Global Fe–O isotope correlation reveals magmatic origin of Kiruna-type apatite-iron-oxide ores

Kiruna-type apatite-iron-oxide ores are key iron sources for modern industry, yet their origin remains controversial. Diverse ore-forming processes have been discussed, comprising low-temperature hydrothermal processes versus a high-temperature origin from magma or magmatic fluids. We present an extensive set of new and combined iron and oxygen isotope data from magnetite of Kiruna-type ores from Sweden, Chile and Iran, and compare them with new global reference data from layered intrusions, active volcanic provinces, and established low-temperature and hydrothermal iron ores. We show that approximately 80% of the magnetite from the investigated Kiruna-type ores exhibit δ 56 Fe and δ 18 O ratios that overlap with the volcanic and plutonic reference materials (> 800 °C), whereas ~20%, mainly vein-hosted and disseminated magnetite, match the low-temperature reference samples (≤400 °C). Thus, Kiruna-type ores are dominantly magmatic in origin, but may contain late-stage hydrothermal magnetite populations that can locally overprint primary high-temperature magmatic signatures. The origin of giant Kiruna-type iron ores has been debated for nearly 100 years. This study employs extensive stable isotope data from Kiruna-type ores worldwide and magmatic and hydrothermal reference materials to show that iconic Kiruna-type ores originate primarily from ortho-magmatic processes.