The fluorite deposits NE of Regensburg, SE Germany—A mineralogical and chemical comparison of unconformity-related fluorite vein-type deposits

A world-class fluorite mining district was located NNE of Regensburg at Nabburg-Wölsendorf, Germany. Together with some smaller mining sites near Bach a.d. Donau, Germany, it provided the main part of the domestic fluorite supply until the late 1980s when the last mines were shut down in NE Bavaria. Both fluorite mining districts are separated from each other by the Bodenwöhr Halbgraben and their mineral assemblages are controlled by deep-seated fault zones. The NW–SE-running “Pfahl”, also called the “Great Bavarian Quartz Lode” (=quartz dyke along a deep-seated shear zone), is of structural control for the vein-type U-, Pb-, Zn-, Cu-, Bi, -Co-, Ni-, As-, and Se mineralization in the Nabburg-Wölsendorf District. Another prominent fault structure, the NNW-trending boundary fault of the NE Bavarian Basement, controls vein-type fluorite deposits with minor Cu-, Zn- and Pb sulfides in the Donaustauf District.Concluding from the REE distribution, primary fluorite mineralization was sourced from granitic and gneissic aquifers or intrastratal solutions, giving rise in both fluorite mining districts to similar mineral assemblages. Two fluorite mineralizations can be distinguished from each other in both mining districts. The primary fluorite mineralization evolved in the aftermaths of the granite intrusion, during the Late Carboniferous and the early Permian, whereas the secondary mineralization or remobilization of fluorite took place until post-Middle Jurassic times. Remobilization, related to the paleogeography of each mineral district, was more intensive in the northern district, because of a moderate erosion. In the southern district, erosion cut deeply into the fluorite mineralization and left behind only relics of minerals indicative of fluorite remobilization.The physical–chemical regime of the fluorite mineralization was interpreted, making use of the common phase diagrams which show Eh and pH along the x- and y axes. The temperature of formation was as high as 200 °C in both districts. The fluid regime became more acidic and more oxidizing from the north toward the south. This physical–chemical change well accords with the paleogeographic situation when the fluorite mineralization took place during the Permo-Mesozoic and with the supergene alteration during the Miocene and Pliocene when the southern part of the basement was undergoing stronger uplift than the northern part. Gangue minerals, e.g., ferroan carbonate and sulfate minerals are proximity indicators marking the distance of the vein-mineralization relative to the horizontal reference plain and implying a depth zonation (unconformity), whereas minerals and elements originating from the hypogene alteration are proximity indicators relative to the vertical reference plains (veins related to deep-seated lineamentary fault zones).