Individual Fluid Inclusions Research Articles

Determination of Cl and Br concentrations in individual fluid inclusions by combining microthermometry and LA-ICPMS analysis: Implications for the origin of salinity in crustal fluids

The origin of salinity in crustal fluids can be deduced from their Cl, Br and Na concentrations and associated Cl/Br and Na/Br ratios. Here, a protocol for quantitative analysis of Cl, Br and Na concentration in individual fluid inclusions of variable size, bulk salinity, Cl, Br and Na contents is presented. It combines microthermometry and laser ablation — inductively coupled plasma mass spectrometry (LA-ICPMS) analysis.The method is based on: (i) microthermometric measurement of ice melting, hydrate melting, halite dissolution and sylvite dissolution temperatures in fluid inclusions; (ii) calibration of LA-ICPMS signals of Br by analyzing Na–Br–Cl standard solutions loaded in silica glass capillaries; (iii) calculation of solute (Cl, Na, Ca, K, Mg) concentration based on LA-ICPMS intensities and the Pitzer thermodynamic model (for inclusions of salinity below 23wt.% equivalent NaCl), or a charge-balance approach (for higher salinity inclusions) and (iv) calculation of uncertainty on Cl, Br and Na concentration and Cl/Br and Na/Br ratios in the fluid.This method is applied to natural fluid inclusions with variable Cl, Br and Na contents as previously determined by microthermometry and bulk crush-leach coupled with ion chromatography analysis. The studied materials consist of: i) low-salinity inclusions from Alpine quartz veins and ii) high-salinity inclusions from the Trimouns talc deposit and from Athabasca Basin uranium deposits. Molar Cl/Br and Na/Br ratios range from 140±50 to 1200±400 and 17±6 to 880±290 in the analyzed inclusions. The calculated uncertainty on Cl/Br and Na/Br ratios in individual fluid inclusions is between 30 and 38%, regardless of the salinity. It appears that the analysis of individual inclusions coupled with uncertainty calculation allows the identification of significant variation of Cl/Br and Na/Br ratios within individual samples that could not be identified from previous bulk crush-leach and ion chromatography analyses. Because Br is a minor solute (molar Cl/Br is between ~100 and ~10000 in most crustal fluids), and because of its high first ionization energy, the determination of Br concentration in individual inclusion by combining microthermometry and LA-ICPMS analysis is most readily achievable for large (>50μm in diameter) inclusions. For smaller inclusions (10 to 50μm in diameter), the applicability of the present method is more sensitive to the Br concentration.

Improvement of the determination of element concentrations in quartz-hosted fluid inclusions by LA-ICP-MS and Pitzer thermodynamic modeling of ice melting temperature

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become an essential analytical tool for the study of paleofluid chemistry through the analysis of individual fluid inclusions. The calculation of major and trace element concentrations in fluid inclusions is usually based on empirical equations whose significance and accuracy are questionable. In addition, methods for estimation of analytical uncertainties element concentration in individual fluid inclusions are lacking. This study describes a method based upon Pitzer’s thermodynamic model for the calculation of major element (Na, K, Mg and Ca) concentrations in low-to moderate-salinity fluid inclusions. A signal processing protocol, used in combination with the new method is also developed to calculate the concentration, for each inclusion, and uncertainty for each major and trace element. In order to validate the proposed method, synthetic and natural fluid inclusions (from Alpine quartz veins) were ablated with a 193nm ArF excimer laser and analyzed with a quadrupole ICP-MS, equipped with an octopole collision–reaction cell. The difference between the calculated and actual element concentration (i.e. accuracy) does not exceed 20% and the calculated relative standard deviation (i.e. precision) for all element concentrations is ∼10% in standards (glasses, solutions in capillary tubes and synthetic fluid inclusions). The element concentrations obtained with this new method for the Alpine fluid inclusions are in good agreement with those previously measured using Laser Induced Breakdown Spectroscopy (LIBS) or crush-leach methods. Finally, the calculated concentrations and associated uncertainties determined for each element in individual fluid inclusions show that the sensitivity of LA-ICP-MS analysis is high enough to reflect small variations of major and trace element concentrations in the Alpine paleofluid, initially considered to have a constant chemistry. The new approach presented in this paper highlights small but significant variations in the paleofluid chemistry that were not previously detected with conventional LA-ICP-MS data processing.

