Evolution of the epithermal mineralization in the Pukanec deposit, Štiavnické vrchy Mts. (Slovakia)

The Pukanec deposit belongs to the historically significant ore field of epithermal Au-Ag mineralization in the Middle Miocene Štiavnica andesite stratovolcano (Western Carpathians, Slovakia). Up to now, the Ag and Au carriers were not properly identified so far. In this paper, we present mineralogical, paragenetic, and geochemical aspects of the ore mineralization and fluid properties for the Pukanec deposit. Five subsequent mineralization stages were recognized, comprising (1) pyrite stage with quartz, K-feldspar, pyrite, and Au-Ag alloys, (2) base-metal stage with pyrite, Au/Ag alloys, johanssenite, rhodonite, calcite, rhodochrosite, galena, sphalerite, chalcopyrite, hessite, and cervelleite, (3) main Ag-bearing stage with pearceite, polybasite and acanthite, (4) late Ag-Cu stage with acanthite, jalpaite, stromeyerite, mckinstryite, and uytenbogaardtite, and (5) supergene stage with acanthite, aurorite, chalcophanite, pyrolusite, hydrohetaerolite, coronadite, and todorokite. K-Ar radiometric dating of K-feldspars from the hydrothermal mineralization yielded an age of 11.66 ± 0.56 Ma (2σ). Results of fluid inclusion microthermometry indicate that ore-forming fluids in the Pukanec Au-Ag deposit underwent phase separation (boiling) at depth, serving as the principal mechanism for early base-metal precipitation. In contrast, fluid mixing prevailed as the dominant precipitation process of later precious metals in shallower levels. Fluids from the central zone exhibit elevated salinities and homogenization temperatures relative to those in marginal zones, signifying a lateral decrease of salinity from the deposit center outward. Boiling represents a key precipitation trigger in the central zone, particularly within shallow, calcite-hosted, base metal-enriched assemblages. The upper zone, characterized by quartz-chalcedony-hosted precious metal (Au-Ag) mineralization, formed primarily through mixing and conductive cooling of fluids. According to average temperatures of various veins the base-metal stage was formed in the range 249–227 °C followed by precious metal stage in the range 239–208 °C.