LA-ICPMS U-Pb dating of titanite: New constraints on multistage geological evolution of the Norrbotten mining district, Sweden

Established techniques for the laser ablation ICP-MS U-Pb analysis of zircon have been developed for applications to common Pb bearing phases, including titanite and allanite. A novel common Pb correction procedure has been developed for deriving mineral ages, and has been tested using well characterised samples. This technique has been applied to samples from the mining district of Norbotten, Sweden, with the aim of constraining the timing of iron oxide-copper-gold (IOCG) mineralization in relation to the formation of Fe-oxide-apatite deposits and the regional geological evolution. Titanites from three of the Fe oxide apatite deposits studied show complex multistage growth textures implying a prolonged metasomatic history. Cores from these samples show ages in the range 2070 to 2000 Ma before present, indicating that the age of the host supercrust sequence in Norbotten may be significantly older than previously thought. The rims of these crystals which show distinct trace element signatures to the cores, as well as unequivocally metasomatic titanites and allanites from other areas give ages in the range ~1875 - 1820 Ma. These ages probably indicate the time of primary mineralisation of both the late stages of Fe oxide and Cu-Au types. Younger ages in the range ~1790 - 1700 Ma are interpreted to relate to subsequent metamorphism of the deposits, and remobilisation of sulphide mineralisation along long lived, regional scale structures.