Gold partitioning in melt-vapor-brine systems

We used laser-ablation inductively coupled plasma mass spectrometry to measure the solubility of gold in synthetic sulfur-free vapor and brine fluid inclusions in a vapor + brine + haplogranite + magnetite + gold metal assemblage. Experiments were conducted at 800°C, oxygen fugacity buffered at Ni-NiO (NNO), and pressures ranging from 110 to 145 MPa. The wt% NaCl eq. of vapor increases from 2.3 to 19 and that of brine decreases from 57 to 35 with increasing pressure. The composition of the vapors and brines are dominated by NaCl + KCl + FeCl₂ + H₂O. Gold concentrations in vapor and brine decrease from 36 to 5 and 50 to 28 μg/g, respectively, and the calculated vapor:brine partition coefficients for gold decrease from 0.72 to 0.17 as pressure decreases from 145 to 110 MPa. These data are consistent with the thermodynamic boundary condition that the concentration of gold in the vapor and brine must approach a common value as the critical pressure is approached along the 800°C isotherm in the NaCl-KCl-FeCl₂-HCl-H₂O system.We use the equilibrium constant for gold dissolution as AuOH⁰, extrapolated from lower temperature and overlapping pressure range, to calculate expected concentrations of AuOH⁰ in our experimental vapors. These calculations suggest that a significant quantity of gold in our experimental vapors is present as a non-hydroxide species. Possible chloridogold(I) species are hypothesized based on the positively correlated gold and chloride concentrations in our experimental vapors. The absolute concentration of gold in our synthetic vapor, brine, and melt and calculated mass partition coefficients for gold between these physicochemically distinct magmatic phases suggests that gold solubility in aqueous fluids is a function of aqueous phase salinity, specifically total chloride concentration, at magmatic conditions. However, though we highlight here the effect of salinity, the combination of our data with data sets from lower temperatures evinces a significant decrease in gold solubility as temperature drops from 800°C to 600°C. This decrease in solubility has implications for gold deposition from ascending magmatic fluids.     

Geology and ore genesis of the late Paleozoic Heijianshan Fe oxide–Cu (–Au) deposit in the Eastern Tianshan,NW China

The Heijianshan Fe–Cu (–Au) deposit, located in the Aqishan-Yamansu belt of the Eastern Tianshan (NW China), is hosted in the mafic–intermediate volcanic and mafic–felsic volcaniclastic rocks of the Upper Carboniferous Matoutan Formation. Based on the pervasive alteration, mineral assemblages and crosscutting relationships of veins, six magmatic–hydrothermal stages have been established, including epidote alteration (Stage I), magnetite mineralization (Stage II), pyrite alteration (Stage III), Cu (–Au) mineralization (Stage IV), late veins (Stage V) and supergene alteration (Stage VI). The Stage I epidote–calcite–tourmaline–sericite alteration assemblage indicates a pre-mineralization Ca–Mg alteration event. Stage II Fe and Stage IV Cu (–Au) mineralization stages at Heijianshan can be clearly distinguished from alteration, mineral assemblages, and nature and sources of ore-forming fluids.Homogenization temperatures of primary fluid inclusions in quartz and calcite from Stage I (189–370 °C), II (301–536 °C), III (119–262 °C) and V (46–198 °C) suggest that fluid incursion and mixing probably occurred during Stage I to II and Stage V, respectively. The Stage II magmatic–hydrothermal-derived Fe mineralization fluids were characterized by high temperature (>300 °C), medium–high salinity (21.2–56.0 wt% NaCl equiv.) and being Na–Ca–Mg–Fe-dominated. These fluids were overprinted by the external low temperature (<300 °C), medium–high salinity (19.0–34.7 wt% NaCl equiv.) and Ca–Mg-dominated basinal brines that were responsible for the subsequent pyrite alteration and Cu (–Au) mineralization, as supported by quartz CL images and H–O isotopes. Furthermore, in-situ sulfur isotopes also indicate that the sulfur sources vary in different stages, viz., Stage II (magmatic–hydrothermal), III (basinal brine-related) and IV (magmatic–hydrothermal). Stage II disseminated pyrite has δ³⁴S_fluid values of 1.7–4.3‰, comparable with sulfur from magmatic reservoirs. δ³⁴S_fluid values (24.3–29.3‰) of Stage III Type A pyrite (coexists with hematite) probably indicate external basinal brine involvement, consistent with the analytical results of fluid inclusions. With the basinal brines further interacting with volcanic/volcaniclastic rocks of the Carboniferous Matoutan Formation, Stage III Type B pyrite–chalcopyrite–pyrrhotite assemblage (with low δ³⁴S_fluid values of 4.6–10.0‰) may have formed at low fO₂ and temperature (119–262 °C). The continuous basinal brine–volcanic/volcaniclastic rock interactions during the basin inversion (∼325–300 Ma) may have leached sulfur and copper from the rocks, yielding magmatic-like δ³⁴S_fluid values (1.5–4.1‰). Such fluids may have altered pyrite and precipitated chalcopyrite with minor Au in Stage IV. Eventually, the Stage V low temperature (∼160 °C) and low salinity meteoric water may have percolated into the ore-forming fluid system and formed late-hydrothermal veins.The similar alteration and mineralization paragenetic sequences, ore-forming fluid sources and evolution, and tectonic settings of the Heijianshan deposit to the Mesozoic Central Andean IOCG deposits indicate that the former is probably the first identified Paleozoic IOCG-like deposit in the Central Asian Orogenic Belt.     

