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

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Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

Iron formations are economically significant, iron- and silica-rich sedimentary rocks that are restricted to Precambrian successions. There are no known modern or Phanerozoic analogues for these deposits that are comparable in terms of areal extent and thickness. Although many aspects of iron formation origin remain debatable, it is generally accepted that secular changes in the style of deposition are genetically linked to plate tectonic processes, mantle plume events, and evolution of Earth's surface environments. Two types of Precambrian iron formations have been recognized based on depositional and tectonic settings. Iron formations formed proximal to volcanic centers are interlayered with or laterally linked to submarine volcanic rocks and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger sedimentary rock-hosted iron formations are developed in passive-margin settings and typically lack a direct association with volcanic rocks. A full gradation between these two end-members exists in the rock record. Texturally, iron formations are divided into two groups. Banded iron formation (BIF) is predominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is more common in middle to late Paleoproterozoic successions, having been deposited in shallow-marine settings after the rise of atmospheric oxygen at ~2.4 Ga. Secular changes in the style of iron formation deposition have been linked to a diverse array of environmental changes. Geochronologic studies emphasize the periodicity in deposition of giant iron formations, which are coeval with large igneous provinces (LIPs). Giant sedimentary rock-hosted iron formations first appeared ~ 2.6 Ga, possibly when the construction of large continents changed the heat flux across the core–mantle boundary. From ~ 2.6 to ~ 2.4 Ga, global mafic-to-ultramafic magmatism culminated in the deposition of giant sedimentary rock-hosted iron formations in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited immediately before a shift from reducing to oxidizing conditions in the ocean–atmosphere system. Counterintuitively, enhanced magmatism at 2.50–2.45 Ga, which likely delivered large amounts of reductants to shallow-marine environments, may have triggered atmospheric oxidation. After the rise of atmospheric oxygen ~ 2.4 Ga, GIF became more abundant in the rock record than BIF. Iron formations largely disappeared ~ 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-scale glaciations, during the so-called snowball Earth events. In the Phanerozoic, deeper-water iron formation deposition became restricted to local areas of closed to semi-closed basins, where volcanic and hydrothermal activity was extensive, such as in back-arc basins. In contrast, episodically deposited, basin-scale Phanerozoic oolitic and pisolitic ironstones are linked to periods of intense magmatic activity and ocean anoxia.

Late Paleoproterozoic iron formations and at least some 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 the production of the conditions necessary for deposition of iron formation. Mantle plume events that led to the emplacement of LIPs also enhanced spreading rates of mid-ocean ridges and resulted in higher growth rates of oceanic plateaus; both processes thus contributed to a higher hydrothermal iron flux to the oceans. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the ocean oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the ocean redox state, iron was deposited mainly proximally, generally as oxides or sulfides; manganese was more mobile. It is concluded that occurrences of BIF, GIF, Phanerozoic ironstones, and hydrothermal sedimentary rocks of exhalative origin (exhalites) surrounding VMS systems are a record of a complex interplay among mantle plume events, plate tectonics, and ocean redox conditions throughout Earth's history, in which mantle heat unidirectionally decreased and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter-term fluctuations.

Iron Formations: Their Origins and Implications for Ancient Seawater Chemistry

Red, supermature quartzites of the Baraboo interval of the Lake Superior region contain detrital zircon that ranges in age from 1782 to 1712 Ma. Deposition clearly occurred after the geon 18 Penokean orogeny. These late Paleoproterozoic sedimentary rocks consist largely of quartz, kaolinite or pyrophyllite, and hematite; detrital feldspar and muscovite are rare or absent. Their Chemical Index of Alteration ranges from 96.8 to 98.6, among the most chemically mature clastic sediments in the geological record. The quartzites are underlain by mature, feldspar‐free paleosols, accounting for the absence of feldspar in the overlying sediments and indicating the presence of first‐cycle quartzose detritus. Such physical and chemical characteristics imply that late Paleoproterozoic deposition in the Lake Superior region occurred in a stable tectonic setting with subdued topographic relief in a warm, humid climate. Folding and low‐grade metamorphism of the quartzites is thought to reflect ∼1630 Ma foreland ...

http://www.geology.wisc.edu/~bsinger/Publications/2003.MedarisetalJGeol/Medaris_Singer_et_al_J_Geol_2003.pdf

Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzites

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_5/b_fdi_18-19/25070.pdf

An accreted continental terrane in northwestern Perú

The Gogebic iron range is an elongate belt of Paleoproterozoic strata extending from the west shore of Lake Gogebic in the upper peninsula of Michigan for about 125 km westward into northern Wisconsin. It is one of six major informally named iron ranges in the Lake Superior region and produced about 325 million tons of direct-shipping ore between 1887 and 1967. A significant resource of concentrating-grade ore remains in the western and eastern parts of the range. The iron range forms a broad, gently southward-opening arc where the central part of the range exposes rocks that were deposited somewhat north of the eastern and western parts. A fundamental boundary marking both the tectonic setting of deposition and the later deformation within the Penokean orogen lies fortuitously in an east-west direction along the range so that the central part of the range preserves sediments deposited north of that boundary, whereas the eastern and western parts of the range were deposited south of the boundary. Thus, the central part of the range provides a record of sedimentation and very mild deformation in a part of the Penokean orogen farthest from the interior of the orogen to the south. The eastern and western parts of the range, in contrast, exhibit a depositional and deformational style typical of parts closer to the interior of the orogen. A second fortuitous feature of the iron range is that the entire area was tilted from 40° to 90° northward by Mesoproterozoic deformation so that the map view offers an oblique cross section of the Paleoproterozoic sedimentary sequence and structures. Together, these features make the Gogebic iron range a unique area in which to observe (1) the lateral transition from deposition on a stable platform to deposition in a tectonically and volcanically active region, and (2) the transition from essentially undeformed Paleoproterozoic strata to their folded and faulted equivalents. Paleoproterozoic strata in the Gogebic iron range are part of the Marquette Range Supergroup. They were deposited unconformably on Neoarchean rocks consisting of a diverse volcanic suite (the Ramsay Formation) which was intruded by granitic rocks of the Puritan Quartz Monzonite. The Marquette Range Supergroup in this region consists of a basal sequence of orthoquartzite (Sunday Quartzite) and dolomite (Bad River Dolomite), both of which are part of the Chocolay Group. The group is preserved only in the eastern and western parts of the range but was probably present throughout before the erosion interval that separated it from the overlying Menominee Group. The Menominee Group consists of basal clastic rocks (Palms Formation) that grade upward into the Ironwood Iron-Formation, which is the principal iron-bearing unit of the range. The Ironwood interfingers with the Emperor Volcanic Complex in the eastern part of the range and with volcanic rocks and gabbro in the western part of the range. The Ironwood is overlain unconformably by the Tyler Formation in the central and western parts of the range and by the Tyler's equivalent, the Copps Formation, in the eastern part of the range. Strata in the central part of the iron range are entirely sedimentary. Deposition occurred in a relatively stable tectonic setting, at least until the deposition of the Tyler Formation. The Tyler consists largely of turbidites deposited in a foreland basin in advance of accreting volcanic arcs to the south. Penokean deformation in the central part of the range was very minor; the evidence of deformation consists of steep faults with small offsets and a few bedding-parallel faults that also have small offsets and that are recognized only in mine workings. In both the eastern and western parts of the iron range, abrupt facies changes mark a passage into a more tectonically and volcanically active belt. These relationships are especially well displayed in the east where a graben, the Presque Isle trough, began to subside during deposition of the Ironwood Iron-Formation. The thickness of the Ironwood increases into the graben and its internal stratigraphy also changes. The most prominent changes in the graben are the presence of a thick volcanic unit, the Emperor Volcanic Complex of the Menominee Group, and comagmatic gabbro sills that interfinger with the Ironwood. In the western part of the range, volcanic rocks and comagmatic gabbro sills are also present in the Ironwood, but a graben that is equivalent to the Presque Isle trough is not evident.

The Gogebic Iron Range - A Sample of the Northern Margin of the Penokean Fold and Thrust Belt

The presence of elongate troughs of Precambrian X (middle Precambrian) rocks in Precambrian W (lower Precambrian) rocks in the western Marquette district of northern Michigan has been known since the late 1800s. However, little data can be brought to bear on estimates of the depth and cross-sectional configuration of these features. For this reason, gravity models and geologic interpretations were made from gravity profiles measured over the Marquette Trough, Republic Trough, and Mitchigan River Trough.

Gravity-model studies, combined with geologic studies, indicate that near Humboldt, Michigan, the Marquette Trough is ∼2,438 m deep and asymetrically shaped, the deepest section being near the southern edge; near the west end of Lake Michigamme, the Marquette Trough is about 1,097 m deep at its northern edge; the Republic Trough is about 1,524 m deep at a point ∼2.4 km northwest of its southeastern end; and the Mitchigan River Trough is probably a fault-bounded westward-dipping monocline ∼610 m deep.

