Mineralogy and fluid inclusion gas chemistry of production well mineral scale deposits at the Dixie Valley geothermal field,USA

At the Dixie Valley geothermal field, Nevada, USA, fluid boiling triggered the precipitation of carbonate scale minerals in concentric bands around tubing inserted into production well 28–33. When the tubing was removed, this mineral scale was sampled at 44 depth intervals between the wellhead and 1227 m depth. These samples provide a unique opportunity to evaluate the effects of fluid boiling on the scale mineralogy and geochemistry of the vapor and liquid phase. In this study, the mineralogy of the scale deposits and the composition of the fluid inclusion gases trapped in the mineral scales were analyzed. The scale consists mainly of calcite from 670–1112 m depth and aragonite from 1125 to 1227 m depth, with traces of quartz and Mg‐smectite. Mineral textures, including hopper growth, twinning, and fibrous growth in the aragonite and banded deposits of fine grained calcite crystals, are the result of progressive boiling. The fluid inclusion noncondensable gas was dominated by CO₂. However, significant variations in He relative to N₂ and Ar provide evidence that the geothermal reservoir consists of mixed source deeply circulating reservoir water and shallow, air saturated meteoric water. Gas analyses for many inclusions also showed higher CH₄ and H₂ relative to CO₂ than measured in gas sampled from this well, other production wells, and fumaroles. These inclusions are interpreted to have trapped CH₄‐ and H₂‐enriched gas resulting from early stages of boiling.     

Source and character of syntaxial hydrothermal calcite veins in Paleoproterozoic crystalline rocks revealed by fine‐scale investigations

Calcite veins in Paleoproterozoic granitoids on the Baltic Shield are the focus of this study. These veins are distinguished by their monomineralic character, unusual thickness and closeness to Neoproterozoic dolerite dykes and therefore have drawn attention. The aim of this study was to define the source of these veins and to unravel their isotopic and chemical nature by carrying out fine‐scale studies. Seven calcite veins covering a depth interval of 50–420 m below the ground surface and composed of breccias or crack‐sealed fillings typically expressing syntaxial growth were sampled and analysed for a variety of physicochemical variables: homogenization temperature (T_h) and salinity of fluid inclusions, and stable isotopes (⁸⁷Sr/⁸⁶Sr, ¹³C/¹²C, ¹⁸O/¹⁶O), trace‐element concentrations (Fe, Mn, Mg, Sr, rare earth elements) and cathodoluminescence (CL) of the solid phase. The fluid‐inclusion data show that the calcites were precipitated mainly from relatively low‐temperature (T_h = 73–106°C) brines (13.4–24.5 wt.% CaCl₂), and the ⁸⁷Sr/⁸⁶Sr is more radiogenic than expected for Rb‐poor minerals precipitated from Neoproterozoic fluids. These features, together with the distribution of δ¹³C and δ¹⁸O values, provide evidence that the calcite veins are not genetic with the nearby Neoproterozoic dolerite dykes, but are of Paleozoic age and were precipitated from warm brines expressing a rather large variability in salinity. Whereas the isotopic and chemical variables express rather constant average values among the individual veins, they vary considerably on fine‐scale across individual veins. This has implications for understanding processes causing calcite‐rich veins to form and capture trace metals in crystalline bedrock settings.     

Fluid control on low‐temperature mineral formation in volcanic rocks of Kahrizak,Iran

