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Erzgebirge ( Krusne Hory ) - Zinnwald / Cinovec, Altenberg, Sadisdorf, Ehrenfriedersdorf, Geyer District, Freiberg, Hammerlein, Niederschlag / Kovarska |
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Germany |
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Sn W Pb Ag
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Super Porphyry Cu and Au
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The tin, tungsten and fluorite-barite deposits of the Erzgebirge (Krusné Hory) metallogenic province are distributed over an area of 8000 km2 in Saxony (eastern Germany) and northern Bohemia (Czech Republic) in central Europe.
The deposits of this region have produced over 300 000 tonnes of Sn, 300 000 tonnes of Pb, 130 000 tonnes of U, 900 000 tonnes of barite, 270 000 tonnes of WO3, 25 000 tonnes of Cu, 1 000 tonnes of Ag and 500 000 tonnes of As2O3.
See also the Bohemian Massif record which describes the setting and origin of the broader massif that includes the Erzgebirge / Krusné hory, and concentrates on its uranium mineralisation.
Mining in the region dates back to at least the 13th century. It was an important source of metals up to and during the second world war and of uranium in addition following that same war. However, after the unification of Germany, the low grades mined rendered most of the deposits un-economic.
See the Krusné hory record for the deposits found in the continuation of the belt into the Czech Republic.
The region is located on the north-western margin of the Bohemian Massif in the Saxothuringian zone of the European Variscides and is related to a partially concealed Late Palaeozoic multiphase granitic batholith intruding an amphibolite facies Neoproterozoic to Carboniferous age metamorphic complex.
The Neoproterozoic basement is composed of migmatitic gneisses and mica schists with abundant interlayers of metamorphosed marls, dolomites, calc-silicates, quartzites, leptynites, ultrabasic and granulitic rocks which were migmatised and granitised during the Variscan orogeny. The overlying Lower Palaeozoic comprises marine clastics (mainly pelitic) and granitic rocks, which are transgressively overlain by Lower Devonian clastics. Middle Devonian clastics and carbonates with interbedded sub-marine spilite-keratophyre volcanics are followed by the Carboniferous Culm facies. The Palaeozoic succession of the Bohemian Massif formed in a basin between the Mid-European plate and the Teplá-Barrandian microplate which opened as a rift in the older Precambrian continental crust. Following the post-Variscan consolidation, the Bohemian Massif formed an emergent continental mass surrounded by Mesozoic and Cenozoic sedimentary cover.
Magmatic rocks within the massif are related to the Archaean to Paleoproterozoic, the Late Neoproterozoic to Lower Palaeozoic Cadomian/Baikalian (700 to 500 Ma), the Lower Palaeozoic Caledonian (500 to 390 Ma) and the Late Palaeozoic Variscan (350 to 300 Ma) orogenies. The bulk of the acid plutonic rocks, particularly the extensive granitic batholiths are partly of Caledonian age, but are predominantly Variscan in the 300 to 270 Ma range.
The granitoid batholiths of the Erzgebirge district outcrop over areas of tens to hundreds of sq. km. Many are composite bodies built up of a succession of temporally, texturally and chemically distinct sub-intrusions.
The distribution of the phases of the Variscan granitoids show that development moved from the core to the margins of the periphery with time, with the youngest penetrating to shallow depths in the Carboniferous and Permian. The 'tin granites' tend to be the late, most acidic members of the evolution of Variscan granites which were intruded at high plutonic to sub-volcanic levels of 5.5 to 0.5 km below the surface. The granite plutons of the Erzgebirge are also characterised by related breccia pipes and sub-volcanic intrusives.
These granites have been divided (Seltmann & Stemprok 1995, Seltmann & Breiter, 1993) into the:
i). Older Granites of 320 to 310 Ma age which are generally characterised by Au, W-Mo and Pb-Zn mineralisation; and
ii). Younger Granites of 305 to 290 Ma age which are accompanied by quartz-cassiterite type Sn-W, F and Li mineralisation. Greisenisation within this group usually takes place in several phases, manifested by the development of silica, zinnwaldite, sericitisation and topaz, suggesting the existence of two greisenisation stages, namely -
• L1 type from 305 to 300 Ma, which host Sn-W pervasive greisen deposits with predominantly rare alkali element associations, characterised by Cs-Rb-Li-Sn-W-Be, in highly specialised granite stocks, developed at upper metasomatic levels, commonly with associated Li micas of the zinnwaldite type;
• L2 type from 300 to 290 Ma, Sn-W-muscovite joint/fracture controlled greisen deposits with no rare alkali accumulations, which are restricted to the apical sub-intrusions and a Sn-W-Cu-Pb association, developed at lower metasomatic levels commonly with associated phengite type muscovite.
