2. Geological setting

The shear zone-bounded uranium deposits consist of peneconcordant lenses or highly disseminated uranium mineralisation evolved in brecciaed shear zones and/or in fractures. The host rocks of these deposits are strongly altered in aceites, exhibiting extensive albitisation, chloritisation and hematitisation of the host rock series. In the recent classification of metamor-

Figure 1. Simplified geological map of the Bohemian Massif with the most significant hydrothermal uranium deposits.

According to their mineral composition, the aceites are very similar to episyenites developed in uranium deposits of theMassif Central and the ArmoricanMassifin France, linked toleucogranitic plutons. Episyenites are defined as igneous-like rocks of syenite composition; displaying cavities produced by hydrothermal dissolution of quartz grains than can ultimately host uranium ore deposits [5]. Both rock types are products of low-temperature alkaline metasomatism associated with a significant input of Na2O and the loss of SiO2. Distinctly different mineral compositions have metasomatic deposits, which originated by high-temperature alkali metasomatism (e.g., Central Ukraine). Metasomatic facies in these uranium deposits include albitites, aegirinites and alkali-amphibole-rich rocks. In the recent international union of geological sciences (IUGS) classification of metamorphic rocks, these metasomatites are classified as fault-related metasomatites, which are common in the Precambrian shields [4]. The aim of this chapter is to present the detailed petrology,mineralogy and geochemistry of aceites and associated uraniummineralisation evolved

phic and metasomatic rocks, these metasomatic rocks are named as aceites [4].

50 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

in shear zone-hosted uranium ore deposits of the Bohemian Massif.

In high-grade metamorphic rocks of the Moldanubian Zone are evolved the Rožná, Olší, Okrouhlá Radouň, Zadní Chodov and Dyleň uranium deposits. The high-grade metamorphic rocks are represented by biotite paragneisses with intercalations of amphibolites, calc-silicate rocks, marbles and lenses of partly serpentinised ultrabasic rocks (dunites).

The Vítkov II and Lhota uranium deposits occur in granitoids of the Bor pluton. An N-S trending, 35-km long magmatic body forms the Bor pluton, which is emplaced in the fault zone, which is a part of regional West Bohemian shear zone. The most voluminous rocks in the Bor pluton are coarse-grained biotite, usually porphyritic biotite granites. In the northern block, older amphibole-biotite granodiorites, tonalites and quartz diorites were also observed. Dykes of two-mica monzogranites, aplites, which predominates in the southern part of the Bor pluton, fill N-S faults. (Figure 2) [6].

Figure 2. Geological map of the Bor pluton, modified from [6].

#### 2.1. Rožná uranium deposit

The Rožná uranium deposit occurs in the uppermost Gföhl unit of the Moldanubian Zone. The host rocks of this ore deposit are formed predominantly by biotite paragneisses, amphibolites and small bodies of calc-silicate rocks, marbles, serpentinites and pyroxenites. The uranium mineralisation is bounded on the longitudinal shear zones (R1–R4). The main part of disseminated uranium mineralisation hosts the Rožná 1 (R1) and Rožná 4 (R4) shear zones. In the less strongly mineralised Rožná 2 (R2) and Rožná 3 (R3) shear zones, numerous pinnate carbonate veins occur. Longitudinal shear zones are segmented by NW-SE and SW-NE faults, which host post-uranium carbonate-quartz-sulphide mineralisation (Figure 3) [1].

a grade 0.1–0.15% U, exceptionally 0.3% U. Disseminated U-mineralisation bound to oblique fault zones is usually hosted by quartz-carbonate-sulphide breccias at intersections with diagonal and longitudinal structures. Compared to other types of mineralisation, the ore bodies are small but contain relatively high-grade ore of average grade 0.8 and up to 20% U in some ore shoots. Total mine production of the Rožná ore deposit was 22,220 tons U with average grade of 0.24% U, mined from 1957 to April 2017 [7]. The Rožná uranium deposit was the last mined

Shear Zone-Hosted Uranium Deposits of the Bohemian Massif (Central European Variscan Belt)

