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.     

Geochemical fingerprinting of hydraulic fracturing fluids from Qusaiba Hot Shale and formation water from Paleozoic petroleum systems,Saudi Arabia

Provenance studies of produced water are essential to trace flow dynamics and reservoir compartmentalization in petroleum systems and to quantify fluid recovery rates from unconventional fracturing. Produced water from a hydraulically fractured well in the Qusaiba Hot Shale in the Northern Exploration Area, Saudi Arabia, was daily monitored and analyzed for water chemistry, and environmental (δ²H, δ¹³C, δ¹⁸O_H2O, δ¹⁸O_SO4, δ³⁴S_SO4, δ³⁷Cl, ⁸⁷Sr/⁸⁶Sr) and cosmogenic isotopes (³H, ¹⁴C, ³⁶Cl), to differentiate from reference fluids of supply water, fracturing fluids, and formation water from adjacent Paleozoic units. Initially, recovered water is composed of fracturing fluids and subsequently replaced by a homogeneous cut of pristine formation water. Formation water is composed of dominant meteoric water (approximately 84 vol%) and minor fossil evaporated seawater. The young ¹⁴C‐apparent age between 6000 and 6700 years BP and depleted δ¹⁸O/δ²H values for the meteoric component confirm the infiltration of surface water into the Qusaiba Hot Shale interval or adjacent units during the Early Holocene Pluvial Period under cooler and wetter climatic conditions than present, which suggest the presence of a very recent, dynamic hydraulic flow system. ³⁶Cl/Cl ratios between 102 × 10^?15 and 31 × 10^?15 are ambiguous and can be attributed to atmospheric recharge close to the coast, mixing of ³⁶Cl‐enriched Quaternary meteoric recharge with ³⁶Cl‐depleted fossil seawater, and/or hypogene production by U‐Th‐enriched host rock. Produced waters from Qusaiba Hot Shale are within the compositional range of Na‐Cl‐type formation water from Paleozoic reservoir units in northern Saudi Arabia with salinities from 30 000 to 130 000 mg l^?1. As a novel technological approach for exploration wells in Northern Saudi Arabia, multi‐isotopic methods were successfully implemented to quantify flowback volumes from hydraulic fracturing, and to fingerprint pristine formation water or pore water in Paleozoic systems on their provenance, residence time, migration pathways, and secondary alteration processes.     

Chemical and isotopic signatures of Na/HCO3/CO2‐rich geofluids,North Portugal

Geochemical and isotopic studies have been undertaken to assess the origin of CO₂‐rich waters issuing in the northern part of Portugal. These solutions are hot (76°C) to cold (17°C) Na–HCO₃ mineral waters. The δ²H and δ¹⁸O signatures of the mineral waters reflect the influence of altitude on meteoric recharge. The lack of an ¹⁸O‐shift indicates there has been no high temperature water–rock interaction at depth, corroborating the results of several chemical geothermometers (reservoir temperature of about 120°C). The low ¹⁴C activity (up to 9.9 pmC) measured in some of the cold CO₂‐rich mineral waters (total dissolved inorganic carbon) is incompatible with the presence of ³H (from 1.7 to 4.1 TU) in those waters, which indicates relatively short subsurface circulation times. The δ¹³C values of CO₂ gas and dissolved inorganic carbon range between ?6‰ and ?1‰ versus Vienna‐Peedee Belemnite, indicating that the total carbon in the recharge waters is being diluted by larger quantities of CO₂ (¹⁴C‐free) introduced from deep‐seated (upper mantle) sources, masking the ¹⁴C‐dating values. The differences in the ⁸⁷Sr/⁸⁶Sr ratios of the studied thermal and mineral waters seem to be caused by water–rock interaction with different granitic rocks. Chlorine isotope signatures (?0.4‰ < δ³⁷Cl < +0.4‰ versus standard mean ocean chloride) indicate that Cl in these waters could be derived from mixing of a small amount of igneous Cl from leaching of granitic rocks.     

