Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic

OREGEO-01958; No of Pages 13 Ore Geology Reviews xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeorev

Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic Miloš René Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holešovičkách 41, 182 09 Prague 8, Czech Republic

a r t i c l e

i n f o

Article history: Received 20 April 2016 Received in revised form 22 September 2016 Accepted 28 September 2016 Available online xxxx Keywords: Central European variscides Uranium mineralisation, aceite Brannerite Geochemistry Mineralogy

a b s t r a c t The Bor pluton belongs to the Variscan granitoid plutons of the Moldanubian zone of the central European Variscides. Along with similar granitoid plutons of the western part of the Bohemian Massif (Leuchtenberg, Babylon, Fichtelgebirge), it is close in its composition to low-F biotite granites of the Saxothuringian zone. This paper investigates the hydrothermal alteration of the Bor pluton in relation to uranium mineralisation. Uranium mineralisation of the Bor pluton is associated with shear zones occurring on the western margin of the pluton on its boundary with high-grade metasediments of the Moldanubian zone (Zadní Chodov) and with metasomatic mineralisation evolved in hydrothermally altered biotite granites of the Bor pluton (Vítkov II, Lhota). Uranium mineralisation in altered granites is accompanied by their intense hematitisation, albitisation, chloritisation and carbonatisation. Hydrothermal alterations of granites were accompanied by the enrichment in U, Na, P, Ti, Mg, Ca and depletion in Si and K. The altered high-grade metasediments occurring in shear-zones on the western boundary of the Bor pluton are enriched in U, Ca, Mg and P and carbonaceous matter. In the Bor pluton area with typical coffinite–uraninite association the presence of brannerite is significant for uranium mineralisation. The observed coffinite is enriched in Y (up to 4.3 wt.% Y2O3) and Zr (up to 2.0 wt.% ZrO2). Unaltered brannerite displays low Ca (0.9–4.5 wt.% CaO), Al (0.0–0.5 wt.% Al2O3), Fe (0.7–2.2 wt.% FeO), Pb (0.0–1.4 wt.% PbO) and Th (0.0–2.8 wt.% ThO2) concentrations. The highly altered brannerite is depleted in U, enriched in Ti, Si and Al. The temperature of the ore stage was estimated using chlorite thermometry and ranged from 122 °C to 258 °C. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Bohemian Massif is part of the European Variscan belt, which hosts a significant quantity of uranium deposits bound on brittle shear zones developed in high-grade metamorphic rocks and/or in granitic rocks (Dill, 1983; Kříbek et al., 2009; René, 2015). These deposits may represent the basement-rock-hosted part of unconformity-type deposits, the overlying sedimentary cover being almost entirely eroded during later geological periods. According to Dahlkamp (1993), these deposits are classified as subunconformity epimetamorphic deposits (e.g., the Beaverlodge district and Fay-Verna deposits in Canada). In the Bohemian Massif, this group of uranium deposits is represented by the Rožná and Okrouhlá Radouň ore deposits in metamorphic rocks series of the Moldanubian zone (Kříbek et al., 2009; René, 2015) and by the Zadní Chodov, Vítkov II, and Lhota ore deposits in area of the Bor pluton. These subunconformity epimetamorphic deposits consist of pen concordant lenses of highly disseminated uranium mineralisation evolved in fractures and/or brecciaed shear zones. The crystalline host rocks (metasediments, granites) of these deposits are strongly altered,

E-mail address: [email protected].

exhibiting extensive albitisation, chloritisation, arigilitisation and hematitisation. These low-temperature metasomatic rocks are named as aceites according to the recent IUGS classification (Fettes and Desmons, 2007). According to their mineral compositions, the aceites are very similar to episyenites evolved in disseminated uranium deposits of the Massif Central and Armorican Massif in France, which are linked with leucogranite plutons (e.g., Cathelineau, 1986). The Bor pluton is one of the Variscan plutons in western part of the Bohemian Massif known for its industrially significant uranium mineralisation. The uranium ore deposits evolved in the area of the Bor pluton offer a unique possibility for complex study of evolution of hydrothermal alterations accompanying uranium mineralisation with some focus on behaviour of titanium during alteration of granitic rocks and high-grade metamorphic rock series. The uranium ore deposits hosted in shear zones are evolved in altered granitic rocks (Lhota, Vítkov II) and in surrounding high-grade metamorphic rocks (Dyleň, Zadní Chodov, Wäldel/Mähring) (Dill, 1985; Kříbek et al., 2009). In comparison with other shear zones hosting uranium mineralisation in Bohemian Massif (Rožná, Okrouhlá Radouň) where coffinite and uraninite predominate, in the uranium mineralisation evolved in the Bor pluton and surrounding high-grade metamorphic rocks brannerite is another significant uranium ore mineral. The

http://dx.doi.org/10.1016/j.oregeorev.2016.09.033 0169-1368/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

2

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

2. Geologic setting

objectives of the presented paper is to present a detailed petrology, geochemistry and mineralogy of the hydrothermally altered rocks of the Bor pluton and associated high-grade metasediments connected with uranium mineralisation.

The Bor pluton consists of a N-S elongated body formed previously by biotite granites. It is emplaced in the shear zone, which is a part of

a

E 12°30´

E12° Selb

Czech Republic

ge

ir lgeb e t h Fic pluton

Cheb

L-L

N 50°

Mariánské lázn

MA Fa

ZCH

Bor pluton

Germany Le

Fl

Ro

Weiden

Kl

Metamorphic rocks Prevariscan granite

Biotite granodiorite

N 49°30´

Biotite granite Two-mica granite

Ba

N

High-fractionated granite 40 km 0 E 15°

E 0°

Harz

i Bohem an Massif

London M SUB VARISCICU

Armorican Massif

1

N

CU

3 2

NUBICUM LDA MO

IS T

Paris

I NG RI HU T XO SA

BR UN OV

RH M EN OHERCYNICU

UL IA

Praha

M

N 50°

N E

W

Vosges

Wien

Schwarzwald

S

N 44°

Massif Central

Shear-zone hosted uranium deposits Faults Variscan thrusts Boundary of basement outcrops

100 km

b

Fig. 1. a Geological map of the granitoids in the western part of the Bohemian Massif ZCH – Zadní Chodov uranium deposits, MA – Mähring/Wäldel uranium deposit, Ba – Babylon pluton, Fa – Falkenberg pluton, Fl – Flossenbürg pluton, Kl – Kladruby pluton, L-L Mariánské Lázně granite pluton, Le – Leuchtenberg pluton, Ro – Rozvadov pluton (after Breiter and Sokol, 1997; Siebel et al., 1997, modified by author). b Sketch map of the Variscan belts of western and central Europe with three shear-hosted uranium deposits in the Bohemian Massif (1 – Rožná, 2 – Okrouhlá Radouň, 3 – Zadní Chodov) (after Kříbek et al., 2009, modified by author).

