Acta Musei Moraviae

Ore minerals from the dolomitic veins in the serpentinite rocks of the Letovice crystalline complex (Czech Republic)

Minerals found in dolomite veins cutting serpentine rocks around Letovice and its surroundings composed remarkably heterogeneous set which is different from any other locality in the Bohemian Massif. The composition of copper minerals (bornite, chalcocite, covellite, chalcopyrite), millerite, dolomite veins and magnetite is similar to the composition of their host ultrabasic rocks. Lead and bismuth (galena, native Bi) are considered in this environment as atypical. Parkerite and wittichenite are transitional – they consist of elements of both groups. Despite being impossible to determine detailed order of crystallization, it was implied by microscopic studies that sulphides Pb–Bi(Cu) are relatively older than copper minerals. Fe–Cr spinelides that are rarely found in dolomite veins on the outline of serpentine rock are possibly relicts of premetamorphic magmatic stage. The formation of “ribbon-like” pure magnetite in the immediate vicinity of sulphides is probably related to redeposition processes of older stages in process of hydrothermal alteration. Chondrite normalized REE pattern of dolomite shows positive Eu anomaly and MREE depletion.

The isotopic composition of sulphur of hydrothermal fluids (δ³⁴S = +4 to +5 ‰ CDT) indicates that sulphur was not derived from meta-ultrabasic rocks. Calculated δ¹⁸O value of hydrothermal fluids is -1,9 to +0,9 ‰ SMOW (temperatures 188–238 °C, i.e. temperatures of homogenisation of primary fluid inclusions); calculated δ¹³C value of hydrothermal fluids is -5 to -6 ‰ PDB. On the formation of Pb–Bi minerals the temperature could be higher (approx. 250 to 300 °C). Secondary fluid inclusions in dolomite contain methane, primary fluid inclusions are relatively high in salinity (17.0 to 20.9 wt.% NaCl equiv.). Such parameters correspond mainly to brine derived from a sedimentary basin. The substance of metals was probably extracted during migration of solutions from its surroundings.