Fluid Inclusion Populations Research Articles

PIT (Petroleum Inclusion Thermodynamic): a new modeling tool for the characterization of hydrocarbon fluid inclusions from volumetric and microthermometric measurements

A recent technique, the confocal scanning laser microscopy, allows for the accurate measuring of the bubble filling degree (Fv) of oil fluid inclusions as a function of the temperature [Pironon, J., Canals, M., Dubessy, J., Walgenwitz, F., Laplace-Builhe, C., 1998. Volumetric reconstruction of individual fluid inclusions by confocal scanning laser microscopy. Eur. J. Mineral. 10, 1143–1150]. These data, combined with measurements of homogenization temperature (Th), give us new constraints for characterizing individually hydrocarbon fluid inclusions. For this purpose, a new modeling tool, PIT (Petroleum Inclusion Thermodynamic), based on applied thermodynamics of natural oils, has been constructed for interpreting volumetric (Fv) and microthermometric (Th) measurements obtained on hydrocarbon fluid inclusions. The software allows us: (1) to reconstruct the paleo-thermobarometric trapping conditions of oils; (2) to model petroleum inclusion composition; and (3) to understand (Fv–Th) differences among a population of fluid inclusions in terms of various pre- or post-trapping processes (inclusion stretching, oil mixing, liquid/vapor de-mixing, oil leaching by a gas, etc.).

Precise geothermometry on fluid inclusion populations that trapped mixtures of immiscible fluids

This study was performed to evaluate a quantitative method based on ^99mTc-DMSA renal planar scintigraphy performed during acute pyelonephritis (APN) to detect kidneys at risk of scarring. <b>Methods:</b> A total of 43 children (5.8 ± 3.6 y old [mean ± SD]) were examined by ^99mTc-DMSA scintigraphy during (DMSA 1) and 8 ± 2 mo after (DMSA 2) APN. Two levels of interpretation were performed independently: first, a semiquantitative analysis to classify the kidneys by considering the evolution between DMSA 1 and DMSA 2 (i.e., to determine which kidneys had developed scarring), and second, an automatic quantitative analysis of DMSA 1 to define and to evaluate a predictive index for kidney evolution from DMSA 1 to DMAS 2. The method consisted of determining an automatic threshold for the kidney and then calculating ratios of the count density in a given isocount <i>n</i>% (region of interest containing all the pixels with a value ≥ <i>n</i>% of the value of the pixel with the maximal activity value) to the count density in a 20% isocount (C_<i>n</i>%) and the number of pixels in a given isocount to the number of pixels in a 20% isocount (S_<i>n</i>%). <b>Results:</b> All kidneys normal at DMSA 1 remained normal at DMSA 2. For the automatic index, the C_70% ratio was considered the best index for the prediction of scarring. When this C_70% ratio was used, a cutoff value of 0.45 was able to predict scarring with a sensitivity of 0.83, a specificity of 0.78, a positive predictive value of 0.85, and a negative predictive value of 0.77. <b>Conclusion:</b> A cutoff value of 0.45 for the C_70% ratio calculated for ^99mTc-DMSA scintigraphy performed during APN may be useful for detecting kidneys at risk of scarring.

Ore-fluid evolution at the Getchell Carlin-type gold deposit, Nevada, USA

Minerals and fluid-inclusion populations were examined using petrography, microthermometry, quadrupole mass-spectrometer gas analyses and stable-isotope studies to characterize fluids responsible for gold mineralization at the Getchell Carlin-type gold deposit. The gold-ore assemblage at Getchell is superimposed on quartz-pyrite vein mineralization associated with a Late-Cretaceous granodiorite stock that intruded Lower-Paleozoic sedimentary rocks. The ore assemblage, of mid-Tertiary age, consists of disseminated arsenian pyrite that contains submicrometer gold, jasperoid quartz, and later fluorite and orpiment that fill fractures and vugs. Late ore-stage realgar and calcite enclose ore-stage minerals. Pre-ore quartz trapped fluids with a wide range of salinities (1 to 21 wt.% NaCl equivalent), gas compositions (H2O, CO2, and CH4), and temperatures (120 to > 360°C). Oxygen- and hydrogen-isotope ratios indicate that pre-ore fluids likely had a magmatic source, and were associated with intrusion of the granodiorite stock and related dikes. Ore-stage jasperoid contains moderate salinity, aqueous fluid inclusions trapped at 180 to 220°C. Ore fluids contain minor CO2 and trace H2S that allowed the fluid to react with limestone host rocks and transport gold, respectively. Aqueous inclusions in fluorite indicate that fluid temperatures declined to ∼ 175°C by the end of ore-stage mineralization. As the hydrothermal system collapsed, fluid temperatures declined to 155 to 115°C and realgar and calcite precipitated. Inclusion fluids in ore-stage minerals have high δDH2O and δ18OH2O values that indicate that the fluid had a deep source, and had a metamorphic or magmatic origin, or both. Late ore-stage fluids extend to lower δH2O values, and have a wider range of δ18OH2O values suggesting dilution by variably exchanged meteoric waters. Results show that deeply sourced ore fluids rose along the Getchell fault system, where they dissolved carbonate wall rocks and deposited gold-enriched pyrite and jasperoid quartz. Gold and pyrite precipitated together as H2S in the ore fluids reacted with iron in the host rocks. As ore fluids mixed with local aquifer fluids, ore fluids became cooler and more dilute. Cooling caused precipitation of ore-stage fluorite and orpiment, and late ore-stage realgar. Phase separation and/or neutralization of the ore fluid during the waning stages of the hydrothermal ore system led to deposition of late ore-stage calcite.

