This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. data demonstrate that a saline magmatic-hydrothermal ore fluid will scavenge significant quantities of metals such as Cu and Au from a silicate melt, and when combined with solubility data for Fe, Cu and Au, it is plausible that the magmatic-hydrothermal ore fluid that continues to ascend from the IOA depositional environment can retain sufficient concentrations of these metals to form iron oxide copper-gold (IOCG) deposits at lateral and/or stratigraphically higher levels in the crust. Notably, this study provides a new discrimination diagram to identify magnetite from Kiruna-type deposits and to distinguish them from IOCG, porphyry and Fe-Ti-V/P deposits, based on low Cr (< 100 ppm) and high V (>500 ppm) concentrations. Trace elements in magnetite from massive iron oxide-apatite deposits indicate a combined formation by igneous and magmatic-hydrothermal processes

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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

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

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Iron oxide copper-gold (IOCG) and iron oxide-apatite (IOA) deposits are major sources of Fe, Cu, and Au. Magnetite is the modally dominant and commodity mineral in IOA deposits, whereas magnetite and hematite are predominant in IOCG deposits, with copper sulfides being the primary commodity minerals. It is generally accepted that IOCG deposits formed by hydrothermal processes, but there is a lack of consensus for the source of the ore fluid(s). There are multiple competing hypotheses for the formation of IOA deposits, with models that range from purely magmatic to purely hydrothermal. In the Chilean iron belt, the spatial and temporal association of IOCG and IOA deposits has led to the hypothesis that IOA and IOCG deposits are genetically connected, where S-Cu-Au–poor magnetite-dominated IOA deposits represent the stratigraphically deeper levels of S-Cu-Au–rich magnetite- and hematite-dominated IOCG deposits. Here we report minor element and Fe and O stable isotope abundances for magnetite and H stable isotope abundances for actinolite from the Candelaria IOCG deposit and Quince IOA prospect in the Chilean iron belt. Backscattered electron imaging reveals textures of igneous and magmatic-hydrothermal affinities and the exsolution of Mn-rich ilmenite from magnetite in Quince and deep levels of Candelaria (&gt;500 m below the bottom of the open pit). Trace element concentrations in magnetite systematically increase with depth in both deposits and decrease from core to rim within magnetite grains in shallow samples from Candelaria. These results are consistent with a cooling trend for magnetite growth from deep to shallow levels in both systems. Iron isotope compositions of magnetite range from δ56Fe values of 0.11 ± 0.07 to 0.16 ± 0.05‰ for Quince and between 0.16 ± 0.03 and 0.42 ± 0.04‰ for Candelaria. Oxygen isotope compositions of magnetite range from δ18O values of 2.65 ± 0.07 to 3.33 ± 0.07‰ for Quince and between 1.16 ± 0.07 and 7.80 ± 0.07‰ for Candelaria. For cogenetic actinolite, δD values range from –41.7 ± 2.10 to –39.0 ± 2.10‰ for Quince and from –93.9 ± 2.10 to –54.0 ± 2.10‰ for Candelaria, and δ18O values range between 5.89 ± 0.23 and 6.02 ± 0.23‰ for Quince and between 7.50 ± 0.23 and 7.69 ± 0.23‰ for Candelaria. The paired Fe and O isotope compositions of magnetite and the H isotope signature of actinolite fingerprint a magmatic source reservoir for ore fluids at Candelaria and Quince. Temperature estimates from O isotope thermometry and Fe# of actinolite (Fe# = [molar Fe]/([molar Fe] + [molar Mg])) are consistent with high-temperature mineralization (600°–860°C). The reintegrated composition of primary Ti-rich magnetite is consistent with igneous magnetite and supports magmatic conditions for the formation of magnetite in the Quince prospect and the deep portion of the Candelaria deposit. The trace element variations and zonation in magnetite from shallower levels of Candelaria are consistent with magnetite growth from a cooling magmatic-hydrothermal fluid. The combined chemical and textural data are consistent with a combined igneous and magmatic-hydrothermal origin for Quince and Candelaria, where the deeper portion of Candelaria corresponds to a transitional phase between the shallower IOCG deposit and a deeper IOA system analogous to the Quince IOA prospect, providing evidence for a continuum between both deposit types.

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Growth in global metal demand has fostered a new age of unconventional mining on the seafloor. In situ pulverization and extraction of seafloor massive sulfide (SMS) deposits is economically attractive due to minimal overburden and high ore grades. However, important environmental questions remain on the significance of localized acid generation via irreversible sulfide mineral oxidation. Data on the reaction kinetics are necessary to estimate anthropogenic acid production during seafloor mining. Laboratory experiments were performed to evaluate the effects of pH, temperature, dissolved oxygen, and surface area on the oxidation rate of pyrrhotite and chalcopyrite in seawater. These minerals were chosen to constrain the range of reaction rates because pyrrhotite oxidizes relatively quickly while chalcopyrite is kinetically slow. The rate laws for the abiotic oxidation of pyrrhotite and chalcopyrite in seawater at 22°C are given in the form: R sp = k (m O2) a (m H+) b