The amphibolite-granulite facies transition in West Uusimaa, S.W. Finland. A fluid inclusion study

This work presents the results of a fluid inclusion study of an amphibolite-granulite facies transition in West Uusimaa, S.W. Finland. Early fluid-inclusions in the granulite facies area are characteristically carbonic (CO₂), in contrast to predominantly aqueous early inclusions in the amphibolite facies area. These early inclusions can be related to peak metamorphic conditions (750-820°C and 3-5 kbar for peak granulite facies metamorphism). Relatively young CO₂ inclusions with low densities (<0.8g/cm³) indicate that the first part of the cooling history of the rocks was characterized by a near isothermal uplift.
N₂-CH₄ inclusions, with compositions ranging between pure CH₄ and pure N₂ (Raman spectral analysis), were found in the whole area. They are probably syn- or even pre-early inclusions. Only nearly critical homogenizing inclusions have been found (low density). Pressure estimates, based on densities of early fluid inclusions, show that the rapid transition of amphibolite towards granulite facies metamorphism is virtually isobaric. Granulite facies metamorphism in West Uusimaa is a thermal event, probably induced by the influx of hot, CO₂-bearing fluids.     

Experimental diffusion of hydrogen into synthetic fluid inclusions in quartz

Abstract Quartz-hosted, synthetic CO₂-H₂O fluid inclusions behave as open systems with respect to diffusional transfer of hydrogen during laboratory-simulated metamorphic re-equilibration at 650, 750 and 825°C and 1.5 kbar total pressure with f_O2 defined by the C-CH₄ buffer. Microthermometry and Raman spectroscopy show that the initial CO₂-H₂O inclusions become CO₂-CH₄-H₂-H_2O
inclusions after diffusive influx of hydrogen from the reducing confining medium. Measurable changes are observed in inclusion compositions after only 15 days of re-equilibration, implying significant hydrogen mobility at still lower temperatures over geological time spans. Results of synthetic inclusion re-equilibrium experiments have profound implications for the interpretation of natural fluid-inclusion data; failure to account for potential hydrogen migration in inclusions from high-temperature geological environments may lead to erroneous estimates of P-T, and/or the compositions of metamorphic fluids.     

On the origin of CO2-rich fluid inclusions in migmatites

Abstract Nearly pure CO₂ fluid inclusions are abundant in migmatites although H₂O-rich fluids are predicted from the phase equilibria. Processes which may play a role in this observation include (1) the effects of decompression on melt, (2) generation of a CO₂-bearing volatile phase by the reaction graphite + quartz + biotite + plagioclase = melt + orthopyroxene + CO₂-rich vapour, (3) selective leakage of H₂O from CO₂+ H₂O inclusions when the pressure in the inclusion exceeds the confining pressure during decompression, and (4) enrichment of grain-boundary vapour in CO₂ by subsolidus retrograde hydration reactions.     

Stable isotopic and fluid inclusion evidence for meteoric fluid penetration into an active mountain belt; Alpine Schist, New Zealand

Calcite and quartz veins have formed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt of New Zealand. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle-ductile transition, which has been raised to 5-6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H₂O + NaCl-CO₂ mixtures at 200-350 C. Bulk fluid composition is 15-20 mol% CO₂ and <4.3 total mol CH₄+ N₂+ Ar/100mol H₂O. Water hydrogen isotopic ratio δD_H2O in the fissure veins spans -29 to -68‰, δ¹⁸O_H2O -0.7 to 8.5‰, and bulk carbon isotopic ratio δ¹³C ranges from -3.7 to -11.7‰. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be generated by oxidation of graphite.     

Magmatic and hydrothermal processes in the Bouvet Triple Junction Region (South Atlantic)

Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K₂ O and TiO_z (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K₂O, TiO₂, H₂O.
Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H₂O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl₂+ H₂O; T = 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO₂ and H₂O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC1₂+ H₂O + CO₂; T = 200–310 °C; P = 900–1700 bar).     

