FMF4 : TUpo06 : PO

Fluid Inclusion Investigations on Fissure Minerals from NEAT Transects (Gotthard and Lötschberg Base Tunnels) Through the Central Alps: Preliminary Results

Josef Mullis (josef.mullis@unibas.ch)¹,
Michael Overstolz (michael.overstolz@stud.unibas.ch)¹,
Meinert K. Rahn (rahn@ruf.uni-freiburg.de)²,
Adolf Peretti (a.peretti@mail.tic.ch)³ &
Peter Amacher³
1 Mineralogisch-Petrographisches Institut, Universität Basel, Bernoullistrasse 30, CH-4056 Basel, Switzerland
2 IMPG, Albert-Ludwigs-Universität, Freiburg, Germany
3 GEO-GRL Micro-Analytic GmbH, CH-6460 Altdorf, Switzerland

Alpine fissures in NEAT boreholes, shafts and tunnels are generally small hydrothermal systems which formed during prograde and/or retrograde Alpine regional metamorphism. Investigation of fluids, fissure minerals and hydrothermally altered host rocks of the extensional veins is ongoing in order to accomplish the following aims: 1. to understand the evolution of deformation, fluid composition, temperature and pressure during uplift and cooling of the Central Alps 2. to recognize the origin and fluid flow paths of mineralizing fluids 3. to document the mass transport from the unaltered host rock into the Alpine fissures under continuously changing conditions 4. modelling of fluidmineral equilibria.

The Alpine fissures sampled are from surface and subsurface locations along the line of the NEAT Gotthard and Lötschberg base tunnels. Microthermometric investigations on fissure quartz yield the following preliminary results: - Higher hydrocarbon-rich and methane-rich fluid inclusions are detected in the exploratory tunnel and boreholes between Fruttigen and Kandersteg (Lötschberg base tunnel). These hydrocarbon dominated fluids were formed and trapped between 150 and 270°C and c. 1 and 2.7 kbars. Fibre, prismatic and skeletal quartz as well as calcite precipitated from these fluids. In the autochthonous cover of the Gastern Massif hydrocarbon rich fluids were replaced by carbon dioxide bearing water-rich fluids filling Alpine fissures nearly completely with calcit. - Water dominated fluids are common in rare earth element mineral bearing Alpine fissures from the service tunnel at Amsteg (Gotthard base tunnel). They were trapped at retrograde PTconditions between 350 and 250°C and 3.4 and 2.2 kbars. Late tectonic events allowed carbon dioxide enriched fluids to enter into the water dominated hydrothermal system precipitating siderite in allready formed Alpine fissures. Water-rich fluids are also dominant in the service tunnel and vertical shaft at Sedrun. South of Sedrun, water-rich fluids are dominant and sometimes contaminated with late tectonically controlled carbon dioxid advection. - Carbon dioxide dominated fluids are found in Tessin habit quartz from the exploratory tunnel at Piora-Mulde. These fluids were trapped in crystals which formed at 430 ±20°C and 2 to 3 kbars (MULLIS et al., 1994).

Mullis J, Dubessy J, Poty B & O’Neil J, Geochim. Cosmochim. Acta, 58, 2239-2267, (1994).