Bohuslav Fojt

The Horní Benešov ore deposit in the Devonian Šternberk-Horní Benešov Belt, Jeseníky Mts, Czech Republic. Part II: Fe-ore occurrences

Abstract The iron ore mineralizations at the Horní Benešov sulphidic deposit are located in the Devonian Šternberk-Horní Benešov Belt which is enveloped by the Lower Carboniferous flysch of the Culm Foreland Basin. The belt consists of metasedimentary and metavolcanic rocks. Horní Benešov is the only site in the Jeseníky Mountains, where the oxidic iron ores occur in close association with the sulphide orebodies of the VMS/SHMS type. The ores are composed of magnetite, Fe-silicates [stilpnomelane, often Ba-dominated, chamosite, berthierine and odinite (in the former literature described as greenalite)], carbonates (calcite, siderite, rhodochrosite and rarely dolomite and ankerite), sulphides (pyrite, sphalerite, galena, arsenopyrite, chalcopyrite, cobaltite and a mineral of the linneite-group with unusual composition), apatite, krauskopfite, barite and scheelite. Fluid inclusion and stable isotope data are in agreement with a low-temperature metamorphic/diagenetic reworking of the ores, which does not exceed 200- 250 °C. These low temperatures are also confirmed by the abundance of clastic material and fossils which both survived metamorphism, occurrence of stilpnomelane, diffusively zoned Fe-Mn carbonate crystals and remnants of undigested material in replacement textures. The associated fluids were lowsalinity (1-5 wt. % NaCl eq.), high-δ18O (+8 to +19 ‰ SMOW) aqueous solutions belonging to the Na-Mg-Cl salt system. The post-metamorphic fluid evolution involved the zero-salinity meteoric waters (with negative δ18O values) and high-salinity Ca-Na-Cl brines. The isotope data suggest participation of sulphur derived from the reduction of marine sulphate and carbon from organic matter during the formation of pyrite and carbonate, respectively. However, the latter originated predominantly from CO2-rich volcanogenic exhalations. The studied iron mineralizations are characterized by the absence of ooids, low abundance of hematite, presence of Mn/Fe carbonates and the absence of basic volcanites in the immediate contact with iron ores. The iron ores differ from the typical Lahn-Dill type and therefore, they may represent the iron-rich distal facies of hydrothermal vents which gave rise to the polymetallic sulphide + barite deposit at Horní Benešov. However, magnetite was formed by the replacement of primary siderite or is inferred to originate, at least partially, from precipitation of Fe-rich fluids.