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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 deposits are a globally important source of copper, gold and critical commodities. However, they possess a range of characteristics related to a variety of tectono-magmatic settings that make development of a general genetic model challenging. Here we investigate micro-textural and compositional variations in actinolite, to constrain the thermal evolution of the Candelaria iron oxide-copper-gold deposit in Chile. We identify at least two mineralization stages comprising an early 675–800 °C iron oxide-apatite type mineralization overprinted by a later copper-rich fluid at around 550–700 °C. We propose that these distinct stages were caused by episodic pulses of injection of magmatic-hydrothermal fluids from crystallizing magmas at depth. We suggest that the mineralisation stages we identify were the result of temperature gradients attributable to changes in the magmatic source, rather than variations in formation depth, and that actinolite chemistry can be used as a proxy for formation temperature in iron oxide-copper-gold systems. 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 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.

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.

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.

Magnetite (Fe3O4) is an iron ore mineral that is globally mined especially for steel production. It is denser (5.15 g/cm3) than Earth’s crust (~2.7 g/cm3) and is expected to accumulate at the bottom of melt-rich magma reservoirs. However, recent studies revealed heterogeneous fluid bubble nucleation on oxide minerals such as magnetite during fluid degassing in volcanic systems. To test if the attachment on fluid bubbles is strong enough to efficiently float magnetite in silicate magma, decompression experiments were conducted at geologically relevant magmatic conditions with subsequent annealing to simulate re-equilibration after decompression. The results demonstrate that magnetite-bubble pairs do ascend in silicate melt, accumulating in an upper layer that grows during re-equilibration. This outcome contradicts the paradigm that magnetite must settle gravitationally in silicate melt.

Experiments to constrain cassiterite dissolution kinetics in rhyolitic melts with 0.1–5.9 wt% H2O were conducted at 1023–1373 K and 0.5 GPa in a piston-cylinder apparatus. Care was taken to minimize convection in the experimental charge. Tin diffusivity was found to be concentration dependent. The diffusion profiles were fit well by assuming that tin diffusivity depends exponentially on tin concentration, which is roughly linearly related to SiO2 or SiO2 + Al2O3 concentration. Tin diffusivity was found to increase with temperature following the Arrhenius relation, with activation energy decreasing from 161 kJ/mol in dry rhyolite to 93 kJ/mol in hydrous rhyolite containing 5.9 wt% H2O. Tin diffusivity increases exponentially with H2O concentration, by about 3.2 orders of magnitude at 1123 K from 0 to 6 wt% H2O, and decreases exponentially with SiO2 concentration, by about 0.7 orders of magnitude when SiO2 concentration increases by 10 wt%. The equation to describe Sn2 + diffusivity in rhyolitic to dacitic melt (64–76 wt% SiO2) at about 0.5 GPa is: ln D Sn = − 18.194 + 0.17 76 – C SiO 2 − 19418 − 1389 w / T , where DSn is in m2/s, CSiO2 and w are SiO2 and H2O concentrations in wt%, and T is in Kelvin. The solubility of cassiterite (or tin concentration at cassiterite saturation) in rhyolitic melts increases strongly with increasing temperature.

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 (δ56Fe, 56Fe/54Fe relative to IRMM-14) of magnetite from the Chilean deposits are: Los Colorados, δ56Fe (±2σ) = 0.08 ± 0.03‰ to 0.24 ± 0.08‰; El Laco, δ56Fe = 0.20 ± 0.03‰ to 0.53 ± 0.03‰; Mariela, δ56Fe = 0.13 ± 0.03‰. The O isotopic composition (δ18O, 18O/16O relative to VSMOW) of the same Chilean magnetite samples are: Los Colorados, δ18O (±2σ) = 1.92 ± 0.08‰ to 3.17 ± 0.03‰; El Laco, δ18O = 4.00 ± 0.10‰ to 4.34 ± 0.10‰; Mariela, δ18O = (1.48 ± 0.04‰). The δ18O and δ56Fe 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., δ56Fe 0.06–0.49‰ and δ18O = 1.0–4.5‰), consistent with a high-temperature origin for Chilean IOA deposits. Additionally, minimum formation temperatures calculated by using the measured Δ18O 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 δ56Fe 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 δ56Fe 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.

The Chahgaz iron oxide-apatite (IOA) deposit is one of the main IOA deposits in the Bafq metallogenic province, Central Iran. The Chahgaz mineral deposit is hosted by Early Cambrian felsic to intermediate, altered subvolcanic to effusive rocks that range compositionally from granite to diorite. Geochemical, geochronologic and tectonomagmatic investigations of various host rock types in the Bafq province indicate that mineralization was the product of Early Cambrian active continental margin processes that evolved calc-alkaline felsic igneous rocks followed by formation of diabase dykes in a back-arc basin environment. Magnetite is present in massive magnetite-rich ore bodies and veinlets that cut the massive ore bodies. Detailed macro- and micro-scopic characterization of mineralized samples and host rocks reveals a paragenetic sequence containing three generations of magnetite that are distinguished from one another compositionally and texturally. The massive ores contain apatite in trace amounts, consistent with IOA deposits globally, and locally exhibit textures that are visually similar to lava flow structures, as described for the El Laco IOA deposit, Chile. The ore bodies contain miarolitic cavities that are filled by calcite, hematite and quartz. The host rocks for the Chahgaz deposit have undergone widespread hydrothermal metasomatism including Na-Ca, K-, Mg-, Si-, sericitic, argillic and carbonatization alteration. The compositions of two generations of magnetite, referred to as Mag1 and Mag2, in massive ore overlap compositions reported for igneous and high-temperature magmatic-hydrothermal magnetite. The third generation of magnetite, which is referred to as Mag3 and is present in veinlets cross-cutting the massive magnetite ore bodies, overlaps compositions reported for low to moderate temperature magmatic-hydrothermal magnetite. Pyrite is present as disseminated grains coeval with Mag1 and micro-fracture filling in the massive magnetite-rich ore bodies. The δ18O values obtained for magnetite from representative samples of massive magnetite Mag1 ore range between 2.18 and 6.32‰ and are consistent with δ18O values reported for igneous and magmatic-hydrothermal magnetite from other deposits in the Bafq district and globally. The δ34S values for pyrite range from 22.54 to 24.94‰ and are consistent with an evaporitic sulfur source; plausibly by magma contamination with evaporitic rocks of the Early Cambrian Volcano-Sedimentary Sequence (ECVSS). The data presented here are consistent with formation of the massive magnetite-rich ore bodies in the Chahgaz IOA deposit by an iron-rich magmatic-hydrothermal fluid.

