Quartz-hosted Inclusions Research Articles

Quartz-hosted inclusions and embayments reveal storage, fluxing, and ascent of the Mesa Falls Tuff, Yellowstone

Quartz-hosted glasses from the Mesa Falls Tuff provide a geochemical window into the pre-eruptive magmatic system from one of Yellowstone's largest-volume caldera-forming eruptions. H2O and CO2 concentrations, along with major and trace elements, were measured in both fully enclosed glass inclusions and partially enclosed embayments in the same quartz crystals. Major elements are largely consistent between the inclusions and embayments, except for K2O and Na2O. Of note, K2O is enriched by ∼1 wt.% in embayment interiors relative to inclusions. Most trace elements are also enriched in the embayment interiors compared to inclusions from the same crystals. Fractionation trends of trace elements are consistent with ∼30-60% crystallization. Quartz-hosted glass inclusions preserve 3.1±0.9 wt.% H2O and 493±227 ppm CO2 whereas embayment interiors have 0.9±0.1 wt.% H2O and 399±229 ppm CO2. The CO2 is roughly similar, but the distinct ∼2 wt.% discrepancy between inclusion and embayment interior H2O contents may have been produced by CO2 fluxing sourced from underplated Yellowstone basalts. H2O gradients within embayments are flat in their interiors and modified by sharp positive gradients near embayment exteriors which were produced by post-eruptive rehydration. CO2 gradients occur as gently sloping concentration gradients that extend inward 150 to 250 μm from the embayment exterior. Finite-difference 1D and 2D diffusion modeling indicates the distribution of H2O and CO2 in embayments was produced by slow, fluid-saturated decompression that preceded rapid ascent during the caldera-forming eruption.

Metasomatized mantle lithosphere and altered ocean crust as a fluid source for orogenic gold deposits

Whether auriferous hydrothermal fluid is generated during crustal metamorphism or by subcrustal processes to form orogenic gold deposits is hotly debated. Noble gas isotope and halogen abundance ratios can provide important constraints to help resolve this controversy. The Danba gold deposit is a rare example of a Jurassic age hypozonal orogenic gold deposit on the western margin of the Yangtze Craton, China. It is characterized by thick barren quartz veins with fractures infilled by thin auriferous quartz veinlets. The syn-gold quartz veinlets are associated with plagioclase and pyrrhotite and are distinguished from the barren quartz vein hosts on the basis of cathodoluminescence, trace-element content and Raman spectroscopy and microthermometry of fluid inclusions. The dominant fluid inclusions in the gold-bearing quartz and plagioclase are H2O–CH4 with vapor fill of ∼ 15–45 vol% and high pressure-corrected trapping temperatures of ∼ 500 °C, low salinities (<4 wt% NaCl equiv.) and high Mg/Fe ratios. Crushing syn-gold quartz and plagioclase released fluid inclusions with 40Ar/36Ar ratios of between 490 and 710, molar Br/Cl ratios of (0.3–0.7) × 10−3 and molar I/Cl ratios of (18–60) × 10−6. Crushing syn-gold pyrrhotite released fluid inclusions with 3He/4He ratios of 0.21–0.38 Ra (Ra = air value, 1.39 × 10−6) that are higher than typical crustal values of < 0.1 Ra, and 40Ar/36Ar of 330–640 that are similar to the gold-related quartz-hosted inclusions. The combined noble gas and halogen data do not favor an auriferous fluid derived from K- and U-rich Proterozoic granitic basement, which is expected to have radiogenic 40Ar/36Ar and 3He/4He ratios. Instead, the data are more consistent with a fluid source from underlying mantle lithosphere containing a high proportion of altered ocean crust (AOC). The auriferous fluid that formed the Danba gold deposit is thus considered to have been derived from a source comprising both metasomatized mantle lithosphere that had been fertilized in a Neoproterozoic subduction event and a portion of relict AOC beneath the western margin of the Yangtze Craton. In contrast with syn-gold fluid inclusions, those in pre-gold barren quartz are H2O–CO2 dominated. Crushing pre-gold quartz released fluid inclusions with 40Ar/36Ar ratios of 2410–5450, Br/Cl ratios of (1.2–1.5) × 10−3 and I/Cl ratios of (60–80) × 10−6 that are distinctly different to the gold-ore stage. The higher 40Ar/36Ar ratios of the pre-gold fluids are consistent with a source for the barren host quartz veins from prograde greenschist- to amphibolite-facies metamorphism of the Proterozoic granitic basement.