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

the important West Bohemian shear zone that separates the Moldanubian and Teplá–Barrandian zones in this area (Zulauf, 1994) (Fig. 1). General strike of these structures is NNE–SSW and they dip 50–70o eastwards. Younger NW–SE striking faults divide the Bor pluton into three separate blocks. The northern one with the Zadní Chodov uranium deposit is cut-off in the south by the Central fault, the central block lies between the Central and Tachov faults and the southern one lies south of the Tachov fault. The most voluminous rocks in the Bor pluton are coarse-grained biotite, usually porphyritic biotite granites. These granites intruded during the Variscan magmatic event (337 ± 6 Ma, U/Pb TIMS analyses on zircon; Dörr et al., 1997). In the northern block of the Bor pluton older amphibole-biotite granodiorites, tonalites and quartz diorites were also observed, whose composition and structure resemble the Oberpfalz redwidzites, described by Troll (1968) and Siebel (1994). Very abundant dyke monzogranites and aplites that fill variously dipping N–S striking shear structures occur in particular in the southern block of the Bor pluton. The youngest dyke rocks of Variscan age are lamprophyres that fill steeply dipping structures N–S direction. It appears that the lamprophyres represented a mechanical and chemically favourable environment that caused local accumulation of uranium mineralisation. 3. Selected uranium deposits The uranium deposits in area of the Bor pluton were previously studied in detail, especially during the exploration and mining activity in this area (1952–1992) (e.g., Doležel et al., 1975; Fiala, 1980 a, b; Arapov et al., 1984). Further petrologic and geochemical studies of this area were also published in later years after cessation of uranium mining (e.g., Siebel et al., 1999; René, 2000). 3.1. The Zadní Chodov uranium deposit The Zadní Chodov uranium deposit was mined from 1952 to 1992 and was ranked among medium-size uranium deposits in the Bohemian Massif. The total mine production of low-grade uranium ore (0.195 wt.% U) was 4151 t U (Kafka, 2003). The deposit, which is located in the northern tectonic block of the Bor pluton, was investigated by mine workings down to levels of 1250 m in a length of over 2.5 km. The contact with the Moldanubian high-grade metasediments is tectonic, comprising several parallel shear structures to the Zadní Chodov fault, which forms the western boundary of the Bor pluton. Migmatised biotite paragneisses of the Moldanubian Varied Group predominate and they contain intercalations of quartzites, amphibolites, calc-silicate rocks and crystalline limestones. Dykes and bodies of amphibole-biotite tonalites and biotite granites cut the paragneisses. Aplite, pegmatite and lamprophyre dykes occur but are less frequent and are particularly developed above and below large shear zones. Uranium mineralisation is associated with the N-S trending zones of the Zadní Chodov fault in areas of their intersection with NW-SE trending fault structures, which form an NW continuation of the Central fault. The Zadní Chodov shear zone has been verified in an extent exceeding 20 km, its thickness in the middle part reaches 500 m, and the dip is 50–75° to the east. This dislocation passes northwards into the Eastern shear zone of the Zadní Chodov deposit (Fig. 2). The infill of the shear zones consists of intensely altered and crushed rocks with chlorite-rich and/or clay-mineral-rich assemblages. Uranium mineralisation was concentrated into three shear zones (CH-1, CH-4, and CH-11; Fig. 3). The thickness of individual shear zones is highly variable from 30 cm to approximately 1–2.5 m. The total thickness of these mineralised shear zones are 50–150 m. The high-grade uranium mineralisation was developed at depths of 440– 960 m beneath the present surface. The most common uranium minerals are coffinite (65 vol.%), uraninite (25 vol.%) and brannerite (10 vol.%). The U/Pb age of brannerite is 185 ± 6 Ma (ID–TIMS) and by Ordynec et al. (1987) is interpreted as the Mesozoic recrystallisation of brannerite occurred in the main ore-stage.

3

3.2. The Vítkov II uranium deposit The Vítkov II uranium deposit was mined from 1961 to 1988 and was ranked among medium-size uranium deposits in the Bohemian Massif. The total mine production of low-grade uranium ore (0.124 wt.% U) was 3973 t U (Kafka, 2003). This uranium deposit occurs in the central tectonic block of the Bor pluton. The deposit is evolved between two shear NNE–SSW zones. The shear zone 0–30 in the east is up to 5–7 m in thickness. Its infill consists of highly crushed altered rocks (albitised biotite granites and lamprophyres), quartz and carbonates. Rich accumulations of U-minerals often occur in its vicinity. The Vítkov shear zone (southern part of the Eastern shear zone evolved in area of the Zadní Chodov deposit) forms the western boundary of the deposit. It is very conspicuous and its thickness reaches several tens of meters. It is infilled by crushed and altered country rocks. The pennate NW–SE dislocations situated between the two shear zones are infilled by dyke biotite and two-mica granites and relatively rarely aplites. The occurrence of uranium mineralisation is controlled by NW-SE and NNWSSE trending faults, which form a dense network of fractures and joints. The surrounding biotite granites are usually intensely altered. The ore bodies comprise U-minerals (coffinite, uraninite, brannerite) finely disseminated in the surrounding altered granitoids. The uranium mineralisation of the main ore stage was dated by Ordynec et al. (1987) at 260 ± 2 Ma (U–Pb, uraninite, isotopic dilution, ID-TIMS). Large uranium ore accumulations are associated with the sectors marked by convergence, branching and intersection of fractures. The ore bodies are grouped into four ore pipes, which are accumulated in environs of the shear zone 0–30. The presence of secondary minerals (becquerelite, curiite, uranophane, phosphouranilite, and autunite) that occur down to a depth of 500–700 m is highly significant for the Vítkov II deposit. The origin of these minerals is related to the weathering oxidized zone of this deposit. The deposit has been verified by exploration mine workings down to a depth of 1000 m. 3.3. The Lhota uranium deposit The small uranium deposit Lhota is situated in the central block of the Bor pluton. The deposit has been verified between 1953 and 1967 and 1975–1989 by five exploration shafts down to a depth of 250 m, and by numerous boreholes down to levels of 300 to 600 m. During these two exploration stages low-grade uranium mineralisation (0.120 wt.% U) with the total amount of 158 t U (Kafka, 2003) was identified. The geology of the area surrounding this uranium deposit consists of mostly coarsegrained biotite granites together with smaller bodies of amphibole-biotite granodiorites and tonalites overlain by remnants of the Moldanubian country rocks. Patches of high-grade metamorphic rocks of the Moldanubian Varied group also form abundant xenoliths in the biotite granites. The rock complex is pierced by NW-SE, partly also N-S trending aplite dykes. The two ore bearing shear structures (Os-2, Os-17) strike NW-SE and dip steeply NE (Fig. 4). The thickness of these mineralised shear zones is 5–18 m. The mineralised shear zones comprise coffinite, uraninite and brannerite. The U/Pb age of uraninite II from post-ore quartz stage is 158 ± 3 Ma (ID–TIMS) (Ordynec et al., 1987). 4. Analytical methods Detailed geochemical and mineralogical investigations of the uranium mineralisation in the Zadní Chodov, Vítkov II and Lhota uranium deposits were carried out on a representative suite of rock samples. These samples represent unaltered host granites and high-grade metasediments, as well as their hydrothermally altered equivalents. The whole-rock composition of the biotite granites, amphibole-biotite granodiorites and tonalites and accompanying high-grade metasediments and their altered equivalents was analysed on 74 samples. The major elements were determined by X-ray fluorescence spectrometry using the PANanalytical Axios Advanced spectrometer at

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

N 49°55´

Dyle

Wäldel/ Mähring Zadní Chodov 2

4

6

8 km

BRD v fault

0

Eas tern zon e

4

Chodo

N 49°50´

Moldanubian zone Teplá-Barrandian zone

0-30

Tachov

fault

Zadní

N

Planá

Vítkov II

Ce

nt

Biotite granites Biotite granodiorites

N 49°45´

ra

lf

au

lt

Lhota

ho

c Ta v

Faults

ul

fa t

Uranium ore deposits

N 49°30´

E12°30´

E12°45´

Fig. 2. Geological map of the Bor pluton (according Holovka and Hnízdo, 1992, modified by author).

Activation Laboratories Ltd., Ancaster, Canada. The FeO content was measured via titration, whereas the loss on ignition (L.O.I.) was determined gravimetrically at the Institute of Rock Structure and Mechanics AS CR, v.v.i. in Prague, Czech Republic. The trace elements in selected samples were determined by ICP-MS (a Perkin Elmer Sciex ELAN 6100 ICP mass spectrometer) at Activation Laboratories Ltd., Ancaster, Canada. The decomposition of the rock samples for ICP-MS analysis involved lithium metaborate/tetraborate fusion. In-lab standard or certified reference materials were used for quality control. The detection limits were approximately 1–3 ppm for Ba, Rb Sr and Zr, 0.01–0.2 ppm for Nb, Th and U and 0.002–0.05 ppm for REE. The elemental mobility during rock alteration was estimated using balance constraints of selected representative rock samples based on the isocon method (Grant, 1986). The elemental organic composition of altered high-grade metasediments was determined using a Flash FA 1112 (Thermo Finnigan) CHNS/O micro-analyzer at Institute of Rock Structure and Mechanics, ASCR, v.v.i in Prague. The minerals were analysed in polished thin sections, and back-scattered electron (BSE) images were acquired to study the examined accessory minerals and the internal structure of individual mineral grains. The elemental abundances of Al, As, Bi, Ca, Ce, Dy, Er, Eu, F, Fe, Gd, Hf, Ho, La, Lu, Mg, Mn, Nb, Nd, P,

