The link between fluids and rank variation in the South Wales Coalfield: evidence from fluid inclusions and stable isotopes

Fluid inclusion and stable isotope data from quartz and carbonate minerals in fracture fillings and ‘ironstone’ nodules from the South Wales Coalfield have been used to characterise the fluids generated during basin evolution and associated coalification. Carbonates grew first, probably at relatively shallow depths and low temperatures (<100°C). The carbonates exhibit a trend of increasing C‐isotopic values across the coalfield, ranging from δ¹³C = ?12‰ VPDB in the SE of the coalfield to 0‰ VPDB in the NW, possibly as a result of increasing methanogenesis in the deeper (NW) parts of the coalfield. Quartz formed at a later stage of basin formation, probably at temperatures between 150 and 200°C. Fluid inclusions in these minerals suggest that burial and coalification of the sediments were associated with mixed aqueous–petroleum fluids. Furthermore, the density of these petroleum fluids decreases towards the NW of the coalfield, where the rank of the associated coal increases to anthracite grade. The study confirms that the composition and temperature of these fluids closely correlate with the variations in coal rank, indicating a possible causal link. The data also give general support to models that propose regional fluid flow in the basin. and are consistent with the erosion of approximately 2 km of section which is not preserved today. A geothermal gradient (at maximum burial) of 45°C km^?1 is proposed, and thus no exceptionally anomalous thermal regime is required to explain coal rank variation.     

Petroleum infiltration of high-grade basement, South Norway: Pressure-Temperature-time-composition (P–T–t–X) constraints

Fluid inclusion data provide pressure–temperature–time–composition (P–T–t–X) constraints for an episode of petroleum infiltration of the crystalline basement in South Norway. Petroleum inclusions associated with pyrobitumen occur in postmetamorphic quartz veins in the Modum Complex. Three groups of fluid compositions have been shown, ranging from CH₄ ± CO₂ to condensates with alkanes up to C₁₅. The range in fluid composition is a result of petroleum decomposition at high temperature. Globular and massive pyrobitumen occurs in the quartz veins or in associated vein systems. Reflectance (%Rm) measurements of 3.20–3.35 correspond to a maximum temperature of 207–214°C for the pyrobitumen associated with group II and III inclusions. Geothermometry of chlorites included in the quartz show results of 226–231°C. Pressure conditions of trapping for all three groups of inclusion fluids have been estimated to 520–985 bar at 220°C. The pressure range is probably a result of fluctuations caused by repeated fracture opening and sealing due to seismic activity coupled with mineral growth. A lack of systematic textural relationships between the three groups of inclusions and similar pressure–temperature estimates for all fluid types indicate trapping at similar times and a process of rapid change. Fluid migration in fractures from an overlying, overpressured sedimentary basin into a dry, crystalline basement best explains the observed P–T–t–X constraints.     

Mobilization of ore fluids during Alpine metamorphism: evidence from hydrothermal veins in the Variscan basement of Western Carpathians,Slovakia

Hydrothermal polymetallic veins of the Gemeric unit of the Western Carpathians are oriented coherently with the foliation of their low‐grade Variscan basement host. Early siderite precipitated from homogeneous NaCl‐KCl‐CaCl₂‐H₂O brines with minor CO₂, while immiscible gas–brine mixtures are indicative of the superimposed barite, quartz–tourmaline and quartz–sulphide stages. The high‐salinity aqueous fluid (18–35 wt%) found in all mineralization stages corresponds to formation water modified by interaction with crystalline basement rocks at temperatures between 140 and 300°C. High brominity (around 1000 ppm in average) resulted from evaporation and anhydrite precipitation in a Permo‐Triassic marine basin, and from secondary enrichment by dissolution of organic matter in the marine sediments at diagenetic temperatures. Sulphate depletion reflects thermogenic reduction during infiltration of the formation waters into the Variscan crystalline basement. Crystallization temperatures of the siderite fill (140–300°C) and oxygen isotope ratios of the parental fluids (4–10‰) increase towards the centre of the Gemeric cleavage fan, probably as a consequence of decreasing water/rock ratios in rock‐buffered hydrothermal systems operating during the initial stages of vein evolution. In contrast, buoyant gas–water mixtures, variable salinities and strongly fluctuating P–T parameters in the successive mineralization stages reflect transition from a closed to an open hydrothermal system and mixing of fluids from various sources. Depths of burial were 6–14 km (1.7–4.4 kbar, in a predominantly lithostatic fluid regime) during the siderite and barite sub‐stages of the north‐Gemeric veins, and up to 16 km (1.6–4.5 kbar, in a hydrostatic to lithostatic fluid regime) in the quartz–tourmaline stage of the south‐Gemeric veins. The fluid pressure decreased down to approximately 0.6 kbar during crystallization of sulphides. U‐Pb‐Th, ⁴⁰Ar/³⁹Ar and K/Ar geochronology applied to hydrothermal muscovite–phengite and monazite, as well as cleavage phyllosilicates in the adjacent basement rocks and deformed Permian conglomerates corroborated the opening of hydrothermal veins during Lower Cretaceous thrusting and their rejuvenation during Late Cretaceous sinistral transpressive shearing and extension.     

