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Karel Breiter and Radek Škoda

Abstract

We studied vertical changes in the chemical composition of zircon from two contrasting Variscan granite systems. The Beauvoir system (Massif Central, France) composed of three successive intrusions (B1, B2, B3) represents typical peraluminous S-type granite extremely enriched in P, F, Li, Rb, Cs, Be, Sn, Nb, Ta, and poor in Zr, Th, REE and Y. The Cínovec system (Krušné hory Mts/Erzgebirge, Czech Republic/Germany) composed of two successive intrusions (protolithionite granite, zinnwaldite granite) is only slightly peraluminous, P-poor, F, Li, Rb, Cs, U, Th, REE, Y, Sc, Sn, W, Nb, Ta-rich granite, which may be classified as A-type. In both localities, the most fractionated intrusions are located on the top of the system. Samples from borehole GPF-1 (Beauvoir) represent an 800 m long vertical section through the entire granite stock, while CS-1 borehole (Cínovec) reached a depth of 1600 m. Chemical compositions of zircons from both granite systems show distinct vertical zonality, but their shape and elemental speciation is highly contrasting. At Beauvoir, zircon shows a remarkable increase in Hf-content from 2-4 wt. % HfO2 (~0.03 apfu Hf) in the deepest B3-unit to 15-19 wt. % HfO2 (up to 0.18 apfu Hf) in the uppermost B1-unit. The highest contents of F, P, and U were detected in the intermediate unit B2 at a depth of 400-600 m. At Cínovec, Hf shows only moderate enrichment from ca. 2 wt. % HfO2 in the deeper protolithionite granite to 5-10 wt. % HfO2 in the uppermost part of the zinnwaldite granite. High contents of Th (3-8 wt. % ThO2) are entirely bound in the uppermost section of the granite copula to a depth of 200 m, but below this level the contents only sporadically exceed 1 wt. % ThO2. Concentrations of U, Y, HREE, Sc and Bi also reach their highest values in the uppermost parts of the zinnwaldite granite, but their decrease downward is much gentler. Extreme enrichment of outer zones of zircon crystals from some granites with Hf or high contents of Th, U, REE, Y, Nb and of some other elements in zircons from other localities is not considered to be a specific phenomenon characterizing melts of A- or S-type granite, but reflects a high degree of fractionation of systems rich in Na and F.

Open access

Viktor Goliáš, Gereltsetseg Tumurkhuu, Pavel Kohn, Ondřej Šálek, Jakub Plášil, Radek Škoda and Jan Soumar

Abstract

Significant uranium mineralization represented by a typical assemblage of uranyl supergene minerals in a quartz-uraninite vein hosted in the exocontact zone of the Variscan-Tanvald granite was found at a new construction site in the municipality of Jablonec n. Nisou. Activities of 222Rn in soil gas reached 1 MBq/m3 around two houses, with a maximum of 3.33 MBq/m3 between them on a uranium ore lens outcrop. The uranium content reaches up to 291 ppm eU (3595 Bq/kg 226Ra), and it is possible to find many ‘hot’ pieces of uranium ore fragments with a high percentage of uranium in the Quaternary cover in this place. This unfavourable situation is a result of an improper spatial planning process. The constructor was given the permission to construct the building even though the construction site did not meet safety requirements and the geological survey had failed. Not only geological prospecting was underestimated, but also the radon risk assessment was undervalued.

Open access

Arno Mücke, Zdeněk Dolníček, Bohuslav Fojt, Jana Hladíková, Marta Pudilová, Jaroslav Reif and Radek Škoda

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.