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Iron oxide copper-gold (IOCG) deposits are major sources of Cu, contain abundant Fe oxides, and may contain Au, Ag, Co, rare earth elements (REEs), U, and other metals as economically important byproducts in some deposits. They form by hydrothermal processes, but the source of the metals and ore fluid(s) is still debated. We investigated the geochemistry of magnetite from the hydrothermal unit and manto orebodies at the Mina Justa IOCG deposit in Peru to assess the source of the iron oxides and their relationship with the economic Cu mineralization. We identified three types of magnetite: magnetite with inclusions (type I) is only found in the manto, is the richest in trace elements, and crystallized between 459° and 707°C; type Dark (D) has no visible inclusions and formed at around 543°C; and type Bright (B) has no inclusions, has the highest Fe content, and formed at around 443°C. Temperatures were estimated using the Mg content in magnetite. Magnetite samples from Mina Justa yielded an average δ56Fe ± 2σ value of 0.28 ± 0.05‰ (n = 9), an average δ18O ± 2σ value of 2.19 ± 0.45‰ (n = 9), and Δ’17O values that range between –0.075 and –0.047‰. Sulfide separates yielded δ65Cu values that range from –0.32 to –0.09‰. The trace element compositions and textures of magnetite, along with temperature estimations for magnetite crystallization, are consistent with the manto magnetite belonging to an iron oxide-apatite (IOA) style mineralization that was overprinted by a younger, structurally controlled IOCG event that formed the hydrothermal unit orebody. Altogether, the stable isotopic data fingerprint a magmatic-hydrothermal source for the ore fluids carrying the Fe and Cu at Mina Justa and preclude significant input from meteoric water and basinal brines.

The Candelaria iron oxide-copper-gold deposit in Chile was formed by superimposed, episodic hydrothermal pulses with contrasting composition and temperature, according to micro-textural and compositional variations in actinolite, a common alteration mineral.

The Plio-Pleistocene El Laco iron oxide-apatite (IOA) orebodies in northern Chile are some of the most enigmatic mineral deposits on Earth, interpreted to have formed as lava flows or by hydrothermal replacement, two radically different processes. Field observations provide some support for both processes, but ultimately fail to explain all observations. Previously proposed genetic models based on observations and study of outcrop samples include (1) magnetite crystallization from an erupting immiscible Fe- and P-rich (Si-poor) melt and (2) metasomatic replacement of andesitic lava flows by a hypogene hydrothermal fluid. A more recent investigation of outcrop and drill core samples at El Laco generated data that were used to develop a new genetic model that invokes shallow emplacement and surface venting of a magnetite-bearing magmatic-hydrothermal fluid suspension. This fluid, with rheological properties similar to basaltic lava, would have been mobilized by decompression-induced collapse of the volcanic edifice. In this study, we report oxygen, including 17O, hydrogen, and iron stable isotope ratios in magnetite and bulk iron oxide (magnetite with minor secondary hematite and minor goethite) from five of seven orebodies around the El Laco volcano, excluding San Vicente Bajo and the minor Laquito deposits. Calculated values of δ18O, Δ17O, δD, and δ56Fe fingerprint the source of the ore-forming fluid(s): Δ17Osample = δ17Osample – δ18Osample * 0.5305. Magnetite and bulk iron oxide (magnetite variably altered to goethite and hematite) from Laco Sur, Cristales Grandes, and San Vicente Alto yield δ18O values that range from 4.3 to 4.5‰ (n = 5), 3.0 to 3.9‰ (n = 5), and –8.5 to –0.5‰ (n = 5), respectively. Magnetite samples from Rodados Negros are the least altered samples and were also analyzed for 17O as well as conventional 16O and 18O, yielding calculated δ18O values that range from 2.6 to 3.8‰ (n = 9) and Δ17O values that range from –0.13 to –0.07‰ (n = 5). Bulk iron oxide from Laco Norte yielded δ18O values that range from –10.2 to +4.5‰ (avg = 0.8‰, n = 18). The δ2H values of magnetite and bulk iron oxide from all five orebodies range from –192.8 to –79.9‰ (n = 28); hydrogen is present in fluid inclusions in magnetite and iron oxide, and in minor goethite. Values of δ56Fe for magnetite and bulk iron oxide from all five orebodies range from 0.04 to 0.70‰ (avg = 0.29‰, σ = 0.15‰, n = 26). The iron and oxygen isotope data are consistent with a silicate magma source for iron and oxygen in magnetite from all sampled El Laco orebodies. Oxygen (δ18O Δ +4.4 to –10.2‰) and hydrogen (δ2H ≃ –79.9 to –192.8‰) stable isotope data for bulk iron oxide samples that contain minor goethite from Laco Norte and San Vicente Alto reveal that magnetite has been variably altered to meteoric values, consistent with goethite in equilibrium with local δ18O and δ2H meteoric values of ≃ –15.4 and –211‰, respectively. The H2O contents of iron oxide samples from Laco Norte and San Vicente Alto systematically increase with increasing abundance of goethite and decreasing values of δ18O and δ2H. The values of δ2H (≃ –88 to –140‰) and δ18O (3.0–4.5‰) for magnetite samples from Cristales Grandes, Laco Sur, and Rodados Negros are consistent with growth of magnetite from a degassing silicate melt and/or a boiling magmatic-hydrothermal fluid; the latter is also consistent with δ18O values for quartz, and salinities and homogenization temperatures for fluid inclusions trapped in apatite and clinopyroxene coeval with magnetite. The sum of the data unequivocally fingerprint a silicate magma as the source of the ore fluids responsible for mineralization at El Laco and are consistent with a model that explains mineralization as the synergistic result of common magmatic and magmatic-hydrothermal processes during the evolution of a caldera-related explosive volcanic system.

