CO2-rich Solutions Research Articles

Interaction between a fractured marl caprock and CO2-rich sulfate solution under supercritical CO2 conditions

Geological CO2 sequestration at pilot-plant scale will be developed at Hontomín (Spain). CO2 will be injected into a limestone reservoir that contains a NaCl- and sulfate-rich groundwater in equilibrium with calcite and gypsum. The caprock site is composed of marl. The present study seeks to evaluate the interaction between the Hontomín marl and CO2-rich sulfate solutions under supercritical CO2 conditions (PTotal=150bar, pCO2=61bar and T=60°C).Flow-through percolation experiments were performed using artificially fractured cores to elucidate (i) the role of the composition of the injected solutions (S-free and S-rich solutions) and (ii) the effect of the flow rate (0.2, 1 and 60mLh−1) on fracture permeability. Major dissolution of calcite (S-free and S-rich solutions) and precipitation of gypsum (S-rich solution) together with minor dissolution of the silicate minerals contributed to the formation of an altered skeleton-like zone (mainly made up of unreacted clays) along the fracture walls. Dissolution patterns changed from face dissolution to wormhole formation and uniform dissolution with increasing Peclet numbers.In S-free experiments, fracture permeability did not significantly change regardless of the flow rate despite the fact that a large amount of calcite dissolved. In S-rich solution experiments, fracture permeability decreased under slow flow rates (0.2 and 1mLh−1) because of gypsum precipitation that sealed the fracture. At the highest flow rate (60mLh−1), fracture permeability increased because calcite dissolution predominated over gypsum precipitation.

Influence of the flow rate on dissolution and precipitation features during percolation of CO2-rich sulfate solutions through fractured limestone samples

Calcite dissolution and gypsum precipitation are expected to occur when injecting CO2 in a limestone reservoir with sulfate-rich resident brine. If the reservoir is fractured, these reactions will take place mainly in the fractures, which serve as preferential paths for fluid flow. As a consequence, the geometry of the fractures will vary leading to changes in their hydraulic and transport properties. In this study, a set of percolation experiments which consisted of injecting CO2-rich solutions through fractured limestone cores was performed under P=150bar and T=60°C. Flow rates ranging from 0.2 to 60mL/h and sulfate-rich and sulfate-free solutions were used.Variation in fracture volume induced by calcite dissolution and gypsum precipitation was measured by X-ray computed microtomography (XCMT) and aqueous chemistry. An increase in flow rate led to an increase in volume of dissolved limestone per unit of time, which indicated that the calcite dissolution rate in the fracture was transport controlled. Moreover, the dissolution pattern varied from face dissolution to wormhole formation and uniform dissolution by increasing the flow rate (i.e., Pe from 1 to 346). Fracture permeability always increased and depended on the type of dissolution pattern.

Methanogenesis and clay minerals diagenesis during the formation of dolomite nodules from the Tortonian marls of southern Spain

Dolomite nodules are widespread within the Tortonian marls of Fortuna and Lorca basins in southern Spain. They occur as large bodies of various forms (round, ovoid, tabular) that are parallel or secant relative to the stratification. They are massive and present sometimes internal conduits that are considered as drains used for the migration of fluids.This study brings new results on the mineralogy and elemental and isotopic geochemistry of these dolomite nodules to better describe the processes that were occurring during their formation. The diagenetic reactions that have driven authigenic dolomite precipitation within the sediments involved on the one hand methanogenesis that produced 13C-poor CH4 and 13C-rich CO2, on the other hand carbonate and silicate weathering by the CO2-rich solutions, that released respectively alkalinity and cations in pore solutions. Moreover, the distribution of the major elements (Si, Al, K, Na, Ca, Mg, Fe) in the marls and dolomite nodules indicates that these elements were redistributed within the sedimentary formation during the diagenetic reactions without external inputs by the circulating fluids. These observations thus confirm the link between authigenic dolomite formation and clay minerals diagenesis in sediments where methanogenesis was active.

