4. Results

#### 4.1. Petrography of altered rocks

In altered high-grade metasediments and granitic rocks of above-mentioned uranium deposits (Rožná, Okrouhlá Radouň, Zadní Chodov, Lhota and Vítkov II), four major stages of hydrothermal alteration can be distinguished, namely pre-ore, ore and two post ore-ore stages. During pre-ore alteration, when main part aceites originated, original biotite from biotite paragneisses (Rožná, Okrouhlá Radouň and Zadní Chodov) and two-mica leucogranites (Okrouhlá Radouň) and/or biotite granites (Vítkov II and Lhota) were altered to chlorite I enriched in Fe. Transformation of biotite was sometimes accompanied by origin of rutile. Original plagioclases were altered into albite I (An0–9). Albitisation is sometimes accompanied by K-feldspatisation, which was found at the Vítkov II uranium deposit in highly altered parts of original biotite granites. The albitisation and K-feldspatisation precede the quartz removal. The transitional zones between unaltered and altered high-grade metasediments and granitic rocks are usually gradational, spanning a few tens of centimetres to 1 m. Commonly, the transitional zone displays a weak red colouring due to the presence of fine-grained hematite laths distributed irregularly in originally albitised plagioclase (albite I). Hydrothermally altered rocks have medium porosities due to the hydrothermal leaching of original quartz (typically 10–15 vol.%). In highly altered high-grade metasediments and granitic rocks, the authigenic generations of albite II occur as epitactic overgrowths on pseudomorphs of albite II. The voids resulted through leaching of quartz were later filled by younger generations of albite (albite III) and chlorite (chlorite III). The newly originated albites II and III have near-endmember composition (An0–0.8). The authigenic chlorites II and III are Mg-enriched (chlorite II Fe/Fe + Mg = 0.12–0.54, chlorite III Fe/Fe + Mg = 0.47–0.50). However, the original metamorphic and/or magmatic textures in the altered high-grade metasediments and/or granitic rocks are usually preserved.

During the ore stage, chlorite II, albite II, III, apatite and uranium minerals (uraninite, coffinite, brannerite) were originated. Uranium mineralisation usually comprises three different morphologic-mineralogical types. The highly altered granitoids of the Okrouhlá Radouň and Vítkov II ore deposits are marked by metasomatic coffinite and/or coffinite-uranium mineralisation. The metasomatic mineralisation is usually coupled with highly intensive albitisation and carbonatisation of granitic rocks. The lenticular-shaped uraninite and uraninite-coffinite mineralisation (Vítkov II and Lhota in the Bor pluton) occurs usually on boundary of granitic rocks with metamorphites. The disseminated uranium mineralisation occurs in the xenoliths of metamorphic rocks (Lhota) and in mineralised shear zones (Rožná, Zadní Chodov and Dyleň). In these uranium deposits, coffinite and brannerite occur predominantly in highly chloritised metamorphites. The suitable sources of Ti in brannerites were probably altered high-grade metasediments and/or amphibolites.

Albite and carbonates are the main constituents of the aceites formed through hydrothermal alteration of granites and high-grade metasediments and occupy 65–85 vol.% of the bulk rocks. The quartz post-ore stage is characterised by filling of voids, created by removal of magmatic and/or metamorphic quartz, by quartz II, origin of quartz veinlets (quartz III), veinlets of chlorite III and origin of younger hematite laths (hematite II).

The carbonate bearing post-ore stage is connected with the origin of calcite and relatively rarely sulphides, selenides and zeolites. Carbonates fill cavities in the altered rocks and/or form fine veinlets in highly altered granitic rocks. Occasionally, dolomite and siderite were found.

#### 4.2. Geochemistry of altered rocks

The decomposition of the rock samples for ICP-MS analysis involved lithium metaborate/