Temperature‐dependent Li isotope ratios in Appalachian Plateau and Gulf Coast Sedimentary Basin saline water

Lithium (Li) concentrations of produced water from unconventional (horizontally drilled and hydraulically fractured shale) and conventional gas wells in Devonian reservoirs in the Appalachian Plateau region of western Pennsylvania range from 0.6 to 17 mmol kg^?1, and Li isotope ratios, expressed as in δ⁷Li, range from +8.2 to +15‰. Li concentrations are as high as 40 mmol kg^?1 in produced waters from Plio‐Pleistocene through Jurassic‐aged reservoirs in the Gulf Coast Sedimentary Basin analyzed for this study, and δ⁷Li values range from about +4.2 to +16.6‰. Because of charge‐balance constraints and rock buffering, Li concentrations in saline waters from sedimentary basins throughout the world (including this study) are generally positively correlated with chloride (Cl), the dominant anion in these fluids. Li concentrations also vary with depth, although the extent of depth dependence differs among sedimentary basins. In general, Li concentrations are higher than expected from seawater or evaporation of seawater and therefore require water–mineral reactions that remove lithium from the minerals. Li isotope ratios in these produced waters vary inversely with temperature. However, calculations of temperature‐dependent fractionation of δ⁷Li between average shale δ⁷Li (?0.7‰) and water result in δ⁷Li_water that is more positive than that of most produced waters. This suggests that aqueous δ⁷Li may reflect transport of water from depth and/or reaction with rocks having δ⁷Li lighter than average shale.     

Origin of palaeo‐waters in the Ordovician carbonates in Tahe oilfield,Tarim Basin: constraints from fluid inclusions and Sr,C and O isotopes

Petrographic features, isotopes, and trace elements were determined, and fluid inclusions were analyzed on fracture‐filling, karst‐filling and interparticle calcite cement from the Ordovician carbonates in Tahe oilfield, Tarim basin, NW China. The aim was to assess the origin and evolution of palaeo‐waters in the carbonates. The initial water was seawater diluted by meteoric water, as indicated by bright cathodoluminescence (CL) in low‐temperature calcite. The palaeoseawater was further buried to temperatures from 57 to 110°C, nonluminescent calcite precipitated during the Silurian to middle Devonian. Infiltration of meteoric water of late Devonian age into the carbonate rocks was recorded in the first generation of fracture‐ and karst‐filling dull red CL calcite with temperatures from <50°C to 83°C, low salinities (<9.0 wt%), high Mn contents and high ⁸⁶Sr/⁸⁷Sr ratios from 0.7090 to 0.7099. During the early Permian, ⁸⁷Sr‐rich hydrothermal water may have entered the carbonate rocks, from which precipitated a second generation of fracture‐filling and interparticle calcite and barite cements with salinities greater than 22.4 wt%, and temperatures from 120°C to 180°C. The hydrothermal water may have collected isotopically light CO₂ (possibly of TSR‐origin) during upward migration, resulting in hydrothermal calcite and the present‐day oilfield water having δ¹³C values from ?4.3 to ?13.8‰ and showing negative relationships of ⁸⁷Sr/⁸⁶Sr ratios to δ¹³C and δ¹⁸O values. However, higher temperatures (up to 187°C) and much lower salinities (down to 0.5 wt%) measured from some karst‐filling, giant, nonluminescent calcite crystals may suggest that hydrothermal water was deeply recycled, reduced (Fe‐bearing) meteoric water heated in deeper strata, or water generated from TSR during hydrothermal water activity. Mixing of hydrothermal and local basinal water (or diagenetically altered connate water) with meteoric waters of late Permian age and/or later may have resulted in large variations in salinity of the present oilfield waters with the lowest salinity formation waters in the palaeohighs.     