Granite-hosted mineral deposits of the New Ross area, South Mountain Batholith, Nova Scotia, Canada: P, T and X constraints of fluids using fluid inclusion thermometry and decrepitate analysis

The 370 Ma peraluminous South Mountain Batholith (SMB) intrudes Meguma Supergroup metasedimentary rocks in Nova Scotia. The New Ross area of the SMB contains polymetallic mineralisation (Sn, W, U, Mo, Cu and Mn) in pegmatite, greisen and vein directly or indirectly associated with highly evolved fractions of the SMB. Eight mineral deposits from this area have several fluid inclusion types hosted by quartz: (1) monophase liquid (L); (2) monophase vapour (V); (3) aqueous, L-V (4) aqueous, L-rich + solids; (5) aqueous, L-rich + halite. Inclusions have irregular to equant shapes and are pseudo-secondary or secondary. The irregularity and variability of L:V ratios within fluid inclusion populations suggest post-entrapment modifications of inclusions (i.e. necking).Thermometric data indicate three distinct fluids in terms of salinity: (1) 19-25 wt. % equiv. NaCl (rarely 14-25 wt. % NaCl equiv.), (2) 29-43 wt. % equiv. NaCl, and (3) 0-9 wt. % equiv. NaCl. Temperatures of first melting and ice/hydrohalife melting indicate CaCl2 in solution. Proximity of the deposits to Meguma Supergroup metasedimentary rocks suggests that this Ca component may be externally derived. The majority of the low-salinity fluid population has the composition of meteoric water. Electron microprobe analyses of artificially decrepitated mounds identify Na, Ca and K as major solutes, with a continuum in terms of compositions. Other solute components in the mounds are Fe and Ba, and a variety of metals of unknown speciation also occur (Cu, Zn, Fe, Ni). Homogenisation temperatures (Th) range from c. 80°C to 370°C, but for inclusion assemblages the range is 10°C to 20°C. Given the 3 kbar depth of emplacement of the SMB, estimated entrapment temperatures are c. 200°C to 550°C. The fluid inclusion data appear to reflect: (1) trapping of mixed Na-K-Ca brines during isobaric cooling in pegmatite and greisen deposits as indicated by large ranges in Th; (2) formation of deposits at different ambient pressures (i.e. depth); and (3) mixing of fluids of different reservoirs (i.e. magmatic, metamorphic, meteoric).

Composition of natural, volatile-rich Na–Ca–REE−Sr carbonatitic fluids trapped in fluid inclusions

A carbonatite-derived fluid trapped under closed-system conditions is preserved as fluid inclusions in quartzitic country rocks of the Kalkfeld carbonatite complex, Namibia. The fluid inclusion population provides a unique opportunity of investigating in situ the composition of a natural, volatile-rich carbonatitic fluid with all components trapped and preserved, and its relationship to the volatile-deficient, parental Kalkfeld carbonatite. Individual fluid inclusions display all intermediate compositions between three end-members: (1) low-density CO 2; (2) a Na,K,Ca,Cl −, HCO 3 −-bearing aqueous brine; and (3) an assemblage of solid phases comprising nahcolite, halite, burbankite, sylvite, fluorocarbonate (rouvilleite ?), cryolite, Mn–Fe–calcite, feldspar/feldspathoid, fluorite, base metal sulfides, and phosphate. Cathodoluminescence imaging of the host quartzites features a network of fluid migration pathways. In situ exsolution of invasive fluids and necking-down processes of fluid inclusions, triggered by rapid cooling of interconnected fluid ponds, caused heterogeneous entrapment of the various fluid components. Consequently, synchrotron–XRF analyses of individual inclusions show a broad range of compositions with Th/U = 1 to 45 and Y/Ho = 1 to 28, but consistent rare-earth element (REE) cn patterns. Analysis of the bulk carbonatitic fluid yielded the following element ratios: Na/K = 1.5 to 2.7, Na/Ca = 1.5, Na/ΣREE = 4 to 17, Fe/Mn = 5.2 to 7.2, Th/U = 3 to 13, and Y/Ho = 25 to 65. An estimation of the volatile components in the fluid population at CO 2 = 20 wt% and H 2O = 20 wt% permits a quantitative assessment of the composition of the Na−Ca−REE−Sr alkali-carbonatitic fluid with 40 wt% H 2O and CO 2, 28 to 29 wt% Na 2O + K 2O, 13 to 16 wt% CaO, 3.0 to 4.1 wt% FeO tot, up to 3 wt% ΣREE, up to 3 wt% Sr, 1 to 1.7 wt% MgO, 1 wt% TiO 2, 0.6 wt% MnO, and Th and Ba reaching 1600 and 8000 ppm, respectively. The REE patterns of individual fluid inclusions and of the bulk fluid extend over two orders of magnitude from La to Lu, and in this respect are similar to those of the parental Kalkfeld carbonatite, but are distinguished by a negative (Eu/Eu∗) cn anomaly of 0.5 to 0.6. The data suggest that this fluid is a direct sample of those expelled during a late stage of carbonatite fractionation. A comparison between this alkali–carbonatitic fluid with the volatile-deficient, sövitic Kalkfeld carbonatite suggests that virtually all alkali metals and Cl, and a major proportion of F, Th, U, and Ti were preferentially partitioned into this fluid. This fluid was also able to accommodate significant concentrations of Rb, Cs, Cu, Pb, and Zr in individual samples. The qualitative sequence: Fe = Mn > Sr = REE > Mg = F > Ba = Y > Ti > Th = U > (Zr,Cu,Pb,Rb,Cs) > K > Na = Cl represents an increasing tendency from left to right to partition into the fluid relative to the crystallizing carbonatite melt. As this fluid migrates through and interacts with invaded host rocks, elements will tend to precipitate in the same qualitative sequence from left to right. This selective precipitation of elements from a migrating fluid accounts for observations made in metasomatized crustal and mantle–derived rocks.

