Metasomatic Mineral Systems with IOA, IOCG, and Affiliated Critical and Precious Metal Deposits: A Review from a Field Geology Perspective

Abstract

Worldwide, a growing list of critical (Bi, Co, Cu, F, Fe, Mo, Ni, P, PGE, REE, W, U, and Zn) and precious metal (Ag and Au) resources have been identified in mineral systems forming Fe-oxide-copper-gold (IOCG) deposits; Fe-oxide-apatite (IOA); Fe-sulfide Cu-Au (ISCG); and affiliated W skarn; Fe-rich Au-Co-Bi or Ni; albitite-hosted U or Au ± Co; and five-element (Ag, As, Co, Ni, and U) vein deposits. This paper frames the genesis of this metallogenic diversity by defining the Metasomatic Iron and Alkali-Calcic (MIAC) mineral system and classifying its spectrum of Fe-rich-to-Fe-poor and alkali-calcic deposits. The metasomatic footprint of MIAC systems consists of six main alteration facies, each recording a distinct stage of mineralization as systems have evolved. The fluid flow pathways and the thermal and chemical gradients inferred from the space–time distribution of the alteration facies within a system are best explained by the ascent and lateral propagation of a voluminous hypersaline fluid plume. The primary fluid plume evolves, chemically and physically, as metasomatism progresses and through periodic ingresses of secondary fluids into the plume. Exploration strategies can take advantage of the predictability and the expanded range of exploration targets that the MIAC system framework offers, the building blocks of which are the alteration facies as mappable prospectivity criteria for the facies-specific critical and precious metal deposits the systems generate. Global case studies demonstrate that these criteria are applicable to MIAC systems worldwide.

1. Introduction

Metasomatic Iron and Alkali-Calcic (MIAC) systems can form Iron-Oxide Copper-Gold (IOCG), Fe, P, or rare-earth (REE) Iron Oxide-Apatite (IOA), as well as affiliated critical and precious metal deposits. The affiliated deposits include, but are not limited to, Iron Sulfide Copper-Gold (ISCG); Fe, W, and polymetallic Pb-Zn skarn; Fe-rich Ni and Fe-rich Au-Co-Bi; albitite-hosted U or Au ± Co; Fe-rich Au; Fe-poor Mo; Fe-poor Cu; and five-element (U, Ag, Ni, Co, As, ±Cu, and ±Bi) vein deposits (

Table A1. Representative examples of the mineral resources and their deposit types within the MIAC systems of Australia. Districts and provinces are located in Figure 1.

Deposits			Total Resources (Unless Indicated Otherwise) 1
Name	Class	Type
Olympic Cu-Au Province (Volcano-Plutonic Environment with Additional Sedimentary Hosts)
Carrapateena	MI-Cu	IOCG	900 Mt at 0.56% Cu, 0.24 g/t Au, 3 g/t Ag [217]
Oak Dam East	MI-Fe	IOA	560 Mt at 41%–56% Fe, 0.2% Cu, 690 ppm U [132]
Oak Dam West	MI-Cu	IOCG	1340 Mt at 0.66% Cu, 0.33 g/t Au [218]
Olympic Dam 2	MI-Cu	IOCG	9640 Mt at 0.58% Cu, 0.19 kg/t U3O8, 0.26 g/t Au, 1.0 g/t Ag+ 1740 Mt at 1.49% Cu, 0.44 kg/t U3O8, 0.58 g/t Au, 3 g/t Ag [217]
Prominent Hill	MI-Cu	IOCG	162 Mt at 0.94% Cu, 0.81 g/t Au, 3.0 g/t Ag (UG sulfide) [217]
Hillside	MI-Cu	Fe-rich skarn-Cu	337 Mt at 0.56% Cu, 0.14 g/t Au [219]
Cloncurry district and host Mount Isa Province (sedimentary basin with additional volcano-plutonic hosts)
Elaine 1	MI-Cu	Skarn-hosted Cu-Au	26.1 Mt at 0.56% Cu, 0.09 g/t Au [220]
Elaine-Dorothy	MAC-U	Skarn-hosted U-REE	0.83 Mt at 280 ppm U3O8, 3200 ppm TREO [221]
Eloise	MI-Cu	ISCG	4.8 Mt at 2.4% Cu, 0.6 g/t Au [222]
Ernest Henry 2	MI-Cu	IOCG	167 Mt at 1.1% Cu, 0.5 g/t Au (historic, pre-mining) [129]
			Current: 101.5 Mt at 1.25% Cu, 0.73 g/t Au [223]
Jericho	MI-Cu	ISCG	19.2 Mt at 2.0% Cu, 0.4 g/t Au [224]
Eva Cu 3	MI-Cu	IOCG	260.7 Mt at 0.42% Cu, 0.04 g/t Au [225]
Kalman	MI-Cu	ISCG	39.2 Mt at 0.53% Cu, 0.27 g/t Au, 1.5 g/t Ag, 0.01% Mo, 2.1 g/t Re [226]
Mary Kathleen	MAC-U	Skarn-hosted U	9.5 Mt at 1300 ppm U3O8 [227]
Merlin	MAC-Mo	MAC Mo	6.4 Mt at 1.5% Mo, 26 g/t Re (reserves) [95]
Mt Elliot-Swan	MI-Cu	IOCG–ISCG	353.7 Mt at 0.6% Cu, 0.35 g/t Au [228]
Mt Dore	MAC-Cu	Breccia-hosted Cu-Au	110.4 Mt at 0.55% Cu, 0.10 g/t Au, 0.05% Pb, 0.30% Zn [228]
Rocklands	MI-Cu	IOCG	56.7 Mt at 0.64% Cu, 294 ppm Co, 0.15 ppm Au, 5.3% magnetite; 178 Mt at 15% magnetite [229]
Tick Hill	MAC-Au	Albitite-hosted Au	0.706 Mt at 22.52 g/t Au (mined) [131]
Valhalla	MI-U	Albitite-hosted U	34.7 Mt at 830 ppm U3O8 [230]
Other representative deposits
Johnnies Reward	MI-Cu	IOCG (at granulite facies)	2.19 Mt at 0.7 g/t Au, 0.4% Cu [231]
Peko (tailings)	MI-Cu	IOCG	3.75 Mt at 1.14 g/t Au, 0.25% Cu, 0.11% Co, 80% magnetite [232]
Savage River	MI-Fe	IOA	471.8 Mt at 68.3% Fe, 0.04% Ni, 0.71% TiO2, 1.43% MgO, 0.007% P, 0.35% V, 0.08% S [233]
Warrego	MI-Fe	IOCG	6.95 Mt at 2.0% Cu, 6.6 g/t Au, 0.32% Bi [5]

¹ Metals in bold are critical minerals on the Canadian list [234]. ² Case studies studied in more detail by the authors marked. ³ Eva Cu includes the Little Eva, Turkey Creek, Blackard, Scanlan, Bedford, Lady Clayre, and Ivy Ann deposits.

,

Table A2. Representative examples of the mineral resources and their deposit types within the MIAC systems of Canada, the United States, and of a MI-Ni in Brazil. The deposits in Idaho and the NICO deposit are representative deposits formed in a sedimentary-dominant environment. The southeast Missouri district Boss, Pea Ridge, and Pilot Knob deposits, as well as the Sue-Dianne deposit, are representative of deposits formed in a felsic-to-intermediate volcano-plutonic dominant environment. Jaguar is representative of deposits formed in an ultramafic-to-mafic volcano-plutonic dominant environment. Deposits or their districts are located in Figure 1.

Deposits			Total Resources (Unless Indicated Otherwise) 1
Name	Class	Type
United States
Ram/Sunshine/Sunshine East, Idaho	MI-Co	MI-Co	5.77 Mt at 0.44% Co, 0.69% Cu, 0.53 g/t Au [123]
Iron Creek, Idaho	MI-Co	ISi-Co	4.45 Mt at 0.19% Co, 0.73% Cu (indicated), 1.23 Mt at 0.08% Co, 1.34% Cu (inferred) [235]
Boss, SE Missouri	MI-Cu	IOCG	40 Mt at 0.83% Cu, 18% Fe, 0.035% Co (historic) [236]
Pea Ridge, SE Missouri 2	MI-Fe	IOA	160.6 Mt at ~53%–55% Fe; 0.2 Mt at 12% TREE (historic) [100]
Pilot Knob, SE Missouri	MI-Fe	IOA	20 Mt at 35 to 40% Fe (produced) [113]
Pumpkin Hollow, Yerrington district	MI-Cu	IOCG	501.7 Mt at 0.452% Cu, 0.07 g/t Au, 1.85 g/t Ag (open pit, measured and indicated), 25.4 Mt at 0.358% Cu, 0.03 g/t Au, 1.37 g/t Ag (open pit, inferred); 49.1 Mt at 1.39% Cu, 0.17 g/t Au, 3.98 g/t Ag, 17.8% Fe (underground, measured and indicated), 26.5 Mt at 1.09% Cu, 0.10 g/t Au, 2.19 g/t Ag, 12.8% Fe (underground, inferred) [237]
Moonlight-Superior project, California	MI-Cu	IOCG	402.83 Mt at 0.31% Cu, 1.85 g/t Ag, 0.012 g/t Au (measured and indicated); 64.59 Mt at 0.31% Cu, 0.77 g/t Ag, 0.005 g/t Au (inferred) [238]
Coles Hill, Virginia	MAC-U	Albitite-hosted U	119 Mt at 0.056% U3O8 (indicated resources) [239]
Buena Vista	MI-Fe	IOA	232 Mt at 18.6% Fe [240]
Canada 2
NICO	MI-Co	IOx-Co	33 Mt at 1.02 g/t Au, 0.12% Co, 0.14% Bi, 0.04% Cu [241]
Sue-Dianne 2	MI-Cu	IOCG	8.4 Mt at 0.80% Cu, 0.07 g/t Au, 3.2 g/t Ag [242]
Michelin	MI-U	Albitite-hosted U	42.7 Mt at 0.098% U3O8 [243]
Upper C Moran Lake	MI-U	Albitite-hosted U	6.9 Mt at 0.034% U3O8, 0.078% V2O5 (indicated, historic 3) + 5.3 Mt at 0.024% U3O8, 0.058% V2O5 (inferred, historic 3) [244]
Josette 2	MI-REE	IOA-REE	6.9 Mt at 2.7% REE2O3 (=1.83% LREE, 0.89% HREE) [245]
Fostung	MI-W	W skarn	12.4 Mt at 0.2% WO3 (historic) [246]
Kiggavik	MAC-U	Albitite-hosted U	10.4 Mt at 0.47 U3O8 [247]
Lac Cinquante	MAC-U	Albitite-hosted U	2.8 Mt at 0.693% U3O8, 20.6 g/t Ag 0.167% Mo, 0.25% Cu (historic) [248]
Werner Lake	MI-Co	Isi-Co	57.9kt at 0.51% Co, 0.25% Cu, 0.27% As, 0.22 g/t Au (indicated); 6.3kt at 0.48% Co, 0.14% Cu, 0.30% As, 0.24 g/t Au (inferred) [249]
Jaguar, Carajás, Brazil	MI-Ni	IOA-Ni	58.6 Mt at 0.96% Ni [250]

¹ Metals in bold are critical minerals on the Canadian list [234]. ² Case studies studied in more detail by the authors. ³ The past-producing five-element, and vein-type mines of the northern Great Bear magmatic zone have produced 2.113 Mt, containing 54.216 Moz of Ag, 13.401 Mlbs of U oxides, 8.4 kt of Cu, 450 g of Rd, 0.13 kt of Ni, and 0.23 kt of Co [56]. ³ Historic estimates stated not to be relied upon by [244] but serving as a case example of V enrichment in deposits of MIAC systems.

and

Table A3. Representative examples of the mineral resources and their deposit types within the MIAC systems of Sweden and Finland. Districts are located in Figure 1.

Deposits			Total Resources (Unless Indicated Otherwise) 1
Name	Class	Type
Sweden
Per Geiger	MI-Fe	IOA-REE	734 Mt at 47.3% Fe, 2.3% P, 0.18% TREO [153]
Kiruna (+Konsuln)	MI-Fe	IOA	1437 Mt at 59.8% Fe, 0.33% P, 0.017% TREO [154]
Malmberget	MI-Fe	IOA	1570 Mt at 53.4% Fe, 0.57% P, 0.022% TREO [154]
Svappavaara	MI-Fe	IOA	785 Mt at 46.2% Fe, 0.47% P [154]
Grangesberg	MI-Fe	IOA	148.3 Mt at 41.3% Fe, 0.81% P [251]
Nautanen North	MI-Cu	IOCG	21 Mt at 1.46% Cu, 0.78 g/t Au, 6 g/t Ag, 99 g/t Mo [252]
Kiskamavaara	MI-Cu	IOCG	7.67 Mt at 0.25% Cu, 0.04% Co [214]
Finland
Hannukainen	MI-Cu	IOCG	221 Mt at 32% Fe, 0.17% Cu, 0.077 g/t Au, 135 ppm Co [253]
Sahavaara	MI-Fe	Fe skarn	86.8 Mt at 39.82% Fe, 1.93% S (measured + indicated) [254]
Juomasuo	MAC-Au	Albitite-hosted Au-Co	2.37 Mt at 0.13% Co, 4.6 g/t Au + 5.04 Mt at 0.12% Co [255]

¹ Metals in bold are critical minerals on the Canadian list [234].

