February 2006

Economic Geology

The Central European silver deposits (Part 8)

By R. J. Cathro

Claim post-dated October 18, 1726, situated near the Bockweiser vein, Harz Mountains, GermanyCourtesy of Georges Beaudoin


History records that the Greeks obtained most of their silver currency from the great mines at Laurium, in the southern part of the country, and that the Phoenicians and Romans took most of theirs from the Cartagena region of Spain. Since the Romans had little need for intensive exploration of the frontiers of their empire, the silver resources of Saxony and Bohemia were not exploited until after the Dark Ages. Their development resulted in the first recorded studies of the origin of mineral deposits. This chapter discusses the history and geology of the four most important silver camps, three in Germany and one in the Czech Republic.

Clausthal-Zellerfeld and St. Andreasberg

Two historic German silver vein camps are located south of Goslar and the Rammelsberg mine in the Western Harz Mountains, Clausthal-Zellerfeld (C-Z), and St. Andreasberg camp. Most of the information that follows is from Von Cotta (1870), Phillips (1884), Walther et al. (1986), Lieber and Leyerzapf (1986), and Mertz et al. (1989).

According to Lieber and Leyerzapf (1986), “Few localities worldwide are as famous among collectors and as rich in history as the Harz Mountains…mineralogically, the most important silver mining area in the Harz is St. Andreasberg.” According to Walther (1986), copper was mined at Braunlage, about five kilometres east of St. Andreasberg, during the Bronze Age, while ASME (1987) reported that there is evidence of mining in the C-Z region south of Goslar as early as the 10th century and in the St. Andreasberg area by the 12th century.

Walther (1986) summarized the importance of this silver district as follows: “Together with the Erzgebirge, [it] provided in earlier centuries not only the main part of silver required for the German State and neighbouring countries, especially for coinage, but was pre-eminent in the development of mining and smelting technology, mining laws and mining traditions … Among [its] technical achievements are the invention of the depth indicator, the water column machine, wire rope, track railway with flanged wheels and dynamite.”

Most of the ore in the C-Z district occurs along a major, steeply dipping, branching vein system that is centred on the adjoining towns of Clausthal and Zellerfeld and extends west for seven kilometres to Bad Grund and east seven kilometres to Altenau. Other minor ore zones occur on parallel vein systems that are situated northerly towards Rammelsberg. The main host rock for the veins is a clastic sedimentary unit, mostly composed of greywacke. The much smaller St. Andreasberg camp is located 16 kilometres southeast of C-Z, at the margin of the Brocken granite pluton.

Small-scale mining commenced about 1200 on the C-Z vein system and lasted for 150 years until the outbreak of the Black Death. Mining didn’t resume until about 1500 and continued intermittently thereafter. The initial production was from secondary (cementation) ore in the upper levels, which changed to primary lead-silver ore at depth. Zinc production didn’t start until 1850. The only mine in the camp still operating in 1984 was in the Grund area, which was discovered near the end of the 18th century and worked continuously since 1831. Total production from the C-Z camp has been estimated as about 2 million tonnes of Pb, 1.25 million tonnes of Zn, and 6,000 tonnes of Ag, of which slightly less than half came from the Grund veins. The deepest mine, Burgstatte, reached a depth of 1,000 metres.

The richer, but much smaller, St. Andreasberg camp, which is about six kilometres long, consists of a wedge-shaped vein system that splits into two main veins that diverge to the west. About 25 steep veins have been worked, of which three are most important. They rarely exceed one kilometre in length and one metre in thickness. Wallrocks are mainly Devonian to Lower Carboniferous clastics and limestone. Its mining history was similar to that of the C-Z camp except that the veins were rediscovered in the 15th century and extraction of secondary Ag-Pb-Cu ore occurred between 1480 and 1620. Between 1530 and 1580, more than 100 rich, near-surface deposits were mined using fire-setting, virtually denuding the Harz forests in the process. Mining of sulphide ore took place from 1650 to 1910. Total production has been estimated at 12,500 tonnes of Pb, 2,500 tonnes of Cu, and 313 tonnes of Ag.

Native silver was found in all the St. Andreasberg mines as sheets, wires, and small crystals frequently coated with argentite. The most prized mineral specimens were rare and beautiful crystals of the silver minerals dyscrasite, pyrargyrite, proustite, stephanite, polybasite, and tetrahedrite, plus native antimony and native arsenic. Spectacular calcite crystals occurred in a wide range of habits having, in total, more than 140 different crystal forms found in nearly 400 different combinations. Zeolites were also common.

