Cross-section through the Rammelsberg deposit. Legend: 1) sandstone, 2) slate, 3) and 4) Wissenbach slate, 5) kneist, 6) ore (banded, massive), 7) grey ore (barite), 8) thrust faults, AL) old orebody, NL) new orebody (Walther, 1986)
Rammelsberg, one of the most historic mines in the world, played an important role in the development of economic geology as a separate field of study. While it was not primarily a silver producer, it led to the discovery and exploitation of the nearby medieval silver mines at Clausthal-Zellerfeld, St. Andreasberg, and Freiberg, which were described in Part 8 of this series (the reader is referred to the map of major silver mining camps in the Central European Mineral Belt on p. 87 of the February 2006 issue of the CIM Magazine). More than a thousand years were required to mine the huge and rich Rammelsberg deposit and to discover its genesis.
The Rammelsberg mine is situated on the northern edge of the Harz Mountains, three kilometres south of the city of Goslar. It lies at the base of a low mountain that reaches an elevation of 670 metres, about 275 metres above the lowlands to the north. Most of the history of the mine that follows is derived from Ramdohr (1962), Morrison (1975), and Lieber and Leyerzapf (1986). While the main historical references on the geological study of the ores are Lindgren and Irving (1911), Newhouse and Flaherty (1930), Ramdohr (1962), Krebs (1976), and Hannak (1981), the definitive reference is Large and Walcher (1999).
Mining of silver is known to have commenced there about 960 or earlier because silver coins that bear the likeness of King Otto, who died in 973, have been found. Legend credits the discovery to the horse Ramelus that exposed the ‘rich’ ore while pawing the ground in 938. The horse was owned, according to different versions of the tale, either by a hunter named Ramm or a German nobleman. Rammelsberg silver became a treasure chest for German rulers, who used it as a source of currency.
According to the Rammelsberg Historical Museum (1993), the ore may have been found as early as the first century, whereas Anger et al. (1966) stated that it “was certainly known in prehistoric time.” Because the ore was an extremely fine-grained intergrowth of complex sulphide minerals, mining of the main orebody was apparently delayed until better metallurgical techniques were developed during the Middle Ages. Mining began in open pits using ladders and gradually moved underground using fire-setting to fracture the rock. The earliest adits have dimensions of about 1.5 metres by 0.6 metres.
Although miners were initially attracted to Rammelsberg by its silver content, and it is often referred to in the literature as a silver mine, none of the descriptions of the ore that are available in English mention the presence of primary silver minerals, except as microscopic constituents that are only recovered as by-products. Although Ramdohr (1962), an acknowledged expert on the deposit, stated that oxidation and secondary enrichment were absent, it seems almost certain that the historic silver production must have come from native silver and/or argentite and other secondary silver minerals in a small oxide zone that formed at the outcrop. No other source for the early silver production, such as younger Ag-rich veins, has been reported. Rammelsberg is actually a zinc-lead mine, with zinc the principal metal. It is now considered to be a classic sedimentary exhalative (sedex) Zn-Pb-Ba-Cu-Fe deposit.
Aside from the limited silver production, the main metal recovered in the early years was copper, which was smelted nearby. However, the complex nature of the ore created smelting problems that resulted in impure metal. Although it was shipped as far away as Belgium and Holland, it wasn’t able to compete against purer metal from elsewhere. The smelter had to be moved frequently to areas with abundant timber fuel because treating each unit of ore required an equivalent unit of firewood. In the early years, there was no market for zinc and it was discarded in a slag assaying about 20 per cent zinc. Until differential flotation was introduced in 1934, it wasn’t possible to separate sphalerite from barite because they were so fine grained and intermixed. Run-of-mine ore containing a total metal content of 25 per cent or better (plus up to 25 per cent barite and ten other metals) was routinely sent directly to the smelter. During much of the 20th century, until it closed in 1988, the mine was operated by Preussag Metall AG. In 1992, it became a UNESCO World Heritage Site.
The early history of the mine reads like the history of Europe, marked by frequent fighting between various princes, kings, and church leaders who shared ownership with the citizens of Goslar and mine financiers. Miners with experience at Rammelsberg began to migrate to the rich silver mines at Freiberg, about 300 kilometres to the east, starting in 1168, and at Clausthal-Zellerfeld, about 10 kilometres southwest of Goslar, about 1180. Mining had advanced to a depth of 150 metres at Rammelsberg by 1250 when caving and an inflow of water led to serious production problems. The first mining regulations in Saxony were adopted there in 1271. A major cave-in killed some 400 miners about 1280 and led to a closure that extended until 1360, during which time the population of Goslar declined from 10,000 to 2,500. Part of that decline was caused by the Bubonic Plague (Black Death), which struck in 1347 and killed one-third of the entire European population.