Interaction between CO2-rich sulfate solutions and carbonate reservoir rocks from atmospheric to supercritical CO2 conditions: Experiments and modeling

A test site for CO2 geological storage is situated in Hontomín (Burgos, northern Spain) with a reservoir rock that is mainly composed of limestone. During and after CO2 injection, the resulting CO2-rich acid brine gives rise to the dissolution of carbonate minerals (calcite and dolomite) and gypsum (or anhydrite at depth) may precipitate since the reservoir brine contains sulfate. Experiments using columns filled with crushed limestone or dolostone were conducted under different P–pCO2 conditions (atmospheric: 1–10−3.5bar; subcritical: 10–10bar; and supercritical: 150–34bar), T (25, 40 and 60°C) and input solution compositions (gypsum-undersaturated and gypsum-equilibrated solutions). We evaluated the effect of these parameters on the coupled reactions of calcite/dolomite dissolution and gypsum/anhydrite precipitation. The CrunchFlow and PhreeqC (v.3) numerical codes were used to perform reactive transport simulations of the experiments.Within the range of P–pCO2 and T of this study only gypsum precipitation took place (no anhydrite was detected) and this only occurred when the injected solution was equilibrated with gypsum. Under the P–pCO2–T conditions, the volume of precipitated gypsum was smaller than the volume of dissolved carbonate minerals, yielding an increase in porosity (Δϕ up to ≈4%).A decrease in T favored limestone dissolution regardless of pCO2 owing to increasing undersaturation with decreasing temperature. However, gypsum precipitation was favored at high T and under atmospheric pCO2 conditions but not at high T and under 10bar of pCO2 conditions. The increase in limestone dissolution with pCO2 was directly attributed to pH, which was more acidic at higher pCO2.Limestone dissolution induced late gypsum precipitation (long induction time) in contrast to dolostone dissolution, which promoted rapid gypsum precipitation. Moreover, owing to the slow kinetics of dolomite dissolution with respect to that of calcite, both the volume of dissolved mineral and the increase in porosity were larger in the limestone experiments than in the dolostone ones under all pCO2 conditions (10−3.5 and 10bar).By increasing pCO2, carbonate dissolution occurred along the column whereas it was localized in the very inlet under atmospheric conditions. This was due to the buffer capacity of the carbonic acid, which maintains pH at around 5 and keeps the solution undersaturated with respect to calcite and dolomite along the column.1D reactive transport simulations reproduced the experimental data (carbonate dissolution and gypsum precipitation for different P–pCO2–T conditions). Drawing on reaction rate laws in the literature, we used the reactive surface area to fit the models to the experimental data. The values of the reactive surface area were much smaller than those calculated from the geometric areas.

Fluid inclusions in late minerals from the paleovalley-type uranium deposits of the West Siberian ore region: Thermochemical characteristics and genetic applications

Comprehensive microthermometric investigations revealed similar temperature ranges (280–120°C) for the formation of late carbonates in the Khokhlovskoe, Semizbai, and Malinovskoe deposits of the West Siberian uranium ore region. A close chemical similarity was definitely established between the solutions of fluid inclusions and thermal nitrogen-methane waters with elevated CO2 concentrations typical of this region in general. It was noted that such CO2-rich mineral waters (Yessentuki no. 4 type) are common in the Mesozoic sequences of the Shadrinsk region, where Transuralian uranium deposits occur, and are similar in composition and temperature to the modern CO2-rich formation waters of the host sequences of the Khokhlovskoe deposit. The mineralogical and geochemical features of newly formed late minerals and uranium ores were considered as the most probable reflection of the exfiltration of such thermal solutions into the host levels. Two late mineral assemblages were distinguished: (1) hematite-calcite and (2) goethite-berthierine and goethite-smectite-chlorite with siderite or goethite-kaolinite-illite with siderite; they occur both in the host sequences and in the underlying basement rocks. The development of the latter assemblage causes a significant change in rock color (bleaching); it is widespread and was observed in all the deposits. It was shown that these altered rocks and uranium ores (especially high-grade) are very similar in mineral and chemical composition to the products of acid leaching and accompanying mineralization, which could be related to low-temperature argillization. It was suggested that exogenic epigenetic processes of ancient soil-bedrock oxidation contributed certainly to the development of uranium mineralization, and the modern character of the uranium ores and their host rocks is related to a large extent to the influence of hydrothermal CO2-rich solutions related to the neotectonic activation of the region. This resulted in the development of their specific mineral and chemical compositions and corresponding technological characteristics. It seems expedient to estimate the possible contributions of exogenic and endogenic factors to the formation of the uranium mineralization rather than oppose the roles of these processes of different stages.

