Bohemian Massif - Joachimsthal, Rozna, Zalesi

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The Bohemian Massif is a part of the Variscan belt of Central Europe and hosts many uranium deposits of various sizes, both in the Czech Republic and adjacent eastern Germany. The total historical uranium production is approximately 350 000 t (OECD-IAEA 2003).

This record details the tectonic history and setting of the Massif and describes selected deposits in the Czeck Republic. Additional deposits may be added with time. See also the Krusne Hory ( Erzgebirge ) which concentrates on the tin-tungsten deposits of part of the massif.

Bohemian Massif - Tectonic Setting

The, east-west elongated, diamond-shaped, 350 x 240 km, Bohemian Massif of Central Europe is a large terrane stretching across the central Czech republic, eastern Germany, southern Poland and northern Austria. It is surrounded by four ranges: the Ore Mountains (Krusné hory, or Erzgebirge) to the northwest, the Sudetes (e.g., the Krkonose, Hrubý and Jeseník) to the northeast, the Bohemian-Moravian Highlands (Ceskomoravská vysocina) to the southeast, and the Bohemian Forest (Sumava) to the southwest. The massif encompasses a number of inliers of crystalline rocks, which are older than Permian and were deformed during the Devono-Carboniferous Variscan/Hercynian Orogeny. Starting in the Late Turonian (early-upper Cretaceous) and culminating during the Paleocene, basement blocks forming the Bohemian Massif were upthrust in response to the build-up of pre- and syn-collisional intraplate stresses originating at the front of the evolving Alpine-Carpathian Orogen, with Mesozoic platform sediments and the Alpine molasses being thrust over the Bohemian Massif in the south, determining its current southern margin (Malkovsky, 1987; Ziegler, 1990; Ziegler and Dèzes, 2007).

The main internal Bohemian Massif was formed during the Variscan Orogeny, a phase of orogenesis and accretion of terranes that resulted from the closure of the Rheic Ocean during the Devonian and Carboniferous (408 to 286 Ma), with the progressive northward movement of Gondwana and its subsequent collision with Laurussia. Most elements of the Bohemian Massif are interpreted to have belonged to the Cadomian microcontinent, one of a number of semi-continuous slivers that had split from the North African part of Gondwana (based on characteristic ~2 Ga detrital zircons, diagnostic of a North African provenance) during the opening of the Rheic Ocean, which also included the Armorican and Massif Central terranes in France, and the Northern Iberian massif in western Spain and Portugal, collectively referred to as the Armorican Terrane Assemblage, referred to hereafter as Cardomia (Linnemann et al., 2007).

The northern margin of the Cardomia is defined by the Rheic suture, separating it from the Avalon-Meguma microcontinent (Avalonia) to the north. Avalonia had split from the Amazon craton section of the northern margin of Gondwana further to the west (based on characteristic 1.3 to 1.0 Ga detrital zircons, diagnostic of an Amazon Craton provenance) during the early Cambrian, and drifted to the north and east as the ocean floor of the Iapetus Ocean was subducted below Laurentia to the north. The Iapetus Ocean had separated three tectonic elements, Laurentia (North America and Greenland) from Baltica (the Baltic and northern European blocks) and the part of northern Gondwana represented by Avalonia. During the lower Palaeozoic, the eastern margin of Avalonia collided with Baltica and both drifted together towards Laurentia as the Iapetus Ocean closed, and the Rheic Ocean opened contemporaneously between Avalonia and Gondwana from the early Ordovician. As the Rheic Ocean continued to open, Cardomia split from Gondwana and drifted north. After Avalonia and Baltica collided with Laurentia, to form Laurussia in the Silurian, the Rheic Ocean began to close in the Devonian (Linnemann et al., 2007; Nanceet al., 2008).

The Cardomia microcontinents contain two elements, produced by the 570 to 545 Ma Cardmonian Orogeny. This commenced with the formation of a continental magmatic arc at the periphery of the West African Craton and a related back-arc basin which opened at ~590 to 570 Ma. Diachronous arc-continent collision caused by oblique vector subduction started first in the east (Bohemian Massif) at ~570 to 560 Ma, and in the west (Iberian Massif) at ~545 Ma. This is represented by a southern Palaeo- to Neoproterozoic basement element of West African craton from Gondwana, and a northern late Neoproterozoic to lower Palaeozoic arc/backarc volcano-sedimentary element (Linnemann et al., 2008; Nanceet al., 2008).