Pr, Sc, Si, Sm, Th, U, W, Y, Yb and Zr were determined using a CAMECA SX 100 electron microprobe operated in wavelength-dispersive mode at the Institute of Geological Sciences, Masaryk University in Brno. The accelerating voltage and beam currents were 15 kV and 20 nA or 40 nA, respectively, and the beam diameter was l to 5 μm. The peak count time was 20 s, and the background time was 10 s for major elements. For the trace elements, the times were 40–60 s on the peaks, and 20–30 s on the background positions. The raw data were corrected using the PAP matrix corrections (Pouchou and Pichoir, 1985). The detection limits were approximately 400–500 ppm for Y, 600 ppm for Zr, 500–800 ppm for REE and 600–700 ppm for U and Th. For the chlorite thermometry, the six-component chlorite solid solution model of Walshe (1986) was used. 5. Results 5.1. Petrography of granitoids of the Bor pluton The amphibole-biotite to biotite granodiorites, tonalites and quartz diorites are equigranular, fine- to medium-grained rocks, sometimes with porphyritic texture. They contain amphibole (Mg-hornblende),

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

5

Shaft 3

W

E Ea

-1 Ch

ste

-11 Ch

rn zo

Ch

ne

-4

2

P-

P-1

500 m Biotite paragneisses

Granodiorites to tonalites

Migmatised paragneisses

Biotite granites

Amphibolites

Mineralised shear zones

Faults

Fig. 3. Schematic geological cross-section of the Zadní Chodov uranium deposit (according Novák and Paška, 1983, modified by author).

SW

OS-2

OS-17

OS-3

NE

100 m Biotite paragneisses

Porphyritic biotite granites

Two-mica granites

Amphibolites

Granodiorites to tonalites

Shear zones

Fig. 4. Schematic geological cross-section of the Lhota uranium deposit (according Novák et al., 1983, modified by author).

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

6

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

biotite (annite, Fe / (Fe + Mg) = 0.53–0.56), plagioclase (An36–46), Kfeldspar, quartz and sometimes also a small amount of pyroxene (diopside). Biotite is enriched in Ti (Ti = 0.51–0.60 apfu, atoms per formula unit). The accessory minerals are represented by apatite, zircon, monazite, titanite and allanite. Biotite granites are composed of very characteristic K-feldspar phenocrysts, up to 8 cm in size. The matrix of biotite granites consists of K-feldspar, plagioclase (An19–29), quartz and biotite (annite, Fe / (Fe + Mg) = 0.59–0.62), which is enriched in Ti (Ti = 0.39–0.59 apfu). Accessory minerals are represented by apatite, zircon, monazite, titanite and rare allanite.

Pre-ore

Stage

I

Post-ore quartz

Ore II

Post-ore carbonate

III

Chlorite Rutile Titanite I

II

III

Albite Muscovite Illite

5.2. Petrography of altered granitoids

Kaolinite

Four major stages of alteration of granites and high-grade metasediments can be distinguished, namely pre-ore, ore and two post-ore stages (Mrázek and Fiala, 1979; Fiala, 1980 a, b; Arapov et al., 1984; Fig. 5). The presented paragenetic sequence of the hydrothermal mineralisation is partly schematic. In altered granitoids albites and carbonates (calcite, dolomite) are dominant, whereas the carbonaceous matter is absent. The clay minerals, especially illite are contra wise enriched in altered metasediments. During pre-ore alteration of granitoids, original biotite was chloritised and altered to chlorite I (Fe / Fe + Mg) = 0.30–0.73 (Fig. 6). Alteration of biotite is commonly accompanied by rutile formation due to the liberation of Ti from the original biotite laths (Fig. 7 a). Original magmatic plagioclases (An26–50) were transformed into albite I (An0.0–0.9) (Fig. 7 b). Albitisation is sometimes accompanied by Kfeldspatisation, which occurs mostly at the Vítkov II uranium deposit with metasomatic uranium mineralisation. In these cases Kfeldspatisation often precedes the quartz removal. In highly altered granites, the authigenic generation of albite II occur as epitactic overgrowths on pseudomorphs of albite I (Fig. 7 c). The transitional zone between the altered and unaltered granitoids is gradational, spanning a few tens of centimetres to one metre. Commonly, the transitional zone displays a weak red colouring due to presence of fine-grained hematite laths distributed irregularly in the original albitised plagioclase (albite I). Hydrothermally altered granitoids have low to medium porosities due to hydrothermal leaching of the original quartz (typically 0.5–5 vol.%). The vugs formed initially through leaching of quartz and were later infilled again by a younger generation of quartz (quartz II) and chlorite (chlorite III) (Fig. 7 d). However, the original magmatic texture in the altered granitoids is usually preserved. The chlorite II (Fe / (Fe + Mg) = 0.13) originated during the ore stage and forms either spherulitic aggregates, filling microscopic cavities in the altered rocks, or cloddy aggregates. Some vugs were filled in the ore stage by albite III. The newly originated albites II and III have near-end-member composition. Furthermore part of albites II and III display anomalous behaviour in optical cathodoluminescence, showing damage in the luminescence spectra. The quartz post-ore stage is characterized by filling of voids created by removal of magmatic quartz, intense chloritisation and albitisation of original granitoids by quartz II, origin of quartz veinlets (quartz III), veinlets of chlorite III, origin of younger hematite laths (hematite II) and origin of younger uraninite lenses and veins. The origin of quartz II is connected to the origin of muscovite II, whereas during the formation of quartz veinlets (quartz III) small veinlets of muscovite III formed simultaneously. The formation of muscovite II is also followed by the growth of clay minerals (illite and kaolinite). The last post-ore stage is reflected in the development of calcite and relatively rarely sulphides (pyrite, chalcopyrite, sphalerite, and galenite), selenides (clausthalite, poubaite, nevskite) and zeolites (laumontite). Calcite fills the cavities in the granites after having replaced quartz and/or fine veinlets in highly altered granitoids. Occasionally, dolomite and siderite were found.

Carbonaceous matter Hematite I

III

II

Quartz Apatite I

II

Brannerite II

I

Coffinite II

I

Uraninite Thorite Calcite Dolomite Pyrite Chalcopyrite Galena Fluorite Selenides Zeolites

Fig. 5. Paragenetic sequence of the hydrothermal mineralisation. Compiled after Mrázek and Fiala, 1979; Fiala, 1980 a, b; Arapov et al., 1984 and results of the present study.

0.8

Fe/(Fe + Mg)

0.6

0.4

0.2

0.0 5.0

5.5

6.0

0.5

Si4+(apfu) Chlorite I Chlorite II Fig. 6. Composition of chlorites.

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

7

Fig. 7. Photomicrographs of hydrothermal altered biotite granites from the Bor pluton. a: Aceite with chlorite I (Chl I) contained minute inclusion of rutile. b. Aceite with albite I (Ab I) and chlorite II (Chl II). c: Aceite with individual generation of albite (Ab I, Ab II). d: Aceite with vugs filled by quartz II (Qz II) and chlorite III (Chl III).

5.3. Petrography of high-grade metasediments High-grade metasediments from the Zadní Chodov uranium deposits consist predominantly of partly migmatised biotite paragneisses,

sillimanite-biotite paragneisses, biotite quartzitic paragneisses and cordierite-biotite paragneisses. The dominant biotite and quartzitic biotite paragneisses are medium- to fine-grained rocks and contain quartz, plagioclase, biotite and K-feldspar. The amount of anorthite in the

Fig. 8. Photomicrographs of hydrothermal altered metasediments from the Zadní Chodov uranium deposit. a: Aceite with coarse-grained aggregates of chlorite II (Chl II). b: Aceite with chlorite I containing fine-grained aggregates of hydrothermal carbonaceous matter (Gr III). c: BSE image of illite (Ilt) accumulation containing irregular grains of hydrothermal apatite (Ap). d: Vein of quartz III (Qz) in highly altered high-grade metasediment.

