Article , Coffinite and Ningyoite from Vein-Type Uranium Deposits of the Bohemian Massif (Central European Variscan Belt)

Miloš René 1,*, ZdenˇekDolníˇcek 2, Jiˇrí Sejkora 2, Pavel Škácha 2,3 and Vladimír Šrein 4 1 Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, 182 09 Prague, Czech Republic 2 Department of Mineralogy and Petrology, National Museum, 193 00 Prague, Czech Republic; [email protected] (Z.D.); [email protected] (J.S.); [email protected] (P.Š.) 3 Mining Museum Pˇríbram, 261 01 Pˇríbram, Czech Republic 4 Czech Geological Survey, 152 00 Prague, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +420-266-009-228

 Received: 26 November 2018; Accepted: 15 February 2019; Published: 19 February 2019 

Abstract: Uraninite-coffinite vein-type mineralisation with significant predominance of uraninite over coffinite occurs in the Pˇríbram, Jáchymov and Horní Slavkov ore districts and the Pot ˚uˇcky, Zálesí and Pˇredboˇriceuranium deposits. These uranium deposits are hosted by faults that are mostly developed in low- to high-grade metamorphic rocks of the basement of the Bohemian Massif. Textural features and the chemical composition of uraninite, coffinite and ningyoite were studied using an electron microprobe. Collomorphic uraninite was the only primary uranium in all deposits studied. The contained variable and elevated concentrations of PbO (1.5 wt %–5.4 wt %), CaO (0.7 wt %–8.3 wt %), and SiO2 (up to 10.0 wt %), whereas the contents of Th, Zr, REE and Y were usually below the detection limits of the electron microprobe. Coffinite usually forms by gradual coffinitization of uraninite in ore deposits and the concentration of CaO was lower than that in uraninites, varying from 0.6 wt % to 6.5 wt %. Coffinite from the Jáchymov ore district was partly enriched in Zr (up to 3.3 wt % ZrO2) and Y (up to 5.5 wt % Y2O3), and from the Pot ˚uˇckyuranium deposit, was distinctly enriched in P (up to 8.8 wt % P2O5), occurring in association with ningyoite. The chemical composition of ningyoite was similar to that from type locality; however, ningyoite from Pot ˚uˇckywas distinctly enriched in REE, containing up to 22.3 wt % REE2O3.

Keywords: vein-type uranium deposits; uraninite; coffinite; ningyoite; Bohemian Massif; Variscides

1. Introduction The Bohemian Massif is the easternmost segment of the European Variscan belt, which hosts a large number of uranium deposits (Figure1)[ 1–3]. The estimated total historical production of uranium, as high as 350,000 tonnes [4], makes the Bohemian Massif the most important uranium ore district in Europe. More than three-quarters of the total uranium extracted in the territory of the Czech Republic (which slightly exceeds 100,000 tonnes) originated from ore deposits developed in fault structures hosted by metamorphic rocks, igneous rocks and/or folded sediments. Two principal types of these basement-hosted deposits can be distinguished. The first type is represented by deposits hosted by shear zones, usually containing low-grade uranium mineralisation disseminated in strongly hydrothermally altered and mylonitized rocks. Typical examples of the shear-zone hosted deposits of the Bohemian Massif include the Rožná and Olší deposits (total production 23,000 t U) and Zadní Chodov, Dyleˇnand Vítkov deposits (9800 t U). The second type includes vein-type deposits, characterized by open-space crystallization of hydrothermal minerals, often giving rise

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very high-grade ores. The most important examples of vein-type uranium mineralisation in the to very high-grade ores. The most important examples of vein-type uranium mineralisation in the Bohemian Massif include Příbram (51,000 t U) and Jáchymov (Joachimsthal; 8500 t U) (Figure 1). BohemianModern Massif data include on the Pˇr chemicalíbram (51,000 composition t U) and of J áprimarychymov uranium (Joachimsthal; minerals 8500 from t U) (FigureCzech uranium1). depositsModern are, datain general, on the very chemical limited. composition Recently, se ofveral primary detailed uranium studies minerals were published from Czech [4–6], uraniumfocussing depositson the nature are, in and general, chemical very composition limited. Recently, of uraninite, several detailedcoffinite studies and brannerite were published from shear-zone [4–6], focussing hosted onuranium the nature deposits, and chemical but published composition data on of uraninite,the chemical coffinite composition and brannerite of uranium from minerals shear-zone from hosted vein- uraniumtype deposits deposits, are but restricted published to datathe Jáchymov on the chemical and Zálesí composition deposits of uranium[7–9]. For minerals ningyoite, from which vein-type was depositsreported are from restricted two Czech to the basement-h Jáchymovosted and Zveináles depositsí deposits [10], [7– 9no]. Forcomp ningyoite,lete modern which analysis was reported has been frompublished. two Czech basement-hosted vein deposits [10], no complete modern analysis has been published. ThisThis paperpaper presentspresents newnew electronelectron microprobemicroprobe datadata onon thethe chemicalchemical compositioncomposition ofof primaryprimary uraniumuranium mineralsminerals (uraninite,(uraninite, coffinitecoffinite and ningyoite) from from six six typical typical vein-type vein-type uranium uranium deposits deposits of ofthe the Czech Czech part part of ofthe the Bohemian Bohemian Massif Massif (i.e., (i.e., Příbram, Pˇríbram, Jáchymov, Jáchymov, Pot Potůčky, ˚uˇcky, Horní Horn Slavkov,í Slavkov, Zálesí Záles andí andPředbo Pˇredboˇrice;Figureřice; Figure 1).1 ).Selection Selection of of sampling sampling sites sites reflects reflects variousvarious geologicalgeological settings, settings, paragenetic paragenetic situations,situations, intensitiesintensities ofof superimposedsuperimposed alterationsalterations andand probablyprobably variousvarious agesages ofof Czech Czech vein-type vein-type uraniumuranium deposits. deposits. OurOur analyticalanalytical workwork coveredcovered aa largerlarger than than average average number number of of samples samples from from the the majoritymajority of of deposits/districts deposits/districts studiedstudied inin orderorder toto alsoalso characterizecharacterize potentialpotential variability variability in in chemical chemical compositioncomposition of of primary primary uranium uranium minerals minerals at at the the deposit/district deposit/district scale.scale. BesidesBesides mineralogists mineralogists and and economic economic geologists, geologists, this th datais data on on chemical chemical composition composition and and texture texture of well-localizedof well-localized uranium uranium ores ores may may also bealso of be interest of interest to specialists to specialists in nuclear in nuclear forensics, forensics, a field thata field is stillthat growing,is still growing, particularly particularly because because there is anthere increasing is an increasing international international interest in interest the compositional in the compositional analysis ofanalysis radioactive of radioactive minerals thatminerals can be that used can to be fingerprint used to fingerprint localities [localities11–16]. [11–16].

FigureFigure 1. 1.Simplified Simplified geological geological map map of of the the Bohemian Bohemian Massif Massif showing showing the the position position of of the the most most significant significant CzechCzech basement-hosted basement-hosted uranium uranium deposits. deposits.

2.2. Geological Geological Setting Setting and and Mineralization Mineralization

2.1.2.1. Pˇr Pířbramíbram Uranium Uranium and and Base Base Metal Metal Districts Districts TheThe Pˇr Pířbramíbram uranium uraniumand andbase basemetal metaldistricts districtsare are located locatedin in the the central central part part of of the the Bohemian Bohemian MassifMassif (Figure (Figure1 ).1). This This ore ore region region consists consists of of two two main main ore ore districts: districts: The The Bˇrezov Březovéé Hory Hory base base metal metal districtdistrict and and the the Pˇr Pířbramíbram uranium uranium district district (Figure (Figure2 )[2) 17[17],], both both being being located located in in a a tectonically tectonically complex complex zonezone thatthat isis situated situated in in between between the the slightly slightly metamorphosed metamorphosed Proterozoic-PalaeozoicProterozoic-Palaeozoic ofof thethe TeplTepláá BarrandianBarrandian unit unit and and the the Variscan Variscan Central Central Bohemian Bohemian plutonic plutonic complex complex (CBPC) (CBPC) [ 18[18].]. The The Bˇrezov Březovéé Hory Hory ˇ basebase metal metal district district is is usually usually subdivided subdivided into into Bohut Bohutín,ín, central central Bˇrezov Březovéé Hory Hory and andCernojamsk Černojamskéé ore ore ˇ depositsdeposits (Figure (Figure2 ).2). The The Bˇrezov Březovéé Hory Hory and andCernojamsk Černojamskéé deposits deposits were were also also shown shown to to contain contain some some ˇ uraniumuranium mineralisationmineralisation (J(Jánskáánská and andCernojamsk Černojamskáá veins) veins) [ 19[19,20].,20]. TheThe PˇrPířbramíbram uraniumuranium districtdistrictis is largely hosted by a Neoproterozoic flysch sequence (Figure 2). The ore veins form 20 vein clusters,

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Minerals 2018, 8, x FOR PEER REVIEW 3 of 21 largely hosted by a Neoproterozoic flysch sequence (Figure2). The ore veins form 20 vein clusters, which were grouped into nine ore knots (Tˇrebsko,Kamenná, Lešetice, Brod, Jerusalém, Háje, Bytíz, which were grouped into nine ore knots (Třebsko, Kamenná, Lešetice, Brod, Jerusalém, Háje, Bytíz, Skalka and Oboˇrištˇe;Figure2)[ 21], and comprise four mineralisation stages (siderite–sulphide, calcite, Skalka and Obořiště; Figure 2) [21], and comprise four mineralisation stages (siderite–sulphide, calcite–uraninite and calcite–sulphide). calcite, calcite–uraninite and calcite–sulphide).

Figure 2. Schematic geological map of the PˇrPříbramíbram base metal and uranium ore districts (modified(modified from KˇrKřííbekbek et al. [[17])17])..

The main ore minerals are uraninite and uraniferous anthraxolite, whereas coffinite coffinite is far less abundant. Bitumens from uraniferous anthraxolite were studied by K Kˇrříbekíbek et al. al. [17] [17] in in greater greater detail. detail. Uraninite fromfrom the the Lešetice Lešetice deposit deposit was was dated date byd Andersonby Anderson [22], [22], giving giving two concordanttwo concordant206Pb/ 206238Pb/U238 andU 207andPb/ 207Pb/235U235 agesU ages (275 (275± ±4 4 Ma Ma and and 278 278± ± 44 Ma).Ma). TheThe uranium minerals occur as veinlets, coatings and pods in calcitecalcite gangue.gangue. Along with thisthis lithologicallithological control, the localizationlocalization of economiceconomic mineralizationmineralization was guided by structural control, displayed by thethe shape of individual faults, the character of the cleavage system and the position relative to the main fault andand foldfold structuresstructures [[21].21]. Approximately 98% of uranium ores were mined from the central partpart of the uranium district (Lešetice, Jerusalem, HHájeáje and BytBytízíz ore segments). The whole P Pˇrříbramíbram uranium district yielded 48,432.2 t U from ores averaging 0.33 wt % U. Parallel mining mining of of base base metal metal and and silver silver produced produced more more than than 6,100 6100 t of Pb, 24002,400 tt ofof ZnZn andand 2828 tt ofof AgAg [[2].2].

