Mesothermal Gold Vein Mineralization of the Seolhwa Mine,Asan Mining District,Korea

Gold mineralization of the Seolhwa mine occurs in a single stage of massive quartz veins which filled the north‐east‐trending fault shear zones in the Jurassic granitoid of 161 Ma within the Gyeonggi Massif. The vein quartz contains three main types of fluid inclusions at 25°C: (i) aqueous type I inclusions (0–15 wt.% NaCl) containing small amounts of CO₂; (ii) gas‐rich (more than 70 vol. %), vapor‐homogenizing, aqueous type II inclusions; and (iii) low‐salinity (less than 5 wt.% NaCl), liquid CO₂‐bearing, type III inclusions. The H₂O‐CO₂‐CH₄‐N₂‐NaCl inclusions represent immiscible fluids trapped earlier along the solvus curve in the temperature range 250–430°C at pressures of ～1 kb. Detailed fluid inclusion chronologies suggest a progressive decrease in pressure during the mineralization. Aqueous inclusion fluids represent either later fluids evolved through extensive fluid unmixing from a homogeneous H₂O‐CO₂‐CH₄‐N₂‐NaCl fluid due to decreases in temperature and pressure, or the influence of deep circulated meteoric waters. Initial fluids were homogeneous H₂O‐CO₂‐CH₄‐N₂‐NaCl fluids as follows: 250° to 430°C, 16–62 mol% CO₂, 5–14 mol% CH₄, 0.06–0.31 mol% N₂ and salinities of 0.4–4.9 wt.% NaCl. The T‐X data for the Seolhwa mine suggest that the hydrothermal system has been probably located nearer to the granitic melt, which facilitated the CH₄ formation and resulted in a reduced fluid state indicated by the predominance of pyrrhotite. Measured and calculated isotopic compositions of the hydrothermal fluids [δ¹⁸O = 5.3–6.5‰; δD =?69 to ?84‰] provide evidence of the CH₄‐H₂O equilibria and further indicate that the auriferous fluids were magmatically derived. Both the dominance of δ³⁴S values of sulfides close to the meteoric reference (?0.6–1.4‰; δ³⁴S_ΣS values of 0.3–1.1‰) and the available δ¹³C data (?4‰) are consistent with their deep igneous source. The Seolhwa mine was probably formed by extensive fracturing and veining due to the thermal expansion of water derived from the Jurassic granitoid melt.     

Effects of Deep Fluids on Hydrocarbon Generation and Accumulation in Chinese Petroliferous Basins

Deep fluids in a petroliferous basin generally come from the deep crust or mantle beneath the basin basement, and they transport deep substances(gases and aqueous solutions) as well as heat to sedimentary strata through deep faults. These deep fluids not only lead to large-scale accumulations of CO_2, CH_4, H_2, He and other gases, but also significantly impact hydrocarbon generation and accumulation through organic-inorganic interactions. With the development of deep faults and magmatic-volcanic activities in different periods, most Chinese petroliferous basins have experienced strong impacts associated with deep fluid activity. In the Songliao, Bohai Bay, Northern Jiangsu, Sanshui, Yinggehai and Pearl Mouth Basins in China, a series of CO_2 reservoirs have been discovered. The CO_2 content is up to 99%, with δ~(13)C_(CO2) values ranging from-4.1‰ to-0.37‰ and ~3He/~4He ratios of up to 5.5 Ra. The abiogenic hydrocarbon gas reservoirs with commercial reserves, such as the Changde, Wanjinta, Zhaozhou, and Chaoyanggou reservoirs, are mainly distributed in the Xujiaweizi faulted depression of the Songliao Basin. The δ~(13)CCH4 values of the abiogenic alkane gases are generally -30‰ and exhibit an inverse carbon isotope sequence of δ~(13)C_(CH4)δ~(13)C_(C2H6)δ~(13)C_(C3H8)δ~(13)C_(C4H10). According to laboratory experiments, introducing external H_2 can improve the rate of hydrocarbon generation by up to 147% through the kerogen hydrogenation process. During the migration from deep to shallow depth, CO_2 can significantly alter reservoir rocks. In clastic reservoirs, feldspar is easily altered by CO_2-rich fluids, leading to the formation of dawsonite, a typical mineral in high CO_2 partial pressure environments, as well as the creation of secondary porosity. In carbonate reservoirs, CO_2-rich fluids predominately cause dissolution or precipitation of carbonate minerals. The minerals, e.g., calcite and dolomite, show some typical features, such as higher homogenization temperatures than the burial temperature, relatively high concentrations of Fe and Mn, positive Eu anomalies, depletion of 18 O and enrichment of radiogenic ~(87)Sr. Due to CO_2-rich fluids, the development of high-quality carbonate reservoirs is extended to deep strata. For example, the Well TS1 in the northern Tarim Basin revealed a high-quality Cambrian dolomite reservoir with a porosity of 9.1% at 8408 m, and the Well ZS1 C in the central Tarim Basin revealed a large petroleum reserve in a Cambrian dolomite reservoir at ~6900 m. During the upward migration from deep to shallow basin strata, large volumes of supercritical CO_2 may extract petroleum components from hydrocarbon source rocks or deep reservoirs and facilitate their migration to shallow reservoirs, where the petroleum accumulates with the CO_2. Many reservoirs containing both supercritical CO_2 and petroleum have been discovered in the Songliao, Bohaiwan, Northern Jiangsu, Pearl River Mouth and Yinggehai Basins. The components of the petroleum trapped with CO_2 are dominated by low molecular weight saturated hydrocarbons.     

