Formation of armenite in the Berisal Complex, Simplon Region, Switzerland

Armenite (BaCa₂Al₆Si₉O₃₀ ·2H₂O) occurs in an outcrop of leucocratic celsian + margarite + quartz + white mica + zoisite gneiss in the Berisal Complex, Swiss Central Alps. The locality is cut by undeformed, late-stage, Alpine quartz veins. Armenite crystals are found at the interface between quartz veins and leucocratic host rock, and in a narrow zone of re-crystallised wall-rock gneiss running parallel to the veins. The veins are 15-30 cm thick, cut the dominant foliation of the host rock at 90°, and have a sub-parallel orientation to the nearby Simplon Fault Zone and other local features of extension and uplift. The zone of wall-rock alteration extends 30-50 cm from the vein along foliation. In this zone, the whole-rock Ba content decreases as does the modal abundance of armenite along foliation away from the vein. The decrease is mirrored by an increase in the modal quantities of zoisite, celsian, quartz and margarite. A detailed petrographic and microthermometric study of material from the vein-gneiss interface has been made, and a fluid P-T path has been constructed. Petrographically, eight events are recognisable in the outcrop, all of which postdate the ductile-to-brittle transition, which the rock passed through during exhumation. Associated with these events, eight fluid-inclusion assemblages have been identified that constrain the retrogression of the rocks and allow identification of the processes that were important in the crystallisation of armenite. The chemical components required for armenite formation occur throughout the gneiss outcrop, but armenite has a limited spatial distribution close to the veins. This suggests that the conditions favourable for armenite formation were restricted to the immediate vicinity of the veins. Three groups were identified amongst the eight fluid-inclusion assemblages: the two oldest ones, found in vein-fill quartz, are aqueous with c. 24 mol% CO₂ and no salt. The second group, associated with armenite formation, are trapped in younger vein-fill quartz, in fracture planes in quartz and armenite, and in two generations of armenite. These inclusions are CO₂-depleted, water-rich, and contain 3-9 wt% NaCl. The third group is water-rich, and is found in very late vein-fill quartz. We conclude that regional fracturing events brought about a considerable decrease in P_fluid which led to an influx of water-rich, salt-bearing fluids. The changes in fluid chemistry led to formation of armenite by in-situ recrystallisation of pre-existing Ba-, Ca- and Al-rich minerals.