Apatite age and composition: A key to the geological history of the Malmberget Iron-Oxide-Apatite (IOA) deposit and the region

Abstract

The Malmberget and Kiruna iron-oxide-apatite (IOA) deposits in northern Sweden are two of the oldest IOA deposits, but their full ore-forming tale is not yet unambiguously constrained. Apatite from Malmberget IOA deposit was analyzed for U–Pb age, Sr isotopes and trace elements for better understanding its formation and evolution. Limited zoning and relatively homogeneous trace element composition of apatite show no signs of hydrothermal alteration, supporting an ortho-magmatic origin of the Malmberget IOA deposit. Apatite Sr isotopes (0.7069–0.7078) indicate a mildly enriched (juvenile) source. The apatite U–Pb age is ~1809 ± 8 Ma, interpreted as the time of amphibolite-facies recrystallization (≥550 °C) of the ore, a process that homogenized the dominantly primary igneous apatite composition. Cooling through the closure temperature of apatite U–Pb system occurred shortly after 1.80 Ga preserving the high-T age. Combined with previous studies, the geological history of the Kiruna and Malmberget IOA deposits can be summarized as: i) both were emplaced at ~1.88 Ga, ii) they suffered regional metamorphism at ~1.80 Ga (low-grade in Kiruna, including hydrothermal overprint), later hydrothermal overprints at iii) c. 1.73 Ga and iv) c. 1.63 Ga, possibly also at v) c. 1.5 Ga and finally clay formation at vi) c. 0.9 Ga.