Fracture-fill calcite as a record of microbial methanogenesis and fluid migration: a case study from the Devonian Antrim Shale, Michigan Basin

The Devonian Antrim Shale is an organic‐rich, naturally fractured black shale in the Michigan Basin that serves as both a source and reservoir for natural gas. A well‐developed network of major, through‐going vertical fractures controls reservoir‐scale permeability in the Antrim Shale. Many fractures are open, but some are partially sealed by calcite cements that retain isotopic evidence of widespread microbial methanogenesis. Fracture filling calcite displays an unusually broad spectrum of δ¹³C values (+34 to ?41‰ PDB), suggesting that both aerobic and anaerobic bacterial processes were active in the reservoir. Calcites with high δ¹³C values (>+15‰) record cementation of fractures from dissolved inorganic carbon (DIC) generated during bacterial methanogenesis. Calcites with low δ¹³C values (13C values between ?10 and ?30‰ can be attributed to variable organic matter oxidation pathways, methane oxidation, and carbonate rock buffering. Identification of ¹³C‐rich calcite provides unambiguous evidence of biogenic methane generation and may be used to identify gas deposits in other sedimentary basins. It is likely that repeated glacial advances and retreats exposed the Antrim Shale at the basin margin, enhanced meteoric recharge into the shallow part of the fractured reservoir, and initiated multiple episodes of bacterial methanogenesis and methanotrophic activity that were recorded in fracture‐fill cements. The δ¹⁸O values in both formation waters and calcite cements increase with depth in the basin (?12 to ?4‰ SMOW, and +21 to +27‰ PDB, respectively). Most fracture‐fill cements from outcrop samples have δ¹³C values between ?41 and ?15‰ PDB. In contrast, most cement in cores have δ¹³C values between +15 and +34‰ PDB. Radiocarbon and ²³⁰Th dating of fracture‐fill calcite indicates that the calcite formed between 33 and 390 ka, well within the Pleistocene Epoch.     

Thermochemical and bacterial sulfate reduction in the Cambrian and Lower Ordovician carbonates in the Tazhong Area,Tarim Basin,NW China: evidence from fluid inclusions,C, S,and Sr isotopic data

Petrographic features, C, O, S, and Sr isotopes were determined, and fluid inclusions (FI) were analyzed on various stages of vug‐ and fracture‐fillings from the Cambrian and Lower Ordovician reservoirs in the Tazhong area, Tarim basin, NW China. The aim was to assess the origin of pyrite and anhydrite and the processes affecting sulfur during diagenesis of the carbonates. Pyrite from seven wells has δ³⁴S values from ?22‰ to +31‰. The pyrites with low δ³⁴S values from ?21.8‰ to ?12.3‰ were found close to fracture‐filling calcites with vapor‐liquid double‐phase aqueous fluid inclusions homogenization temperatures (FI‐Th) from 55.7 to 73.2°C, salinities from 1.4wt% to 6.59wt% NaCl equiv and δ¹³C values from ?2.3‰ to ?14.2‰, indicating an origin from bacterial sulfate reduction by organic matter. Other sulfides with heavier δ³⁴S values may have formed by thermochemical sulfate reduction (TSR) during two episodes. The earlier TSR in the Middle and Lower Cambrian resulted in pyrites and H₂S having δ³⁴S values from 30 to 33‰, close to those of bedded anhydrite and oilfield water (approximately 34‰). The later TSR is represented by calcites with δ¹³C values as light as ?17.7‰ and FI‐Th of about 120–145°C, and pyrite and H₂S with δ³⁴S values close to those of the Upper Cambrian burial‐diagenetic anhydrite (between +14.8‰ and +22.6‰). The values of the anhydrite are significantly lighter than contemporary seawater sulfates. This together with ⁸⁷Sr/⁸⁶Sr values of anhydrite and TSR calcites from 0.7091 to 0.7125 suggests a source from the underlying Ediacaran seawater sulfate and detrital Sr contribution.     

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.     