The upper continental crust, an aquifer and its fluid: hydaulic and chemical data from 4 km depth in fractured crystalline basement rocks at the KTB test site

Detailed information on the hydrogeologic and hydraulic properties of the deeper parts of the upper continental crust is scarce. The pilot hole of the deep research drillhole (KTB) in crystalline basement of central Germany provided access to the crust for an exceptional pumping experiment of 1‐year duration. The hydraulic properties of fractured crystalline rocks at 4 km depth were derived from the well test and a total of 23100 m³ of saline fluid was pumped from the crustal reservoir. The experiment shows that the water‐saturated fracture pore space of the brittle upper crust is highly connected, hence, the continental upper crust is an aquifer. The pressure–time data from the well tests showed three distinct flow periods: the first period relates to wellbore storage and skin effects, the second flow period shows the typical characteristics of the homogeneous isotropic basement rock aquifer and the third flow period relates to the influence of a distant hydraulic border, probably an effect of the Franconian lineament, a steep dipping major thrust fault known from surface geology. The data analysis provided a transmissivity of the pumped aquifer T = 6.1 × 10^?6 m² sec^?1, the corresponding hydraulic conductivity (permeability) is K = 4.07 × 10^?8 m sec^?1 and the computed storage coefficient (storativity) of the aquifer of about S = 5 × 10^?6. This unexpected high permeability of the continental upper crust is well within the conditions of possible advective flow. The average flow porosity of the fractured basement aquifer is 0.6–0.7% and this range can be taken as a representative and characteristic values for the continental upper crust in general. The chemical composition of the pumped fluid was nearly constant during the 1‐year test. The total of dissolved solids amounts to 62 g l^?1 and comprise mainly a mixture of CaCl₂ and NaCl; all other dissolved components amount to about 2 g l^?1. The cation proportions of the fluid (X_Ca approximately 0.6) reflects the mineralogical composition of the reservoir rock and the high salinity results from desiccation (H₂O‐loss) due to the formation of abundant hydrate minerals during water–rock interaction. The constant fluid composition suggests that the fluid has been pumped from a rather homogeneous reservoir lithology dominated by metagabbros and amphibolites containing abundant Ca‐rich plagioclase.     

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.     

Composition and evolution of fluids during skarn development in the Monte Capanne thermal aureole,Elba Island,central Italy

We describe the chemistry of the fluids circulating during skarn formation by focusing on fluids trapped in calcsilicate minerals of the inner thermal aureole of the Late Miocene Monte Capanne intrusion of western Elba Island (central Italy). Primary, CH₄‐dominant, C‐O‐H‐S‐salt fluid inclusions formed during prograde growth of the main skarn‐forming mineral phases: grossular/andradite and vesuvianite. The variable phase ratios attest to heterogeneous entrapment of fluid, with co‐entrapment of an immiscible hydrocarbon–brine mixture. Chemical elements driving skarn metasomatism such as Na, K, Ca, S and Cl, Fe and Mn were dominantly partitioned into the circulating fluid phase. The high salinity (apparent salinity between 58 and 70 wt% NaCl eq.) and the C‐component of the fluids are interpreted as evidence for a composite origin of the skarn‐forming fluids that involves both fluids derived from the crystallizing intrusion and contributions from metamorphic devolatilization. Oxidation of a Fe‐rich brine in an environment dominated by fluctuation in pressure from lithostatic to hydrostatic conditions (maintained by active crack‐sealing) contributed to skarn development. Fluid infiltration conformed to a geothermal gradient of about 100°C km^?1, embracing the transition from high‐temperature contact metamorphism and fluid‐assisted skarn formation (at ca 600°C) to a barren hydrothermal stage (at ca 200°C).     

