ZEOLITES ON FISSURES OF CRYSTALLINE BASEMENT ROCKS IN THE SWISS ALPS

A number of zeolite species have been described from Alpine fissures in crystalline basement rocks of the Swiss Alps. The zeolites typically overgrow earlier minerals of the fissure assemblages, but zeolites also occur as single stage fissure deposits in granite and gneiss. A systematic study of zeolite samples from the Alps showed that the majority of finds originate from three main areas. Two of them are located in gneiss and granite of the southern Aar-Massif and one in granite-gneiss of the Pennine nappes of the Ticino area. In addition to these known localities from the surface, tunnel construction in the Alps provided excellent zeolite material from fissures in basement rocks. At present, a new 53km long Gotthard railroad base tunnel is excavated and offers the unique opportunity to study the in-situ formation of low-temperature minerals. Reported zeolite species from Alpine fissures in granite and gneiss include: chabazite, heulandite, laumontite, scolezite, stellerite, stilbite. The zeolites often occur with other species in the fissures including: quartz, apophyllite and chlorite. Earlier minerals in zeolite-bearing fissures may include: prehnite, adularia, fluorite, hematite and others.

Particularly spectacular is the extensive occurrence of laumontite vein fillings in the new Gotthard rail tunnel (NEAT). Laumontite covers fissure walls as dense mats. Up to 1cm long white needles of laumontite are the last minerals that precipitated from hot water in the fissures and cavities. It overgrows “primary” fissure quartz and chlorite coatings of the fissure walls. Locally, apophyllite overgrows laumontite as euhedral crystals thus it is the last mineral formed in the fissures. Laumontite is the absolutely dominant zeolite of the tunnel fissures, but the mineral is rarely preserved in surface outcrops where chabazite and stilbite dominate. Laumontite in the tunnel forms by precipitation from alkaline hot water that acquired its chemical composition by reaction with primary plagioclase at temperatures of about 160°C. The present day rock temperature in the tunnel is < 44°C. Alpine cooling and exhumation rates for the Aar Massif suggest that fissure laumontite formed about 2-5 Ma ago.