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., backarc 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 oxid...

There are no confirmed microfossils older than 3,500 million years (Myr) on Earth, probably because of the highly metamorphosed nature of the oldest sedimentary rocks 1 . Therefore, studies have focused almost exclusively on chemical traces and primarily on the isotopic composition of carbonaceous material,which has led to controversies regarding the origin of isotopically light reduced carbon 2 . Schists from the approximately 3,700-Myr-old Isua supracrustal belt in southwest Greenland contain up to 8.8 wt% graphitic carbon that is depleted in 13 C, and this depletion has been attributed to biological activity 3,4 . However, because non-biological decarbonation reactions and Fischer-Tropsch-type synthesis 5 can produce reduced Page 1 of 11 carbon with similar isotopic compositions, non-biological interpretations are possible 2 . Apatite with graphite coatings, within iron formations from the ca. 3,830-Myr-old Akilia supracrustal belt in southwest Greenland has been interpreted as the metamorphosed product of biogenic matter 6 , supported by the presence of biologically important heteroatoms within the graphite 7,8 . However, it has been suggested that some graphite in the Akilia iron formations was deposited by metamorphic fluids 8,9 . This latter interpretation is echoed in the Nuvvuagittuq supracrustal belt (NSB) by the presence of poorly crystalline, fluid-deposited graphite that coats apatite 10 , demonstrating that some apatite-graphite occurrences are produced abiotically during fluid remobilization and high-grade metamorphism.The NSB in northeastern Canada represents a fragment of the Earth's primitive mafic oceanic crust.The NSB is composed predominantly of basaltic metavolcanic rocks (Extended Data Fig. 1 Data Fig. 2). The presence of well-preserved, 20-3,000-µm chalcopyrite crystals within the NSB jasper and carbonate iron formations (Extended Data Fig. 3a) demonstrate the lack of postdepositional oxidation.Most NSB rocks were subjected to upper amphibolite-facies metamorphism around 2,700 Myr ago 14,20 . Here we describe parts of the NSB that were less affected by deformation (Supplementary Table 4) and focus on sites where metamorphic grade appears not to have exceeded lower amphibolite facies 17 . This setting is evidenced by local outcrops in the southwestern margins of the belt that preserve primary chert, diagenetic calcite rhombohedra with poikilitic textures, and minerals of low metamorphic grade such as euhedral stilpnomelane and minnesotaite in chert that lack pseudomorphic retrograde textures.

No abstract

No abstract

No abstract

No abstract

No abstract

No abstract