The origin of tin deposits in the Mount Wells region,Pine Creek Geosyncline,Northern Territory

Tabular steeply dipping cassiterite‐bearing lodes in the Mount Wells region are hosted by lower greenschist fades metasediment of the Pine Creek Geosyncline within the contact aureole of late orogenic granitoids. The latter are predominantly I‐type, but S‐type phases are developed near the sediment‐granitoid contact.

Quartz, cassiterite, pyrite, arsenopyrite, chalcopyrite and pyrrhotite are the main minerals. Two types of lodes are present: (i) Sn‐quartz lodes containing 5–10 vol% sulphide minerals; and (ii) Sn‐sulphide lodes containing ～ 70 vol% sulphide minerals. At the surface, the former appear as normal quartz veins and the latter as hematite‐quartz breccia resulting from the collapse of original sulphide‐rich lodes as a consequence of volume reduction due to oxidation and leaching.

Two stages of quartz veining are recognized in both types of lodes. Cassiterite is present in stage I while stage II is composed of barren quartz with minor pyrite. Late stage III carbonate veinlets are present in Sn‐sulphide lodes. The lode‐wallrock contact is sharp with weak alteration effects confined to the fringe of the lodes. The alteration minerals include sericite, quartz, tourmaline, chlorite, pyrite and minor K‐feldspar.

Four types of fluid inclusions are present in vein quartz and cassiterite: Type A (CO₂ ± H₂O ± CH₄); Type B (H₂O+～ 20% vapour); Type C (H₂O+ < 15% vapour) and Type D (H₂O+ < 15% vapour + NaCl). Early ‘primary’ inclusions represented by Types A and B are present in stage I only and have a well‐defined temperature mode at ～300°C and a salinity range of 1–20 wt% eq NaCl. Types C and D inclusions are ‘secondary’ in stage I and primary in stage II and have a temperature mode at 120–160°C and salinities from about 1 to more than 26 wt% eq NaCl. Variable H₂O‐CO₂ ratios of Type A inclusions and homogenization in CO₂ or H₂O phase at near identical temperature indicate entrapment at the H₂O‐CO₂ solvus and define a pressure of ～ 100 MPa. The melting sequence of frozen inclusions suggests that the ore fluids were mainly H₂O‐CO₂‐CH₄‐Na‐Ca‐Cl brines. This is also confirmed by Raman Laser Spectrometry.

Oxygen and sulphur isotope data are consistent with a magmatic origin of the ore fluids. The δD values are up to 20%0 higher than those expected for magmatic fluids and probably resulted from interaction of the latter with the carbonaceous strata. This interpretation is supported by δ¹³C data on the fluid inclusion CO₂.

Fluid inclusions, stable isotope and mineralogical data are used to approximate the physico‐chemical parameters of the ore fluids which are as follows: T 300°C, m Cl～2, fO₂ ～ 10^‐35, mSS ～ 0.01, Sn ～ 1 ppm, Cu ～ 1 ppm and pH ～ 5.5.

It is suggested that fluids of granitic parentage interacted with the enclosing sediment and picked up CO₂, CH₄ and possibly Ca. The granitic phases became reduced due to this interaction and developed S‐type characteristics. Tin was probably partitioned into the CH₄‐bearing reduced fluids. At some stage the fluid overpressure exceeded the lithostatic lode enforcing failure of the carapace and the intruded rocks by hydraulic fracturing causing CH₄ and CO₂ loss resulting in the precipitation of the ore minerals.     

金矿成矿流体特点及深-浅部流体相互作用成矿机制

以中国一些典型金矿研究资料为基础 ,结合前人关于深部流体研究成果 ,分析对比了金矿成矿流体和深部流体的特点 ,总结了深部流体参与金矿成矿作用的主要表现 ,然后计算了深部富气流体的能量传递效应和富水流体在不同深度上的密度变化规律。认为深部富气流体主要是向浅部输送大量热能和部分有利于成矿的物质 ,而浅部富水流体下渗提供足够的水体 ,二者在中地壳的“低速层”附近交汇 ,发生相互作用并作用于围岩 ,在有利条件下演化形成成矿流体 ,最终上升到地壳浅部沉淀成矿。     

Fluid Inclusions,C–H–O–S Isotope and Geochronology of the Bujinhei Pb–Zn Deposit in the Southern Great Xing'an Range of Northeast China: Implication for Ore Genesis