2.2. Jáchymov Jáchymov Uranium District and Pot˚uˇckyDepositPotůčky Deposit 2 The JJáchymováchymov uranium district covers approximately 45 km 2 ofof the the central area of a NE–SW trending antiformalantiformal structurestructure ofof thethe KrušnKrušnéé Hory/ErzgebirgeHory/Erzgebirge Mts.Mts. This This unit unit belongs belongs to the Saxothuringian Zone,Zone, whichwhich is is the the most most complex complex part part of of the the central central European European Variscides Variscides [23 ].[23]. This This ore districtore district is located is located at the at intersection the intersection of two of regional two regional fault zones, fault the zones, NW–SE the strikingNW–SE Gera-J strikingáchymov Gera- faultJáchymov zone andfault the zone ENE–WSW and the ENE–WSW striking Krušn strikingé Hory Kruš faultné zoneHory (Figure fault zone3)[ 24 (Figure]. The host3) [24]. rocks The of host the Jrocksáchymov of the ore Jáchymov district are ore Neoproterozoic district are Neoprote and Cambrianrozoic metamorphicand Cambrian rocks metamorphic of the Jáchymov rocks of series, the overlyingJáchymov theseries, granitic overlying rocks ofthe the granitic Eibenstock-Nejdek rocks of the pluton. Eibenstock-Nejdek Two vein groups pluton. could Two be distinguished vein groups incould the Jábechymov distinguished uranium in district: the Jáchymov the ore-rich uraniu N–S veinsm district: and weakly the ore-rich mineralized N–S orveins barren and E–W weakly veins. mineralizedUranium or mineralizationbarren E–W veins. is bound primarily to the carbonate-uraninite stage, but remobilised coffinite and uraninite also occur in the arsenide and sulphide stages. The published U-Pb ages cover a wide range from 76 to 286 Ma [25]. However, more precise dating from the Niederschlema-Alberoda uranium deposit, which is situated in the German part of the Jáchymov-Gera fault zone, gave ages

Minerals 2019, 9, 123 4 of 23 of 271 ± 6, 190 ± 4 and 120 ± 6 Ma for uranium minerals originating from the carbonate-uraninite, arsenic-sulphide and sulphide stages, respectively [26]. Uranium ores in the Jáchymov ore district have been mined since 1853 for the production of uranium paints and, at the beginning of the 20th century, as a source of radium. However, up to 1945, only 469.5 t U were mined [27]. From 1945 to 1994, 7950 t U were mined from ores containing an average of 0.30 wt %–0.35 wt % U. The main uranium ore production yielded the Rovnost-Eliáš-Eduard (3178.9 t U) and Eva-Barbora (1725.6 t U) ore clusters [2]. Minerals 2018, 8, x FOR PEER REVIEW 4 of 21

FigureFigure 3. Schematic3. Schematic geological geological map map of theof the Jáchymov Jáchymov uranium uranium district district (modified (modified from from Kom Komínekínek et al. et [al.24 ]). [24]). The Pot ˚uˇckydeposit is a small uranium deposit situated NE of the Jáchymov ore district and on theUranium SE edge mineralization of the Johanngeorgenstadt is bound primarily uranium to districtthe carbonate-uraninite in Germany [28]. stage, This uranium but remobilised deposit developedcoffinite and in uraninite rocks of the also Já occurchymov in the series. arsenide Uranium and mineralizationsulphide stages. was The concentrated published U-Pb in NW–SE ages cover and N–Sa wide ore range veins from [29], 76 whose to 286 nature Ma [25]. is veryHowever, similar more to that precise of the dating Jáchymov from the uranium Niederschlema-Alberoda district. Uranium mineralization,uranium deposit, represented which is situated by the calcite–uraninite in the German part stage, of the is formed Jáchymov-Gera by uraninite, fault coffinite zone, gave and ages newly of recognised271 ± 6, 190 ± ningyoite. 4 and 120 Economic± 6 Ma for uranium mineralization minerals was originating concentrated from at the depths carbonate-uraninite, up to 150 m below arsenic- the surface.sulphide From and sulphide 1954 to 1964, stages, the respec deposittively yielded [26]. 323.6Uranium t U [ 2ores,28]. in the Jáchymov ore district have been mined since 1853 for the production of uranium paints and, at the beginning of the 20th century, as 2.3.a source Horn ofí Slavkov radium. Uranium However, District up to 1945, only 469.5 t U were mined [27]. From 1945 to 1994, 7950 t U were minedThe from Horn oresí Slavkov containing uranium an average district of 0.30 occurs wt in%–0.35 Proterozoic wt % U. metamorphic The main uranium rocks ofore the production Slavkov crystallineyielded the unitRovnost-Eliáš-Eduard in the western part (3178.9 of thet U) Bohemianand Eva-Barbora Massif (1725.6 (Figure t 1U)). ore This clusters unit consists[2]. of coarse-grainedThe Potůčky biotite deposit orthogneisses is a small uranium and paragneiss deposit series. situated The NE Slavkov of the crystallineJáchymov ore unit district was subsided and on belowthe SE Variscan edge of granites.the Johanngeorgenstadt Two principal fault uranium systems—NE–SW district in Germany and NW–SE—cut [28]. This this uranium crystalline deposit unit. Uraniumdeveloped mineralization in rocks of the occurs Jáchymov in veins series. parallel Uraniu tom NW–SE mineralization faults and was is concentratedconcentrated inin veinNW–SE clusters and (FigureN–S ore4 ),veins often [29], being whose present nature in ore is very lenses similar and consisting to that of ofthe coffinite, Jáchymov two uranium generations district. of uraninite Uranium andmineralization, rarely, ningyoite represented [10]. The by the mineralization calcite–uraninite showed stage, vertical is formed zoning by uraninite, with quartz–calcite–coffinite coffinite and newly mineralizationrecognised ningyoite. in the upper Economic parts ofmineralization ore veins, whereas was concentrated –uraninite at depths mineralization up to 150 m occurred below the in thesurface. central From parts 1954 of oreto 1964, veins. the The deposit uranium yielded mineralization 323.6 t U [2,28]. mainly occurred in biotite paragneisses and amphibolites, especially at (i) the intersection of veins with faults (ii) a change in vein strike or dip and (iii)2.3. atHorní adjoining Slavkov stringers Uranium [ 24District]. The uranium ores were mined from 1948 to 1962, yielding 2668.3 t U. The mainThe Horní uranium Slavkov production uranium was district concentrated occurs in theProterozoic Barbora metamorphic (779.3 t U), Ležnice rocks (650.9of the tSlavkov U) and ZdaˇrBcrystalline ˚uh(539.1 unit in t U) the ore western clusters part [2]. of the Bohemian Massif (Figure 1). This unit consists of coarse- grained biotite orthogneisses and paragneiss series. The Slavkov crystalline unit was subsided below Variscan granites. Two principal fault systems—NE–SW and NW–SE—cut this crystalline unit. Uranium mineralization occurs in veins parallel to NW–SE faults and is concentrated in vein clusters (Figure 4), often being present in ore lenses and consisting of coffinite, two generations of uraninite and rarely, ningyoite [10]. The mineralization showed vertical zoning with quartz–calcite–coffinite mineralization in the upper parts of ore veins, whereas dolomite–uraninite mineralization occurred in the central parts of ore veins. The uranium mineralization mainly occurred in biotite paragneisses and amphibolites, especially at (i) the intersection of veins with faults (ii) a change in vein strike or dip and (iii) at adjoining stringers [24]. The uranium ores were mined from 1948 to 1962, yielding 2,668.3 t U. The main uranium production was concentrated in the Barbora (779.3 t U), Ležnice (650.9 t U) and Zdař Bůh (539.1 t U) ore clusters [2].

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Figure 4.FigureSchematic 4. Schematic geological geological map of map the Hornof theí HorníSlavkov Slavko uraniumv uranium district district (modified (modified from from Kom Komínekínek et al. [24]). et al. [24]). 2.4. Zálesí Uranium Deposit 2.4. Zálesí Uranium Deposit 2.4.The Zálesí Záles Uraniumí uranium Deposit deposit is located on the north-eastern margin of the Bohemian Massif (Figure1), within theThe Orlica-Zálesí Snie´uraniumznik˙ crystallinedeposit is located complex on (OSCC), the north-eastern an Early Palaeozoicmargin of magmatic–sedimentarythe Bohemian Massif sequence(Figure that1), within wasmetamorphosed the Orlica-Śnieżnik during crystalline the Variscan complex orogeny (OSCC), [30an– Early32]. The Palaeozoic OSCC ismagmatic– formed by metamorphicsedimentary rocks sequence of the that Stronie was metamorphosed and Snie´ znik˙ Groupsduring the (Figure Variscan5). Theorogeny ore deposit[30–32]. occursThe OSCC within is a pocketformed of by metamorphic metamorphic rocksrocks of the the Stronie Stronie and Group, Śnież whichnik Groups is tectonically (Figure 5). thrustThe ore between deposit Snieoccurs´ znik˙ orthogneisseswithin a pocket [32]. Theof metamorphic rocks of the Stronie rocks of Group the Stronie host all Group, of the economic which is mineralization,tectonically thrust which between consists Śnieżnik orthogneisses [32]. The rocks of the Stronie Group host all of the economic mineralization, of 30Śnie individualżnik orthogneisses veins and [32]. two The stockwork rocks of bodies,the Stronie and Group mineralization host all of wasthe economic located mainly mineralization, in the N–S which consists of 30 individual veins and two stockwork bodies, and mineralization was located strikingwhich complex consists quartz-carbonateof 30 individual veins veins and up totwo 25 cmstockwork thick [33 bodies,]. One-third and mine of theralization mined orewas originated located mainly in the N–S striking complex quartz-carbonate veins up to 25 cm thick [33]. One-third of the frommainly two stockworkin the N–S bodies,striking situatedcomplex withinquartz-carbonate the “Central veins Tectonized up to 25 cm Zone”, thick and [33]. was One-third composed of the of a mined ore originated from two stockwork bodies, situated within the “Central Tectonized Zone”, and densemined net ore of subparalleloriginated from veinlets. two stockwork Three mineralization bodies, situated stages within were the distinguished—uraninite, “Central Tectonized Zone”, arsenide and was composed of a dense net of subparallel veinlets. Three mineralization stages were distinguished— and sulphide [34]. Primary uraninite was partly replaced by a coffinite-like mineral. Chemical dating uraninite, arsenide and sulphide [34]. Primary uraninite was partly replaced by a coffinite-like mineral. by electron microprobe gave ages of 232–135 Ma (median 161 Ma), and 95–15 Ma (median 43 Ma) for Chemical dating by electron microprobe gave ages of 232–135 Ma (median 161 Ma), and 95–15 Ma uraninite and “coffinite”, respectively [34]. The uranium ores of the Zálesí deposit were mined from 1959 (median 43 Ma) for uraninite and “coffinite”, respectively [34]. The uranium ores of the Zálesí deposit to 1968 yielding 405.3 t U from ore containing 0.105 wt % U [2]. were mined from 1959 to 1968 yielding 405.3 t U from ore containing 0.105 wt % U [2].

Figure 5. Geological position of the Zálesí uranium deposit (modified from Dolníček et al. [32]). FigureFigure 5. 5.Geological Geological position position of of the the Z Zálesíálesí uraniumuranium deposit (modified (modified from from Dolní Dolnčíekˇceket et al. al. [32]). [32]).

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2.5. PˇredboˇriceUranium Deposit The Pˇredboˇriceuranium deposit represents a small vein uranium deposit occurring in the Krásná Hora-SedlˇcanyOrdovician-Silurian metamorphic islet, in close contact with biotite granodiorites of the CBPC (Figure1). Uranium mineralization occurs as ore lenses or veins in the N–S direction, which are predominantly hosted by the Ordovician hornfelses. The main ore mineral is uraninite and the main gangue minerals are quartz, calcite and barite. The ore mineralization contains about 20 selenides (e.g., , bukovite, , eskebornite, fischesserite, hakite, merenskyite, milotaite, permingeatite and petˇríˇcekite)[35–38]. Uranium ores averaging 0.39 wt % U were mined from 1965 to 1975 and the total mine production was 253.3 t U [2].

