A km-scale illite alteration zone in sedimentary wall rocks adjacent to a hydrothermal fluorite vein deposit

In a study of wall-rock alteration in the 1.4 km long Wurmtal adit at the Kafersteige hydrothermal vein deposit (northern Black Forest, Germany) illite was found to be the only clay mineral within the Bunter sandstone and intercalated claystone. Illite occurs mainly as a detrital mineral in the claystone, whereas it is hydrothermally neoformed in the sandstone, either in the pores or as an alteration product of K-feldspar. The extensive occurrence of authigenic illite along the entire 1.4 km long profile confirms that the fluids migrated far into the sandstone. The authigenic illite formed during a first pulse of high-temperature fluids (T-h of similar to 220 degrees C) with a low salinity (similar to 1 wt.% NaCleq). These fluids also dissolved Sr and Rb from detrital illite of the claystones at the vein. A later hydrothermal pulse with a lower temperature (similar to 70-150 degrees C) and higher salinity (21-28 wt.% NaCleq) silicified the sandstone adjacent to the vein and caused partial substitution of OH- by F- in the structure of the detrital and neoformed illite along the profile. Within analytical error, the K-Ar dates for the neoformed illite of the < 2 mu m fraction are the same along the profile (similar to 150 Ma). During this hydrothermal process, the age of the detrital illite within the claystone was reset from 310 to 190 Ma. The illite-rich < 0.2 mu m fractions yield ages of similar to 142 Ma (sandstone) indicating a Jurassic origin. The uniform age data for illite in the sandstone and in the claystone are probably due to extensive migration of hot fluids through the wall rocks. The hydrothermal fluids are attributed to recycled meteoric water and brines that ascended from the basement into the cover rocks during the opening of the North Atlantic and/or the nearby Tethys area.