After rehabilitation and refinancing between 1407 and 1460, mining resumed until more water problems led to another closure between 1525 and 1566. That was solved with the introduction of a new type of pump, invented at Joachimsthal in 1550, that was capable of lifting water from greater depths. It used metal rods that moved up and down in a shaft and were driven by a waterwheel. In 1572, a noteworthy surveying achievement occurred when a drainage adit, 2,350 metres long, which had been driven with hammers and chisels, was connected to the deep workings. The next complications were the Thirty Years’ War and another plague.
Shothole drilling and blasting were introduced in 1632 to replace fire-setting, but its popularity came slowly because of the cost of gunpowder and the poor quality of drill steel. Between about 1650 and 1830, miners worked a six-day week consisting of 86 hours at the mine, of which 52 were spent working, 30 eating and sleeping, and four praying. Sunday was a holiday. Productivity was about 100 to 200 tonnes per man per year prior to 1700, improving to 450 by 1884. Much of that improvement was due to the introduction of new technology, a lot of it developed in the Harz Mountains, including the man engine in 1833 (described in Part 8), wire rope in 1838, dynamite in 1873, steam power in 1875, and pneumatic drilling in 1876. Fire-setting ended in 1878.
A bizarre episode in European history occurred in 1714, when George Lewis, the great grandson of James I of England, became George I, King of England. He also happened to be the ruling monarch of the Kingdom of Hanover, which included the Harz Mountains. That resulted in control of Rammelsberg and other mines in the Harz Mountains passing to the British Crown. It also meant that many decisions were now made in London. The Kingdom of Hanover belonged to the English kings until Queen Victoria’s reign began in 1837. It then passed to the Duke of Cumberland, a German, until 1866 when it became a province of Prussia (Habashi, 1998).
The Rammelsberg orebody is a syngenetic sulphide and barite deposit that was precipitated in a seafloor depression after a period of geosynclinal and magmatic preparation. It consists of two similar, thinly laminated deposits formed in separate, adjacent basins: the Old Mine, which outcrops for a length of 600 metres and extends to a depth of 310 metres; and the New Mine, discovered by underground exploration in 1859, which extends from 30 metres to 90 metres below surface to about 500 metres. Ramdohr (1962) reported that the mineralization was so fine-grained that individual grains were barely discernible with the naked eye. Several sulphosalt minerals are present in veinlets and as disseminations, including bournonite, jamesonite, boulangerite, and the silver mineral tetrahedrite. The deposits have been subdivided into six different layers, depending on the predominant metals contained within each ore type. From top to bottom, the layers are enriched in barite, lead, zinc, copper, and iron. Silver is associated mainly with copper and lead.
The deposits are conformable within the Middle Devonian Wissenbach shale, dated at 390 Ma, and lie within an overturned syncline formed during the Middle Carboniferous Variscan orogeny, about 340 Ma. At least 20 tuff horizons up to three metres thick occur beneath the ore horizon. The contacts between the deposits and wallrocks are often sheared and faulted and the ore has been locally deformed. A funnel-shaped stockwork zone called ‘kneist,’ which is situated vertically above, but stratigraphically below, the orebodies, contains the same mineral assemblage as the deposits plus arsenopyrite and bismuth minerals. Hot metalliferous hydrothermal solutions, rich in hydrogen sulphide, are believed to have ascended through the kneist to reach the seafloor, where they precipitated as colloidal, rhythmically layered, and rapidly crystallizing layers. The Oker and Brocken granitic plutons, which intruded into the Paleozoic sequence in the Lower Permian (about 290 Ma), are much younger than the mineralization or the deformation. Based on geochemical and isotopic evidence, the metasedimentary rocks are considered to be the source of the metals.
The original size of the orebody, including the barite zone, has been estimated as 27 to 30 million tonnes, of which about 22 million tonnes were mined. The overall grade reportedly averaged about 14% Zn, 6% Pb, 2% Cu, 20% barite, 150 g/t (4.1 oz/ton) Ag, and 1 g/t (0.03 oz/ton) Au (Large and Walcher, 1999).