Magmatic fluids immiscible with silicate melts: examples from inclusions in phenocrysts and glasses, and implications for magma evolution and metal transport

The first occurrence of immiscibility in magmas appears to be most important in the magmatic–hydrothermal transition, and thus studies of magmatic immiscibility should be primarily directed towards recognition of coexisting silicate melt and essentially non-silicate liquids and fluids (aqueous, carbonic and sulphide). However, immiscible phase separation during decompression, cooling and crystallization of magmas is an inherently fugitive phenomenon. The only remaining evidence of this process and the closest approximation of natural immiscible magmatic liquids and vapours can be provided by melt and fluid inclusions trapped in silicate glasses and magmatic phenocrysts. Such inclusions are often used as a natural experimental laboratory to model the process of exsolution and the compositions of volatile-rich phases from a wide range of terrestrial magmas. In this paper several examples from recent research on melt and fluid inclusions are used to demonstrate the significance of naturally occurring immiscibility in understanding some large-scale magma chamber processes, such as degassing and partitioning of metals.     

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.     

Geological setting, nature of ore fluids and sulphur isotope geochemistry of the Fu Ning Carlin-type gold deposits, Yunnan Province, China

Carlin‐type gold deposits in southern China are present in Palaeozoic to Mesozoic siliciclastic and carbonate rocks. The border region of Yunnan, Guizhou and Guangxi Provinces contains gold deposits on the south‐western margin of the Pre‐Cambrian South China Craton in south‐eastern Yunnan Province. The Fu Ning gold deposits host epigenetic, micron‐sized disseminated gold in: (i) Middle Devonian (D₁p) black carbonaceous mudstone at the Kuzhubao gold deposit and (ii) fault breccia zones at the contact between Triassic gabbro (β ) and the Devonian mudstone (D₁p) at the Bashishan gold deposit. The deposits are associated with zones of intense deformation with enhanced permeability and porosity that focused hydrothermal fluid flow, especially where low‐angle N‐S striking thrust faults are cut by NW striking strike‐slip and/or NE striking normal faults. Major sulphide ore minerals in the Fu Ning gold deposits are pyrite, arsenopyrite, arsenic‐rich pyrite, stibnite and minor iron‐poor sphalerite. Gangue minerals are quartz, sericite, calcite, ankerite and chlorite. Hypogene ore grades range from 1 to 7 g t^?1 Au and up to 18 g t^?1 Au at the Kuzhubao gold deposit and are generally less than 3 g t^?1 Au at the Bashishan gold deposit. Sub‐microscopic gold mineralization is associated with finely disseminated arsenic‐rich pyrite in the Stage III mineral assemblage. Two types of primary fluid inclusions have been recorded: Type I liquid–vapour inclusions with moderate‐to‐high liquid/vapour ratios, and Type II inclusions containing moderate liquid/vapour ratios with CO₂ as determined from laser Raman analysis. Temperature of homogenization (T_h) data collected from these primary fluid inclusions in gold‐ore Stage III quartz ranged from 180 to 275°C at the Kuzhubao gold deposit and 210 to 330°C at the Bashishan gold deposit. Salinity results indicate that there were possibly two fluids present during gold deposition, including: (i) an early fluid with 0.8–6.5 wt.% NaCl equivalent, similar to salinity in shear‐zone‐hosted gold deposits with metamorphic derived fluids; and (ii) a late fluid with 11.8–13.4 wt.% NaCl equivalent, indicating possible derivation from connate waters and/or brine sources. CO₂ and trace CH₄ were only detected by laser Raman spectrometry in gold‐ore‐stage primary fluid inclusions. Results of sulphur isotope studies showed that δ³⁴S values for pyrite and arsenopyrite associated with gold‐ore mineralization during Stage III at the Kuzhubao and Bashishan gold deposits are isotopically similar and moderately heavy with a range from +9 to +15 per mil, and also fall into the range of δ³⁴S values reported for Carlin‐type gold deposits. Sulphur isotopes suggest that the Fu Ning gold deposits were formed from connate waters and/or basinal brines. Fluid geochemistry data from the Fu Ning gold deposits suggest a Carlin‐type genetic model, involving fluid mixing between: (i) deep CO₂‐rich metamorphic fluids, (ii) moderately saline, reduced connate waters and/or basinal brines; and (iii) evolved meteoric waters.     