Pyrite is ubiquitous in the world-class iron oxide copper-gold (IOCG) deposits of the Candelaria-Punta del Cobre district, documented from early to late stages of mineralization and observed in deep and shallow levels of mineralized bodies. Despite its abundance, the chemical and isotopic signature of pyrite from the Candelaria-Punta del Cobre district, and most IOCG deposits worldwide, remains poorly understood. We evaluated in situ chemical and isotopic variations at the grain scale in a set of pyrite-bearing samples collected throughout the district in order to characterize and further understand the nature of mineralization in this IOCG system. Our multianalytical approach integrated synchrotron micro-X-ray fluorescence (μ-XRF) mapping of pyrite grains with electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry data, and sulfur isotope determinations using secondary ion mass spectrometry (SIMS) complemented with bulk sulfur isotope analyses of coeval pyrite, chalcopyrite, and anhydrite. Synchrotron μ-XRF elemental concentration maps of individual pyrite grains reveal a strong zonation of Co, Ni, As, and Se. The observed relationships between Ni and Se are interpreted to reflect changes in temperature and redox conditions during ore formation and provide constraints on fluid evolution. Co and Ni concentrations and ratios suggest contributions from magmas of mafic-intermediate composition. Pyrite chemical concentrations reflect potential stratigraphic controls, where the sample from the upper part of the stratigraphy diverges from trends formed by the rest of the sample set from lower stratigraphic levels. The SIMS δ34S values of pyrite (and chalcopyrite) range between –2 up to 10‰, and bulk δ34S values of pyrite range between 4 up to 12‰. The majority of the δ34S analyses, falling between –1 and 2‰, indicate a magmatic source for sulfur and, by inference, for the hydrothermal ore fluid(s). Variation in the δ34S signature can be explained by changes in the redox conditions, fluid sources, and/or the temperature of the hydrothermal fluid. The Se/S ratio combined with δ34S values in pyrite is consistent with mixing between a magmatic-hydrothermal fluid and a fluid with a probable basinal signature. The results of this study are consistent with the hydrothermal fluids responsible for mineralization in the Candelaria-Punta del Cobre district being predominantly of magmatic origin, plausibly from mafic-intermediate magmas based on the Ni-Co content in pyrite. External fluid incursion, potentially from a basinal sedimentary source, occurred late in the evolution of the system, adding additional reduced sulfur as pyrite. There is no evidence to suggest that the late fluid added significant Cu-Au mineralization, but this cannot be ruled out. Finally, the data reveal that trace element ratios coupled with spatially resolved sulfur isotope data in pyrite are powerful proxies to track the magmatic-hydrothermal evolution of IOCG systems and help constrain the source of their contained metals.

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 (>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.

The textures of outcrop and near-surface exposures of the massive magnetite orebodies (>90 vol % magnetite) at the Plio-Pleistocene El Laco iron oxide-apatite (IOA) deposit in northern Chile are similar to basaltic lava flows and have compositions that overlap high- and low-temperature hydrothermal magnetite. Existing models—liquid immiscibility and complete metasomatic replacement of andesitic lava flows—attempt to explain the genesis of the orebodies by entirely igneous or entirely hydrothermal processes. Importantly, those models were developed by studying only near-surface and outcrop samples. Here, we present the results of a comprehensive study of samples from outcrop and drill core that require a new model for the evolution of the El Laco ore deposit. Backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to investigate the textural and compositional variability of magnetite and apatite from surface and drill core samples in order to obtain a holistic understanding of textures and compositions laterally and vertically through the orebodies. Magnetite was analyzed from 39 surface samples from five orebodies (Cristales Grandes, Rodados Negros, San Vicente Alto, Laco Norte, and Laco Sur) and 47 drill core samples from three orebodies (Laco Norte, Laco Sur, and Extensión Laco Sur). The geochemistry of apatite from eight surface samples from three orebodies (Cristales Grandes, Rodados Negros, and Laco Sur) was investigated. Minor and trace element compositions of magnetite in these samples are similar to magnetite from igneous rocks and magmatic-hydrothermal systems. Magnetite grains from deeper zones of the orebodies contain >1 wt % titanium, as well as ilmenite oxyexsolution lamellae and interstitial ilmenite. The ilmenite oxyexsolution lamellae, interstitial ilmenite, and igneous-like trace element concentrations in titanomagnetite from the deeper parts of the orebodies are consistent with original crystallization of titanomagnetite from silicate melt or high-temperature magmatic-hydrothermal fluid. The systematic decrease of trace element concentrations in magnetite from intermediate to shallow depths is consistent with progressive growth of magnetite from a cooling magmatic-hydrothermal fluid. Apatite grains from surface outcrops are F rich (typically >3 wt %) and have compositions that overlap igneous and magmatic-hydrothermal apatite. Magnetite and fluorapatite grains contain mineral inclusions (e.g., monazite and thorite) that evince syn- or postmineralization metasomatic alteration. Magnetite grains commonly meet at triple junctions, which preserve evidence for reequilibration of the ore minerals with hydrothermal fluid during or after mineralization. The data presented here are consistent with genesis of the El Laco orebodies via shallow emplacement and eruption of magnetite-bearing magmatic-hydrothermal fluid suspensions that were mobilized by decompression-induced collapse of the volcanic edifice. The ore-forming magnetite-fluid suspension would have rheological properties similar to basaltic lava flows, which explains the textures and presence of cavities and gas escape tubes in surface outcrops.

