Evolution of an old mining district between 725 and 1630 ad at the boundary between Thüringen and Bayern, SE Germany, using mineralogical and chemical markers, radio-carbon dating, and coal petrography of slags

摘要

Joint investigations of smelting residues, using mineralogical, chemical, and coal petrographic methods as well as radio-carbon dating, are a clue to unravel the history of ancient mining and smelting districts. The data can also contribute to the history of colonization and industrialization of a particular region as well as to constrain and depict human-induced disasters in the past. In the present case, it is a mining area straddling the border between Thüringen (Thuringia) and Bayern (Bavaria), SE Germany. Thermodynamical calculations provide numerical results on the smelting process as to the temperature of formation, proper, and allow for a precise description of the dumping sites as to the redox conditions (Eh) and pH of the fluids under near-ambient conditions. The soluble components involved in these processes may be determined. During the early Middle ages, Sorbian tribes were roaming the area in search of soft iron ores being well exposed near the surface and easily to smelt. During the early modern times, the interest of the miners had shifted towards more complex ore, also containing lead. Gentries in possession of the land were granted mining licenses by the Counts of Nurnberg to exploit these ores. The ore was almost exclusively derived from vein-type deposits rich in ankerite and siderite, which altered into soft Fe ore giving rise to goethite in the gossans. Stratabound SEDEX deposits were only high up on the agenda of miners when they were looking for iron-only deposits. The small size of SEDEX deposits in this region and some difficulties the miners were confronted with during their exploitation did not render this type of ore a real competitor for Fe being mined elsewhere in Germany. Towards modern times, the miners began extracting the byproducts, Pb and Cu, from the siderite veins, and Fe became a residue within the tailings, as it was no longer of economic interest. The miners drifted steadily deeper with a jump in depth of mining at the passage from the High Middle Ages to the Late Middle Ages. Obviously, the miners managed to cope with the hydraulic problems and found more efficacious methods of working in the underground mines. Coal petrographic methods allow for a determination of the temperature of formation and also to constrain the derivation of firewood they used to poke the fire in the smelting furnace. The way the ancient miners produced their charcoal was be described. It was a very inefficient method yielding a low-quality coal with a moderate caloric output from softwood- and hardwood-based organic matter. Thermodynamical calculations based upon Fe phosphates and sulfates show that the slags were laid down always under dry conditions far above wetlands inundated periodically. A blaze at the dumping sites caused an introduction of sulfur in an otherwise sulfur-deficient slag environment after 1275 ad. Therefore, even the secondary minerals within the slags are still of value for a historical environment analysis of the human habitat.