Mineralization in the two camps was markedly different, with St. Andreasberg containing significant amounts of copper and minor zinc, the opposite of C-Z. There was also a sharp contrast between the mode of silver occurrence in the two camps, with the much larger production of C-Z obtained from argentiferous galena. In contrast, most of the silver in the St. Andreasberg ore came from the rich Ag minerals listed above rather than from galena. St. Andreasberg also contained much more fluorite, small amounts of Co and Ni minerals, and well-banded comb textures. Veins at C-Z were stronger structures with greater length and thickness, and significantly lower temperatures of formation.

Ideas about the age and origin of the mineralization changed repeatedly. A connection with the tectonic uplift of the Harz Mountains and the intrusion of granite plutons, including an inferred deep pluton beneath the district, was proven false by age dating adularia from the veins (Mertz et al., 1989). The mineralization was dated at about 120 Ma (Lower Cretaceous), about 170 Ma younger than the intrusions. Geochemical and isotopic studies later showed that the metals and sulphur were derived from the sedimentary wallrocks.

The technological inventions produced in the Oberharz were groundbreaking ideas that were soon adopted around the world. Wire rope was designed by Julius Wilhelm Albert, a government mining official at Clausthal, to replace hemp rope and steel chains in deep shafts. It was first tested there in 1834 at the 484-metre deep Caroline mine. In 1866, Alfred Nobel and Hermann Koch, also a government official at Clausthal, experimented with nitroglycerine. Further development by Nobel led to the commercial use of dynamite.

The ‘man engine’ was invented in 1833 by L. Doerell at Zellerfeld. The pumping practice then in use employed two rods running in opposite directions in the shaft, each completing its stroke, halting, and reversing direction according to the motion of the waterwheel crank. Doerell fitted footplates and handgrips to each rod so that by stepping from one to the other at the turning point, a miner could ride up or down the shaft. While this saved the miners a great deal of time and effort, it would still have been a frightening journey in a dark shaft for someone afraid of heights. A man engine was installed in 1837 in the Samson mine shaft near St. Andreasberg when it was 600 metres deep, but was used for hoisting ore rather than water. It was later extended to the ultimate shaft depth of 810 metres. A working model is still in operation. The Samson mine has been declared a historic monument and the camp now lies within a National Park.

Freiberg

The Freiberg camp is situated in Germany at the northwestern edge of the Erzgebirge (Ore Mountains in English). Most of the historical and geological information on the camp contained here is from Von Cotta (1870), Phillips (1884), Bernard et al. (1968), Lieber and Leyerzapf (1986), and Baumann (1994).

Lieber and Leyerzapf (1986) summarized the early history and significance of the camp as follows: “[Mineral] collectors throughout the world are well acquainted with the beautiful silver minerals from Freiberg, Saxony…Intensive exploration and mining were carried out and an increasingly large number of silver veins became known. The district grew to 20 kilometres north-south and 10 kilometres east-west, 200 square kilometres virtually covered with mining operations.”

Miners from Rammelsberg and the Harz Mountains commenced work at Freiberg in 1168 after native silver was noticed in a cart track by men transporting salt to Bohemia. Silver mining was flourishing by 1181 and it was designated as a ‘free city’ in 1221. Activity declined after the easy surface ore was removed, but a second period of prosperity began in the 14th century after drainage adits were driven beneath the old mines to prevent flooding. The scope of the water problem is indicated by the 2,100 horses and 250 miners who were kept busy lifting water to surface in 1569. Production was often disrupted by frequent regional problems such as wars and plagues that will be discussed later in connection with the Rammelsberg mine.

A third period of intensive activity took place from 1750 through the 1800s. Most of the ore was obtained from narrow, short veins, although others up to five kilometres long and a metre wide were also mined. By 1900, only 28 mines remained active and they closed with the outbreak of the Great War in 1914. A few mines operated temporarily in 1937 and after World War II, but the last mine closed in 1969. Total historic silver production has been reported as about 5,250 tonnes (Lieber and Leyerzapf, 1986), 5,700 tonnes (Freels and S?temprok, 1995), and greater than 7,000 tonnes (Beaudoin and Sangster, 1992). In addition to silver, Cu, Pb, Zn, As, pyrite, and small amounts of Au, U, Cd, Ge, In, Bi, and Sn were also produced.

The Freiberg vein swarm is hosted by an Upper Proterozoic and Cambro-Ordovician sedimentary sequence that was metamorphosed into a gneiss dome. Most of the silver was present as fine disseminations in argentiferous galena. Common silver minerals were native silver, argentite, polybasite, pyrargyrite, stephanite, miargyrite, and tetrahedrite. The latter, called ‘grey copper’ by prospectors, or fahlore (fahlerz in German), was probably the most important. It is a complex mineral in which Ag and Cu can substitute for each other. A rare variety of tetrahedrite, in which the amount of Ag exceeds that of Cu, was named freibergite after the town. Unfortunately, no information was found in English on the silver content of the freibergite at Freiberg. At the Keno Hill camp, Yukon, for comparison, freibergite returned assays as high as 25.5 weight % (255,000 g/t, or 7,500 oz/ton).