Application of boron isotopes to the understanding of fluid–rock interactions in a hydrothermally stimulated oil reservoir in the Alberta Basin, Canada

Boron isotope ratios of reservoir minerals and fluids can be a useful geothermometer and monitor of fluid–rock interactions. In Cold Lake oil sands of northern Alberta, there is a large variation in δ¹¹B of the produced waters generated during steam injection and recovery of oil and water. The higher temperature waters (～ 200 °C) have isotopically light δ¹¹B values (+ 3‰ to + 14‰) and high B contents (～150 p.p.m.). It is inferred that the range of δ¹¹B values of the hydrothermal fluids results from reaction with the reservoir rock, and is a function of the temperature of the fluid–rock interaction. The distinct B geochemistry of the produced waters can be used to show that there is no detectable mixing of the oil recovery waters with the regional formation waters or shallow groundwater aquifers containing potable water. Examination of B isotope ratios of reservoir minerals, before and after steam injection, allows the evaluation of sources of B in the reservoir. The only significant phase containing B is pumice. It shows generally positive δ¹¹B values before steam injection and negative values after steam, with δ¹¹B as low as ? 28‰. Other possibly reactive phases include clay minerals and organic matter, but their abundance is not great enough to impact on the isotopic composition of the produced waters. This information makes it possible to evaluate the boron isotope fractionation equation derived from experimental data ( Williams LB (2000) Boron isotope geochemistry during burial diagenesis. PhD Dissertation. The University of Calgary, Alberta, Canada; Williams LB, Hervig RL, Holloway JR, Hutcheon I (2001a) Boron isotope geochemistry during diagenesis: Part 1. Experimental determination of fractionation during illitization of smectite. Geochimica et Cosmochimica Acta, in press). The results show that the fractionation curve predicts the difference between δ¹¹B of the pumice and hydrothermal fluids in the Cold Lake reservoir. This not only indicates that the reservoir fluids have approached boron isotope equilibrium with the reservoir rock, but also shows that B isotopes provide a useful geothermometer for hydrothermally stimulated oil reservoirs.     

On the homogeneity of fluids forming bedding‐parallel veins

A group of 400–500 m long, bedding‐parallel calcite veins are exposed in the central La Popa Basin of northeastern Mexico. These veins provide a unique opportunity to determine the kilometer‐scale fluid–rock system associated with bedding‐parallel vein formation, and to test for sampling bias in studies that often use one or two samples to constrain the characteristics of regional‐scale paleohydrogeological systems. We use fluid inclusion microthermometry in conjunction with measurements of δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr ratios to constrain the vein‐forming fluid temperatures, compositions and sources, and compare these values along and between the veins to establish the homogeneity of the vein‐forming fluids and fluid–rock system. The δ¹³C values of the veins are close to those of the host rock, and average – 3.96‰ (PDB). The δ¹⁸O values of the veins are typically 1‰ lower than those of the host rocks, and average – 9.54‰ (PDB). Fluid inclusion homogenization temperatures average 137°C and inclusion salinities are all <6 wt% NaCl equivalent. The ⁸⁷Sr/⁸⁶Sr ratios of the veins average 0.70731 and are substantially lower than the values expected for the host rock. Calculated fluid δ¹⁸O values range from 4 to 10‰ (SMOW). The isotopic and microthermometric data indicate the veins most likely formed at depths of 3–4 km when meteoric water mixed with upward migrating, warm basinal brines. Vein microstructures and field characteristics indicate they formed from multiple slip events that most likely were associated with transport of individual fluid pulses that migrated along bedding planes. The large‐scale homogeneity of vein geochemistry is remarkable and demonstrates that only one or two samples would be sufficient to accurately characterize the kilometer‐scale paleohydrogeological system for these veins.     