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Iniciativa Cientifica Milenio, through grant "Millennium Nucleus for Metal Tracing along Subduction". Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT), CONICYT FONDAP: 15090013. CONICYT-FONDEQUIP grants: EQM120098, EQM140009. Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT): 21170857, 72160268. United States Department of Energy (DOE): DE-AC02-06CH11357. National Science Foundation (NSF): EAR 1524394.

Most known porphyry Cu ± Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite–trondhjemite–granodiorite–diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ~2·7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu ± Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ~2·74 Ga, ~2·70 Ga, and ~ 2·69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t) ± SD values of 4·5 ± 0·3, 4·2 ± 0·6, and 4·3 ± 0·4, and δ18O ± SD values of 5·40 ± 0·11 ‰, 3·91 ± 0·13 ‰, and 4·83 ± 0·12 ‰, respectively. These isotopic signatures indicate that, although these magmas are mantle-sourced with minimal crustal contamination, for the St-Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids. Primary barometric minerals (i.e. zircon, amphibole, apatite, and magnetite–ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite–magnetite–quartz buffer as ΔFMQ, using zircon geochemistry indicates that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from ΔFMQ –0·3 ± 0·6 to ΔFMQ +0·8 ± 0·4 and to ΔFMQ +1·2 ± 0·4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of ΔFMQ +1·6 ± 0·3 and ΔFMQ +2·6 ± 0·1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate that amphibole may overestimate the fO2 of intrusive rocks by up to 1 log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e. S6+/ΣS) in primary apatite yielded ≥ΔFMQ −0·3 and ΔFMQ +1·4–1·8 for St-Jude and Clifford, respectively. The magnetite–ilmenite mineral pairs from the Clifford tonalite yielded ΔFMQ +3·3 ± 1·3 at equilibrium temperatures of 634 ± 21 °C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0·1 wt%) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results to indicate that the deposits at Côté Gold and Clifford formed from mildly (~ΔFMQ +0·8 ± 0·4) to moderately (~ΔFMQ +1·5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (~ΔFMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu–Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias.

Magnetite is the main constituent of iron oxide–apatite (IOA) deposits, which are a globally important source of Fe and other elements such as P and REE, critical for modern technologies. Geochemical studies of magnetite from IOA deposits have provided key insights into the ore-forming processes and source of mineralizing fluids. However, to date, only qualitative estimations have been obtained for one of the key controlling physico-chemical parameters, i.e., the temperature of magnetite formation. Here we reconstruct the thermal evolution of Andean IOA deposits by using magnetite thermometry. Our study comprised a > 3000 point geochemical dataset of magnetite from several IOA deposits within the Early Cretaceous Chilean Iron Belt, as well as from the Pliocene El Laco IOA deposit in the Chilean Altiplano. Thermometry data reveal that the deposits formed under a wide range of temperatures, from purely magmatic (~ 1000 to 800 °C), to late magmatic or magmatic-hydrothermal (~ 800 to 600 °C), to purely hydrothermal (

The Mantoverde iron oxide-copper-gold (IOCG) deposit, Chile, contains hundreds of millions of tonnes (Mt) of mineable iron oxide and copper sulfide ore. While there is an agreement that mineralization at Mantoverde was caused by hydrothermal fluid(s), there is a lack of consensus for the role(s) that non-magmatic vs. magmatic fluid(s) played during the evolution of the mineralized system. In order to overcome the hydrothermal overprint at Mantoverde, which is known to disturb most conventional stable isotope systems (e.g., oxygen), we report the first δ56Fe and δ18O pairs for early-stage magnetite and late-stage hematite that provide information on the source reservoir of the hydrothermal fluids. Magnetite δ56Fe values range from 0.46 ± 0.04 to 0.58 ± 0.02‰ and average 0.51 ± 0.16‰ (n = 10; 2σ). Three hematite δ56Fe values were measured to be 0.34 ± 0.10, 0.42 ± 0.09, and 0.46 ± 0.06. Magnetite δ18O values range from 0.69 ± 0.04 to 4.61 ± 0.05‰ and average 2.99 ± 2.70‰ (n = 9; 2σ). Hematite δ18O values range from − 1.36 ± 0.05 to 5.57 ± 0.05‰ and average 0.10 ± 5.38‰ (n = 6; 2σ). These new δ56Fe and δ18O values fingerprint a magmatic-hydrothermal fluid as the predominant ore-forming fluid responsible for mineralization in the Mantoverde system.