Magnetite is the most important iron ore in iron oxide-apatite (IOA) deposits which represent the Cu-poor end-member of the iron oxide-copper–gold (IOCG) clan. Magnetite chemistry has been used as a petrogenetic indicator to identify the geological environment of ore formation and as a fingerprint of the source reservoir of iron. In this study, we present new textural and microanalytical EPMA and LA-ICP-MS data of magnetite from Carmen, Fresia, Mariela and El Romeral IOA deposits located in the Cretaceous Coastal Cordillera of northern Chile. We also provide a comprehensive summary and discussion of magnetite geochemistry from Andean IOAs including Los Colorados, Cerro Negro Norte, El Romeral (Chilean Iron Belt) and the Pliocene El Laco IOA deposit located in the Central Volcanic Zone of the Chilean Andes. Microtextures coupled with geochemical data were used to define and characterize the occurrence of different magnetite types. Magnetite exhibits a variety of textural features including oscillatory zoning, colloform banding, re-equilibration textures, exsolution lamellae and symplectites. The magmatic vs. hydrothermal origin of the different magnetite types and the evolution of IOA deposits can be assessed using diagrams based on compatible trace elements. However, magnetite is very susceptible to hydrothermal alteration and to both textural and compositional re-equilibration during magmatic and superimposed hydrothermal events. Based on the data presented here, we conclude that V and Ga are possibly the most reliable compatible elements in magnetite to trace ore-forming processes in the Andean IOA deposits. Magnetite chemistry reveals different conditions/events of formation for each IOA deposit ranging from high-temperature, low-oxygen fugacity (ƒO2), purely magmatic (>600 °C) conditions; to lower temperature and higher ƒO2 magmatic-hydrothermal (300–600 °C) to low-temperature hydrothermal (

Precious metals and gems were among the first minerals to be valued by civilization and they have kept their attraction for millennia. Even in their raw state, gems and precious metals are easily recognizable as truly special and beautiful. Although other minerals, such as pyrite ( fool's gold ), might be confused for gold, it is rare to confuse gold for another mineral. Its rich color and luster are simply unique. The same is true for the brilliant play of light and color from gemstones. On top of it all, precious metals and gems are rare. We even use special nomenclature to discuss their abundances. Gold and silver abundances are commonly reported in terms of the troy ounce, which equals about 31 g; prices of precious metals are commonly quoted in troy ounces, and we have used them (referred to here as ounces) to describe the metal endowment of individual deposits. Weights of diamonds and other precious gems are measured in an even smaller unit, the carat, which amounts to 0.2 g. Because these non-metric units are widely understood, we use them in parts of this discussion. In spite of their rarity, precious metals and gems are a big business, with world production worth almost $200 billion (Figure 11.1). Gold is the most important of the group, with silver, diamonds, platinum-group elements (PGEs), and the colored gems emerald, ruby, and sapphire following in that order. Gold production increased in the 1990s, but has declined annually since 2000 in all countries except China. Global production of PGEs increased by more than 1,000% from 1960 to 2008, and plateaued thereafter. Production of gem diamonds increased by almost 1,400% at the start of the twenty-first century, dropped slightly during the economic crisis of 2008–2010, and leveled off after that. Mine production of industrial diamonds, which are discussed further in Chapter 13, increased from 1960 to 2008, although with cyclicity, but fell dramatically after 2008 owing to the proliferation of synthetic diamonds for industrial applications.

Magnetite from hydrothermal ore deposits can contain up to tens of thousands of parts per million (ppm) of elements such as Ti, Si, V, Al, Ca, Mg, Na, which tend to either structurally incorporate into growth and sector zones or form mineral micro- to nano-sized particles. Here, we report micro- to nano-structural and chemical data of hydrothermal magnetite from the Los Colorados iron oxide–apatite deposit in Chile, where magnetite displays both types of trace element incorporation. Three generations of magnetites (X–Z) were identified with concentrations of minor and trace elements that vary significantly: SiO2, from below detection limit (bdl) to 3.1 wt%; Al2O3, 0.3–2.3 wt%; CaO, bdl–0.9 wt%; MgO, 0.02–2.5 wt%; TiO2, 0.1–0.4 wt%; MnO, 0.04–0.2 wt%; Na2O, bdl–0.4 wt%; and K2O, bdl–0.4 wt%. An exception is V2O3, which is remarkably constant, ranging from 0.3 to 0.4 wt%. Six types of crystalline nanoparticles (NPs) were identified by means of transmission electron microscopy in the trace element-rich zones, which are each a few micrometres wide: (1) diopside, (2) clinoenstatite; (3) amphibole, (4) mica, (5) ulvospinel, and (6) Ti-rich magnetite. In addition, Al-rich nanodomains, which contain 2–3 wt% of Al, occur within a single crystal of magnetite. The accumulation of NPs in the trace element-rich zones suggest that they form owing to supersaturation from a hydrothermal fluid, followed by entrapment during continuous growth of the magnetite surface. It is also concluded that mineral NPs promote exsolution of new phases from the mineral host, otherwise preserved as structurally bound trace elements. The presence of abundant mineral NPs in magnetite points to a complex incorporation of trace elements during growth, and provides a cautionary note on the interpretation of micron-scale chemical data of magnetite.