Fluid inclusion constraints on the origin of the brines responsible for Pb–Zn mineralization at Pine Point and coarse non‐saddle and saddle dolomite formation in southern Northwest Territories

The Pine Point region is a classic metallogenic mining camp that produced over 58 million short tons of Zn–Pb ore from approximately 40 base‐metal mineralized deposits hosted by Middle Devonian carbonates. The ore deposits are localized in paleokarstic features found in the epigenetic ‘Presqu'ile’ dolomite that preferentially replaced some of the upper barrier limestones. The main ore‐stage sulfides include galena, sphalerite, marcasite, and pyrite. A bulk fluid inclusion chemistry study was carried out on sulfide, coarse non‐saddle and saddle dolomite and calcite samples from the Pine Point and Great Slave Reef deposits, and unmineralized coarse non‐saddle and saddle dolomite samples from Hay West, Windy Point and Qito areas. Molar Cl/Br ratio data from Pine Point indicate the presence of four fluids at different stages of the paragenesis. The fluids trapped in sulfides and ore‐stage dolomites predominately consist of a Br‐rich fluid with a composition similar to that of evaporated seawater (fluid A), and a very Br‐enriched fluid of unknown origin (fluid B). Both these fluids are CaCl₂–NaCl (Na to Ca ratios of 1:10)‐rich brines and have compositions unlike the modern formation waters in the Devonian aquifers in the basin today. A third, relatively Cl‐rich (or Br‐poor), fluid (fluid C) was identified in two samples and may have acquired some chlorinity by dissolving halide minerals. Mixing between the Br‐rich fluid A and a dilute fluid also occurred in the later stages of the paragenesis, resulting in the formation of calcite and native sulfur. Saddle and coarse dolomites not associated with significant sulfide mineralization have a narrow range of halogen compositions similar to fluid A. There is no evidence of fluid B or C in the unmineralized samples. Relative to a modern‐day seawater compositions all the fluids have had some modification of their cation compositions. There is some weak evidence for interactions with clastic units or crystalline basement rocks. It is also possible however, that the evaporative brines could have formed from a relatively CaCl₂‐rich, NaCl‐depleted Devonian seawater, unlike the composition of modern‐day seawater.     

Geochemistry of the Bagnères-de-Bigorre thermal waters from the North Pyrenean Zone (France)

Thermal springs are poorly known in the sedimentary sites of the Pyrenees. In this paper, we describe the ‘Bagnères‐de‐Bigorre’ springs which occur in a remarkably active seismotectonic context. A chemical and isotopic study of 15 spring waters (both cold and thermal, ranging in temperature from 7.0 to 49.9°C), and continuous monitoring of a single spring allow us to characterise water–rock interactions, fluids paths and mixing processes. Three groups of waters are distinguished: (I) SO₄²⁺–Ca²⁺–Cl^– thermal waters (II) SO₄²⁺–Cl^––Ca²⁺ thermal waters and (III) HCO₃^––Ca²⁺ cold shallow waters. Their characteristics suggest interactions with Mesozoic evaporite and carbonate formations. O and D isotopes from thermal waters indicate a local meteoric origin of Atlantic signature and a recharge elevation of 800 to 1000 m, which corresponds to a single feeding area. Their δ¹³C values (?2.8 to ?9.6‰) are consistent with carbonate dissolution, slight fractionation and a surficial organic input leading to δ¹³C depletion. Sr isotopes (0.70751 to 0.70777), Na⁺/Cl^– and (Ca²⁺ + Mg²⁺)/SO₄^2– ratios as well as thermodynamic calculations show that the dissolution of anhydrite and halite‐bearing Triassic layers control the chemical composition of group‐I and ‐II waters. The contrasting trends of cation/Cl^– ratios and TDS of waters from groups I and II suggest the existence of two different circulation paths at depth as well as dilution with surficial waters similar to group III. Calculated mixing proportions show that three waters from group I are diluted from 17 to 66%, whereas all waters from group II are mixed. The aquifer temperature is estimated to be in the range 55–64°C using the retrograde and prograde solubilities of anhydrite and chalcedony, respectively. Accordingly, the mean depth of the reservoir is around 1.7 km, which allows us to constrain the depth of the Triassic layer.     

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.     

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.     