The incorporation of sulfur (S) into the apatite structure and the partitioning of S between apatite and silicate melt (DSap/m) have been proposed to vary systematically as a function of prevailing redox conditions. In this study, we experimentally equilibrated apatite with mafic silicate melt at 1000 °C, 300 MPa and a range of oxygen fugacity (fO2; log fO2 [ΔFMQ] = −1, 0, +0.3, +1.2, and +3 where FMQ is the fayalite-magnetite-quartz mineral redox buffer) to explore the partitioning behavior of S, including different oxidation states of S, between apatite and silicate melt. The data reveal that DSap/m values increase systematically with increasing fO2, from 0.02± 0.01 at log fO2 [ΔFMQ] of −1 to 3.20 ± 0.19 at log fO2 [ΔFMQ] of +3. The bulk S content (∼0.37 and ∼0.28 wt.% S added) imparts a minor influence on DSap/m at reducing conditions. Micro X-ray absorption near edge structure (μ-XANES) spectroscopy at the S K-edge was used to measure, in situ, the oxidation states of S in experimentally crystallized apatite. The S-XANES analyses reveal that with increasing fO2, apatite progressively incorporates S6+ ≫ S2− > S4+ > S1+. The integrated S6+/ΣS peak area ratios and centroid energies (eV) were determined for apatite crystals in apatite from experiments at all fO2 conditions. The orientation effects occurring during S-XANES analyses of apatite were considered by merging spectra from multiple grains with random crystallographic orientation. The S-XANES data reveal that S6+/ΣS peak area ratios and the centroid energies increase systematically with fO2, demonstrating that the S6+/ΣS ratio in apatite can be used as an oxybarometer. The results demonstrate that both the S6+/ΣS and CeV calibration methods are highly sensitive in the redox range of ∼FMQ to ∼FMQ + 1.2 at the conditions and compositions evaluated in this study. As a result, the S-in-apatite oxybarometer is particularly applicable to mafic systems such as mid ocean ridge basalts (MORB), relatively reduced ocean island basalts (OIB), and back-arc basin basalt (BABB). Owing to the ubiquity of apatite in magmatic and magmatic-hydrothermal systems, measuring the concentration and oxidations state(s) of S-in-apatite has the potential to serve as a powerful sulfo- and oxy-barometer for a broad range of natural systems.

The oxidation state of sulfur (S) plays a critical role in the formation of igneous and hydrothermal mineral systems. Constraining the oxidation state of S during mineralization and alteration is a valuable tool for understanding primary versus secondary processes that affect mineral systems. Recent experimental studies demonstrate that apatite, which is a ubiquitous accessory mineral in igneous and hydrothermal ore-forming systems, structurally incorporates multiple oxidation states of S (i.e., S6+, S4+, S2−), and that the abundance of reduced versus oxidized S in apatite is systematically related to oxygen fugacity. In this study, we used micro X-ray absorption near edge structure (μ-XANES) spectroscopy at the S K-edge to measure the oxidation states of S in natural apatite from the Phillips mine magnetite-sulfide mineral deposit (Putnam County, New York). Micro-XANES transects were collected within two apatite grains, starting near the edge of (1) a pyrrhotite inclusion, and (2) an inclusion assemblage consisting of pyrite, ferroan carbonate, pyroxene, and magnetite. Significant compositional and textural variations within the apatite were observed by electron probe micro-analysis (EPMA), wavelength dispersive (WDS) spectroscopy element mapping, and cathodoluminescence (CL) imaging, and used in combination with the μ-XANES data to discuss the formation of the Phillips mine apatite. The μ-XANES analyses reveal that apatite contains variable proportions of S6+, S4+, S1− and S2−, with corresponding peak absorption energies of 2481.7 ± 0.3 eV, 2477.9 ± 0.4 eV, 2471.8 ± 0.1 eV, and 2469.8 ± 0.04 eV, respectively. Notably, this marks the first observation of reduced S species (S2−, S1−) in terrestrial apatite. Peak areas ratios (S6+/∑S) demonstrate systematic variations in the oxidation state of S within the apatite grains. Elevated S6+/∑S peak area ratios typically coincide with higher concentrations of S and rare earth elements within the apatite grains. Several observations, including the presence of multiple oxidation states of S in apatite, and monazite inclusions that record secondary, fluid-mediated dissolution-reprecipitation of apatite, indicate differences in the oxidation of S, thus oxygen fugacity, during primary mineralization and secondary alteration (i.e., metasomatism). We propose a model for the formation of the Phillips mine apatite wherein the primary apatite grains crystallized from a reduced, S-bearing hydrothermal fluid characterized by a low SO2/H2S ratio. Subsequently, metasomatism of apatite in the presence of an oxidized fluid, which contained an elevated SO2/H2S ratio, resulted in the exsolution of rare earth elements from apatite and concomitant growth of monazite, and the structural incorporation of oxidized S (S6+ and S4+) in apatite. This study demonstrates that the oxidation states of S in apatite provide valuable geochemical information regarding the redox evolution of mineralized systems.

Most known porphyry Cu deposits formed in the Phanerozoic and are exclusively associated with moderately oxidized, sulfur-rich, hydrous arc-related magmas derived from partial melting of the asthenospheric mantle metasomatized by slab-derived fluids. Yet, whether similar metallogenic processes also operated in the Precambrian remains obscure. Here we address the issue by investigating the origin, fO2, and S contents of calc-alkaline plutonic rocks associated with the Haib porphyry Cu deposit in the Paleoproterozoic Richtersveld Magmatic Arc (southern Namibia), an interpreted mature island-arc setting. We show that the ca. 1886–1881 Ma ore-forming magmas, originated from a mantle-dominated source with minor crustal contributions, were relatively oxidized (1‒2 log units above the fayalite-magnetite-quartz redox buffer) and sulfur-rich. These results indicate that moderately oxidized, sulfur-rich arc magma associated with porphyry Cu mineralization already existed in the late Paleoproterozoic, probably as a result of recycling of sulfate-rich seawater or sediments from the subducted oceanic lithosphere at that time., Tectonomagmatic conditions in the Precambrian were hypothesized to be unfavorable for porphyry Cu deposit formation. Here, the authors show that metallogenic processes typify Phanerozoic porphyry Cu deposits operated by ~1.88 Ga, reflecting modification of mantle lithosphere by oxidized slab-derived fluids at that time.

Many intermediate to felsic intrusive and extrusive rocks contain mafic magmatic enclaves that are evidence for magma recharge and mixing. Whether enclaves represent records of pro-longed mixing or...

