ALBERT

Description: We present the first systematic characterization of the Fe isotope composition of magmatic pyrrhotite for the application of Fe isotopes to the origin and evolution of magmatic sulfide deposits. Iron isotopes provide constraints on redox, temperature, fluid exsolution, fractional crystallization, and intermineral diffusion at magmatic or subsolidus conditions. Paired with S isotopes, Fe isotopes have proven to be useful tracers of crustal contamination in bulk magmatic sulfide. Relative and specific Fe isotope values exist for major Fe-bearing igneous minerals; however, there is a paucity of well-constrained Fe isotope data for sulfides. Recent studies indicate that pyrrhotite will strongly influence the Fe isotope systematics of magmatic systems, yet there are few published δ56Fe values (56Fe/54Fe in the sample relative to IRMM-14) of natural pyrrhotite. To provide this fundamental information, we report the first dataset for magmatic pyrrhotite from nine deposits of various origins and ages. The δ56Fe value of pyrrhotite samples (n = 17) ranges from –0.55 ± 0.04‰ to +0.05 ± 0.03‰, and reflects the composition of the sulfide, variable degrees of assimilation, and crystallization history of each deposit. Only the impact-related Sudbury deposit shows especially light δ56Fe values for pyrrhotite (–0.89 ± 0.04‰; (–0.62 ± 0.04‰; n = 2). A modeling approach using these new data confirms the potentially strong influence of pyrrhotite on the Fe isotope systematics of a magmatic system. A global dataset of pure individual sulfide minerals will aid in more accurately tracing the processes at play in the formation and evolution of deposits containing magmatic sulfide.

Description: 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.

Description: 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.

Description: 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. © 2017. American Geophysical Union. All Rights Reserved.

Description: Iron-oxide apatite (IOA) deposits are mined for iron (Fe) and can also contain economically exploitable amounts of Cu, P, U, Ag, Co, and rare earth elements (REE). Recently, it has been proposed based on trace element zonation in magnetite grains from the Los Colorados Kiruna-type IOA deposit, Chile, that ore formation is directly linked to a magmatic source. The model begins with the crystallization of magnetite microlites within an oxidized volatile-rich (H2O+Cl) andesitic magma reservoir, followed by decompression, nucleation of fluid bubbles on magnetite microlite surfaces, segregation of a Fe-Cl-rich fluid-magnetite suspension within the magma reservoir, and subsequent ascent of the suspension from the magma chamber via pre-existing structurally enhanced dilatant zones that act as conduits. Emplacement and precipitation of the suspension results in the formation of magnetite grains with core-to-rim features that record a transition from purely igneous to magmatic-hydrothermal conditions within IOA deposits. Here we test this model by using in situ femtosecond laser ablation MC-ICP-MS measurements of Fe isotopes to determine grain-to-grain and intra-grain Fe isotope variations in magnetite grains from the Los Colorados IOA deposit. All in situ δ56Fe values (56Fe/54Fe relative to IRMM-14) plot within the magmatic range (0.06 to 0.50‰), in agreement with previously published bulk Fe isotope analyses in magnetite from the Los Colorados IOA deposit. Different trace element signatures of these magnetite grains indicate an igneous or magmatic-hydrothermal origin, respectively. Although data partly overlap, the assigned igneous magnetites yield on average higher δ56Fe values (0.24 ± 0.07‰; n = 33), when compared to magmatic-hydrothermal magnetites (0.15 ± 0.05‰; n = 26). Some magnetite grains exhibit a distinct core-to-rim trend from higher toward lower δ56Fe signatures. Furthermore, the δ56Fe of the igneous magnetites correlate negatively with trace elements contents typical for igneous formation (Ti, Al, Ga, V, Mn, Zn); igneous magnetites become isotopically heavier with decreasing concentrations of these elements, indicating a trend toward higher δ56Fe in the magnetite with magma evolution. Model calculations of the δ56Fe evolution in melt, magnetite, and fluid further constrain the magmatic-hydrothermal origin of Kiruna-type IOA deposits.

Description: Mineralized quartz veins within Phanerozoic orogenic belts provide important insights into fluid source and transport, source(s) of S, and genetic implications for ore deposit formation. Here, investigations on mineralized veins hosted by the Hog Mountain tonalite, southernmost Appalachians, were performed using scanned electron microscopy, microprobe analysis, and secondary ion mass spectrometry. The small, reduced Hog Mountain tonalite hosts three different mineralized vein types dominated by Au-bearing quartz veins with a simple base metal sulfide assemblage and spatially associated Au phases (electrum 〉 native gold 〉 maldonite) with Bi phases (native bismuth 〉 unnamed Bi3Te ≈ hedleyite 〉 bismuthinite). In contrast, barren to low-grade arsenopyrite and sphalerite-pyrite-arsenopyrite-galena-chalcopyrite (Zn-Fe-As-Pb-Cu) veinlets are rare. Base metal sulfides are stoichiometrically homogeneous with the exception of sphalerite and galena that have noticeable enrichments in Fe and Cd, and Bi and Ag, respectively. Trace element concentrations (Au, Ag, Bi, Te, Se, and transition metals) in major and minor base metal sulfides are generally low. In situ sulfur isotope analyses on base metal sulfides from different vein types show two distinct populations: (1) δ34Ssulfide = 12.9 ± 1.4‰ in low-grade arsenopyrite and Zn-Fe-As-Pb-Cu veinlets and (2) δ34Ssulfide = 7.7 ± 0.9‰ in mineralized quartz veins. Based on mineral chemistry, two fluid phases were responsible for metal deposition at Hog Mountain. Fluid phase I with low-soluble Au formed rare, barren to low-grade arsenopyrite and Zn-Fe-As-Pb-Cu veinlets from highly acidic, relatively high temperature fluid with moderate ƒS2 and variable ƒO2. In contrast, a reduced, near-neutral, low ƒS2 fluid phase II had higher soluble Au concentrations and created the dominant Au-bearing quartz veins in which Au was deposited via scavenging by Bi melts syngenetic at lower temperatures. Isotopic modeling shows that both fluid phases sourced their S from the metasedimentary rocks hosting the tonalite and adjacent, coeval gold deposits, with the tonalite contributing some S to fluid phase II, resulting in lower δ34S values. Our results are consistent with mineral assemblage, mineral chemistry, and sulfur source(s) from other intrusion-hosted Phanerozoic orogenic gold deposits and support the syngenetic Au scavenging model by Bi melts as a viable Au deposition process in orogenic gold deposits.