Geochemical Characteristics and Sources of Ore-forming Fluids of the Mayuan Pb-Zn Deposit, Nanzheng, Shaanxi, China

The Mayuan stratabound Pb-Zn deposit in Nanzheng,Shaanxi Province,is located in the northern margin of the Yangtze Plate,in the southern margin of the Beiba Arch.The orebodies are stratiform and hosted in breciated dolostone of the Sinian Dengying Formation.The ore minerals are primarily sphalerite and galena,and the gangue minerals comprise of dolomite,quartz,barite,calcite and solid bitumen.Fluid inclusions from ore-stage quartz and calcite have homogenization tempreatures from 98 to 337℃ and salinities from 7.7 wt%to 22.2 wt%(NaCl equiv.).The vapor phase of the inclusions is mainly composed of CH_4 with minor CO_2 and H_2S.The δD_(fluid) values of fluid inclusions in quartz and calcite display a range from-68‰ to-113‰(SMOW),and the δ~(18)O_(fluid)values calculated from δ~(18)O_(quartz) and δ~(18)O_(calcite) values range from 4.5‰ to 16.7‰(SMOW).These data suggest that the ore-forming fluids may have been derived from evaporitic sea water that had reacted with organic matter.The δ~(13)C_(CH4) values of CH_4 in fluid inclusions range from-37.2‰ to-21.0‰(PDB),suggesting that the CH_4 in the ore-forming fluids was mainly derived from organic matter.This,together with the abundance of solid bitumen in the ores,suggest that organic matter played an important role in mineralization,and that the thermochemical sulfate reduction(TSR) was the main mechanism of sulfide precipitation.The Mayuan Pb-Zn deposit is a carbonate-hosted epigenetic deposit that may be classified as a Mississippi Valley type(MVT) deposit.     

Hydrogen-alkali exchange between silicate melts and two-phase aqueous mixtures: an experimental investigation

Experiments were performed in the three-phase system high-silica rhyolite melt + low-salinity aqueous vapor + hydrosaline brine, to investigate the exchange equilibria for hydrogen, potassium, and sodium in magmatic-hydrothermal systems at 800 °C and 100 MPa, and 850 °C and 50 MPa. The K ^aqm/melt _H,Na and K ^aqm/melt _H,K for hydrogen-sodium exchange between a vapor + brine mixture and a silicate melt are inversely proportional to the total chloride concentration (ΣCl) in the vapor + brine mixture indicating that HCl/NaCl and HCl/KCl are higher in the low-salinity aqueous vapor relative to high-salinity brine. The equilibrium constants for vapor/melt and brine/melt exchange were extracted from regressions of K ^{a q m / m e l t} _{H , N a} and K ^{a q m / m e l t} _{H , K} versus the proportion of aqueous vapor relative to brine in the aqueous mixture (F^aqv) at P and T, expressed as a function of ΣCl. No significant pressure effect on the empirically determined exchange constants was observed for the range of pressures investigated. Model equilibrium constants are: K ^aqv/melt _H,Na(vapor/melt)=26(±1.3) at 100 MPa (800 °C), and 19( ± 7.0) at 50 MPa (850 °C); K ^aqv/melt _H,K=14(±1.1) at 100 MPa (800 °C), and 24(±12) at 50 MPa (850 °C); K ^aqb/melt _H,b(brine/melt)= 1.6(±0.7) at 100 MPa (800 °C), and 3.9(±2.3) at 50 MPa (850 °C); and K ^aqb/melt _H,K=2.7(±1.2) at 100 MPa (800 °C) and 3.8(±2.3) at 50 MPa (850 °C). Values for K ^aqv/melt _H,K and K ^aqb/melt _H,K were used to calculate KCl/HCl in the aqueous vapor and brine as a function of melt aluminum saturation index (ASI: molar Al₂O₃/(K₂O+Na₂O+CaO) and pressure. The model log KCl/HCl values show that a change in melt ASI from peraluminous (ASI = 1.04) to moderately metaluminous (ASI = 1.01) shifts the cooling pathway (in temperature-log KCl/HCl space) of the aqueous vapor toward the andalusite+muscovite+K-feldspar reaction point. Received: 22 August 1996  / Accepted: 5 February 1997     

Experimental investigation of H2O and Cl− solubilities in F-enriched silicate liquids; implications for volatile saturation of topaz rhyolite magmas

To interpret the degassing of F-bearing felsic magmas, the solubilities of H₂O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C. Chloride solubility in these liquids increases with decreasing H₂O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl⁻ exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate liquids containing as little as 1 wt% H₂O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H₂O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl⁻ in H₂O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H₂O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H₂O versus Cl⁻ in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H₂O and Cl⁻ concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive H₂O and Cl⁻ concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior to eruption. Received: 6 November 1995 / Accepted: 29 January 1998     

Magmatic processes that generated the rhyolite of Glass Mountain, Medicine Lake volcano, N. California

Glass Mountain consists of a 1 km³, compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust. Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain. Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H₂O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5 km³) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of the andesite at pressures of at least 200 MPa (6 km depth) under near H₂O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing magmatic inclusions indicates that crystallization under near H₂O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the crust. Received: 26 March 1996 / Accepted: 14 November 1996     

Fluid Inclusions,Stable Isotopes,and Geochronology of the Haobugao Lead‐Zinc Deposit,Inner Mongolia,China

The Haobugao deposit, located in the southern segment of the Great Xing'an Range, is a famous skarn‐related Pb‐Zn‐(Cu)‐(Fe) deposit in northern China. The results of our fluid inclusion research indicate that garnets of the early stage (I skarn stage) contain three types of fluid inclusions (consistent with the Mesozoic granites): vapor‐rich inclusions (type LV, with V_H2O/(V_H2O + L_H2O) < 50 vol %, and the majority are 5–25 vol %), liquid‐rich two‐phase aqueous inclusions (type VL, with V_H2O/(V_H2O + L_H2O) > 50 vol %, the majority are 60–80 vol %), and halite‐bearing multiphase inclusions (type SL). These different types of fluid inclusions are totally homogenized at similar temperatures (around 320–420°C), indicating that the ore‐forming fluids of the early mineralization stage may belong to a boiling fluid system. The hydrothermal fluids of the middle mineralization stage (II, magnetite‐quartz) are characterized by liquid‐rich two‐phase aqueous inclusions (type VL, homogenization temperatures of 309–439°C and salinities of 9.5–14.9 wt % NaCl eqv.) that coexist with vapor‐rich inclusions (type LV, homogenization temperatures of 284–365°C and salinities of 5.2–10.4 wt % NaCl eqv.). Minerals of the late mineralization stage (III sulfide‐quartz stage and IV sulfide‐calcite stage) only contain liquid‐rich aqueous inclusions (type VL). These inclusions are totally homogenized at temperatures of 145–240°C, and the calculated salinities range from 2.0 to 12.6 wt % NaCl eqv. Therefore, the ore‐forming fluids of the late stage are NaCl‐H₂O‐type hydrothermal solutions of low to medium temperature and low salinity. The δD values and calculated δ¹⁸O_SMOW values of ore‐forming fluids of the deposit are in the range of ?4.8 to 2.65‰ and ?127.3‰ to ?144.1‰, respectively, indicating that ore‐forming fluids of the Haobugao deposit originated from the mixing of magmatic fluid and meteoric water. The S‐Pb isotopic compositions of sulfides indicate that the ore‐forming materials are mainly derived from underlying magma. Zircon grains from the mineralization‐related granite in the mining area yield a weighted ²⁰⁶Pb/²³⁸U mean age of 144.8 ±0.8 Ma, which is consistent with a molybdenite Re‐Os model age (140.3 ±3.4 Ma). Therefore, the Haobugao deposit formed in the Early Cretaceous, and it is the product of a magmatic hydrothermal system.     

Fluid Inclusion,H-O,S, Pb and noble gas isotope studies of the Aerhada Pb-Zn-Ag deposit,Inner Mongolia,NE China

The Aerhada Pb-Zn-Ag deposit is located in the western segment of the Great Hinggan Range Ag-Pb-Zn-Cu-Mo-Au-Fe metallogenic belt in NE China. Orebodies occur mainly as vein type and are hosted by sandstone and siliceous slate. Three stages of primary mineralization, including an early arsenopyrite-pyrite-quartz, a middle polymetallic and silver sulfides-quartz and a late sphalerite-pyrite-calcite-fluorite are recognized. Four types of fluid inclusions have been identified in the ore-bearing quartz and fluorite veins, i.e., liquid-rich, gas-rich, three-phase CO₂ aqueous inclusions, and pure gas or liquid aqueous inclusions. Microthermometric studies on fluid inclusions reveal that homogenization temperatures from early to late stages range from 253° to 430 °C, 195° to 394 °C and 133° to 207 °C, respectively. Fluid salinities range from 2.9 to 14.0 wt.% NaCl equiv. The vapor composition of the ore fluid is dominated by H₂O, CO₂ and CH₄, with minor proportions of N₂. The fluid δ¹⁸O_H2O and δD_H2O values vary from +1.6 to +9.3‰ and −122 to −56‰, respectively, and reflect a magmatic fluid and a meteoric fluid dominant hydrothermal system for the early and late stages of mineralization, respectively. The calculated δ³⁴S_H2S values of hydrothermal fluids in equilibrium with sulfides range from +5.2 to +7.1‰, suggesting a mixed source for sulfur, i.e., the local magmatic and sedimentary rocks. The Pb isotope compositions of sulfides are similar to those of the local magmatic and sedimentary rocks, implying that lead and possibly silver relate to these sources. The noble gas isotope compositions of fluid inclusions hosted in ore minerals suggest that the ore-forming fluids were dominantly derived from a deep mantle source. Fluid mixing and dilution are inferred as the dominant mechanisms for ore deposition. The Aerhada Pb-Zn-Ag deposit can be classified as a medium to low temperature hydrothermal vein type deposit.     

The partitioning behavior of silver in a vapor-brine-rhyolite melt assemblage

The partitioning of silver in a sulfur-free rhyolite melt-vapor-brine assemblage has been quantified at 800 °C, pressures of 100 and 140 MPa and f_O2≈NNO (nickel-nickel oxide). Silver solubility (±2σ) in rhyolite increases 5-fold from 105 ± 21 to 675 ± 98 μg/g as pressure increases from 100 to 140 MPa. Nernst-type partition coefficients describing the mass transfer of silver at 100 MPa between vapor and melt, brine and melt and vapor and brine are 32 ± 30, 1151 ± 238 and 0.026 ± 0.004, respectively. At 140 MPa, values for for vapor and melt, brine and melt, and vapor and brine are 32 ± 10, 413 ± 172 and 0.06 ± 0.03, respectively. Apparent equilibrium constant values (±2σ) describing the exchange of silver and sodium between vapor and melt, , at 100 and 140 MPa are 105 ± 68 and 14 ± 6. The average values (±2σ) for silver and sodium exchange between brine and melt, , at 100 and 140 MPa are 313 ± 288 and 65 ± 12. These data indicate that the mass transfer of silver from rhyolite melt to an exsolved volatile phase(s) is enhanced at 100 MPa relative to 140 MPa, suggesting that decompression increases the silver ore-generative potential of an evolving silicate magma. Model calculations using the new data suggest that the evolution of low-density, aqueous fluid (i.e., vapor) may be responsible for the the silver tonnage of many porphyry-type and perhaps epithermal-type ore deposits. For example, Halter et al. (Halter W. E., Pettke T. and Heinrich C. A. (2002) The origin of Cu/Au ratios in porphyry-type ore deposits. Science296, 1842-1844) used detailed silicate and sulfide melt inclusion and vapor and brine fluid inclusions analyses to estimate a melt volume on the order of 15 km³ to satisfy the copper budget at the Bajo de la Alumbrera copper-, gold-, silver-ore deposit. Using their melt volume estimate with the data presented here, model calculations for a 15-km³ felsic melt, saturated with pyrrhotite and magnetite, suggest that a low-salinity magmatic vapor may scavenge on the order of 7 × 10¹² g of silver from the melt. This quantity of silver exceeds the discovered 2 × 10⁹ g of Ag at Alumbrera. Calculated tonnages for numerous other deposits yield similar results. The excess silver in the vapor, remaining after porphyry formation, is then available to precipitate at lower PTconditions in the stratigraphically higher epithermal environment. These data suggest that silver, and perhaps other ore metals, in the porphyry-epithermal continuum may be derived solely from the time-integrated flux of dominantly low-salinity vapor exsolved from a series of sequential magma batches.     

Modification of fluid inclusions in quartz by deviatoric stress. III: Influence of principal stresses on inclusion density and orientation

Extraction of useful geochemical, petrologic and structural information from deformed fluid inclusions is still a challenge in rocks displaying moderate plastic strain. In order to better understand the inclusion modifications induced by deviatoric stresses, six deformation experiments were performed with a Griggs piston-cylinder apparatus. Natural NaCl–H₂O inclusions in an oriented quartz crystal were subjected to differential stresses of 250–470 MPa at 700–900 °C and at 700–1,000 MPa confining pressure. Independently of the strain rate and of the crystallographic orientation of the quartz, the inclusions became dismembered and flattened within a crystallographic cleavage plane subperpendicular to σ ₁. The neonate (newly formed) inclusions that result from dismemberment have densities that tend towards equilibrium with P _fluid = σ ₁ at T _shearing. These results permit ambiguities in earlier deformation experiments on CO₂–H₂O–NaCl to be resolved. The results of the two studies converge, indicating that density changes in neonate inclusions are promoted by high differential stresses, long periods at high P and high T, and fluid compositions that maximize quartz solubility. Neonates spawned from large precursor inclusions show greater changes in density that those spawned from small precursors. These findings support the proposal that deformed fluid inclusions can serve as monitors of both the orientation and magnitude of deviatoric stresses during low-strain, ductile deformation of quartz-bearing rocks.     

Platinum solubility and partitioning in a felsic melt-vapor-brine assemblage

The solubility and partitioning of Pt in a S-free vapor - brine - rhyolite melt - Pt metal assemblage has been quantified at 800 °C, f_O2=NNO and pressures of 100 and 140 MPa. Vapor and brine were sampled at run conditions by trapping these phases as glass-hosted fluid inclusions as the melt cooled through the glass transition temperature. The vapor and brine were in equilibrium with the melt at the time of trapping and, thus, represent fluids which were sampled at the termination of each experimental run. The microthermometrically determined salinities of vapor and brine are ∼2 and ∼63 wt.% NaCl eq. and ∼9 and ∼43 wt.% NaCl eq. at 100 and 140 MPa, respectively. Platinum solubilities in vapor, brine and glass (i.e., quenched melt) were quantified by using laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS). Equilibrium is discussed with reference to the major and trace element concentrations of glass-hosted fluid inclusions as well as the silicate melt over run times that varied from 110 to 377 h at 140 MPa and 159 to 564 h at 100 MPa. Platinum solubility values (±1σ) in H₂O-saturated felsic melt are 0.28 ± 0.13 μg/g and 0.38 ± 0.06 μg/g at 140 and 100 MPa, respectively. Platinum solubility values () at 140 and 100 MPa, respectively, in aqueous vapor are 0.91 ± 0.29 μg/g and 0.37 ± 0.17 μg/g and in are brine 16 ± 10 μg/g and 3.3 ± 1.0 μg/g. The measured solubility data were used to calculate Nernst-type partition coefficients for Pt between vapor/melt, brine/melt and vapor/brine. The partition coefficient values () for vapor/melt, brine/melt and vapor/brine at 140 MPa are 2.9 ± 1.0, 67 ± 27, and 0.13 ± 0.05 and at 100 MPa are 1.0 ± 0.2, 6.8 ± 2.4, and 0.15 ± 0.05. The partitioning data were used to model the Pt-scavenging capacity of vapor and brine during the crystallization-driven degassing (i.e., second boiling) of a felsic silicate melt over a depth range (i.e., 3-6 km) consistent with the evolution of magmatic-hydrothermal ore deposits. Model calculations suggest that aqueous vapor and brine can scavenge sufficient quantities of Pt, and by analogy other platinum group elements (PGE), to produce economically important PGE-rich magmatic-hydrothermal ore deposits in Earth’s upper continental crust.     

