Nanoinclusions in zoned magnetite from the Sossego IOCG deposit, Carajás, Brazil: Implication for mineral zoning and magnetite origin discrimination

doi:10.1016/j.oregeorev.2021.104453

Ore Geology Reviews

Volume 139, Part A, December 2021, 104453

https://doi.org/10.1016/j.oregeorev.2021.104453 Get rights and content

Highlights

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Chemical heterogeneity in zoned magnetite was revealed by nanogeochemistry.
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Four types of nanoinclusions include amphibole, ilmenite, pyroxene, and Si-rich magnetite.
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Ilmenite was formed by oxy-exsolution of ulvöspinel, other nanominerals were formed by supersaturation + entrapment.
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Compositional zoning in magnetite likely formed by a self-organization process.
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Nanometer mineral assemblages record high-temperature magmatic-hydrothermal process at Sossego.

Abstract

Compositional zoning is common in magnetite from different geological environments, but its formation mechanism remains controversial. Here, we characterize micron- to nano-scale textural and chemical variations in zoned magnetite from the Sossego iron oxide-copper–gold deposit (Carajás, Brazil) using electron probe microanalyzer (EPMA) and transmission electron microscopy (TEM). Lack of porosity and of a reaction front at both the micron and nanometer scales indicates that compositional zoning in magnetite is a pristine texture formed during crystal growth, rather than a secondary texture due to dissolution and reprecipitation reaction. TEM energy dispersive X-ray spectrometry analyses and mapping identify four types of nanoinclusions in zoned magnetite: 1) Mg-Fe-Al silicates, most likely amphibole, 2) Fe-Ti oxides, mainly ilmenite, 3) pyroxene, 4) Si-rich magnetite. The formation of ilmenite nanoinclusions in Sossego magnetite is possibly due to oxy-exsolution of ulvöspinel from Ti-rich magnetite, whereas nanoinclusions of other minerals likely formed by local supersaturation in the fluid boundary layer, followed by magnetite crystal entrapment. Compositional zoning in magnetite likely formed by a self-organization process where fluid composition fluctuations are feedback responses for an evolving fluid system far from equilibrium, rather than cyclic variations in external factors such as temperature and oxygen fugacity. Saturation of silicate minerals results in relative depletion of Si, Ca, and Al in boundary layer fluids, and formation of inclusion-poor zone depleted in these elements. Nanoinclusions in magnetite highlight the importance of textural characterization when using in situ chemical composition to discriminate the origin of magnetite. In addition, the assemblages of nanoinclusions in magnetite can be used to complement the discrimination of magnetite origins.

Graphical abstract

Introduction

The chemical composition of magnetite from a range of rocks and mineral deposits has been widely used to reveal petrogenesis and ore genesis (e.g., Dupuis and Beaudoin, 2011, Dare et al., 2012, Nadoll et al., 2012, Huang et al., 2013, Huang et al., 2019a, Huang et al., 2019b, Knipping et al., 2015, Canil et al., 2016, Canil and Lacourse, 2020), and used as a tool in mineral exploration (e.g., Dupuis and Beaudoin, 2011, Makvandi et al., 2016, Pisiak et al., 2017). However, with increased studies, the empirical diagrams based on trace element contents or ratios of magnetite are considered to have some limitations due to extremely various magnetite chemistry at different scales (Gourcerol et al., 2016, Velasco et al., 2016, Broughm et al., 2017, Wen et al., 2017, Huang et al., 2019b). The various composition of magnetite at the scale of a deposit type, a deposit, or a paragenetic stage, have been ascribed to variations in external factors such as temperature, oxygen fugacity, sulfur fugacity and melt/fluid composition (Dare et al., 2012, Acosta-Gongora et al., 2014, Nadoll et al., 2014, Huang et al., 2016, Huang et al., 2015a, Huang et al., 2015b, Knipping et al., 2015, Liu et al., 2015, Canil et al., 2016, Sun et al., 2017, Li et al., 2018). A magnetite grain of several to hundreds of microns can also show compositional heterogeneity, which has been explained by micro inclusions or chemical zoning in magnetite (Dupuis and Beaudoin, 2011, Huberty et al., 2012, Huang et al., 2014, Xu et al., 2014, Cook et al., 2016, Deditius et al., 2018, Ciobanu et al., 2019, Verdugo-Ihl et al., 2020). However, the mineralogy and distribution of inclusions in magnetite are not well studied, in particular, when these inclusions have nanometer sizes. Zoned magnetite, composed of zones rich or poor in one or several elements, has been found in different types of deposits (Westendorp et al., 1991, Dare et al., 2015, Knipping et al., 2015, Deditius et al., 2018, Ciobanu et al., 2019, Huang and Beaudoin, 2019), and is thus an excellent example for studying element or mineral behavior during crystal growth. Moreover, the detailed textural and compositional studies of zoned magnetite will provide important constraints on their formation mechanism.

