Iron formations are economically important sedimentary rocks that are most common in Precambrian sedimentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that secular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Algoma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic successions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have been linked to diverse environmental changes. Geochronologic studies emphasize the episodic nature of the deposition of giant iron formations, as they are coeval with, and genetically linked to, time periods when large igneous provinces (LIPs) were emplaced. Superior-type iron formation first appeared at ca. 2.6 Ga, when construction of large continents changed the heat flux at the core-mantle boundary. From ca. 2.6 to ca. 2.4 Ga, global mafic magmatism culminated in the deposition of giant Superior-type BIF in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited during the early stage of a shift from reducing to oxidizing conditions in the ocean-atmosphere system. Counterintuitively, enhanced magmatism at 2.50 to 2.45 Ga may have triggered atmospheric oxidation. After the rise of atmospheric oxygen during the GOE at ca. 2.4 Ga, GIF became abundant in the rock record, compared to the predominance of BIF prior to the Great Oxidation Event (GOE). Iron formations generally disappeared at ca. 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global glaciations, the so-called Snowball Earth events. By the Phanerozoic, marine iron deposition was restricted to local areas of closed to semiclosed basins, where volcanic and hydrothermal activity was extensive (e.g., back-arc basins), with ironstones additionally being linked to periods of intense magmatic activity and ocean anoxia.

Late Paleoproterozoic iron formations and Paleozoic ironstones were deposited at the redoxcline where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to produce the conditions necessary for deposition of iron formation. Mantle plume events that led to the formation of LIPs also enhanced spreading rates of midocean ridges and produced higher growth rates of oceanic plateaus, both processes thus having contributed to a higher hydrothermal flux to the ocean. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the oceanic oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the oceanic redox state, iron was deposited only proximally, generally as oxides or sulfides. Manganese, in contrast, was more mobile. We conclude that occurrences of BIF, GIF, Phanerozoic ironstones, and exhalites surrounding VMS systems record a complex interplay involving mantle heat, tectonics, and surface redox conditions throughout Earth history, in which mantle heat unidirectionally declined and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter term fluctuations.

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Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes

Phanerozoic continental growth and gold metallogeny of Asia

The iron oxide copper-gold (IOCG) group of deposits, initially defined following discovery of the giant Olympic Dam Cu-U-Au deposit, has progressively become too-embracing when associated deposits and potential end members or analogs are included. The broader group includes several low Ti iron oxide-associated deposits that include iron oxide (P-rich), iron oxide (F- and REE-rich), Fe or Cu-Au skarn, high-grade iron oxide-hosted Au ± Cu, carbonatite-hosted (Cu-, REE-, and F-rich), and IOCG sensu stricto deposits. Consideration of this broad group as a whole obscures the critical features of the IOCG sensu stricto deposits, such as their temporal distribution and tectonic environment, thus leading to difficulties in developing a robust exploration model. The IOCG sensu stricto deposits are magmatic-hydrothermal deposits that contain economic Cu and Au grades, are structurally controlled, commonly contain significant volumes of breccia, are commonly associated with presulfide sodic or sodic-calcic alteration, have alteration and/or brecciation zones on a large, commonly regional, scale relative to economic mineralization, have abundant low Ti iron oxides and/or iron silicates intimately associated with, but generally paragenetically older than, Fe-Cu sulfides, have LREE enrichment and low S sulfides (lack of abundant pyrite), lack widespread quartz veins or silicification, and show a clear temporal, but not close spatial, relationship to major magmatic intrusions. These intrusions, where identified, are commonly alkaline to subalkaline, mixed mafic (even ultramafic) to felsic in composition, with evidence for mantle derivation of at least the mafic end members of the suite. The giant size of many of the deposits and surrounding alteration zones, the highly saline ore fluids, and the available stable and radiogenic isotope data indicate release of deep, volatile-rich magmatic fluids through devolatization of causative, mantle-derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional-scale fluid flow paths. Precambrian deposits are the dominant members of the IOCG group in terms of both copper and gold resources. The 12 IOCG deposits with >100 tonnes (t) resources are located in intracratonic settings within about 100 km of the margins of Archean or Paleoproterozoic cratons or other lithospheric boundaries, and formed 100 to 200 m.y. after supercontinent assembly. Their tectonic setting at formation was most likely anorogenic, with magmatism and associated hydrothermal activity driven by mantle underplating and/or plumes. Limited amounts of partial melting of volatile-rich and possibly metal-enriched metasomatized early Precambrian subcontinental lithospheric mantle (SCLM), fertilized during earlier subduction, probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts, with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the IOCG deposits. Preservation of near-surface deposits, such as Olympic Dam, is probably due to their formation above buoyant and refractory SCLM, which resisted delamination and associated uplift. Most Precambrian iron oxide (P-rich) or magnetite-apatite (Kiruna-type) deposits have a different temporal distribution, apparently forming in convergent margin settings prior to or following supercontinent assembly. It is only in the Phanerozoic that IOCG and magnetite-apatite deposits are roughly penecontemporaneous in convergent margin settings. The Phanerozoic IOCG deposits, such as Candelaria, Chile, occur in anomalous extensional to transtensional zones in the Coastal Cordillera, which are also the sites of mantle-derived mafic to felsic intrusions that are anomalous in an Andean context. This implies that special conditions, possibly detached slabs of metasomatized SCLM, may be required in convergent margin settings to generate world-class IOCG deposits. It is likely that formation of giant IOCG deposits was mainly a Precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized SCLM. Generally smaller and rarer Phanerozoic IOCG deposits formed in tectonic settings where conditions similar to those in the Precambrian were replicated.