Wolframite-quartz type greisen tungsten deposits (without Sn-Li-F) are restricted to the biotite granites, mostly in the western part of the district, where they are spatially separated from the cassiterite wall rocks greisens.
The majority of the mineralised intrusions were controlled by NW-SE trending fault zones, as were the mineralised structures. Two major environments contain high level mineralisation, namely:
i). subvolcanic intrusions of highly evolved tin granites with Sn-W-Mo, commonly associated with breccia pipes, occurring as greisen and skarn type mineralisation which are limited to a narrow contact zone in the apical sections of the batholith and generally have accompanying Cu - the location of skarn mineralisation being controlled by the availability of suitable carbonate wall rocks, and
ii). deep shear and fracture zones, largely oriented NW-SE, which are related to zones of cataclastic or mylonite brecciation, with U and/or F and are accompanied by Fe sulphides.
Zonation with depth along the Erzgebirge district are illustrated by the following:
• Sub-volcanic levels at depths of around 500 m below the palaeo-surface at the Altenberg tin field in the Eastern Erzgebirge;
• Tin-tungsten mineralisation at the Ehrenfriedersdorf tin (-tungsten) field in the Central Erzgebirge at palaeo-depths estimated at ~2000 m;
• Fluorite (-tin-polymetallic) mineralisation at the Schönbrunn field in the Western Erzgebirge characterised by an intrusive level of around 4000 m below the palaeo-surface.
Uranium mineralisation is less closely related to granitic contacts and has a stronger association with lineaments/fracture zones, being largely confined to the West Erzgebirge/Krusné Hory pluton and the Jáchymov-Gera lineament where most of the significant deposits are located. The most important deposits are within the envelope to the granite, rather than the intrusive itself. In the Czeck part of the massif, there is a zonation from uranium associated with graphite in the surrounding sedimentary hosts, pitchblende in chloritic altered granite and carbonate (dolomite, calcite) veins in the envelope to the granite. At Jáchymov (Joachimsthal) in the Czeck Republic, the paragenesis comprised quartz-sulphide with pyrrhotite, pyrite, sphalerite and galena, followed by pitchblende, initially in a quartz-calcite-pitchblende stage, then a dolomite-selenite phase, both of which are accompanied with silica and hematite alteration of the wall-rock. The early calcite was dolomitised by the second phase.
Significant poly-metallic Pb-Zn-Ag mineralisation is predominantly concentrated in the 40 sq. km Freiberg-Brand and 20 sq. km Halsbrücke-Grobschirma districts, although deposits with lower sphalerite and galena contents occur within the western Krusné Hory pluton.
Fluorite-barite deposits are exploited in the Eastern and Central Erzgebirge/Krusné Hory where they either occur with Pb-Zn-Ag or are developed separately. Associations include i). fluorite-quartz and fluorite; ii). hematite-barite; and iii). fluorite-barite with galena.
A zoning is suggested by Seltmann & Stemprok (1995), comprising an i). east-west band of tin deposits across the centre of the province; ii). a zone of tungsten deposits assymetrically developed to the north-west; iii). uranium mineralisation on the eastern margin of the western pluton of the Erzgebirge/Krusné Hory batholith; iv). Fe skarns in the Central Erzgebirge; v). a Mo zone and an important Pb-Zn-Ag zone in the Eastern Erzgebirge.
Significant deposits within the Erzgebirge district in Germany include the following:
Zinnwald (Cínovec) Sn-W-Li deposit - which is 3 km south of the Altenberg deposit in Germany. This deposit straddles the Czeck-German border. Reserves on the Czeck side, at Cínovec, included 55 Mt @ 0.2% Sn, 0.045% W as cassiterite, wolframite and scheelite, from which 40 000 t of tin were extracted. In addition there were 550 Mt @ 0.18% Rb, 0.26% Li and 0.01% Cs delineated.
The country rock in the region comprises Proterozoic metamorphic complex muscovite-biotite orthogneisses and paragneisses and Lower Palaeozoic phyllite and epiamphibolites to the east and partly migmatised muscovite-biotite paragneiss to the west. These are overlain and cut by the Teplice Rhyolite, composed of extrusive and partially intrusive rhyolites, dacites and ignimbrites and their tuffs with arkosic and Mid-Carboniferous coal interbeds. A thick north-south trending dyke of syenogranite porphyry up to 2 km wide intrudes along or near the contact between the eastern gneisses and the Teplice Rhyolite, while smaller dykes and masses are found elsewhere within the Teplice Rhyolite and basement rocks.
