Multistage hydrothermal vein mineralization in low-grade metamorphosed rocks: Chric locality, Tepla-Barrandian Unit, Bohemian Massif, Czech Republic

A small and isolated hydrothermal vein-type Sb deposit is hosted by a lamprophyre dike which penetrated Neoproterozoic to early Cambrian volcanosedimentary successions near Chric in the Tepla-Barrandian Unit, Czech Republic. Hydrothermal minerals were formed in several consecutive mineralization stages, separated by long pauses in hydrothermal fluid flow. Early Stage I is characterized by the formation of carbonates of dolomite-ankerite series (Carb I) and arsenopyrite with an imprecise Re-Os age of similar to 570 to 510 Ma. Stage I was followed by tectonic movements and brecciation. The second mineralization stage (Stage II) is characterized by the precipitation of quartz (Qtz I) and Sb minerals (predominantly stibnite) while the third stage (Stage III) is marked by the presence of carbonates of dolomite-ankerite series (Carb II) and quartz (Qtz II) along with Cu, Zn, Pb, Ag and Fe sulfides. Lead isotope data of galena indicate a Variscan age of Stages II-III. These stages were followed by precipitation of another generation of carbonate of dolomite-ankerite series (Carb III, Stage IV), quartz (Qtz III, Stage V) and formation of Sb supergene minerals. The earlier reported occurrence of gold was not confirmed in the studied rock samples but three types of gold particles of variable fineness, with one of them indicating a local origin, were found in the heavy mineral concentrate from a local creek. Elevated Au concentrations were found in arsenopyrite and pyrite. The ore-forming hydrothermal fluid of the mineralization stage III was of H-2 O-CO2 type with minor CH4 admixture, medium salinity, and temperatures in the range of 258-350 degrees C with an observed temperature decrease from Stage II to Stage V. Oxygen isotope data of Carb II correspond to high delta O-18(fluid) values (similar to +7.5 parts per thousand VSMOW) with a decrease in the fluid delta O-18 to later stages. An important part of the carbonate carbon was derived from decomposition of organic matter or its high-temperature reaction with hydrothermal fluid. Considering the wide variation in delta S-34 values of sulfides and elevated Sb contents in the TBU black shales, we anticipate that the origin of the Chric mineralization is closely associated with a metal and carbon-sulfur remobilization from the TBU volcanosedimentary successions in response to Cadomian and Variscan tectonothermal events.