Avalonia has been interpreted to represent a similarly aged arc, which had collided with and was accreted to the Amazon Craton section of Gondwana further to the west, and split along the resultant suture zone to form the Avalonian microcontinent (Linnemann et al., 2007).

Laurussia (with Avalonia on its leading edge) and Gondwana sandwiched Cardomia, which was progressively amalgamated into Pangea as the Rheic Ocean closed. The Rhenohercynian Zone, the southern margin of Avalonia in Laurussia, represents a Late Silurian to Lower Devonian (~418 to 400 Ma) magmatic arc and back-arc basin that have been attributed to north-directed subduction beneath Laurussia (e.g., Kroner et al., 2007). The Mid-German Crystalline zone on its southern margin is interpreted to mark the Rheic suture between Avalonia and Cardomia. Following destruction of the Rheic Ocean floor, oblique subduction of thinned Gondwanan continental crust began in the Early Devonian and persisted until the Early Carboniferous. In the process, the Rheic basin was compressed and folded into the Variscan Orogen, with staged closure of sedimentary basins by the subduction of oceanic segments, together with the development of parautochthonous and allochthonous units bounded by northwards directed thrusts. These stacked nappes were associated with low-grade (greenschist) metamorphism (Phillips, 1964; Primmer, 1985) and later, post-orogenic granite magmatism. The resultant Variscan Orogen can be subdivided into a number of tectonic zones within the Bohemian Massif, separated by major thrusts, ranging from the northern (marginal) Rhenohercynian Zone immediate to the north of the massif, to the (internal) Saxothuringian and Moldanubian Zones of Central Europe in the south. From the Permian onward the Variscan mountain belt was eroded and became partly covered by younger sediments, with the exception of the core Variscan massifs.

The basement terranes of the Bohemian Massif comprise three main structural zones, characterised by differing degrees of metamorphism, lithological composition and tectonic styles, namely:
Saxothuringian Zone which forms the northern part of the massif, consists of late Neoproterozoic to Lower Carboniferous marine sediments deposited in the Saxothuringian Basin, a passive back-arc basin to the magmatic arc of the Cardmonian Orogeny. These rocks were slightly metamorphosed during the Variscan/Hercynian orogeny and are found in the northwestern Bohemian Massif, and are tectonostratigraphically underlain by high grade gneisses and granites, outcropping as the competent massifs of the Erzgebirge and Saxonian Granulite Massif. These higher grade rocks were deformed and recrystallised during the Cadomian orogeny and intruded by felsic plutons during the Cambrian and Ordovician. No rocks older than 570 Ma are found within the Saxothuringian Zone, the northern boundary of which is assumed to be the Rheic suture, that juxtaposes the Rhenohercynian Zone of Avalonia/Laurussia to the immediate north (Linnemann, 2007).
Moldanubian Zone forms the central parts of the massif and is characterised by a crystalline rocks with generally higher metamorphic grade (up to amphibolite or granulite facies), when compared to the Saxothuringian Zone, and by voluminous ~520 Ma granitoids. Most of the metamorphic rocks yield ages 615 to 520 Ma. An important feature is the occurrence of inliers of old cratonic basement, e.g., the 1.38 Ga Dobra Gneiss and the 2.05 to 2.1 Ga Svetlik Gneiss. The Moldanubian zone is composed of one allochthonous unit, tectonostratigraphically overlying two parautochthonous units, as follows, in descending order:
i). the Gföhl Unit, a stack of high grade south-vergent crystalline nappes composed of a large body of granitic gneiss associated with migmatites and K-feldspar-sillimanite paragneisses, metasediments, orthogneisses, amphibolites, granitoids and calc-silicates and contains numerous bodies of kyanite-K feldspar granulite (U-Pb zircon ages ~340 Ma; Kröner et al., 2000) and tectonic lenses of eclogite and garnet and/or spinel peridotite;
ii). the Drosendorf Unit, or "Varied Series", a strongly deformed, thick sequence of biotite-plagioclase orthogneisses with garnet and sillimanite, and Proterozoic magmatic crystallisation ages (e.g., Dobra and Svetlik gneisses) overlain by early Palaeozoic metasedimentary cover comprising interlayered calc-silicate rocks, quartzite, intercalations of marble, graphite schists, and numerous bodies of amphibolites, and internal south-vergent thrusting (Franke, 2000; Kribek et al., 2008);
iii). the Svratka (or Ostrong) Unit, or "Monotonous Series", composed of high grade biotite-plagioclase paragneisses interlayered with leucocratic, K feldspar-rich granitic orthogneisses rocks with both igneous (orthogneiss and amphibolite) and sedimentary (paragneiss) protoliths, and lenses of eclogite, which were overprinted by granulite facies metamorphism.
  The "Varied" and "Monotonous Series" are of lower metamorphic grade than the Gföhl unit. Internal thrusting of the Gföhl Unit over the Drosendorf Unit produced widespread metamorphism at ~340 Ma (Franke 2006). Early post-metamorphic durbachites with U-Pb zircon ages between 338 and 335 Ma reflect HP melting in mantle or slab rocks (Kotková et al., 2003). During the early stage of post-orogenic extension in the Moldanubian domain (325 to 300 Ma; Handler et al., 1991) high-grade rocks were, to a variable degree, affected by a greenschist-facies overprint. The Moldanubian zone was thrust over both the Saxothuringian Zone to the northwest, and the Moravo-Silesian Zone to the east (Linnemann et al., 2008).
Moravo-Silesian Zone to the southeast of the massif and includes the Brunovistulian Block, an allochthonous unit thrust over the Moravo-Silesian crystalline rocks. This part of the Bohemian Massif has an Avalonian character and is thought to be part of the southern part of Laurussia. As such, it has been interpreted that the Rheic suture curves from east-west in the north of the massif, to be north-south and to NE-SW around the margin of the Bohemian Massif to separate the Moldanubian and Moravo-Silesian zones (Linnemann et al., 2008).