Petrographic Evidences for the Origin of Iron in IOCG IronDeposits of Kuh-E-Faryadoon and Kouli-Kosh, Southeast Central Iran

Kuh-E-Faryadoon and Kouli Kosh iron deposits are hosted by regional metamorphic rocks of Sanandaj-Sirjan zone around Deh-Bid area. Plugs and domes of acid rocks protrude these metamorphic rocks. A mylonitization event is super imposed on acid rock and metamorphic complexes. Field, petrological and geochemical studies have shown that (1) Iron mineralization have occurred during mylonitization in N45W shear zones. (2) Mineralization have many characteric pecularities of IOCG hydrothermal deposits. (3) Mylonitization is super imposed on regional metamorphism therefore early paragenesis of regional metamorphism are destabilized and new paragenesis are formed. (4) Iron is released through destabilization of biotite, clinopyroxene, epidote, ilmenite and titanomagnetite of mafic metamorphic rock (mainly greenschists) and acid rocks during mylonitization event. Petrographic evidences and relevant reaction are presented and discussed. (5) Iron is mainly and preferentially released from greenschists. (6) Transport of iron by carbonile complexes are proposed based on direct evidence of Co2 rich solutions during mylonitization.

Accessory phases from the Soultz monzogranite, Soultz-sous-Forêts, France: Implications for titanite destabilisation and differential REE, Y and Th mobility in hydrothermal systems

The metaluminous Soultz-sous-Forêts monzogranite, France, is highly evolved and contains elevated concentrations of rare-earth elements (REE), Y and particularly Th. Primary accessory minerals include fluorapatite, allanite-(Ce) and Th-rich titanite. Primary titanite has been altered to anatase+calcite+quartz+synchysite-(Ce)±bastnaesite-(Ce) or anatase+calcite+quartz+monazite-(Ce)+xenotime-(Y)±thorite. Fluorocarbonate-bearing assemblages are restricted to those samples exhibiting minor selective alteration, whereas those containing phosphate-rich assemblages formed in pervasively altered samples that have experienced high fluid/rock ratios. Comparative electron-microprobe analysis of primary and hydrothermally-derived accessory phases found middle REE, Y and Th concentrations depleted in synchysite-(Ce) relative to primary titanite. Such depletions are not seen in phosphate-rich samples containing monazite-(Ce) and xenotime-(Y). Variability in elemental concentrations may be attributed to distinct fluid chemistries and hence, lead to differential mobility during alteration. Following previous experimental work and mineralogical observations, the ingress of CO2-rich solutions was integral for titanite breakdown and the resultant metasomatic assemblage. The influx of CO2-rich fluids concomitantly with chloritisation of biotite produced fluids enriched in FCO3−. We, therefore, hypothesise that after the alteration of titanite, remnant HCO3− or FCO3−-rich fluids were able to mobilise significant proportions of MREE, Y and Th not accommodated into the synchysite-(Ce) structure. Conversely, those samples rich in monazite-(Ce) and xenotime-(Y) retained their REE, Y and Th concentrations due to the presence of aqueous HPO42− derived from apatite dissolution.

XAS evidence for the stability of polytellurides in hydrothermal fluids up to 599 C, 800 bar

Although the crustal abundance of tellurium (Te) is about half of that of gold (Au), several classes of Au deposits are highly enriched in Te. Our understanding of the nature of this Au-Te association is hampered by the lack of experimental studies of Te geochemistry at elevated temperature. We characterized the structure of polytelluride solutions from room temperature to 599 °C at 800 bar using in situ X-ray absorption spectroscopy. Both ab-initio XANES and EXAFS fits show that polytellurides are stable up to the highest temperature, with planar structures (four- or threefold coordination of Te) giving way to linear chains (e.g., Te2-2ion) at temperatures above ~200 °C. This is the first experimental confirmation of the thermal stability of polytelluride species. The data show that polytellurides play an important role in Te transport in reduced S-rich or CO2-rich solutions and vapors.

Analysis of the heat of reaction and regeneration in alkanolamine-CO2 system

The estimation of regeneration heat of absorbent is important because it is a key factor that has an effect on the process efficiency. In this study, thermal stability and regeneration heat of aqueous amine solutions such as monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), N-methyldiethanolamine (MDEA), and 1,8-diamino-pmenthane (KIER-C3) were investigated by using TGA-DSC analysis. The thermal characteristics of the fresh and CO2 rich amine solutions were estimated. The CO2 rich amine solutions were obtained by VLE experiments at T=40 °C. The regeneration heat of aqueous MEA solution was 76.991–66.707 kJ/mol-CO2, which is similar to heat of absorption. The reproducibility of the results was obtained. The regeneration heat of aqueous KIER-C3 20 wt% solution (1.68 M) was lower than that of aqueous MEA 30 wt% solution (4.91 M). Therefore, the KIER-C3 can be used as an effective absorbent for acid gas removal.

Insights into the formation of Fe- and Mg-rich aqueous solutions on early Mars provided by the ALH 84001 carbonates

The chemical and isotopic pattern of the zoned carbonate globules in the ALH 84001 meteorite reveals a unique aqueous environment on early Mars. If the evolution of the fluid composition was dictated primarily by carbonate precipitation, the zoning pattern of the carbonates can constrain the fluid to have had an Mg/Ca mole ratio > ~5.3 and a Fe/Ca mole ratio > ~1 prior to the formation of the carbonates. Chemical equilibrium modeling of water–rock interactions indicates that low temperatures and low pH favor the formation of an aqueous solution with elevated Mg and Fe concentrations. The modeling shows that a sufficiently Fe- and Mg-rich fluid could have formed through low-temperature (< 100 °C) subsurface aqueous alteration of an ALH 84001-type rock at pH 5–7. This range of pH corresponds to an elevated CO 2 fugacity (~ 0.1–1 bar). Formation of ALH 84001 carbonates could have been driven by degassing of CO 2 and corresponding pH increase in near-surface environments during an upwelling of subsurface CO 2-rich solutions. This scenario is consistent with the unaltered nature of the ALH 84001 rock and with chemical and isotopic composition of its carbonates.

Phase behavior of {carbon dioxide + [bmim][Ac]} mixtures

Carbon dioxide solubility {(vapor + liquid) equilibria: VLE} in ionic liquid, 1-butyl-3-methylimidazolium acetate ([bmim][Ac]), has been measured with a gravimetric microbalance at four isotherms about (283, 298, 323, and 348) K up to about 2 MPa. (Vapor + liquid + liquid) equilibria (VLLE: or liquid–liquid separations) have also been investigated with a volumetric method used in our previous works, since the present analysis of the VLE data using our equation-of-state model has predicted the VLLE at CO 2-rich side solutions. The prediction for the VLLE has been confirmed experimentally. CO 2 solubilities at the ionic liquid-rich side show extremely unusual behaviors; CO 2 dissolves in the ionic liquid to a great degree, but there is hardly any vapor pressure above these mixtures up to about 20 mol% of CO 2. It indicates that CO 2 may have formed a non-volatile or very low vapor pressure molecular complex with the ionic liquid. The thermodynamic excess properties (enthalpy, entropy, and Gibbs free energy) of the present system do support such a complex formation. We have conducted several other experiments to investigate the complex formation (or chemical reactions), and conclude that a minor chemical reaction occurs but the complex formation is reversible without much degradation of the ionic liquid.

Solubility of CO2in Room Temperature Ionic Liquid [hmim][Tf2N

Solubility measurements of carbon dioxide in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide have been performed with a gravimetric microbalance at temperatures of about 282, 297, 323, and 348 K and pressures up to about 2 MPa. Two different sources for the ionic liquid are examined in this work: an ultrapure sample from NIST (the IUPAC task force sample) and a commercially available sample. Both samples show nearly identical solubility behaviors, being undistinguishable within experimental uncertainties. Solubility (pressure-temperature-composition) data have been well correlated with an equation-of-state (EOS) model used in our previous works. The EOS model calculations are compared with experimental solubility data for the same system in the literature. The present EOS has predicted partial immiscibility at the CO2-rich side solutions. To prove this prediction, vapor-liquid-liquid equilibrium experiments have been made, and our predictions have been confirmed.

