Hydrothermal,multiphase convection of H2O‐NaCl fluids from ambient to magmatic temperatures: a new numerical scheme and benchmarks for code comparison

Thermohaline convection of subsurface fluids strongly influences heat and mass fluxes within the Earth's crust. The most effective hydrothermal systems develop in the vicinity of magmatic activity and can be important for geothermal energy production and ore formation. As most parts of these systems are inaccessible to direct observations, numerical simulations are necessary to understand and characterize fluid flow. Here, we present a new numerical scheme for thermohaline convection based on the control volume finite element method (CVFEM), allowing for unstructured meshes, the representation of sharp thermal and solute fronts in advection‐dominated systems and phase separation of variably miscible, compressible fluids. The model is an implementation of the Complex Systems Modelling Platform CSMP++ and includes an accurate thermodynamic representation of strongly nonlinear fluid properties of salt water for magmatic‐hydrothermal conditions (up to 1000°C, 500 MPa and 100 wt% NaCl). The method ensures that all fluid properties are taken as calculated on the respective node using a fully upstream‐weighted approach, which greatly increases the stability of the numerical scheme. We compare results from our model with two well‐established codes, HYDROTHERM and TOUGH2, by conducting benchmarks of different complexity and find good to excellent agreement in the temporal and spatial evolution of the hydrothermal systems. In a simulation with high‐temperature, high‐salinity conditions currently outside of the range of both HYDROTHERM and TOUGH2, we show the significance of the formation of a solid halite phase, which introduces heterogeneity. Results suggest that salt added by magmatic degassing is not easily vented or accommodated within the crust and can result in dynamic, complex hydrologies.     

Modelling the Lost City hydrothermal field: influence of topography and permeability structure

The Lost City hydrothermal field (LCHF) is hosted in serpentinite at the crest of the Atlantis Massif, an oceanic core complex close to the mid‐Atlantic Ridge. It is remarkable for its longevity and for venting low‐temperature (40–91°C) alkaline fluids rich in hydrogen and methane. IODP Hole U1309D, 5 km north of the LCHF, penetrated 1415 m of gabbroic rocks and contains a near‐conductive thermal gradient close to 100°C km^?1. This is remarkable so close to an active hydrothermal field. We present hydrothermal modelling using a topographic profile through the vent field and IODP site U1309. Long‐lived circulation with vent temperatures similar to the LCHF can be sustained at moderate permeabilities of 10^?14 to 10^?15 m² with a basal heatflow of 0.22 W m^?2. Seafloor topography is an important control, with vents tending to form and remain in higher topography. Models with a uniform permeability throughout the Massif cannot simultaneously maintain circulation at the LCHF and the near‐conductive gradient in the borehole, where permeabilities <10^?16 m² are required. A steeply dipping permeability discontinuity between the LCHF and the drill hole is required to stabilize venting at the summit of the massif by creating a lateral conductive boundary layer. The discontinuity needs to be close to the vent site, supporting previous inferences that high permeability is most likely produced by faulting related to the transform fault. Rapid increases in modelled fluid temperatures with depth beneath the vent agree with previous estimates of reaction temperature based on geochemical modelling.     

Modeling fluid flow in a heterogeneous, fault-controlled hydrothermal system

Previous studies have shown that most hydrothermal systems discharging at the land surface are associated with faulting, and that the location, temperature and rate of discharge of these systems are controlled by the geometry and style of the controlling fault(s). Unfortunately, the transport of heat and fluid in fault-controlled hydrothermal systems is difficult to model realistically; although heterogeneity and anisotropy are assumed to place important controls on flow in faults, few data or observations are available to constrain the distribution of hydraulic properties within active faults. Here, analytical and numerical models are combined with geostatistical models of spatially varying hydraulic properties to model the flow of heat and fluid in the Borax Lake fault of south-east Oregon, USA. A geometric mean permeability within the fault of 7 × 10⁻¹⁴ m² with 2× vertical/horizontal anisotropy in correlation length scale is shown to give the closest match to field observations. Furthermore, the simulations demonstrate that continuity of flow paths is an important factor in reproducing the observed behavior. In addition to providing some insight into possible spatial distributions of hydraulic properties at the Borax Lake site, the study highlights one potential avenue for integrating field observations with simulation results in order to gain greater understanding of fluid flow in faults and fault-controlled hydrothermal and petroleum reservoirs.     