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

8

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

plagioclases from these high-grade metasediments ranged from An20 to An35. Garnet was found occasionally and accessory minerals are represented by apatite, ilmenite, zircon, monazite, titanite and xenotime. 5.4. Petrography of altered high-grade metasediments The pre-ore stage in high-grade metamorphic rocks is associated with the formation of shear zones. The appearance of shear zones is accompanied by accumulation of carbonaceous matter and/or clay minerals together with chloritisation of original biotite (chlorite I). In the fine-grained infilling of shear zones chlorite I (Fe / (Fe + Mg) = 0.30– 0.67) predominates over illite. Together with chloritisation of biotite muscovite I, hematite and rutile developed. Original plagioclase was albitised (albite I, An0.0–0.9) and monazite was partly altered to Th-depleted monazite and thorite. During the ore stage chlorite II, apatite, coffinite and brannerite appear. Chlorite II (Fe / (Fe + Mg) = 0.12– 0.15) forms well crystallised aggregates in fine-grained infilling of shear zones (Fig. 8 a). Three types of carbonaceous matter were distinguished by microscopic study. Type I that predominate occurs as finegrained, irregular or elongate grains that are dispersed in the inorganic matrix. Type II forms laths that are common in high-grade metasediments with well-developed metamorphic structure. Type III represents hydrothermal carbonaceous matter occurred in small irregular aggregates or in veinlets (Fig. 8 b). Apatite appears in relatively large

irregular grains in this fine-grained infilling of chlorite and clay minerals (Fig. 8 c). In some cases during the ore-stage, very fine veinlets of quartz I developed. The post-ore quartz stage is connected with origin of small veins and veinlets of uraninite II and quartz II (Fig. 8 d). During the postore carbonate stage calcite, dolomite and ankerite appeared, together with a small amount of sulphides (pyrite, chalkopyrite, galenite, and sphalerite) and selenides (e.g. clausthalite, poubaite, nevskite). 5.5. Chemical composition of unaltered granitoid rocks The representative chemical analyses of the unaltered and altered granitoid rocks are shown in Table 1. The amphibole-biotite, biotite granodiorites or quartz diorites are metaluminous to slightly peraluminous rocks (A/CKN – mol. Al2O3 / (CaO + K2O + Na2O) = 0.9–1.1). Compared to the common I-type granites (Collins et al., 1982; Whalen et al., 1987), the amphibole-biotite granodiorite to quartz diorite from the Bor pluton are enriched in Ca (3.0–6.0 wt.% CaO), Mg (1.7–5.6 wt.% MgO), Ba (741–1339 ppm), Sr (257–571 ppm) and Zr (219–472 ppm). They are further enriched in LREE (LaN/YbN = 25– 34) and display a low negative Eu-anomaly (Eu/Eu* = 0.58–0.96). The biotite granites of the Bor pluton are metaluminous to peraluminous rocks (A/CKN = 0.8–1.4). They could be classified as typical I/S-type granites. In comparison to amphibole-biotite granodiorite to quartz diorite they are depleted in Ca (0.5–3.9 wt.% CaO), Mg (0.1–

Table 1 Representative whole-rock analyses of unaltered and altered granitoids. wt.%

Re-619

Re-657

Re-540

Re-665

Re-650

Re-673

Re-649

Re-662

Type

TO

TO

ALTTO

ALTTO

GR

GR

ALTGR

ALTGR

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 L.O.I. Total

55.96 1,31 17.94 1.16 5.40 0.12 3.43 5.89 3.50 2.90 0.54 1.63 99.78

61.86 0.93 15.82 0.50 5.44 0.09 3.77 3.79 2.76 3.33 0.24 1.39 99.92

56.02 0.93 17.09 3.42 4.26 0.13 3.14 7.15 3.09 2.86 0.28 1.39 99.76

56.42 0.46 14.84 2.36 5.43 0.09 7.08 6.16 1.96 2.84 0.22 2.11 99.97

67.25 0.59 15.60 1.20 2.24 0.05 0.10 1.84 4.00 4.85 0.37 1.52 99.61

65.97 0.37 15.90 0.35 2.84 0.03 1.18 2.70 3.62 4.52 0.24 1.68 99.40

63.99 0.66 16.50 1.78 2.75 0.06 0.22 0.10 5.06 4.36 0.43 3.71 99.62

47.46 0.75 12.50 1.90 5.12 0.20 9.06 7.96 1.70 3.33 1.24 8.69 100.21

ppm Ba Rb Sr Y Zr Nb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu LaN/YbN Eu/Eu*

1257 95 571 21 237 11 16 3 66.0 125.90 13.73 47.80 7.00 1.87 5.02 0.65 3.11 0.62 1.68 0.23 1.42 0.21 31.34 0.96

741 160 257 20 219 16 26 6 74.00 120.00 11.70 45.00 7.70 1.30 6.20 0.75 3.70 0.68 2.10 0.27 2.00 0.29 24.95 0.58

1970 107 595 32 347 11 14 3 62.00 113.00 14.20 59.00 10.50 2.40 8.60 1.02 5.20 0.95 2.90 0.36 2.70 0.38 15.48 0.77

1055 96 341 26 207 10 17 4 70.00 96.00 11.60 42.00 7.20 1.40 5.80 0.69 3.40 0.63 1.90 0.24 1.80 0.26 26.22 0.66

1274 160 293 19 312 13 32 5 70.40 132.90 14.32 51.70 8.14 1.42 5.89 0.79 4.04 0.72 1.96 0.28 1.64 0.27 28.94 0.63

1186 91 295 27 300 10 40 1 116.00 158.00 18.50 66.00 10.00 1.40 7.10 0.83 3.90 0.70 2.20 0.25 1.70 0.26 46.00 0.51

613 158 59 24 344 20 31 58 60.10 124.20 13.81 49.20 7.59 1.03 6.17 0.87 4.56 0.88 2.32 0.36 2.19 0.34 18.50 0.46

1399 92 205 18 414 16 35 10 59.60 127.90 16.53 64.50 10.38 2.08 6.42 0.77 3.60 0.69 1.89 0.26 1.64 0.26 24.50 0.78

TO – granodiorites to quartz diorites: Re-619 – biotite granodiorite, Zadní Chodov, borehole Chv-2, 141 m, Re-657 – biotite tonalite, Hlinné near Lhota, borehole Hl-16, 188 m, ALTTO – altered granodiorites to quartz diorites: Re-540 – chloritised biotite tonalite, Pernolec near Lhota, borehole Pe-27, 42 m, Re-665 – chloritised biotite granodiorite, Zadní Chodov, borehole Chv-16, 110 m, GR – biotite granite: Re-650 – porphyritic biotite granite, Hlinné near Lhota, borehole Hl-15, 260 m, Re-673 – coarse grained biotite granite, Zadní Chodov, borehole Chv-17, 379 m, ALTGR – altered biotite granite: Re-649 – altered porphyritic biotite granite, Hlinné near Lhota, borehole Hl-15, 231 m, Re-662 altered porphyritic biotite granite, Zadní Chodov, borehole Chv-15, 110 m. Eu/Eu* = EuN/√[(SmN)*(GdN)].

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

500

change occurred during the alteration, the chemical composition of the altered rocks cannot be compared directly with those of the unaltered rocks. Loss of volume in altered granitoids occurs mostly due to leaching of quartz which has to be accounted for. Thus for the detailed investigation of losses and gains during alteration and the behaviour of selected trace elements, the isocon method developed by Grant (1986) was applied. The resulting scattering of the elements in the isocon plots for the aceites from the studied area suggests that the major and trace elements were mobilised to variable extent. The isocon plot of unaltered and altered biotite granites from the Lhota uranium deposit is displayed in Fig. 10 a. It appears that the altered biotite granite is highly albitised, hematitised and chloritised and carbonatised. Thus, hydrothermal alteration added Na, Fe3+, Mg, Ca, P, Ti, U and depleted Si, K, Sr and Ba.

Fig. 9. REE patterns of biotite granites of the Bor pluton and their hydrothermally altered equivalents. Original data normalised to chondrite according Boynton (1984).

