Contrasting paleofluid systems in the continental basement: a fluid inclusion and stable isotope study of hydrothermal vein mineralization,Schwarzwald district,Germany

An integrated fluid inclusion and stable isotope study was carried out on hydrothermal veins (Sb‐bearing quartz veins, metal‐bearing fluorite–barite–quartz veins) from the Schwarzwald district, Germany. A total number of 106 Variscan (quartz veins related to Variscan orogenic processes) and post‐Variscan deposits were studied by microthermometry, Raman spectroscopy, and stable isotope analysis. The fluid inclusions in Variscan quartz veins are of the H₂O–NaCl–(KCl) type, have low salinities (0–10 wt.% eqv. NaCl) and high T_h values (150–350°C). Oxygen isotope data for quartz range from +2.8‰ to +12.2‰ and calculated δ¹⁸O_H2O values of the fluid are between ?12.5‰ and +4.4‰. The δD values of water extracted from fluid inclusions vary between ?49‰ and +4‰. The geological framework, fluid inclusion and stable isotope characteristics of the Variscan veins suggest an origin from regional metamorphic devolatilization processes. By contrast, the fluid inclusions in post‐Variscan fluorite, calcite, barite, quartz, and sphalerite belong to the H₂O–NaCl–CaCl₂ type, have high salinities (22–25 wt.% eqv. NaCl) and lower T_h values of 90–200°C. A low‐salinity fluid (0–15 wt.% eqv. NaCl) was observed in late‐stage fluorite, calcite, and quartz, which was trapped at similar temperatures. The δ¹⁸O values of quartz range between +11.1‰ and +20.9‰, which translates into calculated δ¹⁸O_H2O values between ?11.0‰ and +4.4‰. This range is consistent with δ¹⁸O_H2O values of fluid inclusion water extracted from fluorite (?11.6‰ to +1.1‰). The δD values of directly measured fluid inclusion water range between ?29‰ and ?1‰, ?26‰ and ?15‰, and ?63‰ and +9‰ for fluorite, quartz, and calcite, respectively. Calculations using the fluid inclusion and isotope data point to formation of the fluorite–barite–quartz veins under near‐hydrostatic conditions. The δ¹⁸O_H2O and δD data, particularly the observed wide range in δD, indicate that the mineralization formed through large‐scale mixing of a basement‐derived saline NaCl–CaCl₂ brine with meteoric water. Our comprehensive study provides evidence for two fundamentally different fluid systems in the crystalline basement. The Variscan fluid regime is dominated by fluids generated through metamorphic devolatilization and fluid expulsion driven by compressional nappe tectonics. The onset of post‐Variscan extensional tectonics resulted in replacement of the orogenic fluid regime by fluids which have distinct compositional characteristics and are related to a change in the principal fluid sources and the general fluid flow patterns. This younger system shows remarkably persistent geochemical and isotopic features over a prolonged period of more than 100 Ma.     

Upper crustal fluid flow in the outboard region of the Southern Alps,New Zealand

The currently active fluid regime within the outboard region of the Southern Alps, New Zealand was investigated using a combination of field observations, carbon‐ and oxygen‐stable isotopes from fault‐hosted calcites and interpretation of magnetotelluric (MT) data. Active faulting in the region is dominated by NE striking and N striking, oppositely dipping thrust fault pairs. Stable isotopic analyses of calcites hosted within these fault zones range from 10 to 25‰δ¹⁸O and from ?33 to 0‰δ¹³C. These values reflect mixing of three parent fluids: meteoric water, carbon‐exchanged groundwater and minor deeper rock‐exchanged fluids, at temperatures of 10–90°C in the upper 3–4 km of the crust. A broad, ‘U‐shaped’ zone of high electrical conductivity (maximum depth c. 28 km) underlies the central Southern Alps. In the ductile region of the crust, the high‐conductivity zone is subhorizontal. Near‐vertical zones of high conductivity extend upward to the surface on both sides of the conductive zone. On the outboard side of the orogen, the conductive zone reaches the surface coincident with the trace of the active Forest Creek Faults. Near‐surface flow is shown to dominate the outboard region. Topographically driven meteoric water interacts, on a kilometre scale, with either carbon‐exchanged groundwater or directly with organic material within Pliocene gravels, resulting in a distinctive low ¹³C signal within fault‐hosted calcites of the outboard region. The high‐strain zone in the lower crust focuses the migration of deeply sourced fluids upward to the base of the brittle–ductile transition. Interconnected fluid is imaged as a narrow vertical zone of high conductivity in the upper crust, implying continuous permeability and possibly buoyancy‐driven flow of deeply sourced fluids to higher levels of the crust where they are detected by the isotopic analysis of the fault‐hosted calcites.     