3. Material and Methods Representative archive samples of ore mineralization from all the above deposits used for this study were collected during exploration and mining of the Czechoslovak Uranium Industry enterprise (recently DIAMO). Some samples were also collected from mine dumps at uranium ore deposits (e.g., Pˇríbram—Bytíz, Jáchymov—Eva, Pot ˚uˇckyand Zálesí). The uranium minerals were analyzed in polished sections. Back-scattered electron (BSE) images were acquired to study the internal fabric of mineral aggregates and individual mineral grains. Chemical analyses were performed using a Cameca SX-100 electron microprobe (Gennevilliers Cedex, France) (National Museum, Prague; ZdenˇekDolníˇcekas analyst) operating in wavelength-dispersive (WDS) mode (15 kV, 10 nA and 2 µm wide beam). The following standards and X-ray lines were used to minimize line overlaps: Al—sanidine (Al Kα), As—clinoclase (As Lα), Ba—barite (Ba Lα), Bi—Bi (Bi Mα), Ca—apatite (Ca Kα), Ce—CePO4 (Ce Lα), Cu— (Cu Kα), Dy—DyPO4 (Dy Lβ), Er—ErPO4 (Er Lα), Eu—EuPO4 (Eu Lα), Fe—hematite (Fe Kα), Gd—GdPO4 (Gd Lα), Ho—HoPO4 (Ho Lβ), La—LaPO4 (La Lα), Lu—LuPO4 (Lu Mβ), Mn—rhodonite (Mn Kα), Na—albite (Na Kα), Nb—Nb (Nb Lα), Nd—NdPO4 (Nd Lβ), Ni—Ni (Ni Kα), P—apatite (P Kα), Pb—vanadinite (Pb Mα), Pr—PrPO4 (Pr Lβ), S—celestite (S Kα), Sc—ScVO4 (Sc Kα), Si—wollastonite (Si Kα), Sm—SmPO4 (Sm Lα), Sr—celestite (Sr Lβ), Tb—TbPO4 (Tb Lα), Th—Th (Th Mα), Ti—TiO2 (Ti Kα), Tm—TmPO4 (Tm Lα), U—UO2 (U Mα), V—V (V Kα), W—scheelite (W Mα), Y—YVO4 (Y Lα), Yb—YbPO4 (Yb Lα), Zn—ZnO (Zn Kα), Zr—zircon (Zr Lα). Peak counting times were 20 s for all elements and one-half of the peak time for each background. Contents of the elements listed, which are not included in tables, were analyzed quantitatively, but had contents below the detection limit (ca. 0.01 wt %–0.04 wt % for most elements, around 0.1 wt %–0.3 wt % for REEs). Raw intensities were converted into concentrations of elements using automated “PAP” matrix-correction software [39]. When reporting the chemical composition of primary uranium minerals, the median values for each deposit were used, as it is less sensitive to extreme values than the mean. The all performed analyses of uraninite, coffinite and ningyoite are published as Supplementary Materials (Tables S1–S3). To represent the full variation, minimum and maximum values are also reported (Tables1–3). For selected elements, the Spearman correlation coefficients were presented (Tables4 and5). These were preferred over Pearson correlation coefficients, as there are more robust to aberrant values.

4. Results The predominant uranium mineral in samples from all uranium deposits studied was uraninite. Coffinite, which was also identified in our samples, was relatively rare. Uranium-bearing anthraxolite contained grains of uraninite and occurred only in the Pˇríbram uranium district (ore clusters Bytíz and Lešetice). Ningyoite was newly found as a very rare uranium mineral in some uraninite-rich dump samples from the Pot ˚uˇckydeposit. Minerals 2019, 9, 123 7 of 23

4.1. Uraninite Uraninite usually occurred in the form of spherical or botryoidal aggregates enclosed in carbonate gangue and/or was associated with quartz. In some cases, uraninite spheroids showed radial fractures that provided pathways for younger hydrothermal solutions. The activity of younger silica-rich Minerals 2018, 8, x FOR PEER REVIEW 7 of 21 solutions resulted in superimposed coffinitization of uraninite. In the Pˇríbram uranium district, uraninite occurred as spherical or botryoidally evolved aggregates, In the Příbram uranium district, uraninite occurred as spherical or botryoidally evolved aggregates, fine veinlets and fine bands within calcite (Figure6A,B). Relatively rare uraninite II occurred in fine veinlets and fine bands within calcite (Figure 6A,B). Relatively rare uraninite II occurred in later later calcite–sulphide stages and formed fine spherical coatings close to older uraninite from the calcite–sulphide stages and formed fine spherical coatings close to older uraninite from the calcite– calcite–uraninite stage (Figure6C). Uraniferous anthraxolite formed irregular fragments, rounded grains uraninite stage (Figure 6C). Uraniferous anthraxolite formed irregular fragments, rounded grains and and veinlets within calcite. Its uranium content was concentrated in numerous inclusions of uraninite veinlets within calcite. Its uranium content was concentrated in numerous inclusions of uraninite (and minor coffinite), which typically showed a disintegrated/cracked nature (Figure6D). (and minor coffinite), which typically showed a disintegrated/cracked nature (Figure 6D).

Figure 6. Back-scatteredBack-scattered electron electron (BSE) (BSE) images of uran uraniumium mineralization with distribution of Ca (apfu, O = 4) 4) in in uraninite. uraninite, P Pˇrříbramíbram uranium district. district. Urn—uraninit (A) Botryoidallye, Cal—calcite, evolved aggregates Ant—anthraxolite. of uraninite; (B) Detailed distribution of Ca in botryoidally evolved uraninite; (C) Detailed distribution of Ca in Inpartly the zonedJáchymov uraninite; uranium (D) Distributiondistrict uraninite f Ca in formed uraninite massive included spherical in anthraxolite. aggregates Urn—uraninite, or thin veinlets liningCal—calcite, wall rocks, Ant—anthraxolite. while the central areas of the veins were filled with dolomite pigmented by Fe- oxides (Figure 7A,B). The spherical morphology of the uraninite aggregates was likewise characteristic for PotInůč theky Jandáchymov Horní uraniumSlavkov. In district the Zálesí uraninite uranium formed deposit, massive uraninite spherical occurred aggregates in the form or thin of spherical veinlets and/orlining wall zoned rocks, aggregates while the (Figure central 8A,B). areas of the veins were filled with dolomite pigmented by Fe-oxides (Figure7A,B). The spherical morphology of the uraninite aggregates was likewise characteristic for Pot ˚uˇckyand Horní Slavkov. In the Zálesí uranium deposit, uraninite occurred in the form of spherical and/or zoned aggregates (Figure8A,B).

Figure 7. BSE images of uranium mineralization from the Jáchymov uranium district with distribution of Ca (apfu, O = 4). Urn—uraninite, Cfn—coffinite, Qz—quartz.

Minerals 2018, 8, x FOR PEER REVIEW 7 of 21

In the Příbram uranium district, uraninite occurred as spherical or botryoidally evolved aggregates, fine veinlets and fine bands within calcite (Figure 6A,B). Relatively rare uraninite II occurred in later calcite–sulphide stages and formed fine spherical coatings close to older uraninite from the calcite– uraninite stage (Figure 6C). Uraniferous anthraxolite formed irregular fragments, rounded grains and veinlets within calcite. Its uranium content was concentrated in numerous inclusions of uraninite (and minor coffinite), which typically showed a disintegrated/cracked nature (Figure 6D).

Figure 6. Back-scattered electron (BSE) images of uranium mineralization with distribution of Ca (apfu, O = 4) in uraninite. Příbram uranium district. Urn—uraninite, Cal—calcite, Ant—anthraxolite.

In the Jáchymov uranium district uraninite formed massive spherical aggregates or thin veinlets lining wall rocks, while the central areas of the veins were filled with dolomite pigmented by Fe- oxides (Figure 7A,B). The spherical morphology of the uraninite aggregates was likewise characteristic Mineralsfor Potůč2019ky, 9and, 123 Horní Slavkov. In the Zálesí uranium deposit, uraninite occurred in the form of spherical8 of 23 and/or zoned aggregates (Figure 8A,B).

FigureFigure 7. BSE images of uranium mineralization from thethe Jáchymov Jáchymov uranium district with distribution ofof CaCa ((apfuapfu,, O O = = 4). 4). ( AUrn—uraninite,) Distribution of Cfn—coffinite, Ca in partly zoned Qz—quartz. uraninite; (B) Distribution of Ca in botryoidally Mineralsevolved 2018, 8 uraninite., x FOR PEER Urn—uraninite, REVIEW Cfn—coffinite, Qz—quartz. 8 of 21

FigureFigure 8.8. BSE images of uranium mineralizationmineralization with distribution of Ca (apfu, O == 4) in uraninite,uraninite, ZZálesíálesí uraniumuranium deposit.deposit. (Urn—uraninite,A) Distribution ofCfn—coffinite, Ca in botryoidally Cal—calcite. evolved uraninite; (B) Intergrowths of uraninite with coffinite. Urn—uraninite, Cfn—coffinite, Cal—calcite. The UO2 content in uraninite from the vein-type ore deposits studied was quite variable, ranging fromThe 65.40 UO wt2 content % to 81.80 in uraninite wt %. All from other the constitu vein-typeents ore varied deposits significantly: studied was the quite PbO variable, content ranging varied fromfrom 65.400.10 wt wt % % to to 10.81 81.80 wt wt %, %. SiO All2 from other 0.0 constituents wt % to 9.98 varied wt %, significantly: FeO from 0.0 the wtPbO % to content 4.15 wt varied% and fromCaO from 0.10 wt0.69 % wt to 10.81% to 8.31 wt %,wt SiO% (Table2 from 1). 0.0 Concentrat wt % to 9.98ions wt of %, P, FeOTh and from Zr 0.0were wt relatively % to 4.15 low: wt % up and to CaO1.11 wt from % 0.69P2O5, wt up % to to 0.12 8.31 wt wt % % ThO (Table2 and1). up Concentrations to 1.79 wt % ofZrO P,2 Th. The and highest Zr were concentration relatively low: of PbO up towas1.11 found wt % in P uraninite2O5, up to from 0.12 P wtříbram % ThO (up2 toand 10.81 up wt to %), 1.79 the wt highest % ZrO concentration2. The highest of concentration CaO was found of PbOin uraninite was found from in Horní uraninite Slavkov from (up Pˇríbram to 8.31 (up wt to %) 10.81 and wtthe %), highest the highest concentration concentration of SiO2 of was CaO found was foundin uraninite in uraninite from the from Předbo Hornřiceí Slavkov deposit (up (up to to 8.31 9.98 wt wt%) %). and The the concentrations highest concentration of REE in the of SiO majority2 was foundof the inuraninites uraninite fromanalyzed the Pˇredboˇricedepositwere below the de (uptection to 9.98 limits wt %). of The the concentrations electron microprobe. of REE in The the majorityconcentrations of the uraninitesof Y were mostly analyzed also were low; below the highest the detection concentrations limits of were the electron found in microprobe. uraninite from The concentrationsZálesí (up to 2.32 of wt Y were % Y2O mostly3; Table also 1). low; the highest concentrations were found in uraninite from Zálesí (up to 2.32 wt % Y2O3; Table1). Table 1. Ranges of selected components in the uraninites studied.