Apatite grains from the Los Colorados iron oxide–apatite (IOA) deposit, the largest IOA deposit in the Chilean Iron Belt (CIB), exhibit significant intracrystalline spatial variability with respect to the concentrations of F, Cl, and OH and trace elements. Statistical interrogation of the compositional data indicates that individual apatite grains contain spatially discrete F-rich and Cl-rich domains. The chemical composition of the F-rich domains is consistent with apatite growth from silicate melts, whereas the chemical composition of the Cl-rich domains is consistent with apatite growth from a magmatic-hydrothermal fluid that cooled as it percolated outward from the Los Colorados fault—the structural control for emplacement of the ore body—into the surrounding brecciated diorite and andesite host rocks. Apatite in the deposit is intimately intergrown with magnetite and actinolite for which trace element, Fe, H, and O stable isotope data indicate a combined magmatic/magmatic-hydrothermal genesis for the deposit. The compositional data for apatite are consistent with a genetic model wherein F-rich apatite cores crystallized with magnetite from silicate melt, followed by exsolution of a magmatic-hydrothermal fluid during decompression of the parent magma. Experimental studies demonstrate that magmatic-hydrothermal volatile phase bubbles preferentially nucleate and grow on the surfaces of apatite and magnetite microlites during decompression of a magma body. Continued degassing of the melt results in the volatile phase sweeping up apatite and magnetite microlites, and forming a magnetite-apatite-fluid suspension that is buoyant in the magma chamber, and ascends from the source magma along faults during regional extension. Halite-saturated fluid inclusions in magnetite, which is paragenetically equivalent to apatite at Los Colorados, indicate that the magmatic-hydrothermal fluid was a brine, which allows this fluid to efficiently scavenge Cl, P, rare earth elements, and other fluid-compatible elements from the silicate melt. During ascent, the XCl/XF ratio of apatite increases as it grows from the evolving Cl-rich magmatic-hydrothermal fluid during decompression and cooling.

The ratio of the halogens F and Cl in apatite is highly sensitive to changes in the composition of an evolving silicate melt or aqueous fluid. For this reason apatite chemistry is widely used as a monitor of halogen behavior in magmatic-hydrothermal systems. Apatite is an ubiquitous mineral in iron oxide – apatite (IOA) mineral deposits, where P and volatiles such as F, Cl, H2O and S play a major role in ore genesis. In this study, we present a combination of textural, micro-analytical and isotopic data for apatite from three Andean IOA deposits (Carmen, Fresia and Mariela) of Early Cretaceous age from the Coastal Cordillera of northern Chile. Apatite textures and compositions show evidence of post-crystallization alteration. Apatite is predominantly zoned with respect to Cl and F, showing a decoupled geochemical behavior between these two elements. Overall, four types of apatite or domains were identified in the analyzed grains based on the XCl-apatite/XF-apatite and XCl-apatite/XOH-apatite ratios determined using the atomic proportions of F, Cl and OH. F-apatite is characterized by a XCl-apatite/XF-apatite 25 and a XCl-apatite/XOH-apatite > 4.5 and up to 3000; Cl-OH-F-apatite has a XCl-apatite/XF-apatite = 0.15–8 and a XCl-apatite/XOH-apatite = 0.25–5.45; and Cl-OH-apatite shows a XCl-apatite/XF-apatite = 8–75 and a XCl-apatite/XOH-apatite = 0.5–3. Carmen apatite is mostly F-apatite, but shows Cl-OH compositions restricted to rims and fractures, whereas apatite from Mariela is dominantly Cl-apatite and Cl-OH-apatite. Apatite from Fresia have variable compositions between Cl-OH-F, Cl-OH- and Cl-apatite, where Cl- and Cl-OH-apatites are characterized by an enrichment of S, Na, Sr and Fe relative to F-apatite. Most notably, S and Na correlate with Cl. In Carmen and Fresia, Cl-OH-apatite is slightly depleted in LREEs, Th and U, a finding consistent with micro-textural evidence of metasomatic processes including coupled dissolution-reprecipitation and formation of secondary monazite and xenotime inclusions. In contrast, apatite from Mariela exhibits no depletion in LREEs and displays a homogeneous distribution of Th and U between the different apatite types, with no monazite and xenotime inclusions. The textural types of apatite identified in this study, coupled with the halogen and trace element composition of apatite, are consistent with modification of primary, F-apatite by interaction with hydrothermal fluids, which led to the formation of Cl-OH-apatite and Cl-apatite. In addition, the initial 87Sr/86Sr ratios and eNd values of apatite (0.7038 to 0.7050 and −0.3 to +6.5, respectively), calculated considering a 130 Ma age, are consistent with a magmatic origin for the primary F-apatite with minimal or no crustal contribution. Thus, textural, geochemical, and isotopic results of apatite support a magmatic-hydrothermal origin for these Andean IOA deposits with variable degrees of metasomatic overprint as evidenced by the formation of Cl-OH and Cl-apatite.

Ti-isotope fractionation on the most Ti-rich minerals on Earth has not been reported. Therefore, we present a chemical preparation and separation technique for Ti-rich minerals for mineralogic, petrologic, and economic geologic studies. A two-stage ion-exchange column procedure modified from the previous literature is used in the current study to separate Ti from Fe-rich samples, while α-TiO2 does not require chemical separation. Purified solutions in conjunction with solution standards were measured on two different instruments with dry plasma and medium-resolution mode providing mass-dependent results with the lowest errors. 49/47TiOL-Ti for the solution and solids analyzed here demonstrate a range of >5‰ far greater than the whole procedural 1 error of 0.10‰ for a synthetic compound and 0.07‰ for the mineral magnetite; thus, the procedure produces results is resolvable within the current range of measured Ti-isotope fractionation in these minerals.