). Hofstra et al. [1] have classified their deposit environment as regional metasomatic instead of the more common sedimentary, magmatic, and metamorphic-related environments. MIAC systems form along fluid-flow pathways through the upper crust, irrespective of the sedimentary, volcano–plutonic (felsic, intermediate, mafic, and ultramafic), and metamorphic host rocks (see the representative host geological environments in Table A1, Table A2 and Table A3) [1,2]. The mineral resource estimates of the deposits include the critical raw materials Bi, Co, Cu, F, Fe, Mo, Nb, Ni, P, Pd, and Pt; and the rare earths (REE), U, Ti (minor), V (minor), W, and Zn (Table A1, Table A2 and Table A3) [2,3,4,5]. The additional base and precious metal resources consist of Ag, Au, Pb, and Re. Potential by-products comprise In, Sb, Sn, Ta, and Te [3,6,7]. The district-scale metallogenic diversity of MIAC systems has been globally observed, as shown by the representative mineral resources of the deposits in Table A1, Table A2 and Table A3.

Prior to the recognition of the MIAC system (see [7] and Section 3), genetic models for IOCG deposits provided schematic cross sections of the ore and hydrothermal alteration envelopes in both magmatic and non-magmatic settings for up to ~5 km in extent [2,8]. The linkages among the extraordinary range of IOCG deposits, and the realm of Fe oxide-rich-to-Fe oxide-poor deposit types within IOCG districts have remained contentious [2,9]. The problem was most acute in poorly exposed districts, where repeated regional-scale events (e.g., orogenic or magmatic) and metal remobilization hampered the recognition of the primary metasomatic affiliations among deposits. To resolve the apparent lack of linkages, the IOCG spectrum was dramatically reduced, while the previously considered affiliated deposits were re-interpreted as formed in distinct geological environments (cf. [9]). Yet, the results from field observations, geochemical investigations, and constraints on the timing of mineralization events required a more holistic mineral system approach to interpret the observed variety of the alteration and mineralization types [4,5,7,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29].

The integration of geological, geochemical, mineralogical, tectonic, and geophysical research on Canadian and global case studies has elucidated the apparent disparity in the metal associations and deposit types within MIAC systems [6,7,21,22,23,24,25,26,27,28,29,30]. Systematic and predictable metal speciation takes place through six main stages during the metasomatic evolution of the system [7]. Each stage is constrained by the formation of distinct alteration facies, each with a diagnostic range of composition to which a distinct suite of deposit types is associated [6,7,20,21,22,23,28]. The suite of metasomatic reactions is inferred to be driven by a compositionally evolving plume of hypersaline fluids derived from different sources (e.g., magmatic, basinal, and formational fluids and salt melts) depending on the host geological environment (see Section 5.1). At the scale of entire districts, in areas representing centers of maximum fluid circulation, a suite of metasomatic reactions thoroughly transforms the textures, the composition, and the mineralogy of all rock types encountered along the flow path into new rock types, i.e., metasomatites. The scientific breakthroughs on the significance of salt melts in MIAC systems help to explain this extreme reactivity of fluid plumes [31,32].

The characterization of MIAC systems in terms of alteration facies by Corriveau and colleagues was shown to be applicable globally, including in the Olympic Cu-Au Province of Australia ([13] and in [6,7,10,18,20,21,22,23,24,25,26,33]). We, here, adopt the Skirrow et al. [34] mappable criteria for IOCG deposits and expand, with a strong focus on the field geology of alteration facies, their capabilities to help interpret the mineral potential for the range of deposit types within MIAC systems. To achieve that, this review paper (1) strengthens the early definition and the concept of a MIAC system [7,22]; (2) refines the classification of MIAC-related mineral deposit types and deposit classes [1,2,5,22,35]; (3) synthesizes the shared geological attributes and space–time relationships of their alteration facies as part of a mineral system as per Wyborn et al. [36]; and, then, (4) adapts the ore deposit model for the Fe-oxide and alkali-calcic deposits (IOA and IOCG) of Corriveau et al. [7] to the broader MIAC system. The energy driver and the fluid and metal sources of the mineral systems are also discussed from a field geology perspective.

2. Case Studies

This paper draws on the field examinations and characterizations of MIAC systems with an alteration facies approach with or by colleagues worldwide (Figure 1), e.g., Canada; the Central Andes and the El Laco deposit in Chili; the Cloncurry district and the Olympic Cu-Au Province in Australia; the Norrbotten district in Sweden; the Middle–Lower Yangtze River Metallogenic Belt and the Kangdian district in China; and the YangYang deposit in the Republic of Korea (Figure 1; see Acknowledgment Section) [10,13,19,20,21,22,23,24,25,26,37,38,39]. Additional examples were drawn from key synthesis [1,2,3,4,5,8,9,33,35,40,41,42,43], special volumes [20,44], books [4,13,39], and other publications. The tables in Section 3.4 summarize the published field relationships among the alteration facies and mineralization used for this review paper.

In Canada, the Great Bear magmatic zone (e.g., the NICO, Sue-Dianne, and Terra deposits), the East Arm Basin, and the Central Mineral Belt provide insights on MIAC systems in volcano–plutonic belts [6,7,8,9,10,11,12,13,14,15,16,17,18,26]. The Romanet Horst (e.g., Delhi Pacific prospect), the Wanapitei district (e.g., Scadding deposit), and the Temagami area (e.g., Belfast prospect) are examples of MIAC systems within sedimentary basins with mafic rocks that predate the MIAC systems but where coeval regional-scale magmatism is not in the immediate vicinity of the districts [22,49,50]. The Bondy gneiss complex and the Josette deposit (Table A2) are metamorphosed examples [6,19,51]. Hundreds of field geology photographs and scans of rock slabs and cobaltinitrate-stained rock slabs have been published for these case studies. The photographs and scans are part of the extensive collections of Natural Resources Canada (e.g., 18,000 for the Great Bear magmatic zone, Canada, most of which are unpublished to date).

The Paleoproterozoic Great Bear magmatic zone in Canada is regarded as the best exposed MIAC province in the world. It consists of series of MIAC systems that were formed within volcanic centers and their subvolcanic intrusions, as well as systems in an older sedimentary basin and a metamorphic basement (Section 4.1). The glacially polished outcrops and the phaneritic grain size and distinctive patina of the metasomatic minerals due to minor micron-to-one-millimeter-scale weathering of outcrop surfaces facilitate the identification of mineral assemblages in the field [11]. The space and time distribution of the lithological units, alteration facies, and mineralization zones have been documented along semi-continuously exposed tilted or differentially uplifted MIAC systems with depth profiles reaching 2-to-3 km [10,11,12,14,15,16,17,18,52,53,54,55,56,57,58,59,60]. A lack of regional metamorphism; the abundance of pre-, syn-, and post-MIAC intrusive bodies; the extensive U-Pb geochronological data on zircon constraining the timing of geological markers; and the 3D exposures from basement and sub-volcanic intrusions to epithermal caps and underlying sedimentary basins provide constraints on the timing of MIAC activity and on the depth to paleosurface profiles of alteration zonation in MIAC systems [16,17,27,61,62].

3. Definitions

3.1. Alteration, Alteration Facies, and Metasomatism

For clarity, the word ‘alteration’ is used when describing specific products of fluid/rock reactions (alteration facies), while acknowledging that the alteration facies are the result of metasomatic reactions, as it relates better to previous work. Alteration facies are a regular set of metasomatic mineral parageneses developed under similar physicochemical conditions, the combination of which covers the entire petrogenetic process associated with the development of the system (see Section 3.4) [11,15]. Consequently, alteration facies are not simply a rock unit with specific physical (and, in this case, also) chemical features, as each facies is also a tangible record of a distinct stage in the evolution of the system. We, herein, use the alteration facies to track down the evolution of the MIAC system.

Alteration is also used as a process term to refer to weaker chemical changes following fluid/rock reactions for the late alteration types that are overprinting MIAC systems and that are not systematically leading to the formation of a new rock type because of strong metasomatism. Metasomatism or metasomatic are used herein to describe fluid–rock interactions that lead to the intense-to-megascopically complete mineralogical and chemical re-equilibration of the host rocks into metasomatites that are typically associated with the high-temperature alteration facies of MIAC systems.

3.2. Definition of MIAC Systems

From here on, we use the original definition of mineral systems of Wyborn et al. [36] for the geologically mappable footprint that records the regional distribution of ore deposition factors and the essential elements that exploration should focus on. Building on this definition, the terminology to name and describe mineral systems with IOCG, IOA, and affiliated deposits is expanded here to the Metasomatic Iron and Alkali-Calcic (MIAC) mineral system from the iron oxide and alkali deposit types of Porter [5] and the iron oxide and alkali-calcic alteration systems of Corriveau et al. [7,15]. This new umbrella name is needed as Fe silicate-, Fe sulfide-, Fe sulfarsenide-, and Fe carbonate-rich deposits and Fe-poor alkali-calcic deposits are increasingly recognized as part of the host mineral system that form polymetallic Fe-oxide deposits [7,22,23,33]. The MIAC system is defined as metasomatic to account for the thorough fluid-driven transformations of host rocks into metasomatites at the deposit and the regional scales [63]. Transformations are mineralogical, chemical, textural, and structural in nature, and they involve the pervasive dissolution of original minerals and the precipitation of new—metasomatic—mineral parageneses forming completely new rock types [7,63,64]. In the process, a wide range of new physical properties are acquired by the metasomatites [30].

The MIAC system is characterized by Fe-rich alteration facies that are typically silica-deficient and associated with Fe-rich mineralization parageneses. Both ferric and ferrous Fe can be stable, and Fe can be concentrated in oxides, silicates, sulfides, sulfarsenides, or carbonates (Section 3.4) [2,3,4,5,7,8,9,10]. The system is classified as alkali and calcic due to the alteration facies rich in alkali metals (K as biotite, K-feldspar, and the white micas of the paragonite-muscovite-phengite suite; Na as albite, oligoclase, scapolite, and the more rare Na-pyroxene or Na-amphibole) and those rich in alkaline earth metals (Ca as amphibole, calcite, dolomite and ankerite, anhydrite, clinopyroxene, epidote, fluorite, garnet and scheelite; Mg as amphibole, chlorite, clinopyroxene, dolomite, phengitic mica and talc; and Ba as barite) [2,3,4,5,8,10,11,35,65]. The outcomes of alkali-calcic metasomatism include the regional corridors of albitite that are diagnostic of these systems and the extensive zones of Na- and K-bearing alteration [2,5,8,28,50,52,54,65,66,67,68,69,70].

3.3. Classification for the Mineral Deposits in MIAC Systems

Two main groups of mineral deposits are defined to organize and group the broad range of mineral deposit types in MIAC systems: Metasomatic Iron (MI) and Metasomatic Alkali-Calcic deposits. Each group is subdivided into classes using the main commodities of economic interest of the deposit, acknowledging that most mineral deposits are polymetallic (Table A1, Table A2 and Table A3).

MI deposits include the deposit classes in which Fe-rich minerals (i.e., Fe oxides, Fe silicates, Fe carbonates, Fe sulfides and Fe sulfarsenides) prevail in the mineralization paragenesis (Table A1, Table A2 and Table A3) [33,35,41,70,71,72,73,74,75,76]. The alteration facies associated with each class of MI deposits are listed in Section 4. These deposits can also have an alkali-calcic component (e.g., Fe-rich silicates like amphibole, biotite, or clinopyroxene; and Fe-poor minerals like apatite, calcite, K-feldspar, or white micas). The MI-W, MI-Fe, MI-Ni, MI-REE, MI-Co, MI-Cu, MI-U, MI-Au, and, possibly, MI-Ag classes are currently documented, each with their own suite of deposit types (Table A1, Table A2 and Table A3; Section 3.4; Figure 2). For example, deposits of the MI-Cu class include the IOCG and ISCG deposit types. The strict definition of IOCG deposit by Williams et al. [2] is retained here to describe that deposit type.

The MI deposit types rich in Fe-oxides (magnetite and hematite) form the largest deposits in global MIAC districts [2,3,9,33,71], including those in which REE from the primary endowments are remobilized into REE mineralization [6,77,78]. Examples include IOCG; IOA (with Fe, Ni, P, REE, or V resources); Fe (magnetite) skarn; MI-Co ± (Au, Bi, and Cu); MI-Ni; and albitite-hosted MI-U deposits (Table A1, Table A2 and Table A3; Section 3.4). In MI deposits, examples of Fe silicate-rich parageneses (amphibole, biotite, clinopyroxene, chlorite, epidote, and garnet) include those of MIAC-related W and Pb-Zn skarn and some of the MI-Co deposits (e.g., Idaho Cobalt Belt [41]). Fe sulfides (pyrrhotite and pyrite) parageneses (e.g., ISCG deposits), Fe sulfarsenide (arsenopyrite) parageneses (e.g. Fe-rich Au-Co-Bi deposits) or Fe carbonates (ankerite and siderite) parageneses also occur [33,79,80].

MAC deposits comprise the deposit classes in which Fe-poor alkali-calcic minerals (e.g., carbonates like dolomite, Fe-dolomite, or calcite; silicates like Mg-chlorite, Mg-amphibole, talc, K-feldspar, albite, and white micas) prevail in mineralization parageneses [7,22,35,81]. MAC deposits can be related to a suite of the alkali-calcic alteration facies formed coevally with the Fe and alkali-calcic alteration facies [81]. Mineralization is often associated with quartz. The specific classes of MAC deposits are defined by their main commodities, with MAC-Au, MAC-Cu, MAC-Mo, and MAC-U deposits being currently identified (Figure 2).