Fluids and halogens at the diagenetic–metamorphic boundary: evidence from veins in continental basins, western Norway

Seven vein types are recognized in three continental Devonian molasse basins (the Hornelen, Kvamshesten and Solund basins) in western Norway. These include calcite‐, quartz‐ and epidote‐dominated veins. The salinities of fluid inclusions from quartz‐dominated veins in the Hornelen and Kvamshesten basins are close to or slightly higher than those for modern seawater, whereas the fluids from quartz‐ and calcite‐dominated veins in the Solund basin range from seawater values to 20 wt % NaCl equivalent. Minerals such as biotite, amphibole, titanite, chlorite and epidote are abundant in the latter veins, and are important constituents of the authigenic mineral assemblages. A combination of fluid inclusion and petrological data suggest that at least some of the veins formed at depths around 12–14 km. The Cl/Br ratios and the salinity of the fluid inclusions can be explained by interactions with evaporites, implying that the sedimentary environment forming the basin fill had the strongest influence upon low‐grade metamorphic fluid Cl and Br contents. Differences in the Cl/I and Na/Br ratios between the Solund basin and the Hornelen and Kvamshesten basins are best explained by local mass transfer between pore fluids and the surrounding rock matrix during burial and increasing temperatures.     

Salinity structure of the central North Slope foreland basin, Alaska, USA: implications for pathways of past and present topographically driven regional fluid flow

Previous studies of the areal variation in heat flow in the National Petroleum Reserve Alaska (NPRA) support the existence of an active topographically driven regional fluid flow regime in this central part of the North Slope foreland basin. Drilling records and wireline logs for over 30 wells drilled in the NPRA provide additional field information, which can be used to further constrain interpretation of the pattern of regional flow of basinal waters within the NPRA. Hydraulic heads estimated from drilling mud weights show that ground water flow occurs generally from south to north, but with divergence to the north‐east and north‐west away from the central part of the NPRA towards coastal areas of elevated shallow heat flow. Salinities calculated from SP logs range from less than 1 g L^?1, to marine values of 35 g L^?1, to hypersaline values of over 150 g L^?1. The entire upper sedimentary section to a depth of 2 km or more in the eastern part of the NPRA has been preferentially flushed with meteoric water through an area corresponding to the sandiest portion of the Nanushuk group. Deeper areas of low salinity occur within the Sadlerochit and Lisburne sections. The pattern of regional flow in the east is complicated, however, by the presence of a large mass of hypersaline water at depth. It is not known whether these brines are being displaced laterally and upward towards the discharge end of the basin or whether fresher waters are simply riding up over the top. Deep, hypersaline waters also occur in fault slices in the Brooks Range and have survived meteoric flushing. The brines were probably formed at the time of deposition of the Lisburne carbonates. The fluid flow regime to the west is different. Low‐salinity waters may be flowing northward underneath this section through the Ellesmerian section and discharging upward nearer the coast. However, sparse well log control severely limits what can be deduced about the details of flow paths in the central and western parts of the NPRA.     

Package FLUIDS. Part 3: correlations between equations of state, thermodynamics and fluid inclusions

The computer package FLUIDS ( Bakker 2003 ) has been revised to calculate fluid properties in pores and inclusions. The programs are provided with a Graphical User Interface (GUI) and are adapted to new operating systems, including MacOS, Windows and Linux. The van der Waals equation of state has been added to the group Loners and is used to illustrate a large variety of calculation procedures for many thermodynamic parameters and properties. The mathematical transformations can be applied to any equation of state with the form p ( V , T , n ), i.e. the pressure of a multi-component fluid is expressed as a function of volume, temperature and amount of substance. The fluid properties that are usually observed in microthermometric analyses of fluid inclusions, such as phase separation, phase coexistence and stability, can be predicted with these equations of state by using its spinodal and critical point calculations, in addition to fugacity calculations of liquid and vapour phases. The computer program LonerW can be freely downloaded from the website: http://fluids.unileoben.ac.at/Computer.html .     

