Compositions of magmatic melts and evolution of mineral-forming fluids in the banska Stiavnica epithermal Au-Ag-Pb-Zn deposit, Slovakia: A study of inclusions in minerals

Abstract

Melt and fluid inclusions were investigated in minerals from igneous rocks and ore (Au-Ag-Pb-Zn) veins of the Stiavnica ore field in Central Slovakia. High H2O (7.1–12.0 wt %) and Cl (0.32–0.46 wt %) contents were found in silicate melt inclusions (65–69 wt % SiO2 and 5.2–5.6 wt % K2O) in plagioclase phenocrysts (An 68–36) from biotite-homblende andesites of the eastern part of the caldera. Similar high water contents are characteristic of magmatic melts (71–76 wt % SiO2 and 3.7–5.1 wt % K2O) forming the sanidine rhyolites of the Vyhne extrusive dome in the northwestern part of the Stiavnica caldera (up to 7.1 wt %) and the rhyolites of the Klotilda dike in the eastern part of the ore field (up to 11.5 wt %). The examination of primary inclusions in quartz and sanidine from the Vyhne rhyolites revealed high concentrations of N2 and CO2 in magmatic fluid (8.6 g/kg H2O and 59 g/kg H2O, respectively). Fluid pressure was estimated as 5.0 kbar on the basis of primary CO2 fluid inclusions in plagioclase phenocrysts from the Kalvari basanites. This value corresponds to a depth of 18 km and may be indicative of a deep CO2 source. Quartz from the granodiorites of the central part of the Stiavnica-Hodrusa complex crystallized from a melt with 4.2–6.1 wt % H2O and 0.24–0.80 wt % Cl. Magmatic fluid cogenetic with this silicate melt was represented by a chloride brine with a salinity of no less than 77–80 wt % NaCl equiv. Secondary inclusions in quartz of the igneous rocks recorded a continuous trend of temperature, pressure, and solution salinity, from the parameters of magmatic fluids to the conditions of formation of ore veins. The gold mineralization of the Svyatozar vein system was formed from boiling low-salinity fluids (0.3–8.0 wt % NaCl equv.) at temperatures of 365–160°C and pressures of 160–60 bar. The Terezia, Bieber, Viliam, Spitaler, and Rozalia epithermal gold-silver-base metal veins were also formed from heterogeneous low-salinity fluids (0.3–12.1 wt %) at temperatures of 380–58°C and pressures of 240–10 bar. It was found that the salt components of the solutions were dominated by chlorides (high content of fluorine, up to 0.45 mol/kg H2O, was also detected), and sulfate solutions appeared in the upper levels. The dissolved gas of ore-forming solutions was dominated by CO2 (0.1–8.4 mol %, averaging 1.3 wt %) and contained minor nitrogen (0.00–0.85 mol %, averaging 0.05 mol %) and negligible methane admixtures (0.00–0.05 mol %, averaging 0.004 mol %). These data allowed us to conclude that the magmatic melts could be sources of H2O, Cl, CO2, and N2. The formation of the epithermal mineralization of the Stiavnica ore field was associated with the mixing of magmatic fluid with low-concentration meteoric waters, and the fluid was in a heterogeneous state.