Figure 2. Relative timing relationships of alteration facies, from 1 to 6, and transitional ones (1–2, 2–1, 2–3). Red to blue arrow: the fluid plume evolving in composition and physical conditions as it precipitates the regular sequence of alteration facies. The inflow fluid at each facies is the outflow fluid of the previous facies (primary fluid plume/inflow fluid + elements dissolved from hosts − elements precipitated = evolving fluid plume). The ingress of magmatic heat and external fluids disrupt the physicochemical conditions of the primary plume. The downward red arrow represents the ingress of magmatic heat and high-temperature fluids, and the upward yellow arrow represents the disruption that occurs due to the ingress of low-temperature fluids (see Section 5 for additional information). Note that heat ingress does not produce additional Facies 1 albitite. The mineral abbreviations are according to Warr [82], with additions for iron oxides (IO), oxides (OX), sulfides (Sulf), silicates (Sia), arsenides (As), sulfarsenides (AsS), and minerals (min.). The Great Bear magmatic zone (GBMZ), the Southern Breccia (S), the Chalco prospect (C) and NICO deposit (N) of the Lou system (Lou), and the Port Radium-Echo Bay (PR-EB) system occur in Canada (Figure 1) [15,17,56,59,60,61,62]. The Olympic Dam (O), Manxman A1 (M), Island Dam (I), Oak Dam East (D east), Oak Dam West (D west), and Bellatrix (B) are from Australia [6,10,23,71,72]. Deposits and prospects are labeled with their facies and metal associations, and the continuous to intermittent path is based on the facies observed (additional information in Section 3.4). Additional abbreviations include the following: giant quartz veins (G), hematite-group (Hem-gp), high temperature (HT), including (inc.), low temperature (LT), metasomatic alkali-calcic (MAC), metasomatic iron (MI), and magnetite-group (Mag-gp). The figure is extensively modified and updated from Fig. 12 of Corriveau et al. [7] and the published components of the original figure reproduced under GAC^®’s Fair Dealing/Fair Use policy.

Figure 2. Relative timing relationships of alteration facies, from 1 to 6, and transitional ones (1–2, 2–1, 2–3). Red to blue arrow: the fluid plume evolving in composition and physical conditions as it precipitates the regular sequence of alteration facies. The inflow fluid at each facies is the outflow fluid of the previous facies (primary fluid plume/inflow fluid + elements dissolved from hosts − elements precipitated = evolving fluid plume). The ingress of magmatic heat and external fluids disrupt the physicochemical conditions of the primary plume. The downward red arrow represents the ingress of magmatic heat and high-temperature fluids, and the upward yellow arrow represents the disruption that occurs due to the ingress of low-temperature fluids (see Section 5 for additional information). Note that heat ingress does not produce additional Facies 1 albitite. The mineral abbreviations are according to Warr [82], with additions for iron oxides (IO), oxides (OX), sulfides (Sulf), silicates (Sia), arsenides (As), sulfarsenides (AsS), and minerals (min.). The Great Bear magmatic zone (GBMZ), the Southern Breccia (S), the Chalco prospect (C) and NICO deposit (N) of the Lou system (Lou), and the Port Radium-Echo Bay (PR-EB) system occur in Canada (Figure 1) [15,17,56,59,60,61,62]. The Olympic Dam (O), Manxman A1 (M), Island Dam (I), Oak Dam East (D east), Oak Dam West (D west), and Bellatrix (B) are from Australia [6,10,23,71,72]. Deposits and prospects are labeled with their facies and metal associations, and the continuous to intermittent path is based on the facies observed (additional information in Section 3.4). Additional abbreviations include the following: giant quartz veins (G), hematite-group (Hem-gp), high temperature (HT), including (inc.), low temperature (LT), metasomatic alkali-calcic (MAC), metasomatic iron (MI), and magnetite-group (Mag-gp). The figure is extensively modified and updated from Fig. 12 of Corriveau et al. [7] and the published components of the original figure reproduced under GAC^®’s Fair Dealing/Fair Use policy.

3.4. Review of Alteration Facies and Mineralization in MIAC Systems

The definition of the alteration facies that form MIAC systems [11,15] follow the definition of the metasomatic facies of Zharikov et al. [83] for the IUGS subcommission on the nomenclature of metasomatic rocks. The alteration facies are differentiated by the appearance or disappearance of specific suites of mineral assemblages, which reflect variations in one (or more) of the physicochemical parameters of the fluid-driven metasomatic reactions that formed the system (Figure 2; Section 3.3). The distinct physicochemical conditions that led to each alteration facies also led to the facies-specific deposit types as illustrated in this section (see also Figure 2).

In mature MIAC systems, metasomatism typically sequentially forms six main alteration facies, with high temperatures (HT) reaching a peak of up to about 1000 °C and low temperatures (LT) reaching about 150 °C (Figure 2; Section 5.1.1; [15] and the references therein; and [31,32]). The systems are globally going through (1) a heating stage with significant leaching of metals and volatiles, (2) a stage where iron and alkali-calcic alteration facies are associated with MI deposits, (3) a stage where alkali-calcic alteration facies are associated with MAC deposits, and (4) a stage where alteration facies are developed under epithermal conditions [55,84] or where mineralized veins are form through remobilization from or are superimposed on pre-existing MIAC systems [21,60,85].

The heating stage of the system produces barren Facies 1a Na, Facies 1b Na-Ca, and Facies 1c skarn (

Table 1. Alteration facies at the heating stage of MIAC systems that serve as a ground preparation for subsequent mineralization. Additional examples of deposits are listed in Table A1, Table A2 and Table A3. Deposits listed or their districts are located in Figure 1.

Alteration Facies	Minerals 1			General Characteristics	Association with Mineralization and Case Examples
	Alteration		Mineralization
	Fe-Poor	Accessory
Facies 1a Na	Ab–Olc; res. Qz	Mnz, Ru, Ttn, Zrn	Barren	Preferential distribution along regional-to-local discontinuities (e.g., lithological contacts, roofs of laccoliths, faults, and shear zones) A regional extent along albitite corridors (≤100 s of km in length and a few km in width) in zones of enhanced crustal permeability Ab replaces most minerals in host rocks, not only feldspars Facies replaces any host rocks Transient porosity and damage zones in albitite favour brecciation and serve as ground preparation for mineralization Fine-grained to locally very coarse-grained where adjacent to coeval intrusions Albitite corridors are diagnostic of a MIAC system Regularly overprinted by all fertile alteration facies, with Fe-poor alkali-calcic alteration components of Facies 5 being most common	Barren when formed, e.g., Southern Province, Great Bear, CAN; Kethri Cu Belt and its Albitite Line, IND; Central Andes, CHL-PER; Cloncurry district and Mt Isa Province, AUS; Norrbotten district, SWE; Kuusamo belt, FIN; and Middle–Lower Yangtze Metallogenic Belt, CHN. Host to subsequent albitite-hosted mineralization. Sources: [10,11,12,13,14,15,22,26,52,54,55,66,67,68,69,70,76,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110].
Facies 1b Na-Ca	Ab–Olc, Anh, Scp; res. Qz	Amp, Cpx, Mnz, Ru, Ttn, Zrn	Barren
Facies 1c Skarn	Mg-Fe Amp (Tr), Mg-Fe Chl, Cpx, Grt, Phl, Tlc, res. Cb		Barren	Localized in carbonate-bearing units Spatially associated with, and pre, syn, or post Facies 1 Na alteration; can cut albitite No causative intrusions; driven by the heat of fluids Fine-grained but can be medium-grained Distal-to-proximal to other MIAC alteration facies	Barren when formed, e.g., Great Bear, CAN; Punt Hill (Olympic Cu-Au Province), AUS; and Middle–Lower Yangtze Metallogenic Belt, CHN. Host to subsequent skarn-hosted mineralization Sources: [14,64,92,111].

¹ The abbreviations follow [82], except for oligoclase (Olc), carbonates (Cb), sulfides (Sulf), silicates (Sia), As (arsenides), AsS (sulfarsenides), minerals (min.), remobilized (rem.), and residual (res.). Abbreviations for countries: AUS—Australia, BRA—Brazil, CAN—Canada, CHL—Chile, CHN—China, FIN—Finland, IND—India, MRT—Mauritania, PER—Peru, SWE—Sweden, VNW—Vietnam, and USA—United States.

). At this stage, host rock elements, including metals and volatiles, are extensively leached and transferred to the fluid plume. In silicate rocks, mineral assemblages record a significant addition of Na to form albitite and other regionally extensive sodic alteration (Facies 1a), and lesser enrichment in Na, which is where Ca remains (Facies 1b) (Table 1). Carbonate rocks are transformed to skarn (Facies 1c) at high temperatures (above ~450 °C) [6] and can be albitized at lower temperatures [49], whereas the significant addition of Na is largely partitioned into silicate lithologies. Onset of Fe enrichment starts at Facies 1–2 HT Na-Ca-Fe (Table 1).

Alteration facies associated with a significant increase of Fe content and the formation of MI deposits are reflected by a prevalence of Fe-rich minerals in the alteration assemblages (through addition of Fe and leaching of other elements). For example, MI-Fe magnetite skarn and IOA deposits are associated with Facies 2–1 Fe-rich skarn and Facies 2 HT Ca-Fe (Figure 3A–D). Prior to the formation of MI-Fe deposits, the Fe enrichment observed in Facies 2–1 Fe-rich skarn can be associated with W mineralization (

Table 2. The field geology attributes of Facies 2 and transitional Facies 2–3, and the associated deposit types of the Metasomatic Iron (MI) class. Abbreviations for minerals and countries as per Table 1.

Alteration Facies	Minerals			General Characteristics	Association with Mineralization and Case Examples
	Alteration		Mineralization
	Fe-Rich Fe-Poor	Accessory	Main Accessory
Facies 1–2 HT Na-Ca-Fe	Amp (Act, Hbl), Cpx, Mag Ab–Olc	Ap, Mnz, Ru, Qz, Ttn, Zrn	Mag, Py	Close to thermal core of MIAC systems Overprint/spatially above and evolving from albitite Proximal/evolving to HT Ca-Fe facies Medium-grained-to-coarse-grained with pseudo-pegmatitic textures Onset of Fe-rich alteration types in certain MIAC systems Bt-Mag-Ab can precipitate and be associated with Py and Pyh, with no or only minor Ccp as it transitions to Facies 3	Proximal to iron oxide (IO) and IOA deposits. Incipient REE mineralization possible. Examples: Great Bear magmatic zone, CAN and Moonta-Wallaroo district (Olympic Cu-Au Province), AUS. Sources: [12,14,55,61,92].
Facies 2–1 Fe-rich skarn Figure 3A	Fe-rich Cpx and Grt, Mag, Pyh res. Cb	Amp, Aln, Ap, Qz, Ttn	Mag, Sch	Close to the thermal core of a MIAC system No causative intrusion; driven by heat of fluids Significant increase of Fe in minerals Fine-to-very-coarse-grained Commonly overprinted by HT Ca-Fe alteration	MI-W, e.g., Fostung (Wanapitei district), CAN and Punt Hill, AUS. Sources: [6,7,111,112].
Facies 2a HT Ca-Fe (Fe, P, or REE endowed) Figure 3B–D	Fe-rich Amp (Act, Hbl), Ep, Mag or Pyh Ap, Qz	res. Cpx and Grt; Ttn	Ap, Mag, REE-min. (e.g., Aln, Adr, Bri, Mnz Flr) Ccp, Py, Thr, Urn; rem. Sch	Thermal core of MIAC systems; peak temperatures recorded in hydrosaline liquid and salt melt inclusions In zones of enhanced crustal permeability proximal to or within local-to-regional structural corridors or above sub-volcanic intrusions (generally of intermediate composition) Ductile behavior is common with the development of a mineral foliation and local folding Can occur as fluidized IOA breccias, including with dyke-like morphologies Fine-to-very-coarse-grained Amp-dominant to Mag-dominant to Ap-dominant Veins, replaces, or overprints and/or is spatially above albitite Infills albitite breccias and can replace its fragments; Mag-selective to pervasive alteration of fragments is common Pyh precipitates instead of Mag with Amp, and it can be the main Fe-rich minerals in geological environments that are rich in graphitic sedimentary units Commonly overprinted by Facies 2–3, 3, and 5a	MI-Fe (IO, IOA) deposits, e.g., El Laco, CHL; Kiruna, Per Geiger, Malberget (Norrbotten district), SWE; Pilot Knob, Pea Ridge (southeast Missouri district), USA; Middle–Lower Yangtze Metallogenic Belt, CHN. Magnetite-skarn as magnetite-dominant HT Ca-Fe overprint on skarn. REE remobilization from IOA apatite forms MI-REE deposits, e.g., Josette, CAN and Pea Ridge (southeast Missouri district), USA. Sources: [6,11,14,27,61,70,77,78,110,113,114,115,116].
Facies 2b HT Ca-(Mg)-Fe (Ni-endowed) Figure 3E	Fe-rich Amp (Act, Hbl), Mag, Pyh, Py Ap, Qz	Bt res. Cpx and Grt	Mlr, Pn Ccp	In or very close to the thermal core of a MIAC system In provinces with abundant mafic–ultramafic units Formed in discrete zones of enhanced crustal permeability Can occur as fluidized breccias Precedes Cu-rich mineralization types	MI-Ni deposits, e.g., Jaguar, Jatobá, GT-34 (Carajás), BRA Sources: [117,118,119,120].
Facies 2–3 HT Ca-K-Fe to Bt-rich HT K-Fe (Bt > Kfs)   Figure 3F	Fe-rich Amp (Act, Hbl), Bt, Mag, Pyh, Sid Ap, Kfs, Qz	res. Cpx and Grt, Flr, Ilm, Ttn	Apy, Co-As/AsS, Bin, Au in As-min. or native Overprint: Bin, Emp, Mdo, native Au/Bi Ccp, Mlr, Mol, Py, Sch	In sedimentary sequences where magmatic events occur In ultramafic-to-mafic environments In zones of enhanced crustal permeability proximal to, or within, local-to-regional structures Commonly stratabound and replacing earlier stratabound Facies 2 Amp-dominant HT Ca-Fe alteration Replacement by Facies 3 biotite-rich K-Fe alteration is common where Cu mineralization occurs Ductile behavior with mineral foliation and local folding Mineralization hosted in, or outside of, the shoulders of chemically reactive rocks (carbonates) Minor-to-absent brecciation associated with mineralization Fine-to-medium-grained	MI-Co deposits, e.g., Guelb Morghein, MRT; NICO, CAN; Sin Quyen, VNM; Blackbird, Ram, Sunshine, Sunshine East, Merle (Idaho Cobalt Belt), USA; and components of Ernest-Henry, AUS. Sources: [7,10,15,17,41,61,78,80,121,122,123,124,125,126].
Facies 2–3 HT Ca-K-Fe (Kfs>Bt)	Fe-rich Amp (Act), Bt, Mag, Pyh, Kfs	Ap, Ttn, Zrn	Ccp, Py, Urn	Evolves from HT Ca-Fe and transitions to and is proximal to a HT K-Fe alteration in felsic and intermediate volcanic and plutonic rocks Incipient brecciation with Amp- or Amp-Mag infill and Kfs-altered fragments and K-feldspar haloes to breccias Amp and Amp-Mag veins with Kfs haloes Fine-to-medium-grained	Halo of MI-Cu deposits. Breccia infill and Kfs-altered ore breccia, e.g., Ernest Henry, AUS Sources: [7,10,15,17,70,126].