Ore-Fluid Evolution of the Sizhuang Orogenic Gold Deposit, Jiaodong Peninsula, China

The Sizhuang gold deposit with a proven gold resource of &gt;120 t, located in northwest Jiaodong Peninsula in China, lies in the southern part of the Jiaojia gold belt. Gold mineralization can be divided into altered rock type, auriferous quartz vein type, and sulfide-quartz veinlet in K-feldspar altered granite. According to mineral paragenesis and mineral crosscutting relationships, three stages of metal mineralization can be identified: early stage, main stage, and late stage. Gold mainly occurs in the main stage. The petrography and microthermometry of fluid inclusion shows three types of inclusions (type 1 H2O–CO2 inclusions, type 2 aqueous inclusions, and type 3 CO2 inclusions). Early stage quartz-hosted inclusions have a trapped temperatures range 303–390 °C. The gold-rich main stage contains a fluid-inclusion cluster with both type 1 and 2 inclusions (trapped between 279 and 298 °C), and a wide range of homogenization temperatures of CO2 occurs to the vapor phase (17.6 to 30.5 °C). The late stage calcite only contains type 1 inclusions with homogenization temperatures between 195 and 289 °C. With evidences from the H–O isotope data and the study of water–rock interaction, the metamorphic water of the Jiaodong Group is considered to be the dominating source for the ore-forming fluid. The ore-fluid belonged to a CO2–H2O–NaCl system with medium-low temperature (160–360 °C), medium-low salinity (3.00–11.83 wt% NaCl eq.), and low density (1.51–1.02 g/cm3). Fluid immiscibility caused by pressure fluctuation is the key mechanism in inducing gold mineralization in the Sizhuang gold deposit.

The controls of post-entrapment diffusion on the solubility of chalcopyrite daughter crystals in natural quartz-hosted fluid inclusions

The presence of chalcopyrite daughter crystals in natural quartz-hosted fluid inclusions that do not dissolve when heated to trapping conditions suggests that inclusions are subject to post-entrapment modifications that affect chalcopyrite solubility. Previous double capsule experiments conducted by Mavrogenes and Bodnar (1994) concluded that the post-entrapment outward diffusion of H2 is responsible for the presence of non-dissolvable chalcopyrite crystals in natural, quartz-hosted brine inclusions. However, recent studies have shown that quartz-hosted inclusions can also be modified by diffusional H+ loss and Cu+ gain. This means that multiple factors may influence chalcopyrite solubility in different fluid inclusion types. In this study, the experimental procedure of Mavrogenes and Bodnar (1994) was recreated in order to rehydrogenate quartz-hosted, chalcopyrite-bearing fluid inclusions from the El Teniente Cu–Mo porphyry deposit, Chile. These inclusions had a range of salinities and densities. Results show that the experimental technique is successful for fluid inclusions that contain relatively small chalcopyrite daughter crystals and have moderate salinities (>5wt.% NaCleq). In contrast, chalcopyrite crystals do not dissolve in low density vapor inclusions even after rehydrogenation. The failure of chalcopyrite crystals to dissolve in these inclusions is attributed to their lower initial pH and higher sulfide concentrations, which led to greater post-entrapment H+ loss and Cu+ gain. This considered, Cu concentrations in moderate to high salinity inclusions are likely to reflect those present at trapping, suggesting that H2 loss is the primary control on the failed dissolution of chalcopyrite. By contrast, Cu concentrations in S-rich vapor inclusions can increase considerably via inward Cu+ diffusion in the presence of an external Cu-bearing fluid and a pH gradient between the inclusion and this fluid (Lerchbaumer and Audétat, 2012; Seo and Heinrich, 2013). In accordance with these studies, the post-entrapment modification of Cu concentrations in vapor inclusions may undermine the apparent importance of phase separated vapors as a key agent of Cu transport and deposition in porphyry systems.