Iron oxide copper-gold (IOCG) and Kiruna-type iron oxide-apatite (IOA) deposits are commonly spatially and temporally associated with one another, and with coeval magmatism. Here, we use trace element concentrations in magnetite and pyrite, Fe and O stable isotope abundances of magnetite and hematite, H isotopes of magnetite and actinolite, and Re-Os systematics of magnetite from the Los Colorados Kiruna-type IOA deposit in the Chilean iron belt to develop a new genetic model that explains IOCG and IOA deposits as a continuum produced by a combination of igneous and magmatic-hydrothermal processes. The concentrations of [Al + Mn] and [Ti + V] are highest in magnetite cores and decrease systematically from core to rim, consistent with growth of magnetite cores from a silicate melt, and rims from a cooling magmatic-hydrothermal fluid. Almost all bulk δ 18 O values in magnetite are within the range of 0 to 5‰, and bulk δ 56 Fe for magnetite are within the range 0 to 0.8‰ of Fe isotopes, both of which indicate a magmatic source for O and Fe. The values of δ 18 O and δD for actinolite, which is paragenetically equivalent to magnetite, are, respectively, 6.46 ± 0.56 and-59.3 ± 1.7‰, indicative of a mantle source. Pyrite grains consistently yield Co/Ni ratios that exceed unity, and imply precipitation of pyrite from an ore fluid evolved from an intermediate to mafic magma. The calculated initial 187 Os/ 188 Os ratio (Osi) for magnetite from Los Colorados is 1.2, overlapping Osi values for Chilean porphyry-Cu deposits, and consistent with an origin from juvenile magma. Together, the data are consistent with a geologic model wherein (1) magnetite microlites crystallize as a near-liquidus phase from an intermediate to mafic silicate melt; (2) magnetite microlites serve as nucleation sites for fluid bubbles and promote volatile saturation of the melt; (3) the volatile phase coalesces and encapsulates magnetite microlites to form a magnetite-fluid suspension; (4) the suspension scavenges Fe, Cu, Au, S, Cl, P, and rare earth elements (REE) from the melt; (5) the suspension ascends from the host magma during regional extension; (6) as the suspension ascends, originally igneous mag-netite microlites grow larger by sourcing Fe from the cooling magmatic-hydrothermal fluid; (7) in deep-seated crustal faults, magnetite crystals are deposited to form a Kiruna-type IOA deposit due to decompression of the magnetite-fluid suspension; and (8) the further ascending fluid transports Fe, Cu, Au, and S to shallower levels or lateral distal zones of the system where hematite, magnetite, and sulfides precipitate to form IOCG deposits. The model explains the globally observed temporal and spatial relationship between magmatism and IOA and IOCG deposits, and provides a valuable conceptual framework to define exploration strategies.

The Andean IOCG clan in the Coastal Cordillera of northern Chile, comprises the iron oxide-Cu-Au sensu stricto, the iron oxide-apatite (IOA), and the stratabound Cu(− Ag) deposits, also known as Manto-type Cu(− Ag) deposits. IOCGs and Manto-type deposits constitute the second source of copper in Chile, after porphyry Cu-Mo systems, whereas IOAs are an important source of iron. Regardless of their economic importance, little is known about their formation and contrasting genetic models have been proposed i.e., an origin from magmatic-hydrothermal fluids exsolved from a cooling intrusion, or formation from non-magmatic basinal brines heated by plutonic bodies. Here we report Re-Os data for five IOCG deposits: Candelaria, Mantoverde, Casualidad, Diego de Almagro, and Barreal Seco, three IOAs: Los Colorados, El Romeral, Carmen, and two Manto-type deposits: Altamira and Franke. Calculated Re-Os ages for some of these deposits are consistent with previously published geochronological data, and indicate an Early to Late Cretaceous age. The calculated ages for the Altamira Manto-type Cu deposit yield a ~ 90 Ma age, indicating that the Cretaceous Manto-type Cu belt of central Chile extends further north than previously thought. Additionally, the calculated initial Os ratios using reported ages show that IOCGs have a more radiogenic signature (2.0-2.4) than IOAs (1.0-1.2) and Manto-type Cu deposits (~ 1.1). A comparison with previously published Os initial ratios for the Mantoverde (0.2) and Candelaria (~ 0.4) IOCGs suggests that the mineralization in Chilean IOCGs is the result of different events with metals derived from diverse sources i.e., the magmatic-hydrothermal system and basement/host rocks. Rhenium and Os concentrations for the various studied deposits vary over a wide range of values at the ppb and ppt level, respectively, with many sulfides and magnetite bearing appreciable no common Os. The Re and Os contents in sulfides (pyrite, chalcopyrite) from IOCGs are low, ranging from 2 to 149 ppb Re and up to 49 ppt Os. Pyrite from IOA deposits show Re and Os concentrations ranging from 10.8 to 214 ppb Re, and Os < 156 ppt, whereas magnetite has lower Re and Os contents (0.7-129 ppb Re, 0.2-3.6 ppt Os). However, rhenium and Os concentrations in magnetite might be attributed to small sulfide inclusions. Chalcocite from Manto-type Cu deposits shows the widest range and highest Re and Os concentrations; 15 to 253 ppb and 34 to 639 ppt, respectively. The Os present in all these deposits is dominated by radiogenic 187Os. A comparison of Re and Os data from different deposit types, show that Andean IOCG and IOA deposits have similar characteristics to Chilean porphyry Cu-Mo systems with low to moderate Re content (< 250 ppb), low Os concentration (< 300 ppt total Os) dominated by radiogenic Os (> 90%Osr), and variable initial Os ratios. Our data supports the idea that Andean IOCG and IOA deposits formed by magmatic-hydrothermal processes with little involvement of basinal brines. In contrast, published Re-Os information from IOCGs in China and Australia argue for the involvement of a much more radiogenic crustal source possibly related to non-magmatic oxidized saline brines that leached basement rocks. We conclude that Andean IOCG (and IOA) deposits in northern Chile were formed mainly by magmatic-hydrothermal processes related to the formation and emplacement of plutonic rocks with moderate contribution from leaching of basement and/or volcanic host rocks. Surface or basin-derived brines as well as sediments appear to play only a minor role in the formation of the Chilean IOCG clan.