). In MIAC systems formed in geological environments with abundant mafic-to-ultramafic rocks, Ni mineralization can be associated with Facies 2 HT Ca-Fe with and without the development in the same MIAC system of Facies 2–3 HT Ca-K-Fe and Facies 3 HT K-Fe (Table 2; Figure 3E). Mineralization in REE is associated with the HT Ca-Fe facies in systems that evolved to form the HT K-Fe alteration facies, such as those observed at the Josette IOA-REE deposit (Table 2 and Table A3) and in the zones of IOA mineralization at the past-producing Terra Ag mine (Table 2; Figure 3D). Facies 2 is thus associated with deposits of the MI-Fe, MI-Ni, and MI-REE classes. Polymetallic Co mineralization with Fe oxide or Fe silicate parageneses with and without Fe sulfarsenides is associated with Facies 2–3 HT Ca-K-Fe and biotite-rich Facies 3 HT K-Fe, forming deposits of the MI-Co class (Figure 3F).

Polymetallic Cu mineralization of the MI-Cu class is primarily associated with Facies 3 HT K-Fe and Facies 5a LT (K, Ba, Ca, Mg, and Na)-(CO₂, F, and H⁺)-Fe, whereas the Facies 4 formed at the transition from magnetite to hematite is commonly barren (

Table 3. Facies 3 HT K-Fe alteration and the onset of Cu mineralization in the MI deposit class. Abbreviations for minerals and countries as per Table 1.

Alteration Facies	Minerals			General Characteristics	Association with Mineralization and Case Examples
	Alteration		Mineralization
	Fe-rich Fe-poor	Accessory	Main Accessory
Facies 3 HT K-Fe (Bt-rich)   Figure 4B,C	Bt, Mag and/or Pyh Kfs, Qz	Ap, Amp, Grt	Brn, Ccp, Py, native Au, Urn	In sedimentary-, volcano-sedimentary-, volcaniclastic-, or mafic and ultramafic-dominant environments Stratabound alteration and mineralization Mineralization in areas of enhanced crustal permeability along a local-to-regional discontinuity Weak-to-absent brecciation associated with mineralization Fine-to-medium-grained Commonly replaces Facies 2–3 or Facies 1 albitite; breccias are rare Pyh and Py can be the main Fe-rich minerals	MI-Cu, including Mag-group IOCG and ISCG, e.g., components of Candelaria, CHL; Dahongshan (Kangdian district), CHN; and components of Ernest-Henry (Cloncurry district), AUS. ISCG, which are typical of geological sequences with graphitic units. Sources: [64,70,74,75,123,126,127,128,129].
Facies 3 HT K-Fe (Kfs-rich)   Figure 4A,F	Bt, Mag Kfs, Qz	Ap, Brt, Flr, Ilm, Mnz, Thr, Ttn, Zrn	Ccp, native Au, Py, Urn Apy, Bn, Mol, Pyh	In felsic-to-intermediate volcanic and plutonic environments Breccia-hosted mineralization marking a transition to brittle conditions In breccias, Kfs replaces fragments and Mag and Sulf precipitate in matrix; with increasing intensity, matrix assemblages replace Kfs-altered fragments; can develop in albitite Mag veins with Kfs haloes cut Facies 1 albitite; fragment in albitite can be Mag-altered in their cores and Kfs-altered at their margin Structurally controlled mineralization in zones of enhanced crustal permeability Fine-to-medium-grained	MI-Cu, including Mag-group IOCG, e.g., components of Candelaria, CHL; Ernest Henry (Cloncurry district), AUS; and Sue-Dianne, CAN. MI-U, including albite-hosted U, e.g., Southern Breccia (Great Bear magmatic zone), CAN. Sources: [7,10,11,14,28,35,51,70,72,87,126,130].

,

Table 4. Facies 4 barren K-felsite and barren-to-mineralized K-skarn. Abbreviations for minerals and countries as per Table 1.

Alteration Facies	Minerals			General Characteristics	Association with Mineralization and Case Examples
	Alteration		Mineralization
	Fe-rich Fe-poor	Accessory	Main Accessory
Facies 4a K-felsite	Kfs			At the Mag-to-Hem transition Commonly brecciated and forming haloes to Fe-oxide breccias Can replace albitite and K-skarn Fine-grained	Haloes to zones of mineralization, e.g., Ernest-Henry (Cloncurry district), AUS. Sources: [14,70].
Facies 4b K-skarn	Ank, Mag Aln, Cpx (Adr-Grs), Ep, Kfs	Cal, Dol	Ccp, Gn, Mol, Py, Sp	In carbonate sedimentary units or carbonate-altered rocks Commonly brecciated Fine-to-medium-grained Can be replaced by K-felsite Overprinted by and host to Facies 5 mineralization	Example: Mile Lake prospect (Great Bear magmatic zone), CAN. Source: [56].

and

Table 5. Facies 5 alteration and associated mineralization in the MI deposit class. Abbreviations for minerals and countries as per Table 1.

Alteration Facies	Minerals			General Characteristics	Association with Mineralization and Case Examples
	Alteration		Mineralization
	Fe-rich Fe-poor	Accessory	Main Accessory
Facies 5a LT (K, Ba, Ca, Na)-(CO2, F, H+)-Fe   Figure 4D,E	Chl, Ep, Hem, Sid Ab, Ank, Brt, Cal, Dol, Flr, Kfs, Msc, Qz	Anh, Ank, Ap, Dol, Tur	Brn, Ccp, Cct, LREE-min., native Au, Py, Urn Apy, Mol, Sch	In felsic-to-intermediate volcanic and plutonic environments Breccia-hosted mineralization Structurally controlled zonation of Cu-Sulf in deposits Fine-to-medium-grained Commonly overprints Facies 2 IOA mineralization and albitite and can form distally from them.	MI-Cu, e.g., Hem-group IOCG Olympic Dam, AUS. MI-U, e.g., albitite-hosted U Michelin (Central Mineral Belt), CAN. Sources: [2,10,33,43,71,107,131,132].
	Aln, Amp (Act, Hst, Stp), Chl, Ep, Grt, Hem, Mag Ab, Ank, Brt, Cal, Dol, Flr, Kfs, Msc, Qz	Ap	Brn, Ccp, Cct, LREE-min., native Au, Py, Urn Gn, Mol, Sp	In environments rich in carbonate units or in mafic-ultramafic units Breccia-, shear-hosted, K-skarn-hosted, or albitite-hosted mineralization Fine-to-medium-grained	MI-Cu (e.g., Hem-group IOCG and K-skarn hosted; Sossego (Carajás), BRA; and Hillside, Punt Hill (Olympic Cu-Au Province), AUS. MI-U, e.g., Moran Lake (Central Mineral Belt), CAN. Sources: [6,7,56,92,107,111].
	Act, Bt, Chl, Pyh, Py Msc, Qz	Ap, Cb, LREE-min., Tur	Brn, Ccp Gn, Mol, Sp	In geological environments with abundant graphitic units Structurally controlled in fault or shear zones Sulfide replacement lodes Fine-to-medium-grained	MI-Cu, e.g., Eloise, Jericho (Cloncurry district), AUS; Delhi Pacific (Romanet Horst), CAN. Sources: [50,133].
Facies 5b Si-rich LT (Ca, Mg)-(K, Na)-Si-Fe; Figure 4G	Amp, Chl, Ep, Hem, Mag, Pyh Ab, Ank, Brt, Dol, Flr, Qz	LREE-min., Kfs, Msc	Native Au, Py Ccp	Breccia, shear-hosted, or replacement zones Can be albitite- or skarn-hosted At the transition from aluminosilicates + Fe min. to Qz + Fe min. Fine-to-medium-grained	MI-Au, e.g., Scadding (Wanapitei district), CAN. MI-Ag. Sources: [7,22,33,80].

). Cu mineralization in MI-Cu deposits can be associated with abundant Fe oxides (e.g., IOCG; Figure 4A,E), Fe silicates (Figure 4B,C), or Fe sulfides (e.g., ISCG; Figure 4D). In some MIAC systems, Facies 3 and Facies 5a are also associated with U mineralization (Figure 4F), which can form deposits of the MI-U class [28]. Facies 5b LT (Ca, Mg, K, Na)-Si-Fe, overlapping with the later stage of the development of Facies 5a, can be associated with Au mineralization (Figure 4G) without significant Cu additions that can form deposits of the MI-Au class.

Facies 5c K/CO₂ and Facies 5d Si/CO₂ represent the alkalic-calcic alteration facies poor in Fe that are largely formed after the Fe-rich alteration facies and are associated with the diversified mineralization types associated with the formation of MAC deposits (Figure 5A–C;

Table 6. Fe-poor alkali-calcic alteration facies forming deposits of the MAC class. Abbreviations for minerals and countries as per Table 1.

Alteration Facies	Minerals			General Characteristics	Association with Mineralization and Case Examples
	Alteration		Mineralization
	Fe-rich Fe-poor	Accessory	Main Accessory
Facies 5c K/CO2   Figure 5B,C	Ank Chl, Cal, Dol (ferroan), Kfs, Msc, Qz, Tur		Ccp, Mol, Native Au, Pyh, Py Brn	Contemporaneous with Fe-rich metasomatism Typically overprints or formed close to earlier corridors of albitite Structurally controlled and generally hosted in fault zones Occur as breccias, replacements, or networks of veins Probable remobilization and reconcentration of metals from earlier mineralized alteration facies Fine-grained-to-pegmatitic with hydrothermal textures	MAC-Cu, e.g., Mount Dore (Cloncurry district), AUS; Alwyn (Wanapitei district), CAN; Pahtohavare (Norrbotten district), SWE. MAC-Mo, e.g., Merlin (Cloncurry district), AUS. Hanging wall of MI-REE, e.g., Per Geijer (Norrbotten district), SWE. Sources: [22,95,96].
Facies 5d Si/CO2   Figure 5A	Cal, Chl, Dol (ferroan), Qz	Ab, Kfs, Msc	Native Au, Pyh, Py, Urn Ccp, LREE-min.		MAC-Au, e.g., Tick Hill (Cloncurry district), AUS; Kish showing (Romanet Horst), CAN. Sources: [50,131].

). The polymetallic Cu mineralization hosted in carbonate–quartz veins or in pervasive zones of silicification associated with K-feldspar replacement is associated with the Fe-poor Facies 5c (Figure 2) (Figure 5B,C). Facies 5d (Figure 2) is associated with albitite-hosted U and albitite-hosted Au and/or Co deposit types (Figure 5A).

As MIAC systems waned at Facies 6, aluminous or quartz-rich alteration were formed under epithermal conditions (

Table 7. Waning stage of MIAC systems at the transition to epithermal conditions and metal remobilization from pre-existing alteration facies of a MIAC system with post-MIAC fluid circulation. Abbreviations for minerals and countries as per Table 1.

Alteration Facies	Minerals		General Characteristics	Association with Mineralization and Case Examples
	Alteration	Mineralization
	Fe-rich Fe-poor	Main
Facies 6 K-Si-Al ± Fe-Ba epithermal caps	Hem Phyllic, sericitic, silicic, advanced argillic, silicification	Py	Apex of system near or at paleosurface Peak acidic conditions and thermal low of a MIAC system Occurs as veins, vein networks, or replacement zones Postdates MIAC and MAC alteration Metal reconcentration from earlier alteration facies and mineralization	Epithermal mineralization, e.g., Gossan Island, Echo Bay Gossan (Great Bear magmatic zone), CAN; Central Andes, CHL [10,55,56,64,84].
Facies 6 LT Fe-(Si,CO2) ± Ba	Hem Brt, Cal, Dol, Qz	As–AsS, Ccp, Gn, Sp, Urn	Postdates MIAC and MAC alteration Veins, vein networks, and breccias in brittle structures Metal signature of the host facies Giant quartz veins (≤ tens of km); more local silica flooding Varied time gap between primary MIAC system and recirculation or addition of fluids and metals	Vein-type mineralization, five-element veins, e.g., Camsell River and Port Radium-Echo Bay districts (Great Bear magmatic zone), CAN; Olympic Cu-Au Province, AUS; Lufilian arc, Africa [21,56,60,85,134,135,136,137].