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.     

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.     

Numerical modeling of the origin of calcite mineralization in the Refugio‐Carneros fault,Santa Barbara Basin,California

Many faults in active and exhumed hydrocarbon‐generating basins are characterized by thick deposits of carbonate fault cement of limited vertical and horizontal extent. Based on fluid inclusion and stable isotope characteristics, these deposits have been attributed to upward flow of formation water and hydrocarbons. The present study sought to test this hypothesis by using numerical reactive transport modeling to investigate the origin of calcite cements in the Refugio‐Carneros fault located on the northern flank of the Santa Barbara Basin of southern California. Previous research has shown this calcite to have low δ¹³C values of about ?40 to ?30‰PDB, suggesting that methane‐rich fluids ascended the fault and contributed carbon for the mineralization. Fluid inclusion homogenization temperatures of 80–125°C in the calcite indicate that the fluids also transported significant quantities of heat. Fluid inclusion salinities ranging from fresh water to seawater values and the proximity of the Refugio‐Carneros fault to a zone of groundwater recharge in the Santa Ynez Mountains suggest that calcite precipitation in the fault may have been induced by the oxidation of methane‐rich basinal fluids by infiltrating meteoric fluids descending steeply dipping sedimentary layers on the northern basin flank. This oxidation could have occurred via at least two different mixing scenarios. In the first, overpressures in the central part of the basin may have driven methane‐rich formation waters derived from the Monterey Formation northward toward the basin flanks where they mixed with meteoric water descending from the Santa Ynez Mountains and diverted upward through the Refugio‐Carneros fault. In the second scenario, methane‐rich fluids sourced from deeper Paleogene sediments would have been driven upward by overpressures generated in the fault zones because of deformation, pressure solution, and flow, and released during fault rupture, ultimately mixing with meteoric water at shallow depth. The models in the present study were designed to test this second scenario, and show that in order for the observed fluid inclusion temperatures to be reached within 200 m of the surface, moderate overpressures and high permeabilities were required in the fault zone. Sudden release of overpressure may have been triggered by earthquakes and led to transient pulses of accelerated fluid flow and heat transport along faults, most likely on the order of tens to hundreds of years in duration. While the models also showed that methane‐rich fluids ascending the Refugio‐Carneros fault could be oxidized by meteoric water traversing the Vaqueros Sandstone to form calcite, they raised doubts about whether the length of time and the number of fault pulses needed for mineralization by the fault overpressuring mechanism were too high given existing geologic constraints.     

Changes in fluid pathways in a calcite vein mesh (Natih Formation,Oman Mountains): insights from stable isotopes

We present a structural, microstructural, and stable isotope study of a calcite vein mesh within the Cretaceous Natih Formation in the Oman Mountains to explore changes in fluid pathways during vein formation. Stage 1 veins form a mesh of steeply dipping crack‐seal extension veins confined to a 3.5‐m‐thick stratigraphic interval. Different strike orientations of Stage 1 veins show mutually crosscutting relationships. Stage 2 veins occur in the dilatant parts of a younger normal fault interpreted to penetrate the stratigraphy below. The δ¹⁸O composition of the host rock ranges from 21.8‰ to 23.7‰. The δ¹³C composition ranges from 1.5‰ to 2.3‰. This range is consistent with regionally developed diagenetic alteration at top of the Natih Formation. The δ¹⁸O composition of vein calcite varies from 22.5‰ to 26.2‰, whereas δ¹³C composition ranges from ?0.8‰ to 2.1‰. A first trend observed in Stage 1 veins involves a decrease of δ¹³C to compositions nearly 1.3‰ lower than the host rock, whereas δ¹⁸O remains constant. A second trend observed in Stage 2 calcite has δ¹⁸O values up to 3.3‰ higher than the host rock, whereas the δ¹³C composition is similar. Stable isotope data and microstructures indicate an episodic flow regime for both stages. During Stage 1, formation of a stratabound vein mesh involved bedding‐parallel flow, under near‐lithostatic fluid pressures. The ¹⁸O fluid composition was host rock‐buffered, whereas ¹³C composition was relatively depleted. This may reflect reaction of low ¹³C CO₂ derived by fluid interaction with organic matter in the limestones. Stage 2 vein formation is associated with fault‐controlled fluid flow accessing fluids in equilibrium with limestones about 50 m beneath. We highlight how evolution of effective stress states and the growth of faults influence the hydraulic connectivity in fracture networks and we demonstrate the value of stable isotopes in tracking changes in fluid pathways.     