The ore deposit is contained within a cupola of the 330 to 295 Ma Cínovec-Zinnwald granite which intrudes the Teplice Rhyolite and is exposed as an oval shaped, north-south elongated 1.3 x 0.3 km outcrop. The western contact is steep, while to the east and south-east it dips at 10 to 30°. The exposed pluton is composed of lithium (zinnwaldite) albite granite (with no plagioclase), representing the upper skin, underlain at a depth of >730 m by medium grained porphyritic granite. In the cupola there are two textural varieties, an earlier porphyritic and later medium grained seriate granite enclosing relics of the porphyritic phase. These lithium granites contain 0.11% Li2O, 4.62% K2O. They contain cassiterite, fluorite, topaz and tantalite as the main accessory minerals, as well as bastnaesite, uranpyrochlore and synchisite. The underlying granite averages 0.05% Li2O, 5.34% K2O, with accessory zircon, tantalite, monazite, xenotime and rutile.
The contact between the lithium-albite granite and the Teplice Rhyolite is marked by irregular developments of a pegmatite-like (Stockscheider) rim. The deposit is composed of:
- Irregular metasomatic greisen and greisenised granite zones from several tens to 100 to 200 m thick following, and located near or at the upper contact of the cupola. It is variably composed of quartz and zinnwaldite with or without topaz, with irregular admixtures of sericite, fluorite and adularia-potash feldspar. Some are intensely hematised.
- Thin, flat greisen zones enclosing quartz veins up to 2 m thick with irregular wall-rock greisenisation within the broader greisen zone described in the previous point. Both the greisen and veins parallel the intrusive contact of the cupola, dipping shallowly to the north, south and east. The ore minerals are cassiterite, wolframite and scheelite. In the greisens, disseminated cassiterite predominates, while in the veins wolframite is roughly equal to, or more abundant than cassiterite.
Altenberg - The Altenberg tin deposit is located in the eastern Erzgebirge, approximately 40 km south of Dresden in Germany and was worked for over 550 years before closing in 1991, by which time approximately 37 Mt of ore had been extracted. The country rock surrounding the deposit comprises the Proterozoic gneiss complex of the eastern Erzgebirge. During the upper Carboniferous (Westphalian) the Teplice quartz porphyry complex was erupted along a 25 km long, NNW-SSE trending deep fissure system, the Teplice-Ulberndorf zone. This eruption was followed by intense brecciation and granite intrusion. One of the latter granite masses was the Altenberg stock, a monzogranite which forms a small dome with a surface exposure of 350 m in diameter. It has steep sides dipping outwards at around 70° and tin mineralisation in its apex. The main stock contains inclusions of and earlier syenogranite and is itself divided into four sub-types, which are in turn cut by a younger, smaller phase which is related to a further phase of brecciation. In the hangingwall of the interior granite there is a 0.3 to 2 m thick marginal pegmatite, underlain by rhythmically layered fine and coarse meta-albite granitic topaz-mica-greisen, the top layers of which are very topaz rich while the flanks contain abundant quartz. Below the greisen is a zone of feldspar alteration, with upper albite and lower potash rich zones.
The greisen zones are characterised by a network of fine mineralised fractures and veins which localised the alteration which grades upwards into a compact, fine to medium grained black to black-green topaz-mica-greisen with quartz, lithium biotite, topaz, some fluorite, and cassiterite with lesser wolframite, molybdenite, arsenopyrite, native bismuth, bismuthinite and other sulphides. The mineralisation was introduced in a number of pulse, but seems to be directly related to faulting and fractures, while the greisens are pervasive. The ore zone is asymmetrically developed in the northern part of the monzogranite adjacent to the country rock contact.
Sadisdorf - The Sadisdorf tin deposit is located in the eastern Erzgebirge approximately 12 km north-west of Altenberg in Germany. The deposit is related to a structurally complicated cataclasite and sub-volcanic complex developed within basement Proterozoic gneiss at the intersection of a NNW trending cataclasite zone and a NE trending brittle fracture zone. It is centred on a multiple sub-volcanic intrusion of tin granite emplaced within a large fluid explosive cataclastic breccia composed of polymict country rock and intrusive clasts from mm to several metres across. The tin granite comprises the early 250x150 m UG1 stock, separated from the country to the north by the downward tapering (from 140 to 90 m) ring dyke of UG2. The third phase UG3 is on the north-western margin of UG1 and occurs as an up to 40 m thick dyke of residual biotite granite. The youngest UG4 leuco-microgranites are conical shaped bodies in the centre of the intrusive complex.
The emplacement of the fluid explosive breccias and sub-volcanic intrusives were all connected to multiple metasomatic events which produced a wide variety of greisens and mineralising phases. The metasomatites are predominantly pseudomorphic greisens, mainly cassiterite bearing mica-greisen with increased topaz and quartz contents. The greisen formed by each individual event is typically zoned where the outer mica-greisen has been overprinted by topaz and an inner quartz-greisen, and less commonly as steeply dipping schlieren to pipe like masses from a few cm to a few metres across.
