alunogen and Mg-Zn-Cu-rich smectite in the fumarolic deposits represent the main mineralogical associations of this zone. The same two facies are defined in the fossil zone, interpreted as related to a superficial epithermal environment, with well crystallized kaolinite, alunite-natroalunite, pyrophyllite, mixed opal-A-opal C/T-quartz and Mg-Zn-Cu-rich Illite/Smectite (I/S). The presence and distribution of Mg-Zn-Cu rich montmorillonites and beidellites in the active and fossil zones are used as indicative of the hot mineralized vector fluids. In the active zone they would be interpreted alternatively as the result of an in situ argillic alteration, overlapping steam-heated typical assemblages and mineral associations formed in the fumarolic zones, or having an “allochthonous” origin, being formed at depth within the superficial part of the clay cap and subsequently transported to the surface during phreatic eruptions or fluidized events. The distribution of the hydrothermal alterations is likely related to the intersection of NW-SE and NE-SW structures, linked to the Pabelloncito graben, a primary feature in the control of the circulation of hydrothermal fluids towards the surface.

Palaeoproterozoic foreland fold-thrust belt structures and lateral faults in the West Troms Basement Complex, northern Norway, and their relation to inverted metasedimentary sequences

Palaeoproterozoic fold-thrust belt structures and steep, lateral shear zones characterize the foreland deformation of Neoarchaean basement tonalites in Vanna, West Troms Basement Complex, northern Norway. Low-grade par-autochthonous and allochthonous cover units (2.4–2.2. Ga) with sandstones and calcareous metapelites exist in separate areas of the foreland. They were formed as intracontinental rift- and/or deltaic shelf deposits, and subsequently intruded by a diorite sill at c. 2.2 Ga. The basement and cover units were folded and inverted along low-angle thrusts and steep reverse faults during two late/post Svecofennian (1.77–1.63 Ga) orthogonal shortening events (D1-D2). The D1 event involved NE-SW shortening, folding, ENE-directed thrusting, and dextral lateral shearing, controlled by pre-existing, N-S striking mafic dykes (c. 2.4 Ga) and basin-bounding normal faults. The D2 event involved SE vergent nappe translation, flat-ramp thrust propagation in a frontal duplex above a basement-seated detachment, and sinistral lateral reactivation in a partitioned orogen-parallel, transpressive setting. Hydrothermal fluid circulation affected all the shear zones. New aeromagnetic data show the basement-involved fold-thrust belt architecture well. The orthogonal Vanna Island fold-thrust belt styles of deformation resemble other inverted rift-basin deposits in northern Fennoscandia, deformed during the Svecofennian Orogeny (1.92–1.79 Ga), Alta-Kautokeino and Karasjok greenstone belts in northern Norway, Central Lapland, Peräpohja, Kittilä and Kuusamo belts of Finland, and in the Norrbotten province of Sweden. Westward younging of the orogenic events explain the younger age span of deformation on Vanna Island.

The extensive hydrocarbon-mediated fixation of hydrothermal gold in the Witwatersrand Basin, South Africa