Fluid circulation related to post‐Messinian extension,Thassos Island,North Aegean

In the North Aegean Domain, Thassos Island contains a Plio‐Pleistocene basin controlled by a large‐scale flat‐ramp extensional system with a potential décollement located at depth within a marble unit. Numerous mineralizations associated with normal faults of Plio‐Pleistocene age are the sign of fluid circulation during extension. Two main generations of fluid flow are recognized, related to Plio‐Pleistocene extension. A first circulation under high‐temperature conditions (about 100–200°C) resulted in dolomitization of marbles near the base of the Plio‐Pleistocene basin. The dolomites are characterized by low δ¹⁸O values (down to 11‰ versus Standard Mean Ocean Water). Some cataclastic deformation affected the dolomites. Hydrothermal quartz that crystallized in extension veins above a blind ramp also has low δ¹⁸O values (about 13‰). This shows that high‐temperature fluids moved up from the décollement level toward the surface. A second downward circulation of continental waters at near‐surface temperature is documented by calcite veins in fault zones and at the base of the Plio‐Pleistocene basin. These veins have O isotope values relatively constant at about 23–25‰ and C isotope values intermediate between the high δ¹³C value of the carbonate host rock (about 1–3‰ versus Peedee Belemnite) and the low δ¹³C value of soil‐derived carbon (?10‰). The calcites associated with the oxidative remobilization of primary sulphide Zn–Pb mineralization of Thassos carbonates have comparable O and C isotope compositions. Hot fluids, within the 100–200°C temperature range, have likely contributed to the weakening of the lower marble unit of Thassos and, thus, to the process of décollement.     

Red calcite: an indicator of paleo‐karst systems associated with bauxitic unconformities

A unique red calcite generation, which fills fractures/cavities, is hosted by Mesozoic carbonates in the Transdanubian Range, Hungary. Solid inclusions are located along growth zones of calcite. Hematite, the most abundant solid inclusion, gives the red colour of it. Outcrop‐scale geometry, mineralogical features and detrital mineral assemblage (hematite, gibbsite, goethite, kaolinite, smectite, illite, Cr‐spinel, monazite, xenotime, zircon, apatite and Ti‐oxide) of calcite precipitates suggest strong correlation between the calcite and nearby karst bauxite deposits. Fluid inclusion petrography and microthermometry (T < 50°C; salinity from 0 to 0.17 NaCl eq. w%) of primary fluid inclusions, and the stable isotope trend of the calcite, following the meteoric water line, clearly indicate vadose and phreatic meteoric origin in a near‐surface karst system. The late Cretaceous to mid‐Eocene unconformity‐related cavity‐filling deposits occur close to the surface; indicating that the most recent Quaternary exhumation re‐exposed those surfaces that existed at the time of calcite mineralization. Thus, red calcite precipitates are interpreted as being speleothems, vestiges of the subterranean part of the pre‐Middle Eocene karst. The infiltrated, fine bauxite particles enclosed by the calcite are the witnesses of the once areally extensive pre‐Middle Eocene bauxitic blanket that became partially eroded by the time of the deposition of the cover beds. Red calcite when found in core samples may provide good evidence on bauxite formation associated with the overlying unconformity, even if it was later removed by erosion. Therefore, presence or absence of red calcite may be used as distinguishing criteria between karst episodes with or without bauxite formation.     