Modeling of hydrogen production by serpentinization in ultramafic‐hosted hydrothermal systems: application to the Rainbow field

The production of hydrogen by serpentinization in ultramafic‐hosted hydrothermal systems is simulated by coupling thermodynamic and dynamic modeling in the framework of a thermo‐hydraulic single‐pass model where a high‐temperature hydrothermal fluid moves preferentially through a main canal of high permeability. The alteration of ultramafic rocks is modeled with a first‐order kinetic formulation, wherein the serpentinization rate coefficient, K_r, takes the form: K_r = A exp(?α(T ? T₀)²). In this formulation, α determines the temperature range of the reaction and T₀ is the temperature at which the serpentinization rate reaches its maximum. This model is applied to the Rainbow hydrothermal system, which is situated on the Mid‐Atlantic Ridge, and characterized by a high temperature, a high mass flux, and a very high hydrogen concentration. The results show that a first‐order kinetic law gives a useful representation of the kinetics of serpentinization. The estimated value for the parameter A in the temperature‐dependent formulation of the serpentinization rate coefficient lies in the range (1–5) × 10^?11 s^?1. This effective parameter is several orders of magnitude lower than the values obtained from small grain‐size experiments, but in agreement with other published modeling studies of natural systems. Numerical simulations show that the venting site is able to produce the observed high concentration of hydrogen during the whole continuous lifetime of the Rainbow site.     

Fluid chemistry in the Solitaire and Dodo hydrothermal fields of the Central Indian Ridge

Fluid chemistry and microbial community patterns in chimney habitats were investigated in two hydrothermal fields located at the Central Indian Ridge. Endmember hydrothermal fluid of the Solitaire field, located ~3 km away from the spreading center, was characterized by moderately high temperature (307°C), Cl depletion (489 mm ), mildly acidic pH (≥4.40), and low metal concentrations (Fe ≤ 105 μm and Mn = 78 μm ). Chloride depletion indicates that the subseafloor source fluid had undergone phase separation at temperatures higher than ~390°C while the metal depletion was likely attributable to fluid alteration occurring at a venting temperature of around 307°C. These different temperature conditions suggested from fluid chemistry might be associated with an off‐spreading center location of the field that allows subseafloor fluid cooling prior to seafloor discharge. The microbial community in the chimney habitat seemed comparable to previously known patterns in typical basalt‐hosted hydrothermal systems. Endmember hydrothermal fluid of the Dodo field, standing on center of the spreading axis, was characterized by high H₂ concentration of 2.7 mm . The H₂ enrichment was likely attributable to fresh basalt–fluid interaction, as suggested by the nondeformed sheet lava flow expansion around the vents. Thermodynamic calculation of the reducing pyrite–pyrrhotite–magnetite (PPM) redox buffer indeed reproduced the H₂ enrichment. The quantitative cultivation test revealed that the microbial community associated with the hydrothermal fluid hosted abundant populations of (hyper)thermophilic hydrogenotrophic chemolithoautotrophs such as methanogens. The function of subseafloor hydrogenotrophic methanogenic populations dwelling around the H₂‐enriched hydrothermal fluid flows was also inferred from the ¹³C‐ and D‐depleted signature of CH₄ in the collected fluids. It was observed that the hydrothermal activity of the Dodo field had ceased until 2013.     

The application of failure mode diagrams for exploring the roles of fluid pressure and stress states in controlling styles of fracture-controlled permeability enhancement in faults and shear zones

Permeability enhancement associated with deformation processes in faults and shear zones plays a key role in facilitating fluid redistribution between fluid reservoirs in the crust. Especially in high fluid flux hydrothermal systems, fracture-controlled permeability can be relatively short-lived, unless it is repeatedly regenerated by ongoing deformation. Failure mode diagrams in pore fluid factor and differential stress space, here termed λ–σ failure mode diagrams, provide a powerful tool for analysing how fluid pressure and stress states drive failure, associated permeability enhancement and vein styles during deformation in faults and shear zones. During fault-valve behaviour in the seismogenic regime, relative rates of recovery of pore fluid factor, differential stress and fault cohesive strength between rupture events impact on styles of veining and associated, fracture-controlled permeability enhancement in faults and shear zones. Examples of vein-rich fault zones are used to illustrate how constraints can be placed, not just on fluid pressure and stress states at failure, but also on the fluid pressurization and loading paths associated with failure and transitory permeability enhancement in faults and shear zones. This provides insights about when, during the fault-valve cycle, various types of veins can form. The use of failure mode diagrams also provides insights about the relative roles of optimally oriented faults and misoriented faults as hydraulically conductive structures. The analysis highlights the dynamics of competition between fluid pressures and loading rates in driving failure and repeated permeability regeneration in fracture-controlled, hydrothermal systems.     