The Bujinhei Pb–Zn deposit is located in the southern Great Xing'an Range metallogenic belt. It is a representative medium‐ to high‐temperature hydrothermal vein type deposit controlled by fractures, and orebodies hosted in the Permian Shoushangou Formation. The hydrothermal mineralization is classified into three stages: pyrite ± arsenopyrite–quartz (Stage 1), polymetallic sulfide–quartz (Stage 2), and polymetallic sulfide–calcite (Stage 3). Fluid inclusion petrography, laser Raman analyses and microthermometry indicate that the liquid‐rich aqueous inclusions (L) and vapor‐rich CO₂ ± CH₄–H₂O inclusions (C) occur in the Stage 1 and as medium‐ to high‐ temperature and low‐ to medium‐salinity NaCl–H₂O–CO₂–CH₄ hydrothermal fluids. The liquid‐rich (L) and rare vapor‐rich CO₂ ± CH₄–H₂O inclusions (C) occur in the Stage 2 with medium‐temperature and low‐salinity NaCl–H₂O ± CO₂ ± CH₄ hydrothermal fluids. The exclusively liquid‐rich (L) fluid inclusions are observed in the Stage 3, and the hydrothermal fluid belongs to medium‐temperature and low‐salinity NaCl–H₂O hydrothermal fluids. The results of hydrogen and oxygen isotope analyses indicate that ore‐forming fluids were initially derived from the magmatic water and mixed with local meteoric water in the late stage (δ¹⁸O_H2O‐SMOW = 6.0 to 2.2‰, δD_SMOW = ?103 to ?134‰). The carbon isotope compositions (?18.4‰ to ?26.5‰) indicate that the carbon in the fluid was derived from the surrounding strata. The sulfur isotope compositions (5.7 to 15.2‰) indicate that the ore sulfur was also primarily derived from the strata. The ore vein No. 1 occurs in fractures and approximately parallel to the rhyolite porphyry; orebodies have a close spatial and temporal relationship with the rhyolite porphyry. The rhyolite porphyry yielded a crystallization age of 122.9  ± 2.4 Ma, indicating that the Bujinhei deposit may be related to the Early Cretaceous magmatic event. Geochemical analyses reveal that the Bujinhei rhyolite porphyry is high in K₂O and peraluminous, and derived from an acidic liquid as a result of strong interaction with hydrothermal fluid during the late magmatic stage; it is similar to A2‐type granites, and formed in a backarc extensional environment. These results indicate that the Bujinhei Pb–Zn deposit was a vein type system that formed in Early Cretaceous and influenced by the Paleo‐Pacific tectonic system. Bujinhei deposit is a representative hydrothermal vein type deposit on the genetic types, and occurs on the western slope of the southern Great Xing'an Range. The ore‐forming fluids were medium‐ to high‐temperature and low‐to medium‐salinity NaCl–H₂O–CO₂–CH₄ hydrothermal fluids, which became medium‐temperature and low‐salinity NaCl–H₂O hydrothermal fluids in later stages, and came from magmatic water and mixed with meteoric water, whereas the ore‐forming materials were mainly derived from the surrounding strata. The LA–ICP–MS zircon U–Pb dating indicates that the Bujinhei deposit formed at the period of late Early Cretaceous, potentially in a backarc extensional environment influenced by the Paleo‐Pacific tectonic system.     

Re-equilibration of CO2 fluid inclusions at controlled hydrogen fugacities

Abstract Natural, pure CO₂ inclusions in quartz and olivine (c. Fo₉₀) were exposed to controlled f_H2 conditions at T= 718–728°C and P_total= 2 kbar; their compositions were monitored (before and after exposures) by microsampling Raman spectroscopy (MRS) and microthermometry. In both minerals exposed at the graphite–methane buffer (f_H2= 73 bar), fluid speciations record the diffusion of hydrogen into the inclusions. In quartz, room-temperature products in euhedral isolated (EI type) inclusions are carbonic phases with molar compositions of c. CO₂(60) + CH₄(40) plus graphite (Gr) and H₂O, whereas anhedral inclusions along secondary fractures (AS type) are Gr-free and contain H₂O plus carbonic phases with compositions in the range c. CO₂(60) + CH₄(40) to CO₂(10) + CH₄(90). EI type inclusions in olivine evolved to c. CO₂(90–95) + CH₄(5–10) without Gr, whereas AS type inclusions have a range of compositions from CO₂(90) + CH₄(10) ± Gr to CH₄(50) + H₂(50) ± Gr; neither H₂O nor any hydrous species was detected by optical microscopy or MRS in the olivine-hosted products. Differences in composition between and among the texturally distinct populations of inclusions in both minerals probably arise from variations in initial fluid densities, as all inclusions apparently equilibrated with the ambient f_H2. These relations suggest that compositional variability among inclusions in a given natural sample does not require the entrapment of multiple generations of fluids. In addition, the absence of H₂O in the olivine-hosted inclusions would require the extraction of oxygen from the fluids, in which case re-equilibration mechanisms may be dependent on the composition and structure of the host mineral. Many of the same samples were re-exposed to identical P–T conditions using Ar as the pressure medium, yielding ambient f_H2= 0.06 bar. In most inclusions, the carbonic fluids returned to pure CO₂ and graphite persisted in the products. Reversal of the mechanisms from the prior exposure at f_H2= 73 bar did not occur in any inclusions but the AS types in olivine, in which minor CO₂ was produced at the expense of CH₄ and/or graphite. The observed non-reversibility of previous mechanisms may be attributed to: (1) slower fluid–solid reactions compared to reactions in the homogeneous fluid phase; (2) depressed activities of graphite due to poor ordering; and/or (3) low ambient f_O2 at the conditions of the second run.     