FT-IR measurements of petroleum fluid inclusions: methane, n-alkanes and carbon dioxide quantitative analysis

A recent advancement in petroleum geochemistry is to model fossil oil composition using microthermometric and volumetric data acquired from individual fluid inclusion analysis. Fourier transform infrared (FT‐IR) microspectroscopy can record compositional information related to gas (CH₄ and CO₂) and alkane contents of petroleum inclusions. In this study, a quantitative procedure for FT‐IR microspectrometry has been developed to obtain, from individual fluid inclusions, mol percentage concentrations of methane, alkanes and carbon dioxide as constraints to thermodynamic modelling. A petroleum inclusion in a sample from the Québec City Promontory nappe area was used as standard to record a reference spectrum of methane. The analytical procedure is based on the measurement of CH₄/alkane and CH₄/CO₂ band area ratios. CH₄/alkane infrared band area ratio is obtained after spectral subtraction of the reference methane spectrum. This area ratio, affected by absolute absorption intensities of methane, methyl and methylene, provides a molar CH₄/alkane ratio. Methyl/methylene ratio (CH₂/CH₃) ratio is obtained following procedures established in previous work. CO₂/CH₄ concentration ratio is estimated from relative absolute absorption intensities. Application to natural inclusions from different environments shows good correlation between FT‐IR quantification and PIT (petroleum inclusion thermodynamic) modelling.     

Evolution of porosity, permeability and fluid pressure in dilatant faults post-failure: implications for fluid flow and mineralization

Mineralization of brittle fault zones is associated with sudden dilation, and the corresponding changes in porosity, permeability and fluid pressure, that occur during fault slip events. The resulting fluid pressure gradients cause fluid to flow into and along the fault until it is sealed. The volume of fluid that can pass through the deforming region depends on the degree of dilation, the porosity and permeability of the fault and wall rocks, and the rate of fault sealing. A numerical model representing a steep fault cutting through a horizontal seal is used to investigate patterns of fluid flow following a dilatant fault slip event. The model is initialized with porosity, permeability and fluid pressure representing the static mechanical state of the system immediately after such an event. Fault sealing is represented by a specified evolution of porosity, coupled to changes in permeability and fluid pressure, with the rate of porosity reduction being constrained by independent estimates of the rate of fault sealing by pressure solution. The general pattern of fluid flow predicted by the model is of initial flow into the fault from all directions, followed by upward flow driven by overpressure beneath the seal. The integrated fluid flux through the fault after a single failure event is insufficient to account for observed mineralization in faults; mineralization would require multiple fault slip events. Downward flow is predicted if the wall rocks below the seal are less permeable than those above. This phenomenon could at least partially explain the occurrence of uranium deposits in reactivated basement faults that cross an unconformity between relatively impermeable basement and overlying sedimentary rocks.     

Retrograde P–T evolution and high temperature–low pressure fluid circulation in relation to late Hercynian intrusions: a mineralogical and fluid inclusion study of the Charroux-Civray plutonic complex (north-western Massif Central, France)

The calc‐alkaline plutonic complex from Charroux‐Civray (north‐western part of the French Massif Central) displays multiphase hydrothermal alteration. Plutonic rocks, as well as early retrograde Ca–Al silicate assemblages, which have crystallized during cooling and uplifting of the plutonic series, are affected by multiphase chlorite–phengite–illite–carbonate alteration linked to intense pervasive fluid circulation through microfractures. The petrographic study of alteration sequences and their associated fluid inclusions in microfissures of the plutonic rocks, as well as in mineral fillings of the veins, yields a reconstruction of the P–T–X evolution of the Hercynian basement after the crystallization of the main calc‐alkaline plutonic bodies. This reconstruction covers the uplift of the basement to its exposure and the subsequent burial by Mesozoic sediments. Cooling of the calc‐alkaline plutonic series started at solidus temperatures (～650°C), at a pressure of about 4 kbar (1 bar = 10⁵ N m^?2), as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data. Cooling continued under slightly decreasing pressure during uplift down to 2–3 kbar at 200–280°C (prehnite–pumpellyite paragenesis). Then, a hot geothermal circulation of CO₂‐bearing fluids was induced within the calc‐alkaline rocks leading to the formation of greisen‐like mineralizations. During this stage, temperatures around 400–450°C were still high for the inferred depths (～2 kbar). They imply abnormal heat flows and thermal gradients of 60–80°C km^?1. The hypothesis of the existence of one large or a succession of smaller peraluminous plutons at depth, supported by geophysical data, suggests that localized heat flows were linked to concealed leucogranite intrusions. As uplift continued, greisen mineralization was subsequently affected by the chlorite–phengite–dolomite assemblage, correlated with aqueous and nitrogen‐bearing fluid circulations in the temperature range of 400–450°C. In a later stage, a continuous temperature decrease at constant pressure (～0.5 kbar) led to the alteration of the dolomite–illite–chlorite type in the 130–250°C temperature range.     