Re-Os analyses of pyrite from the Pueblo Viejo high-sulfidation epithermal deposit (both Monte Negro and Moore ore zones) yield an isochron age of 111.9 ± 3.7 Ma and initial 187 Os/ 188 Os ratio of 0.22 ± 0.18 (mean square of weighted deviates = 84, n = 21). Isochrons grouped by pyrite setting (layered versus vein) or locations yield ages within uncertainty of the overall isochron and with each other and, as such, do not yield temporally distinguishable ages. Sphalerite vein samples as well as shale source rocks show evidence of open system behavior and loss of Re and therefore are not included in the above regression. These results confirm that the Pueblo Viejo deposit is Early Cretaceous in age and coeval with the volcanic Los Ranchos Formation, which hosts the deposit. Recent paleontological age assignments for the Hatillo Formation, which overlies the Los Ranchos Formation, show that it is only slightly younger, opening the possibility that mineralization in the Hatillo Formation could also be related to late stages of Los Ranchos magmatism. Whether these revised Los Ranchos-Hatillo relations provide sufficient cover to account for the paragenetically late formation of pyrophyllite at Pueblo Viejo remains unclear. Recognition that Pueblo Viejo is coeval with the Los Ranchos Formation shows that large-scale, high-sulfidation epithermal mineralization can form from basal island-arc tholeiitic magmatism. A review of available experimental data confirms that these magmas can generate hydrothermal solutions of the necessary composition; favorability of the Los Ranchos Formation might have been enhanced by its bimodal composition. These results suggest that exploration for epithermal precious metal deposits should include basal island-arc tholeiite terranes.

Mutnovsky Volcano, located in Kamchatka, Russia, is a young volcano that has formed a series of four overlapping stratocones over its approximately 80 ka history. Erupted products at Mutnovsky range in composition from basalts to dacites; basalts are the most common. In this study, melt inclusions from representative samples of all erupted compositions from all four eruptive centers were analyzed to investigate the causes of the compositional heterogeneity, melt evolution, and pre-eruptive magma dynamics. Melt inclusions from Mutnovsky were sampled in olivine, plagioclase, orthopyroxene, and clinopyroxene. The melt inclusion data represent a wide range of melt compositions, from basalt through rhyolite. Geochemical modeling of melt inclusion data, combined with field evidence and chemical zoning of plagioclase phenocrysts, indicates that fractional crystallization and magma mixing produced the range of erupted bulk rock compositions. The measured variability of melt inclusion compositions in each host mineral phase indicates that different host minerals trapped unique melts that evolved separately from one another. The melt inclusion data suggest that individual melt portions evolved by fractional crystallization, perhaps in different magma chambers, within the Mutnovsky plumbing system, and were mixed prior to eruption. Our data do not indicate whether the mixing events were the cause of eruption or are simply the manifestation of the eruption process.

Porphyry type ore deposits sometimes contain fluid inclusion compositions consistent with the partitioning of copper and gold into vapor relative to coexisting brine at the depositional stage. However this has not been reproduced experimentally at magmatic conditions. In an attempt to determine the conditions under which copper and gold may partition preferentially into vapor relative to brine at temperatures above the solidus of granitic magmas we performed experiments at 800 degrees C 100 MPa oxygen fugacity (f(O2)(sys)) buffered by Ni NiO and a(S2)(sys) fixed at either 3.5 x 10( 2) by using intermediate solid solution pyrrhotite or 1.2 x 10( 4) by using intermediate solid solution pyrrhotite bornite. The coexisting vapor (similar to 3 wt. NaCl eq.) and brine (similar to 68 wt. NaCl eq.) were composed initially of NaCl + KCl + HCl + H2O with starting HCl set to < 1000 mu g/g in the aqueous mixture. Synthetic vapor and brine fluid inclusions were trapped at run conditions and subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry (LA ICP MS). Our experiments demonstrate that copper and gold partitioned strongly into the magmatic volatile phase(s) (MVP) (i. e. vapor or brine) relative to a silicate melt over the entire imposed range of a(S2)(sys). Nernst style partition coefficients between coexisting brine (b) and melt (m) D b/m (+/ 1 sigma) range from 3.6(+/ 2.2) x 10(1) to 4(+/ 2) x 10(2) for copper and from 1.2(+/ 0.6) x 10(2) to 2.4(+/ 2.4) x 10(3) for gold. Partition coefficients between coexisting vapor (v) and melt D v/m range from 2.1 +/ 0.7 to 18 +/ 5 and 7(+/ 3) x 10(1) to 1.6(+/ 1.6) x 10(2) for copper and gold respectively. Partition coefficients for all experiments between coexisting brine and vapor D b/v (+/ 1 sigma) range from 7(+/ 2) to 1.0(+/ 0.4) x 10(2) and 1.7(+/ 0.2) to 15(+/ 2) for copper and gold respectively. Observed average D b/v at an a(S2)(sys) of 1.2 x 10( 4) were elevated 95(+/ 5) and 15 +/ 1 for copper and gold respectively relative to those at the higher a(S2)(sys) of 3.5 x 10 (2) where D b/v were 10(+/ 5) for copper and 7(+/ 6) for gold. Thus there is an inverse relationship between the a(S2)(sys) and the D b/v for both copper and gold with increasing a(S2)(sys) resulting in a decrease in the D b/v signifying increased importance of the vapor phase for copper and gold transport. This suggests that copper and gold may complex with volatile S species as well as Cl species at magmatic conditions however none of the experiments of our study at 800 degrees C and 100 MPa had a D b/v

We have performed experiments to evaluate Au solubility in natural, water-saturated basaltic melts as a function of oxygen fugacity. Experiments were carried out at 1000 °C and 200 MPa, and oxygen fugacity was controlled at the fayalite-magnetite-quartz (FMQ) oxygen fugacity buffer and FMQ + 4. All experiments were saturated with a metal-chloride aqueous solution loaded initially as a 10 wt% NaCl eq. fluid. The stable phase assemblage at FMQ consists of basalt melt, olivine, clinopyroxene, a single-phase aqueous fluid, and metallic Au. The stable phase assemblage at FMQ + 4 consists of basalt melt, clinopyroxene, magnetite-spinel solid solution, a single-phase aqueous fluid, and metallic Au. Silicate glasses (i.e., quenched melt) and their contained crystalline material were analyzed by using both electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Measured Au concentrations in the quenched melt range from 4.8 μg g−1 to 0.64 μg g−1 at FMQ + 4, and 0.54 μg g−1 to 0.1 μg g−1 at FMQ. The measured solubility of Au in olivine and clinopyroxene was consistently below the LA-ICP-MS limit of detection (i.e., 0.1 μg g−1). These melt solubility data place important limitations on the dissolved Au content of water-saturated, Cl- and S-bearing basaltic liquids at geologically relevant fO2 values. The new data are compared to published, experimentally-determined values for Au solubility in dry and hydrous silicate liquids spanning the compositional range from basalt to rhyolite, and the effects of melt composition, oxygen fugacity, pressure and temperature are discussed.

