Recent developments in element concentration and isotope ratio analysis of individual fluid inclusions by laser ablation single and multiple collector ICP-MS. Ore Geology Reviews 44, 10-38.

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Artículo de publicación ISIKiruna-type iron oxide-apatite (IOA) deposits are an important source of Fe ore, and two radically different processes are being actively investigated for their origin. One hypothesis invokes direct crystallization of immiscible Fe-rich melt that separated from a parent silicate magma, while the other hypothesis invokes deposition of Fe-oxides from hydrothermal fluids of either magmatic or crustal origin. Here, we present a new model based on Fe and O stable isotopes and trace and major element geochemistry data of magnetite from the similar to 350 Mt Fe Los Colorados IOA deposit in the Chilean iron belt that merges these divergent processes into a single sequence of events that explains all characteristic features of these curious deposits. We propose that concentration of magnetite takes place by the preferred wetting of magnetite, followed by buoyant segregation of these early-formed magmatic magnetite-bubble pairs, which become a rising magnetite suspension that deposits massive magnetite in regional-scale transcurrent faults. Our data demonstrate an unambiguous magmatic origin, consistent with the namesake IOA analogue in the Kiruna district, Sweden. Further, our model explains the observed coexisting purely magmatic and hydrothermal-magmatic features and allows a genetic connection between Kiruna-type IOA and iron oxide-copper-gold deposits, contributing to a global understanding valuable to exploration efforts.German Academic Exchange Service (DAAD) Ph.D. grant Society of Economic Geologists University of Michigan Rackham Graduate School U.S. National Science Foundation EAR-1250239 EAR-1264537 Fondecyt 1140780 Millennium Science Initiative grant "Nucleus for Metal Tracing Along Subduction" NC130065 FONDAP 1509001

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Artículo de publicación ISIIron 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 (d56Fe, 56Fe/54Fe relative to IRMM-14) of magnetite from the Chilean deposits are: Los Colorados, d56Fe (±2r) = 0.08 ± 0.03‰ to 0.24 ± 0.08‰; El Laco, d56Fe = 0.20 ± 0.03‰ to 0.53 ± 0.03‰; Mariela, d56Fe = 0.13 ± 0.03‰. The O isotopic composition (d18O, 18O/16O relative to VSMOW) of the same Chilean magnetite samples are: Los Colorados, d18O (±2r) = 1.92 ± 0.08‰ to 3.17 ± 0.03‰; El Laco, d18O = 4.00 ± 0.10‰ to 4.34 ± 0.10‰; Mariela, d18O = (1.48 ± 0.04‰). The d18O and d56Fe 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., d56Fe 0.06–0.49‰ and d18O = 1.0– 4.5‰), consistent with a high-temperature origin for Chilean IOA deposits. Additionally, minimum formation temperatures calculated by using the measured D18O values of coexisting Los Colorados magnetite and actinolite separates (630 C) as well as Fe numbers of actinolite grains (610–820 C) are consistent with this interpretation. We also present Fe isotope data from magmatic magnetite of the Bushveld Complex, South Africa, where d56Fe 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 d56Fe 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.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 1509001