Metasomatic iron and alkali-calcic (MIAC) mineral systems form district-scale metasomatic footprints in the upper crust that are genetically associated with iron oxide–apatite (IOA), iron oxide and iron sulfide copper–gold (IOCG, ISCG), skarn, and affiliated critical and precious metal deposits. The development of MIAC systems is characterized by series of alteration facies that form key mappable entities in the field and along drill cores. Each facies can precipitate deposit types specific to the facies or host deposits formed at a subsequent facies. Defining the spatial and temporal relations between alteration facies and host rocks as well as with pre, syn, and post MIAC magmatic, tectonic, and mineralization events is essential to understanding the evolution of a MIAC system and to evaluating its overall mineral prospectivity. This paper proposes an ontology for MIAC systems that frames the key characteristics of the main alteration facies described and links it to a taxonomy and descriptive lexicons that allow the user to build an efficient data collection system tailored to the description of MIAC systems. The application developed by the Geological Survey of Canada for collecting field data is used as an example. The data collection system, including the application for collecting field data and the lexicons, are applicable to regional- and deposit-scale geological mapping as well as to drill core logging. They respond to the need for the metallogenic mapping of mineral systems and the development of more robust mineral prospectivity maps and exploration strategies for the discovery of critical and precious metal resources in MIAC systems.

In the absence of appropriate tools and a knowledge base for exploring high-grade metamorphic terrains, felsic gneiss complexes at granulite facies have long been considered barren and have remained undermapped and understudied. This was the case of the Bondy gneiss complex in the southwestern Grenville Province of Canada which consists of 1.39–1.35 Ga volcanic and plutonic rocks metamorphosed under granulite facies conditions at 1.19 Ga. Iron oxide–apatite and Cu-Ag-Au mineral occurrences occur among gneisses rich in biotite, cordierite, garnet, K-feldspar, orthopyroxene and/or sillimanite-rich gneisses, plagioclase-cordierite-orthopyroxene white gneisses, magnetite-garnet-rich gneisses, garnetites, hyperaluminous sillimanite-pyrite-quartz gneisses, phlogopite-sillimanite gneisses, and tourmalinites. Petrological and geochemical studies indicate that the precursors of these gneisses are altered volcanic and volcaniclastic rocks with attributes of pre-metamorphic Na, Ca-Fe, K-Fe, K, chloritic, argillic, phyllic, advanced argillic and skarn alteration. The nature of these hydrothermal rocks and the ore deposit model that best represents them are further investigated herein through lithogeochemistry. The lithofacies mineralized in Cu (±Au, Ag, Zn) are distinguished by the presence of garnet, magnetite and zircon, and exhibit pronounced enrichment in Fe, Mg, HREE and Zr relative to the least-altered rocks. In discrimination diagrams, the metamorphosed mineral system is demonstrated to exhibit the diagnostic attributes of, and is interpreted as, a metasomatic iron and alkali-calcic (MIAC) mineral system with iron oxide–apatite (IOA) and iron oxide copper–gold (IOCG) mineralization that evolves toward an epithermal cap. This contribution demonstrates that alteration facies diagnostic of MIAC systems and their IOCG and IOA mineralization remain diagnostic even after high-grade metamorphism. Exploration strategies can thus use the lithogeochemical footprint and the distribution and types of alteration facies observed as pathfinders for the facies-specific deposit types of MIAC systems.