) [7,15,55,71,84]. Many stages of the hydrothermal vein-, breccia-, or shear-hosted mineralization with localized alteration postdated the host MIAC systems. The metal associations are compatible with the remobilization of the primary endowment of the host MIAC system into mineralization zones hosted in discrete veins or stockworks that can reach very high grades. Repeated metal remobilization and addition during regional-scale and post-MIAC events are well documented worldwide [7,21,85].

Most of the MIAC systems exhibited, although at variable levels of maturity and intensity, the full suite of alteration facies. However, depending on local conditions and possible disruptions in the evolution of the fluid plume, some MIAC systems may not exhibit all alteration facies. This could be caused, for example, by (1) changes in the fluid circulation pathways due to active deformation; (2) the absence of rejuvenation of the voluminous fluid plume by external (e.g., basinal, magmatic, and meteoric) fluids during its evolution; (3) the distance between the MIAC system and its primary heat sources, or the lack of extensive magmatism as additional heat sources; or (4) the faulting and telescoping of albitite corridors to the field of LT alteration facies (see discussion in Section 5). In addition, alteration facies can remain non-documented within a system.

For example, extensive and intense Facies 2 HT Ca-Fe alteration and extensive weak overprints of IOCG mineralized Facies 5 LT K-Fe have been reported for the Oak Dam East deposit in Australia [132]. Yet, extensive and intense Facies 3 and 5 HT and LT K-Fe alteration were missing. Based on the typical evolution of MIAC systems, a probable cause was a knowledge gap. This gap has now been filled with the discovery of the giant Oak Dam West IOCG deposit (Table A1). The example of Oak Dam West illustrates that gaps in the sequence of alteration facies in the mapping or drilling record could be important data gaps that need to be filled in. Exploration and mapping strategies can be oriented to search for the missing alteration facies and the zones of mineralization they could host.

4. Geo-Environment of MIAC Systems

4.1. Geological Context

MIAC mineral systems and their associated mineral deposits are developed within a wide range of geological environments and host rocks that include the following: (1) coeval volcanic centers; (2) above, within, or adjacent to coeval sub-volcanic intrusions or more isolated alkaline intrusions, such as carbonatites; (3) within sedimentary basins and volcano-sedimentary basins that predate the MIAC event by a few m.y. or much older ones; (4) pre-MIAC intrusive suites a few m.y. or much older, as well as metamorphic basements; and (5) major and coeval fault zones and their splays [2,5,17,43,62,138,139]. Individual MIAC systems generally have a district-scale footprint that is typically 35 km in length, 15 km in width, and 10 km in depth, with interlinked and extensive zones of intense metasomatism [17,52,55,57,140,141].

Well-exposed MIAC provinces occur in continental magmatic arcs, such as those of the Canadian Great Bear magmatic zone and Central Mineral Belt, the South American Central Andes, the USA southeast Missouri district, the Scandinavian Norrbotten district, the Chinese Middle–Lower Yangtze River Metallogenic Belt, and the Iranian Bafq district [2,3,4,5,6,7,13,25,113,142,143]. The systems are typically distributed along or in proximity to transcrustal fault systems within intra-continental arc basins (e.g., the Central Andes of Peru and Chile [40,143], and the Middle–Lower Yangtze River Metallogenic Belt in China [144]), or in a back-arc setting (e.g., southeast Missouri district, USA [145]). The array of MIAC centers forms metallogenic provinces that can be traced for up to 2000 km in Mesozoic provinces, e.g., the Central Andes [40,141,146]. Archean and Proterozoic provinces are traced continuously for up to 600 km, but tectonic reconstructions link coeval Proterozoic metallogenic provinces into more extensive belts [5,12,56,95,139,147,148].

Systems that form within large sedimentary basins are typically interlinked by extensive fault zones, which are commonly related to the re-activation of rift-related faults [149], and they either have sparse or distant coeval magmatism, such as in the Canadian Romanet Horst (Québec) and Wanapitei district (Ontario); and the Chinese Kangdian district [22,49,50,64,100]. Other MIAC systems occur along fault systems developed in supracrustal rocks among batholiths (e.g., Mount Isa Province in Australia), or they are spatially decoupled from potentially coeval batholiths (e.g., Wanapitei district in Ontario and the Romanet Horst and other systems easterly of the Labrador Trough in Québec) [49,50,66,67,89,100]. Some of the systems developed during orogenesis or were reworked during orogenesis, as illustrated by the Josette REE deposit in Québec [51,116], the Adirondack district in the United States [150], the Tennant Creek system in Australia [33], and some zones of IOCG mineralization in the Norrbotten district in Sweden [21].

Unexposed to poorly exposed MIAC systems and their IOCG and IOA deposits can include giant mining districts, such as the Olympic Cu-Au Province in Australia, the Carajás Mineral Province in Brazil, and the Norrbotten district in Sweden [24,33,42,149,150,151,152,153,154]. Once formed, MIAC systems can become buried by the deposition of younger sedimentary basins (e.g., cover sequences above the Great Bear magmatic zone and the Olympic Cu-Au Province) [42,155], or they can be metamorphosed to mid-to-high grades through orogenesis [19,21,51,152,156,157].

Figure 6 highlights some key components of the geological context of MIAC systems using the well-constrained field geology of the Great Bear magmatic zone, including the metamorphic basement rocks and overlying 1.88 Ga sedimentary basin; the 1.87 Ga volcanic centers and their coeval intrusions; and the extensive 1.866 Ga ignimbritic cover and the multi-phase batholiths ([62] and the references therein).

4.2. Pre-MIAC Sedimentary Basins

Volcanic centers and intrusions coeval with MIAC systems are deposited on or have intruded earlier sedimentary basins that consist of carbonate, siltstone, and wacke, as well as evaporite or scapolite-bearing units interpreted as evaporite [2,42,68,70,86,114,161,162,163,164]. Some of these basins only slightly predate, and underwent short-lived orogenesis before, the MIAC activity (Great Bear magmatic zone) [62]. Other sedimentary basins are nearly coeval with or postdate volcanism and underwent tectonic inversion (Central Andes) [161]. Finally, there are older metamorphosed basins, such as those of the Mount Isa and Curnamona provinces of Australia [82], Kangdian district in China [64], and Romanet Horst in Canada [50]. Such sedimentary basins are common hosts for MI-Co and albitite-hosted mineralization.

4.3. Syn-MIAC Magmatic Flare-Up

Worldwide, magmatism that is coeval with MIAC systems typically forms distinct volcanic and intrusive centers with mafic, intermediate, and felsic compositions of shoshonitic-to-calc-alkaline affinities (e.g., Figure 6) [42,62,64,76,86,114,163]. Mafic igneous rocks are limited, while intermediate-to-felsic igneous rocks are common, and their relative proportions can vary across and among metallogenic belts (Figure 6) [16,34,58,62]. In addition to volcanic and intrusive rocks, syn-magmatic and localized fluvio-lacustrine sedimentary rocks, evaporite, acidic volcanic lake, and carbonate units can also be present (e.g., Olympic Cu-Au Province in Australia, Middle–Lower Yangtze River Metallogenic Belt in China, Great Bear magmatic zone in Canada, and there are also other MIAC provinces worldwide) [10,11,53,64,78,86,162,163,164,165]. The duration of the magmatic events associated with the formation of the primary MIAC systems were in the order of about 5–10 Ma, as described below.

In the Great Bear magmatic zone in Canada (Figure 6), the onset of regional metasomatism between 1876 and 1873 Ma coincided with the emplacement of syn-tectonic granitic dykes and sparse granitic intrusions at the waning stage of a compressional event [15,16,62]. A magmatic flare-up followed between 1873 and 1868 Ma, and it was related with the development of numerous volcanic centers consisting of compositionally diverse volcanic rocks, porphyritic-to-phaneritic sub-volcanic intrusions, and localized syn-volcanic sedimentary units. This magmatism provided one of the fluid sources for the development of MIAC systems [166]. The volcanic centers were preferentially located in extensional pull-apart intra-arc basins that have been interpreted to occur along the entire exposure of the belt [62]. Hildebrand and Bowring [167] interpreted the continental arc to be of low topography. The volcanic centers were interconnected by arc-parallel major fault zones like the one largely obscured by batholith emplacement in the west and the well-preserved Wopmay fault zone in the east [62,160]. Basaltic flows occur in narrow basins along the east-bounding, orogen-parallel Wopmay fault zone (Figure 6) [15,53]. In the other 1.87 Ga basins of the Great Bear magmatic zone, mafic rocks are minor [58,61,158,165].

In the northwest part of the Great Bear magmatic zone, the volcanic centers comprise andesite stratovolcanoes and calderas, and they were formed at ca. 1873 Ma along with their co-magmatic dioritic-to-monzonitic sub-volcanic intrusions and resurgent plutons; these also host IOA and IOCG prospects and past-producing vein-type mines (Figure 6; Camsell River district hosting the past-producing Terra Ag mine) [16,57,58,62]. The subvolcanic intrusions were emplaced as laccoliths and lopoliths, which is a mode of emplacement also observed in the MIAC system of the East Arm Basin coeval with the Great Bear magmatic zone [18,51,52,57,60,165], as well as in the Central Andes [114] and in the Middle–Lower Yangtze River Metallogenic Belt [64,86].

In the southeast of the Great Bear magmatic zone, the volcanic centers evolved in composition from rhyolitic/dacitic-to-dacitic/andesitic compositions. Volcanism was concurrent with the emplacement of many generations of porphyritic-to-phaneritic granitic intrusions [16,27,62]. Mineralization took place at the transition from felsic-to-felsic-intermediate magmatism as Facies 1–2 evolved to Facies 2–3, forming MI-W and then MI-Co mineralization, as observed in the NICO Au-Co-Bi-Cu deposit. These mineralization styles predominantly occur in the sedimentary sequences forming the basement of the volcanic centers [17,62,86,122]. The subsequent phase of dacitic/andesitic magmatism was associated throughout the southeast Great Bear magmatic zone with conditions that formed the Facies 3 HT K-Fe alteration and associated IOCG mineralization, the latter of which preferentially developed in the volcanic or sub-volcanic porphyritic units. Examples include the Summit Peak IOCG prospect overlying the NICO deposit, the Sue-Dianne IOCG deposit, and several IOCG prospects in the vicinities of the Sue-Dianne deposit and in the Fab area [16,56,62,124]. This period of dacitic/andesitic magmatism was also associated with the formation of albitite-hosted MI-U mineralization associated with Facies 3 HT K-Fe, such as the showings along the Southern Breccia trend hosted in albitite that replaced the sedimentary sequences [28,64,86].

In the Olympic Cu-Au Province (Australia), minor basalts, basaltic andesite, andesite, and dacite evolved to abundant rhyolite in the Lower Gawler Range Volcanics (1595 to 1588 Ma) that host some of the IOCG deposits of the province, such as the Prominent Hill deposit [42,163,168,169,170]. At Prominent Hill, this volcanic sequence overlies an earlier clastic sedimentary basin with carbonate reef units [164]. Additionally, localized fluvio-lacustrine sedimentary rocks and acidic volcanic lakes also occur there [163,164]. At the supergiant Olympic Dam Cu-U-Au-Ag deposit, the host is a brecciated granite (Roxby Downs Granite) dated at 1593 ± 0.21 Ma [171]. The hematite associated with mineralization in three IOCG deposits of the province was dated at 1591.27 ± 0.89 Ma at Olympic Dam, 1598.9 ± 6.3 Ma at Wirrda Well, and 1590.6 ± 6.5 Ma at Acropolis [172,173].

In the Central Andes, andesite hosts are predominant and dacite is localized. Volcanism was followed by the emplacement of diorite laccoliths, local gabbro, and some tonalite, granodiorite, and monzonite with magnetite-series, as well as I-type medium-to-high K calc-alkaline affinity [75,114,174]. These units and their basement rocks have undergone extensive metasomatism and mineralization. In the Middle–Lower Yangtze River Metallogenic Belt, calc-alkaline-to-shoshonitic andesite units and their underlying diorite intrusions and sedimentary basins with carbonates, and evaporites host the MIAC centers [64,86,175]. All these case studies share strikingly similar sequences of magmatic evolution.

Some MIAC systems formed at the same time as the emplacement of carbonatite intrusions, such as those found in the Romanet Horst in Canada [49], or they were spatially associated with them, such as at Bayan Obo in China [176]. In many examples, alkaline intrusions, notably carbonatites, or potassic alkaline-to-ultrapotassic intrusions, were emplaced subsequently, e.g., the Johnnies Rewards prospect and the later Mordor complex in Australia [156]; the 1.35 Ga Bondy gneiss complex, and the 1.09–1.07 Ga Kensington-Skootamatta ultrapotassic to shoshonitic suite [19], and the Lac Cinquante U deposit, and the ultrapotassic extrusive and intrusive rocks of the Christopher Island Formation in Canada [47].