Reaction textures in calc-silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica: implications for the retrograde P–T path

Calc-silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica, exhibit a sequence of reaction textures that have been used to elucidate their retrograde P–T path. The highest temperature recorded in the calc-silicates is represented by the wollastonite- and scapolite-bearing assemblages which yield at least 760°C at 6 kbar based on experimental results. The calc-silicates have partially re-equilibrated at lower temperatures (down to 450°C) as evidenced by the successive reactions: (1) wollastonite + scapolite + calcite = garnet + CO₂, (2) wollastonite + CO₂= calcite + quartz, (3) wollastonite + plagioclase = garnet + quartz, (4) scapolite = plagioclase + calcite + quartz, (5) garnet + CO₂+ H₂O = epidote + calcite + quartz, and (6) clinopyroxene + CO₂+ H₂O = tremolite + calcite + quartz.
The reaction sequence observed indicates that a CO₂ was relatively low in the wollastonite-bearing rocks during peak metamorphic conditions, and may have been further lowered by local infiltration of H₂O from the surrounding migmatitic gneisses on cooling. Fluid activities in the Bolingen calc-silicates were probably locally variable during the granulite facies metamorphism, and large-scale CO₂ advection did not occur.
A retrograde P–T path, from the sillimanite stability field ( c. 760°C at 6 kbar) into the andalusite stability field ( c. 450°C at <3 kbar), is suggested by the occurrence of secondary andalusite in an adjacent cordierite–sillimanite gneiss in which sillimanite occurs as inclusions in cordierite.     

Grain boundary migration of water and carbon dioxide during uplift of garnet-zone Alpine Schist, New Zealand

Abstract Deformed quartz veins in garnet-zone schist adjacent to the active Alpine Fault, New Zealand, have fluid inclusions trapped along quartz grain boundaries. Textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins. Many of the inclusions are empty, but some contain carbon dioxide with densities that range from 0.16 to 0.80 g cm⁻³. No water, nitrogen or methane was detected. The inclusions are considerably more CO₂-rich than either the primary metamorphic fluid (<5% CO₂) or fluids trapped in fracture-related situations in the same, or related, rocks (<50% CO₂). Enrichment of CO₂ is inferred to have resulted from selective migration (wicking) of saline water from the inclusions along water-wet grain boundaries after cooling-induced immiscibility of a water-CO₂ mixture. Inclusion volumes changed after loss of water. Non-wetting CO₂ remained trapped in the inclusions until further percolation progressively removed CO₂ in solution. This mechanism of fluid migration dominated in ductile quartz-rich rocks near, but below, the brittle-ductile transition. At deeper levels, hydraulic fracturing is also an important mechanism for fluid migration, whereas at shallower levels advection through open fractures dominates the fluid flow regime.     

Shallow-level metamorphic fluids in a high uplift rat metamorphic belt; Alpine Schist, New Zealand

Abstract Fluids, some of which are CO₂-rich (up to 40 mol.% CO₂) and some of which are highly saline (up to 18 wt% NaCl equivalent), are trapped as fluid inclusions in quartz-calcite (∼ metallic minerals) veins which cross-cut the pumpellyite-actinolite to amphibolite facies rocks of the Alpine Schist. Fluids were commonly trapped as immiscible liquid-vapour mixes in quartz and calcite showing open-space growth textures. Fluid entrapment occurred at fluid pressures near 500 bars (possibly as low as 150 bars) at temperatures ranging from 260 to 330° C. Saline fluids may have formed by partitioning of dissolved salts into an aqueous phase on segregation of immiscible fluids from a low-density CO₂-rich fluid. Calcite deposited by these fluids has δ¹³C ranging from – 8.4 to – 11.5 and δ¹⁸O from + 4 to + 13. Isotopic data, fluid compositions and mode of occurrence suggest that the fluids are derived from high-grade metamorphic rocks. Fluid interaction with wall-rock has caused biotite crystallization and/or recrystallization in some rocks and retrogression of biotite to chlorite in other rocks.
Fluid penetration through the rock is almost pervasive in many areas where permeability, probably related to Alpine Fault activity, has focussed fluids on a regional scale into fractured rocks. The fluid flow process is made possible by high uplift-rates (in excess of 10 mm/year) bringing hot rocks near to the surface.     