Multiple fluid generations in the Sudbury igneous complex: fluid inclusion, Ar, O, H, Rb and Sr evidence

The elliptically zoned Sudbury Structure in northern Ontario is host to world-class nickel–copper–PGE deposits, which are generally regarded as the products of a single magmatic event. However, there is increasing evidence of multi-episodic fluid rock (including Ni–Cu sulphide) interactions in the Sudbury Structure that range from early remobilization of the cooling ores (ca. 1850 Ma), through metamorphism and deformation related to the Penokean Orogeny to neotectonic fracturing (ca. 5–13 Ma). Previous fluid inclusion studies have identified an H 2O–NaCl±CO 2±KCl fluid of highly variable phase ratios intimately associated with the ores, and a distinct population of primary two-phase fluid inclusions associated with post-intrusion metasomatic alteration. This study introduces two new distinct populations of fluid inclusions: (1) a population represented by two-phase CO 2-bearing fluid inclusions hosted along healed-fracture planes in quartz that offset ore minerals, that was trapped between 280–340°C and 1750 to 3500 bars pressure. As the fluid inclusion trails offset and therefore post-date the ore, their genesis is consistent with a late Penokean (1.83–1.89 Ga) or later (ca. 1450) contractional event; (2) a two-phase highly-saline (>16 wt.% NaCl equivalent) fluid hosted within neotectonic (5–13 Ma Rb–Sr) galena–sphalerite–calcite–quartz–chlorite veins. Inclusions of this fluid were trapped at temperatures ranging from 60 to 135°C and maximum confining pressures of 950 bars. Stable isotope data (δO,δD) from biotite, amphibole and epidote from the alteration assemblages of remobilized veins within the Sudbury Structure show distinct evidence of a mixing trend of fluids originating within or near the `magmatic water box' and trending towards a metamorphic fluid of isotopic composition similar to an evolved Sudbury Structure groundwater. Ar–Ar data from the amphiboles within Sudbury Intrusive Complex (SIC) footwall breccia from the Craig Mine are consistent with a mixed Ar–Ar spectrum. The step-heating Ar release spectrum yields ages ranging from ca. 1800 Ma at the low temperature end to approximately 2640 Ma for high temperature steps. These dates are in agreement with the ca. 1850 and 2647 Ma ages for the SIC and Levack gneisses, respectively, and are interpreted as the reinjection of country rocks partially melted by the SIC back into the Sudbury Structure. The isotopic evidence for mixed fluids derived from a combination of a magmatic fluid and a regional groundwater/metamorphic fluid suggests that the highly variable phase-ratio fluid inclusions associated with the ore may be divided into two end-member fluids represented by an exsolved SIC fluid and a regional groundwater/metamorphic fluid. This is consistent with at least five distinct fluids being present during the evolution of the Sudbury structure.

Meteoric water component in magmatic fluids from porphyry copper mineralization, Babine Lake area, British Columbia

Coexisting vapor-rich and high-salinity “magmatic” fluid inclusions from mineralized quartz ± carbonate veins in porphyry Cu systems of the Babine Lake area are depleted in deuterium (D) relative to accepted magmatic isotopic values. The δD compositions of high-salinity fluids from early mineralized veins range from −100‰ to −135‰ (standard mean ocean water), whereas fluid inclusions in late veins and breccias have δD values between −138‰ to −153‰, and unmineralized, regional veins have δD values from −94‰ to −150‰. Multiple δD analyses of fluids from inclusions are within ± 5‰ for 14 of 17 duplicate analyses, which suggests that mixing of fluid-inclusion populations did not contribute significantly to the D-depleted values. A correlation between δ 13 C values of vein carbonate and δD values of inclusion fluids indicates that postentrapment reequilibration did not significantly alter isotopic compositions of fluid inclusions. Incorporation of externally derived low-D material into the mineralizing intrusions, either from the influx into the melt of deeply convected, evolved meteoric fluids or by crustal assimilation of D-depleted country rock, is required to explain the observed D-depleted values of the magmatic fluids.

Fluid inclusion studies of gold deposits in the Gezhen shear zone, Hainan Province, China

The Tuwaishan, Baoban, Erjia, Bumo and other gold deposits in western Hainan occur in Precambrian metamorphic clastic rocks and are structurally controlled by the Gezhen shear zone. Fluid inclusion studies have been carried out of the gold deposits mentioned above. The homogenization temperatures of the whole fluid inclusion population range from 140°C to 370°C, indicating that gold was precipitated mainly at 240–250°C. The salinities are within the range of 2.0–9.2 wt% NaCl equiv. and the pressure of formation of the deposits was estimated at about 270×105−500×105Pa, corresponding to a depth of about 1.1–2.0 km under lithostatic confinement. Chemical studies show that the ore fluid is of the Na+(K+)-Ca2+-Cl−(F−) type. Theδ18O andδD values of the fluid vary from −2.7‰- +4.4‰ and −50‰–−87‰ Evidence developed from fluid inclusions and geological setting indicates that the ore fluid was a mixture of magmatic and meteoric-hydrothermal waters. Changes in chemical composition andδ18O andδD of fluid inclusions from one ore field to another seem to be related with regional tectonism, metamorphism and magmatism.

