Editorial for Special Issue “Footprints of Mineral Systems with IOCG, IOA and Affiliated Critical Metal Deposits: From Metasomatism to Metamorphism”

Metasomatic iron and alkali–calcic (MIAC) systems can form iron oxide copper–gold (IOCG), rare-earth (REE) iron oxide–apatite (IOA), and affiliated critical and precious metal deposits. The affiliated deposits include iron sulfide copper–gold (ISCG), polymetallic skarns, Fe-rich Au-Co-Bi, albitite-hosted U or Au ± Co, and five-element vein deposits. MIAC systems form along fluid-flow pathways through the upper crust regardless of the host rocks, which can be sedimentary, volcanic, plutonic (felsic, intermediate, mafic, or ultramafic), or metamorphic. The diagnostic alteration, facies, and breccias associated with these ore systems reflect the regional chemical and textural modification of the host rocks. This modification is the result of progressive and efficient metasomatic fluid–rock reactions. A holistic mineral system approach is necessary to interpret the observed variety of alteration and mineralization types. MIAC systems are generally investigated using a multidisciplinary approach. The multidisciplinary approach involves meso- and macro-scale morphological observations, mineralogy, petrography, and geochemistry. The different examples of deposits provide effective vectors to mineralization and address critical issues in exploring for these deposits globally by documenting alteration facies, paragenetic sequences, mineral chemistry, structural controls, and geophysical and geochemical signatures, as well as framing them into ore genetic models and deposit classes.

This Special Issue, entitled “Footprints of Mineral systems with IOCG, IOA and Affiliated Critical Metal Deposits: From Metasomatism to Metamorphism”, includes five contributions related to the footprints of minerals systems with ICG, IOA, and affiliated critical metal deposits. These mineral deposits were investigated using field observations and several geochemical methods, such as the determination of major and trace element composition and mineralogy. The first two contributions focus on the importance of field observations in such complex systems (Contribution 1) and on ontology, taxonomy, and lexicons to better structure geological data acquisition (Contribution 2). The next two contributions study the geochemistry of mineral phases that may concentrate nickel and/or platinum group elements in IOCG deposits (Contribution 3) or nickel and cobalt from arsenides in five-element veins (Contribution 4). Finally, the last paper (Contribution 5) presents the characteristics of MIAC systems in terms of alteration facies applicable on a global scale, refining the classification of MIAC-related mineral deposit types and deposit classes. The main features of these articles are reported in the summary below.

The paper by Mumin and Hamilton (Contribution 1) examines the iron oxide copper–gold (IOCG) mineral system in the Mazenod Lake area, located in the Great Bear Lake Magmatic Zone of the Northwest Territories, Canada. This region features extensive exposure of coalescing hydrothermal systems, enabling a detailed analysis of the relationships between geology, structure, alteration, and mineralization. Intermediate and mafic volcanic rocks are altered by weak but ubiquitous metasomatism. Hydrothermal alteration includes various styles such as potassic, sodic, silicification, skarn, and iron oxide–apatite. These processes occurred during negative tectonic inversion in the Wopmay Orogen, generating oblique rift basins and extensive structures unique to the GBMZ. Significant hydrothermal centers are located along crustal faults, highlighting their key role in mineralization.

The paper by Corriveau et al. (Contribution 2) develops an ontology for MIAC systems. The metasomatic iron–alkali–calcic (MIAC) mineral system forms regional-scale metasomatic footprints in the upper crust, genetically associated with apatite–iron oxide (IOA), copper–gold with iron oxides and sulfides (IOCG, ISCG), skarn deposits, and critical and precious metals. These systems develop through specific alteration facies, which may precipitate distinct deposit types or host deposits formed at later stages. Understanding the spatial and temporal relationships between alteration facies, host rocks, and magmatic, tectonic, and mineralization events is essential for assessing global mineral prospectivity. This paper proposes ontology, taxonomy, and descriptive lexicons to structure the collection of geological data on MIAC systems, illustrated by an application developed by the Geological Survey of Canada. These tools are adapted to regional and deposit geological mapping, as well as drill core description, to support metallogenic mapping and exploration strategies for critical and valuable resources.

