Tracking the origin of metasomatic and ore-forming fluids in IOCG deposits through apatite geochemistry (Nautanen North deposit, Norrbotten, Sweden)

The relative contribution of magmatic and non-magmatic fluids to the metasomatic and ore-forming processes in iron-oxide copper-gold (IOCG) deposits is still widely debated. In this study, the petrography, detailed composition and U-Pb ages of various apatite occurrences from the Nautanen North IOCG deposit, Norrbotten, Sweden, were determined to decipher the evolution of fluid sources in the area.The hydrothermal apatite grains grow over muscovite, intergrow with magnetite, amphibole, K-feldspar, chalcopyrite and sericite, and are replaced by epidote, allanite and/or chlorite along the grain margins. Irregular patterns of the apatite grains were revealed by cathodoluminescence imaging. Fluorine in all apatite occurrences is the dominant halogen (1.62-3.58 wt%), chlorine is depleted (up to 0.34 wt%), while bromine and iodine are found in traces (0.7-72 ppm and 0.15-4.2 ppm respectively). The delta 37Cl value of the apatite grains ranges between-0.8 and 3.4 parts per thousand. Uranium-Pb data yield ages between 1.63 and 1.55 Ga (with one exception at 1.49 Ga showing large age errors).Textural evidences show that the apatite grains have been precipitated during the potassic alteration of the D2 event (1.81-1.78 Ga), which is considered a regional, IOCG-related, high-temperature event. Cathodoluminescence textures reveal that all the apatite occurrences have been chemically modified by the late-stage metasomatic and ore-forming fluids during the nucleation of epidote +/- allanite +/- chlorite. The Br/I and delta 37Cl values of the apatite grains can be considered representative of the associated fluid values and show a trend between two end-members, which indicates the contribution and progressive mixing of two different fluids during late-stage ore-related hydrothermal circulation. The ore-forming fluids were mainly issued from exsolved magmatic fluids from S-type bodies, as revealed by the strong affinity of the Br/I ratio of the ore zone with the pegmatite-related apatite. The other end-member associated with the apatite occurrences that co-exist with metasomatic assemblages, is consistent with fluids linked to evaporite dissolution. Apatite dating is interpreted to reflect resetting ages.