Geology of the Nautanen North Cu-Au-Ag-(Mo) Deposit, Norrbotten, Sweden

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Abstract The Nautanen North deposit in the Gällivare-Malmberget area of Norrbotten, Sweden, currently contains a resource of 21 million tonnes (Mt) at 1.46% Cu, 0.78 g/t Au, 6 g/t Ag, and 99 g/t Mo and remains open at depth and along strike. This study, based on extensive examination of drill core, geochemical data, and petrographic analyses, represents the first comprehensive description of the structural controls, hydrothermal alteration facies and paragenetic sequence of mineral precipitation, and styles and relative timing of iron oxide and sulfide mineralization at the deposit. The deposit is localized between bounding shear zones within the Nautanen deformation zone. High grades of Cu occur within discrete zones of brecciation and veining and as mineralized shear bands. Breccias in the northern portion of the deposit developed within a vertically stacked, relay-like zone in response to late deformation. Hydrothermal alteration of the host rocks was initially dominated by Na facies alteration, which was subsequently overprinted by Na-Ca-Fe, HT (high-temperature) Ca-Fe, HT Ca-K-Fe, HT K-Fe, and LT (low-temperature) K-Fe facies alteration. Magnetite mineralization occurred in at least two phases: an early phase during Na and Na-Ca-Fe facies alteration accompanied by apatite that is interpreted to reflect a distal signature of formation of the proximal Malmberget magnetite-apatite deposit and a later phase coincident with HT Ca-Fe to K-Fe alteration, which overlapped with the onset of Cu and Fe sulfide mineralization under HT Ca-K-Fe and K-Fe and LT K-Fe conditions. The Nautanen North deposit is shown to meet key criteria to be classified as an iron oxide copper-gold deposit.