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

Abstract

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 (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.