In altered high-grade metasediments and granitic rocks of above-mentioned uranium deposits (Rožná, Okrouhlá Radouň, Zadní Chodov, Lhota and Vítkov II), four major stages of hydrothermal alteration can be distinguished, namely pre-ore, ore and two post ore-ore stages. During pre-ore alteration, when main part aceites originated, original biotite from biotite paragneisses (Rožná, Okrouhlá Radouň and Zadní Chodov) and two-mica leucogranites (Okrouhlá Radouň) and/or biotite granites (Vítkov II and Lhota) were altered to chlorite I enriched in Fe. Transformation of biotite was sometimes accompanied by origin of rutile. Original plagioclases were altered into albite I (An0–9). Albitisation is sometimes accompanied by K-feldspatisation, which was found at the Vítkov II uranium deposit in highly altered parts of original biotite granites. The albitisation and K-feldspatisation precede the quartz removal. The transitional zones between unaltered and altered high-grade metasediments and granitic rocks are usually gradational, spanning a few tens of centimetres to 1 m. Commonly, the transitional zone displays a weak red colouring due to the presence of fine-grained hematite laths distributed irregularly in originally albitised plagioclase (albite I). Hydrothermally altered rocks have medium porosities due to the hydrothermal leaching of original quartz (typically 10–15 vol.%). In highly altered high-grade metasediments and granitic rocks, the authigenic generations of albite II occur as epitactic overgrowths on pseudomorphs of albite II. The voids resulted through leaching of quartz were later filled by younger generations of albite (albite III) and chlorite (chlorite III). The newly originated albites II and III have near-endmember composition (An0–0.8). The authigenic chlorites II and III are Mg-enriched (chlorite II Fe/Fe + Mg = 0.12–0.54, chlorite III Fe/Fe + Mg = 0.47–0.50). However, the original metamorphic and/or magmatic textures in the altered high-grade metasediments and/or granitic rocks are

During the ore stage, chlorite II, albite II, III, apatite and uranium minerals (uraninite, coffinite, brannerite) were originated. Uranium mineralisation usually comprises three different morphologic-mineralogical types. The highly altered granitoids of the Okrouhlá Radouň and Vítkov II ore deposits are marked by metasomatic coffinite and/or coffinite-uranium mineralisation. The metasomatic mineralisation is usually coupled with highly intensive albitisation and carbonatisation of granitic rocks. The lenticular-shaped uraninite and uraninite-coffinite mineralisation (Vítkov II and Lhota in the Bor pluton) occurs usually on boundary of granitic rocks with metamorphites. The disseminated uranium mineralisation occurs in the xenoliths of metamorphic rocks (Lhota) and in mineralised shear zones (Rožná, Zadní Chodov and Dyleň). In these uranium deposits, coffinite and brannerite occur predominantly in highly chloritised metamorphites. The suitable sources of Ti in brannerites were

probably altered high-grade metasediments and/or amphibolites.

tetraborate fusion.

usually preserved.

4.1. Petrography of altered rocks

56 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

4. Results

In previous papers about shear zone-hosted uranium deposits in the Bohemian Massif [1, 10, 11], chemical composition of unaltered and altered host rocks was described in detail. Also, in those papers, detailed investigations of losses and gains during hydrothermal alteration of host rock series were performed using isocon method [12]. This chapter discusses about geochemistry of unaltered and altered rocks series concentrated on behaviour of selected trace elements, especially REE, Y and Zr.

The chloritised high-grade metasediments from the Rožná and Okrouhlá Radouň uranium deposits without uranium mineralisation are depleted in REE. This depletion is also displayed by lower ΣREE (Rožná 69–98 ppm, Okrouhlá Radouň 106–196 ppm) and high LREE/HREE ratios (4.0–17.6) relative to the unaltered metasediments. In contrast to chloritised high-grade metasediments without uranium mineralisation, mineralised metasediments from the Rožná and Zadní Chodov uranium deposits are enriched in REE (ΣREE = 108–390 ppm), especially in HREE (LREE/HREE 1.2–4.7) (Figures 5 and 6).

Figure 5. REE patterns of the high-grade metasediments and their hydrothermally altered equivalents from the Rožná uranium deposit. Original data normalised to chondrite according to [13].

Figure 6. REE patterns of the high-grade metasediments and their hydrothermally altered equivalents from the Zadní Chodov uranium deposit. Original data normalised to chondrite according to [13].

The behaviour of Y and Zr in mineralised aceites from shear zone-hosted uranium deposits in the Bohemian Massif is variable. Yttrium is enriched in mineralised aceites from the Rožná and Okrouhlá Radouň ore deposits and its behaviour is close to behaviour of HREE in these rocks. Yttrium in these rocks occurs usually in coffinite (up to 3.4 wt.% Y2O3). In altered biotite granites from the Bor pluton, the concentrations of Y are similar to their concentrations in unaltered granitic rocks (Figure 7).

The concentrations of Zr in unaltered and altered rocks from all above-mentioned uranium deposits are similar. In unaltered host rocks from these ore deposits, Zr is concentrated in zircons. However, during hydrothermal alterations of these rocks, zircon is often highly altered and Zr is concentrated in uranium minerals, especially in coffinite.

Figure 7. Plot Y versus Zr for unaltered and mineralised rocks from the Okrouhlá Radouň, Rožná and Zadní Chodov

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Figure 8. BSE image of uraninite (urn) and coffinite (Cfn) around chlorite flakes, Rožná uranium deposit.

uranium deposits.

