Fluid evolution along multistage composite fault systems at the southern margin of the Lower Palaeozoic Anglo‐Brabant fold belt, Belgium

Abstract

Mineralised vein systems have been investigated at nine localities at the southern margin of the Anglo-Brabant fold belt in Belgium. During the late Silurian to early Middle Devonian Caledonian orogeny, shear zones formed, inferred to be associated with granitoid basement blocks in the subsurface. The circulation of a metamorphic fluid, possibly originating in the Cambrian core of the fold belt, along these shear zones resulted in the formation of mesozonal orogenic mineralisation at the southern margin of the Anglo-Brabant fold belt. The fluid had a composition dominated by H 2 O-CO 2 -X-NaCl-KCl. The shear zones form part of a greater fault zone, the Nieuwpoort-Asquempont fault zone, which is characterised by normal faulting that started before the Givetian and by the reactivation of the shear zones. Two fluid generations are associated with this normal faulting. First, a low salinity H 2 O-NaCl(-KCl) fluid migrated through the Palaeozoic rocks after the Silurian. Based on the isotopic composition, this fluid could be a late-metamorphic Caledonian fluid or a younger fluid that originated from the Rhenohercynian basin and interacted with Lower Devonian rocks along its migration path. Second, a high salinity H 2 O-NaCl-CaCl 2 fluid was identified in the fault systems. Similar fluids have been found in southern and eastern Belgium, where they produced Mississippi Valley-type Zn-Pb deposits. These fluids are interpreted as evaporative brines that infiltrated the Lower Palaeozoic basement, from where they were expelled during extensional tectonism in the Mesozoic.

Fluid evolution at the Variscan front in the vicinity of the Aachen thrust

“…Table 1; Behr et al 1987Behr et al , 1993Dewaele et al 2004;Kenis et al 2000;2005;Schroyen and Muchez 2000;Vogtmann-Becker 1990), we consider the primary fluid inclusions studied in the RWTH-1 well similar to Variscan hydrothermal fluids encountered along the entire Variscan front zone. The Stavelot-Venn massif records metamorphic fluids with similar temperatures and salinities (Schroyen and Muchez 2000;Vogtmann-Becker 1990) which makes this area a potential source for the fluids discussed here.…”

“…and homogenisation temperatures B350°C. Fluid inclusion studies carried out in the AngloBrabant fold belt, Ardennes and the Stavelot-Venn massif, and in the Variscan front zone in eastern Belgium recognised a syn-Variscan H 2 O-CO 2 -NaCl-dominated metamorphic fluid of low salinity and homogenisation temperatures B380°C (Table 1, Behr et al 1987;Dewaele et al 2004;Kenis et al 2000Kenis et al , 2005Schroyen and Muchez 2000;Vogtmann-Becker 1990). The post-Variscan hydrothermal ore deposits of the Rhenohercynian domain are derived from 'basement brines' (Behr et al 1993).…”

“…Zones of intensive hydrothermal alteration resulting from fluid-rock reaction are ubiquitous around fault intersections . To date, Variscan metamorphic fluids are fairly well studied in the Venn-Weser-Inde nappe, the Ardennes, the Variscan front zone in Belgium and northern France, and in the Rhenish massif, east of the Rhine river (Behr et al 1987(Behr et al , 1993Dewaele et al 2004;Kenis et al 2000Kenis et al , 2005Muchez et al , 2000Muchez et al , 2002Vogtmann-Becker 1990). However, a systematic fluid study close to the Aachen thrust is still lacking.…”

A new geological map of the outcrop areas of the Brabant Massif (Belgium)

“…These faults are often characterised by a damage zone of brecciated rocks with deformed quartz and/or carbonate veins (Debacker, 2001;Debacker et al, 2003Debacker et al, , 2004a. As shown by Dewaele et al (2004), fault fluids are typically H 2 O-NaCl-KCl fluids. Hence, the combined use of mapping, wall rock deformation, fault throw, fault orientation and fluid chemistry allows a particular fault to be related to the N-AFZ.…”

“…Although the shear zone predates, and is unrelated to the N-AFZ, the stratigraphic distribution within this poorly exposed outcrop area indicates important fault displacements that cannot be attributed to the reverse shear zone. In addition, fluid inclusion studies on veins from boreholes show fractured veins infilled by a H 2 O-NaCl-KCl fluid, typical of the N-AFZ (Dewaele et al, 2004).…”