Arsenian monazite-(Ce) and xenotime-(Y), REE arsenates and carbonates from the Tisovec-Rejkovo rhyolite, Western Carpathians, Slovakia: Composition and substitutions in the (REE,Y)XO 4 system (X = P, As, Si, Nb, S)

A unique REE–Y–(Th)–P–As–(Si)–(Nb)–(S) accessory assemblage was identified in a small body of Lower Triassic A-type rhyolite of the Silicic Superunit near Tisovec-Rejkovo, Central Slovakia. Arsenian monazite-(Ce)–phosphatian gasparite-(Ce) and arsenian xenotime-(Y)–phosphatian chernovite-(Y) solid solutions in association with REE carbonates (bastnäsite, parisite, röntgenite?, synchysite) and rare cerianite-(Ce) form anhedral to subhedral grains and aggregates (≤ 0.3 mm), scattered in the groundass or as intergrowths with zircon and Fe–Ti oxides. Compositions show a wide AsP − 1 substitution in monazite–gasparite and xenotime–chernovite solid solutions; atom. % As/(As+P) = 0.00 to 0.73 and 0.10 to 0.94, respectively. The presence of S in monazite–gasparite s.s. (≤ 0.14 apfu), together with Ca enrichment indicates “clinoanhydrite“ substitution, CaS(REE,Y) − 1 (P,As) − 1 , rather than the incorporation of brabantite, along the CaTh(REE,Y) − 2 exchange vector. Moreover, the Th- and Nb-rich members reveal thorite and fergusonite substitutions, ThSi(Y,REE) − 1 (P,As) − 1 and Nb(P,As) − 1 , respectively. The As-rich REE phases originated probably during the post-magmatic alteration of primary monazite-(Ce) and xenotime-(Y) by As-rich, high fO 2 fluids, whereas cerianite-(Ce) and the REE carbonates are products of a later hydrothermal overprint of the rhyolite by CO 2-rich solutions.

CONTRASTING FLUID TYPES AT THE NEVORIA GOLD DEPOSIT IN THESOUTHERN CROSS GREENSTONE BELT, WESTERN AUSTRALIA: IMPLICATIONS OFAURIFEROUS FLUIDS DEPOSITING ORES WITHIN AN ARCHEAN BANDED IRON-FORMATION

Two main hydrothermal mineral assemblages have been identified in the banded iron-formation-hosted gold mine at Nevoria. The earlier quartz ± garnet ± clinopyroxene ± calcite vein group formed pre- or synmetamorphic, and the later quartz ± pyrrhotite ± pyrite vein group appears to be postmetamorphic and related to gold mineralization. Fluid inclusion characteristics are obviously different in those two vein groups. Microthermometric analysis indicates that the fluids with metamorphic alteration are aqueous, CO2-rich or CO2-absent solutions; no or very small amounts of CH4 were involved in this fluid. Mineralizing fluids were a CH4-CO2-H2O solution. The initial auriferous fluids were CH4 dominant. Heterogeneous trapping, interaction of the hydrothermal fluid with graphite-bearing rocks, or fluid mixing may cause large variations of CH4/CO2 ratios or a XCH4 of CH4-CO2-H2O inclusions, particularly in mineralized quartz-pyrrhotite veins. Phase mixing or separating, resulting in an increase in pH and f O2, together with loss of reduced sulfur by mineral-fluid reactions and precipitation of sulfides, led to the breakdown of the gold-transporting complexes.

Quartz solubility in H 2O-NaCl and H 2O-CO 2 solutions at deep crust-upper mantle pressures and temperatures: 2–15 kbar and 500–900°C