#### 4.3. Mineralogy

Coffinite in shear zone-hosted uranium deposits occurred in the Bohemian Massif usually prevails uranium mineral. In the Rožná and Zadní Chodov ore deposits, coffinite is concentrated in the deepest part of these deposits. The coffinite occurring in these shear zone-hosted ore deposits is commonly intimately associated with flakes of newly originated chlorite II. A majority of analysed coffinites from the Rožná, Okrouhlá Radouň and Lhota uranium deposits are enriched in Y2O3 (up to 3.4 wt.%) and ZrO2 (up to 13.8 wt.%).

Uraninite in shear zone-hosted uranium deposits from the Bohemian Massif usually occurs as colloform aggregates in highly heterogeneous aggregates together with coffinite. In mineralised aceites, both minerals often form rims around chlorite flakes (Figure 8). In the Rožná ore deposit, the SiO2 and UO2 contents vary from UO2+x to USiO4, indicating the variable coffinitisation of uraninite (Figure 9). Almost all uraninite grains and aggregates were replaced by coffinite to a variable degree.

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The behaviour of Y and Zr in mineralised aceites from shear zone-hosted uranium deposits in the Bohemian Massif is variable. Yttrium is enriched in mineralised aceites from the Rožná and Okrouhlá Radouň ore deposits and its behaviour is close to behaviour of HREE in these rocks. Yttrium in these rocks occurs usually in coffinite (up to 3.4 wt.% Y2O3). In altered biotite granites from the Bor pluton, the concentrations of Y are similar to their concentrations in

Figure 6. REE patterns of the high-grade metasediments and their hydrothermally altered equivalents from the Zadní

The concentrations of Zr in unaltered and altered rocks from all above-mentioned uranium deposits are similar. In unaltered host rocks from these ore deposits, Zr is concentrated in zircons. However, during hydrothermal alterations of these rocks, zircon is often highly altered and Zr is

Coffinite in shear zone-hosted uranium deposits occurred in the Bohemian Massif usually prevails uranium mineral. In the Rožná and Zadní Chodov ore deposits, coffinite is concentrated in the deepest part of these deposits. The coffinite occurring in these shear zone-hosted ore deposits is commonly intimately associated with flakes of newly originated chlorite II. A majority of analysed coffinites from the Rožná, Okrouhlá Radouň and Lhota uranium deposits

Uraninite in shear zone-hosted uranium deposits from the Bohemian Massif usually occurs as colloform aggregates in highly heterogeneous aggregates together with coffinite. In mineralised aceites, both minerals often form rims around chlorite flakes (Figure 8). In the Rožná ore deposit, the SiO2 and UO2 contents vary from UO2+x to USiO4, indicating the variable coffinitisation of uraninite (Figure 9). Almost all uraninite grains and aggregates were replaced by coffinite to a

unaltered granitic rocks (Figure 7).

4.3. Mineralogy

variable degree.

concentrated in uranium minerals, especially in coffinite.

Chodov uranium deposit. Original data normalised to chondrite according to [13].

58 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

are enriched in Y2O3 (up to 3.4 wt.%) and ZrO2 (up to 13.8 wt.%).

Figure 7. Plot Y versus Zr for unaltered and mineralised rocks from the Okrouhlá Radouň, Rožná and Zadní Chodov uranium deposits.

Figure 8. BSE image of uraninite (urn) and coffinite (Cfn) around chlorite flakes, Rožná uranium deposit.

heterogeneous and on their rims altered to Ti-enriched brannerite and rutile (Figure 10). Brannerite from the Rožná uranium deposit is enriched in ZrO2 (up to 4.82 wt. %). Brannerites from the Rožná uranium deposit were sometimes decomposed in complex non-stoichio-

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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

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

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

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,

mixing of chemically heterogeneous basinal brines with meteoric water [1, 15].

composed of chlorite I, albite I and hematite framework is characteristic.

alteration of the Ti-rich accessory minerals (titanite and allanite).

5.2. Behaviour of REE, Zr and Y in aceites

metric U-Ti-Si-Zr phases.

5. Discussion

5.1. Origin of aceites

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

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

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 heterogeneous and on their rims altered to Ti-enriched brannerite and rutile (Figure 10). Brannerite from the Rožná uranium deposit is enriched in ZrO2 (up to 4.82 wt. %). Brannerites from the Rožná uranium deposit were sometimes decomposed in complex non-stoichiometric U-Ti-Si-Zr phases.
