Further evidence for a Jurassic mineralizing event in central Europe: K–Ar dating of hydrothermal alteration and fluid inclusion systematics in wall rocks of the Käfersteige fluorite vein deposit in the northern Black Forest, Germany

Abstract

The hydrothermal fluorite vein deposit of `Käfersteige' ranks among the biggest in central Europe. It is located along the suture zone that separates the Moldanubian and Saxothuringian units in the northern Black Forest, and is hosted in Bunter sandstone and underlying granitic basement. K–Ar ages of authigenic illite from the wall rocks give a Jurassic formation age of around 145 Ma for the deposit. Age data scatter down to 80 Ma in illite from the clay gouge and indicate a younger Cretaceous-Tertiary hydrothermal overprint. The pyrophyllite component in authigenic illite from wall rocks and the re-equilibration of illite suggest a formation temperature around 200 °C. Secondary fluid inclusions in quartz from the wall rocks define a syn-mineralization fluid episode involving Ca–Mg–K–Na–Cl-rich brines (about 27 wt% NaCl_eq) with a T _h of about 125 °C, and a post-mineralization Na–Cl-rich fluid overprint with a T _h of about 100 °C. Both generations of fluid inclusions relate to the final event of each cycle, while authigenic illite composition and re-equilibration of illite in the clay gouge may document initial temperatures of formation. The Upper Jurassic fluid system can be traced all over western Europe. It is probably an expression of continent-wide rifting and concomitant regional fluid circulation in connection with major tectonic disturbances, magmatism and abnormal heat flow during the opening of the North Atlantic ocean. The younger barren fluid overprint could be related to the onset of the Alpine orogeny.