Mineral precipitation associated with vertical fault zones: the interaction of solute advection,diffusion and chemical kinetics

This article is concerned with chemical reactions that occur between two interacting parallel fluid flows using mixing in vertical faults as an example. Mineral precipitation associated with fluid flow in permeable fault zones results in mineralization and chemical reaction (alteration) patterns, which in turn are strongly dependent on interactions between solute advection (controlled by fluid flow rates), solute diffusion/dispersion and chemical kinetics. These interactions can be understood by simultaneously considering two dimensionless numbers, the Damköhler number and the Z‐number. The Damköhler number expresses the interaction between solute advection (flow rate) and chemical kinetics, while the Z‐number expresses the interaction between solute diffusion/dispersion and chemical kinetics. Based on the Damköhler and Z‐numbers, two chemical equilibrium length‐scales are defined, dominated by either solute advection or by solute diffusion/dispersion. For a permeable vertical fault zone and for a given solute diffusion/dispersion coefficient, there exist three possible types of chemical reaction patterns, depending on both the flow rate and the chemical reaction rate. These three types are: (i) those dominated by solute diffusion and dispersion resulting in precipitation at the lower tip of a vertical fault and as a thin sliver within the fault, (ii) those dominated by solute advection resulting in precipitation at or above the upper tip of the fault, and (iii) those in which advection and diffusion/dispersion play similar roles resulting in wide mineralization within the fault. Theoretical analysis indicates that there exists both an optimal flow rate and an optimal chemical reaction rate, such that chemical equilibrium following focusing and mixing of two fluids may be attained within the fault zone (i.e. type 3). However, for rapid and parallel flows, such as those resulting from a lithostatic pressure gradient, it is difficult for a chemical reaction to reach equilibrium within the fault zone, if the two fluids are not well mixed before entering the fault zone. Numerical examples are given to illustrate the three possible types of chemical reaction patterns.     

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.     

Source of ore fluids at the Kalahari Goldridge deposit, Kraaipan greenstone belt, South Africa: evidence from Sr, C and O isotope signatures in carbonates

The Kalahari Goldridge deposit is located in the Archaean Kraaipan greenstone belt in the north-west province of South Africa. Gold mineralization in this deposit is hosted within banded iron formation which is flanked by a mafic schist in the footwall and clastic metasedimentary units in the hanging wall. Data from carbonate minerals from mineralized veins and bulk rock from the A and D zone ore bodies have helped to define the ultimate origin of the ore-forming fluids and their migration history. Carbon isotope ratios of carbonates from both the A and D zone ore bodies have tight clustering from −7.6 to −5.3‰ that indicates a unique origin for the ore-forming fluids associated with the mineralization at Kalahari Goldridge. The δ¹⁸O values of the carbonates have been influenced by temperature gradients and variable degrees of fluid–rock interaction promoting oxygen isotope exchange between ore fluid and host rocks. Minimum ⁸⁷Sr/⁸⁶Sr ratio values of 0.70354 in mineralized veins are most consistent with ore-forming fluids being relatively pristine with a mantle origin. Strontium and the corresponding ore-forming fluids were most likely derived from mantle-derived magmatic rocks probably represented by the meta-basaltic rocks that underlie the ferruginous package in the Kraaipan greenstone belt. Strontium isotopic composition of vein carbonates show considerable variation in ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.70354 to 0.73914. This is consistent with an ore fluid composition that has been modified by the addition of radiogenic Sr possibly during passage of fluid through siliciclastic country rock concomitant with the observed hydrothermal alteration.