The Eocene epoch in the Great Basin of western North America was a period of profuse magmatism and hydrothermal activity. During that period, the Carlin-type gold deposits in Nevada were produced, Earth’s second largest concentration of gold after deposits in South Africa. The characteristics of the Carlin-type deposits have been documented, but a widely acceptable explanation for their genesis is outstanding. Here we integrate microanalyses of ore minerals, experimental data that describe metal partitioning, and published age and isotopic data, to suggest that the gold is sourced from magma. We relate gold deposition to a change from shallow subduction to renewed magmatism and the onset of extension. We propose that upwelling asthenosphere impinged on a strongly modified subcontinental lithospheric mantle, generating magmas that released gold-bearing fluids at depths of 10 to 12 km. The rising aqueous fluids with elevated hydrogen sulphide concentrations and a high ratio of gold to copper underwent phase changes and mixed with meteoric water. Within a few kilometres of the surface, the fluids dissolved and sulphidized carbonate wall rocks, leading to deposition of gold-bearing pyrite. We conclude that the large number and size of Carlin-type deposits in Nevada is the result of an unusual convergence of a specific geologic setting, together with a tectonic trigger that led to extremely efficient transport and deposition of gold.

We have performed experiments to constrain the effect of sulfur fugacity (fS2) and sulfide saturation on the fractionation and partitioning behavior of Pt, Pd and Au in a silicate melt–sulfide crystal/melt–oxide–supercritical aqueous fluid phase–Pt–Pd–Au system. Experiments were performed at 800 C, 150 MPa, with oxygen fugacity (fO2) fixed at approximately the nickel–nickel oxide buffer (NNO). Sulfur fugacity in the experiments was varied five orders of magnitude from approximately log fS2 = 0 to log fS2 = � 5 by using two different sulfide phase assemblages. Assemblage one consisted initially of chalcopyrite plus pyrrhotite and assemblage two was loaded with chalcopyrite plus bornite. At run conditions pyrrhotite transformed compositionally to monosulfide solid solution (mss), chalcopyrite to intermediate solid solution (iss), and in assemblage two chalcopyrite and bornite formed a sulfide melt. Run-product silicate glass (i.e., quenched silicate melt) and crystalline materials were analyzed by using both electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. The measured concentrations of Pt, Pd and Au in quenched silicate melt in runs with log fS2 values ranging from approximately 0.0 to � 5.0 do not exhibit any apparent dependence on fS2. The measured Pt, Pd and Au concentrations in mss do vary as a function of fS2. The measured Pt, Pd and Au concentrations in iss do not appear dependent on fS2. The data suggest that fS2, working in concert with fO2, via the determinant role that these variables play in controlling the magmatic sulfide phase assemblage and the solubility of Pt, Pd and Au as lattice bound components in magmatic sulfide phases, is a controlling factor on the budgets of Pt, Pd and Au during the evolution of magmatic systems.

We have performed five separate sets of experiments to elucidate the effects of magnetite, ulvospinel-magnetite solid solution, and pyrrhotite crystallization on the budgets of Au, Cu, and Ag at magmatic conditions. The experiments were done in both hydrous and anhydrous assemblages at temperatures between 800 and 1050 °C, pressures from ambient to 140 MPa, log f O2 from NNO-0.25 to NNO, and log f S2 from −1.5 to −3.0. Nernst-type partition coefficients (±1σ) at 800 °C in a water-saturated assemblage are D AgMt/melt = 2 × 10−4 ± 2 × 10−9, D CuMt/melt = 0.82 ± 0.69, D CuUsp/melt = 26 ± 17, D AuUsp/melt = 50 ± 31, D CuPo/melt = 174 ± 25. Nernst-type partition coefficients (±1σ) at 1050 °C in an anhydrous assemblage are D CuPo/melt ≥ 200, D AgPo/melt = 58 ± 8, D AuPo/melt = 120 ± 50. The calculated values for D AuUsp/melt and D CuUsp/melt indicate that the addition of Ti to magnetite increases significantly the Au- and Cu-scavenging potential of ulvospinel relative to end-member magnetite. Partition coefficients for Cu and Au between pyrrhotite and melt indicate that a temperature change from 1050 to 800 °C in an anhydrous and hydrous assemblage, respectively, results in no observable change in Cu partitioning. The calculated partition coefficients are used to model the effect of crystal fractionation on the concentrations of Ag, Cu, and Au. Model results suggest that the co-crystallization of magnetite and pyrrhotite sequester no more than 2% Ag, 7% Cu, and 37% Au from the melt over the first 25% solidification. If the melt reaches volatile saturation after 25% crystallization, the presence of end-member magnetite and pyrrhotite do not appear to inhibit the Cu-, Au-, and Ag-ore potential of the magma. Ulvospinel-magnetite, however, may reduce the Au concentration in the melt by approximately one-third relative to its initial value that decreases the overall Au available to partition into the volatile phase.

