The Werra-Fulda mining district, underground extension of the CEVP-alkaline magmatic province – New insights in the magmatic evolution and its interaction with evaporitic deposits

The Werra-Fulda mining district located in Central Germany hosts a world-class deposit of sulfate bearing potash salts. The Permian (Zechstein, ca. 255 Ma) evaporite sequence was intruded by mafic melts in the Neogene resulting in magmatic dykes as well as sill structures frequently observed in the subsurface mining galleries. Furthermore, volcanic rocks intersect the overlaying strata (mainly Buntsandstein and Muschelkalk) at localized spots forming distinct mountains. In this study, samples from subsurface mafic dykes and surface volcanic rocks were intensively documented, sampled, and analyzed for their major, minor, and trace element composition. The geochemical analyses allow the characterization of the rocks that show compositions typical for within plate basanites and nephelinites with minor occurrences of phonolitic dykes. Results of the study also indicate the interaction of magma with adjacent potash salts and partly alteration of potassium and sodium concentration. Two distinct trends in magmatic composition are revealed suggesting at least two sources for the magmatic feeder system in the local area. The geochemical results are compared to published data of the area and nearby volcanic complexes in the Rhön, Vogelsberg, Westerwald, and Siebengebirge. Additionally, we tested the fission-track and (U-Th-Sm)/He dating technique by using apatite from more than 100 magmatic rock samples. Preliminary, results provide two magmatic events: one at about 21 Ma and the other at about 13 Ma. Both events are accompanied by localized tectonic reactivation.