Anne
Westhues,
John M. Hanchar, Christopher R. Voisey
Department of Earth Sciences,
Memorial University of Newfoundland, St. John’s, Newfoundland, Canada
Martin J.
Whitehouse
Department
of Geoscience,
Swedish Museum of Natural History, Stockholm, Sweden
George R.
Rossman
Division of Geological and
Planetary Sciences, California Institute of Technology, Pasadena,
California, USA
Richard Wirth
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.