The nature and distribution of fluids during amphibolite facies metamorphism, Naxos (Greece)

On the basis of fluid inclusion evidence, pervasive influx of deep-seated CO₂-rich fluids has been invoked to account for mid- to upper amphibolite facies (M2_B) metamorphism on the island of Naxos (Cyclades, Greece). In this paper, mineral devolatilization and melt equilibria are used to constrain the composition of both syn- and post-peak-M2_B fluids in the deepest exposed levels of the metamorphic complex. The results indicate that peak-M2_B fluids were spatially and compositionally heterogeneous throughout the high-grade core of the complex, whereas post-peak-M2_B fluids were generally water-rich. The observed heterogeneities in syn-M2_B fluid composition are inconsistent with pervasive CO₂-flushing models invoked by previous workers on the basis of fluid inclusion evidence. It is likely that few CO₂-rich fluid inclusions on Naxos preserve fluids trapped under peak metamorphic conditions. It is suggested that many of these inclusions have behaved as chemically open systems during the intense deformation that accompanied the uplift of the metamorphic complex. A similar process may explain the occurrence of some CO₂-rich fluid inclusions in granulite facies rocks.     

Melt and fluid inclusions from volcanic quartz phenocrysts in the Shila gold / base metal ore district,Peru: precursors to ore‐forming epithermal solutions?

ABSTRACT Despite the close association with volcanic activity, the source of metals and ligands in the epithermal ore deposits is still controversial. In order to explore the magmatic–hydrothermal connection further, silicate melt, saline- and water-rich fluids, and CO₂ vapours are documented that are trapped as inclusions in quartz phenocrysts from dacitic dykes associated with epithermal gold/base metal mineralization in the Shila district (Peru). Melt inclusion characteristics, and microthermometric and laser Raman fluid inclusion data are presented. The investigation of melt and fluid inclusions reveals that the volatile phase of magmas might represent the precursors to the early chlorine-rich ore-forming epithermal solutions. Microthermometric investigations in magmatic quartz crystals and data on quartz mineralized veins suggest that the fluid evolution and ore deposition may be the result of several processes including: release of an evolving magmatic fluid, and/or boiling, and/or mixing.     

Gas Measurement of Fluid Inclusions from the Hishikari Epithermal Gold Deposit, Southern Kyushu, Japan, Using Laser Raman Microprobe

Abstract. Laser Raman microprobe analysis was performed on the fluid inclusions from the Honko-Sanjin zone in the Hishikari epithermal gold deposit, southern Kyushu, Japan. Gas concentrations of fluid inclusions through the zone were below detection limits (e.g., 5 mmole/kg H₂O for CO₂), with an exception at shallow portion in which the CO₂/N₂ mole ratio was determined to be 5.3. Boiling of hydrothermal solutions probably separated gases from ore fluids at the deep portion of the deposit, and migration of gases to shallow portion resulted in CO₂-rich steam-heated water and related acid alteration.     

Syndeformational fluid trapping in quartz: determining the pressure-temperature conditions of deformation from fluid inclusions and the formation of pure CO2 fluid inclusions during grain-boundary migration

Abstract Observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain-boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids. Our results imply that pure CO₂ fluid inclusions in metamorphic rocks are likely to be the residue of deformation-recrystallization process rather than representing a primary metamorphic fluid.
Where the temperature of deformation can be deduced by other means, the densities of fluid inclusions trapped during recrystallization, which we call recrystallization-primary fluid inclusions, can be used to constrain the ambient pressure during deformation. Using these constraints, the data imply that the post-metamorphic Hercynian exhumation in Sardinia brought rocks at 300° C to within 3km of the surface. This conclusion is similar to that described for the rapidly uplifted Southern Alps in New Zealand.     

Primary carbonate/CO2 inclusions in sapphirine-bearing granulites from central Sri Lanka

High-density CO₂-rich fluid inclusions from a sapphirine-bearing granulite (Hakurutale, Sri Lanka) have been studied by microthermometry, Raman spectrometry and SEM analysis. Based on textural evidence, two groups of inclusions can be identified: primary, negative crystal shaped inclusions (group I) and pseudo-secondary inclusions, which experienced a local, limited post-trapping modification (group II). Both groups contain magnesite as a daughter mineral, occurring in a relatively constant fluid/solid inclusion volume ratio (vol_solid =0.15 total volume). CO₂ densities for group I and II differ only slightly. Both groups contain a fluid, which was initially trapped at peak metamorphic conditions as a homogeneous (CO₂+MgCO₃) mixture. Thermodynamic calculations suggest that such a fluid (CO₂+15 vol% MgCO₃) is stable under granulite facies conditions. After trapping, magnesite separated upon cooling, while the remaining CO₂ density suffered minor re-adjustments. A model isochore based on the integration of the magnesite molar volume in the CO₂ fluid passes about 1.5–2 kbar below peak metamorphic conditions. This remaining discrepancy can be explained by the possible role of a small quantity of additional water.     

Genesis of Kanggur Gold Deposit in Eastern Tianshan Orogenic Belt, NW China: Fluid Inclusion and Oxygen and Hydrogen Isotope Constraints

Abstract. Primary fluid inclusions in quartz and carbonates from the Kanggur gold deposit are dominated by aqueous inclusions, with subsidiary CO₂-H₂O inclusions that have a constant range in CO₂ content (10–20 vol %). Microthermometric results indicate that total homogenization temperatures have a wide but similar range for both aqueous inclusions (120 to 310C) and CO₂-H₂O inclusions (140 to 340C). Estimates of fluid salinity for CO₂-H₂O inclusions are quite restricted (5.9∼10.3 equiv. wt% NaCl), whereas aqueous inclusions show much wider salinity ranging from 2.2 to 15.6 equivalent wt %NaCl.
The 6D values of fluid inclusions in carbonates vary from -45 to -61 %, in well accord with the published δD values of fluid inclusions in quartz (-46 to -66 %). Most of the δ¹⁸O and δD values of the ore-forming fluids can be achieved by exchanged meteoric water after isotopic equilibration with wall rock by fluid/rock interaction at a low water/rock ratio. However, the exchanged meteoric water alone cannot explain the full range of δ¹⁸O and δD values, magmatic and/or meta-morphic water should also be involved. The wide salinity in aqueous inclusions may also result from mixing of meteoric water and magmatic and/or metamorphic water.     

Temperatures and Oxygen Isotopic Compositions of Hydrothermal Fluids for the Takatori Tungsten-copper Deposit, Japan

Abstract. Fluid inclusion and oxygen isotope studies are performed to obtain temperatures and oxygen isotopic compositions of hydrothermal fluids for the vein-type tungsten-copper deposit at Takatori in Ibaraki Prefecture, Japan. Temperatures of the hydrothermal fluids are calculated from fluid inclusion data. The calculation incorporates the effects of the salinity, gas concentration, and fluid pressure. The fluid temperatures range from 370 to 460C. For these calculations, this study obtains a density equation for H₂O-NaCl-CO₂ solution at the vapor-liquid two-phase boundary. Then the present study combines the obtained equation with the equation of state by Bowers and Helgeson (1983).
The fluid temperatures determined in this study are applied to the calculation of oxygen isotopic compositions of the hydrothermal fluids. The calculation of the oxygen isotopic compositions is based on the oxygen isotope analyses of vein quartz. The oxygen isotopic compositions of vein quartz range from +13.5 to +14.4 % relative to SMOW. Then, the oxygen isotopic compositions of the hydrothermal fluids in equilibrium with the vein quartz are calculated to be from +9.7 to +10.5 %. These δ¹⁸O_fluid values agree with those of magmatic fluids derived from the ilmenite-series granitic rock, which is related to the mineralization. Keywords: Takatori tungsten-copper deposit, fluid inclusion, oxygen isotope, vein quartz, H₂O-NaCl-CO₂ solution, density     