Jáchymov Potůčky Horní Slavkov Locality n = 81 n = 50 n = 23 (wt %) Min. Max. Median Min. Max. Median Min. Max. Median UO2 67.82 86.56 81.80 70.00 86.77 81.80 70.30 81.23 75.54 ThO2 b.d.l. 0.08 0.00 b.d.l. 0.12 0.00 b.d.l. 0.10 0.00 PbO 0.09 10.23 1.83 b.d.l. 6.51 1.53 0.10 4.74 3.71 SiO2 b.d.l. 9.48 1.67 b.d.l. 9.51 1.66 0.25 9.84 5.30 P2O5 b.d.l. 0.36 0.09 b.d.l. 1.11 0.17 b.d.l. 0.18 0.05 Al2O3 b.d.l. 0.73 0.06 b.d.l. 0.58 0.09 b.d.l. 1.00 0.33 ZrO2 b.d.l. 1.79 0.00 b.d.l. 0.19 0.05 b.d.l. 0.03 0.00 CaO 0.69 7.41 4.14 0.85 6.11 2.81 1.39 8.31 4.92 FeO 0.23 4.15 0.59 0.41 3.13 0.86 b.d.l. 4.10 0.89 As2O5 b.d.l. 6.09 0.78 0.08 5.16 0.96 b.d.l. 2.13 0.27 Y2O3 b.d.l. 1.24 0.22 b.d.l. 1.62 0.89 b.d.l. 0.16 0.00 Příbram Zálesí Předbořice Locality n = 101 n = 45 n = 64 (wt %) Min. Max. Median Min. Max. Median Min. Max. Median UO2 69.76 88.03 81.16 65.40 84.60 74.84 70.90 88.38 81.35 ThO2 b.d.l. 0.09 0.00 b.d.l. 0.08 0.00 b.d.l. 0.09 0.00 PbO 0.13 10.81 5.36 0.31 10.28 2.33 0.49 5.60 3.07 SiO2 b.d.l. 6.83 0.68 0.57 9.25 2.51 b.d.l. 9.98 1.03 P2O5 b.d.l. 0.23 0.07 b.d.l. 0.33 0.02 b.d.l. 0.22 0.02 Al2O3 b.d.l. 0.48 0.05 b.d.l. 0.72 0.04 b.d.l. 0.23 0.02 ZrO2 b.d.l. 0.14 0.00 b.d.l. 0.11 0.00 b.d.l. 0.04 0.00 CaO 1.28 6.24 4.56 1.40 5.41 3.27 2.94 7.93 5.08 FeO b.d.l. 1.05 0.07 0.16 2.01 0.58 b.d.l. 1.04 0.03

Minerals 2019, 9, 123 9 of 23

Table 1. Ranges of selected components in the uraninites studied.

Jáchymov Pot ˚uˇcky Horní Slavkov Locality n = 81 n = 50 n = 23 (wt %) Min. Max. Median Min. Max. Median Min. Max. Median

UO2 67.82 86.56 81.80 70.00 86.77 81.80 70.30 81.23 75.54

ThO2 b.d.l. 0.08 0.00 b.d.l. 0.12 0.00 b.d.l. 0.10 0.00 PbO 0.09 10.23 1.83 b.d.l. 6.51 1.53 0.10 4.74 3.71

SiO2 b.d.l. 9.48 1.67 b.d.l. 9.51 1.66 0.25 9.84 5.30

P2O5 b.d.l. 0.36 0.09 b.d.l. 1.11 0.17 b.d.l. 0.18 0.05

Al2O3 b.d.l. 0.73 0.06 b.d.l. 0.58 0.09 b.d.l. 1.00 0.33

ZrO2 b.d.l. 1.79 0.00 b.d.l. 0.19 0.05 b.d.l. 0.03 0.00 CaO 0.69 7.41 4.14 0.85 6.11 2.81 1.39 8.31 4.92 FeO 0.23 4.15 0.59 0.41 3.13 0.86 b.d.l. 4.10 0.89

As2O5 b.d.l. 6.09 0.78 0.08 5.16 0.96 b.d.l. 2.13 0.27

Y2O3 b.d.l. 1.24 0.22 b.d.l. 1.62 0.89 b.d.l. 0.16 0.00 Pˇríbram Zálesí Pˇredboˇrice Locality n = 101 n = 45 n = 64 (wt %) Min. Max. Median Min. Max. Median Min. Max. Median

UO2 69.76 88.03 81.16 65.40 84.60 74.84 70.90 88.38 81.35

ThO2 b.d.l. 0.09 0.00 b.d.l. 0.08 0.00 b.d.l. 0.09 0.00 PbO 0.13 10.81 5.36 0.31 10.28 2.33 0.49 5.60 3.07

SiO2 b.d.l. 6.83 0.68 0.57 9.25 2.51 b.d.l. 9.98 1.03

P2O5 b.d.l. 0.23 0.07 b.d.l. 0.33 0.02 b.d.l. 0.22 0.02

Al2O3 b.d.l. 0.48 0.05 b.d.l. 0.72 0.04 b.d.l. 0.23 0.02

ZrO2 b.d.l. 0.14 0.00 b.d.l. 0.11 0.00 b.d.l. 0.04 0.00 CaO 1.28 6.24 4.56 1.40 5.41 3.27 2.94 7.93 5.08 FeO b.d.l. 1.05 0.07 0.16 2.01 0.58 b.d.l. 1.04 0.03

As2O5 0.06 2.03 0.44 b.d.l. 4.12 0.70 b.d.l. 0.37 0.16

Y2O3 b.d.l. 0.26 0.00 0.04 2.32 0.62 b.d.l. 0.09 0.00 b.d.l.—below detection limit.

4.2. Coffinite Coffinite occurs predominantly as pseudomorphs after uraninite, or in the form of coatings, irregular clusters or small grains in younger cracks hosted by spherical aggregates of uraninite. In the Pˇríbram uranium district, coffinite occurred as irregular clusters or small grains in cracks later filled with calcite and fine younger disseminations along with larger uraninite grains (Figure9A). Uraniferous anthraxolite, together with predominating inclusions of uraninite, sometimes contained small isometric anhedral grains of coffinite (Figure9B). In the J áchymov uranium district, coffinite occurred in concentric and radial fractures in uraninite spheroids as younger thin rims growing over spherical aggregates of uraninite and as pseudomorphs after uraninite spheroids (Figure 10A,B). In the Pot ˚uˇckydeposit, the coffinitized uraninite and coffinite were found as pseudomorphs after older uraninite spheroids and as irregular disseminations among these spheroids. In the Horní Slavkov uranium district, coffinite occurred as small grains in fractures of massive uraninite. For the Zálesí deposit, coffinitized uraninite and radial aggregates of a coffinite-like mineral growing over older Minerals 2018, 8, x FOR PEER REVIEW 9 of 21

Minerals 2018, 8, x FOR PEER REVIEW 9 of 21 As2O5 0.06 2.03 0.44 b.d.l. 4.12 0.70 b.d.l. 0.37 0.16 Y2O3 b.d.l. 0.26 0.00 0.04 2.32 0.62 b.d.l. 0.09 0.00 As2O5 0.06 2.03 0.44 b.d.l. 4.12 0.70 b.d.l. 0.37 0.16 b.d.l.—below detection limit. Y2O3 b.d.l. 0.26 0.00 0.04 2.32 0.62 b.d.l. 0.09 0.00 b.d.l.—below detection limit. 4.2. Coffinite 4.2. CoffiniteCoffinite occurs predominantly as pseudomorphs after uraninite, or in the form of coatings, irregularCoffinite clusters occurs or small predominantly grains in younger as pseudomorphs cracks hosted after by uraninite, spherical oraggregates in the form of uraninite.of coatings, In irregularthe Příbram clusters uranium or small district, grains coffinite in younger occurred cracks as irregu hostedlar by clusters spherical or smallaggregates grains of in uraninite. cracks later In thefilled P řwithíbram calcite uranium and district,fine younger coffinite disseminations occurred as irregualonglar with clusters larger or uraninite small grains grains in (Figurecracks later9A). filledUraniferous with calcite anthraxolite, and fine together younger with disseminations predominating along inclusions with larger of uraninite, uraninite sometimes grains (Figure contained 9A). Uraniferoussmall isometric anthraxolite, anhedral grainstogether of withcoffinite predominating (Figure 9B). inclusions In the Jáchymov of uraninite, uranium sometimes district, contained coffinite smalloccurred isometric in concentric anhedral and grains radial of fractures coffinite in (Figure uraninite 9B). spheroids In the Jáchymov as younger uranium thin rims district, growing coffinite over occurredspherical inaggregates concentric of and uraninite radial fracturesand as pseudomorphs in uraninite spheroids after uraninite as younger spheroids thin rims (Figure growing 10A,B). over In sphericalthe Potůč kyaggregates deposit, theof uraninite coffinitized and uraninite as pseudomorphs and coffinite after were uraninite found asspheroids pseudomorphs (Figure after10A,B). older In Mineralstheuraninite Pot2019ůč kyspheroids, 9 ,deposit, 123 andthe coffinitizedas irregular uraninite disseminatio and nscoffinite among were these found spheroids. as pseudomorphs In the Horní after Slavkov10 older of 23 uraniumuraninite district, spheroids coffinite and occurredas irregular as small disseminatio grains in fracturesns among of thesemassive spheroids. uraninite. In For the the Horní Zálesí Slavkovdeposit, coffinitized uraninite and radial aggregates of a coffinite-like mineral growing over older uraninite uraniniteuranium district, were significant coffinite occurred (Figure as 11 small). The grains coffinitized in fractures uraninite of massive also uraninite. occurred For in the zoned Zálesí uraninite deposit, were significant (Figure 11). The coffinitized uraninite also occurred in zoned uraninite spheroids from spheroidscoffinitized from uraninite the Pˇredboˇricedeposit. and radial aggregates of a coffinite-like mineral growing over older uraninite werethe P řsignificantedbořice deposit. (Figure 11). The coffinitized uraninite also occurred in zoned uraninite spheroids from the Předbořice deposit.

Figure 9.9. BSE images of uranium mineralization with distribution of Si ( apfu,, O O = 4) 4) in in coffinite, coffinite, Příbram uranium district. Urn—uraninite, Cfn—coffinite, Ant—anthraxolite, Cal—calcite, Dol— PˇrFigureíbram 9. uranium BSE images district. of uranium (A) Distribution mineralization of Si with in coffinite distribution enclosed of Si in (apfu calcite;, O (=B 4)) Distributionin coffinite, dolomite. ofPříbram Si in coffinite uranium that district. is enclosed Urn—uraninite, in antraxolite. Cfn—co Urn—uraninite,ffinite, Ant—anthraxolite, Cfn—coffinite, Cal—calcite, Ant—anthraxolite, Dol— Cal—calcite,dolomite. Dol—dolomite.

Figure 10. BSE images of coffinite and coffinitized uraninite with distribution of Si (apfu, O = 4), Jáchymov uranium district. Urn—uraninite, Cfn—coffinite, Cfn-Urn—coffinitized uraninite, Dol— Figure 10. BSE images of coffinite and coffinitized uraninite with distributionapfu of Si (apfu, O = 4), Figuredolomite, 10. Qz—quartz.BSE images of coffinite and coffinitized uraninite with distribution of Si ( , O = 4), Jáchymov uraniumJáchymov district. uranium (A district.) Distribution Urn—uraninite, of Si in partly Cfn—co coffinitizedffinite, Cfn-Urn—coffinitized uraninite; (B) Detailed uraninite, distribution Dol— of Sidolomite, in botryoidally Qz—quartz. evolved uraninite. Urn—uraninite, Cfn—coffinite, Cfn-Urn—coffinitized uraninite,

Dol—dolomite, Qz—quartz.

