Characterization and geological setting of the Nautanen North Cu-Au deposit, Norrbotten, Sweden

Nautanen North is a copper-gold deposit within the northern Norrbotten Ore Province, Sweden. The research presented in this thesis is the first comprehensive description of the structural controls, hydrothermal alteration, paragenesis, styles and relative and absolute timing of both iron oxides and sulfide mineralization at the deposit. The deposit is localized between bounding shear zones within the Nautanen Deformation Zone (NDZ). High grades of copper and gold occur within discrete zones of containing breccias, veins, and shear bands. Alteration facies in the deposit include sodic and high temperature calcic-iron (HT Ca-Fe), high temperature potassic-iron (HT K-Fe), and low temperature potassic-iron facies. Chalcopyrite and magnetite dominate the mineralization assemblage with lesser pyrite, pyrrhotite, and molybdenite. The deposit meets key criteria to be classified as an iron oxide-copper-gold (IOCG) deposit. Re-Os dating of sulfides indicate multi-episodic formation of the deposit. The formation of HT Ca-Fe alteration with sulfide veins occurred at 1819 ± 5 Ma (molybdenite). This age correlates with local magmatism and regional metasomatic alteration and metamorphism within the NDZ and establishes an older age of commencement for the second phase of IOCG mineralization in the province. Stockwork-style sulfide mineralization hosted within HT K-Fe altered rocks is dated to 1785 ± 6 Ma (molybdenite) while breccia formation containing high copper grades is dated to 1747 -1766 Ma (molybdenite/chalcopyrite-pyrite). These dates correlate to regional metamorphism and large-scale granitic emplacement. Vein-hosted sulfide mineralization adjacent to the central ore zone dates to 1720 ± 6 Ma (chalcopyrite-pyrite) which is correlative with a late phase of regional magmatism and minor sulfide mineralization regionally within the NDZ. Styles, microtextures, and mineral chemistry of magnetite from Nautanen North and other deposits within the iron oxide and alkali-calcic alteration systems (IOAA) framework of the local area (Malmberget, Aitik, Nautanen, Liikavaara Berget) display similar microtextures and styles of precipitation. Inclusion-free magnetite is the most common microtexture present; it forms entire crystals and rims to earlier inclusion-bearing magnetite crystals. Enrichment of Ca, K, and Cu within inclusion-free rims highlights the role of couple dissolution processes in the precipitation of magnetite during the regional IOCG event responsible for the formation of Nautanen North. Temperatures of magnetite precipitation of between 412 - 693 °C were calculated for magnetites from Nautanen North and adjacent deposits utilizing the “Mg content in magnetite empirical geothermometer”. The highest temperatures were found at the Malmberget iron oxide-apatite (IOA) deposit (average = 561 °C). Magnetites in the area’s IOCG deposits have an average precipitation temperature of 432 °C. All magnetites in this study either have mineral inclusions or magnetite trace element chemical compositions that indicate precipitation from a hydrothermal fluid at temperatures consistent with those proposed for (IOAA) facies. Comparisons of these new data with published studies on other IOCG and IOA deposits suggest that magnetite in Norrbotten precipitated at lower temperatures than those within other IOAA districts globally.