ISOTOPE SYSTEMATICS OF THE KIRUNA MAGNETITE ORES, SWEDEN .1. AGE OF THE ORE

U-Pb isotope dating of zircons from a granophyric dike complex which crosscuts the Kiirunavaara magnetite-apatite body demonstrates that the ore was emplaced before 1.880 +/- 0.003 Ga. Whole-rock Sm-Nd isotope compositions for five out of six samples from the footwall and hanging-wall complexes lie on a 1.89 +/- 0.09-Ga isochron. The initial ratio of 0.50991 +/- 0.00007 corresponds to epsilon-Nd(T) = -5.6 and suggests a substantial late Archean component in the igneous rocks. Since the ore cannot have been emplaced after the granophyre nor before the host rocks, ore formation is limited to the period between 1.90 and 1.88 Ga. However, the granophyre remains at present in the form of a crosscutting dike, largely unaffected by the ductile and brittle deformation that has affected both the ore and host rocks. This implies that ore formation, igneous activity, and deformation were all confined to a short time interval during this period and suggests that the Kiruna magnetite-apatite ores were generated during orogenesis. This association of orogenesis, emplacement temperatures up to 600-degrees-C, abundant evidence of hydraulic processes during ore formation, and extensive porphyries is reminiscent of other types of intrusion-related, high-temperature, ore-forming processes.