The concentrations of UO2 and SiO2 in coffinite from the vein-type deposits studied were highly variable, ranging from 50.59 wt % to 74.09 wt % and from 10.1 wt % to 28.15 wt %, respectively. All other elements also varied significantly: the PbO content varied from 0.0 wt % to 7.41 wt %, FeO from 0.0 wt % to 5.25 wt %, CaO from 0.69 wt % to 6.49 wt % and P2O5 from 0.0 wt % to 8.79 wt % (Table2). Anomalously high levels of P were found in coffinite from Pot ˚uˇcky(up to 8.79 wt % P 2O5). Concentrations of Th and Zr in coffinite were relatively low: up to 0.21 wt % ThO2 and up to 3.3 wt % ZrO2. The highest concentrations of Pb and Ca were found in coffinite from the Zálesí deposit (up to 7.41 wt % and 5.67 wt % PbO and CaO, respectively). The highest concentration of Si was found in coffinite from the Horní Slavkov uranium district (up to 28.15 wt % SiO2). The concentrations of REE were usually below detection limits. The concentrations of Y in the coffinite analyzed were mostly low. The highest concentration of Y was found in coffinite from the Zálesí deposit (up to 9.41 wt % Y2O3; Table2). Minerals 2019, 9, 123 11 of 23

Minerals 2018, 8, x FOR PEER REVIEW 10 of 21

FigureFigure 11. 11.BSE BSE image image of of coffinitized coffinitized uraninite uraninite with distribution of of Si Si (apfu (apfu, O, O= 4), = 4),Zálesí Záles uraniumí uranium deposit. Urn—uraninite, Cfn-Urn—coffinitized uraninite. deposit. Urn—uraninite, Cfn-Urn—coffinitized uraninite.

The concentrationsTable 2.of ConcentrationUO2 and SiO2 in ranges coffinite of selected from the components vein-type deposits in coffinite. studied were highly variable, ranging from 50.59 wt % to 74.09 wt % and from 10.1 wt % to 28.15 wt %, respectively. All other elements also variedJáchymov significantly: the PbO cont Potent ˚uˇcky varied from 0.0 wt Horn% to 7.41í Slavkov wt %, FeO from Locality 0.0 wt % to 5.25 wt %, CaOn from= 48 0.69 wt % to 6.49 wtn %= 15and P2O5 from 0.0 wt %n to= 128.79 wt % (Table 2). Anomalously high levels of P were found in coffinite from Potůčky (up to 8.79 wt % P2O5). (wt %) Min. Max. Median Min. Max. Median Min. Max. Median Concentrations of Th and Zr in coffinite were relatively low: up to 0.21 wt % ThO2 and up to 3.3 wt UO 50.59 74.09 62.91 57.85 65.96 63.67 53.65 75.20 64.09 % ZrO2. The2 highest concentrations of Pb and Ca were found in coffinite from the Zálesí deposit (up to 7.41 wtThO %2 and 5.67b.d.l. wt % 0.19 PbO and 0.00 CaO, respectively). b.d.l. 0.09 The highest 0.00 concentration b.d.l. 0.21 of Si was 0.00 found in coffinitePbO from the b.d.l.Horní Slavkov 0.12 uranium 0.00 district b.d.l. (up 1.10 to 28.15 0.01wt % SiO b.d.l.2). The concentrations 4.48 0.30 of REE were usually below detection limits. The concentrations of Y in the coffinite analyzed were mostly SiO 10.47 23.43 17.63 12.67 16.68 13.52 10.23 28.15 21.67 low. The highest2 concentration of Y was found in coffinite from the Zálesí deposit (up to 9.41 wt % Y2O3; TableP2O 52). b.d.l. 5.02 0.85 0.46 8.79 7.83 0.12 0.91 0.42

Al2O3 0.06 2.15 0.82 0.04 1.72 0.20 0.44 2.08 1.05 Table 2. Concentration ranges of selected components in coffinite. ZrO2 b.d.l. 3.30 0.02 b.d.l. 0.18 0.02 b.d.l. 0.19 0.00 Jáchymov Potůčky Horní Slavkov LocalityCaO 0.69 4.35 2.39 0.82 4.14 3.75 1.30 3.52 1.71 n = 48 n = 15 n = 12 (wtFeO %) Min. 0.03 Max. 3.90 Median 0.37 Min. 0.63 Max. 5.25 Median 0.99 0.29Min. 2.29 Max. Median 1.12 UOAs2 O5 50.59b.d.l. 74.09 5.25 62.91 0.39 57.85 0.88 65.96 3.98 63.67 1.16 0.1353.65 1.9675.20 0.0364.09 ThO2 b.d.l. 0.19 0.00 b.d.l. 0.09 0.00 b.d.l. 0.21 0.00 Y2O3 b.d.l. 5.46 0.84 0.11 2.46 2.22 b.d.l. 0.33 0.07 PbO b.d.l. 0.12 0.00 b.d.l. 1.10 0.01 b.d.l. 4.48 0.30 Pˇríbram Zálesí Pˇredboˇrice SiO2 10.47 23.43 17.63 12.67 16.68 13.52 10.23 28.15 21.67 Locality P2O5 b.d.l. 5.02n = 20 0.85 0.46 8.n79= 26 7.83 0.12 n0.91= 2 0.42 Al2O3 0.06 2.15 0.82 0.04 1.72 0.20 0.44 2.08 1.05 (wt %) Min. Max. Median Min. Max. Median Min. Max. Mean ZrO2 b.d.l. 3.30 0.02 b.d.l. 0.18 0.02 b.d.l. 0.19 0.00 CaOUO 2 0.6952.48 4.35 69.33 2.39 65.48 0.82 52.80 4. 71.4614 65.353.75 69.06 1.30 72.243.52 70.651.71

FeOThO 2 0.03b.d.l. 3.90 0.15 0.37 0.00 0.63 b.d.l. 5. 0.1125 0.99 0.00 b.d.l. 0.29 0.112.29 0.061.12 As2O5 b.d.l. 5.25 0.39 0.88 3.98 1.16 0.13 1.96 0.03 PbO b.d.l. 1.74 0.56 b.d.l. 7.41 0.46 1.04 4.87 2.96 Y2O3 b.d.l. 5.46 0.84 0.11 2.46 2.22 b.d.l. 0.33 0.07 SiO2 14.40Př 25.69íbram 17.87 10.10 24.07Zálesí 14.06 11.69P 12.49ředbořice 12.09 Locality n = 20 n = 26 n = 2 P2O5 b.d.l. 0.41 0.15 b.d.l. 4.12 0.11 0.20 0.43 0.32 (wt %) Min. Max. Median Min. Max. Median Min. Max. Mean Al2O3 0.22 2.19 0.83 b.d.l. 2.26 0.11 0.19 0.29 0.24 UO2 52.48 69.33 65.48 52.80 71.46 65.35 69.06 72.24 70.65 ThOZrO2 2 b.d.l.b.d.l. 0.15 0.17 0.00 0.00 b.d.l. b.d.l. 0. 0.1511 0.00 0.00 b.d.l. 0.11 0.06 0.06 PbOCaO b.d.l. 1.24 1.74 6.49 0.56 2.45 b.d.l. 0.56 7.41 5.67 0.46 2.76 1.04 5.27 4.87 5.64 5.46 2.96 SiO2 14.40 25.69 17.87 10.10 24.07 14.06 11.69 12.49 12.09 FeO 0.02 1.52 0.44 b.d.l. 1.95 0.16 0.45 0.87 0.66 P2O5 b.d.l. 0.41 0.15 b.d.l. 4.12 0.11 0.20 0.43 0.32 AlAs2O23O 5 0.22b.d.l. 2.19 0.96 0.83 0.26 b.d.l. b.d.l. 2. 4.7626 0.11 0.91 0.19 0.07 0.29 0.14 0.11 0.24 ZrO2 b.d.l. 0.17 0.00 b.d.l. 0.15 0.00 b.d.l. 0.11 0.06 Y2O3 b.d.l. 2.06 0.37 0.15 9.41 1.13 b.d.l. 0.09 0.06 b.d.l.—below detection limit. Minerals 2018, 8, x FOR PEER REVIEW 11 of 21

CaO 1.24 6.49 2.45 0.56 5.67 2.76 5.27 5.64 5.46 FeO 0.02 1.52 0.44 b.d.l. 1.95 0.16 0.45 0.87 0.66 As2O5 b.d.l. 0.96 0.26 b.d.l. 4.76 0.91 0.07 0.14 0.11 MineralsY20192O3 , 9, 123b.d.l. 2.06 0.37 0.15 9.41 1.13 b.d.l. 0.09 0.06 12 of 23 b.d.l.—below detection limit. 4.3. Ningyoite 4.3. Ningyoite Ningyoite was only found in samples from the Pot ˚uˇckyuranium deposit. It occurred as an Ningyoite was only found in samples from the Potůčky uranium deposit. It occurred as an irregular filling of fractures in older uraninite, sometimes together with coffinite (Figure 12A,B). The irregular filling of fractures in older uraninite, sometimes together with coffinite (Figure 12 A,B). The concentration of UO2 in ningyoite was variable, ranging from 20.45 wt % to 35.93 wt %. All other concentration of UO2 in ningyoite was variable, ranging from 20.45 wt % to 35.93 wt %. All other constituents also varied significantly: the P2O5 content varied from 20.31 wt % to 26.07 wt %, REE2O3 constituents also varied significantly: the P2O5 content varied from 20.31 wt % to 26.07 wt %, REE2O3 from 13.54 wt % to 22.60 wt %, CaO from 7.5 wt % to 9.7 wt %, Y2O3 from 1.93 wt % to 4.46 wt %, PbO from 13.54 wt % to 22.60 wt %, CaO from 7.5 wt % to 9.7 wt %, Y2O3 from 1.93 wt % to 4.46 wt %, PbO from 1.31 wt % to 6.56 wt % and FeO from 0.39 wt % to 0.97 wt % (Table3). The concentrations of Th from 1.31 wt % to 6.56 wt % and FeO from 0.39 wt % to 0.97 wt % (Table 3). The concentrations of Th and Zr were usually below detection limits. and Zr were usually below detection limits.