This paper addresses the complex hydrothermal evolution of Metasomatic Iron and Alkali-Calcic (MIAC) mineral systems based on a review of the lithogeochemical footprints of IOA and IOCG deposits in the northern Norrbotten province (Sweden) and the Cloncurry district (Australia). The use of Na-Ca-Fe-K-Mg molar barcodes on a lithogeochemical diagram tailored for these mineral systems allows to depict the evolution of MIAC systems along diagnostic metasomatic paths from high (HT) to low temperature (LT) alteration facies as follows: 1) HT or LT Na alteration (300-1000 degrees C); 2) HT CaFe alteration (400-1000 degrees C); 3) HT KFe alteration (350-450 degrees C); 4) HT K and HT K-Ca-Mg alteration; 5) LT KFe, Na-Ca-Mg-Fe, and/or Na-Ca-Mg alteration (<= similar to 350 degrees C); and 6) epithermal alteration (<= 150 degrees C) and later stage hydrothermal veining. A distinct range of whole rock compositions and metal associations characterizes each alteration facies and can be captured by diagnostic molar barcodes and alteration indices. In northern Norrbotten, the IOA deposits are hosted in HT CaFe alteration facies but regionally intensely albitized regions are overprinted by KFe alteration. The IOCG deposits are hosted in MIAC systems with zones of early Na (Ca) alteration related to the regionally extensive albitite or scapolite alteration (Facies 1) and localized skarns. These are overprinted by HT CaFe alteration (Facies 2) and HT to LT KFe alteration (Facies 3 and 5). The CuAu mineralization is not systematically associated with the iron oxide-rich breccias and the intense K-feldspar- or sericite-rich KFe alteration typical of many IOCG deposits worldwide. Instead, the lesser intensity of alteration and the abundance of mafic and ultramafic rocks in the environment lead to pattern enriched in Mg with relic of amphibole-rich alteration remaining in the assemblage as demonstrated for the Nautanen North IOCG deposit (Sweden). Consequently, the geochemical footprints of the Norrbotten CuAu deposits are distinct from magnetite-group (e.g., Great Bear magmatic zone, Canada) and hematite-group (e.g., Olympic Dam, Australia) IOCG deposits even if they have all the known alteration facies of MIAC systems. Conversely, IOCG deposits in northern Norrbotten show similarities to certain deposits in the Cloncurry district of Australia. In both regions, the IOCG deposits are associated with HT CaFe and KFe alteration facies that commonly overprint early Na and/or NaCa alteration. In northern Norrbotten, IOA deposits are characterized by early Na alteration evolving towards NaCa alteration, then Fe-rich CaFe alteration. These hydrothermal alteration types are subsequently superimposed by later KFe alteration. We conclude that the use of Na-Ca-Fe-K-Mg molar barcodes provides new insights to understand the evolution of MIAC systems and is a powerful approach for unraveling superimposed alteration trends, which can serve as an exploration targeting tool from the district- to the deposit-scale in complex metasomatized areas.

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.

Recent advances in the characterization of metasomatic iron and alkali-calcic (MIAC) systems with associated iron-oxide apatite (IOA) prospects and iron-oxide–copper–gold (IOCG) and metasomatic cobalt deposits of the Great Bear magmatic zone were used to determine if the geochemistry of glacial sediments can unveil pathfinder elements indicative of mineralization and associated alteration. Analysis of variance within bedrock lithogeochemical (n = 707 samples) and till geochemical datasets (n = 92 samples) are compared. Results show that Fe, Co, Ni, Cu, As, Mo, Bi, La, Th, U, and W were identified as potential vectoring elements in different fractions of till due to their anomalous concentrations down-ice of various mineralized outcrops within the study area. For instance, Fe, Co, Cu, and Mo were established as the most useful vectoring elements in the locally derived till (<2 km down-ice) near the Sue Dianne IOCG deposit, and Fe, Co, Ni, Cu, Mo, W, Bi, and U near the Fab IOCG prospect. At the Sue Dianne deposit, the ratios of near-total (4-acid digestion) versus partial (modified aqua regia digestion) concentrations in the silt + clay-sized till fraction (<0.063 mm) for both La and Th reflect the mineralization alteration signature and define a more consistent dispersal train from mineralization compared to element concentrations mapped alone. Additional testing in an area of continuous till cover near an isolated point source is recommended to further develop the elemental ratio method for exploration of MIAC systems.