Ehrenfriedersdorf - The Ehrenfriedersdorf tin-tungsten district is located on the northern margin of the Central Erzgebirge, approximately 70 km south-west of Dresden in Germany. Mining was undertaken in the district for approximately 750 years until cessation of operations in 1990. The country rocks of the district comprise amphibolite facies Neoproterozoic (and older) gneisses and greenschist facies lower Palaeozoic phyllitic to mica schists, meta-carbonates, and a thick sequence of Cambrian rhyolites and meta-extrusives. The grade of metamorphism decreases from SE to NW.
The dominant magmatism in the Ehrenfriedersdorf district is a four phase, post-kinematic Variscan granitoid complex, comprising: i). a fine grained porphyritic granite, occurring as xenoliths in the subsequent two phases; ii). fine-, medium- and coarse-grained porphyritic granites; iii). fine to medium grained, equigranular monzogranite, grading downwards to medium- to coarse-grained equigranular granite; and iv). the youngest, fine- to medium-grained equigranular to seriate granite, typically present as dykes and sills in the earlier phases.
Mineralisation is present in the following forms within the district: a). stringer zones with tin bearing infilling, the oldest phase of mineralisation, occurring as east-west trending swarms from 6 to 10 m wide mainly within the country rock outside of the granites; b). lode type mineralisation as ENE-WSW trending zones of stringers and veins from mm to tens of cms in thickness and lengths of up to several hundred metres. The veins decrease in thickness away from the intrusive contact and are often accompanied by dykes of aplitic micro-granite or meta-aplitic gresisens. The veins carry cassiterite, arsenopyrite, loellingite, wolframite, scheelite and molybdenite in a gangue of quartz with lesser mica, topaz, fluorite, gilbertite, chlorite, apatite, beryl and triplite; c). greisen veins which have a primary magmatic infilling that has been metasomatically overprinted. These veins are found in the immediate contact zone of the intrusive complex with a variety of strike directions and dips. Most are less than 10 cm thick while a few are more than 1 m across. These are only well mineralised if they follow mineralised lode or stringer zones with the same strike direction; d). vein-like greisen zones in the endo-contact of the intrusive complex which are up to several metres in thickness, are similar to the greisen veins of the exo-contact, but are only mineralised in the outer 70 to 80 m of the complex; e). stockwork-like ores, related to SW-NE trending greisenised cupola-ridges in the upper surface of the granite complex. The positions of these highs are related to structures in the country rock. The greisens are mica- and topaz-mica-greisens comprised of mica, quartz and topaz with minor fluorite and triplite with cassiterite, arsenopyite and occasional molybdenite; f). skarn in calc-silicate hornfels interbeds in the country rock, which may be 0.2 to 2 m thick with sulphide and tin mineralisation related to crosscutting veins and veinlets in the vicinity of the granite contact.
Geyer tin district - The 5 x 4 km Geyer mining district is located in the western part of the Erzgebirge, ~25 km south of Chemnitz. Historic tin (and minor tungsten and iron) mining took place in the Geyer district. between 1395 and 1913. Following World War II, extensive exploration targeting skarn-hosted tin mineralization was undertaken in the area south-west of the outcropping Geyersberg stock (Hösel et al., 1996) followed in by further drilling in 2011 and 2012, culminating in the estimation of an indicated resource of 46 Kt of contained tin at an average ore grade of 0.56% Sn in 8.2 Mt of ore (Elsner 2014). This resource is the Geyer SW deposit. Toward the northeast, the Geyer skarn- and greisen-hosted deposit is separated by the NW–SE trending Geyer-Schönfeld fault system from the stockwork-hosted Ehrenfriedersdorf tin deposit (Hösel 1994) which is described separately above.