Fluid composition and evolution in coesite-bearing rocks (Dora-Maira massif, Western Alps): implications for element recycling during subduction

Fluid inclusions and F, Cl concentration of hydrous minerals were analysed in the coesite-pyrope quartzite, the interlayered jadeite quartzite and their country-rock gneiss from the Dora-Maira massif using a combination of microthermometry, Raman spectrometry, synchrotron X-ray microfiuorescence and electron microprobe analysis. Three populations of fluid inclusions were recognized texturally and can be related to distinct metamorphic stages. A low-salinity aqueous fluid occurs in the retrogressed country gneiss and as late secondary inclusions in jadeite quartzite and chloritized pyrope. An earlier secondary population is found in matrix quartz of the jadeite- and pyro-pe-quartzites. This population can be related to the early decompression and so to incipient breakdown of garnet into phlogopite-bearing assemblages. The inclusion fluid is highly saline (up to 84 wt% equivalent NaCl) and contains Na, Ca, Fe, Cu and Zn as major cations. In pyrope quartzite, additional K was found in these brines, which locally coexist with CO2-rich inclusions. The oldest fluid inclusions are preserved in kyanite grains included in fresh pyrope and in pyrope itself. In pyrope, all inclusions have decrepitated and contain magnesite, an Mg-phosphate, sheet-silicate(s), a chloride and an opaque phase, with no fluid preser ved. In contrast, the kyanite inclusions in pyrope preserve primary H2O-CO2 low-salinity fluid inclusions, probably owing to the low compressibility of the kyanite inclusions and host garnet. In spite of in-situ re-equilibration, these inclusions can be interpreted as relics of the dehydration fluid that attended pyrope growth. These correlations between textural and chemical fluid inclusion data and metamorphic stages are consistent with the fluid composition calculated from the halogen content of different generations of phlogopite and biotite. The preservation of different fluid compositions, both in time and space, is evidence for local control and possibly origin of the fluids, in agreement with isotopic data. These results, in particular the absence of CO2 in the jadeite quartzite, are best interpreted in terms of a fluid-melt system evolution. With increasing metamorphism, partitioning of H2O, Na, Ca, Fe and heavy metals into melt (jadeite quartzite) and Mg, Na/K, F, CO2 and P(?) into a residual aqueous fluid can account for depletion in Na, Ca and Fe of the pyrope quartzite. During the retrograde path, a H 2 O rose as melt crystallized, generating the two populations of hypersaline and water-rich fluids that were highly reactive to pyrope. The process of fluid-melt interaction envisioned here coupled with models of melt extraction in subduction zones provides an attractive opportunity for the instantaneous ( < 1 Ma) and selective transport of elements between a downgoing slab and the overlying mantle wedge.

Direct evolution of brine from a crystallizing silicic melt at the Questa, New Mexico, molybdenum deposit

Molybdenum mineralization in the Southwest orebody at the Questa deposit is associated with crystallization of aplite porphyry, explosive brecciation, and exsolution of magmatic hydrothermal fluids. Quartz, K feldspar, fluorophlogopite, and molybdenite precipitated from exsolving aqueous fluids and cemented aplite porphyry and andesite wall-rock clasts forming a magmatic hydrothermal breccia ore.Fluid inclusions in quartz in the breccia matrix trapped aqueous fluids that transported and precipitated molybdenum. Two fluid inclusion populations associated with molybdenite mineralization were identified. The largest population commonly contains liquid, vapor, halite, sylvite, and one or more opaque daughter crystals. These inclusions homogenize between 200 degrees and 500 degrees C and have a prominent mode at 360 degrees to 400 degrees C; salinities vary from 31 to 57 wt percent NaCl equiv. Low first ice-melting temperatures suggest that additional fluid components, possibly calcium or magnesium chlorides, may be present. Approximately 80 percent of these inclusions homogenize by halite dissolution; phase equilibria constraints require that these inclusions trapped fluids in the vapor-absent field. The remaining saline inclusions homogenize to liquid at temperatures above halite dissolution temperatures. Significantly, the saline fluid did not coexist with a low-density aqueous fluid.A second population of inclusions contains fluids with salinities of 2.7 to 25.0 wt percent NaCl equiv and homogenizes by vapor bubble disappearance, vapor bubble expansion, or critical behavior. Textural relationships and phase behavior during microthermometry indicate that these inclusions constitute a single population which formed as pressure and temperature fluctuated at near-critical conditions.Pressure changes related to brecciation followed by crystallization and system resealing produced the observed fluids. Saline fluids were trapped as the system sealed following a major brecciation event. These high-salinity fluids were produced as the melt, under pressure that increased from hydrostatic to lithostatic, partitioned increasing amounts of chlorine into the exsolving aqueous fluid. Renewed fracturing and abrupt pressure decrease to hydrostatic conditions reduced the mass of chlorine partitioning to the exsolving fluid. Lower salinity fluids exsolved at near-critical conditions and were trapped by inclusions that homogenize to liquid, to vapor, or by critical behavior. The lack of cogenetic liquid- and vapor-rich fluid inclusions and phase equilibria constraints indicate that high-salinity fluids (< or = 57 wt % NaCl equiv) were generated directly by the crystallizing magma and were not a product of aqueous fluid immiscibility.