Figure 12.12. BSEBSE imagesimages of of uranium uranium mineralization mineralization with with distribution distribution of Caof Ca (apfu (apfu, O =, O 4) = in 4) ningyoite in ningyoite and uraninite,and uraninite, Pot ˚uˇckyuraniumPotůčky uranium deposit. deposit. (A) DistributionUrn—uraninite, of Ca Ngn—ningyoite. in ningyoite and uraninite, (B) Detailed BSE image of distribution of Ca in ningyoite. Urn—uraninite, Ngn—ningyoite. Table 3. Ranges of contents of selected components in the studied ningyoite. Table 3. Ranges of contents of selected components in the studied ningyoite. Locality Potůčky (wt %) Locality Min Pot ˚uˇcky Max Median UO2 (wt %)20.45 Min. Max. Median35.93 26.58 ThO2 b.d.l. b.d.l. 0.00 UO 20.45 35.93 26.58 PbO 2 1.31 6.56 4.64 ThO2 b.d.l. b.d.l. 0.00 SiO2 b.d.l. 3.03 0.59 PbO 1.31 6.56 4.64 P2O5 20.31 26.07 24.17 SiO2 b.d.l. 3.03 0.59 Al2O3 b.d.l. 0.35 0.09 P2O5 20.31 26.07 24.17 ZrO2 Al2O3 b.d.l.b.d.l. 0.350.30 0.09 0.00 CaO ZrO2 7.50b.d.l. 0.30 9.74 0.00 8.67 FeO CaO0.39 7.50 9.74 0.97 8.67 0.63 As2O5 FeOb.d.l. 0.39 0.970.43 0.63 0.14 Y2O3 As2O5 1.93b.d.l. 0.434.46 0.14 3.15 Y O 1.93 4.46 3.15 REE2O3 2 3 13.54 22.26 16.70 REE2O3 13.54 22.26 16.70 b.d.l.—below detection limit. b.d.l.—below detection limit. 5. Discussion 5. Discussion

5.1. Composition of Uraninite

Uraninite, nominally UO2, occurs in nature as a non-stoichiometric mineral with a highly Uraninite, nominally UO2, occurs in nature as a non-stoichiometric mineral with a highly defective fluoritedefective structure. fluorite structure. The non-stoichiometry The non-stoichiometry and defects and are defects caused are by caused oxidation by oxidation of uranium, of uranium, cationic cationic substitutions and α-decay damage. During the oxidation of4+ U4+ to6+ U6+ in UO2, excess oxygen substitutions and α-decay damage. During the oxidation of U to U in UO2, excess oxygen is incorporated into the structure causing the formation of non-stoichiometric UO2+x, where x is the number of excess interstitial oxygens. This non-stoichiometric mineral always contains cation impurities, e.g., Pb, Ca, Th, Zr, Y and REE. Therefore, its chemical formula may be written better as Minerals 2019, 9, 123 13 of 23

4+ 6+ 3+ 2+ 4− (U 1−x−y−z−vU xREE yM z v)O2−(0.5y)−2v, where x is the amount of excess oxygen equal to U6+, y is the number of trivalent cations (REE + Y), z is the number of divalent cations (Pb, Ca), and v is number of uranium vacancies in the unit formula [40,41]. Concentrations of these intrinsic elements may exceed 20 wt %. Radiogenic Pb is a major impurity in uraninites of great age. Up to 21.4 wt % PbO were found in uraninites from the Proterozoic unconformity-related uranium deposits in Canada, whereas uraninites from the Variscan uranium deposits in France contained only up to 4.9 wt % PbO [42,43]. The concentrations of Pb in uraninites from the Czech vein-type deposits studied mostly ranged from 1.5 wt % to 5.4 wt % PbO. These concentrations are very similar to those in uraninites from vein-type deposits in the German part of the Krušné Hory-Erzgebirge Mts. (Schneeberg, Schlema-Alberoda)—1.5 wt %–5.5 wt % PbO [43,44]. With respect to the Variscan (i.e., Permian) age of uranium mineralization in the Bohemian Massif [22,25], the high contents of PbO, up to 10.81 wt % recorded in some uraninites from the Pˇríbram, Zálesí and Jáchymov deposits, cannot be considered to represent radiogenic lead generated by in-situ decay of uranium. Instead, one can assume co-precipitation of common lead during crystallization of uraninite, and/or redistribution of lead during superimposed hydrothermal alteration of uraninite. The presence of growth zones rich in lead, which have been observed in samples from the Pˇríbram uranium deposit, strongly favours the first possibility. The absence of a statistically significant correlation between the contents of Pb and U observed at all sites studied (correlation coefficients ≤0.48; Table4) implies the involvement of multiple processes affecting the chemical composition of uraninite, including growth of radiogenic lead with time, co-precipitation of common lead during crystallization of uraninite and/or superimposed hydrothermal alteration associated with loss of lead. A substitution of the type 2U4+ = (U6+ + Ca2+) has been proposed in oxidised Ca-rich uraninite [7]. Concentrations of Ca in the uraninites analyzed varied from 0.7 wt % to 8.3 wt % CaO. The highest Ca content in uraninite from the Horní Slavkov uranium district (up to 8.3 wt % CaO) together with a positive correlation between CaO and PbO (correlation coefficient 0.72; Table4), suggests significant oxidation of uraninite (Figure 13). A similar relationship between Ca and Pb was also observed at the Zálesí deposit (correlation coefficient 0.74; Table4), but at other sites no statistically significant correlations between both elements were found (correlation coefficients ≤0.41; Table4). The concentrations of Ca in other vein-type uranium deposits (Great Bear Lake and Algoma, Canada, Armorican Massif and Massif Central, France) ranged from 1.3 wt % to 9.5 wt % CaO [42–45]. The close association of sulphides and bitumens with coffinite is evidence of reducing conditions during coffinitization [41]. With the exception of Horní Slavkov, negative correlations occurred between concentrations of CaO and SiO2 (correlation coefficients from −0.38 to −0.84; Table4), suggesting reducing conditions during coffinitization of uraninite. Concentrations of Si in uraninites from the Variscan uranium deposits in France also varied considerably, from 0.1 wt % to 5.1 wt % SiO2 [42]. The concentrations of Th in uraninites from vein-type uranium deposits were usually very low, typically below detection limits of the electron microprobe [37]. In our samples, the highest concentrations of Th were found in uraninite from the Horní Slavkov district (up to 0.21 wt % ThO2). However, the majority of our uraninite analyses displayed concentrations of ThO2 below the detection limits. Worldwide, the highest concentrations of Th have been found in uraninites from the Precambrian Witwatersrand conglomerate-hosted uranium deposits in South Africa (up to 9.5 wt % ThO2) and the Proterozoic unconformity-related deposits in Canada (up to 3.8 wt % ThO2)[13,43]. The concentrations of Zr in uraninites from vein-type uranium deposits were usually also low, mostly below the detection limit [13]. However, Zr-enriched uraninite was described from a vein-type deposit in North Eastern Egypt, containing up to 2.49 wt % ZrO2 [46]. The Zr-enriched uraninites occurred in sandstone-type uranium deposits [47]. The highest content of Zr was found in amber from the North Bohemian Cretaceous sandstone-type uranium ore district, containing up to 4.97 wt % ZrO2 [48]. In uraninite samples from vein-type deposits of the Bohemian Massif, the Zr concentrations were also low. Enrichment of up to 0.66 wt % ZrO2 was found only in uraninite from Minerals 2019, 9, 123 14 of 23 the Jáchymov uranium district. A similar enrichment in Zr in uraninite from Jáchymov was also found by Frimmel et al. [13]; however, the authors interpreted such elevated contents of Zr in terms of the presence of discrete inclusions of a Zr-mineral.

Table 4. Spearman correlation coefficients for selected elements analyzed in uraninite.

Jáchymov UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO −0.01 1.00 SiO2 −0.72 −0.20 1.00 P2O5 0.14 −0.32 0.14 1.00 CaO 0.49 −0.05 −0.40 0.10 1.00 Al2O3 −0.52 0.06 0.72 −0.06 −0.25 1.00 Y2O3 −0.10 0.02 −0.28 0.12 −0.26 −0.20 1.00

Pot ˚uˇcky UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO 0.35 1.00 SiO2 −0.78 −0.53 1.00 P2O5 −0.87 −0.30 0.74 1.00 CaO 0.75 0.41 −0.84 −0.74 1.00 Al2O3 −0.68 −0.65 0.85 0.64 −0.69 1.00 Y2O3 0.74 0.29 −0.67 −0.73 0.71 −0.66 1.00

Horní Slavkov UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO −0.48 1.00 SiO2 −0.66 0.07 1.00 P2O5 −0.13 −0.22 0.37 1.00 CaO −0.35 0.72 −0.18 −0.23 1.00 Al2O3 −0.83 0.62 0.70 0.02 0.38 1.00 Y2O3 0.02 0.30 0.16 −0.02 0.27 0.23 1.00

Pˇríbram UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO −0.38 1.00 SiO2 −0.48 −0.18 1.00 P2O5 0.13 0.18 −0.14 1.00 CaO 0.51 −0.08 −0.42 0.27 1.00 Al2O3 −0.23 −0.29 0.60 −0.20 −0.23 1.00 Y2O3 −0.22 0.04 0.21 0.12 −0.00 0.03 1.00

Zálesí UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO −0.43 1.00 SiO2 −0.30 −0.51 1.00 P2O5 −0.50 −0.16 0.64 1.00 CaO −0.38 0.74 −0.38 −0.09 1.00 Al2O3 0.34 −0.87 0.59 0.28 −0.73 1.00 Y2O3 0.45 −0.73 0.26 0.07 −0.67 0.68 1.00

The concentrations of Y varied from below detection limit to 2.32 wt % Y2O3, which was found in uraninite from the Zálesí deposit. The elevated concentrations of Y were also found in uraninites from Jáchymov and Pot ˚uˇcky(about 1.2 wt % Y2O3). In uraninites from other vein-type uranium deposits, the Y concentrations were distinctly lower (0.02 wt %–0.59 wt % Y2O3)[43,45]. The highest to-date reported concentration of Y, as high as 0.79 wt % Y2O3, was found in uraninite from the Wittichen uranium deposit in the Black Forest ore district, SW Germany [13]. The highest reported concentrations of Y occurred in uraninites from high-temperature alkali-enriched magmatic rocks and pegmatites (up to 4.3 wt % Y2O3)[43]. At Pot ˚uˇcky, there was a strong positive correlation between Minerals 2018, 8, x FOR PEER REVIEW 13 of 21

P2O5 0.13 0.18 −0.14 1.00 CaO 0.51 −0.08 −0.42 0.27 1.00 Al2O3 −0.23 −0.29 0.60 −0.20 −0.23 1.00 Y2O3 −0.22 0.04 0.21 0.12 −0.00 0.03 1.00 Zálesí UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3 UO2 1.00 PbO −0.43 1.00 SiO2 −0.30 −0.51 1.00 P2O5 −0.50 −0.16 0.64 1.00 CaO −0.38 0.74 −0.38 −0.09 1.00 Al2O3 0.34 −0.87 0.59 0.28 −0.73 1.00 Y2O3 0.45 −0.73 0.26 0.07 −0.67 0.68 1.00

4+ 6+ 2+ MineralsA substitution2019, 9, 123 of the type 2U = (U + Ca ) has been proposed in oxidised Ca-rich uraninite15 of[7]. 23 Concentrations of Ca in the uraninites analyzed varied from 0.7 wt % to 8.3 wt % CaO. The highest Ca content in uraninite from the Horní Slavkov uranium district (up to 8.3 wt % CaO) together with acontents positive of correlation Y and U (correlationbetween CaO coefficient and PbO (correlati = 0.74) coupledon coefficient with strong0.72; Table negative 4), suggests correlations significant Y–Si (correlationoxidation of coefficienturaninite (Figure−0.67) 13). and A Y–P similar (correlation relation coefficientship between−0.73; Ca and Table Pb4 ).was These also trends observed were at thethe Zálesíopposite deposit to those (correlation observed forcoefficient coffinite 0.74; from Table Pot ˚uˇcky(Table 4), but at5 ),other implying sites thatno statistically the contents significant of Y are a primarycorrelations feature between of uraninite both fromelements Pot ˚uˇcky, were which found is not(correlation associated coefficients with superimposed ≤0.41; Table coffinitization. 4). The concentrationsIn contrast, other of studiedCa in other sites didvein-type not display uranium any statisticallydeposits (Great significant Bear Lake correlations and Algoma, between Canada, Y and ArmoricanU (Table4). Massif and Massif Central, France) ranged from 1.3 wt % to 9.5 wt % CaO [42–45].