The genetic link between magmas and ore deposit formation is well documented by studies of fossil hydrothermal systems associated with magmatic intrusions at depth. However, the role of explosive volcanic processes as active agents of mineralization remains unexplored owing to the fact that metals and volatiles are released into the atmosphere during the eruption of arc volcanoes. Here, we draw on observations of the uniquely preserved El Laco iron deposit in the Central Andes to shed new light on the metallogenic role of explosive volcanism that operates on a global scale. The massive magnetite (Fe3O4) ore bodies at El Laco have surface structures remarkably similar to basaltic lava flows, stimulating controversy about their origin. A long-standing debate has endured because all proposed models were constructed based exclusively on samples collected from surface outcrops representing the uppermost and most altered portion of the deposit. We overcome this sampling bias by studying samples retrieved from several drill cores and surface outcrops. Our results reveal complex lithological, textural and geochemical variations characterized by magmatic-like features and, most notably, a systematic increase in titanium concentration of magnetite with depth that account for an evolving system transitioning from purely magmatic to magmatic-hydrothermal conditions. We conclude that El Laco, and similar deposits worldwide, formed by a synergistic combination of common magmatic processes enhanced during the evolution of caldera-related explosive volcanic systems.

Evolved granites and pegmatites are important sources for critical metals, including niobium (Nb) and tantalum (Ta). The ore deposits that are commonly associated with these granitic rocks are commonly enriched in phosphorous (P) and boron (B), yet there are few experimental constraints on the influence of these fluxing compounds on ore forming processes. Here, the effects of phosphorous (P), boron (B), temperature and melt alkalinity on the solubility of manganocolumbite (MnNb2O6) and manganotantalite (MnTa2O6) in evolved, fluid-saturated granitic melts were studied. Solubility experiments were performed at 100 MPa and 800 to 1000 °C, using three haplogranitic melt compositions with alumina saturation indices (ASI) of 0.8 (peralkaline), 1.0 (subaluminous) and 1.2 (peraluminous). The possible effect of P and the influence of B in P-rich granitic systems were investigated by adding 4 wt% P2O5 and 4 wt% P2O5 + 4 wt% B2O3, respectively, to the starting compositions. The addition of P decreased the solubility of manganocolumbite and manganotantalite in peralkaline granitic melts. In agreement with previous studies, we suggest that P was present as network forming alkali phosphate complexes such as MXPOY in the peralkaline melt structure (M = K, Na etc.), resulting in the observed effect of P at ASI = 0.8. The presence of P had a positive effect on the solubility of manganocolumbite and manganotantalite in subaluminous granitic melts, which became less significant in peraluminous systems. This may reflect the formation of complexes such as AlXPOY and (Ta,Nb)XPOY in the melt. Empirical relationships describing the effect of P on manganocolumbite and manganotantalite solubility in the aforementioned haplogranitc melts at 100 MPa and 800 °C were determined. The addition of B to P-rich peralkaline melts had a minor effect on manganocolumbite solubility, but resulted in an increase of manganocolumbite and manganotantalite solubility in subaluminous systems. For peraluminous melts, a minor increase of the solubility of manganocolumbite and manganotantalite with the addition of B was typically observed. We propose that the addition of B to the P-enriched melts resulted in the formation of network-forming boro-phosphate complexes until the B/P molar ratio in the melts reached unity. Once the B/P molar ratio was greater than one, polymerizing MBO2 complexes were probably formed in peralkaline melts, while B acted presumably as a network modifier in subaluminous and peraluminous systems with B/P > 1. In combination with literature data, we determined similar dissolution enthalpies for manganocolumbite and manganotantalite for given a ASI (and pressure), where the dissolution enthalpy increased with ASI from ∼60 kJ/mol at ASI ≈ 0.6 to ∼150 kJ/mol at ASI ≈ 1.2. Cooling is thus likely to be of greater importance for magmatic mineralization in peraluminous compared to peralkaline systems. The primary influence of B and P in peraluminous systems is to act as fluxes, lowering the crystallization temperature of melts, which allows for the crystallization of primary magmatic tantalite.

El Romeral is one of the largest iron oxide-apatite (IOA) deposits in the Coastal Cordillera of northern Chile. The Cerro Principal magnetite ore body at El Romeral comprises massive magnetite intergrown with actinolite, with minor apatite, scapolite, and sulfides (pyrite ± chalcopyrite). Several generations of magnetite were identified by using a combination of optical and electron microscopy techniques. The main mineralization event is represented by zoned magnetite grains with inclusion-rich cores and inclusion-poor rims, which form the massive magnetite ore body. This main magnetite stage was followed by two late hydrothermal events that are represented by magnetite veinlets that crosscut the massive ore body and by disseminated magnetite in the andesite host rock and in the Romeral diorite. The sulfur stable isotope signature of the late hydrothermal sulfides indicates a magmatic origin for sulfur (δ34S between − 0.8 and 2.9‰), in agreement with previous δ34S data reported for other Chilean IOA and iron oxide-copper-gold deposits. New 40Ar/39Ar dating of actinolite associated with the main magnetite ore stage yielded ages of ca. 128 Ma, concordant within error with a U-Pb zircon age for the Romeral diorite (129.0 ± 0.9 Ma; mean square weighted deviation = 1.9, n = 28). The late hydrothermal magnetite-biotite mineralization is constrained at ca. 118 Ma by 40Ar/39Ar dating of secondary biotite. This potassic alteration is about 10 Ma younger than the main mineralization episode, and it may be related to post-mineralization dikes that crosscut and remobilize Fe from the main magnetite ore body. These data reveal a clear genetic association between magnetite ore formation, sulfide mineralization, and the diorite intrusion at El Romeral (at ~ 129 Ma), followed by a late and more restricted stage of hydrothermal alteration associated with the emplacement of post-ore dikes at ca. 118 Ma. Therefore, this new evidence supports a magmatic-hydrothermal model for the formation of IOA deposits in the Chilean Iron Belt, where the magnetite mineralization was sourced from intermediate magmas during the first Andean stage. In contrast, the beginning of the second Andean stage is characterized by shallow subduction and a compressive regime, which is represented in the district by the emplacement of the Punta de Piedra granite-granodiorite batholith (100 Ma) and marks the end of iron oxide-apatite deposit formation in the area.