There is a close spatial relation between high-grade gold mineralization in the Witwatersrand basin and carbonaceous nodules, veins and seams. Hydrocarbons thus may well have been essential in ore genesis. We have sampled four major gold-, uranium- and hydrocarbon-bearing ore horizons, namely the Carbon Leader, Vaal, B and Black reefs, to determine the role of hydrocarbons in the accumulation and hydrothermal fixation of gold. Our multipronged approach included high-resolution scanning and transmission electron microscopy (SEM, TEM), nanotomography with video clips, and geochemical modeling. Post-depositional hydrothermal activity at the peak of regional metamorphism produced an assemblage of quartz, phyllosilicates, brannerite, crandallite, florencite, monazite and gold in all four reefs. The gold, hydrocarbons and associated mineral assemblages are closely related on the micro to nano scale. Gold deposition occurred in interstices, as fracture fillings in detrital minerals, and on the surface of migrated solid hydrocarbon residues. The spherical to elliptical inclusions in the gold consist of an outer pyrobitumen phase and a central void space, partially associated with nanometric gold, uraninite, coffinite and silica. The hydrocarbon-bearing inclusion likely formed by the entrapment of a fossil liquid oil precursor during gold precipitation. The oil was subsequently thermally altered and converted into the final pyrobitumen and gaseous residues. Geochemical calculations to simulate the interaction of an invading hot hydrothermal fluid with the hydrocarbons in the reefs reveal that a very small amount of hydrocarbons will drastically decrease the aqueous solubility of gold and hence cause its instant precipitation.We extend our genetic model for the epigenetic formation of gold in the Witwatersrand. Regional metamorphism promoted the extensive and likely basin-wide circulation of hydrothermal fluids; these were capable of mobilizing substantial amounts of gold. The liquid, gaseous and solid hydrocarbons in the reefs acted as efficient chemical traps for the concentration of gold. Being strong chemical reductants, they caused the rapid precipitation and accumulation of gold on the surface of the fossil oil droplets and already solidified hydrocarbons. The release of the gases from accessible hydrocarbons into the sediments away from their source buffered the redox state of the hydrothermal solutions even at a considerable distance from the pyrobitumen seams and veins, likely resulting in the deposition of gold in the absence of visible hydrocarbons. Although our findings do not explain the ultimate origin and exceptional endowment of gold in the Witwatersrand, we do provide intriguing evidence for the large-scale hydrothermal mobilization, accumulation and fixation of gold mediated by hydrocarbons during post-depositional metamorphism.

Filling Provenance in Fracture Cavity Formation within Aksu Area, Tarim Basin, NW China: Indicators from Major and Trace Element, Carbon-Oxygen, and Strontium Isotope Compositions

Abstract With an aim to increase the understanding about sedimentary environment and isotopic and chemical characteristics of fillings in fracture cavities with multiple compositions, we conducted scanning electron microscope (SEM), fluid inclusion testing (FIT), common and trace element chemistry, full analysis testing, isotopic compositions (δ13C, δ18O, 87Sr/86Sr), and apatite fission track testing to study the formation environment of Aksu area, Tarim Basin. According to outfield and microscope observations, combined with SEM results, three textural and compositional type fractures and cavities were distinguished. Through fine analysis of geochemistry characteristics on fractures, cavities, multiple filling periods, and environments were interpreted. Constrained by rare earth element (REE) pattern diagram, relationships between carbon and oxygen isotopes, strontium isotope, the compositional patterns, and generation environment of the fracture and cavities were determined. The results show that (1) cavity, fracture filling, and wall rock primarily consist of calcite, with a proportion of 56.85%, 80.48%, and 81.00%, respectively. (2) Four fracture sets have been distinguished in the Ordovician limestone of the karst cave, Middle-Late Caledonian (Set 1), Early Hercynian (Set 2), Indo-Yanshanian (Set 3), and Himalayan orogeny (Set 4). Two stages of cave filling deposition are distinguished. Stage I was coeval with the Middle-Late Caledonian Set 1 fractures and is attributable to the circulation of freshwater fluid. Stage II was coeval with the Early Hercynian Set 2 fractures and is attributable to deep hydrothermal fluid circulation. (3) Cavity, fracture filling, and wall rock in Ordovician strata are slightly influenced by diagenesis alteration and territorial supply. Three significant filling stages were distinguished, freshwater fluid with strong oxidizing environment (Middle-Late Caledonian), hydrothermal fluid with authigenic abnormal enrichment (indicating obvious hypoxic sedimentary water, Early Hercynian), and high-temperature hydrothermal fluid from deep earth (primarily influenced by magmatism, Indo-Yanshanian, and Himalayan).