The partitioning of silver in a sulfur-free rhyolite melt–vapor–brine assemblage has been quantified at 800 °C, pressures of 100 and 140 MPa and f O 2 ≈ NNO (nickel–nickel oxide). Silver solubility (±2σ) in rhyolite increases 5-fold from 105 ± 21 to 675 ± 98 μg/g as pressure increases from 100 to 140 MPa. Nernst-type partition coefficients ( D Ag i , j ± 2 σ ) describing the mass transfer of silver at 100 MPa between vapor and melt, brine and melt and vapor and brine are 32 ± 30, 1151 ± 238 and 0.026 ± 0.004, respectively. At 140 MPa, values for D Ag i , j ( ± 2 σ ) for vapor and melt, brine and melt, and vapor and brine are 32 ± 10, 413 ± 172 and 0.06 ± 0.03, respectively. Apparent equilibrium constant values (±2σ) describing the exchange of silver and sodium between vapor and melt, K Ag , Na v / m , at 100 and 140 MPa are 105 ± 68 and 14 ± 6. The average values (±2σ) for silver and sodium exchange between brine and melt, K Ag , Na b / m , at 100 and 140 MPa are 313 ± 288 and 65 ± 12. These data indicate that the mass transfer of silver from rhyolite melt to an exsolved volatile phase(s) is enhanced at 100 MPa relative to 140 MPa, suggesting that decompression increases the silver ore-generative potential of an evolving silicate magma. Model calculations using the new data suggest that the evolution of low-density, aqueous fluid (i.e., vapor) may be responsible for the the silver tonnage of many porphyry-type and perhaps epithermal-type ore deposits. For example, Halter et al. (Halter W. E., Pettke T. and Heinrich C. A. (2002) The origin of Cu/Au ratios in porphyry-type ore deposits. Science 296, 1842–1844) used detailed silicate and sulfide melt inclusion and vapor and brine fluid inclusions analyses to estimate a melt volume on the order of 15 km3 to satisfy the copper budget at the Bajo de la Alumbrera copper-, gold-, silver-ore deposit. Using their melt volume estimate with the data presented here, model calculations for a 15-km3 felsic melt, saturated with pyrrhotite and magnetite, suggest that a low-salinity magmatic vapor may scavenge on the order of 7 × 1012 g of silver from the melt. This quantity of silver exceeds the discovered 2 × 109 g of Ag at Alumbrera. Calculated tonnages for numerous other deposits yield similar results. The excess silver in the vapor, remaining after porphyry formation, is then available to precipitate at lower PTconditions in the stratigraphically higher epithermal environment. These data suggest that silver, and perhaps other ore metals, in the porphyry-epithermal continuum may be derived solely from the time-integrated flux of dominantly low-salinity vapor exsolved from a series of sequential magma batches.

Iron oxide-apatite (IOA) and iron oxide-copper-gold deposits (IOCG) are often spatially and temporally related with one another and with coeval magmatism. However, a genetic model that accounts for observations of natural systems remains elusive, with few observational data able to distinguish among working hypotheses that invoke meteoric fluid, magmatic-hydrothermal fluid, and immiscible melts. Here, we use high-resolution trace element concentrations in magnetite, hematite and pyrite, high-precision Fe and O stable isotope data of magnetite and hematite grains, δD of magnetite and actinolite, and Re and Os in magnetite and pyrite from the Los Colorados IOA and Candelaria and Mantoverde IOCG deposits in the Chilean Iron Belt to elucidate the origin of IOA and IOCG systems. At Los Colorados, Ti, V, Al, and Mn are enriched in magnetite cores and decrease systematically from core to rim, a trend consistent with magmatic and/or magmatic-hydrothermal magnetites. High Co/Ni ratios of pyrite from Los Colorados are also consistent with a magmatic-hydrothermal origin. δD values for magnetite and actinolite indicate a mantle source for H. Values of d56Fe and d18O for magnetite and hematite from all deposits indicate a magmatic source for Fe and O. The Re-Os systematics overlap data from Andean porphyry Cu-Mo deposits and are consistent with a magmatic-hydrothermal origin. Together, the data are consistent with a genetic model wherein 1) magnetite cores crystallize from silicate melt; 2) these magnetite crystals are nucleation sites for aqueous fluid that exsolves and scavenges Fe, P, S, Cu, Au from silicate melt; 3) the magnetite-fluid suspension is less dense that the surrounding magma, allowing ascent; 4) as the suspension ascends, magnetite grows in equilibrium with the fluid and takes on a magmatic-hydrothermal character (i.e., lower Al, Mn, Ti, V); 5) during ascent, magnetite, apatite and actinolite are deposited to form IOA deposits; 6) the further ascending fluid transports Fe, Cu, Au and S toward the surface where hematite, magnetite and sulfides precipitate to form IOCG deposits. This model is globally applicable and explains the observed temporal and spatial relationship between magmatism and formation of IOA and IOCG deposits.