The Geo-mapping for Energy and Minerals (GEM) and Targeted Geoscience Initiative (TGI) programs conducted extensive collaborative research on mineral systems with iron oxide-copper-gold (IOCG) and affiliated deposits in prospective settings of Canada. Regional alteration mapping as well as geochemical and geophysical modelling undertaken under the GEM program documented the evolution of polymetallic metasomatic systems with iron-oxide and alkali-calcic alteration and led to an increased recognition of the mineral potential of poorly explored areas and historic deposits of the Great Bear magmatic zone in the Northwest Territories, thus providing a solid framework for exploration. Early and barren albitite corridors form across the mineral systems and locally host uranium mineralization associated with telescoping of alteration facies by tectonic activity during the metasomatic growth of the systems. Subsequent to albitization, high-temperature Ca-Fe and Ca-K-Fe alteration form iron oxide-apatite (± rare-earth element) mineralization and IOCG variants rich in cobalt and other critical metals, respectively. Systems that further mature to K-Fe alteration form IOCG mineralization and can evolve to mineralized near-surface phyllic alteration and epithermal caps. Transitional facies also host polymetallic skarn mineralization. Rare-earth element enrichments within iron oxide-apatite zones are strongest where remobilization has occurred, particularly along deformation zones. The TGI projects documented the pertinence for a GEM activity in the Great Bear magmatic zone and subsequently synthesized GEM geoscientific data into a system-scale, ore-deposit model, and outlined criteria for mineral resource assessment. This model, and newly developed field-mapping and lithogeochemical tools were shown to be efficient mineral exploration and regional mapping methods in Canada and were also applied to the archetype IOCG deposit, Olympic Dam, and other deposits in the Olympic Cu-Au metallogenic province of Australia. Case examples also include the Romanet Horst in the Trans-Hudson Orogen (second phase of GEM), the Central Mineral Belt in Labrador (TGI), the Wanapitei Lake district in Ontario (private sector exploration results used by TGI), and the Bondy gneiss complex in Quebec (TGI).

Metasomatic iron and alkali-calcic mineral systems include a wide range of deposits with critical and precious metals, notably IOA, IOCG, skarn, albitite-hosted U, iron-rich Co (Au, Bi, Cu), and polymetallic vein deposits. Host systems evolve along diagnostic metasomatic paths with: 1) Na (albitite) and local skarn, 2) high-temperature (HT) Ca-Fe, 3) HT K-Fe, 4) K and K-Ca-Mg (K-skarn), 5) low-temperature (LT) K-Fe and Ca-Mg-Fe-(Si, CO2), and 6) epithermal alteration. In this contribution, we show that chalcopyrite-rich IOCG-style deposits from the Northern Norrbotten, Sweden, are characterized by early Na alteration overprinted by Ca-Fe and K-Fe alteration with a geochemical footprint distinct from magnetite-rich (Great Bear magmatic zone) or hematite-rich (Olympic Dam) IOCG deposits. The IOCG-style deposits from the Northern Norrbotten have similarities with some found in the Cloncurry district in Australia.

Achieving net-zero carbon emissions goals will increasingly rely on critical mineral resources while simultaneously decreasing the extraction, processing and use of hydrocarbons as the primary provider of energy. Canada is well-positioned to contribute to this effort through a series of innovative policy and research initiatives, and it is Canada's goal to be a stable supplier of critical minerals into the future. To this end, Natural Resources Canada and the Geological Survey of Canada invest financial resources into critical mineral research initiatives. This research aims to generate precompetitive baseline geological, geochemical and geophysical data for large, underexplored regions within Canada, whereas targeted studies focus on mineral systems science and improved exploration models for the large variety of critical mineral resources distributed throughout Canada. These research approaches can be combined, digitally, to generate mineral potential models. These ongoing efforts by the Geological Survey of Canada enhance the viability of Canada being (or maintaining its status as) a hub for critical mineral resource development and processing well into the future.

