Low-grade Variscan basement of the Gemeric tectonic unit of the Western Carpathians contains hydrothermal veins distributed parallel to regional Cretaceous cleavage fabrics. The vein mineralization formed during successive fuchsite, siderite-barite, quartz-tourmaline, quartz-sulphide, and cinnabar stages divided by inter-mineralization tectonics.
Medium salinity (17-25 wt% total), high density NaCl-KCl-CaCl2-H2O inclusions in early siderite were trapped at temperatures between 150-300°C. A wider range of salinities (5-35 wt%, relative to H2O) was detected in minerals of the quartz-tourmaline and quartz-sulphide stages. High density gases CO2-rich (±N2, CH4) and essentially pure N2, fluxed the hydrothermal system during crustallization of barite and the superimposed stages.
Siderite crystallized 6-12 km deep at thermal gradients between 20 and 40°C/km, as constrained by fluid inclusion isochores and vertical oxygen isotope gradients. Greater depths (possibly up to 18 km) and lower thermal gradients (12-20 °C/km) during the superimposed stages indicate progressive crustal thickening during the vein evolution. Oxygen isotope ratios in the ore-forming fluids (4-11 ‰ V-SMOW) reflect various fluid/rock ratios in rock-buffered hydrothermal systems. U-Pb-Th, 40Ar/39Ar, and K/Ar geochronology corroborated crystallisation coincidental with the Lower Cretaceous thrusting, and a Late Cretaceous sinistral transpressive shearing and extension.