Ionic association in H2O–CO2 fluids at mid-crustal conditions

The equilibrium constant, K _a, of the association reaction to form ion pairs from charged solute species in supercritical solutions can be calculated from a model based on published equations. Log K _a at constant pressure is a linear function of the inverse in the dielectric constant of the fluid times temperature. The dielectric properties of H₂O and CO₂ at supercritical pressures and temperatures can also be evaluated using the Kirkwood equation. Using Looyenga mixing rules, the dielectric constant of H₂O–CO₂ mixtures can be obtained and the change in log K _a with addition of CO₂ in aqueous solutions evaluated. These changes in log K _a with addition of CO₂ are consistent with measured changes of log K _a with addition of Ar in supercritical H₂O–Ar solutions.
Log K _a of KCl and NaCl increase to an increasing extent as the mole fraction of CO₂ increases in H₂O–CO₂ solutions. For instance, at 2 kbar and constant temperature between 400 and 600° C, log K _a of KCl increases by about two orders of magnitude whilst that of NaCl increases by over four orders of magnitude as the CO₂ mole fraction increases from 0.0 to 0.35. Such changes in log K _a will have dramatic effects on the solubility of minerals in CO₂-rich environments.     

Microthermometric Evidence for the Formation of Ca-rich Hypersaline Brine and CO2–rich Fluid in the Mori Geothermal Reservoir, Japan

Abstract: Fluid inclusions in hydrothermal quartz and anhydrite samples from the Mori geothermal field, southwest Hokkaido, have been studied microthermometrically using heating/freezing stages. Based on homogenization temperatures of fluid inclusions, salinities after correction of effect of freezing-point depression of CO₂ on ice melting temperatures of fluid inclusions, and previous geochemical data by Yoshida (1991), we discuss the geneses of various types of inclusion fluids in the Mori geothermal reservoir.     

Fluid-mineral equilibria in prehnite-pumpellyite to greenschist facies metabasites near Flin Flon, Manitoba, Canada: implications for petrogenetic grids

A sequence of regional metamorphic isograds indicating a range from prehnite-pumpellyite to lower amphibolite facies was mapped in metabasites near Flin Flon, Manitoba. The lowest grade rocks contain prehnite + pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature- X _CO2 and pressure-temperature (constant X _CO2) diagrams were calculated to quantify the effects of CO₂ in the metamorphic fluid on the stability of prehnite-pumpellyite facies minerals in metabasites containing excess quartz and chlorite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids with X _co2 <0.002. For X _co2>0.002, epidote + chlorite + calcite assemblages are stable. Our calculated phase relations are consistent with regional metamorphism in the Flin Flon area in the presence of an H₂O-rich fluid and a more CO₂-rich fluid in the later fault zones. We believe that the potential effects of small amounts of CO₂ in the metamorphic fluid should be assessed when considering the pressure-temperature implications of mineral assemblages in low-grade metabasites.     

Compositions of magmatic hydrothermal fluids determined by LA-ICP-MS of fluid inclusions from the porphyry copper–molybdenum deposit at Butte, MT