Microanalysis of S, Cl, and Br in fluid inclusions by LA–ICP-MS

The accuracy and precision of micro-analysis of sulfur (S), chlorine (Cl) and bromine (Br) in quartz-hosted fluid inclusions by laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS) were tested. A scapolite mineral sample (Sca17) can be used as standard reference material (SRM) for the determination of Cl and Br, and NIST610 glass for S determination in fluid inclusions. We found that laser ablation of quartz and UV irradiation in the ablation cell produced elevated background signals of S and to a lesser amount of Cl and Br due to remobilization of these elements from the inside surfaces of the ablation chamber. Careful cleaning of the ablation chamber with nitric acid and by UV irradiation results in a 10 times lower S contamination signal and improves fluid inclusion analysis by reducing the detection limit for S by 50%. Micro-analysis of liquid and vapor inclusions synthesized in two different laboratories produce good accuracies of S, Cl, and Br. Analytical uncertainties based on numerous analyses of individual synthetic fluid inclusions in one assemblage are 17–44% (RSD) for the sulfur concentration, and 6–26% for Br/Cl ratios. Limits of detection (LOD) in 30 μm diameter liquid inclusions with densities of 0.99–1.02 g/cm 3 are in the range of 60 μg/g for S, 250 μg/g for Cl, and 15 μg/g for Br. LOD's in similar sized vapor inclusions with a density of 0.33 g/cm 3 are at least an order of magnitude poorer. Based on the investigated natural brine inclusion assemblages, the precisions of Br/Cl ratios (4–9% RSD) are adequate to determine the source of salinity in different ore-forming fluids.

Quantification of transient signals in multiple collector inductively coupled plasma mass spectrometry: accurate lead isotope ratio determination by laser ablation of individual fluid inclusions

This work establishes the analytical protocol for accurate Pb isotopic analysis of fast transient signals by multiple-collector ICP-MS instruments. Individual synthetic fluid inclusions of known Pb and Tl isotopic compositions (dissolved SRM 981 with or without SRM 997 from NIST, enclosed in quartz by a hydrothermal crack annealing technique) were liberated by 193 nm UV laser ablation (LA). Data were recorded on Faraday detectors, for which correction schemes for bias in amplifier response (“tau correction”) are presented and evaluated. tau-Corrected Pb isotope data reveal LA-induced isotope fractionation amounting to ∼0.5% amu−1 for Pb isotopes over the course of an entire fluid inclusion ablation. Instrumental mass bias correction was effected within-run using Tl provided by the fluid inclusion itself or admixed to the ablation aerosol via desolvated nebulization. Isotope ratios derived from the transient signals were either based on individual readings or on bulk signal integration, of which the latter produces significantly more accurate data. The external precision achieved by ablating SRM 610 glass with a 60 µm beam is ±0.011% (2 SD, relative) for 208Pb/206Pb and 207Pb/206Pb ratios and ±0.032% for Pb isotope ratios normalized to mass 204 (n = 18). Inclusion-to-inclusion reproducibilities (n = 11; ∼0.1 ng Pb per inclusion) are ±0.05% (2 SD; 208Pb/206Pb and 207Pb/206Pb) and ±0.13% (20xPb/204Pb), respectively; inclusions containing as little as 0.005 ng Pb returned ±0.1% and ±0.8%. These results are accurate as demonstrated by analysis of synthetic fluid inclusions containing SRM 981 Pb. The analytical protocol presented here for measuring isotope ratios on minute analyte quantities by multiple-collector ICP-MS in fast transient signal mode has great potential for applications to geochemical, archaeological, environmental and possibly biochemical problems.

The Genesis of Distal Zinc Skarns: Evidence from the Mochito Deposit, Honduras

The Mochito deposit, located in Honduras, is a distal Zn(-Pb-Ag) skarn in which economic mineralization (sphalerite and subordinate, argentiferous galena) is found in mantos and chimneys dominated by garnet and pyroxene that replaced limestone and a mixed limestone-siliciclastic unit. Except for a few variably altered, unmineralized diabase dikes, evidence of igneous activity is conspicuously absent. The nearest felsic igneous rocks, volcanic rock units, crop out ~13 km from the deposit. Early skarn and skarn proximal to faults consist dominantly of garnet that evolved from grandite, with a composition of ~Ad55 to andradite (≥Ad90), whereas later skarn, or skarn distal to faults, is mainly made up of pyroxene (Hd70). Magnetite and pyrrhotite locally form between the garnet and pyroxene skarns. Analyses of the whole-rock chemistry show that formation of grandite skarn involved large additions of all major elements (except Ca) and most trace elements, whereas formation of andradite skarn (from grandite skarn) involved losses of most of these elements; the notable exceptions are Ca and Fe, which were added. Mass changes in pyroxene skarn were not evaluated because of the heterogeneous nature of the precursor. Primary fluid inclusions in grandite and associated low-iron sphalerite are interpreted to have been trapped at ~370°C and a pressure of 500 bar. These inclusions have a mean salinity of 14 wt percent NaCl equiv. By contrast, primary inclusions in pyroxene and associated high-iron sphalerite were trapped at ~400°C and have a mean salinity of 5 wt percent NaCl equiv. Fluid inclusions could not be observed in andradite. Small proportions of CO2, CH4, and N2 were detected by gas chromatographic analyses of the fluids released by crushing small samples of the host mineral; CO2 was the most abundant of these gases. Based on LA-ICPMS analyses of individual fluid inclusions, Na and Ca were the principal metals in the fluids (median concentrations of 3.2 wt %), followed by K (0.9 wt %) and Mn (0.3 wt %). Median concentrations of ore metals Zn, Pb, and Ag were 6,000, 900, and 50 ppm, respectively. Analyses of phase equilibria and related thermodynamic calculations indicate that the log f O2 during the grandite and pyroxene skarn stages was >–30.3 and <–30.2, respectively. The pH during the grandite stage was ~5.0. In the absence of reliable data on bulk fluid chemistry, a pH for the pyroxene stage could not be estimated. Based on the calculated Si content of the fluid and the mass addition of Si during grandite skarn formation, the fluid/rock ratio was between 500:1 and 1,000:1. Evaluation of the solubility of sphalerite, galena, and argentite based on the physicochemical characteristics of the putative ore fluid indicate that the ore metals were transported dominantly as chloride complexes and deposited in response to an increase in pH. We propose a model in which relatively oxidizing hydrothermal fluids, exsolving from magma at a depth of >4 km, interacted with graphitic limestones in the Mochito graben during an episode of mid-Tertiary intraplate extension. These fluids rose through faults created by the extension and were cooled by the overlying sedimentary succession. Early skarn formed in an environment of high-fluid flux proximal to the faults and was dominated by grandite because of the oxidizing nature of the fluids. With continued heating of the rocks by subsequent pulses of fluid, temperature increased and the locus of interaction expanded into unaltered limestone distal to the faults, where lower fluid/rock ratios and the presence of graphite promoted buffering of the fluid to lower f O2 and formation of pyroxene skarn. Ore mineral deposition (dominantly sphalerite), which began during or after the formation of grandite skarn, reached its maximum after hydrothermal activity was focused in the lower fluid/rock ratio regime of pyroxene skarn formation, occurring in response to the sharp drop in pH that accompanied neutralization of the fluid by limestone.

The magma and metal source of giant porphyry-type ore deposits, based on lead isotope microanalysis of individual fluid inclusions

The global supply of Mo and much of Cu and Au comes from porphyry-type ore deposits associated with hydrous magmas of broadly calc-alkaline composition, thought to be generated by contemporaneous subduction zone processes. Molybdenum is generally considered to be derived from the continental crust while Cu and Au are sourced in the mantle wedge above subducting slabs. Here we show that neither contemporaneous subduction nor derivation of Mo from crustal sources is required to explain the genesis of porphyry–Cu–Mo–Au deposits on Proterozoic lithosphere in the eastern Rocky Mountains. Uniform Pb isotope ratios measured by LA-MC-ICP-MS in individual fluid inclusions from distinct Cu–Au and later Mo ore-forming stages at Bingham Canyon, USA, demonstrate a common metal source. Uranogenic Pb isotope ratios are particularly non-radiogenic (17.494 < 206Pb/ 204Pb < 17.534; 15.553 < 207Pb/ 204Pb < 15.588) and plot to the left of the geochron and above the mantle Pb evolution line. In 207Pb/ 206Pb vs. 208Pb/ 206Pb space, the fluid Pb isotope data cluster at the non-radiogenic end of a mixing line described by > 80 feldspar data from igneous rocks intimately associated with magmatic-hydrothermal ore formation, which extends to modern depleted mantle or upper crust. Forward Monte Carlo simulations require three events for the U–Th–Pb isotope evolution of the fluid: (1) Late Archean formation of enriched crust is followed by (2) preferential extraction of Pb from this aged crust into a subduction fluid characterized by drastically reduced U/Pb that metasomatized lithospheric mantle at ∼ 1.8 Ga. This mantle reservoir then evolved to produce the retarded uranogenic Pb isotope signatures of the Bingham Canyon Cu–Mo–Au deposit in the Cenozoic (3). Similarly retarded uranogenic Pb isotope data characterize the giant porphyry–Mo and Climax-type Mo deposits of Henderson, Questa, Butte, and SE Arizona that occur in Proterozoic sutures of the central and eastern Rocky Mountains. We propose that Cenozoic melting of subcontinental lithospheric mantle metasomatized by subduction fluids during early Proterozoic amalgamation of terranes to the Wyoming Craton provides the metal endowment and subduction flavour to the giant magmatic-hydrothermal Cu–Mo–Au ore deposits in western North America, which together constitute the world's major molybdenum ore province.

