Effects of frictional–viscous oscillations and fluid flow events on the structural evolution and Re–Os pyrite–chalcopyrite systematics of Cu-rich carbonate veins in northern Norway

•Multidisciplinary study of Paleoproterozoic Cu-rich carbonate veins, northern Norway.•Vein emplacement and reactivation under cyclic frictional–viscous oscillations.•Timing of veining and deformation constrained by Re–Os and K–Ar geochronology.•Strain and ingress of fluids can affect the Re–Os pyrite–chalcopyrite system.•Evidence of fluid flow during grain-size-sensitive viscous deformation.

Mesothermal chalcopyrite + pyrite + magnetite-bearing calcite-dominated vein deposits in the Repparfjord Tectonic Window, northern Norway, have been studied to constrain the mechanics of their emplacement and the timing of initial vein formation and reactivation. The veins cut across Paleoproterozoic tholeiitic metabasalts and present textural contrast between their hydrofractured, coarse-grained margins, and the fine-grained mylonitic cores. They formed under overall viscous conditions, although cyclically increased fluid pressures caused transient embrittlement. As mineral precipitation sealed the fractures, deformation was accommodated again viscously leading to mylonitization of the vein's core. Local brecciation of the calc-mylonite demonstrates the cyclicity of this process. Re–Os chalcopyrite–pyrite and K–Ar fault gouge dates define an almost continuous age range from ~ 2540 Ma to ~ 460 Ma. Regression of three Re–Os analyses yields a 2069 ± 14 Ma age (187Os/188Os = 0.18 ± 0.04), interpreted as the age of vein emplacement, sulfide precipitation, and initial frictional–viscous deformation. K–Ar ages are mixed ages that constrain a maximum age of faulting in association with the veins at approximately 460 Ma, hence indicating structural reactivation connected with Silurian Caledonian orogenic compression. The spread in Re–Os model ages reflects this reactivation, wherein renewed strain accommodation and circulating oxidizing fluids caused fracturing, dynamic recrystallization, and isotopic disturbance of the sulfides. The study provides evidence for fluid flow during viscous deformation and demonstrates that strain, and flow of oxidizing fluids, can have a significant yet localized control on the integrity of the Re–Os systematic in pyrite and chalcopyrite.