Regional low-temperature fluid flow indicated by quartz mineralization in Silesicum, NE Bohemian massif

The crystalline rocks of the Silesicum unit of the Bohemian Massif host two principal types of hydrothermal quartz veins. Veins associated with granitoids are primarily quartz with wollastonite and epidote and accompanied by hydrothermal alteration of host rocks whereas others are of the more complex “Alpine-type”. L + V + S (liquid + vapour + solid phase) fluid inclusions within quartz crystals contain muscovite and ± calcite (± haematite) as solid phases and homogenisation temperatures are between 124 and 176 °C. Trapped fluids are uniformly of the H2O–NaCl–CaCl2 ± MgCl2 ± KCl system with a salinity between 9.1 and 26.8 mass % (NaCl + CaCl2). The variable Na/Ca ratios result from different intensity of fluid-rock reactions. The ubiquitous muscovite is a product of fluid-rock interactions. Oxygen isotope evidence indicates that the fluids were a mixture of meteoric and marine waters in deep regional-scale convection systems. The studied type of fluids represents a post-Variscan hydrothermal system and shows the extent of migration pathways in the upper crust on the north-eastern edge of the Bohemian Massif.