Brine-rock interaction in the Athabasca basement (McArthur River U deposit, Canada): consequences for fluid chemistry and uranium uptake

Terra Nova, 22, 303–308, 2010 Abstract The nature of uranium source rocks, transport conditions and deposition processes are still highly controversial for world-class unconformity-related U deposits. This article presents the first detailed chemistry of brines associated with the giant McArthur River U deposit, Canada. LA-ICP-MS analysis of individual fluid inclusions suggests mixing between a Na–Ca–Mg–K–Sr–Ba brine and a Ca–Mg–Na–K–Sr–Ba brine. The brines share a common origin (evaporated seawater) and show evidence for contrasting interaction with basement rocks. The Na-rich brine lost Mg and K in alteration haloes around U ores, while the Ca-rich brine results from Na–Ca exchange and Sr–Ba gain. U concentrations (0.3–530 μg g−1) are anomalously high compared with usual basinal fluids, this indicating that U uptake occurred within basement rocks. The two brine end-members have mixed within the main U deposit area, which could be one of the major driving forces for U deposition.

Rapid Na, Cu exchange between synthetic fluid inclusions and external aqueous solutions: evidence from LA‐ICP‐MS analysis

AbstractThree sets of equilibration experiments (Set 1 to Set 3) were performed in cold‐seal pressure vessels to investigate the compositional modification of quartz‐hosted fluid inclusions after entrapment. Each set of experiment consisted of two stages. In a pre‐run, fluid inclusions containing 5–10 wt% NaCl and selected trace elements were synthesized at 700°C/140–200 MPa. These samples were then loaded into new capsules together with Cu‐bearing solutions and some mineral buffers, and re‐equilibrated at 600–800°C and 70–140 MPa for 6–8 days. LA‐ICP‐MS analysis of individual fluid inclusions reveals that in re‐equilibration experiments in which the outer fluid was composed of KCl (±FeCl2) up to 83% of the original Na content of pre‐existing fluid inclusions were lost, and up to 5660 ppm Cu were gained. Other elements with larger ionic radii (i.e. K, Fe, Ba, Sr) were not exchanged, demonstrating that the inclusions remained physically intact and that Na and Cu were transported through quartz by diffusion. The observed Na loss from pre‐existing fluid inclusions correlates positively with Cu gain, with about 1 Cu atom being gained per 10 Na atoms lost. Thus, Na and Cu (plus probably H) were exchanged by interdiffusion. Remarkably, this processes resulted in up to 10 times higher Cu concentrations in re‐equilibrated inclusions than were present in the outer fluid, i.e. Cu diffused ‘uphill’. Large variations of Cu concentrations relative to the concentration of other elements are common also in natural fluid inclusion assemblages. However, no evidence for a correlation between Cu content and Na content was found so far, suggesting that Cu diffusion in natural samples may be dominated by processes other than Na–Cu interdiffusion.

A review and comparison of fracture mineral investigations and their application to radioactive waste disposal

Abstract A compilation and comparison of fracture mineral studies from the Canadian and Fennoscandian Shields and the French Massif Central shows many similarities indicating larger external control over fracture mineral deposition, with different rock types exerting local controls. The sites investigated represent a wide range of geological settings, and host rock types ranging from felsic intrusive and extrusives to ultramafic intrusives and volcanics that span an age range from 2.5 to 0.36 Ga. Typical fracture minerals found at Canadian Shield sites include calcite, quartz, chlorite and clays, and these do not appear to be dependant on age, erosional depth or geological environment. The Fennoscandian Shield has a much larger variety of fracture filling minerals with epidote, zeolites, prehnite, fluorite, pyrrhotite, Fe oxides, serpentine, graphite, magnesite and barite in addition to the minerals typically found at Canadian Shield sites. The major control on fracture mineral type is most likely variations in rock type, and fluid chemistry and temperature. The C and O isotopic range of calcite is very similar among sites. Late-stage hydrothermal calcite, with strongly depleted δ18O values, is common at many sites. All of the sites have calcite with δ18O isotopic values in the range of −5 to −20‰ PDB, indicative of formation from meteoric water or basinal brines that have undergone varying degrees of water/rock interaction. One Canadian and a few Swedish sites have calcite in the shallower portion of the rock that shows isotopic evidence of dissolution and re-precipitation in equilibrium with the present-day waters. There are some striking similarities in fluid inclusion data among sites. Most sites have an elevated temperature (100–300 °C), low salinity group of fluid inclusions within the NaCl–H2O system, and a lower temperature (50–150 °C), higher salinity group of fluid inclusions within the NaCl–CaCl2–H2O system. Fluid inclusion density plots show some evidence of simple cooling, but most sites show two or more fluids were responsible for calcite formation. The origin of most of these fluids was magmatic/hydrothermal or meteoric water that had undergone varying degrees of water/rock interaction, but basinal brines and seawater were also possible sources. Several techniques and methods have been used to further characterize calcites. Strontium isotopes and rare earth elements can be useful to recognize different families of calcite. Uranium–Th dating has found many old calcites beyond the useful range of the technique, but also some relatively young calcites that may be related to interglacial periods. Where fluid inclusion data exists, formation temperatures were not consistent with a glacial water origin. Crush and leach experiments (with ion and gas chromatography and thermal ion mass spectrometry) have characterized inclusion fluids, but special care must be used to ensure only one generation is sampled at a time. Cathodoluminescence and scanning electron microscopy with energy-dispersive spectrometry has been useful in identifying multiple fluid generations within single calcite samples. Laser ablation and Raman spectrometry are additional techniques that are useful in determining individual fluid inclusion chemistry and isotopes.

