Home – Home – Jornals – Russian Geology and Geophysics 2011 number 11

Primary fluid inclusions in metasomatic magnesite deposits hosted in Palaeozoic basement of the Western Carpathians are filled with lowly to moderately saline aqueous solutions, with locally increased CO₂ concentrations (up to 34 mol.%). Brine inclusions with up to 42 wt.% dissolved salts are less frequent. The K/Na ratios in the fluid inclusion leachates indicate formation temperatures between 180-310 °C in the Gemeric unit and 230-300 °C in the Veporic tectonic unit.
Carbon isotopes in metasomatic magnesite and dolomite show a larger spread than oxygen ones. In some deposits, the ґ¹⁸ O values are almost fixed in various generations of the metasomatic Mg-carbonates, while ґ¹³ C values vary within several ‰. The C- and O-isotope covariation reflects low concentrations of CO₂ (less than several mol. %) in the aqueous fluid precipitating the Mg-carbonates in an open hydrothermal system and high fluid/rock ratios ( w/r > 5). Calculated ґ¹⁸ O_fluid values between 2 and 10‰ (V-SMOW) indicate isotopic exchange of the carbonate-precipitating fluid with crustal silicate rocks and/or marine carbonates at increased temperatures. Calculated ґ¹³ O_fluid values between -5 and 3‰ are thought to reflect dissolution of the metasomatised carbonate as well as the escape of lighter carbon isotope during the CO₂ degassing.
Magnesium-carbonate-precipitating fluids typically contain increased Br concentrations resembling the halite-fractionated residual brines originated by seawater evaporation. However, the extent of the Br enrichment substantially exceeds the buffering capacity of the seawater evaporation and is even greater than that in the spatially associated siderite vein- and replacement-type deposits. Apart fr om the seawater evaporation, superimposed leaching of the organic matter from marine sediments probably played an important role. This mechanism has, however, little effect in open hydrothermal systems. Hence, the mechanism of the additional Br enrichment of the magnesite-forming fluids remains unknown.
The observed stable isotope record is a result of Alpine hydrothermal processes, as evidenced by coarse-grained dolomite with Alpine-type mineral assemblage (rutile, apatite, zircon, muscovite-phengite) identified also in the spatially associated siderite vein- and replacement-type deposits. Another evidence for the Alpine origin is frequently observed primary CO₂-rich aqueous inclusions, up to 50 јm in diameter, which could not survive the Early Cretaceous Alpine metamorphic overprint, as well as different covariation of stable isotopes in siderite deposits, wh ere larger oxygen isotope fluctuations are accompanied by less extensive carbon isotope fractionation. This indicates different precipitation mechanisms, i.e., CO₂-devolatilization in an open system during Mg-metasomatism and devolatilization-absent precipitation of the siderite in a closed system triggered by rising temperature. The associated magnesite and siderite deposits may be consanguineous with respect to the presence of the evaporated seawater component in the ore-forming fluid, but they cannot be coeval because of the different compositions of primary fluid inclusions, hydrologic regimes (open versus closed hydrothermal system), and precipitation mechanisms. The fluid inclusion and stable isotope evidence does not definitely discard genetic models, linking the Mg-metasomatism with infiltration of bitter brines along faults during the Permo-Triassic rifting, but this process must have been entirely obliterated by the Late Alpine (Cretaceous) hydrothermal activity along shear zones formed during Middle-Late Cretaceous transtension-extension of the orogenic wedge. The Permo-Triassic rift-related origin of the magnesite must cope with the problem of complete loss of pristine isotopic signature of the metasomatic Mg-carbonates during the superimposed Alpine hydrothermal activity, contrasting with none or negligible Alpine metamorphic/hydrothermal overprint of the spatially associated Fe-carbonate vein- and replacement-type deposits.