Authigenic sericite record of a fossil geothermal system: the Offenburg trough, central Black Forest, Germany

A fossil geothermal area is hosted by the Carboniferous, Permian and Bunter sandstones of the Offenburg intramontane trough in the central Black Forest. The hydrothermal alteration is identified on the basis of newly formed sericites, which appear as pseudomorphs after feldspar and filling of pore spaces. According to K-Ar dating of sericite, serititization occurred about 145 Ma ago (Jurassic). On the basis of delta(18)O analyses of sericite, sericite composition and vitrinite reflectance, the hydrothermal fluids had temperatures of 150-210degreesC. Because their electrolyte content was low, these fluids are assumed to have derived from meteoric water. A second pulse of electrolyte-rich hydrothermal fluids resulted in quartz overgrowths. Fluid mobilization seems to be linked to the disintegration of Pangaea and to reactivated fault systems extending from the crystalline basement into the intramontane sediments.