Fluid evolution and mineral formation in the Beregovo gold-base metal deposit, Transcarpathia, Ukraine

Abstract

The Beregovo vein-type gold-base metal deposit is hosted by volcanic sedimentary sequences of the Pannonian basin in the Carpathian Neogene volcanic belt. Four stages of mineralization were distinguished: (1) sulfide, (2) quartz-barite, (3) carbonate-quartz, and (4) carbonate-goethite. Homogenization temperature, salinity, and chemical and isotopic composition of fluid inclusions in sphalerite, galena, barite, quartz, and carbonates, combined with the delta 18 O values of quartz, carbonates, and barite, suggest the presence of three major fluid types: (1) deep, relatively high temperature (250 degrees C) and moderate salinity (about 8 wt % NaCl equiv) Na-K-Cl fluid with relatively high delta 18 O (5ppm), delta D (-60ppm), and delta 13 C (-5ppm) values; (2) high-salinity (up to 14 wt % NaCl equiv) Ca-Mg-Na-Cl formation water with higher delta D values (-40ppm); and (3) near-surface steam-heated (about 200 degrees C) sulfate-bicarbonate-dominated fluid delta 18 O from -5 to -1ppm, and delta D from -110 to -56ppm). Low delta 13 C values (-25ppm) for CO 2 in type 2 and 3 fluids reflect a significant contribution of carbon from organic-rich sedimentary wall rocks.The delta 34 S values of sulfide minerals range from -2 to +2.5 for pyrite, -1 to +6 for sphalerite, and -2 to +8 per mil for galena. Two sources of sulfur are identified for the Beregovo sulfide ores. Ores with delta 34 S values close to 0 per mil may have been magmatic and those with higher delta 34 S values (up to 8ppm) perhaps resulted from organic matter (5ppm) in the wall rocks.Two main isotopically and genetically different barite generations were found: hypogene barites with delta 34 S values from 13 to 26 per mil, and supergene barites with values from 4.0 to 5.5 per mil. None of the barites had isotopically equilibrated with the sulfide minerals. If equilibrium in the isotopic exchange between sulfate and water were realized, the delta 18 O values of the fluid would range from -5.0 to -2.0 and -10.5 to -5.9 per mil for the two generations, respectively. For comparison, oxygen isotope composition for present-day precipitation is about -10 per mil.