4.4. Syn- to Post-MIAC Batholith Emplacement

Batholiths are a regionally significant feature of MIAC metallogenic provinces, and they have long been interpreted as a key source of fluids for the development of MIAC systems. However, in certain MIAC provinces, the peak period of batholith development postdated the MIAC activity, as described below. This indicates that the batholiths were not necessarily involved in the development of the MIAC footprints and their primary metal endowment. However, the batholiths post-dating MIAC systems do play a role in recirculation of fluids that can remobilize and concentrate the metals deposited in pre-existing mineralization zones [85,177]. The tonalite, granodiorite, monzodiorite, monzonite, and granite intrusions that constitute batholiths (Figure 6) have continental I- to A-type geochemical affinities [62,139,174].

In the Great Bear magmatic zone, the initial magmatic flare-up associated with peak MIAC activities (1873 to 1868 Ma) transitioned to the emplacement of a batholith (1.866–1.85 Ga) that coincided with the deposition of voluminous dacitic-to-rhyolitic ignimbrite sequences [16,53,62]. Similarly, in the Olympic Cu-Au Province, there was a rapid shift from the syn-MIAC basaltic-to-rhyolitic Lower Gawler Range Volcanic, as well as from the coeval intrusions hosting the Olympic Dam and Carrapateena IOCG deposits (from 1595 to 1588 Ma) to the post-MIAC 1587 Ma dacitic Upper Gawler Range Volcanics [71,142,170]. Batholith emplacement continued for at least another eight million years following the development of the MIAC systems [170].

Granitic magmatism, invoked for the genesis of IOCG deposits in the Carajás district, has also been shown to predate the MIAC systems and their IOCG deposits in the district [149]. In the Central Andes, the batholith is, in part, coeval with the development of MIAC systems [88]. In certain geological provinces, the batholiths can also be spatially decoupled from the MIAC systems. For example, the Killarney granite is located tens of kilometers east of the MIAC systems of the Wanapitei district in Ontario [22,100].

4.5. Structural Context

At the scale of a metallogenic province, individual MIAC systems and their cluster of mineral deposits are connected via arc- or orogen-parallel fault systems and their splay faults that are active during MIAC development. Examples include (1) the Atacama Fault System in the Central Andes [40,88,141,143,161], and (2) the series of fault networks that linked the volcanic centers in the Great Bear magmatic zone, including the Wopmay fault zone [15,16,57,62,160]. Dilatational jogs along such fault systems hosted the volcanic centers, intrusions, and MIAC systems in the Great Bear magmatic zone [62] and the MIAC systems in the Central Andes [114]. Albitite corridors, formed along such faults, commonly host subsequent alteration facies and mineralization [16,28,68,88,89,97,98,104,106].

In the southern Great Bear magmatic zone, a change in the composition of the intrusions was associated with (1) a distinct change from higher-to-lower temperature alteration facies, (2) a transition from a compressional and ductile to an extensional and brittle deformation regime, and (3) the associated mineralization types [7,11,16]. The Fe-rich Au-Co-Bi NICO deposit formed in the waning stages of compressional ductile deformation and at the onset of brittle extension. In contrast, the Sue-Dianne IOCG deposit, the Southern Breccia albitite-hosted MI-U mineralization, and the Chalco and Summit Peak IOCG prospects adjacent to the NICO deposit formed in the initial stage of extension largely under a brittle regime [16]. A transition is observed from biotite-rich Facies 3, in which syn-metasomatism brittle–ductile tectonic fabrics like foliations are developed (in the Southern Breccia) to the K-feldspar rich Facies 3 and Facies 5 that were associated with brecciation in the zones of IOCG mineralization [10].

5. Discussion

In Skirrow et al. [34], Geoscience Australia addressed the parameters that affect IOCG ore formation and derived mineral potential criteria from them. Applied to the MIAC system and its entire range of deposit types, these parameters are as follows: (1) the sources of hypersaline hydrothermal fluids, salt melts, hydrosaline liquids, ligands, cations, metals, and sulfur that precipitate the metasomatites, as well as their primary metal endowment; (2) the energy and the kinetics of reactions that ‘drive’ the system; (3) the crustal and mantle lithospheric architecture, including the fault systems and major discontinuities that serve as fluid pathways; and (4) the ore depositional gradients induced by the spatial and/or temporal changes in the physicochemical parameters of the fluid plume that drive the precipitation of the distinct metal associations of MIAC systems. Field geology provides fundamental insights into all four parameters and is the focus of this discussion. The timing of the primary MIAC system and subsequent reworking is constrained through the dating of pre-, syn- and post-MIAC dykes, intrusions, and hydrothermal veins, as well as the dating of metasomatic minerals [16,17,62,172,173,177,178,179].

5.1. Metal and Fluid Sources: A Regional Field Geology Perspective

The suite of interconnected zones with extreme metasomatism—where replacement prevails over veining and brecciation, regardless of the host rock types and geological environments—indicates that the fluids driving MIAC systems are more aggressive and pervasive than common hydrothermal fluids (cf. the discussion in Tornos et al. [141]). This is acknowledged in Hofstra et al. [1] by the classification of MIAC-related deposits as ‘regional metasomatic’ rather than hydrothermal.

The next sections provide a field geology perspective on the nature of the primary metasomatizing fluid plume and the potential ingresses of external fluids based on the pre-, syn-, and post-MIAC volcano-plutonic and sedimentary events. The scientific breakthrough on the probable involvement of salt melts in the formation of mineral deposits and alteration facies in MIAC systems is also addressed as it helps to comprehend how the fluid plume can result in very intense metasomatism. The role of mantle metasomatism, the participation of mantle fluids, and the relationships between the nature of magmatism and the tectonic environment for establishing the MIAC system are beyond the scope of this paper.

5.1.1. The Primary Sources of Regional Fluid Plumes

The primary MIAC event (as opposed to subsequent remobilization) is commonly coeval with a magmatic flare-up that creates discrete volcanic centers with sub-volcanic intrusions of calc-alkaline to shoshonitic to A-type affinities (e.g., Great Bear magmatic zone, Bafq district, Middle–Lower Yangtze River Metallogenic Belt, and Olympic Cu-Au Province) [42,62,64,75,76,163,170,173,180,181]. Fertile magmas were oxidized and had very high temperatures and a lower degree of fractionation than magmas that postdate MIAC activities [145,182]. These magmas could also assimilate and potentially melt their wall rocks, including evaporite-rich units ([141] and the references therein). Magmatic systems could thus liberate a large quantity of metasomatizing fluids with a wide range of composition that would impart a magmatic signature to fluids trapped as inclusions in MIAC minerals [40,134,166,183,184,185,186].

Metasomatism proceeded broadly coevally along entire metallogenic provinces across pre-MIAC basements, sedimentary basins, and overlying syn-MIAC volcano-sedimentary basins, as well as those within syn-MIAC sub- and intra-volcanic intrusions [16,52,64]. In non tectonically disrupted components of the systems, the spatial and timing relationships among alteration Facies 1 to 6 along exposed, tilted stratigraphic sections record how systems evolved from the depth to paleosurface [15,55,56]. What elements were precipitated from the inflow fluid (i.e., metasomatites) and what was mobilized into the outflow fluids (i.e., the change in composition from hosts to metasomatites) can thus be interpreted. Such information, combined with the tight constraint on the coeval timing and province-scale extent of Facies 1 albitite corridors and the sequential formation of alteration Facies 1 to 6, indicate that (1) the metasomatizing fluids must likely collected into regional fluid plumes to trigger the system, (2) the plume must have been initially very rich in Na, and (3) single-pass infiltration of the fluid plume from the depth to the surface without complex overprinting relationships between the alteration facies, due to tectonic disruption or other regional-scale events, can take place in simple systems (cf. the discussion of single-pass infiltration in [89] and the single fluid model of [43]). The most fertile MIAC systems were commonly disrupted and are characterized by complex overprints of alteration facies as well as telescoping of alteration facies. Field indications of such disruptions could be the superimposition of high-temperature facies on lower-temperature facies or the cyclical formation of the same alteration facies. This is indicative of a dynamic environment in which the fluid plume was periodically rejuvenated by fluids from different sources and reheated from renewed magma emplacement.

The field metasomatic record testifies to the progressive dissolution of most, or even all, minerals from host rocks, as well as to the efficient transport at the local-to-regional scale of the elements dissolved by the primary fluid plume. This includes metals, Fe, S, volatiles, and, in some extreme cases, some of the normally immobile elements such as Al, REE, Ti, and Zr [6,24,27,47,187,188]. Through such intense metasomatism, a diverse range of metasomatized host rocks become sources for metals, volatiles, and fluids. Once captured by the primary plume, these external fluids will have transferred their own igneous, sedimentary, or metamorphic source signatures to the plume, including a basinal signature that is provided by evaporites and metamorphosed evaporites [121,184,185,186]. In parallel, as Ti and Zr are less mobile and—through their relative enrichment—metasomatic titanite, rutile, and zircon can be precipitated in the early formed albitite and, locally, in other alteration facies [31,61,68,177,188]. These early metasomatic minerals can then be dated to assess the timing of the primary metasomatic event [177,188], and the composition of their inclusions can be analyzed to assess the composition of the primary metasomatizing fluids [31]. Zeng et al. [31] discovered the earliest metasomatizing fluid observed to date in MIAC systems: Na-K-Fe-Ca-Cl-SO₄ hydrosaline liquids with Fe-chlorides as the main Fe species within zircon grains that were precipitated during the albitization event predating the formation of IOA deposits of the Middle–Lower Yangtze River Metallogenic Belt. These hydrosaline inclusions have 12–22 wt.% Na, whereasK, Fe, Ca, and S contents are up to 10–18 wt.%. They are enriched in Ti; contain halite, sylvite, Fe-chlorides, anhydrite, fluorite, ±hematite, and pyrite; have magmatic isotopic compositions; and their homogenization temperatures reach 930 °C [31]. Multiphase inclusions in other IOA deposits also record the presence of salt melts (e.g., chloride, sulfate, carbonate, and phosphate melts) that are rich in Na, K, Ca, Fe, Cl, F, and SO₄ and reach ~1000 °C in temperature [32,86,189,190].

The source of the salt melts in the fluid plumes and in IOA deposits remains unclear. As salt melts inclusions have been observed in albitite, their source must predate or be formed coevally with albitization. The exsolution of hypersaline fluids from sub-volcanic intrusions has been proposed as a possible source by Hildebrand [52] and Zhao et al. [86]. Albitite corridors are indeed common at the roof of laccoliths and lopoliths [31,52,57,86]. However, a sub-volcanic intrusion in the Great Bear magmatic zone host enclaves of pervasively albitized wall rocks while being weakly albitized in that area [14]. Hence, at least this intrusion must postdate the onset of intense albitization. In addition, at least one intra-volcanic pluton has albitite, both at its base and top (Tut pluton, [54]), and this and other sub-volcanic intrusions can be albitized themselves [14,16,17,27,52,54]. The combination of these observations indicate that the primary fluid plume must have been sourced from below the sub-volcanic intrusions and not within it, as was proposed earlier [52].

In MIAC provinces where sedimentary sequences prevail, albitite corridors have developed along regional discontinuities within sedimentary basins far from intrusions, as has been observed in the Mt Isa Province west of the Cloncurry district, as well as in the Romanet Horst and the Wanapitei district in Canada [22,50,66]. Magnetic and gravity anomalies directly associated with the metasomatic footprints of the systems extends for tens of km well beyond any single intrusion [160], and they can reach depths of 10 km, whereas the magnetotelluric footprints of the systems extend to the mantle [191]. Zeng et al. [31] interpreted the hydrosaline liquids to be formed by transcrustal magmatic systems during the migration of magmas and through exsolution from sub-volcanic intrusions, with preferential ponding of the fluids above the cupola of the intrusions at about 2 km depth. Such an interpretation is in line with the geophysical footprints of potential magmatic brine accumulation at a ca. 2 km depth in active volcanic settings [192].

The assimilation and melting of evaporite or carbonate rocks by magmas and the subsequent immiscibility of salt melts are also invoked to form IOA deposits [32,190], and they are among the many genetic models for such deposits [141]. Most models are currently irreconcilable with the observed formation of albitites prior to the IOA deposits at the regional scale [7]. However, andesite flows that host IOA deposits show evidence for Fe-rich, as well as Cu-rich immiscible sulfide, melt inclusions [193]. Such melts and their metals may have made their way into the primary fluid plume we interpret to trigger the formation of MIAC systems based on our field geology constraints.

What we envision is that fluids, including salt melts, (1) originated from magmatic sources with potential additional contributions from the assimilation, metasomatic devolatilization, and melting of evaporitic or carbonate sources, and (2) they also collected into giant hypersaline, hot, and corrosive fluid plumes in the mid-crust. Metasomatism was triggered by the fluid plume as it raised concurrently with the magmas that formed the volcanic sequences and sub-volcanic intrusions. The fluid plume and the magmas may have used similar zones of enhanced crustal permeability like crustal-scale fault systems. The plume would then flow and infiltrate along and across major discontinuities in the upper crust, including along discontinuities in which laccolith and lopoliths were emplaced and albitite corridors formed. Additionally, the faults, breccias, and damage zones related to subsidence of multiple cauldrons (e.g., the Camsell River district, Great Bear magmatic zone, Canada [52,53,165], and southeast Missouri district, USA [194]) would further facilitate fluid flow and metasomatism. Fluids that precipitate early within the alteration sequence commonly have a magmatic signature, while subsequent ones show greater interaction with host rocks or as having tapped additional non-magmatic fluid sources [86,164,195].

