Fluid inclusion and stable isotope evidence for the genesis of quartz-scheelite veins, Metaggitsi area, central Chalkidiki Peninsula, N. Greece

Scheelite mineralization accompanied by muscovite and albite, and traces of Mo-stolzite and stolzite occurs in epigenetic quartz vein systems hosted by twomica gneissic schists, and locally amphibolites, of the Paleozoic or older Vertiskos Formation, in the Metaggitsi area, central Chalkidiki, N Greece. Three types of primary fluid inclusions coexist in quartz and scheelite: type 1, the most abundant, consists of mixed H2O-CO2 inclusions with highly variable (20-90 vol.%) CO2 contents and salinities between 0.2 and 8.3 equivalent weight % NaCl. Densities range from 0.79 to 0.99 g/cc; type 1 inclusions contain also traces ( < 2 mol%) of CH4. Type 2 inclusions are nearly 100 vol.% liquid CO2, with traces of CH4, and densities between 0.75 and 0.88 g/cc. Type 3 inclusions, the least abundant, contain an aqueous liquid of low salinity (0.5 to 8.5 equivalent weight% NaCl) with 10-30 vol.% H2O gas infrequently containing also small amounts of CO2 ( < 2 mol%); densities range from 0.72 to 0.99 g/cc. The wide range of coexisting fluid inclusion compositions is interpreted as a result of fluid immiscibility during entrapment. Immiscibility is documented by the partitioning of CH4 and CO2, into gas-rich (CO2-rich) type 1 inclusions, and the conformity of end-member compositions trapped in type 1 inclusions to chemical equilibrium fractionation at the minimum measured homogenization temperatures, and calculated homogenization pressures. Minimum measured homogenization temperatures of aqueous and gasrich type 1 inclusions of 220°-250 °C, either to the H2O, or to the CO2 phase, is considered the best estimate of temperature of formation of the veins, and temperature of scheelite deposition. Corresponding fluid pressures were between 1.2 and 2.6 kbar. Oxygen fugacities during mineralization varied from 10-35 to 10-31 bar and were slightly above the synthetic Ni-NiO buffer values. The fluid inclusion data combined with δ18O water values of 3 to 6 per mil (SMOW) and δ13C CO2- fluid of -1.2 to +4.3 per mil (PDB), together with geologic data, indicate generation of mineralizing fluids primarily by late-to post-metamorphic devolatilization reactions

Numerical model to determine the composition of H2O–NaCl–CaCl2 fluid inclusions based on microthermometric and microanalytical data

Natural fluids approximated by the H2O–NaCl–CaCl2 system are common in a wide range of geologic environments, including sedimentary basins associated with hydrocarbon occurrences and MVT deposits, submarine hydrothermal systems, and other metamorphic, magmatic and hydrothermal environments. We present a comprehensive numerical model and Microsoft® Excel©-based computer program to determine the compositions of fluid inclusions in the H2O–NaCl–CaCl2 system based on microthermometric and microanalytical data. The model consists of six polynomial correlation equations that describe liquid salinity as a function of NaCl/CaCl2 ratio and melting temperature on each of the ice, hydrohalite, halite, antarcticite, CaCl2·4H2O and CaCl2·2H2O vapor-saturated liquidus surfaces. The cotectic and peritectic boundaries are determined from the intersections of the liquidus surfaces. The model is implicitly internally consistent and topologically correct. The model expands upon the compositional range of applicability and the data types that can be used for compositional determination. It reproduces experimental data for all compositions that lie within the H2O–NaCl–CaCl2·4H2O compositional triangle in the H2O–NaCl–CaCl2 system and yields accurate reproductions of the H2O–NaCl and H2O–CaCl2 binaries. Furthermore, in comparison to previously published models, the one presented here eliminates systematic errors, wavy isotherms and cotectic and peritectic curves with local “bumps.