Post-Variscan pervasive dolomitization of the Devonian Brilon Reef Complex, Germany: an example of fault-controlled hydrothermal dolomitization. Matthias Grobe* Hans G. Machel, University of Alberta, Department of Earth and Atmospheric Sciences. 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 (Fig. 1 & 2). Some of these intervals show striking petrographic similarities with pervasively dolomitized hydrocarbon reservoir rocks and host rocks of MVT-deposits around the world. They are characterized by the presence of grey matrix dolomite, coarse crystalline milky-white dolomite (occurring as a recrystallization product of grey matrix dolomite and as saddle dolomite cement) followed by sulfide minerals (sphalerite, galena, pyrite) and late calcite cements. The Brilon Reef Complex serves as a case study to enhance the understanding of the origin and formation of pervasive dolostone, which has important implications for petroleum exploration and production strategies. Several lines of evidence indicate that pervasive dolomitization and associated minor base metal sulfide mineralization in the Brilon Reef Complex was mainly controlled by faults that served as fluid pathways for the episodic injection of hydrothermal dolomitizing and mineralizing fluids during the Mesozoic to Late Cretaceous (?Early Tertiary): (1) The thicknesses of pervasively dolomitized intervals vary considerably from one drill core to the other, ranging from tens of centimeters to hundreds of meters (Fig. 2), generally increasing with increasing proximity to NNW-SSE trending normal faults. (2) Pervasive dolomitization cross-cuts bedding-planes and facies boundaries and postdates the Late CarboniferousVariscan deformation. (3) The weak mineralization with base metal sulfides extends into Upper Cretaceous marginal marine sandstones that unconformably overly the Devonian carbonates. (4) Petrographic and isotopic (8 18 0, 5' 3 C, 87 Sr/ 86 Sr) trends in the limestones immediately adjacent to the pervasively dolomitized intervals are suggestive of hydrothermal alteration by hydrothermal fluids. With decreasing distance from the dolomitized intervals, the size of calcite crystals that make up the limestone matrix increasesfrom2 to 30 |im, crystal shapes changefromeuhedral to anhedral and intercrystalline porosity decreases. The S 8 O-values of the limestones and calcite fossils change drasticallyfromvalues in the range of least-altered Devonian marine calcite (5 I8 0 = -3.8 to -5.8 %o PDB) to strongly depleted values (S l8 0 = -9.0 %o PDB); the 6 ,3 C-values follow the same trend, decreasing from slightly enriched values of up to +5.5 %o PDB to rock-buffered values of about +3.0 %o PDB at the contact to the dolomitized intervals. Sr-isotope compositions correlate well with these trends showing an increase from Givetian to Frasnian marine values ( 87 Sr/ 86 Sr = 0.7079 to 0.7083) to more radiogenic values that approach that of the grey matrix dolomite ("Sr/^Sr = 0.7094), suggesting that the fluid event that led to the formation of the grey matrix dolomite also caused the alteration of the adjacent limestones. (5) Microthermometric data from fluid inclusions of saddle dolomite, sphalerite, and late stage calcite indicate precipitation of these mineral phases from highly saline, warm fluids (16 to 26 equiv. wt.% NaCl, T h = 70 to 120°C). Salinities and homogenization temperatures decrease from saddle dolomite to sphalerite to late stage calcite. (6) After maximum burial during the Carboniferous, the Brilon Reef Complex quickly became exhumed, which is documented by occurrences of reworked terrestrial sediments of Triassic, Jurassic, and Cretaceous age in karst cavities within the Devonian carbonates, indicating that mineralization during the Mesozoic must have taken place under near- surface conditions (< 1000 m of burial). Assuming a geothermal gradient of 30° C/1000 m, the temperatures of the mineralizing fluids obtainedfromfluid inclusion homogenization temperatures (see above) are considerably higher than those of the rocks intruded by the mineralizing fluids. Hence, these fluids are hydrothermal. 7) The Sr-isotope composition of whitish dolomite ranges from 0.7085 to 0.7091. The more radiogenic values are characteristic of recrystallized grey matrix dolomite, while saddle dolomite cements are less radiogenic, indicating that the pervasive dolostone in the Brilon Reef Complex originatedfromat least two fluids with distinctly different Sr-isotope compositions: a radiogenic fluid that led to the formation of grey matrix dolomite and a less radiogenic fluid that resulted in the recrystallization of grey matrix dolomite and the precipitation of saddle dolomite. 8) Late stage calcite cements can be divided into two distinct growth phases: an iron-poor core with 87 Sr/® 6 Sr = 0.7106 and an iron-rich rim with 87 Sr/ 86 Sr = 0.7137, which again indicates the episodic nature of the fault-controlled mineralization in the Brilon Reef Complex. Integration of these data into the regional hydrologic framework of the post-Variscan structural and thermal evolution GEO-TRIAD '98 201 CARBONATE DIAGEIMESIS & SEDIMENTATION (19 ) © 2010 by the Canadian Society of Petroleum Geologists