Carbonaceous material present in ancient rocks can be used as an indicator of life during the time the rocks were formed. In particular, evidence for the existence of life more than 3,800 million years ago might come from mineral associations between apatite and graphite in rocks from southern West Greenland. However, this interpretation is partly based on the assumption that the graphite was formed at the same time as the host rocks, an assumption that has been difficult to prove. Here we investigate the origins of poorly crystalline graphite associated with apatite in metamorphosed banded iron formations from northern Canada that are 3,750 to 4,280 million years old. We measured average δ13Cgraphite values of -22.8+/-1.9‰ (1σ), similar to values from West Greenland sedimentary rocks of comparable age, and that point to a biological source for this carbon. Our microscopic and spectroscopic analyses suggest, however, that the graphite experienced much lower temperatures than the host rocks during metamorphism. We conclude that the poorly crystalline graphite in these rocks was deposited by fluids after peak metamorphism of the banded iron formations. We suggest that the occurrence of carbonaceous material with low δ13C values in Eoarchaean rocks cannot be used to indicate the presence of a microbial biosphere on the earliest Earth unless the syngeneity of the carbonaceous material in the host rock can be confirmed.