Diagenesis of the Amposta offshore oil reservoir (Amposta Marino C2 well,Lower Cretaceous,Valencia Trough,Spain)

Samples from the Amposta Marino C2 well (Amposta oil field) have been investigated in order to understand the origin of fractures and porosity and to reconstruct the fluid flow history of the basin prior, during and after oil migration. Three main types of fracture systems and four types of calcite cements have been identified. Fracture types A and B are totally filled by calcite cement 1 (CC1) and 2 (CC2), respectively; fracture type A corresponds to pre‐Alpine structures, while type B is attributed to fractures developed during the Alpine compression (late Eocene‐early Oligocene). The oxygen, carbon and strontium isotope compositions of CC2 are close to those of the host‐rock, suggesting a high degree of fluid‐rock interaction, and therefore a relatively closed palaeohydrogeological system. Fracture type C, developed during the Neogene extension and enlarged by subaerial exposure, tend to be filled with reddish (CS3r) and greenish (CS3g) microspar calcite sediment and blocky calcite cement type 4 (CC4), and postdated by kaolinite, pyrite, barite and oil. The CS3 generation records lower oxygen and carbon isotopic compositions and higher ⁸⁷Sr/⁸⁶Sr ratios than the host‐limestones. These CS3 karstic infillings recrystallized early within evolved‐meteoric waters having very little interaction with the host‐rock. Blocky calcite cement type 4 (CC4 generation) has the lowest oxygen isotope ratio and the most radiogenic ⁸⁷Sr/⁸⁶Sr values, indicating low fluid‐rock interaction. The increasingly open palaeohydrogeological system was dominated by migration of hot brines with elevated oxygen isotope ratios into the buried karstic system. The main oil emplacement in the Amposta reservoir occurred after the CC4 event, closely related to the Neogene extensional fractures. Corrosion of CC4 (blocky calcite cement type 4) occurred prior to (or during) petroleum charge, possibly related to kaolinite precipitation from relatively acidic fluids. Barite and pyrite precipitation occurred after this corrosion. The sulphur source associated with the late precipitation of pyrite was likely related to isotopically light sulphur expelled, e.g. as sulphide, from the petroleum source rock (Ascla Fm). Geofluids (2010) 10 , 314–333     

Inferring intermediate‐scale fluid flow in a heterogeneous metasedimentary multilayer sequence during progressive deformation: evidence from the Monts d’Arrée slate belt (Brittany,France)

Quartz veins in the early Variscan Monts d’Arrée slate belt (Central Armorican Terrane, Western France), have been used to determine fluid‐flow characteristics. A combination of a detailed structural analysis, fluid inclusion microthermometry and stable isotope analyses provides insights in the scale of fluid flow and the water–rock interactions. This research suggests that fluids were expelled during progressive deformation and underwent an evolution in fluid chemistry because of changing redox conditions. Seven quartz‐vein generations were identified in the metasedimentary multilayer sequence of the Upper Silurian to Lower Devonian Plougastel Formation, and placed within the time frame of the deformation history. Fluid inclusion data of primary inclusions in syn‐ to post‐tectonic vein generations indicate a gradual increase in methane content of the aqueous–gaseous H₂O–CO₂–NaCl–CH₄–N₂ fluid during similar P–T conditions (350–400°C and 2–3.5 kbar). The heterogeneous centimetre‐ to metre‐scale multilayer sequence of quartzites and phyllites has a range of oxygen‐isotope values (8.0–14.1‰ Vienna Standard Mean Ocean Water), which is comparable with the range in the crosscutting quartz veins (10.5–14.7‰ V‐SMOW). Significant differences between oxygen‐isotope values of veins and adjacent host rock (Δ = ?2.8‰ to +4.9‰ V‐SMOW) suggest an absence of host‐rock buffering on a centimetre scale, but based on the similar range of isotope values in the Plougastel Formation, an intraformational buffering and an intermediate‐scale fluid‐flow system could be inferred. The abundance of veins, their well‐distributed and isolated occurrence, and their direct relationship with the progressive deformation suggests that the intermediate‐scale fluid‐flow system primarily occurred in a dynamically generated network of temporarily open fractures.     

