Structural constraints on the formation of Cu-rich mesothermal vein deposits in the Repparfjord Tectonic Window, northern Norway

The Paleoproterozoic metasupracrustal sequence of the Repparfjord Tectonic Window (RTW) within the Caledonian Nappe System in northern Norway hosts several Cu-bearing mesothermal carbonate-quartz veins. Structural analysis shows that the emplacement of the veins is geometrically and kinematically compatible with a phase of NW-SE shortening. Northwest- and southeast-dipping veins are related to brittle-ductile thrust faults, whereas sub-vertical E-W to NE-SW striking veins show a strong component of dextral strike-slip ductile shearing. It is proposed that the veins were emplaced under brittle-ductile conditions in a dextral transpressional shear corridor. Fluid overpressuring possibly led to fracturing of the host greenstones. The subsequent pressure drop led to metal precipitation. The newly-formed veins, which remained weaker than the surrounding metabasalts, localized subsequent fluid circulation and ductile strain increments. Coaxial NW-SE compression is reported for both the Paleoproterozoic Svecofennian and the Silurian Caledonian orogenies. Re-Os analysis on sulfides is in progress and will provide constraints on the age of both vein formation and associated tectonic stress regime.