One of the most noteworthy events in the history of the city was the founding of the Freiberg School of Mines (also referred to as the Mining Academy; Bergakademie in German) in 1765 (Phillips gave the date as 1702). It grew into the premier mining school in Europe, drawing students from all over the world, and soon became famous for the study of vein deposits, including important theories on their genesis. In the words of Baumann (1994): “Wherever mining specialists and metallurgists meet, the name of Freiberg will be known to them. All over the world, this name is associated with the scientific and technological achievements of generations of scientists and mining engineers which for centuries have been part of the cultural heritage of mankind.”

In order to better understand the complex relationship between mineralization and individual veins, several classification systems, based on paragenesis, were developed at the School of Mines over the years. The first systems were presented by Charpentier in 1778 and Werner in 1791. The most recent is Baumann’s widely respected 1964 classification that divided the mineralization contained in 1,100 veins into nine ‘ore associations’ according to their mineralogy, relative age, and vein orientation. That allowed the ore formations to be subdivided into five that accompanied the Hercynian orogeny (320 to 280 Ma), and four later ones that were dated between 250 and 30 Ma. Isotopic studies and age dating showed that a granitic pluton that outcrops on the eastern margin of the camp is not genetically related to the vein mineralization, which was derived from the metasedimentary host rocks.

Whereas Freiberg is essentially a primary silver camp that produced other metals as by-products, several other mining camps are present in the Erzgebirge in which silver was historically important but not as dominant. These include, on the Czech side of the Erzgebirge (called the Krus?né hory), Jachymov (Joachimsthal), which was discovered in 1516, and later became important as a producer of uranium (Cathro, 2005). On the German side, Schneeberg, Annaberg, Marienberg, and Johanngeorgenstadt are also noteworthy examples. According to Freels and S?temprok (1995), total production on the German side of the border was about 8,000 tonnes of Ag and 270,800 tonnes of Pb, and about 2,000 tonnes of Ag and 29,000 tonnes of Pb from the Czech side of the Erzgebirge.


References

AMERICAN SOCIETY OF MECHANICAL ENGINEERS (ASME), 1987. Grube Samson silver mine reversible waterwheel and man-engine: Designation as an international mechanical engineering landmark, 6 p., www.ASME.org/history/brochures/h118.pdf.

BAUMANN, L., 1994. The vein deposits of Freiberg, Saxony. Monograph Series on Mineral Deposits, 31. Gebruder Borntraeger, Berlin-Stuttgart, p. 149-167.

BEAUDOIN, G. and SANGSTER, D.F., 1992. A descriptive model for silver-lead-zinc veins in clastic metasedimentary terranes. Economic Geology, 87, p. 1005-1021.

BERNARD, J.H., RÖSLER, H.J., and BAUMANN, L., 1968. Hydrothermal ore deposits of the Bohemian Massif. Guide to Excursion 22AC, Czechoslovakia and German Democratic Republic. International Geological Congress, XXIII Session, Prague.

CATHRO, R.J., 2005. Joachimsthal, Czech Republic. CIM Bulletin, 1085, p. 73-75.

FREELS, D., S?TEMPROK, M. (editors), HÖSEL, G.,TISCHENDORF, G., WASTERNACK, J., and BREITER, K., 1995. Mineral resources in Erzgebirge-Vogtland/ Krus?né hory. Explanatory Notes to Map 2: Metals, Fluorite/Barite, Occurrences and Environmental Impact. C?esk´y geologick´y ústav, Prague, Sächsisches Landesamt f. Umwel u. Geologie, Bereich Boden u. Geologie, Freiberg,  16 p.

LIEBER, W. and LEYERZAPF, H.,  1986. German silver: An historical perspective on silver mining in Germany. The Mineralogical Record, 17, p. 3-17.

MERTZ, D.F., LIPPOLT, H.J., and SCHNORRER-

KÖHLER, G., 1989. Early Cretaceous mineralizing activity in the St. Andreasberg ore district (Southwest Harz, Federal Republic of Germany). Mineralium Deposita, 24, p.  9-13.

PHILLIPS, J. A., 1884.  A Treatise on Ore Deposits. Macmillan and Co., London, 651 p.

VON COTTA, B., 1870. A Treatise on Ore Deposits (translated from the 2nd German edition by Frederick Prime Jr.). Van Nostrand, New York, 575 p.

WALTHER, H.W., 1986. Federal Republic of Germany. In Mineral Deposits of Europe, Volume 3, Central Europe. Edited by F.W. Dunning and A.M. Evans. The Institution of Mining and Metallurgy and The Mineralogical Society, London, p. 181-191, 230-236.

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