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.     

A comprehensive review of hydrocarbons and genetic model of the sandstone‐hosted Dongsheng uranium deposit,Ordos Basin,China

The Dongsheng uranium deposit, the largest in situ leach uranium mine in the Ordos Basin, geometrically forms a roll‐front type deposit that is hosted in the Middle Jurassic Zhiluo Formation. The genesis of the mineralization, however, has long been a topic of great debate. Regional faults, epigenetic alterations in surface outcrops, natural oil seeps, and experimental findings support a reducing microenvironment during ore genesis. The bulk of the mineralization is coffinite. Based on thin‐section petrography, some of the coffinite is intimately intergrown with authigenic pyrite (ore‐stage pyrite) and is commonly juxtaposed with some late diagenetic sparry calcite (ore‐stage calcite) in primary pores, suggesting simultaneous precipitation. Measured homogenization temperatures of greater than 100°C from fluid inclusions indicate circulation of low‐temperature hydrothermal fluids in the ore zone. The carbon isotopic compositions of late calcite cement (δ¹³C_VPDB = ?31.0 to ?1.4‰) suggest that they were partly derived from sedimentary organic carbon, possibly from deep‐seated petroleum fluids emanating from nearby faults. Hydrogen and oxygen isotope data from kaolinite cement (δD = ?133 to ?116‰ and δ¹⁸O_SMOW = 12.6–13.8‰) indicate that the mineralizing fluids differed from magmatic and metamorphic fluids and were more depleted in D (²H) than modern regional meteoric waters. Such a strongly negative hydrogen isotopic signature suggests that there has been selective modification of δD by CH₄±H₂S±H₂ fluids. Ore‐stage pyrite lies within a very wide range of δ³⁴S (?39.2 to 26.9‰), suggesting that the pyrite has a complex origin and that bacterially mediated sulfate reduction cannot be precluded. Hydrocarbon migration and its role in uranium reduction and precipitation have here been unequivocally defined. Thus, a unifying model for uranium mineralization can be established: Early coupled bacterial uranium mineralization and hydrocarbon oxidation were followed by later recrystallization of ore phases in association with low‐temperature hydrothermal solutions under hydrocarbon‐induced reducing conditions.     

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.     

Basin‐scale fluid movement patterns revealed by veins: Wessex Basin,UK

Calcite veins at outcrop in the Mesozoic, oil‐bearing Wessex Basin, UK, have been studied using field characterization, petrography, fluid inclusions and stable isotopes to help address the extent, timing and spatial and stratigraphic variability of basin‐scale fluid flow. The absence of quartz shows that veins formed at low temperature without an influence of hydrothermal fluids. Carbon isotopes suggest that the majority of vein calcite was derived locally from the host rock but up to one quarter of the carbon in the vein calcite came from CO₂ from petroleum source rocks. Veins become progressively enriched in source‐rock‐derived CO₂ from the outer margin towards the middle, indicating a growing influence of external CO₂. The carbon isotope data suggest large‐scale migration of substantial amounts of CO₂ around the whole basin. Fluid inclusion salinity data and interpreted water‐δ¹⁸O data show that meteoric water penetrated deep into the western part of the basin after interacting with halite‐rich evaporites in the Triassic section before entering fractured Lower and Middle Jurassic rocks. This large‐scale meteoric invasion of the basin probably happened during early Cenozoic uplift. A similar approach was used to reveal that, in the eastern part of the basin close to the area that underwent most uplift, uppermost Jurassic and Cretaceous rocks underwent vein formation in the presence of marine connate water suggesting a closed system. Stratigraphically underlying Upper Jurassic mudstone and Lower Cretaceous sandstone, in the most uplifted part of the basin, contain veins that resulted from intermediate behaviour with input from saline meteoric water and marine connate waters. Thus, while source‐rock‐derived CO₂ seems to have permeated the entire section, water movement has been more restricted. Oil‐filled inclusions in vein calcite have been found within dominant E‐W trending normal faults, suggesting that these may have facilitated oil migration.     

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.     

