5.1. Origin of aceites

The differences in mineralogical composition of aceites in the studied shear zone-hosted uranium deposits from the Bohemian Massif are expressed by different composition of original host rock series (high-grade metasediments vs. granitic rocks) and different tectonic movements on shear zones of individual ore deposits. For altered metasediments (Rožná and Zadní Chodov), high concentrations of chlorite I and clay minerals (illite, kaolinite and smectite) as fillings of shear zones are significant. The clay minerals filling in these uranium deposits differ in composition of the assemblage of these minerals. The Fe-illite predominates at the Rožná ore deposit and in the shear zones at Zadní Chodov chlorite predominates over illite. For aceites evolved in altered granitic rocks (Okrouhlá Radouň, Vítkov II and Lhota), the rock matrix composed of chlorite I, albite I and hematite framework is characteristic.

Uranium in the host high-grade metasediments and granitic rocks of all investigated uranium deposits is essentially hosted in monazite and zircon, in leucocratic granites from the Okrouhlá Radouň ore deposit, and also in xenotime. In barren aceites, monazite and xenotime are usually missing and zircon is often highly altered. Therefore, the source of uranium may be found in the decomposition of uranium-bearing accessories, as is also proposed for the inconformity-type uranium deposits in Canada [14]. The titanium necessary for the origin of brannerite was probably released during chloritisation of Ti-enriched biotite and hydrothermal alteration of the Ti-rich accessory minerals (titanite and allanite).

A prominent hematitisation occurred in aceites from the Okrouhlá Radouň, Vítkov II and Lhota uranium deposits, and deeper parts of the Rožná uranium deposit indicates deep infiltration of oxidised, surface-derived fluids to the crystalline basement during the pre-ore stage. The deep circulation of fluids gave rise to desilification, hematitisation and albitisation of host rock complexes along shear zones. The fluids responsible for origin of aceites differ from earlier low-salinity metamorphic fluids in their generally higher but highly variable salinities (0–25 wt.% NaCleq.). Differences in the salinity of these fluids probably reflect the mixing of chemically heterogeneous basinal brines with meteoric water [1, 15].

#### 5.2. Behaviour of REE, Zr and Y in aceites

Brannerite was found in the Rožná uranium deposit and in uranium deposits from the Bor pluton (Zadní Chodov, Dyleň and Lhota). In mineralized aceites brannerite occurs in form of acicular aggregates and/or irregular grains. Larger brannerite grains are usually

Figure 10. BSE image of brannerite (Brn) and altered brannerite (Abrn) enclosed in finely grained chlorite (Chl) from the

Zadní Chodov uranium deposit.

Figure 9. Chemical composition of coffinite and uraninite from the Rožná uranium deposit (wt.%).

60 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

Rare earth elements, Zr and Y, are usually considered as the immobile elements by hydrothermal alteration of host rock series [16]. However, hydrothermal experiments and some mineralogical research of nature rock series have demonstrated that these elements could be mobile during hydrothermal alterations, especially if the fluids contained strong complexation agents (e.g., fluoride or phosphate anions) [17–19]. The mobility of REE, especially HREE in aceites in the all studied uranium deposits, is suggested by enrichment of HREE in aceites from Rožná, Okrouhlá Radouň and Lhota uranium deposits. The mobility of Zr and Y in the studied aceites is suggested by the occurrence of Zr- and Y-enriched coffinite from the Rožná, Okrouhlá Radouň and Lhota uranium deposits.

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Enrichment in HREE during origin of uranium deposits in shear zone-hosted uranium mineralisation was found in unconformity uranium deposits from Australia and Canada [20– 24]. Coffinites enriched in Zr and Y were found only in the uranium sedimentary deposits in New Mexico, United States [25] and in the natural fission reactor environment of the Oklo uranium deposit, Gabon [26].
