Abstract

High spatial resolution zircon U-Pb geochronological data obtained directly on the Kiirunavaara iron oxide-apatite (IOA) deposit, related orebodies, and host rocks provide new constraints on the timing of mineralization in these deposits. These data raise new arguments in the debate of a magmatic versus hydrothermal/metasomatic genesis of these major (2,500 Mt, 30–70 wt % Fe) Paleoproterozoic deposits. The main orebody at Kiirunavaara contains Ti-poor magnetite and minor (0.05–5 wt % P) apatite, located between a trachyandesite footwall and a rhyodacite hanging wall, which also hosts smaller orebodies (Nukutus, Rektorn, and Tuolluvaara). The pervasive Na and K metasomatism in the host rock is documented by whole-rock geochemical data and cathodoluminescence (CL) microscopy. Zircon U-Pb data for the metavolcanic rocks in the footwall and hanging wall cluster between 1884 ± 4 and 1880 ± 3 Ma. In the footwall, a syenite-aplite system yields ages of 1880 ± 7 and 1881 ± 4 Ma; a granite pluton exposed underground has an age of 1874 ± 4 Ma. Zircons in two ore samples, never directly dated before this study, yield ages of 1877 ± 4 and 1874 ± 7 Ma. Brecciation at the contacts between the ore and host rocks, the tight age at ca. 1880 Ma for most volcanic and plutonic rocks in the footwall and hanging wall, and the marginally younger age for ore at ca. 1877 to 74 Ma, matching the age of the spatially related granite pluton, suggest a magmatic-hydrothermal emplacement model for the Kiruna area IOA ores.

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