Near the village of Mousny, Belgium, a peculiar massive quartz occurrence, composed of multiple large, >m³-size, bodies of milky quartz can be found at the locality known as “Les Blancs Cailloux”. Strikingly, the quartz bodies contain elongated, cleaved, host-rock fragments, still oriented parallel to the regional cleavage attitude.
A detailed petrographical, microstructural and mineralogical study of the vein quartz and a geochemical analysis of fluid inclusions has revealed that the Mousny massive quartz occurrence is genetically linked to the regionally common cleavage-parallel quartz veins. They both show a fluid evolution typical of the metamorphic fluids in the central, epizonal part of the High-Ardenne slate belt.
While the cleavage-parallel veins can be considered to result from mode I extensional fracturing, the genetically linked massive quartz occurrence is seen as being formed in a dilational jog. Within the late-orogenic context of the High-Ardenne slate belt, we favour a model in which the dilational jog is comprised within a weakly south-dipping, extensional shear zone, related to the late-orogenic extensional destabilization of the slate belt, causing a transient enhancement of the structural permeability in this low-permeability mid-crustal environment. The Mousny massive quartz occurrence may in this respect be exemplary for massive quartz occurrences throughout the High-Ardenne slate belt.