Iron Oxide Copper-Gold (IOCG) Deposits through Earth History: Implications for Origin, Lithospheric Setting, and Distinction from Other Epigenetic Iron Oxide Deposits

Magnetite and hematite are common minerals in a range of mineral deposit types. These minerals form partial to complete solid solutions with magnetite, chromite, and spinel series, and ulvospinel as a result of divalent, trivalent, and tetravalent cation substitutions. Electron microprobe analyses of minor and trace elements in magnetite and hematite from a range of mineral deposit types (iron oxide-copper-gold (IOCG), Kiruna apatite–magnetite, banded iron formation (BIF), porphyry Cu, Fe-Cu skarn, Fe-Ti, V, Cr, Ni-Cu-PGE, Cu-Zn-Pb volcanogenic massive sulfide (VMS) and Archean Au-Cu porphyry and Opemiska Cu veins) show compositional differences that can be related to deposit types, and are used to construct discriminant diagrams that separate different styles of mineralization. The Ni + Cr vs. Si + Mg diagram can be used to isolate Ni-Cu-PGE, and Cr deposits from other deposit types. Similarly, the Al/(Zn + Ca) vs. Cu/(Si + Ca) diagram can be used to separate Cu-Zn-Pb VMS deposits from other deposit types. Samples plotting outside the Ni-Cu-PGE and Cu-Zn-Pb VMS fields are discriminated using the Ni/(Cr + Mn) vs. Ti + V or Ca + Al + Mn vs. Ti + V diagrams that discriminate for IOCG, Kiruna, porphyry Cu, BIF, skarn, Fe-Ti, and V deposits.

Discriminant diagrams for iron oxide trace element fingerprinting of mineral deposit types

The inception of plate tectonics on Earth and its subsequent evolution are discussed on the basis of theoretical considerations and observational constraints. The likelihood of plate tectonics in the past depends on what mechanism is responsible for the relatively constant surface heat flux that is indicated by the likely thermal history of Earth. The continuous operation of plate tectonics throughout Earth's history is possible if, for example, the strength of convective stress in the mantle is affected by the gradual subduction of surface water. Various geological indicators for the emergence of plate tectonics are evaluated from a geodynamical perspective, and they invariably involve certain implicit assumptions about mantle dynamics, which are either demonstrably wrong or yet to be explored. The history of plate tectonics is suggested to be intrinsically connected to the secular evolution of the atmosphere, through sea-level changes caused by ocean-mantle interaction.

