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Why at Schneeberg, and not elsewhere? Why in the crystalline basement, and not in the surrounding rocks? How old are the minerals, and where do they come from? Are there more at greater depths, or slightly to the east or west of the place in which they were found? For a mine and for the men who work in it, all these questions are of vital importance and have occupied generations of workers, mining experts and scientists. But definite answers cannot be given to all of them - only to those for which sufficient information, based on geological observations and surveys, is available.
The lead-zinc mineralizations of Schneeberg are not the only ones present. There are similar deposits near Tösens in the high Inn valley and near Annaberg, above Coldrain in Vinschgau, which were once periodically mined. But the mining district of Schneeberg was by far the most profitable.
Geological field surveys at Schneeberg and near Pflersch have shown that the mineralizations are always more or less parallel (concordant) with the schistosity and mineralogical stratification of the embedding rocks. The schistosity, and also the mineralized layers, run ENE-WSW and dip 30-35° north. There are essentially two parallel metalliferous horizons, called by the miners the top or ceiling vein (upper) and the bed vein (lower). The crossing (or cross) vein is another seam which intersects the two deposits at a low angle and links them. These mineralized veins vary in thickness between 1 and 6 m, and their mineral contents are also extremely varied. In addition, they are not continuous, but are often interrupted and displaced by tectonic faults, ranging from a few centimetres to 70 m in some cases. The Medieval miners had already noticed that these mineralized horizons always outcropped near bands of granitic gneiss and that the ore-bodies were particularly rich near dark-grey and black graphitic schists - as we can still find them, for example, on the Schneebergscharte.
All these observations, directly comparable with mineralization processes which are still in progress on the sea floor, as revealed by recent photographs and films, mean that geologists agree about the formation of the Schneeberg mineralization: enrichment in lead and zinc occurred more than 500 MA ago, at the same time as the formation of the sandy-clayey sediments which surround them. In the same way, today, metalliferous muds are still being deposited on the ocean floor in many points. However, there is no definite information as to whether this type of precipitation occurred mainly as a result of inorganic processes or whether bacterial action was involved. The high concentration of sulphides near graphitic sediments indicates that sulphide enrichment occurred in the absence of oxygen. Again, it has still not been clarified whether the acidic volcanic rocks, which are today found both under and above the ore deposits, are in direct relation with the mineralizations, or whether they only enhanced the flow of hydrothermal solutions, which mobilized the minerals. Certainly, the originally muddy sediments were first compressed into sedimentary rocks by the weight of the overlying deposits. Due to immense pressures and temperatures (more than 550°C and 6000 bar) during the Hercynian metamorphism, 300-360 MA ago, the sedimentary rocks were intensely folded and deformed, and then metamorphosed into micaschists, paragneiss and graphitic schists; the metalliferous muds recrystallized and concentrated into a few layers.
The present-day framework of the deposit formed 80-100 MA ago, during the Alpine orogenesis, when the rocks were again subjected to high pressures and temperatures (again exceeding 550°C and 6000 bar) and again folded. Mineralized layers were folded and deformed, metals were mobilized by solutions, and new metalliferous minerals were created. The area surrounding the ore-bodies was also metamorphosed and and quartz-muscovitic schists formed, with high contents of coarse-grained feldspars. In the area in immediate contact with the mineralizations, vein facies formed, consisting of dark schists with large porphyroblasts (crystals) of biotite, with abundant garnet and carbonate, and feldspar/ typically radial in shape, fibrolitic or mixed with the main minerals, tremolite and anthophyllite. There are also neoformations of garnet and albite. In a later orogenetic phase, the entire series of rocks was subjected to weak tectonic movements, which caused the above-mentioned displacements of the mineralized dykes, thus giving rise to considerable problems in excavating and above all in identifying new deposits.