Geochemistry of shallow aquifers and soil gas surveys in a feasibility study at the Rivara natural gas storage site (Po Plain,Northern Italy)

A geochemical survey, in shallow aquifers and soils, has been carried out to evaluate the feasibility of natural gas (CH₄) storage in a deep saline aquifer at Rivara (MO), Northern Italy. This paper discusses the areal distribution of CO₂ and CH₄ fluxes and CO₂, CH₄, Rn, He, H₂ concentrations both in soils and shallow aquifers above the proposed storage reservoir. The distribution of pathfinder elements such as ²²²Rn, He and H₂ has been studied in order to identify potential faults and/or fractures related to preferential migration pathways and the possible interactions between the reservoir and surface. A geochemical and isotopic characterization of the ground waters circulating in the first 200 m has allowed to investigation of (i) the origin of the circulating fluids, (ii) the gas–water–rock interaction processes, (iii) the amount of dissolved gases and/or their saturation status. In the first 200 m, the presence of CH₄-rich reducing waters are probably related to organic matter (peat) bearing strata which generate shallow-derived CH₄, as elsewhere in the Po Plain. On the basis of isotopic analysis, no hints of thermogenic CH₄ gas leakage from a deeper reservoir have been shown. The δ¹³C(CO₂) both in ground waters and free gases suggests a prevalent shallow origin of CO₂ (i.e. organic and/or soil-derived). The acquisition of pre-injection data is strategic for the natural gas storage development project and as a baseline for future monitoring during the gas injection/withdrawing period. Such a geochemical approach is considered as a methodological reference model for future CO₂/CH₄ storage projects.     

Pan-African high-grade metamorphism of southern Sinai,Egypt, fluid inclusion evidence

Fluid inclusions in the leucosomes of Wadi Feiran migmatites showed that CO ₂ , H₂O and (H₂O-CO₂) fluids were likely to have been present when partial melting began in these rocks. Low salinity, aqueous fluid, to a lesser extent, CO₂-rich fluids are the most abundant fluids. The present study suggests that high-density CO₂ inclusions were formed at the earliest stage, while H₂O inclusions were formed at the late stage. In an intermediate stage, low-density CO₂ and H₂O, CO₂ inclusions were formed. At the early stage of uplift and during melt crystallization, the CO₂-bearing vapour was trapped at grain boundaries. At the late stage of uplift, H₂O released at the time of crystallization of the melt was trapped as inclusions.     

The heat capacity of hydrous cordierite above 295 K

The heat capacity of synthetic hydrous cordierite (Mg₂Al₄Si₅O₁₈·nH₂O) has been determined by differential scanning calorimetry (DSC) from 295 to 425 K as a function of H₂O content. Six samples with H₂O contents ranging from 0 to 0.82 per formula unit were examined. The partial molar heat capacity of H₂O in cordierite over the measured temperature interval is independent of composition and temperature within experimental uncertainty and is equal to 43.3 ±0.8 J/mol/ K. This value exceeds the molar heat capacity of gaseous H₂O by 9.7 J/mol/K, but is significantly smaller than the heat capacity of H₂O in several zeolites and liquid H₂O. A statistical-mechanical model of the heat capacity of adsorbed gas species (Barrer 1978) is used to extrapolate the heat capacity of hydrous cordierite to temperatures greater than 425 K. In this model, the heat capacity of hydrous cordierite (Crd·nH₂O) is represented as follows: Cp(Crd · nH₂O) = Cp(Crd)+ n{Cp(H₂O, gas)+ R(gas constant)} (1) An examination of calorimetric data for hydrous beryl, analcime, mordenite, and clinoptilolite (Hemingway et al. 1986; Johnson et al. 1982, 1991, 1992) demonstrates the general applicability of the statistical-mechanical model for the extrapolation of heat capacity data of zeolitic minerals. The heat capacity data for cordierite are combined with the data of Carey and Navrotsky (1992) to obtain the molar enthalpy of formation and enthalpy of hydration of hydrous cordierite as a function of temperature.     

Fluid inclusions in quartz from deep-seated granitic intrusions,south Norway

H₂O, CO₂, and H₂OCO₂ inclusions were observed in quatz from deep-seated granitic intrusions belonging to the Precambrian Farsund plutonic complex, south Norway. These inclusions represent solidus and/or sub-solidus fluids that were present in these rocks at some period between the initial melt and the present. Early CO₂ and H₂OCO₂ inclusions with about 20 mole% CO₂ contain up to 10 mole% CH₄ in the CO₂ phase and have densities from 0.96 to 0.85 g/cc. These inclusions are considered to most nearly approximate solidus vapour phases and suggest conditions of final solidification of the magma at 5 to 6 Kb and 700°C to 800°C. The H₂O inclusions have salinities between 2 and 60 wt%; the majority contain 5 to 20 equivalent wt.% NaCl and have densities from 1.05 to 0.85 g/cc. Microthermometry indicates that other cations such as K⁺, Ca²⁺ and / or Mg²⁺ are present in these aqueous fluids. The H₂O inclusions primarily represent fluids present at a post-magmatic stage of fracturing and healing of these rocks during uplift.     