The Kahrizak volcanic field, south Tehran, in Iran, is composed dominantly of basalt and basaltic andesite that experienced variable degrees of alteration because of the low‐grade metamorphism (stage I) and hydrothermal activity (stage II). Stage I alteration, which occurred in response to the burial of volcanic rocks and their interaction with heated groundwater, is characterized by the formation of low‐temperature zeolite facies minerals in vesicles consisting mainly of fine‐grained mafic phyllosilicate (smectite, chlorite/smectite mixed layer) and zeolites (thomsonite, chabazite, gonnardite, natrolite, analcime, heulandite, and mordenite). Stage II mineralization occurred because of the activity of hydrothermal fluids that formed large crystals of heulandite, stilbite, mesolite/scolecite, natrolite, and analcime along with quartz and calcite in cavities and fractures. The elements necessary for the formation of alteration minerals (i.e. zeolites and mafic phyllosilicates) in Kahrizak were derived from the hydrolysis of olivine and volcanic glass as well as the alteration of plagioclase. Various mineral assemblages formed during stages I and II reflect changes in temperature, pressure, and fluid composition. The change from mafic phyllosilicates to zeolites species is caused by the decrease in Mg and Fe relative to Ca fluid activities. Zeolite assemblages of stage I, known to be formed at lower temperatures, show the general sequential order from older to younger: chabazite, thomsonite, gonnardite, and natrolite. This sequence is consistent with a hypothetical fluid evolution path with increasing Na⁺ relative to Ca²⁺ activity. The change to stage II, which consists of zeolites species (stilbite, scolecite, natrolite, mesolite, analcime, and heulandite) that formed at higher temperatures, can be attributed to a temperature increase and fluid influx caused by hydrothermal activity related to a later magmatic event in the region.     

Hydrothermal palaeofluid circulation in the fracture network of the Baksa Gneiss Complex of SW Pannonian Basin,Hungary

A well‐developed fracture‐filling network is filled by dominantly Ca‐Al‐silicate minerals that can be found in the polymetamorphic rock body of the Baksa Gneiss Complex (SW Hungary). Detailed investigation of this vein network revealed a characteristic diopside→epidote→sphalerite→albite ± kfeldspar→chlorite1 ± prehnite ± adularia→chlorite2→chlorite3→pyrite→calcite1→calcite2→calcite3 fracture‐filling mineral succession. Thermobarometric calculations (two feldspar: 230–336°C; chlorites: approximately 130–300°C) indicate low‐temperature vein formation conditions. The relative succession of chlorites in the mineral sequence combined with the calculated formation temperatures reveals a cooling trend during precipitation of the different chlorite phases (T_chlorite1: 260 ± 32°C →T_chlorite2: 222 ± 20°C →T_chlorite3: 154 ± 13°C). This cooling trend can be supported by the microthermometry data of primary fluid inclusions in diopside (T_h: 276–362°C) and epidote (T_h: 181–359°C) phases. The identical chemical character (0.2–1.5 eq. wt% NaCl) of these inclusions mean that vein mineralization occurred in a same fluid environment. The high trace element content (e.g. As, Cu, Zn, Mn) and Co/Ni ratio approximately 1–5 of pyrite grains support the postmagmatic hydrothermal origin of the veins. The vein microstructure and identical fluid composition indicate that vein mineralization occurred in an interconnected fracture system where crystals grew in fluid filled cracks. Vein system formed at approximately <200 MPa pressure conditions during cooling from approximately 480°C to around 150°C. The rather different fluid characteristics (T_h: 75–124°C; 17.5–22.6 eq. wt% CaCl₂) of primary inclusions of calcite1 combining with the special δ¹⁸O signature of fluid from which this mineral phase precipitated refer to hydrological connection between the crystalline basement and the sedimentary cover.     

Principal features of impact-generated hydrothermal circulation systems: mineralogical and geochemical evidence