The solubility of quartz in H 2O-NaCl solutions was measured at 2, 4.35, 10 and 15 kbar and 500–900°C, and at NaCl concentrations up to halite saturation, usually greater than 75 wt.%. Quartz solubility was also measured in CO 2-H 2O solutions at 10 kbar and 800°C. Solubilities were determined by weight loss of ground and polished quartz crystal fragments which were equilibrated with solutions in Pt envelopes for one to four days and then rapidly quenched. Experiments at 2 kbar were made with externally heated cold-seal apparatus; higher pressure experiments were done in a 3 4 inch-diameter piston-cylinder apparatus with NaCl pressure medium and graphite heater sleeve. Equilibrium solubility was demonstrated in several ways, and the present results reproduce those of Manning (1994) in pure H 2O at selected conditions. At pressures below 4 kbar, NaCl in solution causes an initial “salting-in”, or quartz solubility enhancement, which, at 2 kbar and 700°C, persists to concentrations as great as 70 wt.% NaCl before quartz solubility again becomes as low as in pure H 2O. The maximum solubility occurs at X(H 2O) ∼ 0.9 and is 50% higher than in pure H 2O. At 4.35 kbar and 700°C, however, quartz solubility decreases slightly with initial NaCl concentration, and then begins to drop rapidly with increasing salinity beyond 45 wt.% NaCl. At 10 and 15 kbar there is a steep initial decline in silica molality at all temperatures in the range 500–900°C, leveling off at higher NaCl concentrations. There is thus a pronounced change in solution behavior with pressure, from initial salting-in below 4 kbar to monotonic salting-out above 5 kbar. This pressure-induced change in silica solubility parallels the sharp decrease in H 2O activity in NaCl solutions in the same pressure range found by Aranovich and Newton (1996). Therefore, the pressure-induced change in silica solubility is inferred to be a consequence of the dissociation of the neutral NaCl o complex to Na + and Cl − as solution densities increase above about 0.7 gm/cm 3. At very high salinities, approaching halite saturation, the isobars of quartz solubility as a function of NaCl mole fraction at 700°C converge, indicating that, for hypersaline fluids having the constitution of molten salts, pressure has only a minor effect on quartz solubility. Quartz solubility at 10 kbar shows exponential decline with increasing salinity at all temperatures in the range 500°C to 900°C. This is the expected behavior of a two-component solvent, in which quartz is sparingly soluble in one component. At 10 kbar, isotherms of log silica molality versus H 2O mole fraction are linear between X(H 2O) = 1.0 and 0.5, but begin to curve to lower values at 900°C, where high salinities are attained before halite saturation occurs. This behavior implies that the solute silica species is a hydrate that becomes progressively destabilized at low H 2O concentrations of the solvent. Plots of log silica molality versus log H 2O activity suggest that the solute species is neutral H 4SiO 4 with no additional solvated H 2O molecules, assuming no Na-SiO 2 complexing. The solubility of quartz in CO 2-H 2O fluids at 800°C and 10 kbar is much smaller than in NaCl solutions at the same P,T and H 2O activity. Thermodynamic analysis suggests that the solute species in CO 2-H 2O fluids is H 4SiO 4 with 1–3 solvated H 2O molecules, which is similar to the solute behavior inferred by Walther and Orville (1983) in CO 2 and Ar solutions with H 2O at lower pressures. The present results show that SiO 2 will partition very strongly into a concentrated salt solution in deep crust-upper mantle metamorphic and metasomatic processes, in preference to a coexisting immiscible CO 2-rich fluid. The much greater permeability of silicate rocks for salt solutions than for CO 2-rich solutions, together with the much higher solubility of silica-rich phases in the former, could be an important factor in geochemical segregation processes involving rising and cooling fluids of magmatic or metamorphic origin.

Origin of the fluids associated with granodiorite-hosted, Sb-As-Au-W mineralisation at D�brava (N�zke Tatry Mts, Western Carpathians)

Mineral parageneses of the polymetallic, Sbrich deposit at Dubrava has been formed during five separated stages. A fluid inclusion study demonstrates that the earliest stages with scheelite, molybdenite and arsenopyrite have been associated with immiscible CO2 (± CH4)-rich, low-saline fluids at temperatures between 300 and 400 °C and pressures as much as 2 kbar. Deposition of the main, superimposed ores, stibnite and zinckenite, has been intimately connected with circulation of aqeuous, moderately saline fluids (15.5–23.5 wt% NaCl equiv.) upon epithermal conditions. Salinity of the aqueous fluids associated with tetrahedrite is clustered around 10 wt% NaCl equiv. Quartz from the latest, barite stage has precipitated from aqueous fluids enriched in divalent cations. These fluids are believed to be genetically linked with Triassic evaporite formations preserved in the region. Temperature-salinity diagrams constructed from microthermometry data indicate influx of diluted meteorite water in the stibnite, tetrahedrite and barite stages. Isotopic data are in accordance with model. The δ18O values between −9.3‰ and +1.5‰ have been derived for water in equilibrium with quartz, coexisting with sphalerite, tetrahedrite and barite, thus confirming the participation of isotopically lighter meteoric waters in the mineral-forming solutions. The (δ18O) values between +3.3‰ and +8.5‰ estimated for the water associated with the scheelite and arsenopyrite stages, are suggestive for the majority of metamorphic and/or magmatic water in the mineral-forming, CO2-rich solutions.