1.6 wt.% MgO), Ba (415–1916 ppm), Sr (66–423 ppm) and Zr 100– 391 ppm and also enriched in LREE (LaN/YbN = 15–46), displaying pronounced negative Eu-anomalies (Eu/Eu* = 0.40–0.72). 5.6. Chemical composition of altered granitoid rocks

80

Fe2O3 CaO Al2O3

60

TiO2

40

SiO2

MnO

U

Zr

Ba

Rb Nb

The altered high-grade metasediments occurring in shear-zones of the Zadní Chodov uranium deposits are distinctly enriched in U (up to 4553 ppm), total carbon (up to 3.9 wt.%), organic carbon (up to 1.1 wt.%) and partially depleted in REE, especially in HREE (Fig. 11). The mobility of major elements and selected trace elements in the uranium enriched filling of mineralised shear zone from the Zadní Chodov uranium deposit is displayed in the isocon plot (Fig. 10 b). During hydrothermal alteration of high-grade metasediments and origin of mineralised shear zones the shear zone infill is enriched in U, Ca, Mg, P, Fe3+ and depleted in Ti, Si, K, Ba and Rb. Distinctly intensive tectonic movement on mineralised shear zones of the Zadní Chodov uranium deposit (see Fig. 10 a, b) very probably causes the more intensive

P2O5

100

MgO

MnO

SiO2

Sr

Sr

20

Fe2O3t

Nb

Na2O Al2O3 TiO2

Zr

Th

0 0

b

CaO

U

Rb

0

Th

5.8. Chemical composition of altered high-grade metasediments

ZCH-25 : [scaled] altered rock

P2O5 MgO

K2O

The representative analyses of the unaltered and altered high-grade metasediments are shown in Table 2. The high-grade metasediments are classified as claystones due to their SiO2/Al2O3 and K2O/Na2O ratios (Wimmenauer, 1984). Compared to the average composition of the North American shale composite (NASC) these metasediments have elevated Rb, Th, U, Zr and REE concentrations and are depleted in Ba and Sr.

a

Na2O

20

Re-693 : [scaled] altered rock

100

Albitisation and chloritisation are accompanied by the silica removal from quartz and other silica-rich minerals (biotite), which continued during the argillisation. Concentrations of Al2O3 and Fe2O3 increased during the albitisation, chloritisation and argillisation. During chloritisation the concentrations of FeO and MgO also increased considerably, which is reflected in the appearance of chlorite II, which is richer in magnesium. The content of CaO increases in the granites affected by carbonatisation, reaching up to 8.0 wt.% CaO in altered biotite granites, and it decreases in the rocks affected only by albitisation and chloritisation. The content of Na2O increases in the rocks affected by albitisation (up to 6.1 wt.% Na2O in biotite granites) and is considerably lower in highly argilitised rocks. The content of K2O decreases in most of the altered granites. Higher contents of K2O have only been found in some granites affected by K-feldspatisation. Altered biotite granites are partially depleted in REE, especially in LREE. Some of them are enriched in HREE (Fig. 9). However, where significant mass or volume

5.7. Chemical composition of unaltered high-grade metasediments

80

La Ce Pr NdSm Eu Gd Tb Dy Ho Er Tm Yb Lu

60

10

40

100

20

Sample/Chondrite

Biotite granites Altered biotite granites

3

9

40

60

80

Re-681 : [scaled] original rock

100

0

Ba 20

K2 O

40

60

80

100

ZCH-12 : [scaled] original rock

Fig. 10. a: Isocon plot of the altered biotite granite vs. unaltered biotite granite of the Bor pluton. b: Isocon plot of the shear zone chlorite-clay minerals infill of the shear zone vs. the unaltered biotite paragneiss from the Zadní Chodov uranium deposit. The data of each isocon have been scaled such that all points from the data sets being compared fall between 0 and 100.

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

10

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

chemical and mineralogical alteration of analysed metasediments in comparison with altered biotite granites of the Bor pluton. 5.9. Occurrence and composition of uranium minerals The ore stage in granitoids of the Bor pluton and in high-grade metamorphic rocks is associated with origin of accumulations of uranium minerals (coffinite, uraninite and brannerite). Uranium mineralisation comprises three different morphologic-mineralogical types. The highly altered granitoids in the Vítkov II ore deposit are marked by metasomatic coffinite and/or coffinite–uraninite mineralisation. The metasomatic mineralisation is coupled with highly intensive albitisation and carbonatisation of biotite granites, especially near of the fault 0–30. The lenticular-shaped uraninite and uraninite–coffinite mineralisation (Vítkov II, Lhota) is linked to the contacts of granitoids with xenoliths of the country rocks. The disseminated coffinite or coffinite–brannerite mineralisation occurs in the xenoliths of amphibolites and migmatised paragneisses (Lhota) and in shear zones evolved in high-grade metamorphic rocks (Zadní Chodov). In the both deposits coffinite and brannerite occur predominantly in highly chloritised host rocks. The genetic link between the brannerite mineralisation and host rock is not quite clear. However, it is possible that altered high-grade metamorphic rocks (paragneisses and amphibolites) were more suitable sources of Ti than granites of the Bor pluton. The main uranium mineral in all three investigated uranium deposits is coffinite. These aggregates are distinctly heterogeneous at the microscopic scale. In some cases partly developed alteromorphic uraninite, formed through alteration of coffinite, occurs. Coffinite is enriched in Y (up to 4.3 wt.% Y2O3) and Zr (up to 2.0 wt.% ZrO2). However, for all three investigated uranium deposits in the Bor pluton area (Zadní Chodov, Vítkov II, and Lhota) the uranium mineral of highest economic interest is brannerite. In these deposits brannerite occurs as pin-forming crystal aggregates and/or irregular grains. Larger brannerite grains are usually highly heterogeneous, metamictised and on the rim altered to Ti-enriched brannerite and/or rutile. Smaller brannerite grains are often altered to Ti-enriched brannerite and to rutile (Fig. 12). The compositions of representative unaltered and partly altered brannerite samples are given in Table 3. The range of unaltered and/or partly altered brannerite compositions based on all analyses obtained in this study indicate that these brannerites contain 32.1–38.3 wt.% TiO2, 1.9–34.4 wt.% UO2, 17.4–34.4 wt.% UO3, 0.0–2.8 wt.% ThO2, 0.9– 4.5 wt.% CaO, 0.0–1.4 wt.% PbO, 0.0–0.0.2 wt.% Nb2O5, 0.0–1.4 wt.% SiO2, 0.0–0.5 wt.% Al2O3, 0.7–2.2 wt.% FeO, 0.2–2.2 wt.% Y2O3, 0.1– 0.5 wt.% ZrO2 and 0.0–0.2 wt. REE2O3. The highly altered brannerite is depleted in U, enriched in Ti, Si and Al. The chemical formula of brannerite was initially calculated from the microprobe analyses by normalisation to 6.00 oxygens. On this basis the cation total commonly exceeds the ideal value of 3.00, indicating that a significant amount of the U must be in a higher valence state than the assumed U4+ state. Therefore, the analyses were re-normalised to 3.00 cations and the amount of U6+ necessary to provide charge recalculated. Correlations of Ca vs. U, U vs. Ti, Fe + Al vs. Ti and Si/Ti vs. U/Ti are displayed in Fig. 13. 5.10. Chlorite thermometry The authigenic chlorite data (chlorite II) were used to calculate the temperature of alteration during the ore stage. The chlorite II crystal chemistry indicates that the temperature ranges from 122 °C to 258 °C. 6. Discussion 6.1. Origin and evolution of aceites The most significant textural feature of altered granites in the studied deposits of the Bor pluton (Vítkov II, Lhota) is that the albite I,

chlorite hematite and II framework is filled with younger generations of albite II and III, chlorite II and III and carbonates (calcite, dolomite). However, for the shear zones which are evolved in high-grade metasediments (Zadní Chodov), occurrence of fine-grained chlorite (chlorite I), clay minerals (especially illite) and carbonaceous matter filling is characteristic. The differences in mineralogical and geochemical compositions of both rock environments are expressed by different composition of host rocks and by distinctly more intensive tectonic movement on the most significant shear zones evolved in high-grade metamorphic rocks (Zadní Chodov fault, Eastern shear zone). Carbonaceous matter occurred in shear zones of the Zadní Chodov uranium deposit was found also in the Dyleň uranium deposit (Kříbek, 1981) and the Wäldel/Mähring uranium deposit (Bavaria) where was described as impsonite (Dill and Weiser, 1981). The impsonite from the Wäldel/Mähring uranium deposit could be very probably correlated with the type III carbonaceous matter in the Zadní Chodov uranium deposit. The carbonaceous matter from the Zadní Chodov and Dyleň uranium deposits was by Kříbek (1981) described as anthraxolite. According to isotopic composition study a carbonaceous matter from these uranium deposits, the examined values (δ 13C, PDB = −15 to −25) practically do not differ from isotopic composition of the carbonaceous matter derived from a biological source (Kříbek, 1981). According precise U/Pb dating of uranium bearing minerals (uraninite, coffinite, brannerite; Ordynec et al., 1987), the ore stage (uraninite Table 2 Representative whole-rock analyses of unaltered and altered high-grade metasediments. wt.%