Rožná and Olsí Uranium Deposits

The Rožná and Olsí deposits, which produced 23 000 t of U at average grades of 0.24% U, are located in the east-central part of the Massif, ~100 km ESE of Praha, in the Czeck Republic. They are hosted within the Gföhl Unit of the high-grade Moldanubian domain, close to its boundary with the medium-grade Svratka Crystalline Unit, which is interpreted to be a low-angle thrust (Tajčmanová 2006). The host rocks to the deposits consist mainly of biotite-plagioclase-K feldspar-quartz±garnet±sillimanite±hornblende gneisses and amphibolites with small bodies of calc-silicate gneiss, marble, serpentinite and pyroxenite. The most abundant rock type are fine- to medium-grained biotite gneisses, which show ample evidence of anatexis (U-Pb zircon ages between 340 and 337 Ma; Schulmann et al., 2005). The partial melting of gneisses produced distinctly banded, medium-grained, mostly stromatitic migmatites, while advanced partial melting resulted in the formation of anatectic granitoid gneisses. Hornblende-plagioclase±garnet±biotite±clinopyroxene±quartz±titanite amphibolites, together with small lenses of pyroxenite, occur as bodies that may be several centimetres to several metres thick, and are traceable over distances of as much as several hundred metres. Clinopyroxene-quartz-K feldspar-plagioclase±garnet±hornblende±forsterite±biotite±calcite±scapolite±titanite calc-silicate gneisses form bodies up to 15 m thick and usually occur in association with marbles and amphibolites. Stratabound barite-hyalophane-sulphide and anhydrite-pyrite lenses form a part of the metasedimentary sequence (Kříbek et al., 1996; 2002).

East-west compression at lower crustal levels is interpreted to have produced early, steep, NNW to SSE- and NNE to SSW-striking foliation in rocks of the deposit area (Schulmann et al., 1999; Tajčmanová et al., 2006). These rocks were subsequently exhumed to middle-crustal levels in association with kilometre-scale isoclinal folding with north-south to NE-SW axial planes, an easterly or westerly vergence, and a mineral elongation lineation paralleling the NE-SW fold axes. Longitudinal north-south to NNW-SSE-striking ductile shear zones (the Rožná and Olší shear zones) dip to the WSW at 70 to 90° and strike parallel to the tectonic contact between the Gföhl and the Svratka Crystalline Units. These shear zones are interpreted to have been the result of SW-NE normal and north-south dextral kinematics (Kříbek and Hájek, 2005). Zones of brittle normal deformation are superimposed with associated with graphite- and phyllosilicate-rich coherent and incoherent cataclasites and fault breccias several centimetres to 15 m in thickness. The main longitudinal bounding faults of the Rožná shear zone, Rožná 1 and 4 host the main part of the disseminated uranium mineralisation. The less strongly mineralised Rožná 2 and 3 fault zones, and numerous separate pinnate carbonate veins can be interpreted as synthetic Riedel shear structures.

