Acta Musei Moraviae

Paragenesis of the Zn–Pb ore deposits in the wider neighbourhood of Horní Město village near Rýmařov in Jeseníky Mts. (Czech Republic)

The ~7 km long mineralized zone of the Horní Město ore district is situated in southern section of the Vrbno Group. This Devonian volcano-sedimentary sequence is a part of Rhenohercynian Zone at the eastern margin of the Bohemian Massif. The stratiform sulphidic ores form lenticular bodies embedded in the so called “Horní Město volcanic complex” (URBÁNEK and VALENTA 1989). The host rocks are bimodal volcanites, which are characterized by a distinct predominance of trachytes, less rhyolites, and their pyroclastics. Basic volcanites (containing small iron ore deposits) are already situated out of the mineralized zone in question. The psammitic and psefitic sediments are rare. The whole area was subject of several deformations and metamorphic events during the Variscan orogeny. The metamorphic overprint achieved the lower greenschist facies, namely its chlorite zone. The least metamorphosed rocks are typically trachytes. The geotectonic setting of the studied area is still under debate, but the most recent detailed petrological investigations classify the igneous rocks as “alkaline rock suite derived by back-arc rifting” (JANOUŠEK et al. 2006).

The well elongated lenticular ore bodies contain sulphide minerals, whose distribution may be characterized as irregularly disseminated, laminated, or massiv. Pyrite clearly predominates in all mentioned textural types, sphalerite and galena are less frequent, chalcopyrite is only accessory. The ore locally (mainly in central part of the district) contains relics of original pre-metamorphic structures (collomorphic pyrite, globular aggregates of sphalerite hosted by galena). The metamorphic recrystallization produced granoblastic textures with “triple junction” arrangement of sphalerite and (rarely) pyrite. Moreover, synmetamorphic crystalization of muscovite, albite and Ba-feldspars took place. Post-metamorphic veinlets are the youngest, they composed of silver minerals (proustite, pyrargyrite, stefanite, Sb-pearceite, native silver), jordanite and boulangerite, which are accompanied by remobilized common Fe–Pb–Zn sulphides (FOJT et al. 1992). The metamorphic recrystallization and remobilization of ore is mirrored also in chemical composition of reprecipitated sulphides: the pyrite contains more Ni, Co and Au, the sphalerites are depleted in Fe and Cd and the galenas show elevated content of Ag and Sb. The isotope composition of ore lead clusters at the isochron 0.4 Ga in field between orogenic lead and upper crustal lead.

The gangue is formed mainly by quartz, less calcite, albite, rarely Ba-feldspars, fluorite and cymrite. The veinlets and lenticular metamorphic segregations contain except calcite also carbonates of dolomite group. The lens-like barite bodies are concentrated especially in both flanks of the ore zone. The strontium isotope ratios found in barites (⁸⁷Sr/⁸⁶Sr = 0.7065 to 0.7073) are similar to those of the Middle Devonian seawater. Some parts of fine-grained aggregates of quartz with negligible content of sulphidic minerals are easily comparable with the so called “siliceous rocks” from the Horní Benešov deposit. It is possible that this rock represents type “chert” accompanied with VMS deposits.

The fluid inclusion and stable isotope studies allowed to characterize both the nature of fluids and the formation conditions of individual genetic types of mineralization. Samples from stratiform mineralization exhibit the same parameters of fluid inclusions as mineral phases sampled from metamorphogenic lenslike segregations. Therefore, the fluid inclusion study brought information on metamorphic overprint of stratiform mineralization, not on conditions of its primary precipitation. Two main types of “peak” metamorphic fluids were identified: 1) CO₂–H₂O–NaCl fluids with low salinity (5–7 wt. % NaCl in aqueous solution), low content of CO₂ (2–7 mol. %) and homogenization temperatures ca. 200–300 °C and 2) NaCl–MgCl₂–H₂O fluids with low salinity (5–7 wt. % NaCl eq.) and homogenization temperatures 120–220 °C. The position of isochores of both fluid types in PT-space agrees with predicted PT-estimate of Variscan metamorphic overprint in this area (RENÉ, ŠREIN 2001: 350–400 °C, 4–5 kbars). The calculated positive fluid δ¹⁸O values (+6 to +10 ‰ SMOW) are compatible with the metamorphogenic origin of fluids accompanied with both the recrystalization of stratiform mineralization and the crystalization of minerals from metamorphogenic segregation lenses. The sulphur isotopes probably represent the pre-metamorphic situation. A wide variability of δ³⁴S values of sulphidic sulphur (~32 ‰) is most probably the result of involvement of several sources of sulphur. The origin of isotopically heavy barite sulphur may be found in Devonian marine sulphate.

Samples from younger coarse-grained assemblages and post-metamorphic veinlets showed the clearly different fluid inclusions and stable isotope parameters. Their characteristic features include low homogenization temperatures (90–150 °C), high salinities (21–24 wt. % salts) and Ca–Na–Cl fluid composition. The most inclusions contain fluids rich in NaCl, however, examples with prevailing CaCl₂ were also documented. The sulphide δ³⁴S values exhibit similar scatter like stratiform sulphides which may indicate remobilization of reduced sulphur from pre-existing environment into younger parageneses. A similarity of temperatures derived from sulphur isotope thermometry and homogenization temperatures of fluid inclusions suggest a low-pressure formation conditions. The calculated fluid δ¹⁸O values around 0 ‰ SMOW indicate that the surface waters (meteoric or marine?) were predominating in the hydrothermal system. The ultimate genesis and timing of younger parageneses remain unresolved, but their fluid inclusion and stable isotope parameters are rather comparable to the post-Variscan brines that participated during formation of many types of barren and ore vein-type mineralizations along the whole eastern margin of the Bohemian massif than to pure “metamorphogenic” fluids.

The stratiform polymetallic ore bodies in the Horní Město area are the product of hydrothermal aktivity associated with volcano-sedimentary processes and superimposed weak metamorphic overprint. The ore accumulations may be classified as “distal” VMS (volcanic-associated massive sulfide) deposit, although the massive ores are only subordinate. The mentioned “ore dilution” may be explained by a combination of hydrothermal activity producing sulphide precipitates and deposition of volcanoclastic and/or clastic sedimentary material at the same site and time (cf. HLADÍKOVÁ and KŘÍBEK 1988, HAVELKA 1992). Moreover, the superimposed metamorphic reworking and (to lesser extent) post-metamorphic hydrothermal processes may further contribute to the dispersion of initially cummulated sulphidic mineralization.