The Rammelsberg deposit was so unique that the origin of the metals and their mode of emplacement developed into a controversy that puzzled the world’s best geoscientists for about 200 years. Many German geologists stressed the pronounced layering and tried to show that it was a sedimentary deposit. Others disagreed that it was stratiform because of the disturbed wallrocks at its margins. Other models that were suggested were magmatic, metasomatic, recumbent segregation, ore seam or bed, stockwork, and replacement. Visiting geologists from North America tended to see epigenetic similarities to the massive sulphide deposits they were more familiar with, which were more likely to have been formed by fluids from nearby intrusions or a volcanogenic source. Hannak (1981) has summarized the various theories and contributions on the genesis of the orebody.
Even the great Swedish-American geologist Waldemar Lindgren was fooled by Rammelsberg (Lindgren and Irving, 1911). Lindgren is remembered as one of the founders and giants of modern economic geology, a respected scientist who spent 31 years studying mineral deposits with the U.S. Geological Survey (the last four years as chief geologist) before moving to the Massachusetts Institute of Technology in 1912 to become professor and head of the geology department. In addition to his prolific publishing record, including his textbook Mineral Deposits in 1913, he played a key role in establishing the journal Economic Geology in 1905. As a graduate of the Freiberg Mining Academy (1882), he also had the advantage of being fluent in German when he visited Rammelsberg. In spite of their experience, he and Irving interpreted the origin of the ore incorrectly:
“In our view, the structural relations of the orebody indicate that, without any doubt, the deposit is a bedded vein (a conformable fissure vein). The distribution and structure of the ore is inconsistent with the theory of sedimentary deposition. As far as our experience goes, the structure is unique in ore deposits but as to its interpretation, there can be no reasonable doubt. The structure is that of a dynamo-metamorphic rock. There is no justification, in our opinion, to justify the assumption of sedimentary deposition... In conclusion then, we regard Rammelsberg as of epigenetic origin. The association with barite shows that it was formed at medium depth, probably within a few thousand feet of surface. The source was probably ascending solutions from the neighboring granite batholith three kilometres away.”
The sedimentary nature of the deposit gradually became more accepted as ore microscopy became available; the structure of the deposit and its wallrocks, and the existence of the conduit (stockwork) were revealed by detailed mapping; and geochemical, geochronological, and isotopic studies were conducted. That knowledge, which was impossible to obtain until advances were made in fluid dynamics and geochemistry in the middle of the 20th century, was a major step forward in the study of mineral deposits.
ANGER, G., NIELSEN, H., PUCHELT, H., and RICKE, W., 1966. Sulfur isotopes in the Rammelsberg ore deposit (Germany). Economic Geology, 61, p. 511-536.
HABASHI, F., 1998. The first schools of mines and their role in developing the mineral and metal industries–Part 2. CIM Bulletin, 91, p. 99.
HANNAK, W.W., 1981. Genesis of the Rammelsberg ore deposit near Goslar/Upper Harz, Federal Republic of Germany. In Handbook of Strata-bound and Stratiform Ore Deposits. Edited by K.H. Wolf. Elsevier, 9, p. 551-557.
KREBS, W., 1976. Geology of European stratabound lead-zinc-copper deposits. Notes for a symposium sponsored by the University of Calgary and the Canadian Society of Petroleum Geologists, Calgary, April 8-9, p. 28-47.
LARGE, D. and Walcher, E., 1999. The Rammelsberg massive sulphide Cu-Zn-Pb-Ba deposit, Germany: an example of sediment-hosted, massive sulphide mineralization. Mineralium Deposita, 34, p. 522-538.
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LINDGREN, W. and Irving, J.D., 1911. The origin of the Rammelsberg deposit. Economic Geology, 6, p. 303-313.
MORRISON, T.A., 1975. The history of the Rammelsberg mine. Presentation, Second General Meeting at the Camborne School of Mines, Camborne, Cornwall, United Kingdom, January 8, 24 p.
NEWHOUSE, W.H. and Flaherty, G.F., 1930. The texture and origin of some banded or schistose sulphide ores. Economic Geology, 25, p. 600-615.
RAMDOHR, P., 1962. One thousand years of mining at the Rammelsberg, Harz Mountains, Germany. Proceedings, Geological Association of Canada, 13, p. 13-21.
RAMMELSBERG HISTORICAL MUSEUM, 1993. Available at: www.rammelsberg.de/home.
WALTHER, H.W., 1986. Federal Republic of Germany. In Mineral Deposits of Europe, 3, Central Europe. Edited by F.W. Dunning and A.M. Evans. The Institution of Mining and Metallurgy and The Mineralogical Society, London, p. 194-198.