HYDROTHERMAL EVOLUTION IN THE HOIDAS LAKE VEIN-TYPE REE DEPOSIT, SASKATCHEWAN, CANADA: CONSTRAINTS FROM FLUID INCLUSION MICROTHERMOMETRY AND EVAPORATE MOUND ANALYSIS

The Hoidas Lake rare earth element (REE) deposit, located in northern Saskatchewan, Canada, is a structurally controlled vein-type LREE deposit with allanite-(Ce) and fluorapatite as the main REE carriers. The mineralized veins cut Archean and Paleoproterozoic orthogneisses and supracrustal rocks of the southern Rae Subprovince. The paragenesis includes hyalophane-bearing pegmatite dikes, REE-mineralized allanite-, diopside-, hornblende-, hyalophane-, and titanite-bearing veins, and later breccia veins that contain several stages of apatite. The mineralized veins record hydrothermal alteration with nucleation of monazite and REE-carbonate inclusions in apatite and allanite, respectively. Barren quartz-, carbonate-, and hematite-rich veins represent the final stage of hydrothermal activity. Samples from the hyalophane-bearing pegmatites, apatite breccia veins, and quartz-carbonate veins, used for fluid inclusion petrography and microthermometry, indicate four distinct fluid inclusion assemblages (FIAs): (1) carbonic inclusions showing LCO2 (liquid CO2) + VCO2 (CO2 vapor) phase composition with 20–40 vol.% VCO2 at 0 °C; (2) aqueous inclusions showing L (liquid H2O) + V (H2O vapor) phase composition with 90–100 vol.% V at 20 °C; (3) aqueous inclusions showing L + V + H (halite) phase composition with 15 vol.% V at 20 °C; and (4) aqueous L + V inclusions with 15–20 vol.% V at 20 °C. Type 1 inclusions homogenize to liquid CO2 with Th(CO2) from 3.3 to 30.5 °C. Type 2 V-rich inclusions have high salinities and contain salts other than NaCl. Type 3 L-V-H inclusions have different homogenization behavior in quartz of the hyalophane-bearing pegmatites (L+V+H → L+V → L) compared to quartz-carbonate veins (L+V+H → L+H → L), and Th (total homogenization) values range from 180 to 315 °C with salinities of 30–40 wt.% eq. NaCl. In type 4 inclusions Th ranges from 90 to 290 °C, but for specific samples and FIAs (fluid inclusion assemblages) there is a more limited spread (from 5 to 25 °C). Salinities range from 8 to 24 wt.% eq. NaCl, and the inclusions have variable Na/(Na + Ca). Evaporate mound analysis shows average normalized (to 100%) cation contents of 48% Na, 24% Ca, 6% K, 5% Ba, 4% Mn, 2% Fe, 2% Mg, and 9% Sr for quartz-hosted inclusions in quartz-carbonate veins, and 61% Na, 32% Ca, and 7% K for quartz-hosted inclusions in hyalophane-bearing pegmatite dikes. The thermometric and chemical data suggest that evolution of the Hoidas Lake mineralization involved two fluid types with early entrapment of a carbonic fluid followed by introduction of a mixed Na-Ca-K-(Ba-Mn-Mg-Fe-Sr) aqueous fluid with variable salinities that was responsible for the late alteration of the mineralized veins. Furthermore, the inclusion data provide constraints on entrapment temperature (<310 °C) and also indicate that the pressure was transient (0.5 to 2 kbars) based on the homogenization temperature data for the carbonic (type 1) and aqueous L-V-H (type 3) inclusions.

Selective copper diffusion into quartz-hosted vapor inclusions: Evidence from other host minerals, driving forces, and consequences for Cu–Au ore formation

Recent experimental studies have raised concerns that Cu concentrations in quartz-hosted fluid inclusions from magmatic–hydrothermal ore deposits do not represent pristine concentrations in the trapped fluids, but are modified by post-entrapment diffusional exchange through the host quartz. New microanalyses of fluid inclusions hosted in topaz show significantly lower Cu concentrations in vapor inclusions, compared to otherwise identical inclusions hosted by coexisting quartz, whereas coeval brine (hypersaline liquid) inclusions are very similar independent of host mineral in one sample. Sulfur is present as a major component in all vapor inclusions, as in most porphyry-related vapor inclusions, and Cu never exceeds S, but commonly matches the S content at a molar ratio of Cu:S⩽2 in vapor inclusions hosted by quartz.Univalent ions with a radius smaller than ∼1Å are known to diffuse rapidly through the channels of the quartz structure, parallel to its crystallographic c axis. Since only Cu concentrations differ between topaz- and quartz-hosted inclusions, we hypothesize that Cu+ and H+ re-equilibrate by diffusional ion exchange through these channels, while all other element concentrations remain essentially unchanged. A thermodynamic model considering charge-balanced Cu+H+ exchange and diffusive H2 re-equilibration of an initially Cu-poor but S-rich vapor inclusion with a typical rock-buffered fluid environment outside the host crystal demonstrates a strong chemical driving force for Cu+ to migrate from the surrounding rock into the fluid inclusion during cooling of the system. The driving force for Cu diffusion, against the gradient in total Cu concentration, is the abundant H+ liberated inside the inclusion by dissociation of HCl and particularly by the precipitation of CuFeS2 by reaction with the initially trapped H2S and/or SO2. Gold is not only a much larger ion, but is subject to an opposing driving force, suggesting that high concentrations of this larger ion analyzed in vapor inclusions probably represent true gold concentrations in magmatic–hydrothermal vapor.These findings imply that brine–vapor separation in porphyry deposits does not cause selective Cu transfer to the vapor, but is more likely to destabilize Cu complexes and promote copper ore deposition during decompression and unmixing of the two fluid phases. By contrast, Au may be selectively transferred into the vapor phase, allowing its transport through the deeper porphyry copper deposits to form epithermal gold deposits closer to the earth’s surface.

Evolution of the trachydacite and pantellerite magmas of the bimodal volcanic association of Dzarta-Khuduk, Central Mongolia: Investigation of inclusions in minerals

Using various methods of melt inclusion investigation, including electron and ion microprobe techniques, we estimated the composition, evolution, and formation conditions of melts producing the trachydacites and pantellerites of the Late Paleozoic bimodal volcanic association of Dzarta-Khuduk, Central Mongolia. Primary crystalline and melt inclusions were detected in anorthoclase from trachydacites and quartz from pantellerites and pantelleritic tuffs. Among the crystalline inclusions, we identified hedenbergite, fluorapatite, and pyrrhotite in the trachydacites and F-arfvedsonite, fluorite, ilmenite, and the rare REE diorthosilicate chevkinite in the pantellerites. Melt inclusions in anorthoclase from the trachydacites are composed of glass, a gas phase, and daughter minerals (F-arfvedsonite, fluorite, villiaumite, and anorthoclase rim on the inclusion wall). Melt inclusions in quartz from the pantellerites are composed of glass, a gas phase, and a fine-grained salt aggregate consisting of Li, Na, and Ca fluorides (griceite, villiaumite, and fluorite). Melt inclusions in quartz crystalloclasts from the pantelleritic tuffs are composed of homogeneous silicate glasses. The phenocrysts of the trachydacites and pantellerites crystallized at temperatures of 1060–1000°C. During thermometric experiments with quartz-hosted melt inclusions from the pantellerites, the formation of immiscible silicate and salt (fluoride) melts was observed at a temperature of 800°C. Homogeneous melt inclusions in anorthoclase from the trachydacites have both trachydacite and rhyolite compositions (wt %): 68–70 SiO2, 12–13 Al2O3, 0.34–0.74 TiO2, 5–7 FeO, 0.4–0.9 CaO, and 9–12 Na2O + K2O. The agpaitic index ranges from 0.92 to 1.24. The glasses of homogenized melt inclusions in quartz from the pantellerites and pantelleritic tuffs have rhyolitic compositions. Compared with the homogeneous glasses trapped in anorthoclase of the trachydacites, quartz-hosted inclusions from the pantellerites show higher SiO2 (72–78 wt %) and lower Al2O3 contents (7.8–10.0 wt %). They also contain 0.14–0.26 wt % TiO2, 2.5–4.9 wt % FeO, 9–11 wt % Na2O + K2O, and 0.9–0.15 wt % CaO and show an agpaitic index of 1.2–2.05. Homogeneous melt inclusions in quartz from the pantelleritic tuffs contain 69–72 wt % SiO2. The contents of other major components, including TiO2, Al2O3, FeO, and CaO, are close to those in the homogeneous glasses of quartzhosted melt inclusions in the pantellerites. The contents of Na2O + K2O are 4–10 wt %, and the agpaitic index is 1.0–1.6. The glasses of melt inclusions from each rock group show distinctive volatile compositions. The H2O content is up to 0.08 wt % in anorthoclase of the trachydacites, 0.4–1.4 wt % in quartz of the pantellerites, and up to 5 wt % in quartz of the pantelleritic tuffs. The content of F in the glasses of melt inclusions in the phenocrysts of the trachydacites is no higher than 0.67 wt %, and up to 1.4–2.8 wt % in quartz from the pantellerites. The Cl content is up to 0.2 wt % in the glasses of melt inclusions in the minerals of the trachydacites and up to 0.5 wt % in the glasses of quartz-hosted melt inclusions from the pantellerites. The investigation of trace elements in the homogenized glasses of melt inclusions in minerals showed that the trachydacites and pantellerites were formed from strongly evolved rare-metal alkaline silicate melts with high contents of Li, Zr, Rb, Y, Hf, Th, U, and REE. The analysis of the composition of homogeneous melt inclusions in the minerals of the above rocks allowed us to distinguish magmatic processes resulting in the enrichment of these rocks in trace and rare earth elements. The most important processes are the crystallization differentiation and immiscible separation of silicate and fluoride salt melts. It was also shown that all the melts studied evolved in spatially separated magma chambers. This caused the differences in the character of melt evolution between the trachydacites and pantellerites. During the final stages of differentiation, when the magmatic system was saturated with respect to ore elements, Na-Ca fluoride melts were separated and extracted considerable amounts of Li.

Physico-chemical parameters from melt inclusions for the formation of the massive sulfide deposits in the Altai–Sayan Region, Central Asia

Studies of quartz-hosted melt inclusions from rhyolitic dacite and andesite rocks showed that high-temperature (910–1180°C) felsic melts of normal alkalinity, which correspond to a tholeiitic K–Na series, played an active role during the formation of volcanogenic massive sulfide deposits of the Altai–Sayan Region. High temperatures can be explained by felsic melts degassing in open volcanic systems and heating by ascending basaltic magma. Considerable variation in the H2O content in magmatic systems of the Altai–Sayan volcanogenic massive sulfide deposits (0.08–4.30 wt%) has been established. REE patterns of quartz-hosted inclusions from volcanic rocks of the Altai–Sayan volcanogenic massive sulfide deposits display distinct negative Eu anomalies, which suggest plagioclase fractionation during differentiation of the melt. They are similar to the patterns of quartz-hosted inclusions of the Yaman–Kasy deposit (marginal sea setting) in the South Urals and to island arc rhyolites. Analyses of melt inclusions showed high values of Cu—up to 4100 ppm at the Yubileinoe deposit (Rudny Altai) and up to 3230 ppm at the Kyzyl–Tashtyg deposit (East Tuva). Comparison of data on melt inclusions has shown that some physical and chemical parameters of the magmas that generate volcanogenic massive sulfide and porphyry Cu deposits can be quite similar within certain limits.

Anomalously Metal-Rich Fluids Form Hydrothermal Ore Deposits

Hydrothermal ore deposits form when metals, often as sulfides, precipitate in abundance from aqueous solutions in Earth's crust. Much of our knowledge of the fluids involved comes from studies of fluid inclusions trapped in silicates or carbonates that are believed to represent aliquots of the same solutions that precipitated the ores. We used laser ablation inductively coupled plasma mass spectrometry to test this paradigm by analysis of fluid inclusions in sphalerite from two contrasting zinc-lead ore systems. Metal contents in these inclusions are up to two orders of magnitude greater than those in quartz-hosted inclusions and are much higher than previously thought, suggesting that ore formation is linked to influx of anomalously metal-rich fluids into systems dominated by barren fluids for much of their life.

An evaluation of synthetic fluid inclusions for the purpose of trapping equilibrated, coexisting, immiscible fluid phases at magmatic conditions

We report data that allow us to evaluate the method of trapping immiscible, saline aqueous fluids (i.e., vapor and brine in the NaCl-KCl-HCl-FeCl2-AuHCl2-H2O system) as synthetic fluid inclusions in pre-fractured quartz cores in order to quantify the concentrations of Au, Fe, K, and Na, among coexisting three-phase, immiscible fluids (i.e., haplogranite melt, brine, and vapor) at magmatic conditions. Coexisting vapor and brine were trapped experimentally at 800 °C and 100-110 MPa as synthetic fluid inclusions in both quartz microfractures and quenched silicate melt (i.e., glass), and also sampled indirectly using the recovered quenched aqueous fluid. Quartz-hosted and glass-hosted brine inclusions were analyzed by laser-ablation inductively-coupled-plasma mass spectrometry (LAICPMS) and instrumental neutron activation analysis (INAA), respectively. Quenched aqueous fluid from each experiment containing a quartz core was recovered and analyzed by atomic absorption spectrophotometry (AAS). The composition of aqueous fluids trapped as quartz-hosted inclusions, glass-hosted inclusions, and those recovered after quench yield consistent and precise data, at the 2σ uncertainty level, for the elements of interest. The overlapping Au, Fe, K, and Na concentrations in aqueous fluids trapped and analyzed via three entirely different instrumental techniques (i.e., LAICPMS, INAA, and AAS) suggest strongly that quartz microfractures heal on a slow enough time scale to permit entrapment of fully equilibrated aqueous fluids at our experimental PTX conditions. The data evince clearly that the chemical composition of fluids in quartz microfractures at the time of self-healing represents equilibrium conditions; hence, synthetic fluid inclusions in experiments with low thermal gradients across the charge provide a reasonable estimate of fluid composition at least at the experimental conditions examined in this study.

Mixing and differentiation in the Oruanui rhyolitic magma, Taupo, New Zealand: evidence from volatiles and trace elements in melt inclusions

Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at 26.5 ka from Taupo Volcano, New Zealand. The 530 km3 (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We measured the concentrations of H2O, CO2 and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in seven of eight pumice clasts studied. Variations in dissolved H2O and CO2 in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based on variations of dissolved H2O and CO2 in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would have driven convective mixing.

Thermodynamic modeling of post-entrapment crystallization in igneous phases

Inclusions of quenched silicate liquid in igneous phenocryst phases represent important windows into the pre-eruption chemistry of volcanic rocks. Melt inclusions are subject to a variety of potential modifications after entrapment, which obscure the connection between final inclusion composition, and entrapment conditions. We concentrate on the effects of post-entrapment crystallization (PEC) in the cooling inclusion. PEC is neither an isobaric nor an isochoric process. Pressure decreases between 2 and 27 bars per degree of cooling, depending on the chemistry of melt and host and on the degree of PEC. In the equilibrium case, between about 50% and 65% of this pressure effect is due to thermal expansivity of the liquid, 10–35% from thermal expansivity of the host, and 5–40% from mass transfer between the inclusion and host. This complicates the application of simple element-partitioning schemes for back-calculating the effects of post-entrapment crystallization except in the simplest cases. We present a thermodynamic algorithm for PEC correction. This method is based on the self-consistent thermodynamic model set used in the MELTS software package. The algorithm moves backward through the PEC process, incrementally adding equilibrium crystal composition to the liquid while accounting for consequent variations in pressure and oxygen fugacity. Entrapment conditions are assumed to have been reached when the instantaneous liquidus solid composition most closely matches that of the bulk host crystal. Besides giving information on the degree of PEC and initial inclusion composition, the proposed algorithm can provide constraints on the pressure, temperature and oxygen fugacity at the time of entrapment. Olivine- and orthopyroxene-hosted inclusions from Popocatépetl, Mexico help constrain pre-eruption conditions for mixed magmas from recent eruptive products. Feldspar-hosted inclusions from Satsuma-Iwojima, Japan suggest that these magmas were substantially undersaturated with respect to supercritical vapor phase at the time of entrapment and underwent on the order of 29% post-entrapment crystallization. Quartz-hosted inclusions can potentially be employed in more silicic compositions, but this will require refinement of existing thermodynamic models.