The partitioning of As and Au between rhyolite melt and low-salinity vapor (2 wt% NaCl eq.) in a melt–vapor–Au metal ± magnetite ± pyrrhotite assemblage has been quantified at 800 °C, 120 MPa and f O 2 = NNO . The S-bearing runs have calculated values for the fugacities of H2S, SO2 and S2 of log f H 2 S = 1.1 , log f SO 2 = - 1.5 , and log f S 2 = - 3.0 . The ratio of H2S to SO2 is on the order of 400. The experiments constrain the effect of S on the partitioning behavior of As and Au at magmatic conditions. Calculated average Nernst-type partition coefficients (±1σ) for As between vapor and melt, D As v / m , are 1.0 ± 0.1 and 2.5 ± 0.3 in the S-free and S-bearing assemblages, respectively. These results suggest that sulfur has a small, but statistically meaningful, effect on the mass transfer of As between silicate melt and low-salinity vapor at the experimental conditions. Efficiencies of removal, calculated following Candela and Holland (1986) , suggest that the S-free and S-bearing low-salinity vapor can scavenge approximately 41% and 63% As from water-saturated rhyolite melt, respectively, during devolatilization assuming that As is partitioned into magnetite and pyrrhotite during second boiling. The S-free data are consistent with the presence of arsenous acid, As(OH)3 in the vapor phase. However, the S-bearing data suggest the presence of both arsenous acid and a As–S complex in S-bearing magmatic vapor. Apparent equilibrium constants, log K As ′ ( ± 1 σ ) , describing the partitioning of As between melt and vapor are −1.3 (0.1) and −1.1 (0.1) for the S-free and S-bearing runs, respectively. The increase in the value of K As ′ with the addition of S suggests a role for S in complexing and scavenging As from the melt during degassing. The calculated vapor/melt partition coefficients (±1σ) for Au between vapor and melt, D Au v / m , in S-free and S-bearing assemblages are 15 ± 2.5 and 12 ± 0.3, respectively. Efficiencies of removal ( Candela and Holland, 1986 ) for the S-free melt, calculated assuming that magnetite is the dominant Au-sequestering solid phase during crystallization ( Simon et al., 2003 ), suggest that magmatic vapor may scavenge on the order of 72% Au from a water-saturated melt. Efficiencies of removal calculated for the S-bearing assemblage, assuming pyrrhotite and magnetite are the dominant Au-sequestering solid phases, indicate that vapor may scavenge on the order of 60% Au from the melt. These model calculations suggest that the loss of pyrrhotite and magnetite from a melt, owing to punctuated differentiation during ascent and emplacement, does not prohibit the ability of a rhyolite melt to generate a large-tonnage Au deposit. Apparent equilibrium constants describing the partitioning of Au between melt and vapor were calculated using the mean D Au v / m values for the S-free and S-bearing assemblages; only S-bearing data from runs longer than 400 h were used as shorter runs may not have reached equilibrium with respect only to vapor/melt partitioning of Au. The values for log K Au ′ ( ± 1 σ ) are −4.4 (0.1) and −4.2 (0.2) for the S-free and S-bearing runs, respectively. These data suggest that the presence of S does not affect the mass transfer of Au from degassing silicate melt to an exsolved, low-salinity vapor in a low- f S 2 assemblage (i.e., pyrrhotite–magnetite at NNO) at the experimental conditions reported here. Efficiencies of removal are calculated and used to model the mass transfer of Au from a crystallizing silicate melt to an exsolved, low-salinity vapor phase. The calculations suggest that the model, absolute tonnage of Au scavenged and transported by S-free and S-bearing vapors, from a crystallizing melt, would be comparable and that the time-integrated flux of low-salinity vapor could be responsible for a significant quantity of the Au in magmatic-hydrothermal ore deposits.

The effect of sulfur on the partitioning of Cu in a melt–vapor–brine ± magnetite ± pyrrhotite assemblage has been quantified at 800 � C, 140 MPa, fO2 = nickel–nickel oxide (NNO), log fS2 ¼� 3:0 (i.e., on the magnetite–pyrrhotite curve at NNO), log fH2S ¼� 1:3 and log fSO2 ¼� 1. All experiments were vapor + brine saturated. Vapor and brine fluid inclusions were trapped in silicate glass and self-healed quartz fractures. Vapor and brine are dominated by NaCl, KCl and HCl in the S-free runs and NaCl, KCl and FeCl2 in S-bearing runs. Pyrrhotite served as the source of sulfur in S-bearing experiments. The composition of fluid inclusions, glass and crystals were quantified by laser-ablation inductively coupled plasma mass spectrometry. Major element, chlorine and sulfur concentrations in glass were quantified by using electron probe microanalysis. Calculated Nernst-type partition coefficients (±2r) for Cu between melt– vapor, melt–brine and vapor–brine are D v=m Cu ¼ 63 � 31, D b=m Cu ¼ 240 � 80, and D v=b Cu ¼ 0:27 � 0:10, respectively, in the S-free system. The partition coefficients (±2r) for Cu between melt–vapor, melt–brine and vapor–brine are D v=m Cu ¼ 316 � 22, D b=m Cu ¼ 443 � 68, and D v=b Cu ¼ 0:69 � 0:16, respectively, in the S-bearing system. Apparent equilibrium constants (±1r) describing Cu and Na exchange between vapor and melt and brine and melt were also calculated. The values of K v=m Cu;Na are 34 ± 21 and 128 ± 29 in the S-free and S-bearing runs, respectively. The values of K b=m Cu;Na are 33 ± 22 and60 ± 5 in the S-free and S-bearing runs, respectively. The data presented here indicate that the presence of sulfur increases the mass transfer of Cu into vapor from silicate melt. Further, the nearly threefold increase in D v=b Cu suggests that Cu may be transported as both a chloride and sulfide complex in magmatic vapor, in agreement with hypotheses based on data from natural systems. Most significantly, the data demonstrate that the presence of sulfur enhances the partitioning of Cu from melt into magmatic volatile phases. � 2006 Published by Elsevier Inc.

The decompression rates (r) and the amount and types of volatile species (H2O + Cl ± CO2 ± S) are crucial parameters for the physical behavior of ascending magmas because they affect the formation of crystals and bubbles, as well as bulk viscosities of volcanic suspensions and their eruptive styles (e.g., effusive vs. hazardous explosive eruptions). However, the roles of CO2, S and Cl, in addition to H2O, on decompression induced crystallization (DIC) are still poorly understood. In this study, we investigated the DIC of a trachybasaltic magma, as a function of volatile content (H2O ≈ 5 wt.%; Cl ≈ 0.7 wt.%; ± CO2 ≈ 2000 ppm; ± S ≈ 3000 ppm) and r. Isothermal decompression experiments were conducted at T = 1030 °C and log(fO2) ≈ QFM + 2 (QFM: quartz–fayalite–magnetite buffer) by releasing pressure (P) continuously from 300 to 70 MPa at r = 0.01, 0.1 and 1 MPa/s. The phase assemblages at 300 MPa, before the onset of decompression were composed of 91 to 99 area% melt/glass, 2 to 9 area% clinopyroxene, ≪ 1 area% spinel and ≪ 1 area% bubbles; with higher cpx proportions in the CO2-bearing samples (i.e., at lower water activity). We compare our experimental results with numerical models of equilibrium degassing (using SolEx and DCompress), phase assemblages (using MELTS) and with literature data on phase stabilities in S-bearing and S-free systems. These comparisons reveal that near-equilibrium conditions are reached in all three investigated systems if decompressed at the lowest rate of 0.01 MPa/s before quenching. The crystallization of clinopyroxene during decompression at 0.1 and 0.01 MPa/s is strongly enhanced by the presence of S, whereas spinel shows less significant variations. Plagioclase, biotite and olivine only occur in the S-bearing samples decompressed at a r of 0.01 MPa/s. At r = 0.01 MPa/s (near-equilibrium) a crystallinity of ~ 60 area% was reached in the S-bearing system, while the S-free systems are characterized by a ~ 20 area% crystallinity. The strong influence of S on the crystallinity is mainly explained by an increasing thermal stability of clinopyroxene during decompression to 70 MPa, which is probably due to the lower water activity in the presence of S at P ≤ 70 MPa and the increase of the liquidus T with decreasing water content in the melt (i.e., with decreasing P at volatile saturated conditions). Even though the viscosity of the S-bearing melt is relatively low during decompression, the increasing modal abundance of clinopyroxene (and other phases) can lead to a strong increase of the effective magma viscosity and, thus, can slow down magma ascent within the conduit and limit the escape of bubbles. Subsequently, the magma may either i) solidify at shallow depths or ii) new magma inputs and/or a P increase in the conduit owing to second boiling can induce an explosive eruption. Hence, the obtained experimental results indicate that S-bearing basaltic arc magmas have a higher chance to erupt explosively than S-poor basalts, especially if the ascent rate is relatively low.