Sill emplacement in wet sediments: fluid inclusion and cathodoluminescence studies at Grunehogna, western Dronning Maud Land, Antarctica

The Grunehogna and Kullen sills of western Dronning Maud Land were emplaced into water-saturated, partially lithified sediments of the mid-Proterozoic Ritscherflya Supergroup. Fluid inclusion and optical and scanning electron microscope cathodoluminescence studies of quartz grains within and adjacent to the sills (the ‘sill-sediment interactive zone’, referred to as the ‘interactive zone’) showed that only a single generation of later fluid inclusions is present. The fluids are Na-K-Ca-Cl aqueous solutions with temperatures of homogenization between 95 and 105 °C. Individual quartz particles in the sedimentary rocks above the interactive zone have unique fluid inclusion populations. Petrographic and cathodoluminescence evidence indicated that there are three types of quartz grains in the interactive zone. These consist of (i) a xenocrystic type, (ii) a high-temperature quartz that crystallized either from the magma or as a result of anatectic melting of xenolithic (and/or xenocrystic) material, and (iii) euhedral quartz crystals that have grown in vugs in the peripheral zone. Cathodoluminescence of xenocrystic grains indicated the existence of healed microfractures along which fluid inclusions were concentrated and secondary and authigenic overgrowths on older generation quartz grains had developed. The presence of zonal luminescence patterns of detrital particles, coupled with evidence for equilibrium polygonization in the sedimentary rocks adjacent to the peripheral zone up to about 8 m above the upper contact of the Grunehogna sill suggested local thermal tempering or sintering and recrystallization. Luminescence of detrital quartz particles in the overlying Hogfonna Formation reflected variation in the characteristics of their source terrains, and showed limited evidence of heating by the magma. The interactive zone fluid inclusions are formation waters which penetrated microfractures during diagenesis. We show that no large-scale hydrothermal circulation system is likely to develop following intrusion into a water-saturated environment.

Primary fluids in low-temperature eclogites: evidence from two subduction complexes (Dominican Republic, and California, USA)

Eclogites occur as isolated blocks in melanges of both the Samana Peninsula, Dominican Republic, and the Franciscan Complex, California, USA. In some of these eclogites, fluid inclusions were found in omphacite and sodic-calcic amphibole grains. Textures show that non-planar populations of fluid inclusions formed during growth of clinopyroxene and amphibole. In addition, planar arrays of secondary fluid inclusions are found along healed cracks. Homogenization temperatures to liquid were used to calculate isochores for the fluid inclusions. These data were compared with petrologic geothermobarometry. Temperature conditions of 500–700° C were estimated from garnetclinopyroxene geothermometry. The jadeite contents of omphacite indicate minimum pressures of 8–11 kbar in this temperature range. The P-T estimates agree well with calculated isochores for primary fluid inclusions from the Samana Peninsula, and show some overlap for both primary and secondary fluid inclusions from the Franciscan Complex. Salinities of 1.2–5.3 wt% NaCl equiv. were estimated for both primary and secondary fluid inclusions from Samana and Franciscan eclogites. These data suggest that low-salinity aqueous fluids attended eclogite-facies metamorphism and perhaps retrograde metamorphism in both subduction complexes. The salinities and densities of fluid inclusions in eclogites from the Samana Peninsula and the Franciscan Complex resemble those of counterparts from garnet amphibolites of the Catalina Schist, southern California. An external source for such fluids is suggested by their homogeneous populations coupled with their low salinities. Geologic evidence suggests that the Samana and Franciscan eclogites may have been derived from a Catalina-like source terrane. The Catalina rocks are inferred to have interacted with large volumes of sediment-derived fluid during subduction zone metamorphism at similar P but higher T conditions than those determined for Samana and Franciscan eclogite blocks. These results contrast with data for fluid inclusions from eclogites of the Monviso area, western Alps. The Monviso eclogites yield similar estimates for metamorphic P-T to those obtained in this study, but contain fluid inclusions of brine and of other saline aqueous fluids, all of which are less dense than expected for incorporation at the reported eclogite-facies conditions. The differences between the properties of fluid inclusions from the ecologites and garnet amphibolites of the Samana-Franciscan-Catalina subduction complexes and those of Monviso probably reflect differences between fluid-flow regimes during metamorphism.

Evidence of magmatic CO2-rich fluids in peraluminous graphite-bearing leucogranites from Deep Freeze Range (northern Victoria Land, Antarctica)

Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20–30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO2+H2O inclusions + Al2SiO5 ± brines in garnet (type 1); early CO2-rich inclusions (± brines) in quartz (type 2); early CO2+CH4 (up to 4 mol%)±H2O inclusions + graphite + fibrolite in quartz (type 3); late CH4+CO2+N2 inclusions and H2O inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (≈750°C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (≈700–550°C). Type 4 inclusions were trapped below 500°C. The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO2-rich, possibly with immiscible brines. CO2-rich fluids (±brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T<670°C) led to a continuous decrease off O 2 can entering in the graphite stability field; at the same time, the feldspars reacted with CO2-rich fluids to give secondary fibrolite + graphite. Decrease ofT andf O 2 can explain the progressive variation in the fluid composition from CO2-rich to CH4 and water dominated in a closed system (in situ evolution). The presence of N2 the late stages indicates interaction with external metamorphic fluids.