From deep to shallow fluid reservoirs: evolution of fluid sources during exhumation of the Sierra Almagrera,Betic Cordillera,Spain

The combination of structural, geochemical and palaeotopographic data proves to be an efficient tool to understand fluid transfers in the crust. This study discriminates shallow and deep fluid reservoirs on both sides of the brittle–ductile transition under an extensional regime and points out the role of major transcurrent fault activity in this palaeohydrogeological setting. Palaeofluids trapped in quartz and siderite–barite veins record the transfer of fluids and metal solute species during the Neogene exhumation of the Sierra Almagrera metamorphic belt. Ductile then brittle–ductile extensional quartz veins formed from a deep fluid reservoir, trapping metamorphic secondary brines containing low‐density volatile phases derived from the dissolution of Triassic evaporites. During exhumation, low‐salinity fluids percolated within the brittle domain, as shown by transgranular fluid inclusion planes affecting previous veins. These observations indicate the opening of the system during Serravalian to early Tortonian times and provide evidence for the penetration of surficial fluids of meteoric or basinal origin into the upper part of the brittle–ductile transition. During exhumation, synsedimentary transcurrent tectonic processes occurred from late Tortonian times onwards, while marine conditions prevailed at the Earth's surface. At depth in the brittle domain, quartz veins associated with haematite record a return to high‐salinity fluid circulation suggesting an upward transfer fed from a lower reservoir. During the Messinian, ongoing activity of the trans‐Alboran tectono‐volcanic trend led to the formation of ore deposits. Reducing fluids caused the formation of siderite and pyrite ores. The subsequent formation of galena and barite may be related to an increase of temperature. The high salinity and Cl/Br ratio of the fluids suggest another source of secondary brine derived from dissolved Messinian evaporites, as corroborated by the δ³⁴S signature of barite. These evaporites preceded the main sea‐level drop related to the peak of the salinity crisis (5.60–5.46 Ma).     

Origin of palaeofluids in a normal fault setting in the Aegean region

The province of Burdur (SW Turkey) is seismically an active region. A structural, geochronological, petrographical, geochemical and fluid inclusion study of extension veins and fault‐related calcite precipitates has been undertaken to reconstruct the palaeofluid flow pattern in this normal fault setting in the Aegean region. A palaeostress analysis and U/Th dating of the precipitates reveals the neotectonic significance of the sampled calcites. Fluid inclusion microthermometry of calcites‐filling extension veins shows final melting temperatures (T_m ice) of 0°C. This indicates pure water, most likely of meteoric origin. The oxygen isotope values (?9.8‰ to ?6.5‰ VPDB) and the carbon isotopic composition (?10.4‰ to ?2.9‰ VPDB) of these calcites also show a near‐surface meteoric origin of the fluid responsible for precipitation. The microstructural characteristics of fault‐related calcites indicate that calcite precipitation was linked with fault activity. Final melting temperature of fault‐related calcites ranges between 0 and ?1.9°C. The oxygen isotope values show a broad range between ?15.0‰ and ?2.2‰ VPDB. Several of these calcites have a δ¹⁸O composition that is higher or lower than the oxygen isotopic composition of meteoric calcites in the area (i.e. between ?10‰ and ?6‰ VPDB). The δ¹³C composition largely falls within the range of the host limestones and reflects a rock‐buffered system. Microthermometry and stable isotopic study indicate a meteoric origin of the fluids with some degree of water–rock interaction or mixing with another fluid. Temperatures deduced from microthermometry and stable isotope analyses indicate precipitation temperatures around 50°C. These higher temperatures and the evidence for water–rock interaction indicate a flow path long enough to equilibrate with the host–rock limestone and to increase the temperature. The combined study of extension vein‐ and fault‐related calcite precipitates enables determining the origin of the fluids responsible for precipitation in a normal fault setting. Meteoric water infiltrated in the limestones to a depth of at least 1 km and underwent water–rock interaction or mixing with a residual fluid. This fluid was, moreover, tapped during fault activity. The extension veins, on the contrary, were passively filled with calcites precipitating from the downwards‐migrating meteoric water.     