Preface to “Understanding volcanic processes through geophysical and volcanological data investigations: some case studies from Italian sites (EGU2019 GMPV5.11 session, COV10 S01.11 session)”

Abstract. Volcanic dynamics is driven by the complex interplay between fluid flow (circulation of magmatic and/or hydrothermal fluids) and rock structure (volcano conduits, dykes), the comprehension of which requires both multi-parametric monitoring and modelling of relevant physical and chemical processes of the system. Understanding the factors controlling the dynamics of the processes involved in these interactions is necessary to characterize the overall behaviour of a volcano and the eventual transition mechanisms among stationarity, unrest phases and eruptive styles. The starting point in this context is to have high-quality data of several parameters (seismological, geochemical, geodetic, volcanological), acquired both over years of monitoring activity and focused field experiments. Fundamental contributions come from the use of combined multi-parametric datasets and the adoption of innovative analysis techniques and multi-disciplinary approaches. This Special Issue is addressed to those researchers, who focus their investigations in the field of volcano dynamics. Its main purpose is to shed light on the processes occurring in active volcanic systems over different time scales, with relevant implications for the hazards and the modern monitoring, thus promoting future discussions on this topic. The Issue contains this introducing preface, which describes the Volume aims, and 14 papers, reflecting the main themes. The papers are devoted to the study of some Italian sites, but the proposed approaches are general and therefore applicable to any other volcanic/hydrothermal areas.

Rare element minerals\u2019 assemblage in El Quemado pegmatites (Argentina): insights for pegmatite melt evolution from gahnite, columbite-group minerals and tourmaline chemistry and implications for minerogenesis

The pegmatite district of El Quemado (NW Pampean Ranges, NW Argentina) hosts several Ordovician pegmatite bodies of the LCT (Li, Cs, Ta) type. We present paragenetic assemblages for a set of samples from two of the El Quemado pegmatite groups, Santa Elena and Tres Tetas, and mineral chemistry analyses for gahnite, columbite-group minerals, tourmaline, micas, albite, microcline, and discuss the relation between their major element composition and the degree of evolution of pegmatite melts. The chemical composition of rare element minerals allows recognizing an evolutive trend reaching highly differentiated compositions, with complex paragenetic assemblages including Li-, Zr-, U-, Zn-, P-, Mn- and Ta-bearing minerals. The temperature of crystallization during the magmatic phase was below 400 °C. Non-pervasive hydrothermal alteration, testified by a moderate presence of phyllosilicates, affected the pegmatite bodies. Chlorite geothermometry indicates that the circulation of post-magmatic hydrothermal fluids occurred at a temperature ranging between 200 °C and 250 °C. The mineralogical features recognized in the El Quemado pegmatite rocks have implications for the metallogenesis of the region, suggesting that the pegmatites potentially contributed to the genesis of Ta-Nb oxide placer mineralizations.

Mechanical zonation of rock properties and the development of fluid migration pathways: implications for enhanced geothermal systems in sedimentary-hosted geothermal reservoirs

Oil and gas operations in sedimentary basins have revealed the occurrence of significant temperature anomalies at depth, raising the possibility of major geothermal resource potential in the sedimentary sequences. The efficient development of such a resource may require enhancement by hydraulic stimulation. However, effective stimulation relies on an initial assessment of in situ mechanical properties and a model of the rock response. Here, we examine the distribution of mechanical properties (unconfined compressive strength, UCS; ultrasonic velocity-derived Poisson ratio, ν; and, scratch toughness, Ks) along the cored interval of a sedimentary formation with a known low-to-medium temperature geothermal anomaly in the Permian Basin, U.S. Our results reveal the presence of mechanical stratigraphy along the core, demonstrated by the alternation of distinct soft–hard (i.e.,less stiff-to-stiff) mechanical zone couplets composed of: (1) mechanically softer 0.17-m-thick Zone-A and 0.18-m-thick Zone-C with mean values of UCS = 110 MPa, ν = 0.25, Ks = 1.89 MPa·√m; and (2) mechanically harder 0.41-m-thick Zone-B and 0.15-m-thick Zone-D which show mean values of UCS = 166 MPa, ν = 0.22, and Ks = 2.87 MPa·√m. Although X-ray diffraction analyses of the samples suggest that the entire rock matrix is dominated by dolomite, the harder zones show an abundance of quartz cement (> 30%) and relatively lower phyllosilicate mineral content (< 2%) than the softer zones. Further, we observe that the mechanically harder zones have the greatest occurrences and thicknesses of hydrothermal alterations (anhydrite veins and nodules), indicating that the rock had experienced hydrothermal fluid circulation (basinal brines) in the past. We infer that the mechanical stratigraphy most likely influenced the spatial clustering of fractures that facilitated hydrothermal fluid migration in the past, and provides insight that is relevant for the exploitation of geothermal energy resources in sedimentary basins. We suggest that the harder zones or formation intervals with higher ratios of the hard zones relative to soft zones represent viable targets for hydraulic stimulation of a sedimentary-hosted geothermal reservoir, both for the emplacement of new fractures and the linkage of pre-existing fractures to allow efficient fluid circulation. Our findings in this study provide insight that is relevant for understanding the complexity of pre-existing mechanical heterogeneity in sedimentary-hosted geothermal reservoir targets in other places.

