Magmatic origin of giant 'Kiruna-type' apatite-iron-oxide ores in Central Sweden

Iron is the most important metal for modern industry and Sweden is by far the largest iron-producer in Europe, yet the genesis of Sweden's main iron-source, the 'Kiruna-type' apatite-iron-oxide ores, remains enigmatic. We show that magnetites from the largest central Swedish 'Kiruna-type' deposit at Grängesberg have d18O values between 20.4 and 13.7%, while the 1.9021.88 Ga meta-volcanic host rocks have d18O values between 14.9 and 19%. Over 90% ofthe magnetite data are consistent with direct precipitation from intermediate to felsic magmas or magmatic fluids at high-temperature (δ18Omgt > 10.9‰, i.e. ortho-magmatic). A smaller group of magnetites (δ18Omgt ≤ 10.9%), in turn, equilibrated with high-δ18O, likely meteoric, hydrothermal fluids at low temperatures. The central Swedish 'Kiruna-type' ores thus formed dominantly through magmatic iron-oxide precipitation within a larger volcanic superstructure, while local hydrothermal activity resulted from low-temperature fluid circulation in the shallower parts of this system.