Topography-driven hydrothermal breccia mineralization of Pliocene age at Grimsel Pass, Aar massif, Central Swiss Alps

Fault-bound hydrothermally mineralized breccias with a multistage deformation history occur in crystalline rocks of the Aar massif at Grimsel Pass, Central Swiss Alps. The breccias crop out over 4.5 kni E-W along strike and over 900 m in vertical extent between Trübtensee and Gletsch, and are up to 2 m wide. A characterization of these "Grimsel Breccias" was carried out to elucidate their formation with respect to alpine uplift and fluid circulation history, and to search for possible evidence of past microbial activity. Breccias vary widely in grain size and range from matrix-rich to clast-supported with high porosity in the youngest equivalents. Centimeter-sized voids typically contain stratified geopetal infills of fine-grained hydrothermal minerals. The hydrothermal mineral assemblage is dominated by quartz (including chalcedony), adularia, illite, celadonitic clay minerals, pyrite (As-rich), marcasite, and fine-grained Mo-sulfide. Analyses of bulk rocks (kg) and small subsamples (grams) show significant enrichments of Mo, As, Sb, Au, Cs, Hg, Tl and in some samples of U. Subthermal to thermal springs are currently discharging from the breccia zone at Gletsch (18-19°C) and into a gas pipeline tunnel intersecting the breccia (up to 28°C), indicating ongoing deep fluid circulation in a fracture system related to the Grimsel Breccia. Microbial biomass and Fe-Mn precipitates from thermal springs are enriched in Au, Cs, Sb, Hg, Pb, Mn, W, demonstrating that several of the elements enriched in the breccia are also currently transported and/or redistributed in the active water circulation system. 39Ar/40Ar dat ing of late-stage adularia yielded a middle Pliocene age (3.30±0.06 Ma), indicating formation between 0.3 and 1.2 km below sea level, if current uplift rates of the Aar massif are assumed. The estimated depth of formation is ∼3 km below the palaeosurface. Oxygen isotopes in quartz and adularia, combined with fluid inclusion data, indicate a formation temperature ranging from 160 down to approximately 100 °C at the latest stage. Fluids were of low-salinity with a dominant meteoric component with δ 18O close to -10‰ SMOW. δD values of illite-rich samples are also consistent with formation from meteoric water. Pyrite δ34S shows limited scatter with a slightly negative average of -1.8‰ CDT, consistent with an origin of the sulfide by thermochemical reduction of Triassic sulfate at 220-260 °C at greater depth. A search for signatures of possible microbial activity during breccia formation revealed the presence of extremely fine-grained pyrite and uraninite of potential microbiological origin, some ill-preserved filamentous structures and laminated fabrics potentially related to biofilms, but no indisputable evidence of biological involvement. The hydrothermal breccia mineralization in the Grimsel area demonstrates that meteoric waters penetrated deep into the Aar massif in the Pliocene and caused mineralizations geochemically similar to epithermal ores typically associated with volcanism. The enriched elements probably are derived from a combination of deep sources (Au, As, Sb,Tl) and near-surface oxidized fluids (Mo, U), and element precipitation may be a result of mixing and/or cooling. © 2004 Schweiz. Mineral. Petrogr. Ges.