ZCH-5

ZCH-12

ZCH-6

ZCH-25

ZCH-10

ZCH-11

Rock

BPR

BPR

SHZ

SHZ

ALTBPR

ALTBPR

SiO2 TiO2 Al2O3 Fe2O3 tot. MnO MgO CaO Na2O K2O P2O5 L.O.I. C tot. C org. Total

38.69 0.79 17.28 13.32 0.32 18.08 0.21 0.30 1.60 0.11 8.14 0.25 0.24 99.33

51.94 1.21 20.42 8.15 0.10 5.17 0.14 0.34 4.27 0.12 7.57 0.16 0.15 99.74

44.30 0.33 9.45 12.44 0.24 14.97 1.54 0.23 0.01 0.49 13.91 1.22 1.11 100.24

34.50 0.77 13.30 11.89 0.26 23.05 1.36 0.26 0.10 0.44 12.79 0.63 0.53 99.88

46.91 0.92 21.25 7.52 0.20 8.14 0.14 0.38 4.31 0.10 9.70 0.08 0.07 99.72

58.50 0.34 18.57 5.33 0.06 6.80 0.30 0.36 3.67 0.18 5.88 0.07 0.06 100.12

ppm Ba Rb Sr Y Zr Nb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu LaN/YbN Eu/Eu*

218 61 19 22 171 15 14 32 75.00 136.20 14,51 52.50 8.34 1.95 6.13 0.81 3.94 0.82 2.10 0.31 2.08 0.33 24.31 0.83

858 159 17 39 269 13 16 3 55.20 111.60 12.32 44.80 8.17 1.55 7.62 1.24 6.91 1.47 4.27 0.61 4.10 0.63 9.08 0.60

34 53 46 143 118 195 21 4554 12.80 35.10 5.69 30.60 14.40 7.14 13.50 2.72 15.30 2.74 7.45 1.11 6.90 0.86 1.25 1.57

34 7 49 17 283 55 29 1065 84.90 166.00 18.60 73.40 13.60 3.91 10.30 1.49 7.16 1.44 4.05 0.60 3.69 0.52 15.51 1.01

553 170 24 39 265 18 16 8 49.70 97.30 10.31 36.20 6.24 1.76 5.47 0.94 5.85 1.22 3.74 0.57 3.42 0.53 37.28 0.95

307 158 23 14 209 6 12 4 74.10 118.30 12.01 41.30 5.57 1.42 3.74 0.50 2.67 0.53 1.48 0.21 1.34 0.19 15.51 1.01

BPR – biotite paragneiss: ZCH-5, ZCH-12 – Zadní Chodov, 23. level, gallery V-23, SHZ – infill of shear zone: ZCH-6 – Zadní Chodov, 23. level, gallery V-23, ZCH-25 – Zadní Chodov, 23. level, gallery V-230–6, ALTBPR – altered biotite paragneiss: ZCH-10, ZCH-11 – Zadní Chodov, 23. level, gallery V-23. Eu/Eu* = EuN/√[(SmN)*(GdN)].

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

500

Sample/Chondrite

Unaltered metasediments Altered metasediments 100

10

3 La Ce Pr NdSm Eu Gd Tb Dy Ho Er Tm Yb Lu Fig. 11. REE patterns of altered high-grade metasediments vs. unaltered high-grade metasediments from the Zadní Chodov uranium deposit. Original data normalised to chondrite according Boynton (1984).

259–262 Ma) represents the Variscan uraniferous event, whereas postore quartz and carbonate stages with occurrences of uraninite II (152–158 Ma), coffinite II (130–142 Ma) and brannerite II (140– 185 Ma) are part of the younger Mesozoic events. Similar age for uraninite (295 ± 5 Ma) from the Bavarian uranium ore deposits was published by Dill (1985, 1986). However, Dill (1985) for brannerite gives also the Variscan age (290 ± 70 Ma). In comparison to the French deposits (Poty et al., 1986), those from the western part of the Bohemian Massif (the Fichtelgebirge and Bor pluton area, e.g., Dill, 1985, 1986; Hecht et al., 1991) are characterized by higher activity of Mg-rich solutions and by younger carbonatisation. The lower Fe/Fe + Mg ratio of chlorites from uranium deposits in the western part of the Bohemian Massif when compared to the Massif Central (e.g., Cathelineau, 1986) is related to higher activity of the Mg-rich fluids in the study area. According to Dill (1986) the sources of Mg can be traced back to carbonates and calc-silicate rocks of the Moldanubian Zone. 6.2. Occurrence of brannerite The ore stage in uranium deposits in the Bor pluton area is represented by deposition of coffinite I, brannerite and uraninite I. Occurrence of brannerite for all uranium deposits in the Bor pluton area is significant. Brannerite is neither found in other Variscan uranium deposits in the Bohemian Massif, nor in similar ore deposits in France. Brannerite has been found in aceite-hosted uranium deposits in the Mount Isa uranium district, Quennsland, Australia (Polito et al., 2009; Wilde et al., 2013), and further similar uranium occurrences have

11

Table 3 Representative analyses of brannerite (Zadní Chodov, 23. level, gallery V-23). wt.% UO2 UO3 ThO2 TiO2 FeO CaO MnO SiO2 ZrO2 Sc2O3 Nb2O5 Al2O3 PbO Ce2O3 Pr2O3 Nd2O3 Sm2O3 Gd2O3 Tb2O3 Dy2O3 Er2O3 Yb2O3 Y2O3 Total

22 28.16 25.72 0.24 38.33 1.63 2.92 0.49 0.02 0.49 0.83 0.05 0.00 0.03 0.23 0.16 0.10 0.20 0.10 0.00 0.06 0.00 0.01 0.30 100.07

24 34.32 19.08 0.11 37.57 1.84 2.74 0.39 0.06 0.35 0.36 0.04 0.00 0.00 0.20 0.00 0.16 0.15 0.20 0.02 0.00 0.03 0.02 0.42 98.06

25 28.63 25.44 0.01 37.33 1.28 2.72 0.45 0.24 0.18 0.23 0.11 0.00 0.00 0.25 0.02 0.35 0.24 0.15 0.05 0.22 0.00 0.09 0.51 98.50

27 34.38 21.20 0.06 37.46 1.67 2.70 0.38 0.01 0.05 0.23 0.06 0.00 0.00 0.25 0.03 0.36 0.15 0.08 0.04 0.17 0.07 0.11 0.44 99.90

28 25.68 27.22 0.04 37.39 1.78 2.96 0.48 0.05 0.29 0.35 0.06 0.01 0.01 0.47 0.16 0.71 0.27 0.14 0.00 0.17 0.02 0.06 0.51 98.83

29 29.44 26.38 0.04 36.73 2.02 2.50 0.47 0.02 0.08 0.31 0.09 0.00 0.02 0.25 0.05 0.17 0.14 0.09 0.01 0.13 0.08 0.01 0.44 99.47

31 1.93 38.04 0.33 37.32 2.19 3.58 0.34 5.54 0.21 0.17 0.15 0.45 0.08 0.77 0.04 0.83 0.36 0.29 0.01 0.23 0.09 0.12 0.49 93.56

