This ore field was the largest deposit in the Ore Mountains. It is situated about 20 km south of Zwickau in the western Ore Mountains. Schneeberg was an important mining center since the 15th century, producing silver, cobalt, nickel and bismuth, and the town of Oberschlema was known for its strongest naturally occurring radioactive waters at the beginning of the 20th century. A radium health center was established in the town in 1914.
After the Second World War, Soviet scientists started exploring the old mining areas for uranium, and mining commenced in 1946 even before SAG Wismut was established in 1947. Schneeberg at the southern end of the deposit became Object 03 and Oberschlema Object 02. Subsequently, the blind (no mineralisation reaches the surface) northern part of the ore field (Niederschlema-Alberoda) was discovered and mining started there in 1949. Object 09 was established to mine the Niederschlema-Alberoda part of the ore field.
Mining in Schneeberg continued until 1954 reusing many shafts from previous silver and cobalt mining. Up to that time, about 200 tonnes of uranium was extracted from Schneeberg. Oberschlema was mined to depths of over 700 m. Mining ceased in 1960 after producing more than 7,000 tonnes of uranium. A high density of veins near the surface and the 'wild' mining methods of the 1940s and 1950s caused the complete destruction of Oberschlema. Most houses were so badly damaged and the subsurface so unstable that the entire small town was removed in the 1950s. Today the former mining area hosts a radon health center reestablished in the 1990s. After a restructure of the company in the 1960s, Object 09 became Bergbaubetrieb Aue (mining division Aue). It developed into the single largest producer of uranium within SAG/SDAG Wismut with a peak production of 4,000 tonnes of uranium in 1965. In the last normal production year, 1989, it produced about 550 tonnes. This deposit was the first of all Wismut deposits to be supplied with large modern mining shafts and powerful ventilation in the mid-1950s. The most important shafts where Shaft 38 (Niederschlema), Shaft 366 (Aue-Alberoda) and Shaft 371 (Hartenstein). The latter went into production in 1959 and was the main shaft of the deposit up to the end of production in 1990. Shaft 371 possessed an automated sorting plant used to separate different classes of ore and to increase the overall ore grade. High grade ore was sent directly to the Soviet Union without further processing, while ore containing less than 1% uranium was shipped to the mill at Crossen near Zwickau. This was done until 1980; afterwards all ore was milled, with the sorting plant raising the average ore grade to 0.4% before it was sent to Crossen for processing. An unsuccessful attempt was also made to produce base metals, silver and selenium as by-products. Shaft 371 was connected to the -540 and -990 m level (the depth is related to the Markus Semmler adit dewatering the upper part of the deposit into the Zwickauer Mulde river). Deeper levels were connected by blind shafts. The deepest level opened in 1988 was the -1800 m level, nearly 2,000 m underground, making the mine the deepest in Europe. The mining method used was the same as in the centuries before, although with more modern equipment: galleries were driven along strike of the mineralised vein on a lower and upper level (vertical distance 30 to 45 m). These were connected by small shafts from the lower to the upper level. Then the vein was mined upward using drilling and blasting. The ore was transported to the lower level and brought to the main shafts by rail cars, while the stope was filled with waste rock, as possible. After a vein was mined out, the entrances were sealed to prevent radon from the old stopes from entering areas with active mining. This method was used in all vein type deposits of the Ore Mountains. The total production from Niederschlema-Alberoda was over 72,000 tonnes of uranium. Together with the production from Schneeberg and Oberschlema, production losses and unmined resources, the total uranium content was about 96,000 tonnes of uranium, making Schneeberg-Schlema-Alberoda the largest vein-style uranium deposit in the world.
After the political changes production slowed down in 1990 and eventually stopped on 31 December 1990. Together with the closure of the uranium mine at Pöhla and the tin mines at Altenberg and Ehrenfriedersdorf this ended the 800-year-old history of metal mining in the Ore Mountains. After the end of production, the newly formed Wismut GmbH as successor of SDAG Wismut started restoration of the area. Flooding of the mine started in 1991 and in 1997 the water reached the -540 m level as the uppermost level of shaft 371. At this point, the mine was opened for the public as the deepest tourist mine in Europe. A water treatment plant was erected with a final capacity of 1,300 m3 per hour, removing uranium, radium, arsenic and iron from the contaminated mine water before releasing it into the Zwickauer Mulde river. Underground work, especially in Oberschlema, is being carried out to secure near-surface galleries and shafts from collapse and to provide safe airways for radon-containing mine air to prevent it from uncontrolled movement into populated areas of the region. The huge waste rock dumps were either relocated or recontoured and covered with 80 cm of clay and 20 cm of top soil. In 2008, most of the underground work was finished and nearly all shafts were sealed. Most dumps are poorly restored. Shaft 382 with a depth of 1,400 m will stay open (although it is flooded) and provide a controlled path for radon-bearing air to escape from the mine. There is still some uranium being won as a consequence of the restoration program; it is being purchased by the US with long-term contracts.