In this study, we present electron probe microanalyzer (EPMA) and high-resolution transmission electron microscopy (TEM) analyses of well-studied zoned magnetite from the Sossego iron oxide-copper–gold (IOCG) deposit, Carajás, Brazil. The aim of this study is to: 1) characterize the textural and compositional variation of zoned magnetite; 2) investigate the sitting of trace elements in magnetite; 3) identify the mineralogy and distribution of inclusions (μm to nm scale) in magnetite; and (4) constrain the formation mechanism of inclusion-rich, zoned magnetite. This study highlights that nanometer-scale mineral inclusions are common in zoned magnetite from magmatic-hydrothermal deposits such as IOCG (Ciobanu et al., 2019), IOA (Deditius et al., 2018), and Fe skarns (Yin et al., 2019) and that it is critical to consider these nanometer inclusions to interpret in-situ major and trace element data. The mineral assemblages at the nano-scale have essential implications for the discrimination between igneous and hydrothermal magnetite.

Section snippets

Geology of the Sossego deposit and sample information

Sample in this study was collected from the Sossego Cu-Au deposit in the Carajás Mineral Province of Brazil, one of the world’s largest IOCG metallogenic provinces on Earth (Xavier et al., 2012) (Fig. 1a). The Sossego deposit is hosted by granitic and gabbroic intrusions, and felsic metavolcanic rocks (Monteiro et al., 2008a) (Fig. 1b). The deposit consists of two groups of orebodies, Sequeirinho–Pista–Baiano (SPB) and Sossego–Curral (SC) (Fig. 1b), which have different alteration types and

EPMA analyses

The thin section containing magnetite was examined using optical microscopy and back-scattered electron (BSE) imaging to determine mineral assemblages and microtextures of magnetite grains. High-contrast BSE images and compositional analyses of magnetite were carried out at Université Laval using a CAMECA SX-100 EPMA, equipped with five wavelength-dispersive spectrometers. Analyses were performed using a 5-μm diameter beam, a 15 kV voltage, a current of 20 nA for Fe and 100 nA for minor and

Magnetite petrography and geochemistry

Magnetite grains in the sample 084 are dominated by subhedral crystals with grain size up to 1 mm. The mineral inclusions in magnetite are mainly actinolite, apatite, and chalcopyrite with minor chlorite (Figs. 2c, e, 3a). Magnetite is commonly replaced by chalcopyrite, indicating that magnetite formed before the main Cu mineralization (Fig. 2e). The studied magnetite grain shows oscillatory zoning under high-resolution BSE imaging, and the FIB foil crosscuts the contact between dark gray and

Substitution of trace elements in magnetite

A set of trace elements can be incorporated into magnetite by simple substitution or by coupled substitution. Based on the Goldschmidt’s rule, Mg, Mn, Zn, Co, and Ni can substitute for Fe²⁺ in the octahedral sites and Al, Ga, Cr, and As can substitute for Fe³⁺ in the tetrahedral or octahedral sites (Deer et al., 1992, Dupuis and Beaudoin, 2011, Nadoll et al., 2014).

Elements such as Si, Ca, Na, and K are commonly incompatible in magnetite due to large differences (exceeding ± 15–18% variations)

Conclusions

Zoned magnetite from the Sossego IOCG deposit provides an excellent example for the studies on element incorporation and mineral formation during magnetite growth. Trace elements are incorporated into magnetite by solid solution when fluids are undersaturated in most minerals, but nanoinclusions will precipitate along with magnetite crystallization where silicates and oxides are supersaturated in the boundary layer. The mineral nanoinclusions in zoned magnetite explain the heterogeneous

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank Marc Choquette (Laval U.) for his assistance with EPMA analyses and Hui Yuan and Natalie Hamada (CCEM) for helping FIB-TEM analyses. Yiping Yang (Guangzhou Institute of Geochemistry, CAS) is thanked for interpreting the HRTEM images. This project was funded by “CAS Hundred Talents Program” project (Y9CJ034000) to XWH, the Natural Sciences and Engineering Research Council (NSERC) of Canada, Agnico Eagle Mines Limited, and Ministère de l'Énergie et des Ressources Naturelles du Québec