Multiple Hydrothermal Iron-Formation Upgrading Events in Southeastern São Francisco Craton

Large-scale, hypogene iron mineralization systems developed recurrently in the São Francisco craton in association with two orogenies. During the ca. 2.1–2.0 Ga Trans-Amazonian orogeny, low-temperature and low-to-moderate-salinity metamorphic fluids resulted in carbonatization-related iron enrichment of the early Paleoproterozoic Cauê Iron Formation (IF) of the Minas Supergroup (MSG) as well as short-distance iron mobilization. Monazite from the iron-oxide veins yielded a SHRIMP 232Th/208Pb date corresponding to fluid circulation coeval with the migmatization of the Archean tonalite-trondhjemite-granodiorite crust. During the Ediacaran to Cambrian Brasiliano orogeny, when the São Francisco craton was consolidated in its present configuration, hydrothermal fluids repeatedly mineralized the Cauê IF and the younger IFs of the lower Espinhaço Supergroup. Far-field hydrothermal alteration associated with distinct Fe mineralization episodes widely affected Meso- to Neoproterozoic sequences as well as Archean to Paleoproterozoic terranes of the cratonic core. Circulation of hydrothermal fluids during the Brasiliano orogeny caused desilicification, iron mobilization, and widespread alteration of the rocks. The alteration events are dated with monazite and xenotime intergrown with hematite, anatase, and recrystallized rims of detrital zircons from interlayered quartzites and of igneous zircons from pegmatites, granites, acidic volcanic rocks, and orthogneisses, yielding U-Pb dates between 542 and 493 Ma. U-Pb dates point to five Cambrian hydrothermal events from the waning stages to the aftermath of the Brasiliano orogeny. These hydrothermal fluid circulation events correspond to the emplacement ages of four granite suites and pegmatites that lasted from the final stages of the collision to the collapse of the Araçuaí-West Congo orogen. Episodic circulation of fluids continued for approximately 2 billion years after IF deposition on the eastern São Francisco craton with the formation of high-grade iron ore deposits and resetting of the IF trace-element inventory.

Deformation, Magmatism, and Sulfide Mineralization in the Archean Golden Mile Fault Zone, Kalgoorlie Gold Camp, Western Australia