Uranium mineralisation occurs as: i). disseminated coffinite, uraninite and U-Zr-silicate ore (in decreasing order of abundance) in chloritic, pyritic, carbonatic and graphite-enriched cataclasites within the longitudinal faults; ii). uraninite and coffinite ore in carbonate veins; iii). disseminated coffinite and uraninite ore in desilicified, albitised and hematitic gneiss adjacent to longitudinal faults, and; iv). mostly coffinite ore restricted to the intersection of the longitudinal structures and oblique fault and joint systems.

Three major mineralisation events are recorded at the Rožná uranium deposit, namely:
i). pre-uranium quartz-sulphide and carbonate-sulphide mineralisation, with white mica wall rock alteration K-Ar ages of 304.5±5.8 to 307.6±6.0 Ma, coinciding with the post-orogenic exhumation of the Moldanubian orogenic root and retrograde-metamorphic equilibration of the high-grade metamorphic host rocks. Fluid inclusion, paragenetic and isotope data suggest formation from a reduced low-salinity aqueous fluid at temperatures close to 300°C.
ii). uraniferous hydrothermal event, which is subdivided into three sub-stages -
pre-ore, where K-Ar ages of pre-ore authigenic K feldspar range from 296.3±7.5 to 281.0±5.4 Ma and coincide with the transcurrent reorganisation of crustal blocks of the Bohemian Massif and with latest Carboniferous (Late Stephanian) to Early Permian rifting. Massive hematisation, albitisation and desilicification of the pre-uranium altered rocks indicate an influx of oxidised basinal fluids into the crystalline rocks of the Moldanubian domain. The wide range of fluid inclusions salinities is interpreted to be due to the large-scale mixing of basinal brines with meteoric water, while the cationic composition of these fluids indicates extensive interaction with crystalline rocks. Chlorite thermometry yields temperatures of 260 to 310°C. Uranium is interpreted to have been leached from the Moldanubian crystalline rocks during this substage, accompanied by hydrothermal alteration which followed, or partially temporally overlapped the pre-uranium event alteration.
ore deposition, between 277.2±5.5 and 264.0±4.3 Ma (K-Ar dating of ore-related illite), roughly corresponding to chemical U-Pb dating of authigenic monazite (268±50 Ma). Uranium ore deposition was accompanied by large-scale decomposition of biotite and pre-ore chlorite to Fe-rich illite and iron hydrooxides, suggesting the deposition of uranium ore was mostly in response to the reduction of ore-bearing fluid by interaction with ferrous iron-bearing silicates (biotite and pre-ore chlorite). Thorium data on primary, mostly aqueous, inclusions trapped in carbonates of this substage range from 152 to 174°C, with total salinities from 3.1 to 23.1 wt‰ NaCl eq.
post-ore, corresponding to a gradual reduction of the fluid system, manifested by the appearance of a new generation of authigenic chlorite and pyrite at temperatures of 150 to 170°C (chlorite thermometry). Solid bitumens that post-date uranium mineralisation indicate radiolytic polymerisation of gaseous and liquid hydrocarbons and their derivatives. The origin of the organic compounds can be related to the diagenetic and catagenetic transformation of organic matter in Upper Stephanian and Permian sediments.
iii). post-uranium quartz-carbonate-sulphide mineralisation. K-Ar ages on illite from post-uranium quartz-carbonate-sulphide mineralisation range from 233.7±4.7 to 227.5±4.6 Ma and are consistent with the early Tethys-Central Atlantic rifting and tectonic reactivation of the Variscan structures of the Bohemian Massif. A minor part of the late Variscan uranium mineralisation was remobilised during this hydrothermal event.

