Diversity of fluids in the formation of ore assemblages in the Bohemian Massif (Czechoslovakia)

Abstract

Fluid inclusions in minerals from various ore assemblages in the Czech part of the Bohemian Massif were studied by optical microthermometry. Tin and tungsten vein and greisen deposits related to the early hydrothermal post-magmatic activity of Variscan granitic plutons were formed at temperatures between 300 and 500 °C from chloride solutions, NaCl being the main component. The presence of two unmixing fluids (low salinity ~ 5 wt.% NaCl equiv. and brines 35-40 wt.% NaCl) is typical. CO2 is present only in neglibible amounts (a few mol. %). Hydrothermal Pb-Zn deposits and fluorite-barite veins (without any relationship with magmatism) originated from fluids of moderate to high salinity (9-25 wt.% NaCl equiv.) and complex composition (Na, Ca, Mg salts), which did not contain CO2. Fluorite deposits were formed in the temperature range 110-150 °C, whereas polymetallic ore veins were formed at 100-280 °C. The metamorphosed ores (massive sulphide and gold deposits) contain fluids characterized by the presence of H2O, CO2 and CH4. A high density of CO2 is common (500-960 kg/m3), and corresponding with the metamorphic conditions. A change in the fluid composition with metamorphic grade is evident. As grade decreases, the amount of CO2 falls, methane appears and the amount of water increases. The salinity of fluids varied from 5 to 20 wt.% NaCl equiv. (NaCl predominates).