Abstract

Sediment-hosted copper-silver mineralization in the Raipas Supergroup of the Repparfjord tectonic window of Finnmark, northernmost Norway, is present in two main stratigraphic horizons at Nussir and Ulveryggen. The stratigraphically lower Ulveryggen deposit is hosted by irregular lenses of dominantly fluviatile sandstone and conglomerate, whereas the Nussir deposit, with a strike length of ~9 km, is associated with dolomitic siltstone and dolomite. Both deposits display intense structural control, and apart from the classic stratiform geometry of the disseminated cupriferous sulfides, high-grade mineralization includes abundant foliation parallel and transgressive veinlets, fissures, and irregular lenses and aggregates, with or without quartz, carbonate, chlorite, and biotite gangue. A close association with ductile deformation is apparent. Although scarce, molybdenite occurs intimately associated with the cupriferous sulfides at Nussir, in a paragenetic sequence comprising chalcopyrite-bornite-molybdenite-chalcocite-neodigenite in both the stratiform disseminated and veinlet mineralization.

U-Pb zircon dating of a tuffaceous horizon from the dominantly volcanic Nussir Group, collected 60 m above the ore horizon at Nussir, yielded an age of 2073.1 +23.2/−12.4 Ma that confirms accumulation in an intercontinental rift or island-arc environment during construction of the Paleoproterozoic Columbia supercontinent. Re-Os geochronology on Nussir molybdenite returned ages of 1761 ± 8 and 1768 ± 7 Ma, which represent the timing of the copper-silver mineralization in the area. This age, the oldest direct, unambiguous determination reported for a sediment-hosted copper deposit, implies that the copper mineralization took place ~300 m.y. after the sedimentation and volcanism responsible for the host sequence, and syntectonically with the Svecofennian (Svecokarelian) orogeny in the Fennoscandian Shield.

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