Origin of Zn-Pb Mineralization of the Vein Bt23C, Bytíz Deposit, Příbram Uranium and Base-Metal Ore District, Czech Republic: Constraints from Occurrence of Immiscible Aqueous–Carbonic Fluids

The mineralogical, fluid inclusion, and stable isotope (C, O) study was conducted on a Late Variscan Zn-Pb vein Bt23C, Příbram uranium and base-metal district, Bohemian Massif, Czech Republic. The vein is hosted by folded Proterozoic clastic sediments in exo-contact of a Devonian-to-Lower-Carboniferous granitic pluton. Siderite, dolomite-ankerite, calcite, quartz, baryte, galena, sphalerite, V-rich mica (roscoelite to an unnamed V-analogue of illite), and chlorite (chamosite) form the studied vein samples. The banded texture of the vein was modified by the episodic dissolution of earlier carbonates and/or sphalerite. Petrographic, microthermometric, and Raman studies of fluid inclusions proved a complicated fluid evolution, related to the activity of aqueous fluids and to an episode involving an aqueous–carbonic fluid mixture. Homogenization temperatures of aqueous inclusions decreased from ~210 to ~50 °C during the evolution of the vein, and salinity varied significantly from pure water up to 27 wt.% NaCl eq. The aqueous–carbonic fluid inclusions hosted by late quartz show highly variable phase compositions caused by the entrapment of accidental mixtures of a carbonic and an aqueous phase. Carbonic fluid is dominated by CO2 with minor CH4 and N2, and the associated aqueous solution has a medium salinity (6–14 wt.% NaCl eq.). The low calculated fluid δ18O values (−4.7 to +3.6‰ V-SMOW) suggest a predominance of surface waters during the crystallization of dolomite-ankerite and calcite, combined with a well-mixed source of carbon with δ13C values ranging between −8.2 and −10.5‰ V-PDB. The participation of three fluid endmembers is probable: (i) early high-temperature high-salinity Na>Ca-Cl fluids from an unspecified “deep” source; (ii) late low-salinity low-temperature waters, likely infiltrating from overlying Permian freshwater partly evaporated piedmont basins; (iii) late high-salinity chloridic solutions with both high and low Ca/Na ratios, which can represent externally derived marine brines, and/or local shield brines. The source of volatiles can be (i) in deep crust, (ii) from interactions of fluids with sedimentary wall rocks and/or (iii) in overlying Permian piedmont basins containing, in places, coal seams. The event dealing with heterogeneous CO2-bearing fluids yielded constraints on pressure conditions of ore formation (100–270 bar) as well as on the clarification of some additional genetic aspects of the Příbram’s ores, including the reasons for the widespread dissolution of older vein fill, the possible re-cycling of some ore-forming components, pH changes, and occasionally observed carbon isotope shift due to CO2 degassing.