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Tracing the fluid evolution of the Kiruna iron oxide apatite deposits using zircon, monazite, and whole rock trace elements and isotopic studies

Westhues, Anne and Hanchar, John M. and Voisey, Christopher R. and Whitehouse, Martin J. and Rossman, George R. and Wirth, Richard (2017) Tracing the fluid evolution of the Kiruna iron oxide apatite deposits using zircon, monazite, and whole rock trace elements and isotopic studies. Chemical Geology, 466 . pp. 303-322. ISSN 0009-2541.

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The ore genesis of the Paleoproterozoic iron oxide apatite deposits in the vicinity 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 (ca. 1880 Ma) and related intrusions (ca. 1880 and ca. 1874 Ma), particularly an inclusion-rich rim. 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, and Hf in the host rock zircon grains. The ore zircon crystals have low analytical totals with significant concentrations of Ca, Fe, Y, and P and infrared spectroscopy showed several weight percent of water. These ore zircon grains further show Fe-rich inclusions, zones and/or veins in elemental X-ray maps, and light rare earth elements (LREE) enrichment. Transmission electron microscopy (TEM) shows that the LREE are not due to micro- or nano-inclusions in the zircon, but are likely hosted as LREE oxides in amorphous regions of the grains. Based on these characteristics, the rims on ore zircon grains are interpreted to be of hydrothermal origin. 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. Stark contrasts are visible between the ore (depleted mantle influence) and host rocks (crustal influence) in the whole rock Lu-Hf and Sm-Nd data. 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. The hydrothermal nature of the ore zircon grains and the isotopic signatures point to a hydrothermal influence on the ore formation, with a high temperature magmatic fluid related to the intrusions as most likely heat and fluid source.

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Rossman, George R.0000-0002-4571-6884
Additional Information:© 2017 Elsevier B.V. Received 5 February 2017, Revised 12 June 2017, Accepted 15 June 2017, Available online 17 June 2017. This paper represents one of three parts of the PhD research of the senior author. Many thanks are due to LKAB (Luossavaara-Kiirunavaara Aktiebolag, Kiirunavaara operations) for financial and logistical support, especially K. Holme for making the project possible, and many others for fruitful discussions and help during sampling, including, but not limited to, U.B. Anderson, H. Rutanen, and C. Debras. Olof Martinsson is thanked for introduction to the study area and assistance during field work. The project was financially supported by an NSERC discovery grant to JMH, and graduate student funding by MUN, and student research and travel grants by SEG and MAC to AW. Numerous people are thanked for discussions and assistance in the labs including M. Wilson, S.J. Piercey, G. Layne, G.R. Dunning, S. Strong, P. King, R. Lam, S. Jantzi, C.M. Fisher, S.E. Phillips S.G. Broughm, and L. Hewa at MUN, K. Lindén at the Swedish Museum of Natural History. NordSIMS operates as a Nordic infrastructure of which this is publication 514. J. Anderson, A. Hallberg, S. Bergman, J.A. Perdåhl and others at the Geological Survey of Sweden are thanked for support during initial field work and discussions. We thank Lluís Fontboté and Daniel Harlov for their detailed reviews and Klaus Mezger for editorial handling of the manuscript.
Funding AgencyGrant Number
Luossavaara-Kiirunavaara AktiebolagUNSPECIFIED
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Memorial University of NewfoundlandUNSPECIFIED
Subject Keywords:Iron oxide apatite; Norrbotten; Kiruna; Paleoproterozoic; Zircon; Monazite; Sm-Nd; U-Pb; Hydrothermal alteration; LA-ICPMS; FTIR spectroscopy; TEM
Record Number:CaltechAUTHORS:20170907-081032344
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Official Citation:Anne Westhues, John M. Hanchar, Christopher R. Voisey, Martin J. Whitehouse, George R. Rossman, Richard Wirth, Tracing the fluid evolution of the Kiruna iron oxide apatite deposits using zircon, monazite, and whole rock trace elements and isotopic studies, Chemical Geology, Volume 466, 5 September 2017, Pages 303-322, ISSN 0009-2541, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:81231
Deposited By: Tony Diaz
Deposited On:07 Sep 2017 17:02
Last Modified:03 Oct 2019 18:40

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