Fe–O stable isotope pairs elucidate a high-temperature origin of Chilean iron oxide-apatite deposits

Abstract

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

Unraveling the origin of the Andean IOCG clan: A Re-Os isotope approach

“…Sulfides in IOA deposits are present as minor phases, it follows then that sulfides will concentrate the available Re and Os (and other metals) when in competition with magnetite. This is consistent with reported high base and precious metals (Co, Ni, Cu, Ag, Au) content within pyrite in comparison with magnetite from Los Colorados IOA (Knipping et al, 2015b;Reich et al, 2016). The variable Re and Os concentrations observed in some magnetite samples might be due to the presence of minute sulfide inclusions as observed in studied polished sections (Fig.…”

“…Recent geochemical and mineralogical studies (Knipping et al 2015a;2015b;Bilenker et al, 2016;Reich et al, 2016) including trace element analyses, and Fe and O stable isotope data of magnetite provide evidence of a genetic link between magnetiteapatite (sulfide poor) deposits and IOCGs. These works proposed a genetic model for IOA deposits that invokes magnetite concentration by buoyant segregation or flotation of earlyformed magmatic magnetite-bubble pairs.…”

Kiruna-Type Iron Oxide-Apatite (IOA) and Iron Oxide Copper-Gold (IOCG) Deposits Form by a Combination of Igneous and Magmatic-Hydrothermal Processes: Evidence from the Chilean Iron Belt

“…Those authors compare their data to O and Fe stable isotope compositions of orthomagmatic and hydrothermal magnetite from other mineral deposits and conclude that the most plausible explanation for the formation of the Kiruna-type IOA deposits in Sweden is precipitation of magnetite from a magmatic-hydrothermal fluid. The Fe and O isotope data reveal that magnetite mineralization in Kiruna-type ore deposits in the Grängesberg mining district occurred at temperatures from 600° to 900°C, consistent with temperatures of mineralization for the massive magnetite ore at Los Colorados (i.e., the Western dike) reported by Bilenker et al (2016). Further, Weis (2013) reports that late-stage hydrothermal magnetite precipitated at temperatures below 400°C, also consistent with data for late-stage, hydrothermal, disseminated and veinlet magnetite from Los Colorados (Fig.…”

“…The highest Fe # of 0.72 and lowest Fe # of 0.23 yield model temperatures of 620° and 810°C at 100 and 200 MPa, respectively. For detailed tables with EPMA and LA-ICP-MS data, see Bilenker et al (2016).…”

“…The δ 18 O values for Pea Ridge and Pilot Knob that are elevated relative to purely orthomagmatic magnetite are explained byChildress et al (2016) to reflect crystallization of magnetite from a silicate melt. SeeBilenker et al (2016) for detailed data for samples from Chile and Sweden,Childress et al (2016) for data for PeaRidge and Pilot Knob, and Childress et al (writ. commun.)…”