AbstractThe Golden Mile fault zone is a key controlling structure to the estimated 75 Moz gold endowment of the Kalgoorlie gold camp in the Yilgarn craton of Western Australia. The earliest structures in the fault are F1 folds that developed during D1 recumbent-fold and thrust deformation (&amp;lt;2685 ± 4 Ma). These F1 folds are overprinted by a pervasive NW- to NNW-striking S2 cleavage related to sinistral shearing beginning with 2680 ± 3 Ma D2a sinistral strike-slip and culminating with ca. 2660 Ma D2c sinistral-reverse movement. The majority of deformation in the fault zone correlates to ca. 2675 Ma D2b deformation, which is characterized by sinistral-normal kinematic indicators. Late, ca. 2650–2640 Ma D3 dextral-reverse kinematic indicators overprint the earlier D2 structures. Pyrrhotite-chalcopyrite-pyrite-sphalerite-galena assemblages were emplaced throughout the D2 event within NE-trending D2a tensile fractures, NW- to NNW-striking D2b normal faults and associated breccias, and NW- to NNW-striking D2c low-angle veins, with the latter D2b and D2c structures correlating to the Fimiston and Oroya mineralization types, respectively. All D2a-, D2b-, and D2c-related sulfides in the Golden Mile fault zone show similarly restricted δ34S (~1.0–4.5‰) and elevated Δ33S (~2.0–3.0‰) values that reflect strong local sulfur contribution from shales of the Lower Black Flag Group and host-rock buffering of hydrothermal fluids related to the Fimiston and Oroya mineralization events. This host-rock buffering decreased fluid fO2, favoring the development of pyrrhotite-pyrite stable sulfide assemblages and causing respective decreases and increases in fluid Au-Te and Pb-Bi-Sb concentrations. At the camp scale, the Golden Mile fault zone exerted a primary control on the distribution of porphyry dikes and gold deposits; however, magma and hydrothermal fluid circulation was favored in adjacent, higher-order structural sites due to the fault zone’s incompetent rheology and tendency for ductile deformation and diffuse fluid flow. Other Archean examples such as Au deposits of the Larder Lake-Cadillac deformation zone in the Superior craton illustrate that this type of diffuse fluid flow in large-scale crustal fault zones can result in disseminated economic mineralization. However, this study highlights that host-rock effects on fluid chemistry in large-scale crustal fault zones exercises a strong control on a fluid’s propensity to form ore. The results of this study emphasize that both the rheology and chemistry of rocks within and adjacent to large-scale deformation zones act as important controls on the formation of gold ore in Archean terranes.

Rapid alteration of fractured volcanic conduits beneath Mt Unzen

The nature of sub-volcanic alteration is usually only observable after erosion and exhumation at old inactive volcanoes, via geochemical changes in hydrothermal fluids sampled at the surface, via relatively low-resolution geophysical methods or can be inferred from erupted products. These methods are spatially or temporally removed from the real subsurface and thus provide only indirect information. In contrast, the ICDP deep drilling of the Mt Unzen volcano subsurface affords a snapshot into the in situ interaction between the dacitic dykes that fed dome-forming eruptions and the sub-volcanic hydrothermal system, where the most recent lava dome eruption occurred between 1990 and 1995. Here, we analyse drill core samples from hole USDP-4, constraining their degree and type of alteration. We identify and characterize two clay alteration stages: (1) an unusual argillic alteration infill of fractured or partially dissolved plagioclase and hornblende phenocryst domains with kaolinite and Reichweite 1 illite (70)-smectite and (2) propylitic alteration of amphibole and biotite phenocrysts with the fracture-hosted precipitation of chlorite, sulfide and carbonate minerals. These observations imply that the early clay-forming fluid was acidic and probably had a magmatic component, which is indicated for the fluids related to the second chlorite-carbonate stage by our stable carbon and oxygen isotope data. The porosity in the dyke samples is dominantly fracture-hosted, and fracture-filling mineralization is common, suggesting that the dykes were fractured during magma transport, emplacement and cooling, and that subsequent permeable circulation of hydrothermal fluids led to pore clogging and potential partial sealing of the pore network on a timescale of ~ 9 years from cessation of the last eruption. These observations, in concert with evidence that intermediate, crystal-bearing magmas are susceptible to fracturing during ascent and emplacement, lead us to suggest that arc volcanoes enclosed in highly fractured country rock are susceptible to rapid hydrothermal circulation and alteration, with implications for the development of fluid flow, mineralization, stress regime and volcanic edifice structural stability. We explore these possibilities in the context of alteration at other similar volcanoes.