Relationship of brines in the Kinnarot Basin,Jordan‐Dead Sea Rift Valley

Element ratios and water stable isotopes reveal the presence of only two independent deep brines in the Kinnarot Basin, Israel: the evaporite dissolution brine of Zemah‐1 and the inferred Ha’on mother brine (HMB) with low and high Br/Cl ratios, respectively. HMB is considered to be a representative of the Late Pliocene evaporated Sedom Sea. The freshwater‐diluted evaporation brine emerges as Ha’on brine on the eastern shore of Lake Tiberias and is also identified in the pore water of lake sediments. HMB is converted into Tiberias mother brine (TMB) by dolomitization of limestones and alteration of abundant volcanic rocks occurring along the western side of the lake. The Ha’on and Tiberias brines, both characterized by high δD and δ¹⁸O values, are similar in Na/Cl and Br/Cl ratios but are dissimilar in Br/K ratios because these brines were subjected to different degrees of interactions with rocks and sediments. Excepting the brine from KIN 8, all brines from the Tabigha area including the nearby off‐shore Barbutim brine are related to the TMB. The brine KIN 8 and all brines from the Fuliya and Hammat Gader areas are related to the HMB. The brine encountered in wildcat borehole Zemah‐1 is generated by halite‐anhydrite/gypsum dissolution and is independent from the HMB system.     

Origin of deep saline groundwaters in the Vienne granitic rocks (France): constraints inferred from boron and strontium isotopes

As part of a preliminary geological characterization programme to assess the feasibility of an underground laboratory in granitic rock, a series of 17 deep boreholes (maximum depth, 900 m) was drilled by ANDRA in the Vienne district, France. A salinity gradient was demonstrated in the granitic waters with concentrations varying from approximately 1 g L^?1 at 150 m depth at the top of the basement (beneath the sedimentary cover) to 10 g L^?1 in the deeper part (from 400 to 600 m depth). Sr and B isotope ratios were measured in order to better understand the origin of the salinity and to evaluate the degree of water–rock interaction in the system. The results obtained were compared to those of mineral spring waters emerging from the granitic basement in the Massif Central. Evidence in support of a significant marine contribution include: (i) the Cl–Br investigations agree with a marine origin for the saline groundwaters without evolution from seawater; (ii) the ⁸⁷Sr/⁸⁶Sr ratio of the Vienne deep groundwaters (0.7078–0.7084) is in agreement with a palaeo‐seawater isotopic signature; (iii) measured δ¹¹B values for the deepest brine samples are enriched in ¹¹B (up to 36.1‰) relative to the granitic springs. The combined use of δ¹¹B, Cl, B, Br, Sr contents and ⁸⁷Sr/⁸⁶Sr ratios makes it possible to define and quantify a mixing model between marine and crustal end‐members in order to explain the origin of the deep saline groundwaters in the Vienne granitic rocks.     

Fluid inclusion and stable isotope constraints on the genesis of the Jian copper deposit,Sanandaj–Sirjan metamorphic zone,Iran

The Jian copper deposit, located on the eastern edge of the Sanandaj–Sirjan metamorphic zone, southwest of Iran, is contained within the Surian Permo‐Triassic volcano‐sedimentary complex. Retrograde metamorphism resulted in three stages of mineralization (quartz ± sulfide veins) during exhumation of the Surian metamorphic complex (Middle Jurassic time; 159–167 Ma), and after the peak of the metamorphism (Middle to Late Triassic time; approximately 187 Ma). The early stage of mineralization (stage 1) is related to a homogeneous H₂O–CO₂ (XCO₂ > 0.1) fluid characterized by moderate salinity (<10 wt.% NaCl equivalent) at high temperature and pressure (>370°C, >3 kbar). Early quartz was followed by small amounts of disseminated fine‐grained pyrite and chalcopyrite. Most of the main‐ore‐stage (stage 2) minerals, including chalcopyrite, pyrite and minor sphalerite, pyrrhotite, and galena, precipitated from an aqueous‐carbonic fluid (8–18 wt.% NaCl equivalent) at temperatures ranging between 241 and 388°C during fluid unmixing process (CO₂ effervescence). Fluid unmixing in the primary carbonaceous fluid at pressures of 1.5–3 kbar produced a high XCO₂ (>0.05) and a low XCO₂ (<0.01) aqueous fluid in ore‐bearing quartz veins. Oxygen and hydrogen isotope compositions suggest mineralization by fluids derived from metamorphic dehydration (δ¹⁸O_fluid = +7.6 to +10.7‰ and δD = ?33.1 to ?38.5‰) during stage 2. The late stage (stage 3) is related to a distinct low salinity (1.5–8 wt.% NaCl equivalent) and temperatures of (120–230°C) aqueous fluid at pressures below 1.5 kbar and the deposition of post‐ore barren quartz veins. These fluids probably derived from meteoric waters, which circulated through the metamorphic pile at sufficiently high temperatures and acquire the characteristics of metamorphic fluids (δ¹⁸O_fluid = +4.7 to +5.1‰ and δD = ?52.3 to ?53.9‰) during waning stages of the postearly Cimmerian orogeny in Surian complex. The sulfide‐bearing quartz veins are interpreted as a small‐scale example of redistribution of mineral deposits by metamorphic fluids. This study suggests that mineralization at the Jian deposit is metamorphogenic in style, probably related to a deep‐seated mesothermal system.     

