Iron Formations: Their Origins and Implications for Ancient Seawater Chemistry

Abstract

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.