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SHORT-TERM HYDROTHERMAL EFFECTS ON THE ‘CRYSTALLINITIES’ OF ILLITE AND CHLORITE IN THE FOOTWALL OF THE AACHEN-FAILLE DU MIDI THRUST FAULT – FIRST RESULTS OF THE RWTH-1 DRILLING PROJECT

¹ Institute of Mineralogy and Economic Geology, RWTH Aachen University, Wüllnerstrasse 2, 52056 Aachen, Germany
² Clay and Interface Mineralogy, RWTH Aachen University, Wüllnerstrasse 2, 52056 Aachen, Germany
³ Lehr und Forschungsgebiet Geologie und Endogene Dynamik, RWTH Aachen University, 52056 Aachen, Germany
⁴ Ministère de l’Industrie, des Mines et du Développment Technologie – CAPM – BP. 15620, Yaoundé, Cameroun

^* E-mail address of corresponding author: sindern{at}rwth-aachen.de

Investigation of material from three core sections of the RWTH-1 drill-hole in the Wurm syncline of Aachen, Germany, shows mineralogical and structural evidence of intensive hydrothermal activity in the footwall of the Aachen thrust. Mineral and microstructural data indicate minimum temperatures of 200–250°C. CIS_{illite 001} values of 0.45–0.61 ( °2) and insignificant amounts of smectite indicate a late diagenetic grade for illite pointing to temperatures <200°C. Chlorite, mainly formed in veins and cleavage planes, has CIS_{chlorite 002} values between 0.35 and 0.26 ( °2) which only in part point to anchizonal grade. In contrast to these illite and chlorite data, maximum temperatures up to 370°C can be expected based on comparison with recently published fluid inclusion and mineral thermometric data. Illite is neither significantly affected by the hydrothermal event nor by deformation, and mirrors the burial history of the Wurm syncline.

Chlorite grew syntectonically as is shown by bent and predominantly stretched sheets which do not, however, have deformed structures. Syntectonic hydrothermal growth by incipient nucleation along crystal edges limited domain size and thus also the CIS_{chlorite 002} values. The hydrothermal event did not last long enough to allow further crystal growth. The retarded CIS_illite and CIS_chlorite grades can be best explained by limited duration (probably <5000 y) of the hydrothermal event which for a short time reached epithermal temperatures. The hydrothermal fluid flow was caused by dewatering of sedimentary rocks during thrusting and tectonic thickening within the Variscan orogen and it was focused along the Aachen thrust which represents the frontal Variscan thrust.

Key Words: Aachen • Chlorite • CIS • Crystallinity • Fluid Flow • Hydrothermal • Illite • Integral Breadth • Variscan Orogeny • Wurm Syncline