Fluid mixing as the mechanism of formation of the Dajing Cu-Sn-Ag-Pb-Zn ore deposit, Inner Mongolia

Dajing Cu-Sn-Ag-Pb-Zn ore deposit, in the Inner Mongolia Autonomous Region of China, is a fissure-filling hydrothermal ore deposit. The δ D values of quartz-hosted inclusion water are centered at −100‰ — −130‰. The δ 34 S values of sulfide ore minerals and δ 13 C values of carbonate gangue minerals vary from −0.3‰ to 2.6‰ and from −2.9‰ to −7.0‰, respectively. Integrated isotopic data point to two major contributions to the mineralizing fluid that include a dominant meteoric-derived groundwater, and sulfur and carbon species from hypogene magma. Linear trends are exhibited on the gaseous H 2 O versus CO 2 plot, and plots of CO, N 2 , CH 4 , and C 2 H 6 . It is shown by quantitative simulation that magma degassing cannot explain the linear trends. Hence, these linear trends are interpreted in terms of mixing of CO 2 -rich magmatic fluid with meteoric-derived groundwater. The groundwater circulated in Paleozoic sedimentary rocks and absorbed CO, N 2 , CH 4 , C 2 H 6 and radiogenic Ar from organic matter. Cooling effects resulting from mixing have caused the precipitation of ore minerals.

The evolution of aqueous–carbonic fluids in the Amba Dongar carbonatite, India: implications for fenitisation

Primary and pseudosecondary fluid inclusions hosted by apatite in calciocarbonatite at Amba Dongar, India, and primary fluid inclusions in quartz of the surrounding fenitised sandstones provide an important opportunity to reconstruct the evolution of fluids in a carbonatite setting and determine the composition of the fenitising fluids. Five types of fluid inclusions are present in apatite, namely aqueous liquid–vapour (LV), two-phase aqueous–carbonic (VL), three phase aqueous–carbonic (LLV), one or two solid-bearing aqueous (LVS), and multisolid-bearing aqueous (LVMS) fluid inclusions. Electron microprobe analyses of the solids in opened inclusions suggest that they comprise, calcite, strontianite, ankerite, arcanite (K 2SO 4), albite, K-feldspar and, possibly, pyrite, and hematite. Fluid inclusions in calcite of the calciocarbonatite are either secondary or of indeterminate origin and comprise LV, VL, and LLV types. Quartz-hosted inclusions are either secondary in detrital grains or primary, where they occur in overgrowths, and are mainly of the LV type; small proportions of VL and LLV inclusions are also present. Inclusions were rarely observed in K-feldspar (fenite), and where present are all secondary and similar to those in quartz. LV, LVS, and LVMS inclusions in apatite homogenise at temperatures from 142 to 342, 406 to 594 and 422 to 628 °C, respectively, while LV inclusions in calcite, quartz (primary), and K-feldspar homogenise at temperatures of 120 to 192, 122 to 262 and 183 to 228 °C, respectively; homogenisation temperatures of quartz-hosted inclusions increase with decreasing proximity to the carbonatite, i.e., with increasing intensity of fenitisation. Homogenisation temperatures were not determined for VL or LLV inclusions. All fluid inclusion types in apatite have similar ranges of salinity, from approximately 5 to 15wt.% NaCl eq. Those in calcite have a slightly different range of salinity, from 2 to 13 wt.% NaCl. eq., while inclusions in quartz and K-Feldspar have salinities of 7 wt.% NaCl eq. or lower. The carbonic phase of VL and LLV inclusions is CO 2-dominated, with the contents of other gases (CH 4 and/or N 2), being highest and lowest in apatite- and quartz-hosted inclusions, respectively. A carbonic phase is also present in some LVS inclusions, and contains the highest proportions of CH 4 and/or N 2. Analyses of decrepitate residues indicate that LVS inclusions have low Na/Na+K and high S/S+Cl ratios and contain minor concentrations of Fe. Residues of LVMS inclusions also have low Na/Na+K and high S/S+Cl ratios, albeit higher and lower, respectively, than those of LVS inclusions, but differ from LVS inclusions mainly in having high apparent concentrations of Si and Al, which together with Ca and Sr, likely represent contributions from daughter minerals or trapped solids. LV inclusion residues have relatively high Na/Na+K and low S/S+Cl ratios, particularly in quartz. The trend of Na/Na+K vs. S/S+Cl defined by fluid inclusion residues suggests that the fluids evolved in the sequence LVS⇒(LVMS)⇒LV (ap)⇒LV (qtz). This is supported by isochoric projections, which indicate that LVS fluids and accompanying carbonic fluids were the first to be trapped (at temperatures of >700 °C, the carbonatite solidus, and pressures>4 kilobars (kb)), LVMS fluids were trapped at similar temperatures, but pressures<2 kb, LV (ap) inclusions were trapped at lower temperature and LV (qtz) inclusions were trapped at temperatures of <250 °C and a pressure of about 100 bars. A model is proposed in which a CO 2–CH 4-bearing, S- and K-rich aqueous fluid, preserved as LVS fluid inclusions, exsolved from the calciocarbonatite at a depth of >10 km and a temperature≥700 °C. As this fluid rose to higher crustal levels, it exsolved its carbonic component, which was trapped as LLV and VL fluid inclusions, and mixed with oxidising meteoric or formational waters. The LVS fluid became more dilute and was trapped as LV fluid inclusions in apatite, while the carbonic fluid evolved by converting its CH 4 to CO 2. When the carbonatite magma reached a depth of 3–5 km, it exsolved its last aliquots of aqueous fluid, which were solute-rich, had intermediate K/Na and S/Cl ratios and probably contained significant concentrations of Ca, Al, and Si. These fluids, which were trapped as LVMS inclusions, invaded the surrounding sandstones and replaced quartz with K-feldspar producing potassic fenite. During the waning stages of hydrothermal activity, the system was dominated by a low salinity (NaCl) meteoric or formation water, which redeposited silica as overgrowths on corroded quartz relicts in the fenites.