Fluid mixing induced by hydrothermal activity in the ordovician carbonates in Tarim Basin,China

Permian hydrothermal activity in the Tarim Basin may have been responsible for the invasion of hot brines into Ordovician carbonate reservoirs. Studies have been undertaken to explain the origin and geochemical characteristics of the diagenetic fluid present during this hydrothermal event although there is no consensus on it. We present a genetic model resulting from the study of δ¹³C, δ¹⁸O, δ³⁴S, and ⁸⁷Sr/⁸⁶Sr isotope values and fluid inclusions (FIs) from fracture‐ and vug‐filling calcite, saddle dolomite, fluorite, barite, quartz, and anhydrite from Ordovician outcrops in northwest (NW) Tarim Basin and subsurface cores in Central Tarim Basin. The presence of hydrothermal fluid was confirmed by minerals with fluid inclusion homogenization temperatures being >10°C higher than the paleo‐formation burial temperatures both in the NW Tarim and in the Central Tarim areas. The mixing of hot (>200°C), high‐salinity (>24 wt% NaCl), ⁸⁷Sr‐rich (up to 0.7104) hydrothermal fluid with cool (60–100°C), low‐salinity (0 to 3.5 wt% NaCl), also ⁸⁷Sr‐rich (up to 0.7010) meteoric water in the Ordovician unit was supported by the salinity of fluid inclusions, and δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr isotopic values of the diagenetic minerals. Up‐migrated hydrothermal fluids from the deeper Cambrian strata may have contributed to the hot brine with high sulfate concentrations which promoted thermochemical sulfate reduction (TSR) in the Ordovician, resulting in the formation of ¹²C‐rich (δ¹³C as low as ?13.8‰) calcite and ³⁴S‐rich (δ³⁴S values from 21.4‰ to 29.7‰) H₂S, pyrite, and elemental sulfur. Hydrothermal fluid mixing with fresh water in Ordovician strata in Tarim Basin was facilitated by deep‐seated faults and up‐reaching faults due to the pervasive Permian magmatic activity. Collectively, fluid mixing, hydrothermal dolomitization, TSR, and faulting may have locally dissolved the host carbonates and increased the reservoir porosity and permeability, which has significant implications for hydrocarbon exploration.     

Structural controls on hydrothermal flow in a segmented rift system, Taupo Volcanic Zone, New Zealand

Vigorous hydrothermal convection transfers 10 times the average continental heat flow through the central Taupo Volcanic Zone (TVZ), a region of active extension (approximately 8 mm year^?1) and productive rhyolitic volcanism. Over 20 high‐temperature (>250°C) geothermal fields occur within Quaternary pyroclastic basins, with convective circulation to depths of 7–8 km presumably extending through basement rocks. Parallel‐striking normal faults, fractures and dikes dissect the convective regime, interacting with fluids to either enhance or restrict flow according to the relative permeability of structure and host rock. In the basement, high bulk permeability is maintained by focussed flow through faults and associated fractures well oriented for reactivation in the prevailing stress field. In contrast, distributed flow through fault‐bounded compartments prevails within Quaternary basins, masking any signal of deeper structural control. Exceptions occur where more competent rocks are exposed at the surface. As in narrow magmatic rifts elsewhere, the extensional fabric is partitioned into discrete rift segments linked along strike by accommodation zones. Eighty per cent of TVZ geothermal fields correlate spatially with rift architecture, with 60% located in accommodation zones. We suggest that segmented rift fabrics generate bulk permeability anisotropy that is to some extent predictable, with rift segments characterized by enhanced axial flow, and accommodation zones characterized by locally enhanced vertical permeability that is tectonically maintained. This provides a plausible explanation for the common occurrence of geothermal fields within accommodation zones and their notable absence within densely faulted rift segments. Maintenance of structural permeability in zones of active hydrothermal precipitation necessarily requires repeated brittle failure. Geothermal plumes therefore exploit tectonically maintained permeability within accommodation zones, with rift segments functioning mostly as drawdown regions. The influence of rift architecture on flow paths has important implications for geothermal extraction and epithermal mineral exploration within the TVZ and other structurally segmented hydrothermal systems, both active and extinct.     

