Abstract: Petrographic and Geochemical Evidence for Fault-Controlled Hydrothermal Mineralization of the Brilon Reef Complex, Germany

Intervals of pervasive dolostone associated with minor base metal sulfide mineralization have been encountered in drill cores from the Devonian Brilon Reef Complex in the northeastern Rhenish Schiefergebirge, Germany. Because of striking petrographic similarities with pervasively dolomitized hydrocarbon reservoir rocks and host rocks of MVT deposits around the world, the Brilon Reef Complex serves as a case study to enhance the understanding of the origin and formation of these deposits.

Both pervasive dolomitization and sulfide mineralization in the Brilon Reef Complex postdate the Variscan deformation and are related to deep-reaching Saxonian normal faults. Mineralization occurs preferentially in and around breccia zones and is characterized by the presence of grey matrix dolomite, replacive coarse crystalline whitish dolomite, saddle dolomite cement followed by sulfide minerals, and late calcite cements. In some intervals 'zebra' dolomite fabrics occur.

Microthermometric data from fluid inclusions of saddle dolomite, sphalerite, and late stage calcite indicate precipitation of these mineral phases from highly saline fluids (16 to 22 equiv. wt.% NaCI, T[h] = 70 to 120 degrees C) of NaCI-(MgCI[2])CaCI[2] composition. Isotopic (delta{18}O, delta{13}C) and trace element trends in the limestones immediately adjacent to the mineralized intervals are suggestive of hydrothermal alteration and can be utilized as a predictive tool to locate mineralization.

Integration of these data into the regional hydrologic framework suggests that the mineralizing fluids may share a common origin with present-day brines in the Munsterland Basin directly north of the Brilon Reef Complex. Surface-controlled basin-wide groundwater movements, together with hydrothermal convection caused by increased heat flow during the Mesozoic and early Tertiary, may have led to a migration of brines along fault systems resulting in dolomitization and sulfide mineralization in the Brilon Reef Complex.   

AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid