Metal source and tectonic setting of iron oxide-copper-gold (IOCG) deposits: Evidence from an in situ Nd isotope study of titanite from Norrbotten, Sweden

Abstract

Titanite occurs as a widespread accessory phase in mineralised zones and alteration associated with iron oxide-copper gold (IOCG) and iron oxide-apatite (IOA) deposits of Norrbotten County, Sweden, and is a major host of the REE in these deposits. In situ analyses of SmNd isotope ratios in titanites previously analysed for UPb geochronology and trace element composition confirms previous interpretations of grain scale isotopic heterogeneity. Initial Nd-isotope ratios expressed relative to CHUR range from ~−3 to −8 in IOA deposits, from ~−1 to −9 in IOCG deposits, and from +2 to −4 in the most Cu-rich, deformed IOCG deposits of the Nautanen Deformation zone. Within individual IOA deposits εNd varies relative to CHUR: from −3.1 to −4.0 at Valkommen (Malmberget), from −1.4 to −5.7 in grain cores, and −7.2 to −8.2 in grain rims rim at Gruvberget; and from −3.0 to −6.0 in grain cores and from −5.8 to −7.1 in grain rims at Luossavaara. In IOCG deposits at Rakkurijärvi εNd varies from −6.1. to −7.1, and in deformed IOCG deposits at Nautanen from −1.3 to −2.3. These values are consistent with the derivation of the REE, and potentially economically enriched metals, from the local volcanic sequence, either via granitic melts, or directly by leaching by metasomatic fluids. The most Cu-rich deposits reflect the involvement of more basic protoliths. The age distribution of these deposits suggest IOA deposit formation during the collisional phase of the Svecofennian orogeny (~1.9–1.8Ga), and IOCG mineralisation during this phase and during post-orogenic collapse (~1.8–1.7Ga), whilst model ages indicate the ultimate enrichment of the continental crust in these metals during pre-collisional extensional and subduction-related basic magmatism. These processes underscore the importance of continental cycles both in producing the preserved geological record of orogenic ore deposition, and in the generation of fertile continental crust, from which metals can be mobilised by subsequent events.