Genesis of the Silvermines zinc-lead-barite deposit, Ireland; fluid inclusion and stable isotope evidence

The Silvermines zinc-lead-barite deposit occurs adjacent to a major normal fault system within a sequence of Mississippian (Tournaisian) sandstones, shales, limestones, and dolomites. It comprises large synsedimentary-syndiagenetic and smaller, underlying epigenetic orebodies.Fluid inclusions within the epigenetic minerals are dominated by a simple liquid-vapor type which in places contains one to five solid phases, the commonest of which is calcite. Single-phase liquid inclusions are commonest in barite; single-phase vapor and vapor-liquid inclusions are occasionally found in quartz, barite, and carbonates, although there is no indication of extensive boiling.Fluid inclusion salinities range from 8 to 28 equiv wt percent NaCl with modes at 14 and 19 wt percent. Leachate studies show the fluids to have high sodium concentrations with lesser, variable amounts of potassium and calcium and uniformly low magnesium concentrations (K/Na = 0.03-0.23; Ca/Na = 0.03-0.28; Mg/Ca = 0.1-0.67). Fluid inclusions within synsedimentary-syndiagenetic barite are highly saline, indicating the possible presence of a brine pool at the site of ore deposition.Homogenization temperatures range from 50 degrees to 260 degrees C, with most between 140 degrees and 220 degrees C. There is a mode at 190 degrees C and a subsidiary group at 140 degrees C. Required pressure corrections are less than 5 degrees C. If the pressure within the epigenetic ore zones was purely hydrostatic, as is reasonable, a minimum seawater depth of approximately 80 m is required to prevent boiling of some fluids.For the whole deposit there is a negative correlation between homogenization temperature and salinity for quartz-hosted inclusions, which is interpreted as the involvement of two (or possibly three) fluids in the mineralizing process. We suggest that high-temperature, 8 to 12 equiv wt percent NaCl fluids mixed with lower temperature, 18 to 22 equiv wt percent NaCl fluids, both within and below the ore zone during mineralization.Fluid inclusion waters have delta D values of -24 to -49 per mil for quartz, -23 to -29 per mil for dolomite, -41 to -55 per mil for barite, -46 per mil for sphalerite, and -58 per mil for galena. The delta 18 O values of the mineralizing fluids, calculated from mineral values, range from 1.1 to 7.7 per mil for quartz and 1.8 to 5.7 per mil for dolomite. These data best represent aqueous fluids that have equilibrated with the geosynclinal sequence and granites underlying the deposit. They also suggest that local Carboniferous surface and ground waters may have contained a deuterium-depleted (meteoric?) component.The fluid inclusion evidence presented refutes a model involving the expulsion of Carboniferous formation waters and indicates little, if any, involvement of magmatic waters. The geologic, geochemical, and fluid inclusion evidence best fits a model involving the convection of Carboniferous surface waters in the putative thick geosynclinal sequence and associated granitoids below the deposit, with a contribution from lower Paleozoic formation waters.Mineral precipitation resulted from fluid mixing in the ore zone with some contribution from carbonate wall-rock reactions and possibly boiling.