Fluids associated with hydrothermal dolomitization in St. George Group,western Newfoundland,Canada

Dolomite reservoirs are increasingly recognized as an important petroleum exploration target, although the application of a hydrothermal dolomite exploration model to these reservoirs remains controversial. The St. George Group of western Newfoundland consists of a sequence of dolomitised carbonates, with significant porosity development (up to 30%) and petroleum accumulations. Fluid inclusion microthermometry and bulk fluid leach analyses indicated that fluids responsible for matrix dolomitization (associated with intercrystalline porosity) and later saddle dolomitization are CaCl₂ ± MgCl₂ rich, high salinity (up to 26 eq. wt% NaCl) brines. Integration of fluid inclusion data with thermal maturation histories from the St. George Group show that these dolomites formed at temperatures higher than the ambient rock temperature, and are therefore hydrothermal in origin. Bulk leach analyses show that dolomitization is associated with influxes of postevaporitic brines (±Cl enriched magmatic fluids) late in the diagenetic history of these carbonates. This dolomitization is possibly Devonian in age, during a period of significant magmatic activity, extensional tectonics and development of hypersaline basins. Petrographic and geochemical similarities between Paleozoic hosted hydrothermal dolomitization in western Newfoundland, eastern Canada and the northeastern United States are consistent with a regional‐scale hydrothermal dolomitization event late in the diagenetic history of these carbonates. Geofluids (2010) 10 , 422–437     

Hydrothermal control on organic matter alteration and illite precipitation, Mt Isa Basin, Australia

Abundant illite precipitation in Proterozoic rocks from Northern Lawn Hill Platform, Mt Isa Basin, Australia, occurred in organic matter‐rich black shales rather than in sandstones, siltstones and organic matter‐poor shales. Sandstones and siltstones acted as impermeable rocks, as early diagenetic quartz and carbonate minerals reduced the porosity–permeability. Scanning and transmission electron microscopy (SEM and TEM) studies indicate a relation between creation of microporosity–permeability and organic matter alteration, suitable for subsequent mineral precipitation. K–Ar data indicate that organic matter alteration and the subsequent illite precipitation within the organic matter occurred during the regional hydrothermal event at 1172 ± 50 (2σ) Ma. Hot circulating fluids are considered to be responsible for organic matter alteration, migration and removal of volatile hydrocarbon, and consequently porosity–permeability creation. Those rocks lacking sufficient porosity–permeability, such as sandstones, siltstones and organic matter poor shales, may not have been affected by fluid movement. In hydrothermal systems, shales and mudstones may not be impermeable as usually assumed because of hydrocarbons being rapidly removed by fluid, even with relatively low total organic carbon.     

A simulation of the hydrothermal response to the Chesapeake Bay bolide impact

Groundwater more saline than seawater has been discovered in the tsunami breccia of the Chesapeake Bay Impact Crater. One hypothesis for the origin of this brine is that it may be a liquid residual following steam separation in a hydrothermal system that evolved following the impact. Initial scoping calculations have demonstrated that it is feasible such a residual brine could have remained in the crater for the 35 million years since impact. Numerical simulations have been conducted using the code HYDROTHERM to test whether or not conditions were suitable in the millennia following the impact for the development of a steam phase in the hydrothermal system. Hydraulic and thermal parameters were estimated for the bedrock underlying the crater and the tsunami breccia that fills the crater. Simulations at three different breccia permeabilities suggest that the type of hydrothermal system that might have developed would have been very sensitive to the permeability. A relatively low breccia permeability (1 × 10^?16 m²) results in a system partitioned into a shallow water phase and a deeper superheated steam phase. A moderate breccia permeability (1 × 10^?15 m²) results in a system with regionally extensive multiphase conditions. A relatively high breccia permeability (1 × 10^?14 m²) results in a system dominated by warm‐water convection cells. The permeability of the crater breccia could have had any of these values at given depths and times during the hydrothermal system evolution as the sediments compacted. The simulations were not able to take into account transient permeability conditions, or equations of state that account for the salt content of seawater. Results suggest, however, that it is likely that steam conditions existed at some time in the system following impact, providing additional evidence that is consistent with a hydrothermal origin for the crater brine.     