Fluid regimes during late stages of a continental collision: Physical, chemical, and stable isotope measurements of fluid inclusions in fissure quartz from a geotraverse through the Central Alps, Switzerland

Fluid evolution during neo-alpine metamorphism during late stages of the continental collision between Europe and Africa was studied by analyzing fluid inclusions in alpine fissure quartz collected in forty-nine localities along a geotraverse through the Central Alps, Switzerland. The methods employed include microthermometry, micro-Raman spectroscopy, K/Na thermometry, and stable isotope analysis. Early fluid inclusions provide evidence of close to peak metamorphic temperatures of the late Tertiary or neo-alpine metamorphic event. Fluid composition evolved along the geotraverse from north to south as follows: higher hydrocarbons were dominant in the low- and medium-grade diagenetic zones, methane was the main volatile in the high-grade diagenetic and low-grade anchizone, water dominated in the highgrade anchizone and low-grade epizone, with CO 2 > 10 mol% in the high-grade epizone and in the mesozone. Higher hydrocarbons and CH 4 were the products of kerogen maturation and cracking of preexisting petroleum. Large water supplies originated from the dehydration of cooler metasedimentary rocks that were overthrust by crystalline basements of the Lepontines, Aar, and Gotthard massifs. Carbon isotope analyses suggest that the CO 2 component was derived from oxidation of graphitic matter, especially in the vicinity of sulfate-bearing metasediments and from decarbonation reactions. In the Aar and Gotthard massifs as well as in the Helvetic Axen nappe and its underlying North Helvetic flysch, high fluid pressures prevailed and favored nappe transport. By contrast, in the southern Lepontine area, very low early fluid pressures were probably related to dry rocks and scarce metasediments, and to high geothermal gradients that resulted from intense uplift and erosion between 26 and 18 Ma. Retrograde fluid evolution was recorded by a succession of fluid inclusion populations in each alpine fissure. It was controlled by uplift and cooling and characterized by decreasing contents of volatiles and an increase in δ 18O of host quartz. Tectonic activity led to episodic pressure drops of at least 0.5 to 2 kbar and promoted fluid unmixing, channelized flow, and rapid growth of skeletal quartz. Channelized rather than pervasive fluid migration at temperatures < 450° C and under conditions of brittle deformation is documented by episodic increases in salinity and by fluid flushing through the massifs. There is stable isotope evidence for involvement of meteoric water only in late-crystallizing quartz. Formation of Alpine fissures and fissure minerals was the result of a unique coincidence of late continental collision (< 450° C), fluid expulsion from overthrust metasediments, uplift, and erosion.

Chemistry of hydrothermal quartz in the post-Variscan “Bavarian Pfahl” system, F.R. Germany

Chemical analyses of hydrothermal quartz by means of instrumental neutron activation, coulometric, dewatering, and decrepitation analysis are combined with previously published microthermometric and isotopic data in order to unravel the processes involved in the formation of the post-Variscan hydrothermal system “Bavarian Pfahl”. The Bavarian Pfahl is interpreted as a fossil zone of tectonic activity and large-scale channelized fluid movement in the upper continental crust. The predominance of K relative to Ca and Na in quartz as well as in fluid inclusions strongly supports the hypothesis that incongruous dissolution of feldspar during wall-rock alteration and K-metasomatism during fluid ascent along the shear zone accompanied by intense silicification are characteristic features of important fluid pathways in the crust. The element contents detected in quartz along a 70 km long profile can be correlated with variable wall-rock compositions. High concentrations of Ba, As, Rb, Sr, and Cs are found in the vicinity of sedimentary basins at the western margin of the Bohemian Massif. Elevated element contents in this part of the profile probably result from intense water-rock interaction within the adjoining sedimentary basin. High Ba-contents in the vicinity of the fluorite-barite mining district of Nabburg/Wölsendorf indicate a genetic relationship between both mineralizations. Low Cl/Br-ratios can be generated either by interaction with carbonaceous sediments in these basins or by interaction with graphite in basement rocks. The calculated salinities of the fluid inclusions vary widely, but correlate well with microthermometric data, and indicate mixing of basement or sedimentary brines with high salinity and meteoric water with low salinity. The effects of polyphase cataclasis, a result of seismic pumping, on the fluid inclusion population and the mechanical properties of quartz have been studied by combined dewatering and decrepitation analyses. Despite minor regional variations in the chemical composition of quartz along the profile, the overall uniform element pattern supports homogeneity of the hydrothermal system over distances of at least 70 km and probably over the total length of about 150 km. The fluid volume required for the formation of the Bavarian Pfahl is estimated to correspond to the volume of the Baltic Sea (approx. 23,000 km 3). In analogy with hydrothermal springs in the vicinity of recently active shear zones and faults the lifetime of the hydrothermal system “Bavarian Pfahl” is estimated to be about 160,000 years.

