Hydrothermal circulation at 1.8 Ga in the Kiruna area, northern Sweden, as revealed by apatite geochemical systematics

•A ∼1.8 Ga hydrothermal event in the Kiruna area, northern Sweden is constrained through apatite geochemical systematics.•The geological evolution in the Kiruna IOA deposits is summarized.•An example of complex apatite interpretation provided via cross validation of apatite ages, isotopes, trace elements and micro texture.•Repeatedly hydrothermal events helped form numerous monazites in and around apatite, remobilizing REEs in the Kiruna IOA deposits. The Kiruna area in northern Sweden hosts the classic Kiruna iron-oxide-apatite (IOA) deposit. The area has experienced complex tectonic processes, including basin formation, inversion, and hydraulic fracturing events from ∼1.9 Ga and possibly down to ∼1.5 Ga. However, the nature, timing and links of hydrothermal events related to this tectono-thermal evolution are unclear, impairing our full understanding of IOA formation processes. Here, we use multiple analytical methods to study apatite from the Kiirunavaara orebody. Electron microscopy, cathodoluminescence images, and rare earth element contents of the studied apatite indicate alteration by hydrothermal fluids. U-Pb Tera-Wasserburg lower intercept age of the altered apatite is 1800 ± 36 Ma, indicating a contemporaneous hydrothermal event in the central Kiruna area during coeval basin inversion. Sr isotopes obtained in these altered apatites, ranging from 0.709 to 0.713, may represent intermediate Sr isotope values between primary apatite and secondary fluid resetting. The relatively high but scattered 87Sr/86Sr ratios suggest that country rocks of the IOA deposits and other regional intermediate-felsic igneous rocks with old or enriched components are most likely the source of the fluids. Previous studies have revealed two hydrothermal events in the IOA ores at roughly 1.73 Ga and 1.63 Ga. Thus, fluids sourced from the host rocks of the regional IOA deposits and regional granitoids have circulated episodically at 1.80 Ga, 1.73 Ga, 1.63 Ga and possibly 1.5 Ga in the Kiruna area and have partly redistributed rare earth elements formed during the primary 1.88 Ga IOA formation event.