Origin and evolution of hydrothermal fluids in the Taochong iron deposit,Middle–Lower Yangtze Valley,Eastern China: Evidence from microthermometric and stable isotope analyses of fluid inclusions

The Middle–Lower Yangtze River Valley is one of the most important metallogenic belts in China, hosting numerous Cu–Fe–Au–Mo deposits. The Taochong deposit is located in the northern part of the Fanchang iron ore district of the Middle–Lower Yangtze River metallogenic belt. The Fe-orebody is hosted by Middle Carboniferous to Lower Permian limestones. Skarns and Fe-orebodies occur as tabular bodies along interlayer-gliding faults, at some distance from the inferred granitic intrusions. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity—the skarn, iron oxide (main mineralization stage), and carbonate stages—all contributed to the formation of the Taochong iron deposit. The skarn stage is characterized by the formation of garnet and pyroxene, with high-temperature, hypersaline hydrothermal fluids with isotopic compositions similar to those of typical magmatic fluids. These fluids were probably generated by the separation of brine from a silicate melt instead of the product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by actinolite, chlorite, quartz, calcite and hematite. The hydrothermal fluids at this stage are represented by saline fluid inclusions that coexist with vapor-rich inclusions with anomalously low δD values (− 66‰ to − 94‰). The decrease in ore fluid δ¹⁸O_water with time and decreasing depth is consistent with the decreases in fluid salinity and temperature. The fluid δD values also show a decreasing trend with decreasing depth. Both fluid inclusion and stable isotopic data suggest that the ore fluid during the main period of mineralization was evolved by the boiling of various mixtures of magmatic brine and meteoric water. This process was probably induced by a drop in pressure from lithostatic to hydrostatic. The carbonate stage is represented by calcite veins that cut across the skarn and orebody, locally producing a dense stockwork. This observation indicates the veins formed during the waning stages of hydrothermal activity. The fluids from this stage are mainly represented by a variety of low-salinity fluid inclusions, as well as fewer high-salinity inclusions. These particular fluids have the lowest δ¹⁸O_water values (− 2.2‰ to 0.4‰) and a wide of range of δD values (− 40‰ to − 81‰), which indicate that they were originated from a mixture of residual fluids from the oxide stage, various amounts of meteoric water, and possibly condensed vapor. Low-temperature boiling probably occurred during this stage.We also discuss the reasons behind the anomalously low δD values in fluid inclusion water extracted by thermal decrepitation from quartz at high temperatures, and suggest that calcite data provide a possible benchmark for adjusting low δD values found in quartz intergrown with calcite.     

Compositional re-equilibration of fluid inclusions in quartz

Compositional modifications to salt-water fluid inclusions in quartz were observed following exposure to disequilibrium conditions in a series of laboratory experiments in which samples containing inclusions of known composition were annealed at 3 kbar and 600≤T≤825°C in the presence of fluids having different compositions for time intervals ranging from a few days to one month. Changes in inclusion compositions following re-equilibration were monitored using salt crystal dissolution temperatures and/or IR (infra red) spectroscopy. The behaviors of both synthetic and natural fluid inclusions were studied. The synthetic samples were re-equilibrated under P _int=P _conf conditions to minimize stress in the crystal surrounding the inclusions, and were subjected to both f _H2O ^int f _H2O ^conf and f _H2O ^int f _H2O ^conf . After re-equilibration for four days at T≥600°C, most inclusions displayed significant compositional changes Without decrepitation. Salinity variations as large as ≈ 25 wt% were inferred for brine inclusions exposed to f _H2O ^int ≠f _H2O ^conf for 16 days at 825°C. The majority of our observations are consistent with the net diffusion of water toward the reservior having the lowest μH₂O; i.e., synthetic NaCl−H₂O fluid inclusions exhibited increased Tm(NaCl)s (implying lower relative H₂O contents) when re-equilibrated in the presence of fluids having lower μH₂O, whereas, similar (and, in some cases, the same) inclusions exhibited decreased Tm(NaCl)s (implying higher H₂O contents) after exposure to fluids having higher μH₂O. The behavior of natural salt-water fluid inclusions during re-equilibration was generally consistent with corresponding observations on synthetic samples verifying that compositional changes are not restricted to synthetic inclusions. Our results clearly show that there was chemical communication between fluids trapped as inclusions in quartz and the external fluid reservoir. Additionally, it is evident that although applied stress can significantly enhance the re-equilibration rate, strain in the crystal host around the inclusions resulting from large pressure differentials between the internal and confining fluids is not a necessary prerequisite for compositional change. Finally, because significant compositional changes can be induced in brine inclusions in quartz during shortterm exposure to non-equilibrium conditions at 600≤T≤825°C in the laboratory, it is likely that similar changes may result at much lower temperatures during exposure of natural rocks to non-equilibrium conditions over geologic time.     

