Fluid inclusion and stable C-O isotope constraints on the origin of metasomatic magnesite deposits of the Western Carpathians, Slovakia

Abstract

Primary fluid inclusions in metasomatic magnesite deposits hosted in Palaeozoic basement of the Western Carpathians are filled with of low-to-moderately saline aqueous solutions, with locally increased CO 2 concentrations (up to 34 mol.%). Brine inclusions with up to 42 wt.% of dissolved salts are less frequent. 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 larger spread than those of oxygen. In some deposits, the δ 18O values are almost fixed in various generations of the metasomatic Mg-carbonates, while δ 13C values vary within several ‰. The C, O-isotope covariation reflects low concentrations of CO 2 (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 δ 18O 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 δ 13C 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 CO 2 degassing. Mg-carbonate-precipitating fluids typically contain increased Br-concentrations resembling the halite-fractionated residual brines originated by seawater evaporation. However, extent of the Br-enrichment substantially exceeds the buffering capacity of the seawater evaporation and it is even greater than that in the spatially associated siderite vein- and replacement-type deposits. Apart from the seawater evaporation, superimposed leaching of the organic matter from marine sediments probably played important role. This mechanism has, however, little effect in open hydrothermal systems. Hence, mechanism of the aditional 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 are frequently observed primary CO 2-rich aqueous inclusions, up to 50 μm in diameter, which could not survive the Early Cretaceous Alpine metamorphic overprint, as well as different covariation of stable isotopes in siderite deposits, where larger oxygen isotope fluctuations are accompanied by less extensive carbon isotope fractionation. This indicates different precipitation mechanisms, i.e., CO 2-devolatilization in 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 co-sanguineous with respect to the presence of the evaporated seawater component in the ore-forming fluid, but they cannot be coeval owing to different composition 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 Permo-Triassic rifting, but this process must have been entirely obliterated by the late Alpine (Cretaceous) hydrothermal activity along shear zones formed during middle-to-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.