The Geyer district is dominated by the SW-NE striking and NW dipping Cambrian Jáchymov Group which comprises a succession of paragneisses, mica schists and phyllites, with intercalations of dolomitic to calcitic marbles and quartzites (Hösel 1994). This sequence was metamorphosed to a low- to medium-grade during the Variscan Orogeny and subsequently intruded by a composite granitoid suite that is found at varying depth below the surface over the entire area (Bolduan 1963; Hermann 1967; Hösel et al. 1996; Tischendorf et al. 1965; Tischendorf and Förster 1990). The contact between these granitoids and the metamorphic country rocks is almost horizontal, generally occurring at a depth of ~150 to 450 m, except for the composite, stock-like Geyersberg Intrusion (known as the 'Binge'), which forms a prominent exposure in the eastern part of the district. The flanks of this stock are almost vertical to a depth of 180 m, where they shallow significantly. This stock is mainly composed of two phases, an older granitic suite that has a fine- to coarse-grained porphyritic texture and high biotite content (Bolduan 1963) and is not associated with significant greisen alteration nor Sn mineralisation (Hösel et al., 1996). The younger main intrusive stage cross-cuts the older intrusion and is characterised as an equigranular, low-biotite topaz-bearing granite (Bolduan 1963). It contains up to 4 wt.% fluorine and hosts abundant disseminated cassiterite and lithium-bearing mica (Bolduan 1963; Breiter et al., 1999; Hösel 1994; Hösel et al., 1996) and an intrusive breccia body. To a depth of ~450 m, the textures of the intrusive units merge into a fine-grained, porphyritic granite, that coincides with an increase in biotite and a decrease in topaz (Hösel 1994). The main intrusions are cross-cut by rhyolitic dykes (Bolduan 1963; Hösel et al.,1996), whilst 'stockscheider' pegmatites are found at the contact between the individual intrusions and at the contact with the surrounding meta-sedimentary units. Hornfels and mica schists with contact metamorphic biotite, cordierite and andalusite are abundant in the contact aureole, which is developed over a width of ~50 m outward from the intrusive contact (Hösel et al., 1996).
The skarn mineralogy in the Geyer district is predominantly composed of clinopyroxene and garnet, with minor wollastonite, epidote, vesuvianite, magnetite and fluorite, the relative abundances of which are highly variable (Hösel et al., 1996). This assemblages is overgrown or replaced by chlorite, actinolite, fluorite and cassiterite and other base metal sulphides. High grade, skarn-hosted disseminated cassiterite is intimately associated with magnetite-rich domains, which are mainly found within 450 to 300 m outward from the granite contact (Hösel et al., 1996). The bulk of the tin resources (i.e., ~80%) is hosted by skarn altered to chlorite-actinolite and by fluorite-quartz or fluorite-chlorite veins cutting the skarns and metasedimentary units with a maximum thickness of 15 cm (Hösel et al., 1996).
Minor fluorite-quartz, fluorite-barite, native metal-arsenide, and Mn-Fe veins are found in the Geyer area (Bolduan 1963). These are predominantly younger veins, of Mesozoic and Cenozoic age, not interpreted to be genetically related to the main Sn mineralization (Burisch et al., 2021, 2022; Guilcher et al. 2021; Haschke et al., 2021; Kuschka 1994).
The bulk of the historic production was from the Geyer Binge greisen which was focused on the 'Binge', the exposed, 230 m diameter, 'stock-like', Geyersberg granite (Hoth and Wolf 1986). Greisen alteration and associated cassiterite mineralisation was predominantly developed along NE-SW and NW-SE striking, steeply and flat dipping veins that formed a dense stockwork within the granite. These vein swarms comprised individual veins, ranging in length from 2 to 6 m with a variable alteration halo (Hösel et al., 1996). The alteration halo and veins were mainly composed of topaz, lithium-mica and cassiterite (Bolduan 1963). Most of the cassiterite is hosted within hydrothermal veins and related greisen alteration. Molybdenite and wolframite also occurs in some of these veins, but predate the cassiterite. A variety of hydrothermal veins cross-cut the greisen alteration and cassiterite veining, and are composed of quartz, fluorite, hematite and minor barite. These typically occur in different orientation or as the latest vein infill and are genetically unrelated, based on their mineralogy and cross-cutting relationships (Hösel 1994).
The bulk of the tin mineralisation within the Geyer SW skarn is hosted by exoskarns that mainly replaced marble, but also mica schists that were immediately adjacent to the marble units. Most of the marble units within the deposit area are affected by skarn alteration to some degree, although some drill holes intersected macroscopically unaltered marble.
Three distinct skarn units are recognised at Geyer SW, which are from NW to SE, the 1, 2 and 3 units that dip at 30 to 40°NW, parallel to the metamorphic layering. Each is composed of several skarn layers interpreted to represent former carbonate bands (Hösel et al., 1996), forming laterally discontinuous lens-shaped mineralised bodies. The lowest, or No. 3 skarn unit, contains two discrete skarn layers that can be traced over a strike length of 3 km, extending down to the granite contact at depth, without any recognition of endoskarn. The lower of these two layers is 5 to 20 m thick and is composed of two individual skarn bodies and one orebody. The upper of the layers in the No. 3 skarn unit has a thickness of 15 to 40 m and comprises several skarn bodies replacing marble and adjacent mica schists. Two discrete orebodies have been recognised within this upper layer. The middle, or No. 2 skarn unit, has a strike length of 1.5 km and pinches out at a depth of ~180 m above the granite contact (Hösel et al., 1996). It is characterised by a constant thickness of ~20 m and contains several individual skarn bodies, although the only orebody occurs in the lower part of the unit. The upper, or No. 1 skarn unit has been traced over a lateral distance of >3 km, and extends down to the granite contact. It contains two individual skarn layers. As with the lower skarn unit, no endoskarn has been encountered in association with this skarn unit. The lower of the two layers has a total thickness that varies between 7 and 25 m, and consists of three skarn bodies and one orebody that has a thickness of between 1 and 6 m. These bodies are separated by unaltered mica schists. The upper of the two layers in the No. 1 skarn unit is also separated from the other by unaltered mica schists and has a relatively constant thickness of between 15 and 20 m, enclosing several laterally discontinuous lens-shaped skarn bodies and one discrete orebody in it's lower part. Many other smaller skarn bodies have been identified in the district, although these are typically of only minor thickness and do not host significant tin mineralisation.