Structure of H2O-saturated peralkaline aluminosilicate melt and coexisting aluminosilicate-saturated aqueous fluid determined in-situ to 800 °C and ∼800 MPa

The structure of H₂O-saturated silicate melts and of silicate-saturated aqueous solutions, as well as that of supercritical silicate-rich aqueous liquids, has been characterized in-situ while the sample was at high temperature (to 800 °C) and pressure (up to 796 MPa). Structural information was obtained with confocal microRaman and with FTIR spectroscopy. Two Al-bearing glasses compositionally along the join Na₂O•4SiO₂-Na₂O•4(NaAl)O₂-H₂O (5 and 10 mol% Al₂O₃, denoted NA5 and NA10) were used as starting materials. Fluids and melts were examined along pressure-temperature trajectories of isochores of H₂O at nominal densities (from PVT properties of pure H₂O) of 0.85 g/cm³ (NA10 experiments) and 0.86 g/cm³ (NA5 experiments) with the aluminosilicate + H₂O sample contained in an externally-heated, Ir-gasketed hydrothermal diamond anvil cell.Molecular H₂O (H₂O°) and OH groups that form bonds with cations exist in all three phases. The OH/H₂O° ratio is positively correlated with temperature and pressure (and, therefore, fugacity of H₂O, f_H2O) with (OH/H₂O°)^melt > (OH/H₂O°)^fluid at all pressures and temperatures. Structural units of Q³, Q², Q¹, and Q⁰ type occur together in fluids, in melts, and, when outside the two-phase melt + fluid boundary, in single-phase liquids. The abundance of Q⁰ and Q¹ increases and Q² and Q³ decrease with f_H2O. Therefore, the NBO/T (nonbridging oxygen per tetrahedrally coordination cations), of melt is a positive function of f_H2O. The NBO/T of silicate in coexisting aqueous fluid, although greater than in melt, is less sensitive to f_H2O.The melt structural data are used to describe relationships between activity of H₂O and melting phase relations of silicate systems at high pressure and temperature. The data were also combined with available partial molar configurational heat capacity of Qⁿ-species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H₂O content.     

CO2 solubility in dacitic melts equilibrated with H2O-CO2 fluids: Implications for modeling the solubility of CO2 in silicic melts

The solubility of CO₂ in dacitic melts equilibrated with H₂O-CO₂ fluids was experimentally investigated at 1250°C and 100 to 500 MPa. CO₂ is dissolved in dacitic glasses as molecular CO₂ and carbonate. The quantification of total CO₂ in the glasses by mid-infrared (MIR) spectroscopy is difficult because the weak carbonate bands at 1430 and 1530 cm⁻¹ can not be reliably separated from background features in the spectra. Furthermore, the ratio of CO_2,mol/carbonate in the quenched glasses strongly decreases with increasing water content. Due to the difficulties in quantifying CO₂ species concentrations from the MIR spectra we have measured total CO₂ contents of dacitic glasses by secondary ion mass spectrometry (SIMS).At all pressures, the dependence of CO₂ solubility in dacitic melts on x^fluid_CO2,total shows a strong positive deviation from linearity with almost constant CO₂ solubility at x_CO2fluid > 0.8 (maximum CO₂ solubility of 795 ± 41, 1376 ± 73 and 2949 ± 166 ppm at 100, 200 and 500 MPa, respectively), indicating that dissolved water strongly enhances the solubility of CO₂. A similar nonlinear variation of CO₂ solubility with x_CO2fluid has been observed for rhyolitic melts in which carbon dioxide is incorporated exclusively as molecular CO₂ (Tamic et al., 2001). We infer that water species in the melt do not only stabilize carbonate groups as has been suggested earlier but also CO₂ molecules.A thermodynamic model describing the dependence of the CO₂ solubility in hydrous rhyolitic and dacitic melts on T, P, f_CO2 and the mol fraction of water in the melt (x_water) has been developed. An exponential variation of the equilibrium constant K₁ with x_water is proposed to account for the nonlinear dependence of x_CO2,totalmelt on x_CO2fluid. The model reproduces the CO₂ solubility data for dacitic melts within ±14% relative and the data for rhyolitic melts within 10% relative in the pressure range 100-500 MPa (except for six outliers at low x_CO2fluid). Data obtained for rhyolitic melts at 75 MPa and 850°C show a stronger deviation from the model, suggesting a change in the solubility behavior of CO₂ at low pressures (a Henrian behavior of the CO₂ solubility is observed at low pressure and low H₂O concentrations in the melt). We recommend to use our model only in the pressure range 100-500 MPa and in the x_CO2fluid range 0.1-0.95. The thermodynamic modeling indicates that the partial molar volume of total CO₂ is much lower in rhyolitic melts (31.7 cm³/mol) than in dacitic melts (46.6 cm³/mol). The dissolution enthalpy for CO₂ in hydrous rhyolitic melts was found to be negligible. This result suggests that temperature is of minor importance for CO₂ solubility in silicic melts.     