Ore-forming fluids associated with granite-hosted gold mineralization at the Sanshandao deposit, Jiaodong gold province, China

The Sanshandao gold deposit, with total resources of more than 60 t of gold, is located in the Jiaodong gold province, the most important gold province of China. The deposit is a typical highly fractured and altered, disseminated gold system, with high-grade, quartz-sulphide vein/veinlet stockworks that cut Mesozoic granodiorite. There are four stages of veins that developed in the following sequence: (1) quartz-K-feldspar-sericite; (2) quartz-pyrite±arsenopyrite; (3) quartz-base metal sulfide; and (4) quartz-carbonate. Fluid inclusions in quartz and calcite in vein/veinlet stockworks contain C-O-H fluids of three main types. The first type consists of dilute CO₂–H₂O fluids coeval with the early vein stage. Molar volumes of these CO₂–H₂O fluid inclusions, ranging from 50–60 cm³/mol, yield estimated minimum trapping pressures of 3 kbar. Homogenization temperatures, obtained mainly from CO₂–H₂O inclusions with lower CO₂ concentration, range from 267–375 °C. The second inclusion type, with a CO₂–H₂O±CH₄ composition, was trapped during the main mineralizing stages. These fluids may reflect the CO₂–H₂O fluids that were modified by fluid/rock reactions with altered wallrocks. Isochores for CO₂-H₂O±CH₄ inclusions, with homogenization temperatures ranging from 204–325 °C and molar volumes from 55 to 70 cm³/mol, provide an estimated minimum trapping pressure of 1.2 kbar. The third inclusion type, aqueous inclusions, trapped in cross-cutting microfractures in quartz and randomly in calcite, are post-mineralization, and have homogenization temperatures between 143–228 °C and salinities from 0.71–7.86 wt% NaCl equiv. Stable isotope data show that the metamorphic fluid contribution is minimal and that ore fluids are of magmatic origin, most likely sourced from 120–126 Ma mafic to intermediate dikes. This is consistent with the carbonic nature of the fluid, and the cross-cutting nature of those deposits relative to the host Mesozoic granitoid.Editorial handling: R.J. Goldfarb     

The isotopic and chemical composition of CO2-rich thermal waters in the Mont-Dore region (Massif-Central,France)

Chemical and isotope compositions of fluid samples, collected between 1974 and 1986 from 52 springs or shallow boreholes located in the Mont-Dore region (Massif Central, France), were examined. Some springs and wells were sampled several times during this period. The fluids emerge from Quaternary volcanic rocks or Paleozoic granite at temperatures between 4 and 62°C, and the origin of the H₂O is meteoric. The waters can be classified into three groups: bicarbonate fluids, mixed bicarbonate-chloride fluids (with a mineralization up to 8 g/l), and acid-sulfate fluids. Only two fluids contain sufficient Cl⁻ to be considered as ‘mature’ waters. Previous work has demonstrated that they all contain partly mantle-derived CO₂ gas, and that the CO₂-rich gas phase and bicarbonate-chloride waters are separated at substantial depth.Mineralized fluids circulate at depth and undergo several processes, such as cooling or dilution with recent freshwater, during their ascent to the surface. Therefore, the CO₂-rich gas phase can be partly dissolved in the freshwater, or in deep fluids after their dilution. This process leads to the dissolution of surrounding rocks; such dissolution is discussed on the basis of major-element concentrations (Na, K, Ca, Mg), as well as the Sr 87/86 isotope ratio. Dissolution of S-bearing minerals has also been demonstrated. The presence of the CO₂-rich gas phase also leads to isotope exchange between CO₂ and H₂O. Some mineralized fluids are less affected by these processes than others, in which case they display the chemical and isotopic characteristics of the original deep fluids.It was shown that the applicability of geothermometer calculations for these waters is hampered by several processes that modify the chemical composition. However, some geothermometers can be used for estimating the temperature of the deep fluids using the chemical composition of the less modified fluids. They indicate that fluids emerging from volcanic rocks in the Dordogne valley reach temperatures of around 100–130°C at depth, while the temperature of the fluid that issues from the granite at Saint-Nectaire is 160–175°C at depth.     

Production of metaluminous melt during fluid‐present anatexis: an example from the Maghrebian basement,La Galite Archipelago,central Mediterranean

Garnet brought to the surface by late Miocene granitoids at La Galite Archipelago (Central Mediterranean, Tunisia) contains abundant primary melt and fluid inclusions. Microstructural observations and mineral chemistry define the host garnet as a peritectic phase produced by biotite incongruent melting at ~800 °C and 0.5 GPa, under fluid‐present conditions. The trapped melt is leucogranitic with an unexpected metaluminous and almost peralkaline character. Fluid inclusions are one phase at room temperature, and contain a CO₂‐dominated fluid, with minor H₂O, N₂ and CH₄. Siderite and an OH‐bearing phase were identified by Raman and IR spectroscopy within every analysed inclusion, and are interpreted as products of a post‐entrapment carbonation/hydration reaction between the fluid and the host during cooling. The fluid present during anatexis is therefore inferred to have been originally richer in both H₂O and CO₂. The production of anatectic melt with a metaluminous signature can be explained as the result of partial melting of relatively Al‐poor protoliths assisted by CO₂‐rich fluids.     