This Geyer District summary is drawn from Meyer, et al., 2023).
Hämmerlein - The Hämmerlein skarn Sn deposit is located in the western part of the Erzgebirge district and is hosted by early Palaeozoic schists and gneisses, and is closely associated with one of the most voluminous granite intrusions of the Erzgebirge, the 25 x 22 km, 320 to 325 Ma Eibenstock granite. The principal ore mineral is cassiterite which is disseminated in the four types of skarn in the deposit, namely the garnet, amphibole, pyroxene and magnetite assemblages. The cassiterite is generally fine grained and dominantly occurs in the amphibole skarn.
Freiberg - Silver mining near Freiberg was described as early as 1168 AD, and continued until 1968 AD. More than 1000 Permian and Cretaceous age hydrothermal polymetallic ore veins cut the Proterozoic crystalline basement in the eastern Erzgebirge, some 30 km SW of Dresden in eastern Germany. In the main part of the Freiberg district this basemen consists of biotite-plagioclase orthogneiss and biotite-muscovite paragneiss forming an oval-shaped dome-like structure, surrounded by mica schist, phyllite, a serpentinite-gabbro-amphibolite, and additional gneiss units (Swinkels et al., 2021). The east of the district is dominated by the Late Variscan Niederbobritzsch biotite granite (Tichomirowa 1997) and the ~320 Ma Tharandter Wald Volcanic Complex (Breitkreuz et al., 2021, 2009). The Freiberg district hosts a dense network of mineralised veins exposed over an area of ~30 x 30 km (Baumann, 1965; Müller, 1901). The veins are associated with two roughly perpendicular systems, one trending north-south, the other WNW-ESE. Within each there are two tectono-structural elements, shear zones and feather fractures. The shears have large strike extents and steep dips, with thicknesses of up to 6 m. The feather fractures are less extensive and dip at shallow angles. The mineralisation comprises coarse to banded sulphides which are filled with a variety of sulphides, sulphosalts and native silver. The ore related sulphides include arsenopyrite, pyrite, sphalerite, chalcopyrite, tetrahedrite and galena with lesser freibergite, jamesonite, miargyrite, pyargyrite, proustite and native silver, in a gangue of quartz, carbonates, barite and fluorite. Systematics in the mineralogy and fluid inclusions of the veins suggest cooling and boiling of a low-salinity magmatic-hydrothermal fluid as the main precipitation mechanisms (Bauer et al., 2019; Burisch et al., 2019b; Swinkels et al., 2021). Systematics of the mineralogy and fluid inclusions of the veins suggest cooling and boiling of a low-salinity magmatic-hydrothermal fluid as the main precipitation mechanisms (Bauer et al., 2019; Burisch et al., 2019b; Swinkels et al., 2021). The mineralogical paragenesis with pyrite, base metal sulphides, tetrahedrite-tennantite, and Ag sulphosalts, the high Ag/Au, and fluid characteristics such as low salinity (<4% eq. [NaCl]), and the presence of reduced sulphur (Bauer et al., 2019; Burisch et al., 2019b; Swinkels et al., 2021), suggest ore formation at an intermediate sulphidation state (Einaudi et al., 2003; Hedenquist et al., 2000; Simmons et al., 2005; White and Hedenquist, 1990). Geochronological data constrain the age of epithermal mineralisation to 276 ±16 Ma (Ostendorf et al., 2019). Similar deposits are known at Annaberg 45 km to the south-west of Freiberg in the Central Erzgebirge.
Niederschlag - The Niederschlag fluorite-barite deposit is located close to the town of Oberwiesenthal in the Western Erzgebirge of Germany, and commenced operation in 2013. It is an ~ 3 to 12 m thick, steeply dipping vein with a strike length of ~1 km, that extends from Germany across the border into the Czech Republic. On the Czech side, the vein is referred to as the Kovarska deposit.