The Central Mineral Belt (CMB) in Labrador, Canada, hosts multiple U (+/- base +/- precious metal) showings, prospects and deposits in metamorphosed and variably hydrothermally altered Neoarchean to Mesoproterozoic, igneous and sedimentary rocks. Previous work has recognized U mineralization locally associated with Fe-Ca and alkali metasomatism typical of metasomatic iron oxide and alkali-calcic alteration systems (IOAA) that host iron oxide-copper-gold (IOCG) and affiliated critical metal deposits. However, the type, extent and temporal or genetic relationships between the diverse Fe, Ca and alkali metasomatism and the regionally distributed U mineralization remains poorly understood. Combined unsupervised machine-learning and classification of alteration from a large geochemical dataset distinguish the main alteration phases in the CMB, identify compositional changes related to U mineralization, and infer lithological/mineralogical information from samples with censored (i.e., missing), limited and/or inaccurate metadata. Weak to intense Na and Na + Ca-Fe (Mg) metasomatism in the southwest (Two-Time and Moran Lake areas) and eastern (Michelin area) portions of the CMB pre-dates U mineralization and Fe-oxide breccia development, similar to albitite-hosted U and IOCG deposits globally. Rare earth elements and spider diagrams highlight both preservation and disruption of normally immobile elements. Principal component and cluster analysis indicate significant variations in Fe-Mg +/- Na contents in the rocks from combinations of Na, Ca, Fe, and Mg-rich alteration, while protolith REE signatures can be locally preserved even after pervasive albitization-hematization. Cluster analysis identifies mineralized felsic and mafic rocks in the Michelin deposit and Moran Lake area, facilitating inference of relevant lithological/mineralogical information from samples lacking or with limited meta-data. The methods outlined provide rapid and relatively inexpensive means to optimize identification of mineral systems within large geochemical datasets, verify drill core or field observations, highlight potentially overlooked alteration, and refine economic mineral potential assessments. Based on our results and previous work, we suggest the mineral potential of the southwestern and eastern CMB needs to be re-assessed with modern exploration models for IOAA ore systems and their iron oxide-poor variants.

First posted June 4, 2021 For additional information, contact: Director, Geology, Geophysics, and Geochemistry Science CenterU.S. Geological SurveyMS 973, Box 25046Denver, CO 80225 A challenge for the global economy is to meet the growing demand for commodities used in today's advanced technologies. Critical minerals are commodities (for example, elements, compounds, minerals) deemed vital to the economic and national security of individual countries that are vulnerable to supply disruption. The national geological agencies of Australia, Canada, and the United States recently joined forces to advance understanding and foster development of critical mineral resources in their respective countries through the Critical Minerals Mapping Initiative (CMMI). An initial goal of the CMMI is to fill the knowledge gap on the abundance of critical minerals in ores. To do this, the CMMI compiled modern multielement geochemical data generated by each agency on ore samples collected from historical and active mines and prospects from around the world. To identify relationships between critical minerals, deposit types, deposit environments, and mineral systems, a unified deposit classification scheme was needed. This report describes the scheme developed by the CMMI to classify the initial release of geochemical data. In 2021, the resulting database—along with basic query, statistical analysis, and display tools—will be served to the public through a web-based portal managed by Geoscience Australia. The database will enable users to trace critical minerals through mineral systems and identify individual deposits or deposit types that are potential sources of critical minerals.

Australia's and China's resources (e.g. Olympic Dam Cu-U-Au-Ag and Bayan Obo REE deposits) highlight how discovery and mining of iron oxide copper-gold (IOCG), iron oxide±apatite (IOA) and affiliated primary critical metal deposits in metasomatic iron and alkali-calcic (MIAC) mineral systems can secure a long-term supply of critical metals for Canada and its partners. In Canada, MIAC systems comprise a wide range of undeveloped primary critical metal deposits (e.g. NWT NICO Au-Co-Bi-Cu and Québec HREE-rich Josette deposits). Underexplored settings are parts of metallogenic belts that extend into Australia and the USA. Some settings, such as the Camsell River district explored by the Dene First Nations in the NWT, have infrastructures and 100s of km of historic drill cores. Yet vocabularies for mapping MIAC systems are scanty. Ability to identify metasomatic vectors to ore is fledging. Deposit models based on host rock types, structural controls or metal associations underpin the identification of MIAC-affinities, assessment of systems' full mineral potential and development of robust mineral exploration strategies. This workshop presentation reviews public geoscience research and tools developed by the Targeted Geoscience Initiative to establish the MIAC frameworks of prospective Canadian settings and global mining districts and help de-risk exploration for IOCG, IOA and affiliated primary critical metal deposits. The knowledge also supports fundamental research, environmental baseline assessment and societal decisions. It fulfills objectives of the Canadian Mineral and Metal Plan and the Critical Mineral Mapping Initiative among others. The GSC-led MIAC research team comprises members of the academic, private and public sectors from Canada, Australia, Europe, USA, China and Dene First Nations. The team's novel alteration mapping protocols, geological, mineralogical, geochemical and geophysical framework tools, and holistic mineral systems and petrophysics models mitigate and solve some of the exploration and geosciences challenges posed by the intricacies of MIAC systems. The group pioneers the use of discriminant alteration diagrams and barcodes, the assembly of a vocab for mapping and core logging, and the provision of field short courses, atlas, photo collections and system-scale field, geochemical, rock physical properties and geophysical datasets are in progress to synthesize shared signatures of Canadian settings and global MIAC mining districts. Research on a metamorphosed MIAC system and metamorphic phase equilibria modelling of alteration facies will provide a foundation for framework mapping and exploration of high-grade metamorphic terranes where surface and near surface resources are still to be discovered and mined as are those of non-metamorphosed MIAC systems.