Zálesí U-Ni-Co-As-Ag/Bi deposit

The Zálesí deposit is located on the northeastern margin of the Bohemian Massif, ~130 km ENE of Praha, in the Czeck Republic. It lies within the Orlica-Śniežnik crystalline complex (OSCC), an Early Palaeozoic magmatic-sedimentary sequence, that was metamorphosed to amphibolite facies grade (550 to 650°C, 5 to 8 kbars; Klemd et al., 1995) during the Variscan orogeny (Borkowska et al., 1990; Turniak et al., 2000), accompanied by magmatism, producing syn- and post-kinematic granitoid intrusions and scarce lamprophyric dykes. While most of the Bohemian Massif was uplifted and eroded during the Permian to Jurassic (Chlupáč et al., 2002), to the north, extensive marine sedimentation of Permian and Mesozoic age laps onto the Massif and is preserved in the epicontinental Polish Basin (Dadlez 1997; Marek and Pajchlowa 1997). During the Tertiary, orogenic activity in the Carpathians reactivated the old fault structures far inside the eastern margin of the Bohemian Massif. In addition, local Pliocene to Pleistocene volcanism occurred, which at Zálesí, is represented by a small body of nepheline basanite (Fediuk and Fediuková 1985).

Two principal structural units of the Orlica-Śniežnik crystalline complex (OSCC) are represented in the vicinity of the Zálesí deposit, the:
Stronie Group, essentially a parametamorphic rock sequence that is reported to be a Cambrian component of the core to the OSCC (Don et al., 1990; Skácel 1995; 2004), mainly comprising biotite-plagioclase paragneisses and mica schists, locally with intercalations of marbles, quartzites, graphitic rocks, amphibolites and calc-silicate rocks.
Śniežnik Group, in contrast, is monotonous, composed of the Śniežnik muscovite-biotite orthogneiss, representing a ~500 Ma granitic precursor (Borkowska and Dörr 1998; Turniak et al., 2000).

The Zálesí ore deposit occurs within a shallowly dipping (~30 to 40°N) pocket of metamorphic rocks of the Stronie Group, that has been tectonically thrust into Śniežnik orthogneisses, representing an isoclinal fold with a core of marble, mica schist and minor amphibolite, sandwiched between two limbs composed of amphibole schist and amphibolite. Much of the core is occupied by the large "Central Tectonised Zone" (sensu Šuráň and Veselý 1982) containing brecciated rock that constitutes the important ore-bearing structure. All of these rocks have typically been strongly tectonised and include a significant, northward dipping, graphite-rich structural dislocation that follows the footwall contact between the Stronie parametamorphic rocks and the underlying Śniežnik orthogneisses. This structure, and numerous discordant mylonite zones, are older than the mineralisation, although some younger reactivation is locally apparent. Two lamprophyric (kersantite) dykes postdate the metamorphic foliation but predate hydrothermal mineralisation.

Stronie Group rocks host all 30 individual veins and two stockwork bodies that constitute the economic mineralisation. This mineralisation occurs mainly as north-south trending, 5 to 25 cm thick, complex quartz-carbonate veins (Zeleñák 1968). These veins typically have an irregular morphology, having undergone syn- and post-ore deformation, dislocation and cataclasis. Vein commonly have banded, brecciated, cockade and drusy textures, sometimes overprinted by metasomatic replacement and local remobilisation. Around one third of the mined ore was from two stockwork bodies, situated within the "Central Tectonised Zone", formed by a dense net of subparallel veinlets hosted by brecciated paragneiss, mica schist and marble (the T2 body), and by amphibolite (the T3 body). Both stockworks and veins have similar mineral associations.

The mineralisation took place in three main stages:
i). uraninite, the earliest stage, which comprises palisade quartz, botryoidal or vein uraninite, clausthalite, black fluorite and calcite containing isolated grains of sulphides, mainly chalcopyrite. The quartz-calcite gangue often has a red colouration from disseminated hematite. Uraninite is partially replaced by a non-stoichiometric coffinite-like mineral.
ii). arsenides, which includes native silver and bismuth, mono-, di- and tri- arsenides, and sulpharsenides of Fe, Co and Ni, hosted by a carbonate (calcite and dolomite) gangue. Native Bi forms skeletal crystals while native Ag occurs as dendrites that were often subsequently leached away. The arsenide mineralisation reflects the typical chemical evolution of the hydrothermal system, characterised by a). an increase of the arsenic/metal ratio (mono- to di- to tri-arsenides) and b). elemental fractionation of metals (Ni to Co to Fe). The di- and tri-arsenides have a detailed growth zonation related to substitution of Ni and Co, less Fe, sometimes emphasised by tiny inclusions of sulpharsenides.
iii). sulphides, commonly comprising Fe-Pb-Zn-Cu sulphides hosted in a calcite gangue. In complex veins that contain minerals of all three mineralisation stages, the products of the sulphide stage may alter and replace older mineral phases, especially arsenides.