Relationship between CO2-dominated fluids, hydrothermal alterations and gold mineralization in the Red Lake greenstone belt, Canada

Abstract The Red Lake greenstone belt is one of the foremost Au mining camps in Canada and hosts the world-class Campbell-Red Lake Au deposit. Belt-scale hydrothermal alteration is characterized by proximal ferroan dolomite zones associated with Au mineralization surrounded by distal calcite zones, both being accompanied by potassic alterations (sericite, muscovite, and biotite). At the Campbell-Red Lake and Cochenour deposits Au mineralization (in particular high-grade ore) is associated with silica and sulfides (especially arsenopyrite) that replace carbonate ± quartz veins and the host rocks. The prevalence of carbonic fluid inclusions and rare occurrence of aqueous-bearing inclusions in carbonate–quartz–Au veins in the Campbell-Red Lake deposit, and the consistency of homogenization temperatures of carbonic inclusions within individual fluid inclusion assemblages (FIA), have been interpreted to indicate that H2O-poor, CO2-dominated fluids were responsible for the carbonate veining and Au mineralization. Further studies of fluid inclusions in carbonate–quartz veins within and outside the deformation zone hosting the Campbell-Red Lake deposit (the Red Lake Mine trend) including the Cochenour Au deposit, the Redcon Au prospect, and outcrops in the distal calcite zone also reveal the dominance of carbonic fluid inclusions in vein minerals. These studies indicate that CO2-dominated fluids were flowing through fractures during carbonate vein formation and Au mineralization both within and outside major structures. The carbonic fluid may have been initially undersaturated with water, or it may have resulted from phase separation of an H2O–CO2–NaCl fluid. In the latter case, phase separation modeling indicates that the initial fluid likely had X CO 2 values larger than 0.8. Calculations based on hydrothermal mineral assemblages indicate X CO 2 values in the host rocks from 0.025 to 0.85, reflecting a change from CO2-dominated fluids in the fractures (veins) to H2O-dominated fluids in the host rocks away from the fractures. The CO2-dominated fluids were likely advected from granulite facies in the deeper crust, whereas the H2O-dominated fluids were derived from the ambient host rocks of amphibole to greenschist facies. Calculations based on CO2 requirements and source constraints indicate that the mineralizing fluids were likely two orders of magnitude more enriched in Au than the commonly assumed values of a few μg/L, which may explain why the Campbell-Red Lake deposit has a very high-grade of Au (average 21 g/t for the whole deposit and 81 g/t for the Goldcorp High-Grade zone). Fluid inclusion data suggest that the carbonate veining and Au mineralization likely took place at depths from 7 to 14 km. The development of crustiform–colloform structures in the carbonate ± quartz veins, which was previously interpreted to indicate relatively shallow environments, may alternatively have been related to extremely high fluid pressures and the CO2-dominated nature of the fluids, which could have enhanced the brittle properties of the rocks due to their high wetting angles.