Compositional zoning of fluid inclusions in the Archaean Junction gold deposit,Western Australia: A process of fluid ‐ wall‐rock interaction?

The Junction gold deposit, in Western Australia, is an orogenic gold deposit hosted by a differentiated, iron‐rich, tholeiitic dolerite sill. Petrographic, microthermometric and laser Raman microprobe analyses of fluid inclusions from the Junction deposit indicate that three different vein systems formed at three distinct periods of geological time, and host four fluid‐inclusion populations with a wide range of compositions in the H₂O–CO₂–CH₄–NaCl ± CaCl₂ system. Pre‐shearing, pre‐gold, molybdenite‐bearing quartz veins host fluid inclusions that are characterised by relatively consistent phase ratios comprising H₂O–CO₂–CH₄ ± halite. Microthermometry suggests that these veins precipitated when a highly saline, >340°C fluid mixed with a less saline ≥150°C fluid. The syn‐gold mineralisation event is hosted within the Junction shear zone and is associated with extensive quartz‐calcite ± albite ± chlorite ± pyrrhotite veining. Fluid‐inclusion analyses indicate that gold deposition occurred during the unmixing of a 400°C, moderately saline, H₂O–CO₂ ± CH₄ fluid at pressures between 70 MPa and 440 MPa. Post‐gold quartz‐calcite‐biotite‐pyrrhotite veins occupy normal fault sets that slightly offset the Junction shear zone. Fluid inclusions in these veins are predominantly vapour rich, with CO₂?CH₄. Homogenisation temperatures indicate that the post‐gold quartz veins precipitated from a 310 ± 30°C fluid. Finally, late secondary fluid inclusions show that a <200°C, highly saline, H₂O–CaCl₂–NaCl–bearing fluid percolated along microfractures late in the deposit's history, but did not form any notable vein type. Raman spectroscopy supports the microthermometric data and reveals that CH₄–bearing fluid inclusions occur in syn‐gold quartz grains found almost exclusively at the vein margin, whereas CO₂–bearing fluid inclusions occur in quartz grains that are found toward the centre of the veins. The zonation of CO₂:CH₄ ratios, with respect to the location of fluid inclusions within the syn‐gold quartz veins, suggest that the CH₄ did not travel as part of the auriferous fluid. Fluid unmixing and post‐entrapment alteration of the syn‐gold fluid inclusions are known to have occurred, but cannot adequately account for the relatively ordered zonation of CO₂:CH₄ ratios. Instead, the late introduction of a CH₄–rich fluid into the Junction shear zone appears more likely. Alternatively, the process of CO₂ reduction to CH₄ is a viable and plausible explanation that fits the available data. The CH₄–bearing fluid inclusions occur almost exclusively at the margin of the syn‐gold quartz veins within the zone of high‐grade gold mineralisation because this is where all the criteria needed to reduce CO₂ to CH₄ were satisfied in the Junction deposit.     

Properties of fluids attending variable recrystallization of quartzite during contact metamorphism in the White‐Inyo Range,California

Fluid inclusions in the metamorphic aureole of the Eureka Valley‐Joshua Flat‐Beer Creek (EJB) pluton in the White‐Inyo Range, California, reveal the compositions and origin of fluids that were present during variable recrystallization of quartzite with sedimentary grain shapes to metaquartzite with granoblastic texture. Metamorphosed sedimentary formations, including quartzites, marbles, calcsilicates and schists, became ductile and strongly attenuated in the aureole during growth of the magma chamber. The microstructures of quartzites have an unusual distribution in that within ~250 m from the pluton, where temperatures exceeded 650 °C, they exhibit relict sedimentary grain shapes, only small amount of grain boundary migration (GBM), and crystallographic preferred orientations (CPOs) dominated by <a> slip. At distances >250 m, quartzites were completely recrystallized by GBM and CPOs are indicative of prism [c] slip, characteristics that are typically associated with H₂O‐assisted, high‐T recrystallization. The lack of extensive GBM in the inner aureole can be attributed to rapid replacement of H₂O by CO₂ produced by reaction of quartz grains with calcite cement that also produced interstitial wollastonite. Fluid inclusions in the inner aureole generally occur in margins of quartz grains and are either wholly aqueous (Type 1) or also contain H₂S, CO₂ and CH₄ (Type 2). Type 2 inclusions occur only in some stratigraphic layers. In both inclusion types, NaCl and CaCl₂, in variable proportions, dominate the solutes in the aqueous phase, whereas FeCl₂ and KCl are less abundant solutes. The solutes indicate attainment of a degree of equilibrium with carbonates and schists that are interbedded with the quartzites. Some Types 1 and 2 inclusions in the inner aureole show evidence of decrepitation due to high amounts of strain and/or heating suffered by the host rocks, which suggests that they represent pore fluids that existed in the rocks prior to contact metamorphism. In addition to Type 1 inclusions, outer aureole quartzites also contain inclusions that contain CO₂ vapour bubbles in addition to aqueous phase (Type 3). These inclusions only occur in interiors of granoblastic quartz that was produced by large amounts of GBM. The aqueous phase has identical ranges of first melting and final ice melting temperatures as Type 1 inclusions, suggesting that they have the same solute compositions. These inclusions are thought to represent the interstitial pore H₂O that promoted recrystallization of quartz and reacted with graphite to produce CO₂. Absence of significant amounts of CH₄ in Type 3 inclusions is attributed to elevated fO₂ that was buffered by mineral assemblages in interbedded schists. As opposed to the large amount of CO₂ that was produced by the wollastonite‐forming reaction in the inner aureole to inhibit GBM, the amount of CO₂ produced in the outer aureole by reaction between H₂O and graphite was apparently insufficient to inhibit recrystallization of quartz.     