Syntectonic infiltration by meteoric waters along the Sevier thrust front,southwest Montana

Structural, petrographic, and isotopic data for calcite veins and carbonate host‐rocks from the Sevier thrust front of SW Montana record syntectonic infiltration by H₂O‐rich fluids with meteoric oxygen isotope compositions. Multiple generations of calcite veins record protracted fluid flow associated with regional Cretaceous contraction and subsequent Eocene extension. Vein mineralization occurred during single and multiple mineralization events, at times under elevated fluid pressures. Low salinity (T_m = ?0.6°C to +3.6°C, as NaCl equivalent salinities) and low temperature (estimated 50–80°C for Cretaceous veins, 60–80°C for Eocene veins) fluids interacted with wall‐rock carbonates at shallow depths (3–4 km in the Cretaceous, 2–3 km in the Eocene) during deformation. Shear and extensional veins of all ages show significant intra‐ and inter‐vein variation in δ¹⁸O and δ¹³C. Carbonate host‐rocks have a mean δ¹⁸O_V‐SMOW value of +22.2 ± 3‰ (1σ), and both the Cretaceous veins and Eocene veins have δ¹⁸O ranging from values similar to those of the host‐rocks to as low as +5 to +6‰. The variation in vein δ¹³C_V‐PDB of ?1 to approximately +6‰ is attributed to original stratigraphic variation and C isotope exchange with hydrocarbons. Using the estimated temperature ranges for vein formation, fluid (as H₂O) δ¹⁸O calculated from Cretaceous vein compositions for the Tendoy and Four Eyes Canyon thrust sheets are ?18.5 to ?12.5‰. For the Eocene veins within the Four Eyes Canyon thrust sheet, calculated H₂O δ¹⁸O values are ?16.3 to ?13.5‰. Fluid–rock exchange was localized along fractures and was likely coincident with hydrocarbon migration. Paleotemperature determinations and stable isotope data for veins are consistent with the infiltration of the foreland thrust sheets by meteoric waters, throughout both Sevier orogenesis and subsequent orogenic collapse. The cessation of the Sevier orogeny was coincident with an evolving paleogeographic landscape associated with the retreat of the Western Interior Seaway and the emergence of the thrust front and foreland basin. Meteoric waters penetrated the foreland carbonate thrust sheets of the Sevier orogeny utilizing an evolving mesoscopic fracture network, which was kinematically related to regional thrust structures. The uncertainty in the temperature estimates for the Cretaceous and Eocene vein formation prevents a more detailed assessment of the temporal evolution in meteoric water δ¹⁸O related to changing paleogeography. Meteoric water‐influenced δ¹⁸O values calculated here for Cretaceous to Eocene vein‐forming fluids are similar to those previously proposed for surface waters in the Eocene, and those observed for modern‐day precipitation, in this part of the Idaho‐Montana thrust belt.