The paper by Campo Rodriguez et al. (Contribution 3) investigates the enrichment in nickel (Ni) and platinum group elements (PGEs) from the Jatobá IOCG deposit. Enrichment in PGEs is rare in IOCG deposits but is observed in some deposits in the Carajás mineral province, Brazil. The Jatobá deposit, located in the southern part of the Carajás copper belt, displays such mineralization. This detailed study explores the genesis of the Ni-PGE ores at Jatobá. The sulfides, mainly pyrite, contain Ni and measurable concentrations of palladium and platinum. Several platinum group minerals, such as merenskyite and sperrylite, are identified, often localized in fractures and pyrite grain boundaries. Two genetic models are proposed, including the initial precipitation of Ni-PGE sulfides in mafic rocks, followed by remobilization by IOCG-type fluids or the leaching of Ni and PGE from ultramafic rocks by these fluids. This discovery underscores the potential for new Ni-PGE-enriched deposits in the Carajás region and in similar settings.

The paper by Somarin et al. (Contribution 4) describes arsenides and sulfarsenides from the Port Radium U-Cu-Ni-Co-Ag deposit, located in the Great Bear Lake Magmatic Zone of the Northwest Territories, Canada. The Echo Bay and Port Radium volcano sedimentary sequence formed a regional metasomatic mineral system that developed a wide variety of mineralization styles (IOCG, polymetallic skarns, and five-element veins). This system precipitated various hydrothermal alteration facies, ranging from albitic to magnetite–actinote–apatite, potassic ± albitic, and phyllic and propylitic. Rare-to-minor sulfide minerals, formed during this alteration, characterize the primary MIAC system. The main ore mineralization occurred during the epithermal phase of the MIAC system, when various arsenide/sulfarsenide minerals formed. Epithermal veins are spatially associated with magnetic anomalies due to the high magnetite content, which can be used as an exploration tool. In addition, micro-analytical data and compositional variations in individual minerals are presented, shedding further light on the complex mineralogy of the Fe-Co-Ni-As-S mineralization. Three unique minerals are also presented. The results of this research will not only contribute to understanding the complex mineralogy of the Port Radium vein mineralization but may also help define tools to facilitate the exploration of similar deposits in the region and around the world.

The paper of Corriveau and Montreuil (Contribution 5) presents the characteristics of MIAC systems in terms of alteration facies applicable on a global scale. Here, the authors adopt Skirrow et al.’s [1] mappable criteria for IOCG deposits and extend their capabilities to interpret mineral potential for the range of deposit types within MIAC systems, with an emphasis on the field geology of alteration facies. To achieve this, this review article (1) reinforces the initial definition and concept of an MIAC system; (2) refines the classification of MIAC-related mineral deposit types and deposit classes; (3) synthesizes the shared geological attributes and spatio-temporal relationships of their alteration facies within the framework of a mineral system; and (4) adapts the ore deposit model for IOA and IOCG deposits to the larger MIAC system. The energy engine and fluid and metal sources of mineral systems are also discussed from a field geology perspective.

Overall, the contributions published in the Special Issue of Minerals, entitled “Footprints of Mineral systems with IOCG, IOA and Affiliated Critical Metal Deposits: From Metasomatism to Metamorphism”, have confirmed the ability of field observations, mineralogy, and geochemistry to integrate and deepen the knowledge obtained from the studies of mineral systems with IOCG, IOA, and affiliated critical metal deposits. We are particularly pleased to present contributions related to the importance of acquiring and structuring data geological data from field data, the importance of the growth of metals critical in MIAC systems, and a global classification from the mineral deposits that can be encountered in these systems.

As the Guest Editors of this Special Issue, we hope you enjoy reading these varied and excellent contributions. Our sincere thanks go to the Minerals staff for their help and guidance and to all reviewers for their hard work.