Signatures of sulfide remobilization in the Kiruna mining district: a SIMS sulfur isotope and LA-ICP-MS trace element study

The Kiruna mining district in northern Norrbotten, Sweden, is situated in a region that is broadly described as an iron oxide-copper-gold (IOCG) district. Kiruna is well known for the iron oxide-apatite (IOA) deposits (Kiruna-type), but also hosts several Cu-bearing prospects including the Viscaria and Eastern Pahtohavare stratabound-stratiform Cu-(Fe-Zn), the Rakkurijärvi IOCG, and the Pahtohavare epigenetic Cu ± Au prospects. However the genesis of the Cu mineralization in the district has not been studied from a structural perspective in relation to the tectonic evolution of the region. Utilizing recent structural characterization of the Svecokarelian orogenic cycle as a framework, Cu- and Fe-bearing sulfides have been identified from the pre-, early, and late orogenic phases and assessed for primary and overprinting signatures with in situ δ34S and trace element data. Results indicate that the youngest stage of epigenetic mineralization (late orogenic) in the Pahtohavare area precipitated vein-hosted pyrite from a strongly Co-enriched hydrothermal fluid with δ34S values between 2‰ and 6 ‰. The late mineralization event occurred syn- to post-folding of the host rocks together with remobilization of sulfides from the younger Eastern Pahtohavare deposit. This remobilization is characterized by a fractionation of sulfur isotopes towards heavier values and dispersion of Co and Ni data towards higher and lower concentrations, respectively, and indicates redistribution of metals occurred as a result of the late orogenic mineralization and deformation. This implies a distinct Cu-forming event occurred in the late Svecokarelian orogeny both that introduced new metals and redistributed (and possibly incorporated) metals from a preexisting source. Metamorphic and low temperature remobilization is also recorded in samples from the district characterized by increases in Co and Ni, respectively.