The ore-related mineral assemblages were formed at low temperatures (130 to ~80°C and locally lower) and low pressures (<100 bars), and from aqueous hydrothermal fluids with significantly variable salinity (0 to 27 wt.% salts) and chemical composition. Dolníček et al., 2009, conclude that early fluids of the uraninite stage contain a small admixture of a clathrate-forming gas, possibly CO2. Salinity levels correlate with oxygen isotope signatures of the fluids, suggesting mixing of brines [δ18O of ~2‰ relative to standard mean ocean water (SMOW)] with meteoric waters (δ18O of ~-4‰ SMOW). These fluids are characterised by very variable halogen ratios (molar Br/Cl = 0.8 x 10-3 to 5.3 x 10-3, molar I/Cl = 5.7 x 10-6 to 891 x 10-6) indicating a dominantly external origin for the brines, i.e., from evaporated seawater, which mixed with iodine-enriched halite dissolution brine. They further conclude that the cationic composition of these fluids indicates extensive interaction of the initial brines with their country rocks, likely associated with leaching of sulphur, carbon and metals. The brines possibly originated from Permian-Triassic evaporites in the neighbouring Polish Basin, infiltrated into the basement during post-Variscan extension and finally expelled along faults giving rise to the vein-type mineralisation. Cenozoic reactivation by low-salinity, low δ18O (around -10‰ SMOW) fluids of mainly meteoric origin, that resulted in partial replacement of primary uraninite by coffinite-like mineral aggregates.

Jáchymov/Joachimsthal U-Ag-Bi-Co-Ni deposit

The Joachimsthal deposit lies within the Krušné hory Mountains of the Variscan Saxothuringian Zone within the Czech Republic, close to the NW border with Germany. The sequence in these mountains is composed of two components: i). Krušné hory Crystalline Complex composed of Proterozoic metamorphic rocks, and ii). lower Palaeozoic volcano-sedimentary units, metamorphosed during the Cadomian orogenic cycle to phyllite, mica-schist and gneisses. Large Cambrian (524±10 and ~550 Ma) granite bodies in the district were transformed into red orthogneisses during the Variscan orogeny. In the central parts of the Krušné hory (Erzgebirge) Mountains, parautochthonous metasedimentary basement has been overthrust by a crustal nappe of orthogneiss, in which eclogites in the structural base of the orthogneiss show Variscan equilibration at 700 to 650°C and 25 kbar. Phengite-garnet-kyanite mica-schists, also near the base of the orthogneiss thrust unit, yield estimates of 640°C and 22 kbar. The mountains have a complex Variscan structural history of folding, nappe formation and faulting. Faulting in the Joachimsthal area includes, NE-SW, NW-SE, east-west and north-south, with the NW-SE set being the most prominent, possibly reflecting deep crustal features. These faults have remained active into the Quaternary (Ondrus et al., 2003).

The dominant tectonic feature in the Joachimsthal district is the east-west trending Klínovec antiform, with steep flanks that are complicated by parasitic folds. The metamorphic country rocks are intruded by, and represent an envelope of the extensive subsurface, but only locally outcropping, Upper Carboniferous granites of the post-metamorphic Krušné hory pluton, and by an associated suite of dykes that include granite porphyry, aplite, pegmatite and lamprophyres. The Krušné hory pluton is an inhomogeneous, tongue-like body, 25 km wide x 10 to 12 km vertical thickness, composed of several successive intrusions. The oldest comprises minor diorite and dominant porphyritic biotite granite (with Th>U) from 332 to 323 Ma, and a younger complex of leucogranites (with U>Th). Apical parts of the latter are greisenised with associated Sn mineralisation. In addition, veins of quartz-cassiterite and wolframite are also noted in fractures and faults separate from the greisens, reflecting a high temperature hydrothermal phase. Skarn developments are found associated with the older biotite granite, including magnetite ores, sphalerite, chalcopyrite and exceptionally cassiterite. The most recent igneous activity was Oligocene to Miocene alkali basalts, tuffs, volcanic breccias and minor trachyte (Ondrus et al., 2003).

