Mobilization of ore fluids during Alpine metamorphism: evidence from hydrothermal veins in the Variscan basement of Western Carpathians, Slovakia

摘要

Hydrothermal polymetallic veins of the Gemeric unit of the Western Carpathians are oriented coherently with the foliation of their low-grade Variscan basement host. Early siderite precipitated from homogeneous NaCl-KCl-CaCI_2-H_2O brines with minor CO_2, while immiscible gas-brine mixtures are indicative of the superimposed barite, quartz-tourmaline and quartz-sulphide stages. The high-salinity aqueous fluid (18-35 wt%) found in all mineralization stages corresponds to formation water modified by interaction with crystalline basement rocks at temperatures between 140 and 300℃. High brominity (around 1000 ppm in average) resulted from evaporation and anhydrite precipitation in a Permo-Triassic marine basin, and from secondary enrichment by dissolution of organic matter in the marine sediments at diagenetic temperatures. Sulphate depletion reflects thermogenic reduction during infiltration of the formation waters into the Variscan crystalline basement. Crystallization temperatures of the siderite fill (140-300℃) and oxygen isotope ratios of the parental fluids (4-10‰) increase towards the centre of the Gemeric cleavage fan, probably as a consequence of decreasing water/rock ratios in rock-buffered hydro-thermal systems operating during the initial stages of vein evolution. In contrast, buoyant gas-water mixtures, variable salinities and strongly fluctuating P-T parameters in the successive mineralization stages reflect transition from a closed to an open hydrothermal system and mixing of fluids from various sources. Depths of burial were 6-14 km (1.7-4.4 kbar, in a predominantly lithostatic fluid regime) during the siderite and barite sub-stages of the north-Gemeric veins, and up to 16 km (1.6-4.5 kbar, in a hydrostatic to lithostatic fluid regime) in the quartz-tourmaline stage of the south-Gemeric veins. The fluid pressure decreased down to approximately 0.6 kbar during crystallization of sulphides. U-Pb-Th, ~(40)Ar/~(39)Ar and K/Ar geochronology applied to hydrothermal muscovite-phengite and monazite, as well as cleavage phyllosilicates in the adjacent basement rocks and deformed Permian conglomerates corroborated the opening of hydrothermal veins during Lower Cretaceous thrusting and their rejuvenation during Late Cretaceous sinistral transpressive shearing and extension.