High temperature hydrothermal alteration and amphibole formation in Gakkel Ridge abyssal peridotites

Alteration mineral assemblages in abyssal peridotites offer insights into the temperature and pressure conditions during hydrothermal fluid circulation in the oceanic lithosphere. Abundant hydrothermal venting has been documented along the ultraslow spreading Gakkel Ridge and peridotites have been extensively sampled from the ridge, yet these peridotites have not been systematically evaluated for the nature and extent of their alteration. Here, we present an analysis of the alteration of 40 Gakkel peridotites from 19 dredges spanning ~600 km of the ridge within the Sparsely Magmatic and Eastern Volcanic Zones. We use a petrographic-based alteration scoring system to assign a rank to each sample of 1 (unaltered) to 5 (altered) in 0.5 step intervals, based on the alteration scale of Birner et al. (2016). Gakkel peridotites cover the full range of alteration, from essentially unaltered to completely serpentinized, but are generally moderately to highly altered (average score of 3.5). The extent of alteration is independent of peridotite lithology and is not systematically different between the two zones. Serpentine is present in all samples, magnetite occurs in all samples with alteration scores >2.5, and 27% of samples contain carbonate veins that cross-cut all other minerals. In 20% of samples, all with alteration scores >2.5, tremolite + talc occurs as rims around pyroxene, while chlorite occurs as haloes around spinel. A gabbro-veined lherzolite also contains pargasite replacing pyroxene, with later overprinting by tremolite-talc intergrowths.The abundance of tremolite, chlorite, and talc in Gakkel samples is much lower than reported for talc-tremolite-chlorite schists from oceanic core complexes, where fluid flow through the lower crust leads to open-system behavior. At Gakkel, our petrological observations suggest that alteration was nearer to closed-system conditions, which can be explained by the absence of crust along some sections of the ridge. Pseudosection modeling with Perple_X, which assumes closed-system behavior, indicates that pyroxene reacts to form tremolite + chlorite + talc at temperatures >500 °C, corresponding to fluid flow to depths >20 km for the Gakkel Ridge. The sample with pargasitic amphibole requires temperatures >750 °C to form. Overall, our results suggest that hydrothermal alteration of Gakkel peridotites, and more generally of the lithospheric mantle in regions that lack significant crust, results in different alteration mineral abundances and relationships than in regions where the mafic crust is involved in alteration.

Talc schist deposits from central Cameroon: Mineralogical and physico-chemical characterization

Talc schist deposits are reported in central Cameroon region, and different alteration zones were determined depending on differences related to the texture and color of the host rock. For the purpose of determining mineralogical and physicochemical characteristics of talc schist, a total of 45 samples from the sites studied were collected and analyzed using an array of physico-chemical methods.Talc schist samples are mainly composed of variable amount of talc associated with plagioclase and clay minerals. Three groups of mineral association were determined: i) talc with small amount of chlorite and traces of kaolinite, ii) talc with high amount of chlorite, iii) variable amount of talc with plagioclase and clay minerals. The talc schist samples have homogeneous grain-sizes and are classified as silty sand or sandy silt.The occurrence of Ni and Cr in the talc schists are consistent with the composition of the talc deposits formed in relation to ultramafic rocks. The genesis of talc is due to hydrothermal fluid circulation in ultramafic rocks, attested by the occurrence of S and P.

New Insights into Hydrothermal Fluid Circulation Affected by Regional Groundwater Flow in the Asal Rift, Republic of Djibouti