Geologic Interpretation of Gravity Profiles in the Western Marquette District, Northern Michigan

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New observations on the age and structure of Proterozoic quartzites in Wisconsin

Bedrock geologic map of the southern part of the Diorite and Champion 7 1/2 minute quadrangles, Marquette County, Michigan

The Wisconsin magmatic terranes constitute a major easttrending belt of mainly subduction-related Early Proterozoic volcanic and plutonic rocks and associated volcanogenic sedimentary rocks in northern Wisconsin. Magmatic rocks within the terranes have been subdivided, on the basis of lithology and structure. into two subterranes: the Pembine-Wausau terrane on the north and the Marshfield terrane on the south. Rocks in · the Monico area are part of an island arc suite of the PembineWausau terrane that developed and was tectonically deformed during convergence and docking with Archean rocks of a continental margin to the north. In most areas the magmatic terranes are covered by a thick blanket of Pleistocene deposits. As a result. volcanic stratigraphy and structure within the volcanic belt are poorly known. However. outcrops are relatively abundant locally in the Monico area. which is one of the few places where stratigraphic and structural relations can be studied in the field. Rocks exposed in the Monico area consist of banded gneisses. greenschist-grade metavolcanic rocks with well-preserved primary features. synvolcanic mafic and felsic intrusions. posttectonic granitic rocks. and a Middle Proterozoic diabase dike that is probably related to volcanic rocks of the Keweenawan Supergroup. The northern part of the map area consists of banded gneisses of uncertain age. The gneisses are overlain to the south by a bimodal suite of mafic pillow lavas and pillow breccias intercalated with dacitic to rhyolitic flows, breccias, tuffs. and welded tuffs. Numerous pillows consistently indicate that the volcanic sequence faces stratigraphically. south. Mafic and felsic volcanic units are interbedded on both a small and large scale with the felsic component generally increasing southward. The volcanic sequence is tectonically repeated by major shear zones that are approximately parallel to bedding. A large subvolcanic pluton in the northern part of the map area was emplaced approximately along the contact between the older gneisses and the overlying volcanic sequence. The pluton is a quartz (and feldspar) porphyry with a gray aphanitic matrix. Tuffaceous layers and subtle layering suggest the porphyry is. at least in part. extrusive in eastern exposures. but abundant xenoliths in the western part suggest that in this area it is intrusive. Chemically, the quartz porphyry is almost identical to volcanic rocks associated with the Pelican River massive sulfide deposit that occurs within the map area. A post-tectonic pink to red granite, dated at 1. 739 Ma. is exposed in the southern part of the map area. The granite body U.S. Geological Survey and University of Wisconsin-Oshkosh, Oshkosh, WI 54901, emeritus 2U.S. Geological Survey and Western Illinois University, Macomb, IL 61455, emeritus is bulbous in shape and cuts the general volcanic stratigraphy. An unmetamorphosed Middle Proterozoic (Keweenawan) dike with an associated significant low magnetic anomaly transects the entire map area in a N. 70° E. direction. The dike appears to be part of a regional swarm emplaced near the beginning of the Midcontinent Rift. · Structurally the area is characterized by polyphase deformation resulting from the series of events related to docking of the island arc with a continental margin of mostly Archean rocks to the north. Among the oldest structural fabrics recognized is a north-trending gneissic foliation (SA1) in the northwestern part of the map area. North-trending foliation was not observed elsewhere in the map area, nor has it been reported elsewhere in the Wisconsin magmatic terranes. A widespread east-trending foliation (S1) is well developed in the volcanic rocks to the south of the gneissic rocks. S1 is, in turn, cut by a northeast-trending foliation (S2) consisting of discrete, isolated shear zones. The S2 fabric is cut by well-defined, east-trending, north-side-up shear zones (S3) that are responsible for the repetition of the volcanic succession. A conspicuous, N.88° E.trending foliation, along the southern margin of the gneiss. is tentatively interpreted to be an s3 structure that overprints the north-trending structure in the gneiss. 0 1, 0 2, and 03 structures are truncated by the red granite in the southern part of the map area, and are crosscut by the N. 70° E.-trending undeformed diabase dike.

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John S. Klasner

Papers

The Gogebic Iron Range - A Sample of the Northern Margin of the Penokean Fold and Thrust Belt

Geologic Interpretation of Gravity Profiles in the Western Marquette District, Northern Michigan

New observations on the age and structure of Proterozoic quartzites in Wisconsin

Bedrock geologic map of the southern part of the Diorite and Champion 7 1/2 minute quadrangles, Marquette County, Michigan

Geology and Tectonic Significance of Early Proterozoic Rocks in the Monico Area, Northern Wisconsin