Geochemical composition of sedimentary rocks and imprint of hydrothermal fluid flow at the Variscan front – an example from the RWTH-1 well

The RWTH-1 well within Aachen offers outstanding conditions for the study of Devonian to Carboniferous sedimentary rocks in the NW Rhenohercynian close to the Brabant Massif. The geochemical data obtained from core and cutting samples provide information on the depositional history (Lower Devonian, Siegenian, to Upper Carboniferous, Namurian to Westphalian) of the NW-Rhenohercynian basin and set constraints on the mineralogical and hydrothermal control on trace element abundance in the northern Eifel region near Aachen (Germany) at the Variscan front close to the Aachen thrust. The concentrations of V, Ni, Rb, Sr, Cu and Ba are mainly determined by the amount of illite and in part by feldspar. However, for Sr there are significant contributions from carbonates and Ni is controlled by a detrital component in the Lower Devonian rocks. The elements Cr and Zr are controlled by chromite and zircon. Chromite, however, is not significant in the Upper Devonian Condroz beds. Chlorine has been observed as most susceptible to weathering in surface samples and loss from cutting samples probably due to destruction of fluid inclusions. Lower Devonian and Upper Carboniferous units have similar chemical characteristics indicating similar sources of detritus and in part re-sedimentation of Lower Devonian detritus in Upper Carboniferous times. The high Chemical Index of Alteration points to intensive weathering in the source area and/or recycling during transport, which was also described for the eastern part of the Rhenohercynian basin (Schulz-Dobrick & Wedepohl 1983). The idea of an ophiolitic source of detritus situated in the NE of the Rhenohercynian basin expressed by Schulz-Dobrick & Wedepohl (1983) and others can be supported in this study by the abundance of Cr and Ni, mainly in the Lower Devonian. A marked change for the sedimentary input is recorded in the Upper Devonian (Famennian shales and Condroz beds) in the mineralogical and chemical data. Here, chemically mainly unweathered rocks with felsic composition dominate. In line with literature data on the occurrence of the Condroz beds the Brabant Massif to the NW which was affected by rapid uplift must have hosted this source. Chemical evidence for fluid flow is detected around fault zones indicated in the RWTH-1 well. Input of Cu, Ba and NH4 is attributed to this Variscan hydrothermal event for which no clear chemical evidence exists in the Upper Carboniferous rocks of the RWTH-1 well and in surface outcrops. Rather, fluid flow is focussed along larger thrust faults in the footwall of the Aachen thrust.
Schulz-Dobrick, B. & Wedepohl, K.H. (1983): The chemical composition of sedimentary deposits in the Rhenohercynian Belt of central Europe. –In: Martin, H. & Eder, F.W. (Eds.): Intracontinental Fold Belts: 211-229, Berlin, Heidelberg (Springer).

Sindern, S., Warnsloh, J.M., Trautwein-Bruns, U., Chatziliadou, M., Becker, S., Hilgers, C. , Kramm, U. and Schneider, J. (2008) Geochemical composition of sedimentary rocks and imprint
of hydrothermal fluid flow at the Variscan front – an example from the RWTH-1 well. SDGG, 60, 160.