John M. Hanchar, Christopher R. Voisey
Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada
Department of Geoscience, Swedish Museum of Natural History, Stockholm, Sweden
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany
The ore genesis of the Paleoproterozoic iron oxide apatite deposits around the town of Kiruna in northern Sweden is poorly understood, despite a century-long mining history and 2500 Mt of iron ore with grades of 30 to 70 wt.% Fe produced in the region to date. Zircon grains from the ore, recently dated at ca. 1874 Ma, show very different appearances compared to zircon from surrounding host rocks (1880 Ma) and related intrusions (1880 and 1874 Ma), particularly an inclusion-rich rim that is interpreted to be of hydrothermal origin. In contrast, zircon from the host rocks and a proximal granite intrusion exhibit typical igneous growth zoning. Electron microprobe results show near stoichiometric composition for Zr, Si, Hf, Ca, Fe, Y, and P in the host rock zircon grains. In contrast, the ore zircon crystals have low analytical totals and are shown to contain several weight percent water with infrared spectroscopy. These zircon grains further show Fe-rich inclusions, zones and/or veins in elemental X-ray maps, and light rare earth elements (LREE) enrichment; features that are not present in zircon from the metavolcanic host rocks. Uranium-Pb in monazite from the ore, measured by SIMS, suggests a secondary event influencing the area at ca. 1624 Ma, a period of known geologic activity in Fennoscandia. Electron microprobe X-ray mapping of these monazite grains shows no zoning and relatively low U and Th concentrations.
Whole rock Lu-Hf and Sm-Nd data show stark contrasts between the ore (depleted mantle influence) and host rocks (crustal influence). The depleted mantle signature of the ore could be related to the Kiruna greenstone group as a potential source region for the iron. The Sm-Nd isotopic composition of monazite from the ore shows a crustal influence, and indicates that the younger event has not disturbed the whole rock Sm-Nd signature of the ore. Combining the hydrothermal features of the ore zircon grains and the isotopic signatures points to a hydrothermal influence on the ore formation, with a high T magmatic fluid related to the intrusions as most likely heat and fluid source.