Ore metal redistribution by hydrocarbon–brine and hydrocarbon–halide melt phases, North Range footwall of the Sudbury Igneous Complex, Ontario, Canada

We report methane-dominant hydrocarbon (fluid) inclusions (CH₄±C₂H₆–C₂H₂, C₃H₈) coexisting with primary brine inclusions and secondary halide melt (solid NaCl) inclusions in Au–Pt-rich quartz-sulfide-epidote alteration veins associated with the footwall-style Cu–PGE (platinum-group element)–Au deposits at the Fraser Mine (North Range of the Sudbury Igneous Complex). Evidence for coentrapment of immiscible hydrocarbon–brine, and hydrocarbon–halide melt mixtures is demonstrated. A primary CH₄–brine assemblage was trapped during quartz growth at relatively low T (min. T _trapping∼145–315°C) and P (max. P _trapping∼500 bar), prior to the crystallization of sulfide minerals in the veins. Secondary inclusions contain solid halite and a mixture of CH₄, C₂H₆–C₂H₂ and C₃H₈ and were trapped at a minimum T of ∼710°C. The halite inclusions may represent halide melt that exsolved from crystallizing sulfide ores that texturally postdate (by replacement) early alteration quartz hosting the primary, lower T brine–CH₄ assemblage. Laser ablation ICP-MS analyses show that the brine, hydrocarbon and halide melt inclusions contain significant concentrations of Cu (0.1–1 wt% range), Au, Bi, Ag and Pt (all 0.1–10 ppm range). Cu:Pt and Cu:Au ratios in the inclusions are significantly (up to 4 log units) lower than in the host alteration veins and adjacent massive sulfide ore veins, suggesting either (1) early Cu loss from the volatiles by chalcopyrite precipitation or (2) enhanced Au and Pt solubilities relative to Cu at the temperatures of entrapment. Concentration ratios between coexisting brine and CH₄ inclusions are lower for Cu, Au, Bi and Ag than for other elements (Na, Ca, Fe, Mn, Zn, Pb) indicating that during interaction with the brine, the hydrocarbon phase was enriched in ore metals. The high concentrations of ore metals in hydrocarbon, brine and halide melt phases confirm that both aqueous and non-aqueous volatiles were carriers of precious metals in the Sudbury environment over a wide range of temperatures. Volatile evolution and magmatic sulfide differentiation were clearly part of a single, continuous process in the Sudbury footwall. The exsolution of H₂O-poor volatiles from fractionated sulfide liquid may have been a principal mechanism controlling the final distribution of PGE and Au in the footwall ore systems. The study reports the first measurements of precious metal concentrations in fluid inclusions from a magmatic Ni–Cu–PGE environment (the Sudbury district). Electronic Supplementary Material Supplementary material is available for this article at     

Geology,Fluid Inclusions,and Isotopic Geochemistry of the Jinman Sediment‐Hosted Copper Deposit in the Lanping Basin,China

The Jinman Cu polymetallic deposit is located within Middle Jurassic sandstone and slate units in the Lanping Basin of southwestern China. The Cu mineralization occurs mainly as sulfide‐bearing quartz–carbonate veins in faults and fractures, controlled by a Cenozoic thrust–nappe system. A detailed study of fluid inclusions from the Jinman deposit distinguishes three types of fluid inclusions in syn‐ore quartz and post‐ore calcite: aqueous water (type A), CO₂–H₂O (type B), and CO₂‐dominated (type C) fluid inclusions. The homogenization temperatures of CO₂–H₂O inclusions vary from 208°C to 329°C, with corresponding salinities from 0.6 to 4.6 wt.% NaCl equivalent. The homogenization temperatures of the aqueous fluid inclusions mainly range from 164°C to 249°C, with salinities from 7.2 to 20.2 wt.% NaCl equivalent. These characteristics of fluid inclusions are significantly different from those of basinal mineralization systems, but similar to those of orogenic or magmatic mineralization systems. The H and O isotope compositions suggest that the ore‐forming fluid is predominantly derived from magmatic water, with the participation of basinal brine. The δ³⁴S values are widely variable between ?9.7 ‰ and 9.7 ‰, with a mode distribution around zero, which may be interpreted by the variation in physico‐chemical conditions or by compositional variation of the sources. The mixing of a deeply sourced CO₂‐rich fluid with basinal brine was the key mechanism responsible for the mineralization of the Jinman deposit.     