Brine history indicated by argon, krypton, chlorine, bromine, and iodine analyses of fluid inclusions from the Mississippi Valley type lead-fluorite-barite deposits at Hansonburg, New Mexico

Argon, krypton, chlorine, bromine, and iodine were measured in a homogeneous population of high-salinity hydrothermal fluid inclusions from the Tertiary-age Mississippi Valley-type (MVT) lead-fluorite-barite deposits at Hansonburg, New Mexico to establish new types of evidence for the history of both the fluid and the major dissolved salts. Noble gases and halogens in fluid inclusions containing 10 −10–10 −9 L of brine ( Cl= 3 molal) were analyzed by laser microprobe noble-gas mass spectrometry ( lmngms) on neutron-irradiated samples. The concentrations of 36Ar ( 4.7 × 10 −8 molal) and 84Kr 1.8 × 10 −9 molal) in the fluid inclusions are equal to those of fresh surface waters in equilibrium with air at approximately 20 ± 5°. The mole ratios of Br/Cl ( 1.2 × 10 −4 ) and I/Cl ( 1–2 × 10 −6 ) are among the lowest measured in any natural waters, similar to those of modern brines formed by dissolution of Permian NaCl-bearing evaporites in southeast New Mexico. 40Ar/ 36Ar ratios (600) are twice that of air, and indicate that the fluid inclusions had excess radiogenic 40Ar ( 1.4 × 10 −5 molal) when trapped. The amount of excess 40Ar appears to be too large to have been acquired with Cl by congruent dissolution of halite-bearing evaporites, and possibly too small to have been acquired with Pb by congruent dissolution of granitic basement rocks with Proterozoic K Ar ages. From the lmngms data, combined with published Pb and S isotope data, we infer the following sequence of events in the history of the Hansonburg MVT hydrothermal brine: (1) the brine originated as relatively dilute meteoric water, and it did not gain or lose atmospheric Ar or Kr after recharge; (2) the originally dilute fluid acquired the bulk of its Cl and sulfate in the subsurface after recharge by dissolving halite-bearing Permian? marine evaporites; (3) the high salinity brine then acquired most of its Pb and excess radiogenic 40Ar from interactions with aquifer rocks other than evaporites, possibly clastic sedimentary rocks or basement rocks with Phanerozoic K Ar “ages”; and (4) the brine deposited fluorite without having boiled or degassed.

A model for epigenetic Ba-Pb-Zn mineralization in the Appalachian thrust belt, Quebec; evidence from fluid inclusions and isotopes

Ba-Pb-Zn vein and disseminated deposits occur within a 125-km section of the Taconic thrust belt of the lower Saint Lawrence region of Quebec, Canada. The deposits are hosted by Cambrian sandstone and conglomerate and are not obviously associated with igneous rocks.There are two main fluid inclusion populations, one with homogenization temperatures between 60 degrees and 190 degrees C and salinities between 15 and 32 wt percent NaCl equiv, and another with homogenization temperatures and salinities of 220 degrees to 390 degrees C and 2 to 32 wt percent NaCl equiv, respectively. Primary fluid inclusions and all but one of the fluid inclusions in sphalerite are restricted to the low-temperature-high-salinity fluid inclusion population. The data for sphalerite define a weak trend of decreasing salinity with homogenization temperature. The high-temperature inclusions are interpreted to reflect the influx of a postmineralization fluid and stretching of earlier trapped inclusions.The delta 34 S values for galena and sphalerite in the Saint Fabien deposit range from 8.2 to 10.7 per mil and from 13.0 to 14.1 per mil, respectively. In the other occurrences delta 34 S values for galena are as low as 1.5 per mil. Temperatures obtained from four sphalerite-galena pairs were 86 degrees , 96 degrees , 163 degrees , and 276 degrees C. The delta 34 S values for barite range from 17.5 to 27.5 per mil. There is no correlation between the sulfur isotope ratios of galena and sphalerite and those of barite. The data suggest that there were two sources of sulfur, one from diagenetic pyrite or organic matter and the other from sulfate.delta 18 O (sub V-SMOW) values for carbonate minerals in the Saint Fabien deposit range from 15.1 to 18.8 per mil. If a temperature of 110 degrees C is assumed, the delta 18 O (sub V-SMOW) of the corresponding water was -2.4 to +3.2 per mil which suggests a seawater source. In the other occurrences delta 18 O (sub V-SMOW) are, on average, 2 per mil lower. The delta 13 C values in the Saint Fabien deposit range between -1.9 and +0.9 per mil, but in the other occurrences are as low as -11.4 per mil. These data indicate variable contributions of carbon from seawater and organic matter.Strontium isotope ratios for dolomite and barite range from 0.71195 to 0.71646; ratios for coexisting calcite are consistently higher. All values are significantly more radiogenic than Cambro-Ordovician seawater (0.7092-0.7078). The high values are similar to those interpreted for the Cambro-Ordovician sedimentary succession. However, the wide range of values for barite suggests that this strontium was mixed with strontium from a source with a seawater signature.The geochemical data are consistent with the transport of metals and reduced sulfur in a basinal brine. A model is proposed in which metalliferous fluids were expelled from the Cambro-Ordovician sedimentary succession near the end of, or immediately after, the Taconic orogeny (Late Ordovician to earliest Silurian), migrated under gravity to zones of extension, and mixed with ground waters that had interacted with evaporites. Sulfide deposition is attributed to an increase in pH and barite deposition to an increase in sulfate activity, both of which occurred as a result of fluid mixing. In terms of timing, geologic parameters, and depositional processes, the Ba-Pb-Zn mineralization discussed in this paper is similar to certain Mississippi-Valley-type Pb-Zn deposits and may record a major lower Paleozoic fluid expulsion event in the Appalachian orogen.