Pyrophyllite formation in the thermal aureole of a hydrothermal system in the Lower Saxony Basin,Germany

A combined clay mineralogical, fluid inclusion, and K‐Ar study of Upper Jurassic metasediments at the Gehn (Lower Saxony Basin, Germany) provides evidence for a transient hydrothermal event during Upper Cretaceous basin inversion centered on a prominent gravimetric anomaly. Kaolinite and smectite in Oxfordian pelitic parent rocks that cap a deltaic sandstone unit were locally transformed into pyrophyllite, 2M₁ illite, R3 illite–smectite, chlorite, and berthierine at the Ueffeln quarry. The pyrophyllite‐bearing metapelites lack bedding‐parallel preferred orientation of sheet silicates and experienced peak temperatures of about 260–270°C consistent with microthermometric data on quartz veins in the underlying silicified sandstones. The presence of expandable layers in illite–smectite and high Kübler Index values indicate that the thermal event was rather short‐lived. K‐Ar dating of the <0.2 μm fraction of the pyrophyllite‐bearing Ueffeln metapelite yields a maximum illitization age of 117 ± 2 Ma. Lower trapping temperatures of aqueous fluid inclusions in quartz veins and the absence of pyrophyllite in metapelites of the Frettberg quarry in a distance of about 2.5 km from the Ueffeln quarry infer maximum paleotemperatures of only 220°C. The highly localized thermal anomaly at Ueffeln suggests fault‐controlled fluid migration and heat transfer that provided a thermal aureole for pyrophyllite formation in the metapelites rather than metamorphism due to deep burial. A pH neutral hydrothermal fluid that formed by devolatilization reactions or less likely by mixing of meteoric and marine waters that interacted at depth with shales is indicated by the low salinity (3–5 wt. % NaCl equiv.) of aqueous inclusions, their coexistence with methane–carbon dioxide‐dominated gas inclusions as well as carbon, hydrogen, and oxygen isotope data. The upwelling zone of hydrothermal fluids and the thermal maximum is centered on a gravimetric anomaly interpreted as an igneous intrusion (‘Bramsche Massif’) providing the heat source for the intrabasinal hydrothermal system.     

Monitoring fluid chemistry in iron oxide–copper–gold‐related metasomatic processes,eastern Mt Isa Block,Australia

Most researchers in the Proterozoic eastern Mt Isa Block, NW Queensland, Australia, favour magmatic fluid and salt sources for sodic‐(calcic) alteration and iron oxide–copper–gold mineralization. Here we compare spatial, mineralogic and stable isotope data from regional alteration assemblages with magmatic and magmatic‐hydrothermal interface rocks in order to track chemical and isotopic variations in fluid composition away from inferred fluid sources. Tightly clustered δ¹⁸O values for magnetite, quartz, feldspar and actinolite for igneous‐hosted samples reflect high temperature equilibration in the magmatic‐hydrothermal environment. In contrast, these minerals record predominantly higher δ¹⁸O values in regional alteration and Cu–Au mineralization. This dichotomy reflects partial equilibration with isotopically heavier wallrocks and slightly lower temperatures. Increases in Si concentrations of metasomatic amphiboles relative to igneous amphiboles in part reflect cooling of metasomatic fluids away from igneous rocks. Variations in X_Mg for metasomatic amphiboles indicate local wallrock controls on amphibole chemistry, while variations in X_Cl/X_OH ratios for amphiboles (at constant X_Mg) indicate variable aH₂O/aHCl ratios for metasomatic fluids. Biotite geochemistry also reflects cooling and both increases and decreases in aH₂O/aHCl for fluids away from plutonic rocks. Decreased aH₂O/aHCl ratios for metasomatic fluids reflect in part scavenging of chlorine out of meta‐evaporite sequences, although this process requires already saline fluids. Local increases in aH₂O/aHCl ratios, as well as local decreases in δ¹⁸O values for some minerals (most notably haematite and epithermal‐textured quartz), may indicate ingress of low salinity, low δ¹⁸O fluids of possible meteoric origin late in the hydrothermal history of the region. Taken together, our observations are most consistent with predominantly magmatic sources for metasomatic fluids in the eastern Mt Isa Block, but record chemical and isotopic variations along fluid flow paths that may be important in explaining some of the diversity in alteration and mineralization styles in the district.     