Figure 13. DistributionDistribution of Ca and Pb in uraninite from the Horní Horní Slavkov uranium district. 5.2. Composition of Coffinite The close association of sulphides and bitumens with coffinite is evidence of reducing conditions duringCoffinite, coffinitization nominally [41]. USiO With4.nH the2O( exceptionn = 0–2) isof an Horní orthosilicate Slavkov, which negative may havecorrelations a highly occurred variable 6+ 2+ 4+ 4+ betweenchemical concentrations composition. The of tetravalentCaO and SiO uranium2 (correlation can be partlycoefficients substituted from − by0.38 U to, Ca−0.84;, Zr Table, Th 4),, Ysuggesting3+ and REE reducing3+, while conditions atoms of during silicon cancoffinitization be alternated of uraninite. with P5+ ,Concentrations As5+,V5+,S6+ andof Si OHin uraninites− groups, fromincluding the Variscan the possibility uranium of vacancies deposits in theFrance tetrahedral also varied site [considerably,40,49–51]. from 0.1 wt % to 5.1 wt % SiO2 In[42]. the vein-type uranium deposits studied, coffinite often represented a younger uranium mineral arisingThe from concentrations replacement of of Th uraninite. in uraninites Many from of our vein-type microprobe uranium analyses deposits of coffinite, were usually especially very those low, typicallyfrom Záles belowí, yielded detection SiO limits2 concentrations of the electron too microp low torobe be [37]. assigned In our to samples, the coffinite the highest formula concentrations (U/Si > 1). ofBecause Th were in found some casesin uraninite SiO2 and from UO the2 contentsHorní Slavkov varied district continuously (up to 0.21 around wt % compositions ThO2). However, close the to majorityUSiO4 and of UO our2+ xuraninite, these transitional analyses phasesdisplayed could concentrations be explained asof mixturesThO2 below of variable the detection proportions limits. of Worldwide,coffinite and the uraninite highest [7 ,concentrations52] or possibly of better Th have by the been gradual found enrichment in uraninites of Si from in uraninite the Precambrian during its Witwatersrandcoffinitization. Similarly,conglomerate-hosted gradual coffinitization uranium deposits of uraninite in South was describedAfrica (up by to Leroy9.5 wt and% ThO Holliger2) and [the45] Proterozoicfrom vein-type unconformity-related uranium deposits ofdeposi the Massifts in Canada Central (up in France.to 3.8 wt The % ThO wide2) variability[13,43]. in U/Si ratios of uraniniteThe concentrations and coffinite of and Zr “patchy”in uraninites textures from of vein-type both minerals uranium observed deposits in the were BSE usually images also has beenlow, mostlydocumented below by the Fojt detection et al. [9] limit at the [13]. Záles However,í deposit. Zr-enriched The authors uraninite interpreted was these described phenomena from in a termsvein- typeof repeated deposit coffinitization in North Eastern of uraninite Egypt, containing and uraninitization up to 2.49 of wt coffinite. % ZrO2 The[46]. distinctly The Zr-enr lowiched totals uraninites of some of our microprobe analyses of coffinite (86.4 wt %–95.3 wt %) are probably due to the presence of water. In coffinite from Jáchymov the presence of H2O was confirmed directly by infrared spectroscopy [7]. The concentration of Ca in the coffinites analyzed was lower than those in uraninites and varied from 0.6 wt % to 6.5 wt % CaO. Similar concentrations of Ca were also found in coffinite from vein-type uranium deposits in the Armorican Massif and Massif Central in France [45,53]. Positive correlations observed between the contents of Ca and U, together with negative correlations between Ca and Si contents in coffinites from Pˇríbram and Horní Slavkov (Table5) illustrates a greater affinity of Ca for uraninite than coffinite. At other sites, no such correlations occurred, perhaps due to superimposed alteration. Lead is usually released from uraninite during coffinitization and does not enter the coffinite structure [7,45]. This is consistent with our data, showing lower concentrations of Pb in coffinite than Minerals 2019, 9, 123 16 of 23 in uraninite. No statistically significant correlations occurred between contents of Pb and contents of U or Si in all coffinites studied (correlation coefficients ≤0.48; Table5). The concentrations of Th in our coffinites were low, from below detection limits to 0.21 wt % ThO2. In coffinite from the Armorican Massif in France, the concentration of Th was below the detection limit of the electron microprobe [54]. Thorium-enriched coffinites were found in hydrothermal Fe–Cu–Au–Ag–U deposits at Olympic Dam (Australia), showing up to 3.61 wt % ThO2 [53]. However, highly Th-enriched coffinite occurred in conglomerate-type uranium deposits from the Witwatersrand, South Africa, which contained up to 44.6 wt % ThO2 [55]. The concentrations of Zr in our coffinites were usually below detection limits, but in some coffinite samples from the Jáchymov ore district, elevated contents of up to 3.3 wt % ZrO2 were found. Elevated concentrations of Zr in coffinite from the Jáchymov deposit were also found by Janeczek (0.7 wt %–0.8 wt % ZrO2)[7]. Zr-enriched coffinite occurred in some shear-zone hosted hydrothermal uranium deposits in the Bohemian Massif (Rožná, Okrouhlá Radouˇn)with contents of ZrO2 up to 13.8 wt % [6]. Some coffinites from the vein-type uranium deposits studied were enriched in Y. In all cases, these uranium deposits contained superimposed five-element (Ag–Bi–As–Co–Ni) mineralization. The highest Y concentrations were found in coffinites from the Jáchymov uranium district (up to 5.5 wt % Y2O3) and the Zálesí deposit (up to 9.4 wt % Y2O3) (Figure 14). In both cases there was a positive correlation between Y and P (correlation coefficients 0.69 and 0.75, respectively; Table5), and therefore the following substitution mechanisms could have been involved:

2U4+ = 2Y3+ + P5+ and/or U4+ + Si4+ = Y3+ + P5+ (1)

Y enriched coffinites containing up to 9.0 wt % Y2O3 were described from sandstone-hosted uranium deposits [56,57]. The coffinites from shear-zone hosted uranium deposits of the Bohemian Massif (Rožná, Okrouhlá Radouˇn)contained up to 3.4 wt % Y2O3 [6]. The P contents in coffinites analyzed varied from below detection limits to the elevated values recorded in coffinites from the Zálesí deposit (up to 4.1 wt % P2O5), Jáchymov district (up to 5.0 wt % P2O5) and the Pot ˚uˇckydeposit (up to 8.8 wt % P2O5). The P-rich coffinite from Pot ˚uˇckyoccurred in association with Ca-U4+ phosphate ningyoite. P-enriched coffinite has been described from sandstone-related uranium deposits in the past, especially from Russia [56–60]. The highest content of P was found in coffinite from hydrothermal Fe–Cu–Au–Ag–U uranium deposits at Olympic Dam, Australia (up to 11.4 wt % P2O5)[53]. Other P-enriched coffinites occurred in a natural fission reactor at Bangombé, Gabon (up to 8.9 wt % P2O5)[58] and in the Khianga uranium district in the north-eastern Transbaikal region, Russia (up to 11 wt % P2O5)[60]. Enrichment of P in coffinites can be explained by substitution mechanisms coupled with Y and REE [53] and/or following substitution reactions, implying the existence of a solid solution between coffinite and ningyoite [57]:

4+ 4+ 2+ 5+ U + Si = 2Ca + 0.8P + 0.2 (2)

U4+ + Si4+ = 2Ca2+ + P5+ + (OH)− (3)

With respect to the good correlation between P2O5 and Y2O3 contents in P-enriched coffinites from the Jáchymov district and Zálesí deposit (correlation coefficients 0.69 and 0.75, respectively; Table5), the P enrichment in these coffinites can be explained by substitution reactions involving Y. By contrast, in P-rich coffinites from Pot ˚uˇckythere was a very good correlation between CaO and P2O5 (correlation coefficient 0.81; Table5), which favours a solid solution between coffinite and ningyioite. Minerals 2019, 9, 123 17 of 23

Table 5. Spearman correlation coefficients for selected elements analyzed in coffinite.

Jáchymov UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO 0.01 1.00 SiO2 −0.33 −0.18 1.00 P2O5 −0.61 −0.47 0.25 1.00 CaO −0.12 −0.26 −0.29 0.36 1.00 Al2O3 0.08 0.34 0.37 −0.63 −0.44 1.00 Y2O3 −0.72 −0.23 −0.01 0.69 0.09 −0.46 1.00

Pot ˚uˇcky UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO 0.09 1.00 SiO2 −0.05 −0.31 1.00 P2O5 −0.54 −0.15 0.06 1.00 CaO −0.23 −0.33 0.08 0.81 1.00 Al2O3 0.16 −0.30 0.51 −0.23 −0.23 1.00 Y2O3 −0.42 −0.04 −0.32 0.22 0.12 −0.54 1.00

Horní Slavkov UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO −0.04 1.00 SiO2 −0.80 −0.22 1.00 P2O5 −0.64 −0.14 0.84 1.00 CaO 0.74 −0.03 −0.73 −0.61 1.00 Al2O3 −0.42 0.09 0.35 −0.08 −0.18 1.00 Y2O3 0.10 −0.19 0.32 0.20 −0.32 0.00 1.00

Pˇríbram UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO 0.13 1.00 SiO2 −0.48 0.10 1.00 P2O5 0.39 −0.30 −0.32 1.00 CaO 0.54 0.16 −0.70 0.17 1.00 Al2O3 −0.41 0.08 0.72 −0.55 −0.46 1.00 Y2O3 −0.60 −0.03 0.49 −0.66 −0.48 0.63 1.00

Zálesí UO2 PbO SiO2 P2O5 CaO Al2O3 Y2O3

UO2 1.00 PbO 0.02 1.00 SiO2 −0.45 −0.48 1.00 P2O5 −0.73 −0.42 0.57 1.00 CaO −0.38 0.30 −0.17 0.16 1.00 Al2O3 −0.11 −0.23 0.74 0.22 −0.06 1.00 Y2O3 −0.40 −0.70 0.79 0.75 −0.06 0.57 1.00 Minerals 2018, 8, x FOR PEER REVIEW 16 of 21

UO2 1.00 PbO 0.02 1.00 SiO2 −0.45 −0.48 1.00 P2O5 −0.73 −0.42 0.57 1.00 CaO −0.38 0.30 −0.17 0.16 1.00 Minerals 2019, 9, 123 Al2O3 −0.11 −0.23 0.74 0.22 −0.06 1.00 18 of 23 Y2O3 −0.40 −0.70 0.79 0.75 −0.06 0.57 1.00