The Jáchymov mining district is bounded by a series of faults. The southern margin is delineated by a segment of the 150 km long, 80°SE dipping, ENE-WSW striking, Krušné hory Mountains Fault that parallels the range of the same name. This structure is up to 300 m wide, filled with ferruginous, mylonitic clay. Some 8 km to the north, the curvilinear Northern Fault that changes from east-west in the west to ESE to the east marks the northern limits of the district. The eastern margin is marked by the NNW-SSE Plavno Fault and 6 km to the west the parallel Meridional Fault, that curves into the NW-SE trending Central fault to define the northern half of the western margin. These faults all post-date the intrusion of the Krušné hory pluton. The NNW-SSE to NW-SE trending faults, including the Plavno, Meridional and Central, and a parallel structure between the two, the Panorama Fault, play the major role in faulting within the district, with horizontal displacements of up to 600 m and a vertical component of 150 to 400 m. Lower order, relatively narrow, east-west faults are filled with mylonitic, drusy quartz, dolomite, siderite, sulphides, arsenides and native metals (Ondrus et al., 2003).

Ore deposition is controlled by both structural and lithological factors. The favourable lithological controls include layers of crystalline limestone, as well as graphitic and pyrite-rich rocks, and the distance from a granite contact. Ore-bearing veins within the district mostly strike at between 30 and 300°, while those oriented east-west only contain minor amounts of ore, but was concentrated at vein intersections or coalescences, and in offshoots of the main veins. Several hundred ore veins and ore offshoots branch from the main fault zones. Veins have been classified as either Morning veins, which are east-west and either barren or only weakly mineralised, and Midnight veins which strike from NW to NE (known as the north-south veins), and include most ore veins, which are oriented nearly perpendicular to the main structure of the whole region, the Klínovec antiform. These veins have been grouped into six ore vein clusters (Ondrus et al., 2003).

The Jáchymov ore district is characterised by ores of the tin-tungsten, uranium and "five element association" Ag-Bi-Co-Ni±U. Mineralisation was deposited in a complex paragenetic sequence of stages, involving several chemically specialised phases, accompanied by repeated opening of fractures, and by fracturing of older vein minerals, as follows:
i). Sn-W sulpharsenide stage, which is a high temperature pneumatolitic and hydrothermal phase (~300°C), related to the younger leucogranite complex of the Variscan granites, and is rare in the Jáchymov ore district. This phase is restricted to the east-west veining (Morning veins) and to the apical greisens of leucogranites and is only rarely and weakly expressed in the 'north-south veins' (Midnight veins);
ii). Uranium formation - comprising a carbonate-uraninite stage, deposited into fractures in the older biotite granite and in the metamorphic complex in the 'north-south veins' (Midnight veins). This mineralisation has been dated at 300 to 240 Ma (Baumann et al., 2000) and as such is post-Variscan. Consequently the connection with the Variscan granites is assumed to be only structural, rather than genetic. The first phase of this mineralisation is an ore-free quartz vein phase of chalcedonic, ferruginous, grey and amethystine quartz found on fracture walls, accompanied by fine chalcopyrite and microscopic pyrite. This was followed by the main carbonate-uraninite stage, which is present in variable amounts in a large number of the NW-SE and north-south veins, related to oxidised, neutral to weakly alkaline hydrothermal fluids at ~200°C. The main mineral is colloform botryoidal uraninite, with pink dolomitic carbonate, sporadic dark-violet fluorite, grey to black quartz and pyrite. Uraninite was repeatedly remobilised through the paragenesis of subsequent mineralisation stages. Uraninite cemented fragments of the earlier quartz and penetrated it in thin veinlets alone or accompanied by carbonates. The uraninite has a high REE content. Other associated arsenides and sulphides (mostly chalcopyrite and galena) are younger than the uraninite and most likely the product of the subsequent stage (Ondrus et al., 2003);
iii). Ag-Ni-Co-Bi-As formation - comprising arsenide, arsenic-sulphide and sulphide stages depositing mineralisation into previously partly mineralised open fractures from the two preceding stages. This stage has been dated at 240 to 100 Ma (Ondrus et al., 2003). The early arsenide stage represents the bulk of the total Ag-Ni-Co-Bi-As formation, which is represented by a smaller number of veins than the uranium mineralisation, but is more varied and complex. Hydrothermal fluids strongly reducing, neutral to weakly acid, and medium to low temperature, between 250 and 200°C. The principal gangue is semi-transparent to strongly pigmented quartz. The arsenic-sulphide stage involved arsenic, sulphur and some bismuth, associated with oxidised neutral to weakly acid hydrothermal fluids that varied from early 200 to 180°, to late ~60°C. This stage occurs in a large number of veins, over a wide vertical interval, either as independent veins, or associated with minerals from the arsenide, or younger base metals stages. The most common ore mineral is arsenic accompanied by silver, sometimes with realgar and löllingite in a dolomitic carbonate gangue. Antimonial minerals are prevalent, but less common than arsenic and arsenides. The sulphide stage is widely represented. although the accumulations are minor, either developed independently or deposited on earlier ores, filling contraction fractures in uraninite, and marginal, fractured or leached volumes of arsenide stage ores. The principal minerals are galena, sphalerite, chalcopyrite, pyrite, marcasite and sometimes arsenopyrite, with predominant calcite gangue which occurs with either disseminated, or alternating bands of sulphide. The Post ore stage is indistinct and includes Mn-rich calcite, fluorite-barite, or opaline-quartz.