Feldspars and normative feldspar constituents of bulk magma show complexes and ‘anorogenic’ granites, is associated with hotspot and continental rift environments. A major chartrends supportive of fractional crystallization in the three main types of hy-normative intraplate suites that contain qz-oversaturated acteristic of this magmatism is the production of suites of compositionally diverse rocks that are temporally and rocks: ocean island and continental alkalic suites, ocean island tholeiitic suites and continental tholeiitic suites. These suites are spatially associated. Much is still unknown about the processes that have led to the diversity of magmas within characterized by the presence of a single feldspar in each suite member, a shift of this feldspar from plagioclase to alkali feldspar, such suites or to the global diversity of intra-plate suite types. and K enrichment of alkali feldspar with decreasing temperature in the trachytic members. The modal feldspars provide evidence for a Many ocean island and continental hotspot (and continental rift) related suites are characterized by rocks with reaction relationship between feldspars and indicate a build-up of magmatic volatile content towards saturation with progressive high alkali/silica ratios that fall into the alkalic field of Irvine & Baragar (1971; Fig. 1) regardless of their state fractionation of a parental magma having low initial volatile content. The feldspar and normative feldspar evolutionary paths are unique of silica saturation. Such alkalic suites range from those that are strongly silica undersaturated, containing rocks for each of the three suite types but similar for different suites within the same type. This characteristic extends to the felsic members, such as nephelinites (or leucitites in the more potassic making it easy to distinguish between rhyolitic or granitic rocks suites), to mildly silica-undersaturated suites [the Kenfrom the different suite types. The feldspars in natural volcanic nedy series of Miyashiro (1978)] containing alkali basalts, suites commonly show evidence for a polybaric history, particularly silica-undersaturated trachytes and phonolites, and, in the least-evolved suite members. Late-stage feldspars of the finally, to hy-normative suites [the Coombs trend of intermediate members and feldspars of the most evolved members Miyashiro (1978)] that include olivine basalts, trachytes show paths indicative of significantly lower temperature and pressure and rhyolites. Hy-normative alkalic suites that contain regimes. silica-oversaturated rocks on ocean islands are widespread and include those from Ascension Island (e.g. Harris, 1983), the Samoan Islands (MacDonald, 1968), Terceira, Azores (e.g. White et al., 1979) and Clarion Island, Mexico

Although studies have proposed that iron oxide-apatite (IOA) deposits may represent the deeper roots of some Andean iron oxide copper-gold (IOCG) systems, their genetic links remain obscure and controversial. A key question when considering an integrated genetic model is whether a magmatic-hydrothermal fluid that precipitates massive magnetite will continue transporting significant amounts of dissolved Fe, Cu, and Au after IOA precipitation. Here we provide new geochemical data for accessory pyrite from the Los Colorados IOA deposit in the Chilean iron belt that confirm the role of this sulfide as a relevant repository for economic metals in IOA deposits. Pyrite occurs at Los Colorados as disseminated grains and as veinlets associated with magnetite and actinolite that postdate the main igneous magnetite stage. Electron probe microanalysis (EPMA) data for pyrite show anomalously high Co and Ni concentrations (up ~3.9 and ~1.5 wt %, respectively) and relatively high As contents (100s of ppm to a maximum of ~2,000 ppm). When combined with results from secondary ion mass spectrometry (SIMS) spot analyses, pyrite data show significant amounts of Cu that range from sub-ppm values (~100 ppb) up to 1,000s of ppm, plus nonnegligible concentrations of Zn, Pb, Cd, Sb, Se, and Te (up to ~100 ppm). The highest contents of Cu measured (wt % level) most likely record the presence of Cu-bearing submicron-sized mineral inclusions. Contents of Au and Ag are up to ~1 and 10 ppm, respectively, with maximum concentrations that can rise up to ~800 ppm Au and ~300 ppm Ag due to the presence of submicron-sized inclusions. The high Co/Ni ratios of pyrite from Los Colorados are consistent with a magmatic-hydrothermal origin associated with a greater mafic affinity, compared to pyrite from porphyry Cu deposits. Furthermore, the geochemical signature of Los Colorados pyrite shares important similarities of composition and microtexture with the few published data for pyrite from IOCG deposits (e.g., Ernest Henry, Australia, and Manto Verde, Chile). These findings, combined with recent geochemical and isotopic studies that support an igneous origin for the dike-shaped magnetite orebodies at Los Colorados, point to a magmatic source of mafic to intermediate composition for the contained metals, and support the hypothesis that IOA systems can source Fe-Cu-Au-rich fluids. Based on experimental studies, these IOA-derived fluids may continue transporting significant amounts of metals to form IOCG mineralization at shallower levels in the crust.