Geochemistry and Evolution of Mississippi Valley-Type Mineralizing Brines from the Tri-State and Northern Arkansas Districts Determined by LA-ICP-MS Microanalysis of Fluid Inclusions

The Tri-State and Northern Arkansas districts of the Ozark plateau, North America, are both classic examples of Mississippi Valley-type (MVT) mineralization, formed by continent-scale basinal brine migration as a result of the uplift of the Arkoma foreland basin in response to the Early Permian Ouachita orogeny. The chemistry of the fluids responsible for both sulfide mineralization and gangue precipitation in these districts was studied by quantitative microanalysis of individual fluid inclusions in quartz and sphalerite using 213-nm laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Using halogen systematics, an evaporative seawater origin for the brines was determined, but higher Br concentrations suggest that the sphalerite-hosted “ore fluids” underwent a significantly greater degree of evaporation in the initial stages of fluid evolution compared to brines hosted by gangue phases. Metal contents of the brines responsible for quartz and dolomite precipitation are low compared to modern basinal brines, but many of the fluid inclusions trapped in sphalerite in both districts contained anomalously high metal concentrations, suggesting that mineralization involved incursion of a metal-rich fluid of distinct geochemistry. Examination of the multicomponent chemical characteristics revealed that dolomitization was probably an important process in the early chemical evolution of fluids that infiltrated both districts. In the Tri-State district, precipitation of sulfides was most likely driven by mixing of the metalliferous fluid with another brine, possibly rich in reduced sulfur. In northern Arkansas the compositional variations observed are best explained by local dissolution of the carbonate host rock. This may have been the process that ultimately drove sulfide deposition through fluid neutralization and reduction. Alternatively, the digestion of the host rock may have been the result of locally generated acidity produced by the deposition of sulfides. The discovery of anomalously metal-rich fluids linked to mineralization suggests that these deposits are not simply the product of typical basin evolution, helping to explain the abundance of MVT mineralization in some forelands, whereas others are barren. It is likely that a significant portion of the history of the hydrothermal flow system was characterized by the precipitation of barren gangue assemblages from metal-poor brines, with metalliferous fluids only being expelled from a specific stratigraphic package at a distinct stage of basin evolution.

Modification of gas speciation in quartz-hosted fluid inclusions by stray laser radiation during LA-ICPMS analysis

Laser-ablation inductively coupled-plasma mass spectrometry (LA-ICPMS) is an established method of analyzing the elemental composition of individual fluid inclusions in minerals. While the method affords good spatial control of ablation pits, we have found that stray light from the UV (193 nm) laser may permanently modify the gas species in adjacent fluid inclusions. We have used Raman spectroscopy to analyze natural fluid inclusions in quartz before and after ablating test pits. New molecular species (CO, H 2 , and O 2 ) appeared in the vapor phases of inclusions near the pits, resulting in complex, disequilibrium gas mixtures (e.g., CO 2 -CH 4 -H 2 O-CO-H 2 -O 2 ) and formation of iron oxyhydroxides in brine inclusions. Ablation for 40 s with a laser fluence of ~24 J/cm 2 on the sample produced the new gas species in inclusions at various distances up to 210 μm from the pits, depending on pit diameter. We attribute the changes in gas speciation to molecular photodissociation induced by stray UV-laser light. We have no evidence that this artifact changes the bulk elemental composition or density of the affected inclusions. The problem for fluid inclusion analysis is that phase-transition temperatures (e.g., clathrate hydrate dissociation, partial homogenization of the carbonic phases, and total homogenization) are highly sensitive to the mixture of gas species present. This means that parameters such as gas compositions, bulk salinity, and minimum entrapment temperatures are likely to be misinterpreted if inclusions are analyzed by microthermometry or Raman spectroscopy in samples that have already undergone laser ablation.

An experimental study of the solubility of molybdenum in H 2O and KCl–H 2O solutions from 500 °C to 800 °C, and 150 to 300 MPa

The solubility of molybdenum (Mo) was determined at temperatures from 500 °C to 800 °C and 150 to 300 MPa in KCl–H 2O and pure H 2O solutions in cold-seal experiments. The solutions were trapped as synthetic fluid inclusions in quartz at experimental conditions, and analyzed by laser ablation inductively coupled plasma mass spectrometry (LA ICPMS). Mo solubilities of 1.6 wt% in the case of KCl-bearing aqueous solutions and up to 0.8 wt% in pure H 2O were found. Mo solubility is temperature dependent, but not pressure dependent over the investigated range, and correlates positively with salinity (KCl concentration). Molar ratios of ∼1 for Mo/Cl and Mo/K are derived based on our data. In combination with results of synchrotron X-ray absorption spectroscopy of individual fluid inclusions, it is suggested that Mo-oxo-chloride complexes are present at high salinity (>20 wt% KCl) and ion pairs at moderate to low salinity (<11 wt% KCl) in KCl–H 2O aqueous solutions. Similarly, in the pure H 2O experiments molybdic acid is the dominant species in aqueous solution. The results of these hydrothermal Mo experiments fit with earlier studies conducted at lower temperatures and indicate that high Mo concentrations can be transported in aqueous solutions. Therefore, the Mo concentration in aqueous fluids seems not to be the limiting factor for ore formation, whereas precipitation processes and the availability of sulfur appear to be the main controlling factors in the formation of molybdenite (MoS 2).

