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

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Uraninite, Coffinite and Ningyoite from Vein-Type Uranium Deposits of the Bohemian Massif (Central European Variscan Belt) minerals Article Uraninite, 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 mineral in all deposits studied. The uraninites 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 Minerals 2019, 9, 123; doi:10.3390/min9020123 www.mdpi.com/journal/minerals MineralsMinerals2019 2018, 9, ,8 123, x FOR PEER REVIEW 22 of of 23 21 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, Minerals 2019, 9, 123 3 of 23 Minerals 2018, 8, x FOR
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