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Initiation and Evolution of Plate Tectonics on Earth: Theories and Observations

Volcanogenic Massive Sulfide Deposits

Volcanogenic massive sulphide deposits of the Archean, Noranda District, Quebec

The Blake River Group of the Abitibi Greenstone Belt and its unique VMS and gold-rich VMS endowment

The Paleoproterozoic Flin Flon mining district is one of the world’s most prolific volcanogenic massive sulfide (VMS) camps and includes a single stratigraphic interval that hosts the 85.5 million tonne (Mt) Flin Flon, 777, and Callinan Zn-Cu-(Au) deposits. Rapid seafloor burial of the VMS hydrothermal system by a thick succession of pillowed basalt resulted in the hanging-wall strata being affected to varying degrees by the still upward migrating fluids. This hanging-wall alteration hydrothermal fingerprint allows delineation of the regionally metamorphosed paleohydrothermal system, and its characterization has the potential to lead to discovery of buried, stacked, or structurally displaced mineralization.

Evidence for the presence of continued seafloor hydrothermal activity above the Flin Flon-Callinan VMS horizon is observed in the pillowed flows, interlayered hyaloclastite-rich flow tops, and also within finely bedded interflow volcaniclastic sediment. A 30% to 60% metalliferous exhalative component was detected through geochemical and mineral analysis in interpillow volcaniclastic rocks, chert, and epidosite in the hanging-wall sequence. The regional distribution of Fe- and Mg-rich chlorite, epidote-clinozoisite, biotite-annite, actinolite-hornblende-ferrotschermakite, and stilpnomelane and albite-oligoclase modifies metamorphic isograds and defines discrete vertical fluid pathways controlled by synvolcanic growth faults and associated sill-dike swarms. Silica-enriched hanging-wall alteration zones are proximal to Fe-Ti basalt sills and occur as discrete hanging-wall zones parallel to the plunge of the 62 Mt Flin Flon deposit. Anomalous concentrations of Hg, Sb, Ag, Pb, Te, As, Au, and Bi form within these hanging-wall halo alteration zones, indicating migration of the more volatile metals present in the underlying VMS deposits. Synvolcanic depressions, dike swarms, and hydrothermal-metamorphic fluid corridors are detectable through trace element anomalies, trace mineral chemistry, and 18O isotope geochemistry. Oxygen isotope analysis of the Flin Flon-777-Callinan VMS hanging-wall strata defines a number of high δ O18 anomalies extending 1,200 m above that indicate that <300°C subseafloor hydrothermal activity continued after burial of the massive sulfide deposits. Coupled with the geochemical and mineral chemical anomalies, this is indicative of the presence of continued, relatively low temperature hydrothermal fluid “leakage” from a robust seafloor hydrothermal event that generated the VMS deposits.

A combination of techniques, including mineral chemistry, isotope, and trace element data, is demonstrated to be successful in identifying and delineating zones of hanging-wall hydrothermal alteration in greenschist- to amphibolite-grade metamorphic rocks of the Flin Flon mining camp. Use of these, coupled with mapping to define periods of quiescence as marked by horizons of sedimentary rocks in the hanging-wall basalts of the Hidden Formation, has the potential to lead to discovery of deeply buried deposits on the Flin Flon horizon or deposits at higher stratigraphic levels.

Our findings indicate that the basaltic hanging wall on the Flin Flon-777-Callinan hydrothermal system was an efficient cap on the system, with vestiges of continued hydrothermal fluid flow detected in the interpillow and interflow components. These volumetrically minor components are critical sampling media and are pertinent to global exploration for detection of VMS mineralization buried beneath thick mafic volcanic sequences.

Hanging-Wall Vectoring for Buried Volcanogenic Massive Sulfide Deposits, Paleoproterozoic Flin Flon Mining Camp, Manitoba, Canada

Stratigraphy of volcanogenic massive sulphide-hosting volcanic and volcaniclastic rocks of the Flin Flon Formation, Flin Flon (NTS 763K12 and 13), Manitoba and Saskatchewan

A. G. Galley

Papers

Volcanogenic Massive Sulfide Deposits

Volcanogenic massive sulphide deposits of the Archean, Noranda District, Quebec

The Blake River Group of the Abitibi Greenstone Belt and its unique VMS and gold-rich VMS endowment

Hanging-Wall Vectoring for Buried Volcanogenic Massive Sulfide Deposits, Paleoproterozoic Flin Flon Mining Camp, Manitoba, Canada

Stratigraphy of volcanogenic massive sulphide-hosting volcanic and volcaniclastic rocks of the Flin Flon Formation, Flin Flon (NTS 763K12 and 13), Manitoba and Saskatchewan