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Like most deposits in the western Ore Mountains, the Schneeberg-Schlema-Alberoda deposit is situated on the Gera-Jachymov fault zone. This major geological structure with a length of several hundred kilometres strikes NW-SE, running from the central Bohemian Massif in the Czech Republic to central Germany. The main element of this fault zone in the Ore Mountains is the Roter Kamm ('Red Ridge') fault carrying a young quartz-hematite mineralisation, but no uranium. This fault forms the boundary between the Schneeberg and Oberschlema deposits, having a vertical displacement of about 400 m. The second controlling element for the deposit are Variscan granites, which underlie the deposit. The mineralised veins are within the exocontact of the granite, although the oldest uranium mineralisation is about 20 Ma younger than those granites. The third controlling factor is a heavily deformed unit of Ordovician to Silurian meta-sedimentary rocks. This so-called Loessnitz-Zwoenitz-Trough runs east–west, broadening toward the east. This rock unit is formed by quartzites, meta-black shales, amphibolites and skarns. It is contained within phyllites, which form the major rock type in this area of the Ore Mountains. The rocks of the Loessnitz-Zwoenitz-Trough are called the 'productive series', because they carry nearly all of the mineralisation. Although the vein structures can be followed from the productive series into the phyllites and granites, no significant mineralisation has been observed outside of it. The ore field contains about 1,800 mineralised veins. Uranium-bearing veins run approximately northwest to southeast, parallel to the Roter Kamm fault.
The oldest type of mineralisation consists of quartz, calcite, fluorite, pitchblende and minor hematite. The age of this primary mineralisation is about 270 Ma and it was the most important type in Oberschlema. About 100 Ma later a second uranium mineralisation formed. Veins of this type contain dolomite, calcite, pitchblende, minor sulphides and selenides (especially clausthalite). This mineralisation event did not bring new uranium into the deposit but remobilised uranium from the older mineralisation. These dolomite-uranium veins were the major uranium formation in Niederschlema-Alberoda. A third uranium-bearing vein type contains quartz, calcite, Co- and Ni-arsenides, native bismuth, silver and pitchblende. Like the dolomite veins, there was no input of new uranium but only remobilisation. These were the most important veins in Schneeberg, not only for uranium but also for Ag and Co-Ni production. Telescoping is also a common feature with different types of mineralisation being found stuck together in the same part of the vein.
Beside the uranium-bearing mineralisation types, there are about one dozen different styles of other mineralisation ranging from Sn-W, Pb-Zn, fluorite-barite to quartz-hematite. They had no economic importance but contributed to the large variety of the several hundred minerals known from the deposit. Especially in the oxidation zones of Schneeberg and Oberschlema, many rare secondary minerals are found, with many of them being newly discovered there. For example, in samples from a single ore shot in Schneeberg five new uranium minerals were discovered in 1871, namely walpurgite, zeunerite, troegerite, uranospinite and uranosphaerite.
Pöhla (mining division Aue)
Pöhla is a small village in the western Ore Mountains at the base of the Fichtelberg, the highest mountain in eastern Germany. Mining in the area started centuries ago but mining activity remained minor in the area until after World War II. Wismut started exploration in the 1940s and mined small mineralisations in the area containing a few tonnes of uranium. A shaft was sunk at Pöhla-Globenstein but found only very little uranium mineralisation. However, a magnetite-skarn mineralisation was discovered and shaft ownership was transferred to an east German iron company for exploration purposes. During exploration in the 1960s, drillings showed radioactive anomalies in areas named Hämmerlein and Tellerhäuser south of Pöhla. It was decided to explore these deposits underground using an adit, which was started in 1967. The portal of the adit was in a valley outside Pöhla. At about 3 km the adit cut the mineralisation at Hämmerlein. It showed that there was just a minor uranium mineralisation there, which was mined out during exploration phase resulting in a production of 15 tonnes of uranium. However, significant tin mineralisation was discovered in skarns and schists. Further, the skarns also contained zinc, magnetite, indium and cadmium. This mineralisation was intensively explored during the 1970s and studies were made to set up a major tin mine. However, although resources were high and ore grades were better than in the producing tin mines in East Germany at that time, the mineralogy was very complex, preventing the usage of proven milling technologies. Wismut developed a special milling technology for the complex tin ore but it proved to be too expensive and the project was stopped.
From Hämmerlein, the main adit was driven further during the 1970s to investigate the mineralisation at Tellerhäuser, which was cut at about 7.5 km adit length. This uranium mineralisation was much larger than at Hämmerlein and two blind shaft were sunk to deeper parts of the deposits. Mining started in 1983 and 1,200 tonnes of uranium were produced until the end of production in 1990. Like Hämmerlein, Tellerhäuser also showed a significant tin mineralisation in skarns as well as magnetite. Some of the magnetite was mined as an additive to concrete for nuclear power plants constructed by the East German government in the 1980s. Small amounts of silver were also extracted, but high arsenic contents of the silver ore made processing very expensive and production of silver was stopped in the late 1980s. An ore shoot with massive native arsenic and silver was made in 1990 shortly before the end of uranium mining. Some tons of the material were mined, but no processing was done.