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The Olší uranium deposit is also evolved in the NE part of the Moldanubian Zone. The host rocks of this uranium deposit consist mainly of plagioclase-biotite and amphibole-bearing paragneisses with small lenses of K-rich granitoids and serpentinites. Longitudinal N-S and NNW-SSE striking ductile shear zones (O1, O2 and O3) dip to W at an angle of 45–70. Longitudinal shear zones are crosscut by ductile to brittle NW-SE and SW-NE striking fault zones that host lenses of contrasted uranium mineralisation. The main uranium ore bodies are evolved on the N-S longitudinal shear zones. The Variscan calcite-chlorite-uraninite association and the Mesozoic albite-chlorite-coffinite association represent the uranium mineralisation. Total mine production of the Olší ore deposit

The Okrouhlá Radouň uranium deposit is situated in a NNW-SSE striking shear zone on the NE margin of the Klenov pluton, which is a part of the Moldanubian plutonic complex. The host rock series comprises partly migmatised biotite paragneisses and sillimanite-biotite paragneisses of the Moldanubian Zone and two-mica leucogranites of the Klenov pluton. Granites that formed a series of the NE-SW to NNE-SSW elongated sheets or larger irregular bodies with sheeted margins intruded into the high-grade metasediments. The sheets are

The most significant mineralised structure in this area, the Radouň shear zone, was explored along a strike length of approximately 3 km and to a depth of 650 m. The highest grade uranium mineralisation was developed at depths of 250–400 m beneath the present surface. The thickness of the mineralised shear zone is highly variable, ranking from 30 cm to approximately 7 m. The thickest portion of this zone was observed in the southern part of the uranium deposit, where a shear zone was developed in altered two-mica leucogranites and in highly hydrothermally altered, partly migmatised biotite paragneisses. The shear zone is infilled with cataclasites formed by host rock breccias, which were altered to clay-mineral-rich and chlorite-rich assemblages containing a disseminated uranium mineralisation comprising mainly coffinite and lesser amounts of uraninite. Total mine production of the Okrouhlá Radouň uranium deposit was

The Zadní Chodov uranium deposit, which is located in the northern tectonic block of the Bor pluton, was developed by mine workings down to a level of 1250 m with a length of over 2.5 km. The host rock series are formed by migmatised biotite paragneisses of the Moldanubian Varied

was 2922.2 tons U with average grade of 0.10% U, mined from 1959 to 1989 [7].

1339.5 tons U with average grade of 0.084% U, mined from 1972 to 1990 [7].

uranium deposit in the Central Europe.

2.3. Okrouhlá Radouň uranium deposit

2.4. Zadní Chodov uranium deposit

mostly parallel to the foliation in the metasediments.

2.2. Olší uranium deposit

Uranium mineralisation could be divided into (1) disseminated coffinite > uraninite > U-Zrmineralisation evolved in altered rocks (aceites) of the mineralised shear zones (R1–R4), (2) uraninite > coffinite mineralisation in carbonate veins, (3) disseminated coffinite > uraninite in aceites adjacent to main shear zones (R1, R4) and (4) mostly coffinite mineralisation bound to the intersection of shear zones with an younger NW-SE and SW-NE faults. Ore lenses of disseminated uranium mineralisation in fault zones R1 and R4 are 3.5-m thick on average, ore grade is around 0.5% U, up to 10% U locally. Ore bodies in ore zones R2 and R3 host a large number of carbonate veins up to 2-m thick with U-mineralisation of the average grade 0.6% U. Ore bodies in aceites with predominance of coffinite on uraninite and U-Zr silicate have U-mineralisation of

Figure 3. Schematic cross section of the Rožná uranium deposit, modified from [1].

a grade 0.1–0.15% U, exceptionally 0.3% U. Disseminated U-mineralisation bound to oblique fault zones is usually hosted by quartz-carbonate-sulphide breccias at intersections with diagonal and longitudinal structures. Compared to other types of mineralisation, the ore bodies are small but contain relatively high-grade ore of average grade 0.8 and up to 20% U in some ore shoots. Total mine production of the Rožná ore deposit was 22,220 tons U with average grade of 0.24% U, mined from 1957 to April 2017 [7]. The Rožná uranium deposit was the last mined uranium deposit in the Central Europe.