The Asal Rift hosts a lake located in a depression at 150 m below sea level, where recharge is influenced by regional groundwater flow interacting with the Ghoubbet Sea along the coast of Djibouti. This regional groundwater flow is believed to influence hydrothermal fluid circulation, which we aim to better understand in this study, having the objective of developing concepts for geothermal exploration in the area. To this end, magnetotelluric data acquired in the Asal Rift were processed and analyzed. 1D inversion models of electrical conductivity were interpolated for interpretation. These data were then used to build a 2D hydrogeological model, allowing multiphase flow and heat transfer simulations to be performed, considering the regional groundwater flow near the surface and the site topography, in order to confirm the preferred path of fluid flow. Geophysical data analysis indicates the presence of normal faults, notably the H fault, which may act as a conduit for the circulation of hydrothermal fluids and where the hanging wall can be a hydrogeological barrier within the hydrothermal system of the Asal Rift. The results from the 2D numerical flow and heat transfer modelling show the importance of groundwater flow responsible for thermal springs located at the periphery of Asal Lake. Reservoir temperature inferred by means of geothermometry ranging from 200 to 270 °C was shown to correspond to simulated temperature at potential reservoir depth. Moreover, simulated temperature between 600 and 1700 m depth is close to the temperature profile measured in the geothermal well Asal 6 of the area, with less than 20 °C difference. Simulations indicate that hydrothermal fluid circulation is likely influenced by the regional groundwater flow controlled by the topography and the major water bodies, the Ghoubbet Sea and Asal Lake, feeding buoyant fluids interacting with a deep magmatic source and where tectonic activity created normal faults offering a preferred path for fluid circulation.

Evidence of hydrothermal fluid circulation driving elemental mass redistribution in an active fault zone

Important fault zone processes can be discerned from the characterization of fracture damage and chemical transformations associated with active seismic sources. To characterize the 2010 M7.2 El Mayor-Cucapah rupture zone, continuous samples of fault core and 23 samples of damaged rock were collected perpendicular to strike of the Borrego fault. Samples were analyzed for clay mineralogy, bulk geochemistry, and bulk and grain density from which porosities and volumetric strains were derived. Prior to the Borrego fault forming, the tonalitic protolith, containing chlorite, epidote, and titanite, was subjected to temperatures of ∼330–340 °C during deuteric alteration. Rocks within the damage zone are partially pulverized and contain abundant cataclastic seams. Porosity and volumetric strain peak in zones 1.5 m–10.5 m from the core. Within these zones, losses in Ca and P mass, increases in Mg and Na mass, along with the conservation of Fe and Si mass are consistent with oxidizing acidic conditions at < 200 °C. Gains in LOI are attributed to increases in clay content. The above data support a model of Mg- and Na-rich oxidizing fluid circulation within the damage zone of the Borrego fault.

Tectono-Sedimentary Evolution of the Uranium Deposits of the DASA Graben, Northern Niger

This study presents the structural evolution of the N70° trending Paleozoic-Mesozoic DASA graben, which is considered a sub-basin of the Tim Mersoï basin, and is located in northern Niger. The DASA graben is a uranium-rich trough discovered in recent exploration surveys. A tectono-sedimentary analysis of the DASA graben was implemented with a combined use of satellite imagery, field observations, borehole data analysis and available literature. This graben was affected from the Carboniferous to the Early Cretaceous by two major tectonic periods. The first period was an uplifting stage, which prevailed during the Carboniferous-Permian times and the second, ranging from the Triassic to the Early Cretaceous, corresponds to a rifting episode. The particularity of the DASA graben is that the sediments contain very high uranium grades. Lithological and tectonic factors controlled the emplacement of the uranium mineralization in the graben. The successive fracturing phases that affected the DASA graben were associated with a greater circulation of hydrothermal fluids and would have favoured higher grades of uranium mineralization.