Experimental study of the behavior of copper and zinc in a boiling hydrothermal system

Vapor-liquid partitioning coefficients for Cu and Zn in aqueous fluid were determined under boiling conditions at 500–650 °C and 35–100 MPa in sulfur-bearing and sulfur-free systems. A synthetic fluid inclusion technique was used to sample the experimental system of coexisting vapor-rich and liquid-rich fluid inclusions; the Cu and Zn concentrations in individual fluid inclusions were then analyzed by synchrotron radiation X-ray fluorescence. The vapor-liquid distribution constant of Cu [K D m Cu(vapor) / m Cu(liquid) ], where = m denotes the molality of metal, is found to be strongly dependent on the sulfur content in the experimental solutions. In sulfur-bearing systems, Cu preferentially partitions into the vapor phase, whereas Zn preferentially fractionates into the hypersaline liquid. The K D values for Cu and Zn obtained in this study correspond well with those obtained from natural fluid inclusions in hydrothermal ore deposits. The results suggest that differential volatility of metals is an important factor affecting the enrichment of certain metals in different hydrothermal ore deposits.

Determination of sulfur in fluid inclusions by laser ablation ICP-MS

The quantification capabilities for sulfur microanalysis in quartz-hosted fluid inclusions were investigated with laser ablation (LA) inductively coupled plasma quadrupole mass spectrometry (ICP-Q-MS) and ICP sector field mass spectrometry (ICP-SF-MS) allowing resolution of sulfur from polyatomic interferences. A scapolite mineral sample was used to determine the sulfur concentration in NIST SRM 610 (570 ± 70 µg g−1), which was further validated using EPMA and then used as standard reference material for the fluid inclusion analysis. The sulfur concentration in an assemblage of brine inclusions from a quartz–molybdenum vein was determined to be 5900 ± 2000 µg g−1 measuring 17 inclusions with the ICP-SF-MS and 13 inclusions with the ICP-Q-MS instrument. The agreement between the two ICP-MS instruments for sulfur was ∼5% and well within the overall precision of 35% relative standard deviation. The precision and accuracy was not limited by interferences, but by a so far unknown sulfur contamination source when ablating the host mineral quartz. Due to this contamination, a careful baseline correction is necessary which is described and discussed in detail. Nevertheless, the method developed to determine sulfur maintains the multi-element capabilities for individual fluid inclusions. Limits of detection for sulfur are correlated with the inclusion mass and were found to be ∼ 30–100 µg g−1 for 60 μm inclusions.

Natural gas leakage of Mizhi gas reservoir in Ordos Basin, recorded by natural gas fluid inclusion

Abundant natural gas inclusions were found in calcite veins filled in fractures of Central Fault Belt across the centre of Ordos Basin. Time of the calcite veins and characteristics of natural gas fluid inclusion were investigated by means of dating of thermolum luminescence (TL) and analyzing stable isotope of fluid inclusion. Results show that natural gas inclusion formed at 130–140°C with salinity of 5.5 wt%–6.0 wt% NaCl. It indicates that natural gas inclusion is a kind of thermal hydrocarbon fluid formed within the basin. Method of opening inclusion by heating was used to analyze composition of fluid inclusion online, of which the maximal hydrocarbon gas content of fluid inclusion contained in veins is 2.4219 m3/t rock and the maximal C1/ΣC i ratio is 91%. Laser Raman spectroscopy (LRS) was used to analyze chemistry of individual fluid inclusion in which the maximal hydrocarbon gas content is 91.6% compared with little inorganic composition. Isotope analysis results of calcite veins show that they were deposited in fresh water, in which the δ 13CPDB of calcite veins is from −5.75‰ to 15.23‰ and δ 18OSMOW of calcite veins is from 21.33‰ to 21.67‰. Isotope results show that δ 13C1 PDB of natural gas fluid inclusion is from −21.36‰ to −29.06‰ and δDSMOW of that is from −70.89‰ to −111.03‰. It indicates that the gas of fluid inclusion formed from coal source rocks and it is the same as that of natural gas of Mizhi gas reservoir. Results of TL dating show that time of calcite vein is (32.4±3.42)×104 a, which is thought to be formation time of gas inclusion. It indicated that natural gas inclusion contained in calcite veins recorded natural gas leakage from Mizhi gas reservoir through the Central Fault Belt due to Himalayan tectonic movement.