Episodic fluid flow within continental rift basins: some insights from field data and mathematical models of the Rhinegraben

Vitrinite reflectance data from a petroleum exploration well in the northern Upper Rhinegraben show an unusual vertical maturity trend. Above and below a 500 m thick marl layer the vitrinite reflectance levels are consistent with modern, conductive, geothermal gradients. Between about 1000 and 1500 m depth, however, vitrinite reflectance levels are significantly elevated (about 0.6%Ro). This anomaly cannot be explained with one‐dimensional conductive or conductive–convective heat transfer models, and thermal effects of sedimentation or igneous intrusion seem implausible for this geological setting. The thermal anomaly that formed this maturation anomaly must have been hydrothermal in origin, two‐dimensional in nature, and persisted long enough to elevate the vitrinite reflectance values within this marl unit, yet it must have dissipated before the thermal perturbation would have altered the organic matter below and above the unit. In this study, we propose that the vitrinite reflectance anomalies were caused by a transient thermal inversion induced by episodic, lateral flow of hot (130–160°C) groundwater along conductive fractures and bedding planes. Heat flow constraints suggest that fluids must have moved rapidly up a vertical feeder fault from a depth of at least 3.6 km before migrating laterally. To test this hypothesis, we present a suite of simple, idealized mathematical models of groundwater flow, heat transfer, thermal degradation of kerogen and vitrinite systematics to explore the episodic flow that could have produced the observed thermal anomaly. In these simulations, a single, horizontal aquifer is sandwiched between two less permeable units: the total dimensions of the vertical section model are 4 km thick by 10 km long. The top of the aquifer coincides with the position of the observed thermal maturity anomaly in the Rhinegraben. Boundary conditions along the left edge of this aquifer were varied through time to allow for the migration of hot fluids out into the basin. Inflow temperature, horizontal velocity, duration and frequency of flow and thickness of the aquifer were varied. We found that a thermal maturity anomaly could only be produced by a rather restrictive set of hydrothermal conditions. It was possible to produce the observed vitrinite reflectance anomaly by a single hydrothermal flow event of 130°C fluid migrating laterally into the aquifer at a rate of 1 m a^?1 for about 10 000 years. The anomaly is spatially confined to near the left edge of the basin, near the feeder fault. If the flow event lasted longer than 100 000 years, then the maturation anomaly disappeared as the lower confining unit approached steady‐state thermal conditions. It is possible that such an event occurred about 5 million years ago in response to increases in fault permeability associated with far field Alpine tectonism.     

Potential of palaeofluid analysis for understanding oil charge history

Fluid inclusion data, particularly the distribution of hydrocarbon fluid inclusions and their chemistry, can provide insights into oil charge in a petroleum-prospective region. Examples from the UK Atlantic margin show how we can understand thermal regime, timing and chemistry of oil charge. Data from the UK Atlantic margin based on fluid inclusion temperature profiles shows anomalously high temperatures which are highest at the top of the Triassic–Eocene sequence. This is interpreted as a product of hot fluid flow, probably reflecting hydrothermal activity related to intrusion of sills at depth. The preservation of high temperatures also implies rapid migration from depth through fracture systems. Ar–Ar analysis of oil-bearing K-feldspar cements, and petrographic studies of oil inclusion distribution help delimit timing and migration pathways for the hot fluid charge and later fluid migration events. Coupled with compositional data for oils measured destructively (organic geochemistry) or non-destructively (fluorescence), these approaches allow the development of oil charge histories based on real data rather than theoretical modelling.     