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This sodic-calcic alteration is characterized by variable amounts of albite and actinolite (e.g., Sequeirinho ore body in the Sossego deposit; Monteiro et al., 2008a; Monteiro et al., 2008b; Xavier et al., 2012) or scapolite. Similar high-temperature alteration assemblages in the Salobo deposit (de Melo et al., 2017; Campo-Rodríguez et al., 2021; Huang and Beaudoin, 2021) were used to suggest emplacement at relatively deep crustal levels (e.g., Réquia et al., 2003). Ferric-calcic (magnetite-apatite-actinolite) or ferric-potassic (biotite-magnetite) alteration zones represent the locus of main copper precipitation controlled by ductile structures.
The Jaguar nickel deposit represents an important and unconventional discovery of hydrothermal Ni resources (58.9 Mt @ 0.95% Ni) associated with magnetite and apatite within the Carajás Mineral Province, Northern Brazil. The Jaguar deposit shares several similarities with iron oxide copper–gold (IOCG) deposits of the Carajás Mineral Province, especially regarding the structural control, and the hydrothermal alteration. Although IOCG and Kiruna-type iron oxide-apatite (IOA) deposits are closely associated in several geological provinces, this association has not yet been fully constrained in the Carajás Mineral Province. The Jaguar deposit occurs near mafic–ultramafic intrusions, along a regional fault zone, and is hosted either by granitic (northern portion) or felsic subvolcanic (southern portion) rocks. It is characterized by sub-vertical zones hosting a biotite-chlorite (Bt-Chl) hydrothermal alteration with ductile structures and disseminated sulfides (pyrite and millerite) and magnetite. These zones are overprinted by sulfide-magnetite-apatite-bearing breccias, which represent the main mineralization bodies. The sulfide assemblage in the mineralized zones is dominated by pyrite, pentlandite and millerite, with minor sphalerite and chalcopyrite.
We have investigated the concentration of trace elements in the magnetite and sulfides from the host rocks, different alteration facies and the mineralization at the Jaguar deposit by LA-ICP-MS. Magnetite composition ranges from higher Al, Mn, Ti and V contents, for at least part of those in host rocks (granitic and felsic subvolcanic) and Bt-Chl alteration, into lower contents in the magnetite-apatite alteration, associated with the main Ni mineralization. Magnetite with higher Ti and V contents in host rocks is associated with discrete hydrothermal alteration pockets, have high Ni/Cr ratios and plot in high temperature hydrothermal fields in multi-element diagrams. Therefore, we support that these features are compatible with a hydrothermal origin. Anomalously high-Ni contents in magnetite point that the Neoarchean mafic–ultramafic layered intrusions of the Carajás Province represent potential Ni sources for hydrothermal remobilization. Anomalously high V contents in magnetite suggest lower fO₂ conditions upon the formation of the deposit. The concentration of trace elements in magnetite associated with the Jaguar deposit is similar to those from other IOCG deposits from the Carajás Province (but with higher V and Ni contents), especially the Sossego mine in the Southern Copper Belt, indicating that it could represent a Ni-rich member of the regional-scale IOCG mineral system at Carajás. Pyrite and chalcopyrite have a composition similar to those from magmatic deposits, when using previously proposed discriminant diagrams, but this is also the case for other IOA and IOCG deposits. However, low PGE contents (Ru, Rh, Ir and Os) in pyrite from the Jaguar deposit support a hydrothermal origin. Our results highlight that the classifications provided by available discriminant diagrams are not unequivocal. We suggest that multi-element diagrams represent a complementary approach to binary plots and provide a more comprehensive classification for the use of indicator minerals.
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The formation of these nano-inclusions further led to the local unsaturation of these trace elements in the magnetite-fluid interface, followed by the formation of nano-inclusions-free zones. This is similar to other hydrothermal deposits worldwide (e.g., Los Colorados IOA deposit; Deditius et al., 2018; Sossego IOCG deposit; Huang and Beaudoin, 2021). Mag-1r has similar Ti contents to Mag-1 m (0.01–0.05 wt%; Appendix II), indicating a possibly stable temperature for the Mag-1r formation.
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Magnetite with oscillatory zoning is commonly interpreted to result from cyclic fluctuations in fluid composition, temperature or oxygen fugacity (Dare et al., 2015; Knipping et al., 2015; Deditius et al., 2018; Huang et al., 2018; Huang and Beaudoin, 2019). Huang and Beaudoin (2021) interpreted oscillatory zoning in magnetite from the Sossego IOCG deposit, Brazil as a result of a self-organization process where variable fluid composition is a feedback response for an evolving fluid system far from equilibrium, rather than the variations in temperature and oxygen fugacity. However, oscillatory zoning in magnetite from the Baishiya iron skarn deposit was interpreted as the product of CDR (Yin et al., 2017; Yin et al., 2019), where replicate formation of Al-rich magnetite, Al-rich nanometer-sized lamellae, and zinc spinel nanoparticles in the reaction front results in the trace element-rich zones.