Mineral association and mineralogical criteria for the formation conditions of A B-F-Sn-Bi skarn in damoshan, Gejiu tin field, Southwest China

The Damoshan deposit is a small B-F-Sn-Bi exoskarn deposit and contains a distinctive mineral assemblage comprising andradite, vesuvianite, calcite, diopside, magnetite, hematite, nordenskioldine, cassiterite, varlamoffite, schoenfliesite, native bismuth, eulytite, bismite and bismuthite, in which the occurrence of eulytite is the first reported in China. Textures of the mineral parageneses show that andradite, vesuvianite and diopside were the earliest phases formed during metasomatism, i.e., the skarn forming stage. Then nordenskioldine, magnetite and native bismuth, perhaps together with eulytite, were precipitated at the stage of retrograde alteration. The minerals varlamoffite, schoenfliesite, hematite, bismite and bismuthite were probably the product of supergene alteration. The minerals were analyzed by means of electron microprobe. The data on the coexisting phases and their compositions show that during the metasomatism reduced F- and Sn-rich primary mineralizing solutions reacted with highly oxidized carbonates of the Gejiu Formation, producing a high Fe2+ /Fe3+ skarn (vesuvianite-fluorite skarn) near the contact of granite, and a low Fe2+ /Fe3+ skarn (andradite skarn) in the outer zone of the skarn body in which andradite is extremely tin-bearing up to 5.14 wt.% SnO2). In the retrograde alteration stage, B-rich, but F- and Si-deficient mineralizing solutions replaced the tin-bearing andradite, forming an association of nordenskioldine and magnetite. No sulphides were deposited at this stage because of the oxidization ambient conditions in the andradite skarn. In the supergene oxidation zone, the nordenskioldine was dissolved into varlamoffite and calcite, the native bismuth was transformed into bismite or bismuthite, and the magnetite was altered into hematite under the action of the CO2-rich supergene solutions.

Co–Ni arsenide deposits, with accessory gold, in ultramafic rocks from Morocco

A type of Co–Ni arsenide deposit with accessory gold and no silver can be recognized in ultramafic massifs of upper mantle origin. The Bou Azzer deposits (50 000 t Co) are distributed along the borders of serpentinite massifs in an Upper Proterozoic ophiolite (Anti-Atlas, Morocco). They consist of quartz–carbonate lenses with Co (Ni–Fe) arsenides. Gold is related to the skutterudite (8–15 ppm average content). In the Alpine lherzolite massif of Beni Bousera (Rif, Morocco) are found small chromite–Ni arsenides veins (300 t Ni) with accessory gold. They are associated with late orthopyroxene–vermiculite dikes. Several occurrences of As, arsenides, and gold-bearing carbonate rocks are found in serpentinites from various ophiolites. It is proposed that gold was leached by As- and Co2-rich hydrothermal solutions related to the serpentinization of mantle peridotites.

Evolution of gold-bearing veins in dykes of the Woods Point dyke swarm, Victoria

The Woods Point dyke swarm comprises hundreds of narrow, subparallel igneous dykes and dozens of pipe-shaped dyke bulges within strongly deformed early Palaeozoic turbidites of the Melbourne trough. Porpylitic alteration accompanied dyke emplacement and was followed by microfracturing induced by high fluid pressures, involving CO2 of magmatic origin, as the dykes solidified. Further stress caused through-going faults having ladder and other patterns. Isotopic studies suggest that metamorphically or geothermally-derived solutions filled the faults and other fractures with quartz and carbonate and altered immediately adjacent dyke rock. However earlier-formed vein and wall rock carbonates retained their magmatic isotopic composition. Fluid inclusions indicate vein deposition began at approximately 400°C with salinities up to 9 weight percent NaCl. Nine sulfide minerals and gold were deposited in the veins after ankerite, sericite and albite, while quartz deposition continued through all stages. Sulfur isotopic determinations indicate the vein sulfur could not have been derived from adjacent sedimentary rocks, nor exclusively from the dykes. Metamorphic waters of marine origin is a viable source for sulfur. Saline and CO2-rich alkaline solutions reacted with the dyke wall rocks and probably evolved chemically prior to deposition of gold. Vug carbonates deposited by meteoric water that leached vein carbonates mark the end of vein formation.