Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Jozef Vlasáč, Tomáš Mikuš, Juraj Majzlan, Martin Števko, Adrián Biroň, Marek Szczerba, Rastislav Milovský, Peter Žitňan

Mineralogy and evolution of the epithermal mineralization in the Rudno nad Hronom-Brehy ore deposit, Štiavnické vrchy Mts. (Slovakia)

Journal of Geosciences, volume 69 (2024), issue 1, 21 - 47

DOI: http://doi.org/10.3190/jgeosci.380

The Rudno nad Hronom-Brehy ore deposit in Slovakia represents an important locality of epithermal precious metal mineralization in the Central Slovak Volcanic Field. The main ore structures in the area are Anna, Goldschram, Filip, Johan de Deo and Priečna veins. In this work, we present mineralogical, paragenetic and geochemical aspects of the ore mineralization, hydrothermal alteration patterns, fluid inclusions, isotopic composition of sulfur (δ ³⁴S) and K-Ar age of ore mineralization. Four mineralization stages were recognized, the third one being split into two substages. (1) Pyrite stage with quartz, K-feldspar, arsenopyrite and pyrite. (2) Base-metal stage with sphalerite, galena, chalcopyrite, Au-Ag alloys and famatinite. (3a) Early Ag stage with tetrahedrite-(Zn), tetrahedrite-(Fe), argentotetrahedrite-(Zn), tetrahedrite-(Cd), argentotetrahedrite-(Cd) and greenockite. (3b) Late Ag stage with pyrargyrite, polybasite, pearceite, cupropolybasite, cupropearceite, acanthite and galena. (4) Late Ag-Cu stage with bornite, stromeyerite, mckinstryite, chalcocite, digenite, covellite and uytenbogaardtite. Veins are rich in silver with an average Ag : Au ratio of 85 : 1; in some parts of the veins, Ag content reaches up to 1950 ppm and Au up to 42.7 ppm. The neutral to alkaline style of hydrothermal alteration is represented by K-feldspar, quartz, carbonates, smectite, interstratified illite/smectite, and chlorite (clinochlore, chamosite). This assemblage indicates a low sulfidation origin of the mineralization formed at 177-224 °C. The δ³⁴S values from the base-metal stage varies from +2.8 to +3.5 ‰ for chalcopyrite from the northern part of the Priečna vein, from +1.9 to +2.6 ‰ for galena, from +4.9 to +5.2 ‰ CDT for pyrite from the Anna vein. These values indicate a relatively homogeneous sulfur source, most likely related to an igneous or mixed igneous and host-rock source. Fluid inclusions in quartz associated with the base-metal stage have low salinity (1.1-1.6 wt. % NaCl eq.) and homogenization temperatures of 176-250 °C. The available data suggest that the base-metal stage was accompanied by cooling and dilution owing to a meteoric fluid. The temperature estimated from the tetrahedrite thermometer of the early Ag stage is ~170-205 °C. According to the paragenetic relationship and mineral stability, the deposition temperatures in the late Ag stage did not exceed 160 °C. The late Ag-Cu stage formed at temperatures of <93 °C. The results of K-Ar radiometric dating from the hydrothermal alteration returned an average age of 12.5±0.3 Ma. The studied mineralization is possibly related to the initial stage of resurgent horst tectonic activity and rhyolite volcanism of the fifth stage of Štiavnica stratovolcano formation.