Any hypervelocity impact generates a hydrothermal circulation system in resulting craters. Common characteristics of hydrothermal fluids mobilized within impact structures are considered, based on mineralogical and geochemical investigations, to date. There is similarity between the hydrothermal mineral associations in the majority of terrestrial craters; an assemblage of clay minerals–zeolites–calcite–pyrite is predominant. Combining mineralogical, geochemical, fluid inclusion, and stable isotope data, the distinctive characteristics of impact‐generated hydrothermal fluids can be distinguished as follows: (i) superficial, meteoric and ground water and, possibly, products of dehydration and degassing of minerals under shock are the sources of hot water solutions; (ii) shocked target rocks are sources of the mineral components of the solutions; (iii) flow of fluids occurs mainly in the liquid state; (iv) high rates of flow are likely (10^?4 to 10^?3 m s^?1); (v) fluids are predominantly aqueous and of low salinity; (vi) fluids are weakly alkaline to near‐neutral (pH 6–8) and are supersaturated in silica during the entire hydrothermal process because of the strong predominance of shock‐disordered aluminosilicates and fusion glasses in the host rocks; and (vii) variations in the properties of the circulating solutions, as well as the spatial distribution of secondary mineral assemblages are controlled by temperature gradients within the circulation cell and by a progressive cooling of the impact crater. Products of impact‐generated hydrothermal processes are similar to the hydrothermal mineralization in volcanic areas, as well as in modern geothermal systems, but impacts are always characterized by a retrograde sequence of alteration minerals.     

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.     

A fluid inclusion record of magmatic/hydrothermal pulses in acid Salar Ignorado gypsum,northern Chile

Salar Ignorado is a shallow acid saline lake hosted by a small intervolcanic basin high in the Andes Mountains of northern Chile. Modern surface waters have 3.3–4.1 pH, 0.5–3% total dissolved solids (TDS) and are actively precipitating gypsum crystals. The gypsum crystals trap the acid saline water as fluid inclusions, providing a record of recent surface water characteristics. Salar Ignorado gypsum contains three distinct types of primary fluid inclusions, which result from growth of the gypsum from surface waters. Petrography and microthermometry were performed on 27 gypsum crystals from Salar Ignorado to gain an understanding of recent water chemistry of the salar. One 18.3‐cm‐long gypsum crystal, hosting primary fluid inclusions along 28 successive growth bands, was the focus for fluid inclusion studies and allowed a record of high‐resolution chemical trends. This crystal showed a change in parent fluids during growth, from low salinity, to high salinity, back to low salinity. At the bottom of the crystal, the lowest six fluid inclusion assemblages have salinities of 1.7–5.1 eq. wt. % NaCl. The next nine fluid inclusion assemblages have significantly higher salinity (18.6–27.4 eq. wt. % NaCl) inclusions. The twelve fluid inclusion assemblages near the top of the crystal have low salinity (0.9–8.3 eq. wt. % NaCl) like those at the bottom of the crystal. The high‐salinity fluid inclusions in the middle of this gypsum crystal are interpreted to have formed during a pulse of magmatic/hydrothermal fluids to the surface, perhaps during local active volcanism. Secondary evidence of a magmatic influence on surface waters includes hydrogen sulfide and high molecular weight solid hydrocarbons within some fluid inclusions. This study is among the first detailed fluid inclusion studies of gypsum and suggests that fluid inclusions in gypsum can be paleo‐hydrogeologic proxies.     

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

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

Fluid 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.     

Precipitation of fracture fillings and cements in the Buntsandstein (NW Germany)

The relationship between fracturing and fracture filling in opening‐mode fractures in the Triassic Buntsandstein in the Lower Saxony Basin (LSB; NW Germany) has been studied by an integration of petrographic and structural analysis of core samples, strontium isotope analysis and microthermometry on fluid inclusions. This revealed the relationship between the timing of the fracturing and the precipitation of different mineral phases in the fractures by constraining the precipitation conditions and considering the possible fluid transport mechanisms. The core was studied from four different boreholes, located in different structural settings across the LSB. In the core samples from the four boreholes, fractures filled with calcite, quartz and anhydrite were found, in addition to pore‐filling calcite cementation. In boreholes 2 and 3, calcite‐filled fractures have a fibrous microstructure whereas in borehole 1, fractures are filled with elongate‐blocky calcite crystals. Anhydrite‐filled fractures have, in all samples, a blocky to elongate‐blocky microstructure. Fractures that are filled with quartz are observed in borehole 2 only where the quartz crystals are ‘stretched’ with an elongated habit. Fluid inclusion microthermometry of fracturing‐filling quartz crystals showed that quartz precipitation took place at temperatures of at least 140°C, from a fluid with NaCl–CaCl₂–H₂O composition. Melting phases are meta‐stable and suggest growth from high salinity formation water. Strontium isotopes, measured in leached host rock, indicate that, in boreholes 2 and 3, the fluid which precipitated the calcite cements and calcite‐filled fractures is most likely locally derived whereas in borehole 1, the ⁸⁷Sr/⁸⁶Sr ratios from the pore‐filling cements and in the elongate‐blocky calcite‐filled fracture can only be explained by mixing with externally derived fluids. The elongate‐blocky anhydrite‐filled fractures, present in boreholes 1, 3 and 4, precipitated from a mixture of locally derived pore fluids and a significant quantity of fluid with a lower, less radiogenic, ⁸⁷Sr/⁸⁶Sr ratio. Taking into account the structural evolution of the basin and accompanying salt tectonics, it is likely that the underlying Zechstein is a source for the less radiogenic fluids. Based on the samples in the LSB, it is probable that fibrous fracture fillings in sedimentary rocks most likely developed from locally derived pore fluids whereas elongate‐blocky fracture fillings with smooth walls developed from externally derived pore fluids.     

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

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

Evolution of fluid chemistry during travertine formation in the Troll thermal springs,Svalbard, Norway

Arctic hydrothermal springs at Bockfjorden, Svalbard, have isotope and trace element signatures indicative of derivation from glacial melt waters with minor contribution from seawater. Downstream gradients in water chemistry, isotopic composition and carbonate precipitation rates have been documented for the Troll spring and travertine terrace system. The main controls on the downstream evolution of these parameters are carbon dioxide degassing, calcite precipitation, evaporation and biological activity. The carbonate precipitation rates not only show an approximately parabolic dependence on the calcite supersaturation levels, but depend also on local hydrodynamics. Downstream loss of light isotopes of oxygen and hydrogen can be explained as an effect of evaporation, as estimated using chloride as a conservative marker. Biological activity affects nitrate and bromide concentrations and influences the morphology of calcite precipitates.     

Spatial coupling between spilitization and carbonation of basaltic sills in SW Scottish Highlands: evidence of a mineralogical control of metamorphic fluid flow

In a geochemical and petrological analysis of overprinting episodes of fluid–rock interaction in a well‐studied metabasaltic sill in the SW Scottish Highlands, we show that syn‐deformational access of metamorphic fluids and consequent fluid–rock interaction is at least in part controlled by preexisting mineralogical variations. Lithological and structural channelling of metamorphic fluids along the axis of the Ardrishaig Anticline, SW Scottish Highlands, caused carbonation of metabasaltic sills hosted by metasedimentary rocks of the Argyll Group in the Dalradian Supergroup. Analysis of chemical and mineralogical variability across a metabasaltic sill at Port Cill Maluaig shows that carbonation at greenschist to epidote–amphibolites facies conditions caused by infiltration of H₂O‐CO₂ fluids was controlled by mineralogical variations, which were present before carbonation occurred. This variability probably reflects chemical and mineralogical changes imparted on the sill during premetamorphic spilitization. Calculation of precarbonation mineral modes reveals heterogeneous spatial distributions of epidote, amphibole, chlorite and epidote. This reflects both premetamorphic spilitization and prograde greenschist facies metamorphism prior to fluid flow. Spilitization caused albitization of primary plagioclase and spatially heterogeneous growth of epidote ± calcic amphibole ± chlorite ± quartz ± calcite. Greenschist facies metamorphism caused breakdown of primary pyroxene and continued, but spatially more homogeneous, growth of amphibole + chlorite ± quartz. These processes formed diffuse epidote‐rich patches or semi‐continuous layers. These might represent precursors of epidote segregations, which are better developed elsewhere in the SW Scottish Highlands. Chemical and field analyses of epidote reveal the evidence of local volume fluctuations associated with these concentrations of epidote. Transient permeability enhancement associated with these changes may have permitted higher fluid fluxes and therefore more extensive carbonation. This deflected metamorphic fluid such that its flow direction became more layer parallel, limiting propagation of the reaction front into the sill interior.     