Tourmaline crystallized during iron oxide and alkali-calcic alteration and IOCG mineralization in the Great Bear magmatic zone, Canada. Within the samples analyzed, tourmaline occurs as small crystals in the groundmass of hydrothermal breccias (Southern Breccia and Contact Lake), or as large prismatic crystals in quartz (+/- oremineralogy) veins (Contact Lake, DeVries Lake, and NICO). The tourmaline from these localities is predominantly shorlitic in composition, with minor dravitic, uvitic and feruvitic components. The boron isotopic composition ranges from -15 parts per thousand to -5 parts per thousand, with the majority of values between -15 parts per thousand and -10 parts per thousand. The lack of variation among samples supports a common fluid source across the belt for the metasomatic-hydrothermal systems. These results are isotopically similar to global magmatic and non-marine evaporitic reservoirs, and lighter than seawater and marine sedimentary rocks. The delta B-11 results support a single-fluid IOCG deposit model for the systems studied, wherein metasomatic-hydrothermal fluids derived by magmatism ascend and evolve over time through fluid-rock interactions.

This year the meeting kicked off on Monday with a Special Session in honour of Charlie Gower entitled “Tectonics and Mineral Potential of Proterozoic Orogenic Belts”. This day-long session was followed-up with an evening Public Lecture by Toby Rivers and Fried Schwerdtner on “Gravitationally-Driven Extensional Collapse of a Proterozoic Large Hot Orogen: The Grenville Example.” Tuesday featured a General Session with presentations on a wide range of geoscience topics.

The Paleoproterozoic East Arm Basin of Canada hosts polymetallic vein, iron oxide–apatite (IOA), and potential iron oxide–copper–gold (IOCG) mineral occurrences, mainly associated with a belt of ca. 1.87 Ga intermediate-composition sills termed the Compton intrusions. Advances in our knowledge of the East Arm Basin and of IOA and IOCG deposits within the broader context of iron oxide and alkali-calcic alteration systems enables a new regional analysis of this mineralization and facilitates comparison of these mineral occurrences and host rocks to the nearby Great Bear magmatic zone IOCG districts. The Compton intrusions and co-magmatic Pearson Formation volcanic rocks are comparable in age and composition to intrusive plus volcanic rocks of the Great Bear magmatic zone that host IOA–IOCG mineralization. Taking into account fault displacements, emplacement of Compton intrusions and Pearson Formation volcanic rocks are also consistent with the architecture of modern arcs, supporting a direct relationship with the Great Bear subduction zone. Trace element patterns of uraninite contained in IOA occurrences of the East Arm Basin are also similar to the patterns of uraninite from the Great Bear magmatic zone occurrences, consistent with both regions having experienced similar iron oxide and alkali-calcic alteration and mineralization. Our new results indicate that exploration for IOA, IOCG, and affiliated deposits in the East Arm Basin should focus on delineating increased potassic-iron alteration types and fault/breccia zones associated with these systems through field mapping and application of geochemical, radiometric, magnetic, and gravity surveys.