We analyzed 85 fluid inclusions from seven samples from the porphyry Cu–Mo deposit in Butte, MT, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The Butte deposit formed at unusually great depth relative to most porphyry deposits, and fluid inclusions in deep veins trapped a low-salinity, CO₂-bearing, magmatically derived, supercritical fluid as a single aqueous phase. This fluid is interpreted to be the parent fluid that cooled, decompressed, unmixed, and reacted with wall rock to form the gigantic porphyry Cu deposit at Butte. Few previous analyses of such fluids exist.Low-salinity, aqueous fluids from the earliest veins at Butte are trapped in deep veins with biotite-rich alteration envelopes (EDM veins). These veins, and the Butte quartz monzonite surrounding them, host much of the Butte porphyry Cu mineralization. Twenty fluid inclusions in one EDM quartz vein are dominated by Na, K, Fe (from 0.1 to 1 wt.%) and contain up to 1.3 wt.% Cu. These inclusions contain only small amounts (tens of ppm) of Pb, Zn, and Mn, and typically contain Li, B, Ca, As, Mo, Ag, Sn, Sb, Ba, and W in less than detectable quantities. The abundance of Cu in early fluids indicates that a low-salinity, Cu-rich, aqueous ore fluid can be directly produced by aqueous fluid separation from a granitic magma. Similar inclusions (eight) in an early deep quartz–molybdenite vein with a K-feldspar selvage have similar compositions but contain significantly less Cu than most inclusions in the biotite-altered vein. Analyzed inclusions in both veins contain less than detectable concentrations of Mo even though one is molybdenite-bearing.Low-salinity, CO₂-bearing aqueous fluids are also trapped in pyrite–quartz veins with sericitic selvages. These veins cut both of the above vein types and contain inclusions that were trapped at lower pressure and temperature. Thirty-nine inclusions in two such veins have compositions similar to early fluids, but are enriched by up to a factor of 10 in Mn, Pb, and Zn relative to early fluids, and are slightly depleted in Fe. Many of these inclusions contain as much or more Cu than early fluids, although little chalcopyrite is found in or around pyrite–quartz veins.Eighteen halite-bearing inclusions from three veins from both chalcopyrite-bearing and barren veins with both K-silicate and sericitic selvages were analyzed as well. Halite-saturated inclusions are dominated by Na, K, Fe, and in some inclusions Ca. Whereas these inclusions are significantly enriched in Ca, Mn, Fe, Zn, and Pb, fluids in all three veins contain significantly less Cu than early, high temperature, low-salinity inclusions.Analyses of all inclusion types show that whereas bulk-salinity of the hydrothermal fluid must be largely controlled by the magma, fluid–rock interactions have a significant role in controlling fluid compositions and metal ratios. Cu concentrations range over an order of magnitude, more than any other element, in all four samples containing low-salinity inclusions. We infer that variations are the result of fluid trapping after different amounts of fluid–rock reaction and chalcopyrite precipitation. Enrichment, relative to early fluids, of Mn, Pb, and Zn in fluids related to sericitic alteration is also likely the result of fluid–rock reaction, whereby these elements are released from biotite and feldspars as they alter to sericite. In halite-bearing inclusions, concentrations of Sr, Ca, Pb, and Ba are elevated in inclusions from the pyrite–quartz vein with sericitic alteration relative to halite-bearing inclusions from unaltered and potassically altered samples. Such enrichment is likely caused by the breakdown of plagioclase and K-feldspar in the alteration envelope, releasing Sr, Ca, Pb, and Ba.     

Thermochemical sulphate reduction in the Upper Devonian Cairn Formation of the Fairholme carbonate complex (South-West Alberta, Canadian Rockies): evidence from fluid inclusions and isotopic data

The Fairholme carbonate complex is part of the extensively dolomitized Upper Devonian carbonate reefs in west-central Alberta. The studied formations contain moulds (up to 10 cm in diameter), which are filled partially with (saddle) dolomite, quartz and calcite cements. These cements precipitated from a mixture of brines that acquired high salinity by dissolution of halite and brines derived from evaporated sea water. The fluids were warm (homogenization temperature of primary fluid inclusions of 76 to 200 °C) and saline (20 to 25 wt% NaCl equivalent) and testify to thermochemical sulphate reduction processes. The latter is deduced from S in solid inclusions, CO₂ and H₂S in volatile-rich aqueous inclusions and depleted δ¹³C values down to −26‰ Vienna Pee Dee Belemnite. High ⁸⁷Sr/⁸⁶Sr values (0·7094 to 0·7110) of the cements also indicate interaction of the fluids with siliciclastic sequences. The thermochemical sulphate reduction-related cements probably formed during early Laramide burial. Another (younger) calcite phase, characterized by depleted δ¹⁸O values (−23·9‰ to −13·9‰ Vienna Pee Dee Belemnite), low Na (27 to 37 p.p.m.) and Sr (39 to 150 p.p.m.) concentrations and non-saline (∼0 wt% NaCl equivalent) fluid inclusions, is attributed to post-Laramide meteoric water.