Publikationsserver der RWTH Aachen University
Fluid evolution, vein formation and alteration associated with a low angle shear zone at the northern Variscan fold-and-thrust belt in the vicinity of the Aachen Geothermic-drilling, Western Germany
Abstract
dc:descriptionAnalyses of fluid inclusions in hydrothermal veins and the alteration mineralogy were performed on samples from the RWTH-1 geothermic well. A detailed textural, structural and fluid inclusion study of distinct vein generations has been performed to correlate the temperature-pressure conditions of vein formation with specific deformation episodes. The drill site is located in the town of Aachen, Germany and reached a final depth of 2544 m. The Lower and Upper Devonian rocks of the RWTH-1 drill are located within the Aachen fold and thrust belt system. The carbonates and siliciclastic rocks are crosscut by hydrothermal veining and cataclastic to mylonitic deformation processes. Vein formation is related to a low angle shear zone. Veins in some places form a regular and dense network, often displaying a conjugate system. Three vein types formed synchronous with the progressive deformation (D1) such as, the quartz-carbonate ± chlorite veins, the carbonate ± chlorite veins, and the quartz ± chlorite veins. These hydrothermal veins are a few millimetres to centimetres thick. Vein structures are formed in the transition from brittle to ductile deformation. Brittle deformation is indicated by dilational jogs, tension gashes and pseudotachylites. Ductile deformation is indicated by the undulous extinction of the quartz and carbonate grains, deformation twinning of carbonates and an initial stage of subgrain development in some of the quartz crystals at the vein margin. High temperature fluid activity led to a hydrothermal alteration by chlorite, extending laterally a few millimetres into the host rock. Fluid inclusion in quartz and carbonate veins from core samples were studied to characterise the temperature and composition of these fluids, as well as the isotopic composition of vein minerals and the fluid inclusion trapped therein. The maximum homogenisation temperatures of <390°C of primary fluid inclusions and the calculated pressures of 1.5 to 2 kbar derived from fluid inclusions formed during the progressive deformation; reflect the maximum P-T conditions during the hydrothermal overprint. The eutectic temperature from various fluid inclusion groups is around -21.7°C, indicating that these fluids have a H2O-Na-(K)-Cl composition. The hydrothermal fluids are water-dominant solutions of salinities between <2 and 9 wt% NaCl-eq and contain the gases CO2, CH4 and N2. This fluid composition is typical for the Variscan fluids in middle Europe. Such high temperatures from primary fluid inclusions have not been reported for the German Variscides west of the river Rhine before. The temperature [Th] and fluid composition of these fluids is equivalent to those of the “Tectonic Brines” in the Variscan basement of the Rhenohercynian terrain east of the river Rhine (Behr et al., 1993). These fluid parameters are also in agreement with fluid inclusion studies on Variscan fault and thrust belts in eastern Belgium and northern France (Muchez et. al., 2000; Schroyen et al., 2000). The recorded high temperatures in conjunction with the mm-scale hydrothermal alteration adjacent to the host rock indicate small scale fluid/rock interaction within the system and point to a focused fluid flow. These fluid rock interactions imply a short-term, episodic fluid transport due to tectonic opening movements along the shear zones. Stable isotopes of oxygen and hydrogen were used to constrain the origin of the fluids. The calculated delta 18O and delta D isotopic composition from chlorite separates of vein assemblages as well as the alteration zone and additionally the delta 18O values of quartz, calcite and dolomite in conjunction with the delta D isotopic composition of fluid inclusions therein characterised a metamorphic origin of the fluid source. This study demonstrates the importance of fluid inclusion research for the reconstruction of the palaeofluid flow and the recognition of multiple fluid flow stages during the evolution of the Devonian strata in the Variscan fold-and-thrust belt. The abundance of extensional fractures (e.g. dilatancy structures) and the cogenetic pseudotachylite can be related to seismic activity during the time of hydrothermal veining. Seismic energy and the resulting brittle deformation are responsible for the development of permeabilities along the shear zones leading to hydrothermal fluid flow controlled by seismic pumping. The migration of fluids along faults towards the SW-NE trending Midi-Aachen thrust system represents a simple explanation for the hydrothermal veining. In contrast, investigations of fluid inclusion on post Variscan Pb-Zn deposits crosscut the Variscan structures and these fluids migrated along the post Variscan NW - SE trending faults. The Aachen Geothermic-drilling is the missing link between studies on thrust and fault zones in the Eifel thrust belt and the Faille du Midi thrust belt to the west (eastern Belgium) and thrust belts in the Rheinisches-Schiefergebirge to the east of the Variscan front.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2008
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Lögering, Markus Josef
- Contributors dc:contributor
-
- Meyer, Franz Michael
Subjects
dc:subject × 6Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng