Giant carbonate veins as traces of hydrothermal fluid pathways in exhumed analogues of geothermal reservoir

The potential geothermal reservoir of Western Germany is represented by Devonian carbonates underlying a vast portion of North Rhine-Westphalia at depths between 1.3 km and 6 km. To reduce uncertainty regarding the structures that can control hydrothermal fluid flow in the buried Devonian reservoir, here we studied the orientation and textures of giant calcite-dolomite veins in Devonian carbonates exposed in the Steltenberg and Donnerkuhle quarries, near Hagen. These veins represent a fossil geothermal system and can provide a snapshot on paleo-structure/stress orientations that controlled geothermal fluid circulation in the potential Devonian geothermal reservoir. Calcite-dolomite veins are mainly subvertical, N-S- to NNW-striking, and up to 10 meters thick (common thicknesses between a few tens of centimeters and 2-3 meters). Accordingly, these orientations represent past flow directions of hydrothermal fluids. Subvertical, or steeply SSW- to NNE-plunging vein intersections were additional enhanced, pipe-like fluid flow directions. Veins probably filled strike-slip and extensional faults generated during late to post-Variscan orogeny. Veins formed by multiple crack-and-seal events through hydrothermal fluid flow and mineral precipitation and opened under a main E-W to ENE extension direction. Calcite crystals grown roughly perpendicular to vein walls and show elongated-blocky to blocky textures, indicating precipitation into open fractures that remained open until their sealing. Vein filling is also characterized by host rock slivers that are not in contact each other, floating within calcite-dolomite crystals. All this evidence suggests that newly created fractures were zone of low pressure, where the surrounding fluids were suddenly sucked into, and the subsequent pressure drop favored the fast precipitation of calcite-dolomite crystals keeping clasts in suspension, as typically occur in co-seismic hydrothermal breccias. Such textures and large vein opening are also common in veins from epithermal mineral deposits and points towards lithostatic load and low differential stress during vein development at shallow depth environments.