33 28.98 25.44 0.14 37.38 1.97 3.27 0.47 0.30 0.33 0.22 0.06 0.03 0.00 0.25 0.04 0.43 0.21 0.08 0.00 0.04 0.11 0.07 0.41 100.23

apfu U4+ U6+ Ti4+ Fe2+ Ca2+ Mn2+ Si4+ Zr4+ Sc3+ Al3+ Ce3+ Nd3+ Sm3+ Gd3+ Y3+ O

0.41 0.34 1.84 0.09 0.20 0.03 0.00 0.02 0.05 0.00 0.01 0.00 0.00 0.00 0.01 6

0.52 0.26 1.87 0.10 0.19 0.02 0.00 0.01 0.02 0.00 0.00 0.00 0.00 0.00 0.01 6

0.43 0.36 1.84 0.07 0.19 0.02 0.02 0.01 0.01 0.00 0.01 0.01 0.01 0.00 0.02 6

0.51 0.29 1.85 0.09 0.19 0.02 0.00 0.00 0.01 0.00 0.01 0.01 0.00 0.00 0.02 6

0.38 0.37 1.82 0.10 0.21 0.03 0.00 0.01 0.02 0.00 0.01 0.02 0.01 0.00 0.02 6

0.44 0.38 1.81 0.11 0.18 0.03 0.00 0.00 0.02 0.00 0.01 0.00 0.00 0.00 0.02 6

0.03 0.48 1.68 0.11 0.23 0.01 0.33 0.01 0.01 0.03 0.02 0.02 0.01 0.01 0.02 6

0.41 0.34 1.81 0.11 0.23 0.03 0.02 0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.01 6

been reported in Aricheng South, Guyana (Alexandre, 2010) and in metasomatised metasedimentary rock series of the Central Ukrainian uranium province (Cuney et al., 2012). Brannerite occurs also as accessory minerals in granites and pegmatites (Lumpkin et al., 2012). The chemical composition of brannerite is nominally UTi2O6. However, in brannerite from the above mentioned uranium deposits U may be replaced by Ca, Th, Y and REE, while Si, Al and Fe can replace

Fig. 12. Back-scattered electron images of brannnerite (Brn), altered brannerite (Abrn) and rutile (Rt) enclosed in fine-grained chlorite (Chl).

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

12

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

Ti as a result of oxidation and partial hydration (Smith, 1984). The presence of Pb in its structure is mainly due to the decay of U. Negative correlation between Fe + Al and Ti in unaltered brannerite from the Bor pluton area (Fig. 13 b) implies that a coupled substitution must exist in order to account for charge balance. For altered brannerite negative correlation between U and Ti is highly significant (Fig. 13 c). Similar negative correlation was found also in altered brannerites from brannerite bearing occurrence in Guyana (Alexandre, 2010). The unaltered brannerite from the Bor pluton area in comparison to brannerite occurred in the above-mentioned aceite-hosted deposits in Australia and Guyana is depleted in Si and Al. However, altered brannerites from the Bor pluton area and uranium occurrence of Aricheng South in Guyana (Alexandre, 2010) are enriched in Si (Fig. 13 a, d). Brannerites from metasomatic uranium deposits of the Central Ukrainian uranium province are enriched in Si and Ca, when compared to brannerite from the Bor pluton area (Cuney et al., 2012), while brannerites which occurred as accessories in granites and pegmatites are enriched in Th (Lumpkin et al., 2012). The titanium necessary for the formation of brannerite in the Bor pluton shear zones was probably released during chloritisation of Tienriched biotite and hydrothermal alteration Ti-rich accessories (titanite, allanite), which occurs in unaltered high-grade metasediments and granitoids of this area. In particular high concentration of titanite in metasediments (Zadní Chodov, Lhota), granodiorites and tonalites of the Bor pluton (Zadní Chodov, Vítkov II) was significant source of Ti for brannerite origin. However, Dill (1985) proposed rutile and hydrated Ti-oxides that originated during pre-ore alteration stage by chloritisation of biotite as a possible source of Ti for brannerite from similar uranium deposits in Bavaria (e.g., Wäldel/Mähring). The

source of uranium was either the metamorphic rock series (Dill, 1986) or granites (Fiala, 1986). 6.3. Thermometry of hydrothermal alterations The temperature of the ore stage could be estimated from chlorite thermometry applied to chlorite II, and yielded a range from 122 to 258 °C. Similar temperatures, also estimated from chlorite thermometry were found for the uranium mineralisation at the Hebanz uranium deposit, occurring in the Fichtelgebirge pluton (Hecht et al., 1991). The temperatures of the post-ore quartz stage estimated from fluid-inclusion thermometry on samples form the Vítkov II and Zadní Chodov uranium deposits (Topp, 1993) suggest on significant temperature fluctuation occurred during post-ore stage from 66 °C to 260 °C. 7. Conclusion The uranium mineralisation in the Bor pluton area is evolved in the two distinctly different rocks environment, namely in granitoids of the Bor pluton and in high-grade metasediments of the Moldanubian zone. Uranium deposits coupled with granitoids of the Bor pluton are accompanied by an intense removal of quartz, hematitisation, chloritisation and albitisation. The metasomatic coffinite and/or coffinite–uraninite mineralisation mineralisation evolved especially in the Vítkov II uranium deposit is coupled with highly intensive albitisation and carbonatisation of biotite granites in vicinity of the fault 0–30. These alterations were accompanied by the depletion in Si, K, Sr, Ba, REE and by enrichment in U, Na, P, Ti, Mg and Ca. The altered high-grade metasediments occurring in shear-zones on the western

0.4

0.2

a

b

Fe + Al

Ca

0.3

0.2

0.1

0.1

0.0

0.0

0.0

0.2

0.4

0.6

1

2

3

4+

U

Ti

1.0

0.3

c

d

0.8

Si/Ti

U4+ + U6+

0.2 0.6

0.4

0.0 0.2

-0.1

0.0 1

2

3

0.0

0.1

Brannerite

0.2

0.3

U4+/Ti

Ti Altered brannerite Fig. 13. Chemical composition of brannerite and altered brannerite.

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033

M. René / Ore Geology Reviews xxx (2016) xxx–xxx

boundary of the Bor pluton contain especially high concentrations of chorite, clay minerals (predominantly illite) and carbonacesous matter. There are enriched in U, Ca, Mg, P, Fe3+ and depleted in Ti, Si, K, Ba, Rb, U and REE. The distinctly higher alteration of high-grade metasediments in area of the Zadní Chodov uranium deposit as alteration occurred in albitised granites of the Bor pluton was caused by different mineralogical composition of the both host rock series and by more intensive tectonic movements along the Zadní Chodov fault. Disseminated coffinite or coffinite–brannerite mineralisation occurred in shear zones which evolved in high-grade metamorphic rocks on the western boundary of the Bor pluton. Origin of brannerite was induced by high concentrations of Ti in biotite and occurrence of titanite in high-grade metasediments and granodiorites to tonalites of the Bor pluton. Brannerite, which occurs in examined uranium mineralisations, is present in unaltered and highly altered grains with variable concentrations U4+, U6+, Ti, Al, Ca and low concentrations of Th, Y and REE. The distinctly higher concentrations of brannerite in altered metasediments of the Zadní Chodov uranium deposits or in vicinity of altered metamorphic rocks (paragneisses, amphibolites) in the Lhota uranium deposit is very probably controlled by higher concentrations of suitable Ti sources in these rocks (Ti-enriched biotite, titanite). The temperature of the ore stage was estimated from chlorite thermometry and ranged from122 °C to 258 °C. Acknowledgements Thanks are due to my colleagues from DIAMO Co. for their help in sample collection. I thank Drs. K. Romanidis and V. Fiala from DIAMO Co. for a number of suggestions and recommendations. I wish also to thank A. Szameitat for her constructive remarks and English corrections. Editor-in-Chiew, Prof. F. Piranjo, associate editor Prof. H. Dill and two anonymous reviewers I thanked for their perceptive reviews of the manuscript, valuable comments and recommendations. The research for this paper was carried out thanks to the support of the long-term conceptual development research organisation RVO: 67985891. References Alexandre, P., 2010. Mineralogy and geochemistry of the sodium metasomatism-related uranium occurrence of Aricheng South, Guyana. Mineral. Deposita 45, 351–367. Arapov, J.A., et al., 1984. Czechoslovak uranium deposits. SNTL, Prague in Czech. Boynton, W.V., 1984. Geochemistry of the rare earth elements: meteorite studies. In: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 63–114. Breiter, K., Sokol, A., 1997. Chemistry of the Bohemian granitoids. Geotectonic and metallogenic implications. Sbor. Geol. Věd, LG 31, 75–96. Cathelineau, M., 1986. The hydrothermal alkali metasomatism effects on granitic rocks: quartz dissolution and related subsolidus changes. J. Petrol. 27, 945–965. Collins, W.J., et al., 1982. Nature and origin of A-type granites with particular reference to Southeastern Australia. Contrib. Mineral. Petrol. 80, 189–200. Cuney, M., Emetz, A., Mercadier, J., Mykchaylov, V., Shunko, V., Yuslenko, A., 2012. Uranium deposits associated with Na-metasomatism from Central Ukraine: a review of some of the major deposits and genetic constraints. Ore Geol. Rev. 44, 82–106. Dahlkamp, F.J., 1993. Uranium Ore Deposits. Springer Verlag, Berlin. Dill, H., 1983. Lagerstättengenetische Untersuchungen im Bereich der Uranerz-Struktur Wäldel/Mähring (NE-Bayern). Geol. Rdsch. 72, 329–352. Dill, H., 1985. Die Vererzung am Westrand der Böhmischen Masse – Metalogenese in einer ensialischen Orogenzone. Geol. Jahrb. Reihe D 73, 1–461. Dill, H., 1986. Fault Controlled Uranium Black Ore Mineralization From the Western Edge of the Bohemian Massif (NE-Bavaria, FR Germany). Vein-type Uranium Deposits, TECDOC–361. pp. 303–323. Dill, H., Weiser, T., 1981. Eine Molybdänsulfid-Impsonit-Mineralisation aus den Uranvorkommen Wäldel/Mähring (Oberpfalz). N. Jb. Miner. Mh. 1981 (10), 452–458. Doležel, M., et al., 1975. About metasomatic uranium deposits of the Bohemian Massif. Geol. rud. mestorozd. 6, 42–52 in Russian. Dörr, W., et al., 1997. Dating of collapse related plutons along the West- and Central Bohemian shear zones (European Variscides). Terra Nostra 97, 31–34.

