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REFERENCES Demars, C., Pagel, M., Deloule, E., and Blanc, P., 1996, Cathodoluminescence of quartz from sandstones: Interpretation of the UV range by determination of trace element distributions and fluid inclusion P-T-X properties in authigenic quartz: American Mineralogist, v. 81, p. 891-901. Juhasz-Bodnar, K., 1999, Diagenesis and pore-water evolution of the Permo-Carboniferous Al Khlata Formation, Interior Oman Sedimentary Basin, Sultanate of Oman [phD thesis]: Bern, Universität Bern. Landtwing, M.R., and Pettke, T., 2005, Relationships between SEM-cathodoluminescence response and trace-element composition of hydrothermal vein quartz: American Mineralogist, v. 90, p. 122-131. Perny, B., Eberhardt, P., Ramseyer, K., Mullis, J., and Pankrath, R., 1992, Microdistribution of Al, Li and Na in ² quartz: Possible causes and correlation with short-lived cathodoluminescence: American Mineralogist, v. 77, p. 534-544. .11 PVTx properties of H 2 O-NaCl fluids using Brillouin scattering spectroscopy Mantegazzi Davide*, Sanchez-Valle Carmen*, Driesner Thomas** *Inst. for Mineralogy and Petrology, ETH Zurich, CH-8092 Zurich, (davide.mantegazzi@erdw.ethz.ch) ** Inst. of Isotope Geochemistry and Mineral Resources, ETH Zurich, CH-8092 Zurich Saline-rich aqueous fluids play a very important role in a wide range of geological events in the Earth’s crust and mantle. These processes include, for example, metamorphic reactions, the magma production in the mantle wedge beneath active volcanoes at subduction zones, the transport of chemical components in several geological settings, the alteration of the seafloor and the formation of ore deposits. Mixtures in the H 2 O-NaCl binary system are a good proxy for these saline-rich aqueous fluids. Essential for quantitative modelling of phase equilibria related to such geological processes is the knowledge of the thermodynamic properties of these fluids at geologically relevant pressure and temperature conditions. Despite the important role of these geological fluids, very few experimental data on aqueous saline-rich fluids other than pure H 2 O are available at pressures higher than 0.1-0.5 GPa. For instance, the equations of state (EoS) for H 2 O-NaCl are restricted to 0.5 GPa. Therefore, the interpretation of subduction-related fluid processes and other fluid-mediated geological events requires the extrapolation of thermodynamic properties through over an order of magnitude in pressure. In this contribution we present the PVTx properties of H 2 O-NaCl binary mixtures with different NaCl concentrations up to high P-T conditions, calculated from sound velocity measurements in the external heated membrane diamond anvil cell using Brillouin scattering spectroscopy. Brillouin scattering spectroscopy allows the direct measurement of sound velocities of compressional waves (V P ) propagating in the fluid and hence, the evaluation of the equation of state ρ(P,T). The obtained sound velocities and densities are compared with available data in the literature. The results are combined with previous experimental and theoretical EoS to provide the thermodynamical properties of the most relevant aqueous systems involved in geological fluid-mediated processes in the Earth’s crust and mantle. REFERENCES Wagner, W., and Pruss, A. 2002: The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use, J. Phys. Chem. Ref. Data, 31, 387–535. Abramson, E., and Brown, M.J. (2004): Equation of state of water based on speeds of sound measured in the diamond-anvil cell, Geochim. Cosmochim. Acta, 68, 1827-1835. Sanchez-Valle, C., and Bass, J.D. (2007) Equation of state of H 2 O from sound velocity measurements in the diamond anvil cell, Geochim. Cosmochim. Acta, 71, A873-A873. Driesner, T. 2007: The system H 2 O-NaCl. Part II: Correlations for molar volume, enthalpy, and isobaric heat capacity from 0 to 1000 degrees C, 1 to 5000 bar, and 0 to 1 X-NaCl, Geochim. Cosmochim. Acta, 71, 4902-4919. 223 Symposium 7: Geofluids and related mineralization: from shallow to deep
22 Symposium 7: Geofluids and related mineralization: from shallow to deep .12 Low-Salinity fluids at the Bajo de la Alumbrera porphyry Cu-Au deposit, Argentina Dimitri L. Meier *, Christoph A. Heinrich *, Marcel Guillong * & M. Florencia Marquez-Zavalía ** *Institute of Isotope Geochemistry and Mineral Resources, ETH Zürich, Clausiusstrasse 25, CH-8092 Zürich (christoph.heinrich@erdw.ethz.ch) ** CONICET, IANIGLA – CCT Mendoza, Av. Ruiz Leal s/n - Parque Gral. San Martin, ARG-5500 Mendoza New evidence from several major porphyry copper and epithermal deposits concerning the role of low-salinity fluids has been published. In contrast to earlier interpretations, the parental magmatic fluid is considered to have an intermediate density and salinity. Several metals including copper and gold are mainly transported by a vapor-like fluid, not by a highsalinity brine. This vapor can then contract and carry significant amounts of metal to the low-temperature epithermal regime. The Bajo de la Alumbrera porphyry copper-gold deposit in north western Argentina is well studied in terms of geology, alteration geochemistry and fluid characteristics, but low-salinity fluids are not documented extensively. In this study, samples particularly from the earliest and latest veins have been taken, to test these new findings at Alumbrera. The aim of the study was not a complete reassessment of the fluid evolution but to fill gaps of previous studies. The mean to investigate fluid characteristics are inclusions in vein minerals, whose salinities and temperatures are measured by microthermometry, and which are then chemically analyzed using LA-ICP-MS. A very copper-rich, intermediate-density fluid with high homogenization temperatures, as well as high-density vapor inclusions, have been found and analyzed, surprisingly not in the earliest but in a texturally late vein. Nevertheless, similarities to the original magmatic fluids from other deposits indicate that also at Alumbrera the parental fluid was not a high-salinity brine, as inferred by previous studies, but a fluid with intermediate density and salinity. In late veins, aqueous fluids have been investigated which could easily be the ore fluids for an epithermal deposit, as for example the nearby Farallón Negro deposit. The analytical method has been adapted to measure gold with very low detection limits, and indeed, gold has been detected with a high confidence in those very low-salinity fluids. In addition to that, attempts to characterize the fluid belonging to relatively late pyrite-chalcopyrite veins occurring in many deposits but never investigated properly have been made, but compositional results are sparse. However, microthermometry indicates aqueous fluids with low salinity being the type of fluid responsible for those veins. Few boiling assemblages have also been analyzed, showing that in a phase separation copper, sulfur and gold partition preferentially into the vapor phase, confirming observations on other deposits. The transition from the vapor-like fluid to an aqueous liquid, called contraction, which is assumed to be responsible for high gold concentrations in ore fluid for epithermal deposits, has not been found in fluid inclusions. The results presented here are not sufficient to document the whole fluid evolution of the Bajo de la Alumbrera deposit, more samples and analyses would be necessary. Nevertheless, evidence has been found that the new models can be applied to Alumbrera as well and low-salinity fluids indeed are responsible for many processes forming porphyry copper deposits.
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