Despite many studies reporting the presence of S-bearing apatite in igneous and hydrothermal systems, the oxidation states and incorporation mechanisms of S in the apatite structure remain poorly understood. In this study, we use ab initio calculations to investigate the energetics and geometry of incorporation of S with its oxidation states S 6+ , S 4+ , and S 2− into the apatite end-members fluor-, chlor-, and hydroxylapatite, [Ca 10 (PO 4 ) 6 (F,Cl,OH) 2 ]. The relative stability of different oxidation states of S in apatite is evaluated by using balanced reaction equations where the apatite host and a solid S-bearing source phase (e.g., gypsum for S 6+ and troilite for S 2− ) are the reactants, and the S-incorporated apatite and an anion sink phase are the products. Here, the reaction energy of the balanced equation indicates the stability of the modeled S-incorporated apatite relative to the host apatite, the source, and sink phases. For the incorporation of S into apatite, coupled substitutions are necessary to compensate for charge imbalance. One possible coupled substitution mechanism involves the replacement of La 3+ + PO 4 3− ↔ Ca 2+ + SO 4 2− . Our results show that the incorporation of SO 4 2− into La- and Na-bearing apatite, Ca 8 NaLa(PO 4 ) 6 (F,Cl,OH) 2 , is energetically favored over the incorporation into La- and Si-bearing apatite, Ca 9 La(PO 4 ) 5 (SiO 4 )(F,Cl,OH) 2 (the difference in incorporation energy, Δ E rxn , is 10.7 kJ/mol). This thermodynamic gain is partially attributed to the electrostatic contribution of Na + , and the energetic contribution of La 3+ to the stability of SO 4 2− incorporated into the apatite structure. Co-incorporation of SO 4 2− and SO 3 2− is energetically favored when the lone pair electrons of SO 3 2− face toward the anion column site, compared to facing away from it. Full or partial incorporation of S 2− is favored on the column anion site in the form of [Ca 10 (PO 4 ) 6 S] and [Ca 20 (PO 4 ) 12 SX 2 )], where X = F, Cl, or OH. Upon full incorporation (i.e., replacing all column ions by sulfide ions), S 2− is positioned in the anion column at z = 0.5 (halfway between the mirror planes at z = ¼ and z = ¾) in the energy-optimized structure. The calculated energies for partial incorporation of S 2− demonstrate that in an energy-optimized structure, S 2− is displaced from the mirror plane at z = ¼ or ¾, by 1.0 to 1.6 A, depending on the surrounding species (F − , Cl − , or OH − ); however, the probability for S 2− to be incorporated into the apatite structure is highest for chlorapatite end-members. Our results describe energetically feasible incorporation mechanisms for all three oxidations states of S (S 6+ , S 4+ , S 2− ) in apatite, along with structural distortion and concurring electronic structure changes. These observations are consistent with recently published experimental results (Konecke et al. 2017) that demonstrate S 6+ , S 4+ , and S 2− incorporation into apatite, where the ratio of S 6+ /∑S in apatite is controlled by oxygen fugacity ( f O 2 ). The new computational results coupled with published experimental data provide the basis for using S in apatite as a geochemical proxy to trace variations in oxygen fugacity of magmatic and magmatic-hydrothermal systems.