Fluorite-barite mineralisation at Niederschlag is hosted by the latest Neoproterozoic to Early Palaeozoic (570 to 460 Ma) sedimentary and igneous units of the Saxo-Thuringian Zone, part of the Variscan Orogenic belt (Rötzler and Plessen 2010). This succession was deformed and metamorphosed during the Palaeozoic collision between Gondwana and Laurussia (Kroner and Romer 2013). Peak metamorphism was at ~340 Ma (Kröner and Willner 1998).
Numerous hydrothermal veins with different styles of mineralisation are recognised in the Niederschlag-Bärenstein district, which covers an area of ~10 x 15 km. These veins host different stages and styles of mainly uneconomic mineralisation, which include Sn-quartz, polymetallic-Pb-Zn-(Cu)-quartz, U-carbonate-(quartz), fluorite-quartz, fluorite-barite-Pb-Zn-Cu sulphides, and Ag-Bi-Co-Ni-As-carbonate assemblages. The Niederschlag fluorite-barite vein deposit is associated with the discrete, NNW-SSE striking, Scheibenberg-Niederschlag-Kovářská Fault that is mineralised over a strike length of ~ 2.5 km. The economically significant fluorite resource is restricted to a ~1 km-long length of the structure (Kuschka 2002). The upper levels of the deposit transect a diverse suite of rock types, including garnet-muscovite schist, paragneisses, marble and quartzite, as well as graphite-rich schist, all of which lie within a high-pressure low-temperature metasedimentary nappe structure of the passive Gondwana margin. These hosts have protolith ages of 460 to 500 Ma (Mingram 1998; Rötzler and Plessen 2010). Approximately 200 m below the current land surface, the host lithology grades from carbon-rich metasedimentary units to para- and orthogneisses.
Mineralisation at Niederschlag has a vertical zonation that ranges from veins predominantly containing a five element assemblage of U-Ag-Co-Ni-As minerals associated with fluorite, barite, carbonates and quartz at shallow levels (Kuschka 2002). As the depth increases, the abundance of fluorite increases at the expense of barite and U-Ag-Co-Ni-As minerals. At ~70 m below the surface, fluorite and barite predominate. The latter assemblage is being exploited at Niederschlag.
Two distinct stages of fluorite mineralisation are recognised. The first of these, Fluorite I (after Kuschka 2002) is fine-grained, vari-coloured and intergrown with quartz, forming colloform bands and complex breccia bodies. Intense, mainly silica and fluorite, alteration occurs as vein selvages and intensely altered fragments of host rock within the vein, closely associated with fluorite I mineralisation.
Fluorite II cross cuts fluorite I and is clearly younger. It is coarse grained, forming massive aggregates and/or continuous bands, intergrown with variable barite, and accompanied by quartz and minor tetrahedrite, sphalerite, galena and chalcopyrite. Whilst Stage I mineralisation is present in approximately equal quantity/thickness and composition throughout the entire known vertical depth profile of the Niederschlag vein, Stage II mineralisation has a systematic increase of fluorite and red barite while white barite decreases with depth (Kuschka 2002). At levels shallower than 200 m below the surface, white barite is the most abundant mineral of Stage II, whilst below this depth, fluorite II becomes more abundant (Kuschka 2002). This change in mineralogy roughly coincides with the lithological change from micaschist to gneiss.
An intense 0.2, up to 3 m thick alteration halo sandwiches the Niederschlag vein (Kuschka 2002), characterised by strong silicification of the wall rock, and is invariably associated with stage I fluorite. Immediately adjacent to the vein, the primary minerals of the host rock may be completely replaced by quartz, with only minor kaolinite. Thin marble units are the most intensely altered, being entirely replaced by fluorite and quartz (Kuschka 2002).
Stage II associated alteration is significantly less pronounced, usually only affecting a few mm to cms of the vein selvage (Kuschka 2002), mostly comprising clay minerals that have selectively replaced metamorphic micas. Cenozoic (~26 Ma) phonolitic dykes cut the Niederschlag vein, producing a distinct discolouration and recrystallisation of fluorite in the immediate contact (a few to a few tens of cm) with the dyke (Friedländer 2019), overprinting the fluorite, but only very locally. This relationship affords a minimum age for the mineralisation.
Inferred resources at the Niederschlag deposit in Germany were last reported by (Kuschka 2002) as ~ 1.15 Mt of fluorite and 0.56 Mt of barite.
This Niederschlag summary is drawn from Haschke, et al., 2021).
- The main record (except where noted) is largely summarised from Seltmann & Stemprok (1995), and Seltmann & Breiter, (1993).
The most recent source geological information used to prepare this decription was dated: 2024.
Record last updated: 3/1/2024
This description is a summary from published sources, the chief of which are listed below.