SRXRF experiments and analytical methods of mineral individual fluid inclusions

This paper focuses on the micro-beam and trace element non-destructive experiment and analytical method of mineral fluid inclusions by synchrotron radiation X-ray fluorescence (SRXRF) microprobe at Beijing Synchrotron Radiation Facility (BSRF). The experimental instrument, measurement process and calculating method are introduced. A set of oil-and gas-containing typical mineral fluid inclusions taken from the Tazhong and Lunnan oilfields in the Tarim Basin were analyzed non-destructively. The trace element contents in the fluid inclusions may provide guidance for oil and gas exploration and development.

Paleo-fluid composition determined from individual fluid inclusions by Raman and LIBS: Application to mid-proterozoic evaporitic Na–Ca brines (Alligator Rivers Uranium Field, northern territories Australia)

Detailed determination of the composition of individual fluid inclusions in the H 2O–NaCl–Ca(Mg)–Cl 2 system from microthermometry data may be difficult because (i) the melting of salt hydrates is often unclear, (ii) salts or salt hydrates are often metastable, and iii) eutectic temperatures are often difficult to relate precisely to a unique salt–water system. These problems can be circumvented by using a Raman microprobe to identify the salt hydrates and to measure the chlorinity, by using Laser Induced Breakdown Spectroscopy (LIBS) to determine cation ratios, and by cross-checking the data from these independent techniques at each step in the determination of the fluid inclusion composition. Data from individual inclusion analysis are then completed by crush–leach technique data on bulk samples. This methodology was applied to a case study, the mineralized faults and associated breccia located in the Alligator Rivers Uranium Field (middle Proterozoic Kombolgie Subgroup, Northern Territory, Australia). Faults are cemented by euhedral quartz containing a variety of fluid inclusions with a wide range of ice-melting temperatures, from near 0 °C down to about − 50 °C. The approach has permitted the identification of three types of fluids: an Na-rich brine with variable Mg concentrations, a Ca-rich brine with low Mg concentrations, and a lower salinity fluid. The Ca and Na-brines mixed with each other and a less saline end-member. On the basis of Cl/Br and cation ratios, the Na-brine is interpreted as a primary brine, resulting from the evaporation of seawater. It is inferred that the Na-brine evolved chemically during its interaction with the Ca-rich lithologies from Proterozoic basement, forming subsequently a Ca-brine through Na–Ca exchange reactions. Thus, the fault system, which affects both the base of the Kombolgie sandstone and its basement, represents active drainage zones where different fluid reservoirs were connected, and thus a place where fluid mixing was highly favored.

A XANES study of Cu speciation in high-temperature brines using synthetic fluid inclusions

Cu K-edge X-ray absorption near edge structure (XANES) spectra were recorded from individual synthetic brine fluid inclusions as a function of temperature up to 500 °C. The inclusions serve as sample cells for high-temperature spectroscopic studies of aqueous Cu-Cl speciation. Cu + and Cu 2+ can both be identified from characteristic pre-edge features. Mixed oxidation states can be deconvoluted using linear combinations of Cu + and Cu 2+ spectra. This work illustrates how complex Cu XANES spectra can be interpreted successfully. Cu 2+ is the stable oxidation state in solution at room temperature and Cu + at high temperatures. The change in oxidation state with temperature was completely reversible. Cu + was found to occur exclusively as the linear species [CuCl 2 ] - in solutions containing KCl with Cu:Cl ratios up to 1:6. In the absence of K + , there is evidence for higher order coordination of Cu + , in particular the tetrahedral complex [CuCl 4 ] 3- . The importance of such complexes in natural ore-forming fluids is yet to be determined, but may explain the vapor-phase partitioning of Cu as a Cl complex from a Cl-rich brine.

Fluid mineralization of the Dajing Sn-polymetal deposit: Evidence from LA-ICP-MS analysis of individual fluid inclusions

LA-ICP-MS analyses on single fluid inclusions trapped in fluorite and quartz from the Sn-Cu orebody of the Dajing deposit and from the Sn orebody of the Huanggangliang deposit were conducted to trace the source and evolution of Cu-rich and Sn-rich fluids in the Dajing deposit using the Rb/Sr and Sn/Cu ratios of the fluids. The results show that the Cu-rich fluids in the Dajing deposit, relatively rich in Na and probably derived from deep-seated basic magma, contain Cu several to several tens of times more than Sn, and Sr several to several tens of times more than Rb, while the Sn-rich fluids, relatively rich in K and probably derived from high level granitic magma, contain Sn several to several tens of times more than Cu, and Rb several to several tens of times more than Sr. It is suggested that the Sn-rich and the Cu-rich fluids were mixed at the stage of moderate to low temperature and low salinity, resulting in the formation of Cu-Sn orebodies at the central part of the Dajing deposit.