Copper diffusion plays an important role in natural processes, such as metal transport during the formation of magmatic-hydrothermal porphyry-type ore deposits and Cu isotope fractionation during tektite formation. Copper diffusion data in natural silicate melts, however, are limited. In this study, chalcocite (Cu 2 S) “dissolution” experiments were carried out using chalcocite-rhyolite diffusion “couples” to study Cu (and S) diffusion in rhyolitic melts. Instead of chalcocite dissolution as initially expected, our experiments show that Cu is transferred from the chalcocite crystal to the rhyolitic melt, and Fe is transferred from the rhyolitic melt to chalcocite, whereas the S concentration profile in the rhyolitic melt is essentially flat. From the Cu and Fe exchange profiles in the rhyolitic melts, Cu diffusivities and Fe diffusivities are obtained and reported. Copper diffusivity in rhyolitic melts containing 0.10 to 5.95 wt% H 2 O at temperatures of 750 to 1391 °C and pressures of 0.5 to 1.0 GPa can be described as: D Cu Rhy = exp [ - ( 14.75 ± 0.35 ) - ( 0.23 ± 0.10 ) w - ( 11647 ± 491 ) - ( 698 ± 117 ) w T ] , which allows the estimation of an activation energy for diffusion in dry rhyolitic melts to be 96.8 ± 4.1 kJ/mol. In the above equation, diffusivity ( D ) is in m 2 /s, T is the temperature in K, w is the H 2 O concentration in the rhyolitic melts in wt% and all errors reported are at 1σ level. Combining Cu diffusion data from this study and previous data in basaltic melt gives a general equation for Cu diffusivity in natural silicate melts: D cu = exp [ - ( 17.3 ± 0.9 ) + ( 3.8 ± 1.5 ) ( Si + Al-H ) - ( 4403 ± 1094 ) + ( 9700 ± 1921 ) ( Si + Al-H ) T ] , where Si+Al-H is the cation mole fraction of Si plus Al minus H in the silicate melt on a wet basis. Iron diffusivities obtained in this study, in anhydrous to 6 wt% H 2 O rhyolite, are combined with previous data to get a general equation for Fe diffusion in rhyolitic melts: D Fe Rhy = exp [ - ( 16.1 ± 1.7 ) - ( 19859 ± 2541 ) - ( 1218 ± 135 ) w T ] . Our data demonstrate that Cu diffusion is faster than H 2 O or Cl in rhyolitic melts containing 6 wt% water, which indicates that the scavenging and transport of Cu by a magmatic volatile phase during formation of porphyry-type ore deposits is not limited by diffusion of Cu. Based on our experimental data, Cu diffusivity is almost four orders of magnitude higher than Zn in anhydrous rhyolitic melts, which supports the explanation of more diffusive loss of Cu leading to more fractionated Cu isotopes than Zn isotopes in tektites.

The Cortez Hills breccia zone, a Nevada Carlin-type gold deposit located in the Cortez district on the Battle Mountain-Eureka trend, formed within a high-grade polylithic breccia, and grades at the center of the breccia zone are locally in excess of 1 oz/ton Au. Sample transects from low grade or below detection ( Sb, and diminished Hg and Tl compared to the ore stage. Evolved ore-stage fluids coincidently precipitated sulfosalt minerals—initially, unusual Cu- and Hg-rich aktashite (Cu6Hg3As4S12) and Tl- and Hg-rich christite (TlHgAsS3) and, later, realgar as Cu and Zn in hydrothermal fluids—and chalcopyrite and sphalerite in the host rocks, respectively, were consumed. Calcite-only veins indicate termination of the hydrothermal system. Small fragmented clasts of realgar with chemically distinct Au-bearing, evolved ore-stage pyrite rims found in brecciated rock along a faulted dike-Wenban contact provide the first evidence in a Carlin-type gold deposit of precipitation of an Au-bearing pyrite rim on a late ore-stage mineral. These rare rims likely formed in response to fault movement along the dike-Wenban contact or by dike injection that allowed deep, Au-bearing fluids to access a higher and cooler part of the system where late ore-stage minerals had begun to form. These rims further highlight the normal textures in Carlin-type gold deposits where ore-stage pyrite was overgrown by late ore-stage sulfosalt minerals, demonstrating a single hydrothermal event followed by a single cooling event. This paragenesis is consistent with previously modeled deposit formation time frames of ~45,000 to 15,000 years. Two styles of breccia were identified within the deposit. Replacement, matrix-supported breccia, in which an illite-rich, punky matrix encloses Au-bearing pyrite, jasperoid, and variable relict calcite, formed during intense ore-stage alteration and mineralization. The fine-grained ore and alteration replacement minerals did not require significant open space to form. Later evolved and late ore-stage sulfosalt minerals and calcite that are visible at the hand-sample scale provide the earliest evidence for the development of significant open space. Hydrothermal collapse allowed the incursion of cooler meteoric water into the system, which—along with the retrograde solubility of calcite—provided coupled mechanisms driving significant calcite dissolution, collapse brecciation, and formation of the Cortez Hills breccia zone. Dissolution and collapse brecciation increasingly concentrated ore-stage pyrite as the hydrothermal system transitioned to the late ore stage, contributing to high Au grades.

We present δ56Fe (56Fe/54Fe relative to standard IRMM-014) data from whole rock and magnetite of the Upper and Upper Main Zones (UUMZ) of the Bushveld Complex. With it, we assess the role of fractional crystallization in controlling the Fe isotopic evolution of a mafic magma. The UUMZ evolved by fractional crystallization of a dry tholeiitic magma to produce gabbros and diorites with cumulus magnetite and fayalitic olivine. Despite previous experimental work indicating a potential for magnetite crystallization to drastically change magma δ56Fe, we observe no change in whole rock δ56Fe above and below magnetite saturation. We also observe no systematic change in whole rock δ56Fe with increasing stratigraphic height, and only a small variation in δ56Fe in magnetite separates above magnetite saturation. Whole rock δ56Fe (errors twice standard deviation, ±2σ) throughout the UUMZ ranges from -0.01 ±0.03‰ to 0.21 ±0.09‰ (δ56FeaverageWR = 0.10 ±0.09‰; n=21, isotopically light outlier: δ56FeWR = -0.15‰), and magnetites range from 0.28 ±0.04‰ to 0.86 ±0.07‰ (δ56FeaverageMgt = 0.50 ±0.15‰; n=20), similar to values previously reported for other layered intrusions. We compare our measured δ56FeWR to a model that incorporates the changing normative mineralogy, calculated temperatures, and published fractionation factors of Fe-bearing phases throughout the UUMZ and produces δ56FeWR values that evolve only in response to fractional crystallization. Our results show that the Fe isotopic composition of a multiply-saturated (multiple phases on the liquidus) magma is unlikely to change significantly during fractional crystallization of magnetite due to the competing fractionation of other Fe-bearing cumulus phases. This article is protected by copyright. All rights reserved.