Fluid Inclusions and C?H?O?S?Pb Isotopes: Implications for the Genesis of the Zhuanshanzi Gold Deposit on the Northern Margin of the North China Craton

The Zhuanshanzi gold deposit lies in the eastern section of the Xingmeng orogenic belt and the northern section of the Chifeng‐Chaoyang gold belt. The gold veins are strictly controlled by a NW‐oriented shear fault zone. Quartz veins and altered tectonic rock‐type gold veins are the main vein types. The deposits can be divided into four mineralization stages, and the second and third metallogenic stages are the main metallogenic stages. In this paper, based on the detailed field geological surveys, an analysis of the orebody and ore characteristics, microtemperature measurement of fluid inclusions, the Laser Raman spectrum of the inclusions, determination of C? H? O? S? Pb isotopic geochemical characteristics, and so on were carried out to explore the origin of the ore‐forming fluids, ore‐forming materials, and the genesis of the deposits. The results show that the fluid inclusions can be divided into four types: type I – gas–liquid two‐phase inclusions; type II – gas‐rich inclusions; type III– liquid inclusions; and type IV – CO₂‐containing three‐phase inclusions. However, they are dominated by type Ib – gas liquid inclusions and type IV – three‐phase inclusions containing CO₂. The gas compositions are mainly H₂O and CO₂, indicating that the metallogenic system is a CO₂? H₂O? NaCl system. The homogenization temperature of the ore‐forming fluid evolved from a middle temperature to a low temperature, and the temperature of the fluid was further reduced due to meteoric water mixing during the late stage, as well as a lack of CO₂ components, and eventually evolved into a simple NaCl? H₂O hydrothermal system. C? H? O? S? Pb isotope research proved that the ore‐forming fluids are mainly magmatic water during the early stage, with abundant meteoric water mixed in during the late stage. Ore‐forming materials originated mostly from hypomagma and were possibly influenced by the surrounding rocks, suggesting that the ore‐forming materials were mainly magmatic hydrothermal deposits, with a small amount of crustal component. The fluid immiscibility and the CO₂ and CH₄ gases in the fluids played an active and important role in the precipitation and enrichment of Au during different metallogenic stages. The deposit is considered a magmatic hydrothermal deposit of middle–low temperature.     

Evolution of metamorphic volatiles during exhumation of microdiamond-bearing granulites in the Western Gneiss Region, Norway

Fluid inclusions in garnet, kyanite and quartz from microdiamond-bearing granulites in the Western Gneiss Region, Norway, document a conspicuous fluid evolution as the rocks were exhumed following Caledonian high- and ultrahigh-pressure (HP–UHP) metamorphism. The most important of the various fluid mixtures and daughter minerals in these rocks are: (N₂ + CO₂ + magnesian calcite), (N₂ + CO₂ + CH₄ + graphite + magnesian calcite), (N₂ + CH₄), (N₂ + CH₄ + H₂O), (CO₂) and (H₂O + NaCl + CaCl₂ + nahcolite). Rutile also occurs in the N₂ + CO₂ inclusions as a product of titanium diffusion from the garnet host into the fluid inclusions. Volatiles composed of N₂ + CO₂ + magnesian calcite characterise the ambient metamorphic environment between HP–UHP (peak) and early retrograde metamorphism. During progressive decompression, the mole fraction of N₂ increased in the fluid mixtures; as amphibolite-facies conditions were reached, CH₄ and later, H₂O, appeared in the fluids, concomitant with the disappearance of CO₂ and magnesian calcite. Graphite is ubiquitous in the host lithologies and fluid inclusions. Thermodynamic modelling of the metamorphic volatiles in a graphite-buffered C-O-H system demonstrates that the observed metamorphic volatile evolution was attainable only if the f _O2 increased from c. −3.5 (±0.3) to −0.8 (±0.3) log units relative to the FMQ oxygen buffer. External introduction of oxidising aqueous solutions along a system of interconnected ductile shear zones adequately explains the dramatic increase in the f _O2. The oxidising fluids introduced during exhumation were likely derived from dehydration of oceanic crust and continental sediments previously subducted during an extended period of continental collision in conjunction with the Caledonian orogeny. Received: 15 December 1997 / Accepted: 25 May 1998     

Calculated fluid evolution path versus fluid inclusion data in the COHN system as exemplified by metamorphic rocks from Rogaland, south-west Norway