Multiple generations of magnetite are common in many magmatic-hydrothermal Fe deposits, which can be formed by different mechanisms. This study investigates magnetite from the Alemao IOCG deposit (Brazil) characterized by inclusion-rich cores (Mag-1) and inclusion-poor rims (Mag-2), using electron probe microanalyzer and high-resolution transmission electron microscopy (HRTEM), to understand its formation mechanism and implications for Fe and Cu mineralization. Electron probe microanalysis shows that Mag-1 has Si and Mg contents of 0.28–1.15 wt% and 0.11–0.49 wt%, respectively, higher than Mag-2. Mineral nanoinclusions of minnesotaite, chlorite, quartz, and minor Ti-rich magnetite in the Mag-1 account for the relatively high Si and Mg in the Mag-1, as measured by electron probe microanalysis. The crystal orientation of nanoinclusions relative to the host Mag1 indicates formation by growth entrapment, where they crystallized from boundary layer supersaturated fluids under non-equilibrium conditions.
Identification of a reaction front, spatial coupling of parent and product phases at micro- and nano-meter scales support the formation of Alemao Mag2 by coupled dissolution and reprecipitation (CDR). Variations in fluid composition rather than fluctuations in conditions such as temperature and oxygen fugacity are the factors inducing CDR. A conceptual model is proposed for the formation of Alemao magnetite where Fe-rich fluids formed inclusion-rich primary magnetite and CDR reactions modified the texture and chemical composition of primary magnetite to form a core rich, and a rim poor, in nanoinclusions. During CDR, impurities of Si, Ca, Al, Mn, and Mg and nanoinclusions are removed from the primary magnetite and formed product magnetite with higher Fe. The progressive enrichment of Cu from Mag-1 to Mag-2 during CDR indicates that a Cu-rich fluid related to the Cu-Au mineralization is responsible the formation of Cu-rich Mag-2 and chalcopyrite. Therefore, micron- to nano-scale characterization of magnetite with multiple generations can provide important constraints on fluid evolution in ore system.
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Citation Excerpt :
Hu et al. (2014) considered that Si4+ and Al3+ can enter the magnetite crystal lattice and substitute Fe3+, and divalent elements (e.g., Mg, Mn, and Ca) can enter Si-rich magnetite via charge balance substitution of Si4+ for Fe3+. Huang and Beaudoin (2021) inferred that Si enters magnetite by the coupled substitution of 2Fe3+ ↔ Si4+ + Fe2+, and K can be structurally bonded in magnetite. In Fig. 10a and b, the Dingjiashan and Fengyan magnetite have flat LA-ICP-MS time-resolved signals for Na, K, Ca, Al, Mn, and Si, suggesting that these elements occur mainly in solid solution.
Central Fujian is an important polymetallic (Zn-Pb-Ag-Cu-Mo-Au) mineral province in southeastern (SE) China, yet its mineralization is still disputed between Proterozoic volcanogenic massive sulfide (VMS) or Mesozoic skarn-type. The Dingjiashan and Fengyan are two representative of them, with the ores mainly hosted in the Meso-Neoproterozoic Longbeixi Formation marble and schists. For the magnetite from Dingjiashan and Fengyan, their hydrothermal mineral assemblage and their low V, Al, Cr, Ni, Ga, and HFSEs contents (cf. igneous magnetite) are all typical of hydrothermal origin. LA-ICP-MS time-resolved depth profile and inter-element correlation suggest that: 1) Mn, Ti, V, Ni, and Cr are mainly present in solid solution; 2) Na, Mg, Al, K, Si, and Ca occur both as solid solution and the micro-silicate inclusions and/or halite-bearing fluid inclusions; and 3) Zn, Co, Cu, Ag, Pb, and Bi are intimately associated with sulfide micro-inclusions. For both Dingjiashan and Fengyan magnetite, the Ti, V, Al, Ga, and Mn contents are temperature-controlled, whilst Ti, V, Cr, and Sn contents are redox-controlled. Meanwhile, the Ca and Mg contents are controlled by fluid-rock interactions, and the Mn enrichment is inherited from the fluid composition. Coexisting mineral phases have likely exerted major geochemical control on the magnetite samples. We compiled a large published geochemical dataset on magnetite from well-studied VMS and skarn deposits worldwide. Compared with VMS-hosted magnetite, skarn-hosted magnetite has relatively high median contents for Ca, Al, Ge, Sn, Mn, and Mg, and low median contents for Si, Ga, Ti, V, Ni, and Cr. Overall, the Dingjiashan and Fengyan magnetite samples have similar trace element compositions (esp. Mn and V) to those of skarn-hosted (rather than VMS-hosted) magnetite. Magnetite geochemistry thus suggests that both the Dingjiashan and Fengyan are best classified as stratabound, distal Zn-Pb skarn deposits. Our findings highlight the effectiveness of magnetite trace element geochemistry in differentiating ore deposit types, which contributes to regional Zn-Pb ore exploration and research.

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