The relevance of vein texture in understanding the past hydraulic behaviour of a crystalline rock mass: reconstruction of the palaeohydrology of the Mecsekalja Zone,south Hungary

This study reconstructs the palaeohydrogeologic evolution of the shallow‐to‐moderate Mesozoic subsidence history for the Mecsekalja Zone (MZ), a narrow metamorphic belt in the eastern Mecsek Mountains, Hungary. Brittle deformation of the MZ produced a vein system with a cement history consisting of five sequential carbonate generations and one quartz phase. Vein textures suggest different fluid‐flow mechanisms for the parent fluids of subsequent cement generations. Combined microthermometric and stable‐isotope measurements permit reconstruction of the character of subsequent fluid generations with different flow types, as defined by vein textures, yielding new information regarding the hydraulic behaviour of a metamorphic crystalline complex. Textural observations and geochemical data suggest that fracture‐controlled flow pathways and externally derived fluids were typical of some flow events, while percolation through the rock matrix and the relationship to the Cretaceous volcanism and dyke emplacement were typical of others. The difference in the mode of calcite deposition from pervasive fluids (i.e. pervasive carbonatisation along grain boundaries versus deposition in antitaxial veins) between two calcite generations related to the volcanism inspired a stress‐dependent model of antitaxial vein growth. Textural and isotope variations in a vein generation produced by the same parent fluid indicate rock‐dependent hydraulic behaviour for different rock types, distinct action of the contemporaneous fracture systems and different extents of fluid–rock interaction. Cathodoluminescence microscopy and fluid‐inclusion microthermometry shed light on the possible role of hydraulic fracturing in the formation of massive calcite. The time of formation was estimated from the isotope composition of the oldest calcite generation and its presumptive relationship with the sedimentary sequences to the north, whereas microthermometry permitted conciliation of the reconstructed flow sequence with the Mesozoic subsidence history of the Mórágy Block (including the MZ).     

Origin of dolomites in the Cambrian (upper 3rd‐Furongian) formation,south‐eastern Sichuan Basin,China

The origin of large‐scale ancient dolomite is one of the most hotly debated topics in sedimentology. The Loushanguan group of the upper 3rd‐Furongian Cambrian series on the south‐eastern margin of the Sichuan Basin consists of numerous dolomites, and the origins of these dolomites have never been reported previously although they are probably good hydrocarbon reservoirs. Based on a systematic analysis of petrology, fluid inclusions, carbon and oxygen isotopes, trace elements and rare earth elements (REEs), this study provides some unique insights into the origins of the dolomites. Four dolomite types have been identified in the study area: dolomicrite, fabric‐retentive oolitic dolomite, fabric‐obliterative dolomite and saddle dolomite cement. In the dolomicrite and fabric‐retentive oolitic dolomite, high Sr contents (with respect to the fabric‐obliterative dolomite) and the lack of two‐phase aqueous inclusions suggest that they formed at shallow‐to‐intermediate burial depths at low temperatures (<50–60°C). Carbon and oxygen isotopes and seawater‐like REE+Y characteristics of the dolomicrite and fabric‐retentive oolitic dolomite indicate that the dolomitizing fluids were evaporated seawater or slightly modified seawater. The obliteration of the original sedimentary fabric and relatively low δ¹⁸O and Sr values compared to the fabric‐retentive dolomite indicate that fabric‐obliterative dolomite formed at intermediate‐to‐deep burial diagenesis. The chemical composition approaches pure dolomite and the REE+Y characteristics are similar to those of the fabric‐retentive dolomite, indicating that the fabric‐obliterative dolomite was formed due to the recrystallization of the previously formed fabric‐retentive dolomite at elevated burial depths and temperatures. High fluid inclusion homogenization temperatures (115–150°C), low δ¹⁸O values, nonplanar‐a crystals and seawater‐like REE+Y characteristics suggest that saddle dolomite cement formed by reprecipitation of dolomite that related to seawater‐driven and deep burial fluid. In the study area, dolomicrite and fabric‐retentive oolitic dolomite may have been formed by penecontemporaneous or seepage‐reflux dolomitization during early‐stage diagenesis. Subsequently, during progressive burial, most of the fabric‐retentive dolomite was converted into fabric‐obliterative dolomite by recrystallization. This study confirms that fabric‐obliterative dolomite was the main dolomite type, and although deeply buried, these Cambrian dolomites most likely have preserved coeval seawater geochemical signals.     

