GEOCHRONOLOGY OF THE KIRUNA IRON-ORES AND HYDROTHERMAL ALTERATIONS

The magnetite-apatite iron ores at Kiruna are hosted in a suite of alkaline volcanic and subvolcanic rocks that was formed in a short time interval of about 20 m.y. between 1900 and 1880 Ma. U-Pb dating of titanite from magnetite-titanite dikes in the footwall of the Luossavaara orebody indicates that the age of the magnetite-apatite ores is 1888 +/- 6 Ma. Titanite-actinolite-calcite assemblages in amygdules are related to the hydrothermal and metasomatic alteration of the wall rocks and the ore. The U-Pb titanite age of the amygdule fill is 1876 +/- 9 Ma. The geographic distribution of the alteration suggests that it is not related to the formation of the iron ores. The youngest major magmatic units in the Kiruna area are 1792 +/- 4 Ma syenites that intruded the supracrustal rocks. The Kiruna area underwent a weak metamorphic overprint during the Caledonian orogeny. Our data are inconsistent with a hydrothermal event at ca. 1.5 Ga as has been suggested earlier.