Theory of the Number of Hexagonally Distributed Points in a Given Circle and Its' Application to Study Fluid Inclusion Population

Abstract The equation, N = 2πX2/√3, has been derived to find out the number of hexagonally distributed points (N), in a given circle of radius 'R' and the point-point distance 'd'. where X = R/d. For a given set of microscopic conditions, the above equation is applied to construct 4 standard circular graphic charts. An, attempt has been made to use these charts to study the population of fluid inclusions, by visual comparison, in the quartz samples intimately associated with the sulphides collected from the Mosaboni mine of Bihar.

Hydrocarbon-bearing fluid inclusions in fluorite associated with the Windy Knoll bitumen deposit, UK

Hydrocarbon-bearing fluid inclusions in fluorite, associated with an outcropping bitumen deposit at Windy Knoll, Derbyshire, have been analysed in situ using a combination of microthermometry, Fourier transform infrared (FTIR) microspectrometry, and ultraviolet (UV) microscopy. The inclusions in these samples can be considered as a series with two endmembers: aqueous inclusions containing a low-density vapour phase and inclusions containing liquid “oil” with no detectable aqueous phase. The majority of the inclusions are mixed types containing both aqueous and liquid hydrocarbon phases. Although microthermometry distinguishes at least two different aqueous fluids with varying homogenization temperatures and salinities, the oil fraction is cogenetic and trapped together with just one fluid, a low-salinity, low-calcium brine with an average homogenization temperature of 134°C. The majority of the liquid hydrocarbon-bearing inclusions fluoresce bright blue under UV illumination with peaks around 475 nm, characteristic of paraffinic oils. The FTIR spectra of these inclusions are dominated by peaks assigned to aliphatic C—H bonding. However, inclusions have also been found which display a fluorescence typical of the red-shift associated with less mature oils. The FTIR spectra display peaks assigned to CO, C—O, and O—CH 2 bonding. This study presents new data on the in-situ analysis of hydrocarbon-bearing fluid inclusions from this important area of natural petroleum seepage and ore mineralization. The results suggest a direct link between the fluid inclusion populations, the outcropping bitumens, and fluorite deposition.

The porphyry gold deposit at Marte, northern Chile

A porphyry gold deposit has been documented for the first time at Marte in the Maricunga belt of the Andean Cordillera, northern Chile. Exploration conducted from 1981 through 1987 resulted in definition of 66 metric tons of contained gold. The deposit entered production in late 1989 as an open-pit, heap leach operation.The gold deposit is part of a linear, calc-alkaline volcanoplutonic arc constructed during the mid- to late Miocene as eastward-directed subduction was shallowing. The consequent compression of continental basement underlying the deposit and its environs created a series of high-angle, reverse faults. Stock emplacement and gold mineralization took place within a coeval andesitic stratovolcano at 13 to 14 Ma. The hornblende-biotite diorite stock is subdivided into three phases: two mineralized porphyries and a weakly porphyritic, late mineralization microdiorite. Much of the microdiorite constitutes the matrix of an intrusion breccia, which is transected locally by a closely related hydrothermal breccia.Gold mineralization is coincident with a stockwork of quartz veinlets surrounded by chloritesericite-clay alteration of intermediate argillic type. Pyrite and iron oxides are both present in dominantly disseminated form, with each making up as much as 10 vol percent of the rock. Hematite exceeds magnetite in abundance and was generated in large part by hypogene martitization. Minor chalcopyrite and even lesser quantities of molybdenite, bornite, tennantite, and enargite also occur. Gypsum after anhydrite commonly exceeds 5 vol percent and calcite and tourmaline exist as traces. Isolated remnants of hydrothermal biotite and alkali feldspar attest to the former presence of K silicate alteration, which is believed responsible for introduction of much of the veinlet quartz, magnetite, and gold. Chlorite-dominated alteration affected the late-stage microdiorite and its associated breccias. An erosional remnant of the andesitic volcanic roof to the stock is largely replaced by a chalcedony- and alunite-rich advanced argillic assemblage, which is part of a formerly more extensive tabular alteration zone. Abundant pyrite, gypsum, native sulfur, and barite and trace amounts of enargite and subeconomic gold are present in this advanced argillic cap.The relative impermeability of the cap caused development of an inverted supergene profile in which sulfide-bearing ore is underlain by jarositic leached ore. The steep, elongate orebody is made up of both ore types, which do not differ in gold content. However, the hypogene copper content, about 550 ppm, was more than halved by supergene oxidation. The gold ore is poor in silver (Ag/Au = 0.44), but highly anomalous in molybdenum as well as copper and weakly anomalous with respect to lead, arsenic, and mercury. The advanced argillic cap is marked by pronounced Au, Bi, Hg, Tl, Pb, Mo, and As anomalies, which contrast with a broad zinc halo to the orebody.Preliminary fluid inclusion studies suggest that the auriferous stockwork is a product of boiling fluids which ranged in temperature from 155 degrees to 375 degrees C and in salinity from 2 to 20 wt percent NaCl equiv. A reconstruction of the volcanic edifice combined with pressure estimates based on the fluid inclusion data implies that mineralization took place about 600 to 700 m below the paleosurface. The geologic, alteration, and mineralization characteristics of the Marte gold deposit are closely similar to those of gold-rich porphyry copper deposits, especially those in the Philippines. The only significant mineralogic difference is a deficiency of copper at Marte. The fluid inclusion population at Marte is also more reminiscent of porphyry rather than epithermal environments, although high-salinity inclusions common in porphyry copper deposits were not encountered in the few samples studied.