Production from the key deposits of the Bohemian Massif

Vein-type deposits, spatially associated with granite
   Schlema-Alberoda, 88 000 t U;
   Johanngeorgenstadt deposit, 3600 t U;
   Pöhla-Tellerhäuser-Hämmerlein, 3800 t U;
   Annaberg deposit, 496 t U;
   Jáchymov/Joachimsthal, 8500 t U;
   Horní Slavkov, 500 t U;
   Príbram, 51 000 t U;
   Lower Silesia, 500 t U, Kowary, Kletno, and Radoniów deposits
Granite-hosted, vein- or fault-type deposit
   Grosschloppen deposit, never exploited;
Fault- or shear zone-type deposits, with no evident association with granite
   Mähring deposit, 100 t U;
   West Bohemian area, 9800 t U, Zadní Chodov, Dylen, and Vitkov II deposits;
   Okrouhlá Radouê deposit, 1300 t U;
   Rozná-Olsí, 23 000 t U (average grade 0.24% U), Rozná and Olsí deposits ;
   Jasenice-Pucov, 450 t U;
   Licomerice deposit, 350 t U (non-granite-related fault/shear zone-type deposit);
   Zalesi, 400 t U, mined between 1958 and 1968;

The most recent source geological information used to prepare this summary was dated: 2009.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Dolnicek Z, Fojt B, Prochaska W, Kucera J and Sulovsky P,  2009 - Origin of the Zalesí U–Ni–Co–As–Ag/Bi deposit, Bohemian Massif, Czech Republic: fluid inclusion and stable isotope constraints: in    Mineralium Deposita   v.44 pp. 81-97
Dolnicek Z, Rene M, Hermannova S and Prochaska W,  2014 - Origin of the Okrouhla Radoun episyenite-hosted uranium deposit, Bohemian Massif, Czech Republic: fluid inclusion and stable isotope constraints: in    Mineralium Deposita   v.49 pp. 409-425
Kribek B, Zak K, Dobes P, Leichmann J, Pudilova M, Rene M, Scharm B, Scharmova M, Hajek A, Holeczy D, Hein U F and Lehmann B,  2009 - The Rozna uranium deposit (Bohemian Massif, Czech Republic): shear zone-hosted, late Variscan and post-Variscan hydrothermal mineralization: in    Mineralium Deposita   v.44 pp. 99-128
Ondrus P, Veselovsky F, Gabasova A, Drabek M, Dobes P, Maly K, Hlousek J and Sejkora J,  2003 - Ore-forming processes and mineral parageneses of the Jachymov ore district: in    Journal of the Czech Geological Society   v.48, Issue 3-4, pp. 157-192
Ondrus, P, Veselovsky F, Gabasova A, Hlousek J and Srein V,  2003 - Geology and hydrothermal vein system of the Jachymov (Joachimsthal) ore district: in    Journal of the Czeck Geological Society   v.48, Issue 3-4, pp. 3-18

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