13

Fettes, D., Desmons, J. (Eds.), 2007. Metamorphic Rocks. A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks. Cambridge University Press, Cambridge. Fiala, V., 1980a. a. Die hydrothermale Verwandlung des Bor - Granits. I. Folia Mus. Rer. natur. Boh. occident. Geol. 16, 1–28. Fiala, V., 1980b. b. Die hydrothermale Verwandlung des Bor - Granits II. Folia Mus. Rer. natur. Boh. occident. Geol. 17, 1–31. Fiala, V., 1986. Conception of the unified structure of deposits UDZČ enterprise. Hornická Příbram ve vědě a technice. Ložisková geologie, pp. 365–375 in Czech. Grant, J.A., 1986. The isocon diagram—a simple solution to Gresens equation for metasomatic alteration econ. Geol. 81, 1976–1982. Hecht, L., Spiegel, W., Morteani, G., 1991. Multiphase alteration including disseminated uranium mineralization in quartz-depleted granites (episyenites) of the Fichtelgebirge (Northeastern Bavaria, Germany). In: Poty, B. (Ed.), Source, Transport and Deposition of Metals. Proc. SGA Meeting, pp. 53–56. Holovka, D., Hnízdo, E., 1992. Final Report About Exploration on Uranium. Bor Massif. DIAMO, Příbram, Unpublished Report. pp. 1–280 in Czech. Kafka, J. (Ed.), 2003. Czech Ore and Uranium Mining Industry. Anagram, Ostrava in Czech. Kříbek, B., 1981. The role of organic substances in the process of hydrothermal mineralization. Geol. Capath. 32, 605–614. Kříbek, B., et al., 2009. The Rožná uranium deposit (Bohemian Massif, Czech Republic): shear zone-hosted, late Variscan and post-Variscan hydrothermal mineralization. Mineral. Deposita 44, 99–128. Lumpkin, G.R., Leung, S.H.F., Ferenczy, J., 2012. Chemistry, microstructure, and aplha decay damage of natural brannerite. Chem. Geol. 291, 55–68. Mrázek, P., Fiala, V., 1979. Die hydrothermalen Verwandlungsminerale der Lagerstätte Zadní Chodov. Folia Mus. Rer. natur. Boh. occident. Geol. 14, 1–15. Novák, J., Paška, R., 1983. Final Report About Exploration of the Eastern Shear-Zone on the Zadní Chodov-Eastern Uranium Deposit. Uranium industry, Příbram, pp. 1–56 unpublished report. in Czech. Novák, J., Paška, R., Kovalová, M., Novická, Z., 1983. Final report about exploration works in the Hlinné district in the years 1975–1982. Uranium industry, Příbram, pp. 1–76 unpublished report. in Czech. Ordynec, G.E., et al., 1987. Alters of the uranium mineralization of deposits at the WestBohemian ore district. Geol. Hydrometalurg. Uranu 11, 23–64 in Czech. Polito, P.A., Kyser, T.K., Stanley, C., 2009. The Proterozoic, albitite-hosted, Valhalla uranium deposit, Queensland, Australia: a description of the alteration assemblage associated with uranium mineralisation in diamond drill hole V39. Mineral. Deposita 44, 11–40. Poty, B., et al., 1986. Uranium Deposits Spatially Related to Granites in the French Part of the Hercynian Orogens. Vein Type Uranium Deposits, TECDOC-361. pp. 215–260. Pouchou, J.L., Pichoir, F., 1985. “PAP” (φ-ρ-Z) procedure for improved quantitative microanalysis. In: Armstrong, J.T. (Ed.), Microbeam Analysis. San Francisco Press, San Francisco, pp. 104–106. René, M., 2000. Petrogenesis of the Variscan granites in the western part of the Bohemian Massif. Acta Montana 116, 67–83. René, M., 2015. Rare-earth, yttrium and zirconium mobility associated with the uranium mineralisation at Okrouhlá Radouň, Bohemian Massif, Czech Republic. Eur. J. Mineral. 27, 57–70. Siebel, W., 1994. Inferences about magma mixing and thermal events from isotopic variation in redwitzites near the KTB-site. Ger. Cont. Deep. Drill. Prog. KTB Rep. 94–3, 157–164. Siebel, W., et al., 1997. Granitoid magmatism of the NW Bohemian massif revealed: gravity data, composition, age relations and phase concept. Geol. Rundsch., Suppl. 86, 45–63. Siebel, W., et al., 1999. Petrogenesis of contrasting granitoid plutons in western Bohemia (Czech Republic). Mineral. Petrol. 65, 207–235. Smith Jr., D.K., 1984. Uranium mineralogy. In: de Vivo, B., Ippolito, F., Capaldi, G., Simpson, P.R. (Eds.), Uranium Geochemistry, Mineralogy, Geology, Exploration and Resources. The Institution of mining and metallurgy, London, pp. 43–88. Topp, J., 1993. Flüssigkeitseinschluss-Untersuchungen am Bohrkernmaterial der KTB bis 4000 Meter. Göttinger Arb. Geol. Paläont. 61, 1–58. Troll, G., 1968. Gliederung der redwitzitischen Gesteine Bayerns nach Stoff- und Gefügemerkmalen, 1. Die Typlokalität von Marktredwitz in Oberfranken. Bayerische Akad. Wiss. Abh. 35, 1–87. Walshe, J.L., 1986. A six-component chlorite solid solution model and the conditions of chlorite formation in hydrothermal and geothermal systems. Econ. Geol. 81, 681–703. Whalen, J.B., Currie, K.L., Chappell, B.W., 1987. A-type granites: geochemical characteristics, discrimination and petrogenesis. Contrib. Mineral. Petrol. 95, 407–419. Wilde, A., et al., 2013. Geology and mineralogy of uranium deposits from Mount Isa, Australia: implications for albitite uranium deposit models. Minerals 3, 258–283. Wimmenauer, W., 1984. Das prävarskische Kristallin im Schwarzwald. Fortschr. Mineral. 62, 69–86. Zulauf, G., 1994. Ductile normal faulting along the West Bohemian shear zone (Moldanubian/Teplá–Barrandian boundary): evidence for late Variscan extension collapse in the Variscan Internides. Geol. Rundsch. 83, 276–292.

Please cite this article as: René, M., Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic, Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.09.033