Integration of multiple sulfur isotopes with structural analysis unveils the evolution of ore fluids and source of sulfur at the Kanowna Belle Archean orogenic gold deposit, Yilgarn Craton, Western Australia

The Kanowna Belle deposit is a world-class Archean orogenic gold system that witnessed multiple fluid episodes over a protracted deformation history. The hydrothermal fluid circulation episodes at the Kanowna Belle deposit initiated with the precipitation of early gold-bearing carbonate-famatinite-pyrite-telluride-electrum veins (V1a). These early veins were subsequently folded during a NE-SW shortening event (D1bKB) that led to the development of sericite-chlorite-pyrite stringers (V1b) and foliation (S1KB) dated at c. 2658 ± 9 Ma (U-Pb, xenotime). D1bKB structures are overprinted by quartz-carbonate-sericite-pyrite-gold veins (V2) controlled by the reverse faulting formed as a result of N-S shortening during D2KB. A subsequent deformation event (D3aKB) is related to sinistral shearing produced under ENE-WSW shortening and associated with the development of the Troy lodes and deposition of quartz-pyrite-sericite-gold veins (V3a) dated at c. 2628 ± 9 Ma (U-Pb, xenotime). The application of multiple sulfur isotope analyses of sulfides related to the different mineralization events resolves the hydrothermal fluid isotopic evolution through time. Despite the ore mineralogy differences of the V1, V2, and V3 vein sets, their associated sulfides yield small positive ∆33S (+ 0.1 to + 0.4‰; n = 231) values with two outliers (∆33S = + 0.5‰ and + 0.6‰) across all lithology types. The constant value of MIF-S through the three temporally different gold mineralization episodes implies that sulfur was derived from a single homogenized source of sulfur distal from the deposition site, irrespective of the Au endowment. The consistent small positive ∆33S sulfur isotope signature may support that the Archean orogenic gold system sourced sulfur and possibly hydrothermal fluids from a mantle/magmatic dominated source that homogenized with crustal sulfur at depth prior to gold deposition.

Distribution and intensity of High-Temperature Low-Pressure metamorphism across the Pyrenean-Cantabrian belt: constraints on the thermal record of the pre-orogenic hyperextension rifting

Whereas a straightforward link between crustal thinning and geothermal gradients during rifting is now well established, the thermal structure of sedimentary basins within hyperextended domains remains poorly documented. For this purpose, we investigate the spatial distribution of rift-related High-Temperature Low-Pressure (HT/LP) metamorphism recorded in the preserved hyperextended rift basins inverted and integrated in the Pyrenean-Cantabrian belt. Based on Vitrinite Reflectance (Ro) data measured in 169 boreholes and more than 200 peak-metamorphic temperatures (Tmax) data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM) added to ∼425 previously publishedTmaxdata, we propose a new map depicting the spatial distribution of the HT/LPmetamorphism of the Pyrenean-Cantabrian belt. We also provide three regional-scale geological cross-sections associated with RoandTmaxdata to constrain the distribution of paleo-isograds at depth. Based on these results, we show that the impact of rift-related metamorphism is restricted to the pre- and syn-rift sequence suggested by the depth profiles of Rovalues measured in different tectonostratigraphic intervals (pre-, syn- and post-rift and syn-convergence sediments). However, a small strip of early orogenic sediments (Santonian in age) appears also affected by high temperatures along the North Pyrenean Frontal Thrust and above the Grand Rieu ridge, which we attribute to the percolation of hot hydrothermal fluids sourced from the dehydration of underthrust basement and/or sedimentary rocks at depth during the early orogenic stage. The map shows that the HT/LPmetamorphism (reaching ∼500 °C) is recorded with similar intensity along the Pyrenean-Cantabrian belt from the west in the Basque-Cantabrian Basin to the east in the Boucheville and Bas-Agly basins, for similar burial and rift-related structural settings. This thermal peak is also recorded underneath the northern border of the Mauléon Basin (calibrated by wells). It suggests that the high temperatures were recorded at the basement-sediment interface underneath the most distal part of the hyperextended domain. At basin-scale, we observe in the Basque-Cantabrian, Mauléon-Arzacq and Tarascon rift segments an asymmetry of the thermal structure revealed by different horizontal thermal gradients, supporting an asymmetry of the former hyperextended rift system. Using our results, we compare the Pyrénées to the Alps that also recorded hyperextension but no HT/LPmetamorphic event and suggest that the high-temperature record within the basins depends on high sedimentation rate promoting a thermal blanketing effect and circulation of hydrothermal fluids.