Spring temperatures in the Sagehen Basin, Sierra Nevada, CA: implications for heat flow and groundwater circulation

Heat flow in the Sierra Nevada, CA, is low despite its young geologic age. We investigate the possibility that advective heat transport by groundwater flow leads to an underestimate of heat flow in the Sierras based purely on borehole measurements. Using temperature and discharge measurements at springs in Sagehen Basin, we find that groundwater removes the equivalent of approximately 20–40 mW m⁻² of geothermal heat from the basin. This is comparable with other heat flow measurements in the region and indicates that, in this basin, at least, groundwater does transport a significant amount of geothermal heat within the basin. Additionally, we use estimates of the mean residence time of water discharged at the springs along with hourly temperature records in springs to provide constraints on groundwater flow depths within the basin. An analytical model based on these constraints indicates that the heat removed by groundwater may represent 20% to >90% of the total heat flow in the basin. Without better constraints on the regional hydrogeology and the depth of circulation, we cannot determine whether the heat discharged at the springs represents a change in the mode of heat transfer, i.e. from conduction to advection at shallow depths (<100 m) or whether this is a component of heat transfer that should be added to measured conductive values. If the latter is true, and Sagehen Basin is representative of the Sierras, basal heat flow in the Sierra Nevada may be higher than previously thought.     

Hydrogeology of the Krafla geothermal system,northeast Iceland

The Krafla geothermal system is located in Iceland's northeastern neovolcanic zone, within the Krafla central volcanic complex. Geothermal fluids are superheated steam closest to the magma heat source, two‐phase at higher depths, and sub‐boiling at the shallowest depths. Hydrogen isotope ratios of geothermal fluids range from ?87‰, equivalent to local meteoric water, to ?94‰. These fluids are enriched in ¹⁸O relative to the global meteoric line by +0.5–3.2‰. Calculated vapor fractions of the fluids are 0.0–0.5 wt% (~0–16% by volume) in the northwestern portion of the geothermal system and increase towards the southeast, up to 5.4 wt% (~57% by volume). Hydrothermal epidote sampled from 900 to 2500 m depth has δD values from ?127 to ?108‰, and δ¹⁸O from ?13.0 to ?9.6‰. Fluids in equilibrium with epidote have isotope compositions similar to those calculated for the vapor phase of two‐phase aquifer fluids. We interpret the large range in δD_EPIDOTE and δ¹⁸O_EPIDOTE across the system and within individual wells (up to 7‰ and 3.3‰, respectively) to result from variable mixing of shallow sub‐boiling groundwater with condensates of vapor rising from a deeper two‐phase reservoir. The data suggest that meteoric waters derived from a single source in the northwest are separated into the shallow sub‐boiling reservoir, and deeper two‐phase reservoir. Interaction between these reservoirs occurs by channelized vertical flow of vapor along fractures, and input of magmatic volatiles further alters fluid chemistry in some wells. Isotopic compositions of hydrothermal epidote reflect local equilibrium with fluids formed by mixtures of shallow water, deep vapor condensates, and magmatic volatiles, whose ionic strength is subsequently derived from dissolution of basalt host rock. This study illustrates the benefits of combining phase segregation effects in two‐phase systems during analysis of wellhead fluid data with stable isotope values of hydrous alteration minerals when evaluating the complex hydrogeology of volcano‐hosted geothermal systems.     

Fracture mineralization and fluid flow evolution: an example from Ordovician–Devonian carbonates,southwestern Ontario,Canada

Petrography, geochemistry (stable and radiogenic isotopes), and fluid inclusion microthermometry of matrix dolomite, fracture‐filling calcite, and saddle dolomite in Ordovician to Devonian carbonates from southwestern Ontario, Canada, provide useful insights into fluid flow evolution during diagenesis. The calculated δ¹⁸O_fluid, ΣREE, and REE_SN patterns of matrix and saddle dolomite suggest diverse fluids were involved in dolomitization and/or recrystallization of dolomite. The ⁸⁷Sr/⁸⁶Sr ratios of dolomite of each succession vary from values in the range of coeval seawater to values more radiogenic than corresponding seawater, which indicate diagenetic fluids were influenced by significant water/rock interaction. High salinities (22.4–26.3 wt. % NaCl + CaCl₂) of Silurian and Ordovician dolomite–hosted fluid inclusions indicate involvement of saline waters from dissolution of Silurian evaporites. High fluid inclusion homogenization temperatures (>100°C) in all samples from Devonian to Ordovician show temperatures higher than maximum burial (60–90°C) of their host strata and suggest involvement of hydrothermal fluids in precipitation and/or recrystallization of dolomite. A thermal anomaly over the mid‐continent rift during Devonian to Mississippian time likely was the source of excess heat in the basin. Thermal buoyancy resulting from this anomaly was the driving force for migration of hydrothermal fluids through regional aquifers from the center of the Michigan Basin toward its margin. The decreasing trend of homogenization temperatures from the basin center toward its margin further supports the interpreted migration of hydrothermal fluids from the basin center toward its margin. Hydrocarbon‐bearing fluid inclusions in late‐stage Devonian to Ordovician calcite cements with high homogenization temperatures (>80°C) and their ¹³C‐depleted values (approaching ?32‰ PDB) indicate the close relationship between hydrothermal fluids and hydrocarbon migration.     