The Southern Breccia metasomatic uranium (U) showings are located 1 km south of the NICO deposit, an iron oxide-copper-gold (IOCG) deposit, in the Great Bear magmatic zone of Canada. The timing of both occurrences is tightly constrained to 1873-1868 Ma, linking formation of the albitite-hosted U to development of IOCG mineralization. During this period, regional iron oxide and alkali-calcic metasomatism formed: Na (albite), high-temperature Ca-Fe (amphibole + magnetite), high-temperature Ca-K-Fe (amphibole + magnetite + biotite +/- K-feldspar), high-temperature K-Fe (K-feldspar/ biotite + magnetite), K (K-feldspar +/- biotite), and low-temperature K-Fe-Mg (K-feldspar + hematite + chlorite) assemblages. Primary uraninite and brannerite occur within high-temperature K-Fe alteration composed of magnetite + K-feldspar +/- biotite-cemented breccias developed in earlier albitite. The chemistry of primary uraninite supports precipitation from high-temperature, magmatic-derived fluids, as previously proposed for the NICO deposit. Field relationships, petrography, whole-rock geochemistry, and geochronology indicate that alteration of the Southern Breccia corridor host rocks was coeval with early alteration at NICO, whereas U mineralization postdated Au-Co-Bi at NICO. The linkage of the Southern Breccia to the regional iron-oxide and alkali-calcic alteration system that generated the NICO deposit presents a new driver for formation of albitite-hosted U deposits and highlights an exploration target in IOCG districts globally.

The Southern Breccia metasomatic uranium (U) showings are located 1 km south of the NICO deposit, an iron oxide-copper-gold (IOCG) deposit, in the Great Bear magmatic zone of Canada. The timing of both occurrences is tightly constrained to 1873–1868 Ma, linking formation of the albitite-hosted U to development of IOCG mineralization. During this period, regional iron oxide and alkali-calcic metasomatism formed: Na (albite), high-temperature Ca-Fe (amphibole + magnetite), high-temperature Ca-K-Fe (amphibole + magnetite + biotite ± K-feldspar), high-temperature K-Fe (K-feldspar/biotite + magnetite), K (K-feldspar ± biotite), and low-temperature K-Fe-Mg (K-feldspar + hematite + chlorite) assemblages. Primary uraninite and brannerite occur within high-temperature K-Fe alteration composed of magnetite + K-feldspar ± biotite-cemented breccias developed in earlier albitite. The chemistry of primary uraninite supports precipitation from high-temperature, magmatic-derived fluids, as previously proposed for the NICO deposit. Field relationships, petrography, whole-rock geochemistry, and geochronology indicate that alteration of the Southern Breccia corridor host rocks was coeval with early alteration at NICO, whereas U mineralization postdated Au-Co-Bi at NICO. The linkage of the Southern Breccia to the regional iron-oxide and alkali-calcic alteration system that generated the NICO deposit presents a new driver for formation of albitite-hosted U deposits and highlights an exploration target in IOCG districts globally.

Trace element compositions of magnetite and hematite from 16 well-studied iron oxide–copper–gold (IOCG) and iron oxide apatite (IOA) deposits, combined with partial least squares-discriminant analysis (PLS-DA), were used to investigate the factors controlling the iron oxide chemistry and the links between the chemical composition of iron oxides and hydrothermal processes, as divided by alteration types and IOCG and IOA deposit subtypes. Chemical compositions of iron oxides are controlled by oxygen fugacity, temperature, co-precipitating sulfides, and host rocks. Iron oxides from hematite IOCG deposits show relatively high Nb, Cu, Mo, W, and Sn contents, and can be discriminated from those from magnetite + hematite and magnetite IOA deposits. Magnetite IOCG deposits show a compositional diversity and overlap with the three other types, which may be due to the incremental development of high-temperature Ca–Fe and K–Fe alteration. Iron oxides from the high-temperature Ca–Fe alteration can be discriminated from those from high- and low-temperature K–Fe alteration by higher Mg and V contents. Iron oxides from low-temperature K–Fe alteration can be discriminated from those from high-temperature K–Fe alteration by higher Si, Ca, Zr, W, Nb, and Mo contents. Iron oxides from IOA deposits can be discriminated from those from IOCG deposits by higher Mg, Ti, V, Pb, and Sc contents. The composition of IOCG and IOA iron oxides can be discriminated from those from porphyry Cu, Ni–Cu, and volcanogenic massive sulfide deposits.