Invasion of a karst aquifer by hydrothermal fluids: evidence from stable isotopic compositions of cave mineralization

Mineral deposits in the Cupp‐Coutunn/Promeszutochnaya cave system (Turkmenia, central Asia) record a phase of hydrothermal activity within a pre‐existing karstic groundwater conduit system. Hydrothermal fluids entered the caves through fault zones and deposited sulphate, sulphide and carbonate minerals under phreatic conditions. Locally, intense alteration of limestone wall rocks also occurred at this stage. Elsewhere in the region, similar faults contain economic quantities of galena and elemental sulphur mineralization. Comparisons between the Pb and S isotope compositions of minerals found in cave and ore deposits confirm the link between economic mineralization and hydrothermal activity at Cupp‐Coutunn. The predominance of sulphate mineralization in Cupp‐Coutunn implies that the fluids were more oxidized in the higher permeability zone associated with the karst aquifer. A slight increase in the δ³⁴S of sulphate minerals and a corresponding δ³⁴S decrease in sulphides suggest that partial isotopic equilibration occurred during oxidation. Carbonate minerals indicate that the hydrothermal fluid was enriched in ¹⁸O (δ¹⁸O_SMOW ～ + 10‰) relative to meteoric groundwater and seawater. Estimated values for δ¹³C_DIC (δ¹³C_PDB ～ ? 13‰) are consistent with compositions expected for dissolved inorganic carbon (DIC) derived from the products of thermal decomposition of organic matter and dissolution of marine carbonate. Values derived for δ¹³C_DIC and δ¹⁸O_water indicate that the hydrothermal fluid was of basinal brine origin, generated by extensive water–rock interaction. Following the hydrothermal phase, speleothemic minerals were precipitated under vadose conditions. Speleothemic sulphates show a bimodal sulphur isotope distribution. One group has compositions similar to the hydrothermal sulphates, whilst the second group is characterized by higher δ³⁴S values. This latter group may either record the effects of microbial sulphate reduction, or reflect the introduction of sulphate‐rich groundwater generated by the dissolution of overlying evaporites. Oxygen isotope compositions show that calcite speleothems were precipitated from nonthermal groundwater of meteoric origin. Carbonate speleothems are relatively enriched in ¹³C compared to most cave deposits, but can be explained by normal speleothem‐forming processes under thin, arid‐zone soils dominated by C4 vegetation. However, the presence of sulphate speleothems, with isotopic compositions indicative of the oxidation of hydrothermal sulphide, implies that CO₂ derived by reaction of limestone with sulphuric acid (‘condensation corrosion’) contributed to the formation of ¹³C‐enriched speleothem deposits.     

Role of compressive tectonics on gas charging into the Ordovician sandstone reservoirs in the Sbaa Basin,Algeria: constrained by fluid inclusions and mineralogical data

Structure‐ and tectonic‐related gas migration into Ordovician sandstone reservoirs and its impact on diagenesis history were reconstructed in two gas fields in the Sbaa Basin, in SW Algeria. This was accomplished by petrographical observations, fluid inclusion microthermometry and stable isotope geochemistry on quartz, dickite and carbonate cements and veins. Two successive phases of quartz cementation (CQ1 and CQ2) occurred in the reservoirs. Two phase aqueous inclusions show an increase in temperatures and salinities from the first CQ1 diagenetic phase toward CQ2 in both fields. Microthermometric data on gas inclusions in quartz veins reveal the presence of an average of 92 ± 5 mole% of CH₄ considering a CH₄‐CO₂ system, which is similar to the present‐day gas composition in the reservoirs. The presence of primary methane inclusions in early quartz overgrowths and in quartz and calcite veins suggests that hydrocarbon migration into the reservoir occurred synchronically with early quartz cementation in the sandstones located near the contact with the Silurian gas source rock at 100–140°C during the Late Carboniferous period and the late Hercynian episode fracturing at temperatures between 117 and 185°C, which increased in the NW‐direction of the basin. During the fracture filling, three main types of fluids were identified with different salinities and formation temperatures. A supplementary phase of higher fluid temperature (up to 226°C) recorded in late quartz, and calcite veins is related to a Jurassic thermal event. The occurrence of dickite cements close to the Silurian base near the main fault areas in both fields is mainly correlated with the sandstones where the early gas was charged. It implies that dickite precipitation is related to acidic influx. Late carbonate cements and veins (calcite – siderite – ankerite and strontianite) occurred at the same depths resulting from the same groundwater precipitation. The absence of methane inclusions in calcite cements result from methane flushing by saline waters.     

