Petrology and lithogeochemistry of the host rocks to the Nautanen Cu-Au deposit, Gällivare area, northern Sweden

Sammanfattning: The Nautanen Cu-Au deposit is located 75 km north of the Arctic circle, and 10 km northeast of the town of Gällivare in northern Sweden. Nautanen is just 8 km west of the giant Malmberget apatite-iron ore deposit and 15 km north of one of Europe’s largest copper producers, the Aitik Cu (Au) porphyry deposit. Nautanen is described as an Iron Oxide Copper Gold (IOCG) deposit or hybrid IOCG and contains a core of high grade resources of 0.63 Mt with 2.36% Cu, 1.3 ppm Au, 11 ppm Ag, or using a lower cut-off some 3 Mt with 0.75% Cu, 0.52 ppm Au, and 2 ppm Ag. Nautanen is hosted in strongly altered and tectonically deformed Svecofennian volcanoclastic rocks on a major shear zone, the Nautanen Deformation Zone (NDZ). Alteration is dominated by scapolite, K-feldspar, epidote, sericite, and amphibole. Nautanen host rocks consist of steeply dipping, repeating layers of shoshonitic andesites, that were formed in a continental arc setting. The Nautanen rocks have lithogeochemical values characteristic of the low titanium, low zirconium Porphyrite Group of metavolcanics found in Norrbotten and are lithogeochemically similar to the volcanic host rocks to the Aitik deposit. The high amount of magnetite and locally apatite at Nautanen, suggests a possible connection to the Malmberget apatite-iron ore. Titanite and allanite ages of c. 1.78 Ga at Nautanen correspond to the timing of IOCG overprinting noted at Aitik. Similar high salinity fluid inclusions as those noted as porphyry-related in the Aitik deposit have also been observed in Nautanen, indicating that porphyry type mineralization may have been present at the Nautanen locality as well. Multiple episodes of circulation of fluids and remobilization of ore minerals along the NDZ between Nautanen and Aitik have probably occurred. Variations in tectonic processes, alteration, and magma fractionation have likely led to the small differences seen between the lithogeochemically indicated origins of the host rocks to the Nautanen and Aitik deposits.