Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 177, Issue -, Pages 94-104Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2016.01.009
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Funding
- Society of Economic Geologists
- UM Department of Earth & Environmental Sciences
- NSF [EAR 1264537, EAR 1264560, EAR 1250239]
- Fondecyt [1140780]
- Millennium Science Initiative grant Nucleaus for Metal Tracing Along Subduction [NC130065]
- FONDAP [15090013]
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Iron oxide-apatite (IOA) ore deposits occur globally and can host millions to billions of tons of Fe in addition to economic reserves of other metals such as rare earth elements, which are critical for the expected growth of technology and renewable energy resources. In this study, we pair the stable Fe and O isotope compositions of magnetite samples from several IOA deposits to constrain the source reservoir of these elements in IOAs. Since magnetite constitutes up to 90 modal% of many IOAs, identifying the source of Fe and O within the magnetite may elucidate high-temperature and/or lower-temperature processes responsible for their formation. Here, we focus on the world-class Los Colorados IOA in the Chilean iron belt (CIB), and present data for magnetite from other Fe oxide deposits in the CIB (El Laco, Mariela). We also report Fe and O isotopic values for other IOA deposits, including Mineville, New York (USA) and the type locale, Kiruna (Sweden). The ranges of Fe isotopic composition (delta Fe-56, Fe-56/Fe-54 relative to IRMM-14) of magnetite from the Chilean deposits are: Los Colorados, delta Fe-56 (+/- 2 sigma) = 0.08 +/- 0.03% to 0.24 +/- 0.08%; El Laco, delta Fe-56 = 0.20 +/- 0.03% to 0.53 +/- 0.03%; Mariela, delta Fe-56 = 0.13 +/- 0.03%. The O isotopic composition (delta O-18, (18) O/O-16 relative to VSMOW) of the same Chilean magnetite samples are: Los Colorados, delta O-18 (+/- 2 sigma) - 1.92 +/- 0.08% to 3.17 +/- 0.03%; El Laco, delta O-18 - 4.00 +/- 0.10% to 4.34 +/- 0.10%; Mariela, delta O-18 = (1.48 +/- 0.04%). The delta O-18 and delta Fe-56 values for Kiruna magnetite yield an average of 1.76 +/- 0.25% and 0.16 +/- 0.07%, respectively. The Fe and O isotope data from the Chilean IOAs fit unequivocally within the range of magnetite formed by high-temperature magmatic or magmatic-hydrothermal processes (i.e., delta Fe-56 0.06-0.49% and delta O-18 = 1.0-4.5%), consistent with a high-temperature origin for Chilean IOA deposits. Additionally, minimum formation temperatures calculated by using the measured Delta O-18 values of coexisting Los Colorados magnetite and actinolite separates (630 degrees C) as well as Fe numbers of actinolite grains (610-820 degrees C) are consistent with this interpretation. We also present Fe isotope data from magmatic magnetite of the Bushveld Complex, South Africa, where delta Fe-56 ranges from 0.28 +/- 0.04% to 0.86 +/- 0.07%. Based on these data and comparison to published Fe and O stable isotope values of igneous magnetite, we propose extending the magmatic/high-temperature delta Fe-56 range to 0.86%. Considering that the Chilean IOAs and Kiruna deposit are representative of IOA deposits worldwide, the Fe and O stable isotope data indicate that IOAs are formed by high-temperature (magmatic) processes. (C) 2016 Elsevier Ltd. All rights reserved.
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