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The chemical composition of fluid inclusions in quartz crystals from Alpine fissure veins was determined by combination of microthermometry, Raman spectroscopy, and LA-ICPMS analysis. The veins are hosted in carbonate-bearing, organic-rich, low-grade metamorphic metapelites of the Bundnerschiefer of the eastern Central Alps (Switzerland). This strongly deformed tectonic unit is interpreted as a partly subducted accretionary wedge, on the basis of widespread carpholite assemblages that were later overprinted by lower greenschist facies metamorphism. Veins and their host rocks from two locations were studied to compare several indicators for the conditions during metamorphism, including illite crystallinity, graphite thermometry, stability of mineral assemblages, chlorite thermometry, fluid inclusion solute thermometry, and fluid inclusion isochores. Fluid inclusions are aqueous two-phase with 3.7-4.0 wt% equivalent NaCl at Thusis and 1.6-1.7 wt% at Schiers. Reproducible concentrations of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, B, Al, Mn, Cu, Zn, Pb, As, Sb, Cl, Br, and S could be determined for 97 fluid inclusion assemblages. Fluid and mineral geothermometry consistently indicate temperatures of 320 ± 20°C for the host rocks at Thusis and of 250 ± 30°C at Schiers. Combining fluid inclusion isochores with independent geothermometers results in pressure estimates of 2.8-3.8 kbar for Thusis, and of 3.3-3.4 kbar for Schiers. Pressure-temperature estimates are confirmed by pseudosection modeling. Fluid compositions and petrological modeling consistently demonstrate that chemical fluid-rock equilibrium was attained during vein formation, indicating that the fluids originated locally by metamorphic dehydration during near-isothermal decompression in a rock-buffered system. Keywords Fluid inclusions * LA-ICPMS * Low-grade metamorphism * Accretionary wedge * Veins * Fluid-rock equilibrium * Graphitic metasediments, Introduction The chemical composition of metamorphic fluids is of fundamental importance for predicting mineral stability, evaluating the pressure-temperature history of rocks, and assessing rates and scales of mass transfer during [...]

The process responsible for the gold mineralization in the Witwatersrand Basin, South Africa, remains controversial; the debate is between a detrital placer enrichment in laterally extensive conglomerate layers, and hydrothermal introduction by metamorphic fluids long after sedimentation. While textural and geochronological data may be ambiguous, a first-order geological observation severely delimits the mechanism and timing of gold emplacement. The individual gold reefs are cut and displaced by numerous mafic dikes, which, based on geochemical data presented here, are feeders to flood basalts immediately overlying the Witwatersrand. The displacements preclude fracture-controlled fluid focusing over tens of kilometers along a single conglomerate horizon during or after regional metamorphism. Instead, basin-scale gold enrichment occurred during sedimentation or early diagenesis, well before local redistribution by metamorphic fluids.

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-Fe[Cl.sub.2]-AuH[Cl.sub.2]-[H.sub.2]O 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 [degrees]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 2[sigma] 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. Keywords: Analysis (chemical), fluid inclusions, experimental petrology, magmatic-hydrothermal fluids, element partitioning, immiscible fluids, igneous petrology, vapor, brine

Fluid-phase stability relations combined with thermodynamic modeling using fluid-inclusion analyses and new gold-solubility experiments lead to an integrated geological interpretation linking epithermal gold mineralization and porphyry-style ore formation to the cooling of hydrous magma chambers. The essential chemical requirement for gold transport to low temperatures is an initial excess of sulfide over Fe in the magmatic fluid, which is best achieved by condensing out Fe-rich brine from a buoyant, low- to medium-salinity vapor enriched in volatile S. This vapor can contract directly to an aqueous liquid, by cooling at elevated pressure above the critical curve of the salt-water fluid system. Physical and chemical conditions are matched when magmatic fluid is released through a gradually downward-retracting interface of crystallizing magma beneath a porphyry stock, predicting the consistent zoning and overprinting relations of alteration and mineralization observed in magmatic hydrothermal systems. Keywords: gold solubility, magmatic deposits, vapor; brine, epithetreal, porphyry, thermodynamic modeling.

The solubility of Au metal in rhyolite melt and coexisting magnetite has been determined at 800 [degrees]C, 140 MPa, and [f.sub.o2] [approximately equal to] NNO in a Au-metal saturated, sulfur-free, vapor-brine-silicate melt system. Whole crystals of magnetite suspended within quenched rhyolite glass were analyzed by LA-ICP-MS. These data yield a solubility of Au in magnetite on the order of 2 [micro]/g. The solubility of Au metal in rhyolite melt hosting the magnetite crystals is on the order of 500 ng/g. These data indicate a partition coefficient for Au between magnetite and melt, [D.sup.Mt/melt.sub.Au] [approximately equal to]4. Using reasonable estimates of the mass fraction of magnetite that crystallizes in crustal magmatic systems, we modeled the proportion of Au sequestered by magnetite during fractional crystallization. We considered fractionation in two steps: the idealized derivation of a rhyolite by fractionation of basalt at depth, and the closed-system crystallization of the rhyolite in a magma chamber below the site of ore deposition. Magnetite sequesters 14-54% Au from basaltic parent melt that produces a rhyolite melt via crystal fractionation, at modal abundances of 1-5%. Less then 4% Au is sequestered from a crystallizing rhyolite melt because of low modal abundances of magnetite (