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.

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