Electron backscatter diffraction investigation of length‐fast chalcedony in agate: implications for agate genesis and growth mechanisms

Agates of volcanic origin contain a range of silica minerals, with chalcedony and quartz arranged in concentric bands. Although agates are abundant worldwide, little is known about the genesis of their characteristic banding patterns. Current hypotheses suggest the bands result either from precipitation from convecting siliceous hydrothermal influxes or by in situ crystallization of a silica gel. This study combines the use of a variety of analytical techniques, including electron backscatter diffraction (EBSD), cathodoluminescence (CL), and Fourier transform infrared (FT‐IR) spectroscopy, to characterize the silica minerals present and investigate their spatial and crystallographic relationships in the banding arrangement. Microstructural and spectroscopic observations reveal that chalcedony bands are composed of amorphous silica that also contains nanocrystalline and later‐formed microcrystalline quartz. Nano‐ and microcrystalline quartz grew with a‐axes perpendicular to the growth substrate, typical of length‐fast chalcedony. The bands formed as a result of discrete influxes of silica‐rich fluid. Within these individual bands, there is a sequence of minerals: chalcedony‐A (with amorphous silica and nanocrystalline quartz) → chalcedony‐MQ (with microcrystalline quartz) → quartz. This sequence is reflected in the degree of crystallinity, crystal orientations and water content and is analogous to a diagenetic cycle; the initial chalcedony portion of the band commences with amorphous silica with nanocrystalline quartz followed by fibrous microcrystalline quartz crystals; chalcedony then grades into larger equiaxial mesoquartz crystals. This paragenetic sequence suggests a viable model for the growth of chalcedony in agates.     

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.     

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.     

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.     

Phase‐field modeling of epitaxial growth of polycrystalline quartz veins in hydrothermal experiments

Mineral precipitation in an open fracture plays a crucial role in the evolution of fracture permeability in rocks, and the microstructural development and precipitation rates are closely linked to fluid composition, the kind of host rock as well as temperature and pressure. In this study, we develop a continuum thermodynamic model to understand polycrystalline growth of quartz aggregates from the rock surface. The adapted multiphase‐field model takes into consideration both the absolute growth rate as a function of the driving force of the reaction (free energy differences between solid and liquid phases), and the equilibrium crystal shape (Wulff shape). In addition, we realize the anisotropic shape of the quartz crystal by introducing relative growth rates of the facets. The missing parameters of the model, including surface energy and relative growth rates, are determined by detailed analysis of the crystal shapes and crystallographic orientation of polycrystalline quartz aggregates in veins synthesized in previous hydrothermal experiments. The growth simulations were carried out for a single crystal and for grain aggregates from a rock surface. The single crystal simulation reveals the importance of crystal facetting on the growth rate; for example, growth velocity in the c‐axis direction drops by a factor of ~9 when the faceting is complete. The textures produced by the polycrystal simulations are similar to those observed in the hydrothermal experiments, including the number of surviving grains and crystallographic preferred orientations as a function of the distance from the rock wall. Our model and the methods to define its parameters provide a basis for further investigation of fracture sealing under varying conditions.