Fracture sealing and fluid overpressures in limestones of the Jabal Akhdar dome,Oman mountains

Fractures are important conduits for fluid flow in the Earth's crust. To better understand the spatial and temporal relations among fracturing, fracture sealing, and fluid flow, we have studied fractures, faults, and veins in a large dome (Jabal Akhdar) in the Oman mountains. Our work combines the results of meso‐ and microstructural analyses and stable isotope analyses. Seven generations of fractures and veins have been identified in the carbonate‐dominated dome. The earliest generations of veins developed during extension and subsidence of the Mesozoic basin. These veins formed in the inclined segments of bedding‐parallel stylolites and in extensional fractures that are perpendicular to bedding (#1 and #2, respectively). These extension‐related veins are truncated by bedding‐parallel veins (#4) that formed during top‐to‐north bedding‐parallel shear of both the northern and southern limbs of the dome. These veins are consistent with a change in stress regime and may be related to an earlier generation of strongly deformed pinch‐and‐swell veins (#3) that are exposed locally on the southern limb of the dome. Normal faults contain a set of en‐echelon tension gashes (#5) and veins emplaced in dilational jogs along the fault planes (#6). In the northern part of the dome, veins (#7) associated with thrusts post‐date the normal faults. Samples of veins and their host rocks were analyzed to provide information on fluid‐rock interaction in the dome and the scale(s) of fluid movement. Oxygen isotope values range from +16.2 to +29.3‰; carbon isotope values range from 0 to +3.6‰. The results of the structural and isotopic analyses are consistent with the early veins (#2–#5) having precipitated from overpressured fluid in a isotopically rock‐buffered system. During normal faulting (#5 and #6), a more open system allowed external fluid to infiltrate the dome at drained conditions and precipitate the youngest sets of veins (#6 and #7).     

Textural and isotopic evidence on the fluid source and transport mechanism of antitaxial fibrous microstructures from the Alps and the Appalachians

The source of fluid‐forming veins is of great importance in order to understand the hydraulic system acting in the earth's crust. The study of syntectonic antitaxial veins is one of the few methods by which the opening history can be deduced from rocks, and thus these veins are of primary importance in determining rock kinematics. Antitaxial veins were taken from black shales in two different tectonic settings in the Helvetic Alps, Switzerland, and the Taconic Appalachians, New York State. These syntectonic extension veins are regularly spaced and are oriented sub‐normal to bedding. The vein microstructure displays a symmetry around the median line in the centre of the vein, and a symmetry in cathodoluminescence banding parallel to the vein–wall interface, which suggests transport along bedding‐parallel dissolution planes from both vein‐walls. Antitaxial veins nucleated in transgranular fractures, but evidence for ongoing multiple crack‐seal increments is lacking; rather, veins grew continuously keeping close contact to the vein‐wall. Radiogenic ⁸⁷Sr/⁸⁶Sr ratios are higher in the surrounding matrix than in the vein, and higher than the corresponding seawater data in all samples. Variations are small and calcite in both the vein and the host rock were derived from the same source of fluid in the Helvetic samples. Mass balance of Sr suggests that the amount of calcite is too small in the surrounding host rock to be derived locally. Stable oxygen compositions are heavier in the host rock than in the veins, with overall low variation in both δ¹⁸O and δ¹³C values in the Mesozoic Helvetic samples. Data point to a rock‐buffered system, the precipitate most likely derived from an external source. The lower Palaeozoic Appalachian veins have lesser δ¹⁸O values than the host rock, similar to the Helvetic veins. Radiogenic ⁸⁷Sr/⁸⁶Sr data and a large heterogeneity in stable isotope values indicate an open system. Microstructural and isotopic evidence suggests that the antitaxial veins were formed by pervasive fluid flow, with the solute at least partly derived from an external source.     

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.