The positive results regarding base metals at Hämmerlein and Tellerhäuser led to a reinvestigation of tin, tungsten and further resources at Globenstein and significant mineralisation was discovered in the 1980s. But like Hämmerlein, the complex mineralogy prevented further projects so far.
After production ceased, the area was recultivated, including flooding the mine, removing the mine buildings and covering the waste rock area. A water treatment plant was set up to remove uranium, radium and arsenic from the mine water. Because of the low water flow from the mine, it was possible to replace the initial active chemical water treatment plant with a passive biological unit (wetland). The adit is still open to the Hämmerlein part of the deposit and is operated by a private society as a visitor's mine. Wisutec GmbH, a daughter company of the Wismut GmbH, holds the exploration license for the Pöhla-Globenstein area at the moment (2013).
Königstein (Königstein mining division)
Königstein is situated about 40 km southeast of Dresden in the Elbe Sandstone Mountains. Königstein castle is one of the major attractions in this touristic area. Exploration for uranium started in the early 1960s for sandstone hosted uranium mineralisation similar to the ones discovered further south in the Czech Republic. Finally a major mineralisation was discovered in 1963, hosted by Cretaceous sandstones with disseminated uranium minerals in Rollfront-type ore bodies. However, there are also small veins containing barite and pitchblende and the nature of this deposit is under discussion. Most likely it is a combination of a sedimentary style and hydrothermal mineralisation. Construction of the mine started immediately after the discovery and two main shaft along with several ventilation shafts were sunk to depths up to 300 m. Mining started in 1967 and it developed into one of the main producers of SDAG Wismut in the 1970s, with more than 1,000 tonnes of uranium per year in this decade. The ore mined was transported by a cable way down to the Elbe river valley from where it was transported by rail to the mill at Seelingstädt. Besides conventional mining using drilling, blasting and transport of the ore to the mill, already in 1969 studies were started on unconventional production methods for low grade ore using leaching. The low permeability of the sandstone prevented usual in-situ leaching from drillings. Therefore, different methods were developed to blast the ore underground, seal the blasted blocks, and press sulphuric acid into them. Heap leaching was also carried out as well as uranium extraction from mine water before it was released into the Elbe River. In 1984 conventional ore production ceased and only unconventional methods were used from that year on. Production sunk to about 450 tonnes uranium per year, but the Königstein mining division produced at the lowest costs of all Wismut mining divisions. The uranium bearing solution from leaching was transported by rail to the Seelingstädt mill where the final concentrate was produced. Total production of uranium until 1990 was about 18,000 tonnes, with 12,250 tonnes by conventional mining and 5,750 tonnes by unconventional methods.
In 1990 production ceased, as in the other mining divisions. However, the hydrogeological situation was difficult because of the high amount of uranium-bearing solution in the sandstone units hosting the mineralisation. Above that unit there are three aquifers supplying water to the cities of Pirna and Dresden. Therefore, large amounts of water had to be treated before the mine could finally be flooded. The uranium extracted in this cleaning process was sold to an American company in solution form. The total production of uranium from the mine water treatment between 1991 and 2008 can be estimated to be 1,000 t.
Freital / Dresden-Gittersee ('Willi Agatz' mining division)
This mining area covers parts of the cities Dresden (part Gittersee) and Freital. Mining for hard coal in the area is known to date from the 16th century. In 1949 Wismut studied radioactive anomalies in parts of the coal field and took over some mines. Mine ownership alternated several times between Wismut and the local hard coal mining company. In the early 1950s, when the coal field belonged to the hard coal mining company, two new main shafts in Dresden-Gittersee were sunk to depths of about 700 m and the mine was renamed 'Willi Agatz', after a leader of the East German communist party. In 1968 production of coal for energy production was eventually stopped and the mine was transferred a last time to SDAG Wismut. From that time on coal was only mined for its uranium content. The deposit was depleted in 1989 after producing about 3,700 tonnes of uranium since 1949 and 40 million tonnes of hard coal since the 16th century. It was the last Saxonian hard coal mine to be shut down. The uranium-bearing coal was milled at small local plants in Freital when Wismut first was active in the area in the 1950s. After 1968 the coal was milled at Crossen.
Restoration of the area after 1991 was carried out by the Königstein division of Wismut GmbH. The deposit is connected by a several kilometer long adit to the Elbe River in Dresden. This adit was built in the 19th century to ship coal directly from the underground part of the mines to the river, although it was never used for this purpose. However, the adit had to be refurbished by Wismut GmbH as one of the major tasks at the deposit in order to provide for secure dewatering of the mining area. Furthermore, the dumps had to be covered, including ash dumps from the firing of uranium-bearing coal in former times.
Seven coal seams are present in the Permian (Rotliegend) volcano-sedimentary rocks of the Döhlen basin. Three of these seams carry in restricted parts a uranium mineralisation, which was mined by the Wismut. Uranium was introduced into the coal-forming organic material in early stages by surficial waters. The source for the uranium was most likely local volcanic rock units. At a later stage, some remobilisation took place, producing uranium mineralised veinlets in the host rocks of the coal seams. The average ore grade was 0.11% uranium.
