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

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 δ³⁷Cl value of the apatite grains ranges between −0.8 and 3.4‰. 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 δ³⁷Cl 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.