Abstract Fluid evolution paths in the COHN system can be calculated for metamorphic rocks if there are relevant data regarding the mineral assemblages present, and regarding the oxidation and nitrodation states throughout the entire P-T loop. The compositions of fluid inclusions observed in granulitic rocks from Rogaland (south-west Norway) are compared with theoretical fluid compositions and molar volumes. The fluid parameters are calculated using a P-T path based on mineral assemblages, which are represented by rocks within the pigeonite-in isograd and by rocks near the orthopyroxene-in isograd surrounding an intrusive anorthosite massif. The oxygen and nitrogen fugacities are assumed to be buffered by the coexisting Fe-Ti oxides and Cr-carlsbergite, respectively. Many features of the natural fluid inclusions, including (1) the occurrence of CO₂-N₂-rich graphite-absent fluid inclusions near peak M2 metamorphic conditions (927° C and 400 MPa), (2) the non-existence of intermediate ternary CO₂-CH₄-N₂ compositions and (3) the low-molar-volume CO₂-rich fluid inclusions (36–42 cm³ mol^?1), are reproduced in the calculated fluid system. The observed CO₂-CH₄-rich inclusions with minor N₂ (5 mol%) should also include a large proportion of H₂O according to the calculations. The absence of H₂O from these natural high-molar-volume CO₂-CH₄-rich inclusions and the occurrence of natural CH₄-N₂-rich inclusions are both assumed to result from preferential leakage of H₂O. This has been previously experimentally demonstrated for H₂O-CO₂-rich fluid inclusions, and has also been theoretically predicted. Fluid-deficient conditions may explain the relatively high molar volumes, but cannot be used to explain the occurrence of CH₄-N₂-rich inclusions and the absence of H₂O.     

Biodegradation of CO2, CH4 and volatile organic compounds (VOCs) in soil gas from the Vicano–Cimino hydrothermal system (central Italy)

We investigated the effect of microbial activity on the chemistry of hydrothermal fluids related to the Vicano–Cimino system, central Italy. The database included the composition and δ¹³C CO₂ and δ¹³C CH₄ values for soil gas from an area characterized by intense degassing of fluids having a deep origin. The δ¹³C CH₄ values along vertical profiles in the soil indicated that CH₄ was controlled by microbial oxidation occurring at shallow (< 50 cm) depth, where free O₂ was available. This was consistent with the vertical gradients of CH₄, H₂S and O₂ concentrations. The δ¹³C CO₂ values in soil gas, characterized by a composition similar to that of the hydrothermal fluids, were not significantly influenced by biodegradation. On the contrary, gas strongly affected by air contamination showed a significant δ¹³C CO₂ fractionation. Microbial activity caused strong consumption of hydrothermal alkanes, alkenes, cyclics and hydrogenated halocarbons, whereas benzene was recalcitrant. Oxygenated compounds from hydrocarbon degradation consisted of alcohols, with minor aldehydes, ketones and carboxylic acids. A predominance of alcohols at a high rate of degassing flux, corresponding to a short residence time of hydrothermal gas within the soil, indicated incomplete oxidation. N-bearing compounds were likely produced by humic substances in the soil and/or related to contamination by pesticides, whereas α-pinene traced air entering the soil. The study demonstrates that microbial communities in the soil play an important role for mitigating the release to the atmosphere of C-bearing gases, especially CH₄, through diffuse soil degassing, a mechanism that in central Italy significantly contributes to the discharge of CO₂-rich gas from deep sources.     

Role of fluids in migmatites: CO2-H2O fluid inclusions in leucosomes from the Deep Freeze Range migmatites (Terra Nova Bay, Antarctica)

ABSTRACT The metasedimentary sequence of the Deep Freeze Range (northern Victoria Land, Antarctica) experienced high-T/low-F metamorphism during the Cambro-Ordovician Ross orogeny. The reaction Bt + Sil + Qtz = Grt + Crd + Kfs + melt was responsible for the formation of migmatites. Peak conditions were c. 700–750° C, c. 3.5–5 kbar and x_H2Oc. 0.5). Distribution of fluid inclusions is controlled by host rock type: (1) CO₂-H₂O fluid inclusions occur only in graphite-free leucosomes; (2) CO₂–CH₄± H₂O fluid inclusions are the most common type in leucosomes, and in graphite-bearing mesosomes and gneiss; and (3) CO₂–N₂–CH₄ fluid inclusions are observed only in the gneiss, and subordinately in mesosomes. CO₂–H₂O mixtures (41% CO₂, 58% H₂O, 1% Nad mol.%) are interpreted as remnants of a synmig-matization fluid; their composition and density are compatible P–T–a_H2O conditions of migmatization (c. 750° C, c. 4 kbar, x_H2Oc. 0.5). CO₂-H₂O fluid in graphite-free leucosomes cannot originate via partial melting of graphite-bearing mesosomes in a closed system; this would have produced a mixed CO₂–CH₄ fluid in the leucosomes by a reaction such as Bt + Sil + Qtz + C ± H₂O = Grt + Crd + Kfs + L + CO₂+ CH₄. We conclude that an externally derived oxidizing CO₂-H₂O fluid was present in the middle crust and initiated anatexis. High-density CO₂-rich fluid with traces of CH₄ characterizes the retrograde evolution of these rocks at high temperatures and support isobaric cooling (P–T anticlockwise path). In unmigmatized gneiss, mixed CO₂–N₂–CH₄ fluid yields isochores compatible with peak metamorphic conditions (c. 700–750° C, c. 4–4.5 kbar); they may represent a peak metamorphic fluid that pre-dated the migmatization.