Fluid inclusion and isotopic constraints on the origin of ore‐forming fluid of the Jinman–Liancheng vein Cu deposit in the Lanping Basin,western Yunnan,China

Extensive quartz–carbonate–Cu sulfide veins occur in clastic rocks and are spatially related to Paleocene granites in the western border of the Lanping Basin, western Yunnan, China. Abundant aqueous‐carbonic fluid inclusions occur in these veins but their origin is debated. In the Jinman–Liancheng deposit, individual primary inclusion groups contain either exclusively liquid‐rich inclusions (Gl), or coexisting liquid‐rich and vapor‐rich inclusions (Glv). Microthermometry and estimate of CO₂ content indicate that type Gl inclusions either have homogenization temperatures (Th) 238–263°C and contain c. 3.9–5.5 mole % CO₂, or have Th 178–222°C and contain c. 1.6–3.2 mole % CO₂. Type Glv inclusions are thought to represent samples of fluid unmixing that occurred at 183–218°C. At that time, the liquid phase in the unmixing fluid may contain c. 2.0–3.3 mole % CO₂. As such, the correlation of CO₂ content with Th for type Gl inclusions is thought to be caused by fluid unmixing with decreasing temperature and subsequent CO₂ escape. δ¹⁸O and δD values of the parent water mainly fall in the field below that of primary magmatic water, indicative of fluid derivation from degassed (in open system) magmatic water, with no contributions from basinal or meteoric water. Initial Sr isotopic compositions of hydrothermal carbonates suggest that the fluid was magmatic, probably derived from the Paleogene granites. δ¹³C_PDB values (?4‰ to ?7‰) of the hydrothermal carbonates and δ³⁴S_V‐CDT values of sulfides (mainly ?11‰ to +5‰) indicate that the carbon and sulfur can be derived from (degassed) magma and/or nonmagmatic sources. The CO₂‐rich and magmatic‐water‐derived fluid at Jinman–Liancheng differs from the CO₂‐poor and basinally derived fluid in sediment‐hosted stratiform Cu (SSC) deposits, which suggests that there are no genetic linkages between the vein Cu and SSC deposits in the Lanping Basin.     

Factors controlling free thermal convection in faults in sedimentary basins: implications for the formation of zinc–lead mineral deposits

Stratiform sediment‐hosted Zn–Pb–Ag mineral deposits constitute about 40% of the Earth's zinc resources ( Allen 2001 ), and in most cases their genesis involves the discharge of basinal brines near or on the seafloor through syndepositional faults ( Sangster 2002 ). From the point of view of base metal exploration, it is therefore essential to identify all possible faults that formerly carried the upwelling ore‐forming solutions during mineralising events. This paper presents a numerical investigation of the relative importance of various physical parameters in controlling fluid discharge, recharge and heat transport in faults. A two‐dimensional, free convection of pure water, hydrogeological model is developed for the McArthur basin in northern Australia based on the surface geology, known stratigraphic and structural relationships and regional geophysical interpretations. Numerical experiments and sensitivity analyses reveal that faults with strong initial heat input, due to depth of penetration or magmatic activity, are the most likely candidates to carry discharge fluids to the sites of metal precipitation. Deeper, wider and more permeable faults are more likely to behave as the fluid discharge pathways, whereas shallow, narrow or less permeable faults act as marine water recharge pathways. Compared with these fault‐related factors, aquifer physical properties are less important in determining fluid flow patterns and the geothermal regime. These results are an important step in understanding hydrothermal fluid flow in sedimentary basins in order to develop effective exploration criteria for the location of stratiform Zn–Pb–Ag deposits.