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Selected References:
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Bauer, M.E., Burisch, M., Ostendorf, J., Krause, J., Frenzel, M., Seifert, T. and Gutzmer, J., 2019 - Trace element geochemistry of sphalerite in contrasting hydrothermal fluid systems of the Freiberg district, Germany: insights from LA-ICP-MS analysis, near-infrared light microthermometry of sphalerite-hosted fluid inclusions, and sulfur isotope geo: in Mineralium Deposita v.54, pp. 237-262.
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Bauer, M.E., Seifert, T., Burisch, M., Krause, J., Richter, N. and Gutzmer, J., 2019 - Indium-bearing sulfides from the Hammerlein skarn deposit, Erzgebirge, Germany: evidence for late-stage diffusion of indium into sphalerite: in Mineralium Deposita v.54, pp. 175-192.
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Burisch, M., Hartmann, A., Bach, W., Krolop, P., Krause, J. and Gutzmer, J., 2019 - Genesis of hydrothermal silver-antimony-sulfide veins of the Braunsdorf sector as part of the classic Freiberg silver mining district, Germany: in Mineralium Deposita v.54, pp. 263-280.
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Haschke, S., Gutzmer, J., Wohlgemuth-Ueberwasser, C.C., Kraemer, D. and Burisch, M., 2021 - The Niederschlag fluorite-(barite) deposit, Erzgebirge/Germany - a fluid inclusion and trace element study: in Mineralium Deposita v.56, pp. 1071-1086.
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Kern, M., Kastner, J., Tolosana-Delgado, R., Jeske, T. and Gutzmer, J., 2019 - The inherent link between ore formation and geometallurgy as documented by complex tin mineralization at the Hammerlein deposit (Erzgebirge, Germany): in Mineralium Deposita v.54, pp. 683-698.
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Lefebvre, M.G., Romer, R.L. and Glodny, J., 2017 - Two stages of skarn formation in the Hammerlein tin-skarn deposit, western Erzgebirge, Germany: in SY04 - Critical and precious metal deposits: theory, experiment and nature; A symposium to recognize the work of A.E. Williams-Jones, Mineral Resources to Discover - 14th SGA Biennial Meeting August, 2017 Proceedings, v.4, pp. 1305-1308.
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Meyer, N., Markl, G., Gerdes, A., Gutzmer, J. and Burisch, M., 2024 - Timing and origin of skarn-, greisen-, and vein-hosted tin mineralization at Geyer, Erzgebirge (Germany): in Mineralium Deposita v.59, pp. 1 - 22. doi.org/10.1007/s00126-023-01194-8.
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Ostendorf, J., Henjes-Kunst, F., Seifert, T. and Gutzmer, J., 2019 - Age and genesis of polymetallic veins in the Freiberg district, Erzgebirge, Germany: constraints from radiogenic isotopes: in Mineralium Deposita v.54, pp. 217-236.
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Romer R L, Thomas R, Stein H J and Rhede D, 2007 - Dating multiply overprinted Sn-mineralized granites - examples from the Erzgebirge, Germany : in Mineralium Deposita v42 pp 337-359
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Romer, R.L., Muller, A., Seltmann, R., Wenzel, T., Rotzler, J., Forster, H.-J. and Robler, R., 2012 - Granites of the Erzgebirge - relation of magmatism to the metamorphic and tectonic evolution of the Variscan Orogen: in Romer, R.L., Forster, H.-J., Kroner, U., Muller, A., Robler, R., Rotzler, J., Seltmann, R., and Wenzel, T., (Eds.), 2012 Granites of the Erzgebirge GFZ German Research Centre for Geosciences, Guidebook to Eurogranites 2012 fieldtrip, October 7 to October 13, 2012, Scientific Technical Report 12/15, 131p.
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Swinkels, L.J., Burisch, M., Rossberg, C.M., Oelze, M., Gutzmer, J. and Frenzel, M., 2021 - Gold and silver deportment in sulfide ores - A case study of the Freiberg epithermal Ag-Pb-Zn district, Germany: in Minerals Engineering, v.174, 16p. doi.org/10.1016/j.mineng.2021.107235.
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Swinkels, L.J., Schulz-Isenbeck, J., Frenzel, M., Gutzmer, J. and Burisch, M., 2021 - Spatial and Temporal Evolution of the Freiberg Epithermal Ag-Pb-Zn District, Germany: in Econ. Geol. v.116, pp. 1649-1668.
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Wolff, R., Dunkl, I., Kempe, U. and von Eynatten, H., 2015 - The Age of the Latest Thermal Overprint of Tin and Polymetallic Deposits in the Erzgebirge, Germany: Constraints from Fluorite (U-Th-Sm)/He Thermochronology : in Econ. Geol. v.110, pp. 2025-2040
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