Genesis of Recent silicic magmatism in the Medicine Lake Highland,California: evidence from cognate inclusions found at Little Glass Mountain

We use granular inclusions and phenocrysts in the Little Glass Mountain rhyolite flows to estimate temperature, pressure and the fugacities of O₂, H₂ and H₂O. The compositions of magnetite-ilmenite are used to estimate temperature and oxygen fugacity. Fugacities of H₂ and H₂O are estimated from the compositions of associated biotite-sanidine-magnetite. P_Total depends on the compositions of magnetite -ferrosilite-silica. Lastly, hydrothermal experiments were conducted at the estimated T, P and f_O2 to establish the beginning of melting of the most evolved of the inclusions in CO₂-H₂O fluids.The data suggest that the most evolved inclusions formed at ～ 830°C, a total pressure of 5200 bars, f_O2 of 10^?13 and P_H2O ～ 1000 bars. Of these variables total pressure is most difficult to estimate accurately. The values of T, P etc., previously stated produce a maximum estimate of the depth of equilibration between host magma and the inclusions whereas, assuming P_H2O = P_Total yields a minimum estimate. The physical conditions together with texture suggest a plutonic origin at a minimum depth of 3.4 km but no deeper than 15–18 km beneath the Medicine Lake Highland.The composition and mineralogy suggest that the rhyolite was derived from the dacite by crystal fractionation. The relation between dacite and associated basaltic or andesitic rocks is uncertain. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios (essentially 0.7040 for both inclusions and lavas) do not require involvement of crustal rocks. A source in the uppermost mantle or lower lithosphere is considered most probable for the parental liquid which gave rise to the dacite.     

A detailed fluid inclusion study in silicified breccias from the Kombolgie sandstones (Northern Territory, Australia): inferences for the genesis of middle-Proterozoic unconformity-type uranium deposits

The relative chronology and detailed chemistry of paleofluids circulating at the base of the Kombolgie Sub-basin were investigated in the East Alligator River district (Northern Territory, Australia), where world-class unconformity-type uranium deposits are located. The chemistry of fluid inclusions was determined using in-situ analysis (Raman microprobe and laser-induced breakdown spectroscopy [LIBS]) and by observing the melting sequences by microthermometry. This study revealed the occurrence of three distinct fluids: (i) a sodium-rich brine that corresponds to a diagenetic fluid percolating at the bottom of the Kombolgie sandstones at a temperature close to 150±15 °C; (ii) a calcium-rich brine, probably corresponding to a residual brine in evaporitic environment that has evolved by fluid–rock interactions with the basement lithologies; and (iii) a low salinity fluid, heated in the basement, injected into the base of the sandstone cover. H₂ and O₂ and/or traces of CH₄ were detected in the vapor phase of some fluid inclusions, especially in the low salinity ones in quartz breccia samples taken above mineralized areas. Hydraulic brecciation of the sandstone was associated with a pressure decrease favoring fluid mixing and the subsequent cementation of breccias. According to the fluid inclusion study and other geologic constrains, the minimum thickness of the Sub-Kombolgie Basin is estimated at 4 km. Drusy quartz breccias with evidence of fluid mixing are quite common at the base of the Kombolgie Basin, but not necessarily linked to U-mineralization. However, it is proposed that the presence of gases such as H₂ and O₂ in fluid inclusions, which results from water radiolysis, constitutes an indicator of gas linked to significant U concentrations deeper in the basement rocks.     

Occurrences of Ore Minerals and Fluid Inclusion Study on the Kingking Porphyry Copper–Gold Deposit, Eastern Mindanao, Philippines

The Kingking deposit is a gold‐rich porphyry copper deposit and the southernmost deposit at the eastern Mindanao mineralized belt, Philippines. It is underlain by Cretaceous–Paleogene sedimentary and volcanic rocks that are intruded by mineralized Miocene diorite porphyries and by barren Miocene–Pliocene dacite and diorite porphyries. The main alteration zones in the deposit are the inner potassic zone and the outer propylitic zone. The biotite‐bearing diorite and hornblende diorite porphyries are the primary host rocks of mineralization. Two dominant copper minerals, bornite and chalcopyrite, which usually occur as fracture fillings, are associated with fine crystalline quartz veinlet stockworks in the mineralized diorites. Minor secondary covellite, chalcocite and digenite are also observed. The primary Cu‐Fe sulfide phases initially deposited from ore fluids consisted of bornite solid solution (bnss) and intermediate solid solution (iss), which decomposed to form the bornite and chalcopyrite. Peculiar bornite pods that are different from dissemination and are associated with volcanic rock xenoliths in biotite‐bearing diorite porphyry are noted in a drill hole. These pods of bornite are not associated with quartz veinlet stockworks. Fluid inclusion analyses show three types of inclusions contained in Kingking samples: two‐phase fluid‐rich and vapor‐rich inclusions and polyphase hypersaline inclusions from porphyry‐type quartz veinlet stockworks. The liquid–vapor homogenization temperatures (T_H) and the dissolution temperature of halite daughter crystals (T_M) from the polyphase hypersaline inclusions predominantly range from 400°C up to >500°C. The wide range of T_H and T_M may be due to heterogeneous trapping of variable ratios of vapor and brine. For some inclusions, T_H > T_M and in some cases, T_H < T_M, indicating that some of the brine was supersaturated or saturated with NaCl at the time of entrapment. Calculated salinity of the polyphase hypersaline inclusions ranges from 40 to 60% NaCl equivalent. Temperature and vapor pressure of mineralized fluid were estimated to be 400°C and 16 MPa.