5.1.2. The Secondary Sources: Insights from Metasomatized Host Rocks and Geological Environments

The nature of the host rocks metasomatized across the upper crust, combined with the field relationships among metasomatic, igneous, metamorphic, and sedimentary rocks, provide indications of the fluids and metals added to the main fluid plume as metasomatism proceeded. The metasomatic mineral dissolution during regional-scale albitization in Facies 1 occurs in all rock types (Table 1). Such pervasive dissolution of compositionally diverse host rocks must release, into the plume, a wide variety of volatiles, metals, cations, and ligands, including those found in the more reactive carbonate and evaporite units. The self-propagating dissolution process implies that, at each stage, the elements of the minerals unstable in host rocks become available and mobilized in solution and precipitate, at a subsequent alteration stage, sustaining the metal recharge of the main fluid plume.

Pre-MIAC sedimentary basins can host SEDEX mineralization [138], which may provide a source for Pb and Zn, whereas their mafic rocks can become a source for Cu and Zn [99], leading to MIAC deposits with Zn resources such as the Mount Dore Cu-Au-Pb-Zn deposit in the Cloncurry district, Australia (Table A1) [196]. In addition, pre- and syn-MIAC sedimentary basins can contain formation waters [197]. Their evaporites, including those metamorphosed with scapolite, can provide volatiles and S once metasomatized, and they could also release sulfate melts if assimilated or partially melted by magmas [66,190,198]. Scapolite and sulfate minerals are not the sole markers of evaporite and saline host rocks. For example, scapolite is largely absent in the metasomatized sedimentary rocks of the Great Bear magmatic zone and the fluid inclusions among MIAC mineralization in the region largely recorded magmatic fluid sources [121,122,166]. Yet, albitite can have amphibole-rich components that pseudomorph the nodular textures and bedding structures typical of nodular anhydrite beds, which suggests the presence of an evaporite sequence (E in Fig. 6; Fig. 10A,B in [11]). The pseudomorphing of nodular textures has also been observed in the Norrbotten district [152]. Stromatolite-bearing units were metasomatized to magnetite and albite (Fig. 10C–G in [11]); their stratigraphic location within the Grouard Lake sequence (S in Figure 6) [53] indicates that they may have precipitated within a saline lake in a volcanic cauldron [7,10,11]. Such lakes have been interpreted as a key fluid source for IOCG deposits [164].

The metasomatism of tens of km long carbonate sedimentary units and of graphitic schists (e.g., Cloncurry district, the Great Bear magmatic zone, Romanet Horst, and Norrbotten district) [11,17,49,66,90] requires the release of CO₂ to the fluid plume. Fluid signatures in deposits hosted in such settings are commonly magmatic [66,122,183,195], but later-stage alteration zones have also recorded extensive interaction between fluids and host rocks [195]. Considering the geological environments in which MIAC plumes ascend and propagate, it is likely that the plumes incorporated other fluids encountered along the fluid pathways as described above, as well as the additional magmatic fluids liberated by syn-MIAC intrusions and dyke swarms [16,31,52,124]. As the fluid plume leached metals and volatiles from a wide diversity of host rocks on a regional scale, the outflow fluids acquired a mixed and evolving source signature; hence, the commonly observed record of distinct fluid compositions at different stages of mineralization in MIAC systems [33,66,67,121,183,184,185,186,199,200,201,202]. Distinct fluid compositions at the different stages of mineralization are intrinsic to the metasomatic evolution of the system and do not imply that distinct fluid sources created the distinct stages of the MIAC system (see [203]). However, external ingresses of fluids that were not originally present in the evolving fluid plume (e.g., fluids of magmatic, basinal, metamorphic, and meteoric sources) probably did occur and changed the composition of the parental fluid plume transiently or permanently, depending on the volume of the fluid ingress. The access to external fluids may have been essential to the formation of giant mineral deposits [164,202].

5.2. Energy Driver of Fluid Flow

Coeval MIAC systems developed into 600 to 2000 km long metallogenic belts [40,62,139,146,147,152,204], and they are rooted into a metasomatized mantle through transcrustal magma systems and fault zones at the margins of thick lithospheres [9,33,34]. Consequently, the conditions that triggered, drove, and sustained, in the same geological province, the relatively coeval formation of MIAC systems over 100s of km cannot be a local phenomenon but required a significant and regional thermal event that likely originated from the mantle. This thermal event triggered the formation of a belt-scale transcrustal magmatic system (with a generation of salt melts?). In many geological provinces, the initial flare-up of the MIAC activities event was associated with oxidized calc-alkaline to shoshonitic magmatism, which formed discrete volcanic centers. At the later stages and after development of the MIAC systems, the magmatic system expanded to form belt-scale batholiths made of intrusions with a prevailing A-type signature. This is observed in the Central Andes, the Great Bear magmatic zone, the Middle–Lower Yangtze River Metallogenic Belt, and other settings globally [34,62,144,161,174,180,205]. Syn-MIAC magmas were, in part, sourced in the mantle and have high temperatures with trapped melt inclusions that indicate temperatures of up to 1145 °C in andesite [193] in systems where a very high-temperature IOA mineralization prevails and with zircon crystallization temperatures reaching ~950 °C in systems where IOCG mineralization is the most developed [182].

The magmatic systems, in addition to the fluid plumes, can transfer, at the scale of the metallogenic belt, significant heat from the mantle to the upper crust to drive (or energize, once triggered) the coeval formation of multiple MIAC systems [206]. In addition, the salt melts and hydrosaline liquids inclusions observed within albitite, skarn and IOA deposits indicate that salt melts and hydrosaline liquids probably coexisted in the fluid plume with hydrothermal fluids and considerably enhanced the fluid’s ability to carry, mobilize, and transport cations, volatiles, and metals, as well as even the most incompatible elements such as Ti. Do the salt melts form the primary fluid plume or are they added to a primary fluid plume that is hydrothermal in origin? Are the salt melts sourced in the mantle or are they only derived at the crustal level? Whatever their sources, the presence of salt melts into the hypersaline fluid plumes can sustain their extreme heat and corrosive ability to dissolve most minerals and precipitate metasomatites with mineral assemblages that reached typical of upper amphibolite-to-granulite metamorphic facies (e.g., hypersthene) [117] and textures typical of intrusive rocks [11].

5.3. Structural–Stratigraphic Architecture and Porosity Creation That Enable Fluid Flow

The plumes of fluids—and the salt melts they incorporate—are interpreted to collect and pool in the middle crust. A progressive switch from a compressional to an extensional regime that is commonly documented in MIAC provinces could have promoted the migration of the fluid plumes along transcrustal faults toward the upper crust, like the orogen-parallel Atacama Fault System along which multiple MIAC systems formed (~2000 km) [40]. Such fault systems are commonly rooted in the mantle [33,34,191]. As the plumes reached the upper crust, based on the distribution of alteration facies (and their geophysical footprints), they laterally propagated along fault systems and the interconnected crustal discontinuities that included, for example, damage, fractured, and brecciated zones [88,89,93,94,160,207]. The fluids also used other discontinuities, such as more permeable chemically reactive or brittle layers in stratified units (e.g., sedimentary rocks), major contacts between geological units (like the upper contact of sub-volcanic intrusions with their volcanic ejecta), and pre-existing zones of permeability (like the breccia zones in the walls of calderas) [52,91].

From the micro to the regional scale, the infiltration and propagation of the fluid plumes from the major discontinuities into the host rocks were also controlled by the metasomatic dissolution and reprecipitation of minerals and the creation of transient porosity [63,68]. For example, an albitite is highly porous and remains so unless that albitite was recrystallized above later sub-volcanic intrusions [14,28,52,63,68] or through orogenesis [19]. Permeability channels may also have formed through volume loss during metasomatic devolatilization (e.g., CO₂ release through albitization of the carbonates in the lower temperature system of the Romanet Horst and through development of skarn or of magnetite alteration at the expense of carbonates in the Great Bear magmatic zone and Moonta-Wallaroo region of Australia [11,17,89,92]). This is a result of the thermodynamic and kinetic disequilibrium between corrosive fluids and host rocks [203].

Along fault zones, pre-MIAC fracturing can form zones of damage and brecciation in the host rocks that facilitate the formation of regional albitite corridors. Once established and aided by the intrinsic porosity of the albitite, these corridors become preferential sites for further development of damage and breccia zones as the fault zones are reactivated. The corridors of brecciated albitite become preferential pathways for subsequent Fe-rich to Fe-poor mineralization (Table 1, Table 2, Table 3, Table 4, Table 5, Table 6 and Table 7) [28,68,93,109,110,133,207].

The volcano–plutonic environment also creates extensive breccias that aid fluid flow. An example is the large breccias with fragments of the sub-volcanic intrusions spalled from the walls of a caldera during the caldera collapse in the Camsell River district in the Great Bear magmatic zone [165]. These breccias were infilled by hematite. At the Olympic Dam deposit, extensive brecciation of the host granite has been attributed to the collapse and hydraulic brecciation of a granitic intrusion cupola during the uplift that was followed by depressurization, leading to the exsolution of orthomagmatic fluids and precipitation of hematite [173,178].

5.4. Depositional Environment: Alteration Facies as Predictive Indicators of Mineralization

The confluence of the multi-scale, fluid-driven metasomatic processes, notably the infiltrative replacement, veining, and brecciation at each facies, induces the coupling and decoupling of elements and metals into distinct metal associations and the genesis of facies-specific mineralization types (Figure 7). In contrast, structural or lithological/chemical traps concentrate the deposition of metals to form mineral deposits [22,94,207], whereas, at the scale of the metallogenic belt, deposits are associated with deep, transcrustal fault zones and their splays (Section 5.3; [208,209]). Hence, a critical window for ore genesis is where fertile alteration facies encounter chemically reactive geological units or favorable structural traps. The shape of the resulting mineralization zones becomes a function of the original distribution of the host rocks in the geological sequence and of the geometry of the major discontinuities along which the fluids have circulated, such as sedimentary units and fault zones.

Field observation indicates that mineralization is varied but localized in MIAC systems that exhibit a linear and simple evolution of alteration facies, which is interpreted as indicating an absence of significant rejuvenation of the fluid plume by a voluminous ingress of external fluids. MIAC systems, with a full spectrum of alteration facies and with indications of overprinting or telescoping of alteration facies, are interpreted as evidence for cyclical rejuvenation of the fluid plume and are found to commonly host deposits with varied mineralization styles. Yet, even in a MIAC system with polyphase and complex alteration patterns, the overall evolution and the relative timing among alteration facies remain largely the same as the one depicted in Figure 7. An example of the cyclical formation of distinct facies and renewed mineralization has ben observed between Facies 2 and 2–3 at the NICO deposit, Canada [15].

The relationships described between the alteration facies and deposit types in Figure 7 are those observed from the metal associations that are associated with each alteration facies. Of note, each alteration facies can also become a preferential host to the mineralization associated with an overprinting alteration facies. During an exploration program, identifying the alteration facies associated with the main mineralization event(s) can optimize exploration as it indicates what mineralization types are most likely to be present in the tested zone of the system. For example, many IOCG exploration programs failed, as the drill targets were magnetic bodies of Facies 2 HT Ca-Fe alteration, which are not fertile for Cu mineralization. The association between alteration facies and mineralization and of the overprinting relationships between alteration facies in mineralization zones are best documented through careful macroscopic descriptions of the mineral assemblages on outcrops or drill cores in the field. Supplementing the field descriptions with techniques like cobaltinitrate staining or micro-XRF analysis of rock slabs and drill cores can considerably improve the descriptions [11,14,22]. Discriminating alteration facies using microscopic observations in thin sections is often difficult as the scale is too small to clearly define overprinting relationships between different alteration facies.

In all systems, the corridors of intensely Na-altered rocks with abundant and extensive zones of albitite are intrinsic to the development of the MIAC systems. They also serve as a diagnostic and distinctive marker of the onset of a MIAC system. Though albitization zones can be extensive in other mineral systems, such as volcanic massive sulfide mineral systems, the hosts are not as extensively and intensively albitized to pervasive albitite [6].

Prior albitization events can occur in sedimentary basins due to the internal circulation of saline basinal fluids (e.g., Cloncurry district and Mount Isa Province) [82,211]. Some mineral assemblages of Facies 5 can also contain abundant albite following the progressive recharge of the fluid plume in Na from Facies 2 to 4 [22,68]. Facies 5 albite-carbonate alteration can be associated with Cu-Au mineralization and albite-chlorite with Au mineralization [22]. Here, the emphasis on what is a diagnostic of, and a mappable prospectivity criteria for, a MIAC system is placed on the regional-scale development of early albitite, intense scapolite-albite, and intense oligoclase-rich corridors, not simply on the presence of albitized rocks.

Facies 1 Na can also act as a metallogenic preconditioning factor for a wide range of MIAC deposit types. Arrested fronts of albitization record the mobilization of dissolved cations into subsequent alteration facies and their incremental development as metasomatism intensifies [14]. Albitite also records the progression of the primary fluid flow along fractures (e.g., haloes along fractures), damage zones (e.g., coalescing fracture haloes and albitization network), intrusive contacts, preferential stratigraphic sequences, and regional-scale fault zones and splays, many stemming from a transcrustal fault network [62,91,94,140,160,208].

In carbonate rocks, skarns form coevally with the albitite zones, and the fluid signature can be similar [91], whereas an albitite can form instead of a skarn in low-temperature environments, as observed in the Romanet Horst [49,50]. Albitite (and other regional Na alteration) and associated skarn predate IOA mineralization at deposit-to-regional scales (e.g., the Great Bear magmatic zone, Middle–Lower Yangtze River Metallogenic Belt, Norrbotten district, and the Central Andes; Figure 1) [6,7,31,52,55,88,90,205]. Albitization can also slightly predate the emplacement of sub-volcanic intrusions [14].