Figure 14.14. Distribution of P and Y in coffinitecoffinite fromfrom thethe JJáchymováchymov uraniumuranium districtdistrict andand thethe ZZálesíálesí uranium deposit.deposit. 5.3. Composition of Ningyoite Y enriched coffinites containing up to 9.0 wt % Y2O3 were described from sandstone-hosted uraniumNingyoite, deposits nominally [56,57]. The (U,Ca,Ce) coffinites2(PO from4)2.1–2 shea Hr-zone2O, is ahosted phosphate uranium from deposits the rhabdophane of the Bohemian group 3+ withMassif the (Rožná, general Okrouhlá formula Radou AXO4ň.1–2) contained H2O, where up to A 3.4 = wt Ca, % REE, Y2O3 Th, [6].U, Fe , and X = P, S [61]. The presenceThe ofP contents significant in contentscoffinites of analyzed P and Ca varied in the fr coffiniteom below from detection Bangomb limitsé (Gabon) to the suggestselevated avalues solid solutionrecorded between in coffinites coffinite from and the ningyoiteZálesí deposit [58]. (up In theto 4.1 Pot wt ˚uˇckyuranium % P2O5), Jáchymov deposit, district ningyoite (up to was 5.0 found wt % inP2O close5) and association the Potůčky with deposit P-rich (up coffinite to 8.8 wt (Figure % P2 O125).). The The P-rich ningyoite coffinite was from firstly Pot establishedůčky occurred in the in Ningyassociationô-tôg éwithsandstone-type Ca-U4+ phosphate uranium depositningyoite. (Honshu P-enriched Island, coffinite Japan) [62 has]. Later, been ningyoite described was from also foundsandstone-related in some other uranium sandstone-type deposits uranium in the depositspast, especially in Canada, from Kazakhstan, Russia [56,57–60]. Uzbekistan, The Russia highest and Bulgariacontent of [63 P–65 was]. In found the Bohemian in coffinite Massif, from ningyoite hydrothermal occurred Fe–Cu–Au–Ag–U in the sandstone-type uranium North deposits Bohemian at CretaceousOlympic Dam, uranium Australia district (up to [61 11.4] and wt in % some P2O5) basement-hosted[53]. Other P-enriched vein andcoffinites shear-zone occurred hydrothermal in a natural uraniumfission reactor deposits at Bangombé, (Horní Slavkov, Gabon Já chymov,(up to 8.9 Rožn wt %á )[P210O].5) [58] and in the Khianga uranium district in the north-easternThe chemical Transbaikal composition region, of ningyoite Russia (up from tothe 11 wt uranium % P2O5 deposits) [60]. Enrichment described of above P in wascoffinites thus farcan insufficientlybe explained described. by substituti Publishedon mechanisms works usually coupled present with chemicalY and REE analyses [53] and/or containing following only concentrationssubstitution reactions, of U, P, implying Ca and Fe th withoute existence determination of a solid solution of REE between contents coffinite [10,61–63 and]. Ningyoite ningyoite from[57]: the original locality in Japan contained 23.3 wt %–50.8 wt % UO , 6.1 wt %–11.5 wt % CaO, 4.8 wt % U4+ + Si4+ = 2Ca2+ + 0.8P5+ + 20.2 (2) FeO, 16.8 wt %–29.4 wt % P2O5 and 5.4 wt %–9.3 wt % H2O[62]. The composition of ningyoite from 4+ 4+ 2+ 5+ − the North Bohemian uranium districtU + was Si as= follows: 2Ca + 42.5 P wt + %–49.9(OH) wt % UO2, 11.6 wt %–15.8 wt(3) % CaO, 0.3 wt %–6.0 wt % FeO and 20.0 wt %–26.1 wt % P2O5 [61]. The composition of ningyoite fromWith the ZdaˇrB respect ˚uhore to the clustergood correlation in the Horn betweení Slavkov P2O uranium5 and Y2O district3 contents was: in26.3 P-enriched wt %–41.7 coffinites wt % from the Jáchymov district and Zálesí deposit (correlation coefficients 0.69 and 0.75, respectively; UO2, 9.3 wt %–18.6 wt % CaO and 23.5 wt %–31.2 wt % P2O5 [10]. The chemical composition of Table 5), the P enrichment in these coffinites can be explained by substitution reactions involving Y. ningyoite from the Pot ˚uˇckyuranium deposit (20.5 wt %–35.9 wt % UO2, 7.5 wt %–9.4 wt % CaO, By contrast, in P-rich coffinites from Potůčky there was a very good correlation between CaO and 20.3 wt %–26.1 wt % P2O5) is very similar to the composition of ningyoite from the original site in Japan.P2O5 (correlation The chemical coefficient composition 0.81; of Table ningyoite 5), which from uraniumfavours depositsa solid solution in the former between Soviet coffinite Union wasand neverningyioite. published, but according to Doynikova [66] the Ca/U ratio of ningyoites analyzed (ideally 1:1) varied from sample to sample, and generally Ca prevailed twice as often as U. The Ca/U ratio of ningyioite from Pot ˚uˇckyvaried from 1.1 to 2.0 with a median value of 1.6. The REE concentrations were only analyzed in ningyoite from the Ningyô-tôgé uranium deposit, where contents between 1.2 wt % and 3.8 wt % REE2O3 and only moderate LREE/HREE fractionation (LaN/YbN = 0.6–8.6) were found [67]. The total REE concentrations in ningyoite from Pot ˚uˇckywere Minerals 2018, 8, x FOR PEER REVIEW 17 of 21

5.3. Composition of Ningyoite

Ningyoite, nominally (U,Ca,Ce)2(PO4)2.1–2 H2O, is a phosphate from the rhabdophane group with the general formula AXO4.1–2 H2O, where A = Ca, REE, Th, U, Fe3+, and X = P, S [61]. The presence of significant contents of P and Ca in the coffinite from Bangombé (Gabon) suggests a solid solution between coffinite and ningyoite [58]. In the Potůčky uranium deposit, ningyoite was found in close association with P-rich coffinite (Figure 12). The ningyoite was firstly established in the Ningyô-tôgé sandstone-type uranium deposit (Honshu Island, Japan) [62]. Later, ningyoite was also found in some other sandstone-type uranium deposits in Canada, Kazakhstan, Uzbekistan, Russia and Bulgaria [63–65]. In the Bohemian Massif, ningyoite occurred in the sandstone-type North Bohemian Cretaceous uranium district [61] and in some basement-hosted vein and shear-zone hydrothermal uranium deposits (Horní Slavkov, Jáchymov, Rožná) [10]. The chemical composition of ningyoite from the uranium deposits described above was thus far insufficiently described. Published works usually present chemical analyses containing only concentrations of U, P, Ca and Fe without determination of REE contents [10,61–63]. Ningyoite from the original locality in Japan contained 23.3 wt %–50.8 wt % UO2, 6.1 wt %–11.5 wt % CaO, 4.8 wt % FeO, 16.8 wt %–29.4 wt % P2O5 and 5.4 wt %–9.3 wt % H2O [62]. The composition of ningyoite from the North Bohemian uranium district was as follows: 42.5 wt %–49.9 wt % UO2, 11.6 wt %–15.8 wt % CaO, 0.3 wt %–6.0 wt % FeO and 20.0 wt %–26.1 wt % P2O5 [61]. The composition of ningyoite from the Zdař Bůh ore cluster in the Horní Slavkov uranium district was: 26.3 wt %–41.7 wt % UO2, 9.3 wt %–18.6 wt % CaO and 23.5 wt %–31.2 wt % P2O5 [10]. The chemical composition of ningyoite from the Potůčky uranium deposit (20.5 wt %–35.9 wt % UO2, 7.5 wt %–9.4 wt % CaO, 20.3 wt %–26.1 wt % P2O5) is very similar to the composition of ningyoite from the original site in Japan. The chemical composition of ningyoite from uranium deposits in the former Soviet Union was never published, but according to Doynikova [66] the Ca/U ratio of ningyoites analyzed (ideally 1:1) varied from sample to sample, and generally Ca prevailed twice as often as U. The Ca/U ratio of ningyioite from Potůčky varied from 1.1 to 2.0 with a median value of 1.6. MineralsThe2019 REE, 9, 123concentrations were only analyzed in ningyoite from the Ningyô-tôgé uranium deposit,19 of 23 where contents between 1.2 wt % and 3.8 wt % REE2O3 and only moderate LREE/HREE fractionation (LaN/YbN = 0.6–8.6) were found [67]. The total REE concentrations in ningyoite from Potůčky were distinctly higher (13.5 wt %–22.3 wt % REE2O3) with significant enrichment in middle REE and a distinctly higher (13.5 wt %–22.3 wt % REE2O3) with significant enrichment in middle REE and a similar LaN/YbN ratio (0.8–11.7) (Figure 15). similar LaN/YbN ratio (0.8–11.7) (Figure 15).

Figure 15. Chondrite-normalised rare rare earth earth elem elementent patterns patterns for for ningyoite from the Potůč ˚uˇckyandky and Ningyô-tôgéNingyô-tôgé uranium deposits. Chondrite Chondrite values values ar aree from from Anders Anders and and Grevesse Grevesse [68], [68], data for Ningyô-tôgéNingyô-tôgé deposit are from Muto [[67].67].

6. Conclusions Uraninite, coffinite coffinite and and ningyoite ningyoite mineralization mineralization from hydrothermal from hydrothermal veins of the veins Příbram, of the Jáchymov, Pˇríbram, JHorníáchymov, Slavkov Horn oreí Slavkov districts ore and districts the Pot andůč theky, Pot Zálesí ˚uˇcky, and Záles Příedboand Pˇredboˇriceuraniumřice uranium deposits deposits have been have been studied in this paper. These Late Variscan deposits are situated in low-grade to high-grade metamorphic rocks and/or folded sedimentary rocks of the basement of the Bohemian Massif. Uraninite always represented the primary uranium phase at these localities, whereas coffinite and ningyoite originated mostly during superimposed alteration of primary uraninite. The uraninites analyzed contained variable concentrations of Pb (mostly 1.5 wt %–5.4 wt %, locally up to 10.8 wt % PbO), Ca (0.7 wt %–8.3 wt % CaO), and Si (up to 10.0 wt % SiO2). The contents of Th, Zr, REE and Y were low, mainly below the detection limits of the electron microprobe. The highest concentration of Th was found in uraninite from the Horní Slavkov ore district (up to 0.2 wt % ThO2), the highest concentration of Zr (up to 0.7 wt % ZrO2) contained uraninite from the Jáchymov ore district and the highest concentration of Y was found in uraninite from the Zálesí uranium deposit (up to 2.3 wt % Y2O3). Coffinite from the vein-type deposits studied usually emerged through gradual coffinitization of uraninite. The concentrations of CaO were lower than those in uraninites and varied from 0.6 to 6.5 wt %. Coffinite from the Jáchymov ore district was partly enriched in Zr (up to 3.3 wt % ZrO2). Coffinites from uranium deposits containing superimposed “five-element” (Ag–Bi–As–Co–Ni) mineralization were enriched in Y (up to 2.5 wt %, 5.5 wt % and 9.4 wt % Y2O3 for Pot ˚uˇcky, Jáchymov and Zálesí deposits, respectively). Coffinite from the Pot ˚uˇckyuranium deposit was distinctly enriched in P (up to 8.8 wt % P2O5) and occurred in association with the very rare ningyoite. Ningyoite was found together with P-rich coffinite in veinlets cutting altered uraninite in samples from Pot ˚uˇcky. The composition of ningyoite was similar to that of the type locality in Japan; however, ningyoite from the Pot ˚uˇckyuranium deposit was distinctly enriched in REE (up to 22.3 wt % REE2O3).

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/9/2/123/s1, Table S1: Electron microprobe analyses of selected elements in all uraninites analyzed, Table S2: Electron microprobe analyses of selected elements in all coffinites analyzed, Table S3: Electron microprobe analyses of selected elements in all ningyoites analyzed. Author Contributions: M.R. wrote a significant part of paper, Z.D. analyzed all samples on the electron microprobe, J.S. contributed some samples from the Jáchymov uranium district, P.S. contributed some samples Minerals 2019, 9, 123 20 of 23 from the Pˇríbram uranium district and V.Š. contributed some samples from the Jáchymov uranium district and all samples from the Pot ˚uˇckyuranium deposit. Funding: This research was funded by Czech Science Foundation, grant number [19–05360S] and by Ministry of Culture, Czech Republic, grant number [DKRVO 2018/02]. Acknowledgments: The research for this paper was carried out thanks to the support of the long-term conceptual development research organisation of IRSM AS CR (RVO 67985891) and projects of the National Museum (DKRVO 2019-2023/1.II.a, 00023272). We are grateful to two anonymous reviewers for numerous comments and recommendations that helped to improve this paper. We wish also to thank Annika Szameitat and John Brooker for their English corrections. Conflicts of Interest: The authors declare no conflict of interest.

References

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