The linkages between alteration Facies 2 to 6 and their facies-specific metal associations and deposit types provide mappable mineral potential criteria for the deposits formed in MIAC systems. The Na facies transitions from Na-Ca (albite-scapolite) to HT Na-Ca-Fe assemblages, where albite, amphibole, and magnetite prevail, and then to Facies 2 HT Ca-Fe alteration. The Na to Na-Ca-Fe facies are barren, but the Na-Ca-Fe facies, which marks the first stage of Fe ingress in a MIAC system, can indicate potential proximity to a mineralized Fe-rich alteration facies. With the progressive addition of Fe in the system, the Facies 1 skarn also transitions to the Skarn+Fe facies that can be associated with W mineralization.

Facies 2 mineral assemblages are dominated by amphibole, magnetite, and, more rarely, apatite, and they also replace host rocks or precipitate as veins and cement breccias within the zones of albitite and the least-altered host rocks. The HT Ca-Fe facies has the largest footprint in carbonate-bearing sedimentary sequences, where zones of skarn (clinopyroxene and garnet) may form early, pre- to syn-albitite but are progressively replaced by Facies 2 assemblages. Areas of interest at the regional scale are delineated by Facies 1 to 2, which also host magnetite skarn and iron oxide-apatite (IOA) deposits. The latter are commonly spatially associated with dioritic intrusions and andesite sequences, such as in the Middle–Lower Yangtze River Metallogenic belt and the Great Bear magmatic zone [31,52,64,212]. With heat ingress from high-temperature diorite, IOA mineralization at Facies 2 HT Ca-Fe alteration can thus precipitate under very high temperatures, most being between 600 and 1000 °C (Section 5.2).

Systems that evolve toward conditions forming Facies 3 within sedimentary rocks can precipitate the MI-Co deposits, with metal associations of Au, Co, and Bi, at Facies 2–3 HT Ca-K-Fe alteration, such as the NICO deposit in the Great Bear magmatic zone [17,122]. Brecciation is typically minor. In contrast, extensive brecciation is associated with Facies 3 HT K-Fe (magnetite-K-feldspar/biotite), Facies 4a K-skarn (clinopyroxene, garnet, and K-feldspar), Facies 4b K-felsite (K-feldspar), and the Facies 5 low-temperature K-Fe ± Ca, Mg, H+, CO₂, Si, and Ba (sericite, K-feldspar, hematite, chlorite, carbonate, and epidote). The formation of MI-Cu deposits, including IOCG and ISCG, and of MI-U deposits is associated with Facies 3 and 5a (Figure 7). The IOCG deposits associated with Facies 3 can also be enriched in Co or include Co resources, such as at the Ernest-Henry Cu-Au deposit in the Cloncurry district with Co reaching 2 wt.% in pyrite [126] and the Kiskamavaara Cu-Co deposit in the Norrbotten district [213,214]. Biotite-rich HT K-Fe alteration is also associated with the formation of MI-Co deposits, such as the Ram deposit in the Idaho Cobalt Belt [123]. Facies 6 low-temperature K, Si, Al ± Ba, and Fe were found to be associated with the formation of vein systems, sericitic and phyllic alteration, and epithermal lithocaps [56,57,60,84].

One of the primary challenges of doing visual depictions of alteration zonation in MIAC systems is that the geometry in 3D of the zonation of the alteration facies can vary considerably among MIAC systems based on the geometries of the main discontinuities and the common tectonic disruption of the system during metasomatism. For example, the geometry of alteration zones in a MIAC system formed in predominantly layered sequences (e.g., sedimentary or volcaniclastic) will be considerably different from the geometry of alteration zones in a MIAC system formed in a brittle–ductile shear zone, or from a MIAC system forming a breccia complex at the intersection of two brittle faults in an intrusive complex. This level of geometrical complexity (e.g., discordant alteration zonation) can further be compounded by faulting and with new infiltration stages of the fluids from the plume or from the ingress of external fluids that could be circulating along crosscutting discontinuities.

5.5. Perspectives on Knowledge and Data Gaps

5.5.1. The Need to Acquire Regional Geological Data on MIAC Systems

As per Wyborn et al. [36], truly mappable criteria are, first, boots on the ground. For MIAC systems, there is a strong need to incorporate, in regional mapping programs, the routine description of observable alteration facies that record energy, fluid, and metal sources, as well as the potential geological, chemical, and structural pathways, traps, and thermal gradients, for metals and fluids. This can only be achieved by having appropriate taxonomies and lexicons to cover the realm of geological attributes that are characteristic of each alteration facies ([10,46] and in preparation). This includes lexicons to describe the extensive replacement, veining, and brecciation zones in MIAC systems and their relationships to host rock types and to coeval or later magmatism, sedimentation, metamorphism, and tectonic activities. Considering the realm of Fe oxide-rich to Fe oxide-poor to Fe-poor deposit types, exploration strategies must also move beyond an IOCG-centric approach and integrate the varied information available at district scale on prospects and alteration facies to assess the prospectivity of the MIAC system explored. The selection of geophysical methods to detect areas of mineralization needs to consider the mineralogical expression of Fe enrichments in a targeted mineralized alteration facies. For example, geophysical methods used to detect Fe oxides may not be suitable if mineralization is associated with Fe sulfides or Fe silicates.

Once formed, any MIAC system can be metamorphosed during orogenesis [157]. The Bondy gneiss complex, Grenville Province, Canada [19], and the Johnnies Rewards deposit in the Northern Territory, Australia [156], are examples of MIAC systems that were metamorphosed at granulite facies. The atypical bulk rock compositions of the alteration facies in MIAC systems, where metasomatism is intense, produce diagnostic metamorphic mineral assemblages with very atypical mineral contents in the metamorphosed metasomatites [19,157]. Notably, Facies 1 albitite are metamorphosed to plagioclase-dominant gneisses; Facies 2 HT Ca-Fe to amphibole-, garnet-, or magnetite-rich gneisses; Facies 3 and 5 HT and LT K-Fe to varied garnetite units, orthopyroxene gneisses, and semipelite-like gneisses; Facies 6 argillic and advanced argillic alteration to metapelite-like gneisses; and tourmaline alteration to rocks that were originally interpreted as being metaexhalites [19,157]. The intense leaching of Na or K in the metasomatized rocks significantly limited partial melting and enabled the preservation of some protolith textures and of the bulk-rock composition of the metasomatites [19,215]. However, the intense deformation typical of high-grade metamorphic belts can transpose the discordant alteration zones of the metamorphosed metasomatites into layered amphibole-, biotite-, cordierite-, garnet-, magnetite-, sillimanite-, or tourmaline-rich gneisses that resemble paragneisses (i.e., gneisses with a sedimentary origin) [19,215]. Nevertheless, the combination of lithological associations and mineral contents—and the variation thereof—remain diagnostic of metamorphosed metasomatites, facilitating the recognition, interpretation, and exploration of MIAC systems in high-grade metamorphic belts.

Both the Johnnies Rewards prospect and the Bondy gneiss complex have been subjected to multiple genetic interpretations. The lithogeochemical distribution of the samples from the Bondy gneiss complex in the AIOCG diagram, as well as in other alteration diagrams and their ranges of Na-Ca-Fe-K-Mg molar proportions [19], are typical of the combined iron and alkali–calcic enrichments of MIAC systems and are distinct from those of other mineral systems [19]. Of note, both case examples are within regions with potassic alkaline intrusions (the Mordor complex and the Kensington Skootamatta potassic alkaline intrusion and its minette dyke, respectively) [156,216]. Such spatial associations between metamorphosed MIAC systems and recognizable potassic and ultrapotassic intrusions provide new research avenues on settings prospective for critical minerals.

5.5.2. Beyond the Predictability of Metal Associations and Deposit Types at Each Facies

The holistic mineral system model presented in Figure 7 provides a robust framework for the regional evolution of metal associations and deposit types within MIAC systems according to the space–time distribution of their alteration facies. Deposits, prospects, and alteration facies thus serve as vectors to mineralization, whether they are rich or all the way to poor in Fe oxides and Fe, furthering the realm of mappable prospectivity criteria and the ability to assess the mineral prospectivity in geological provinces with potential for MIAC systems. What controls the metal endowment of the mineralized zones of MIAC systems remains to be resolved. Many researchers suggest that pre to syn-MIAC geodynamic environments and mantle fertility may play a crucial role on the overall metal endowment of a MIAC system [9]. The metal content of the geological sequence replaced by a MIAC system also play a crucial role as a source of metals for ore deposition [10,11,24]. Certain compositional characteristics of the geological sequence may favor the formation of certain deposit types. For example, MI-Co deposits are preferentially formed in geological sequences with abundant sedimentary rocks. The influx of additional metals and fluids from the geological sequence to the plumes as metasomatism progresses can also carry the distinctive isotopic signatures of the host rocks into the alteration zones, as demonstrated for Fe isotopes by Emproto et al. [200].

The metal endowment of the primary fluid plume is more difficult to assess, as even the source signatures detected in the fluid and melt inclusions in albitite must have undergone modification from the sodic metasomatism of their host. The salt melts cannot also directly precipitate Fe oxide magmas without the main fluid plume initiating the regional sodic metasomatism that is first based on the timing relationships between Facies 1 and Facies 2. Deposit models for IOA deposits need to account for the distribution of albitite corridors along deep-seated fault zones and other discontinuities, including above and locally below sub-volcanic intrusions. Breakthroughs in the understanding of immiscible or exsolved salt and sulfate melts, however, provide key insights on how the parental hypersaline fluid plumes can achieve the high temperatures needed to form igneous-like metasomatic textures, as well as the mineral assemblages akin to upper amphibolite through to granulite facies conditions.

6. Conclusions

In the last few decades, the absence of a unifying framework to explain the observed metallogenic diversity of Fe-rich but Fe oxide-poor, as well as Fe-poor alkali–calcic deposits that are spatially associated with IOCG deposits, has resulted in significant confusion on the significance of these deposits, how to differentiate them from sensu stricto IOCG deposits, how to name and classify them, and how to assess their mineral potential. This paper exemplifies that IOCG deposits are one of many deposit types that can be formed into the district-scale metasomatic mineral systems that are defined as Metasomatic Iron and Alkali-Calcic (MIAC) mineral systems.

From the extensively published field geology attributes of alteration facies, it was possible to address how, in a short time and throughout a geological province, a series of interconnected MIAC systems forming diagnostic alteration facies and mineralization types can be developed. A combination of hydrosaline liquids, salt melts, and hypersaline hydrothermal fluids are interpreted to have been derived from transcrustal magmatic systems (and mantle?) and to have accumulated and collected in discrete areas of enhanced crustal permeability in the mid crust to form a series of large fluid plumes along the metallogenic provinces. Regional field geology observations indicate that the parental fluid plume is generally magmatic in origin and can transport metals, such as Cu and Au.

Series of fluid plumes ascended toward the upper crust along a main transcrustal-, continental arc-, or orogen-parallel fault system, reaching 2000 km in length. In many MIAC systems, the ascent of the fluid plumes occurred concurrently with the syn-MIAC magmas that formed discrete volcanic centers and shallow intrusive complexes. The spatial overlap between MIAC and magmatic systems can enable the periodic transfer of heat- and magma-derived fluids to the fluid plume that evolves in the upper crust.

As the plumes reached the upper crust, they infiltrated and percolated vertically and laterally along faults, within reactive or permeable geological units, and along stratigraphic and intrusive discontinuities. Their ascent and percolation in the upper crust was found to be associated with a suite of metasomatic reactions that dissolved the original minerals in the host rocks and precipitated new (metasomatic) ones. The dissolved elements (including metals and volatiles) were added to the compositionally evolving fluid plume. As the fluid plumes percolated into the upper crust, fluids originating from upper crustal sources, like sedimentary basins, caldera lakes, and meteoric waters, could also mix with the fluid plumes and contribute metals.

The chain of metasomatic reactions that formed the sequence of diagnostic alteration facies with distinctive metal associations, gave way to a predictable series of facies-specific critical and precious metal deposit types, which are rich to poor in Fe. Deposit classes are specific to each alteration facies, whereas commodities may precipitate in more than one facies. Deposit classes are thus defined to highlight the main commodities of deposits to respond to the commonly commodity-driven exploration. Accordingly, MIAC systems host Facies 1–2 MI-W (W-skarn); Facies 2 MI-Fe (Fe-skarn, IO, and IOA), MI-REE (remobilized IOA-REE), and MI-Ni; Facies 2–3 MI-Co; Facies 3 and 5a MI-Cu (magnetite- and hematite-group IOCG, ISCG, and other MI-Cu), MI-U (including albitite-hosted IO-U), and rarer MI-Co; Facies 5b MI-Au; Facies 5c and 5d MAC-Au, MAC-Cu, MAC-Mo, or MAC-U (which are typically albitite-hosted); and, finally, Facies 6 epithermal mineralization and later-stage five-element and other mineralized veins. The distinct alteration facies and their mineralization types can have a depth to surface distribution and/or be overprinted on top of each other, but the global diachronous formation of IOA (earlier) and IOCG (later) remain.

Dynamic tectono-magmatic environments favoring episodic rejuvenation of the fluid plumes or its lateral offshoots with the addition of external fluids and heat can increase the potential of the resulting MIAC systems to host large mineral deposits. Each time new fluids circulate in older mineralized alteration facies, metals can be remobilized and reconcentrated locally remaining within their own alteration facies, or they can be channeled away along fault zones.

Once formed, any MIAC system can then be metamorphosed, with examples at granulite facies being preserved in Canada and Australia.