Petrographic studies reveal that structures of possible organic origin and those believed to be relict microfossils are extremely abundant in many Precambrian iron-formations in the United States, Canada, Australia, and South Africa. The structures are mainly spheroids and ellipsoids that range from about 5–40μ in diameter. Particular forms, however, usually have a narrower size range. Some of the structures appear to be similar to certain forms reported by Barghoorn and Tyler (1965) from the Gunflint chert; others presumably are new forms not previously reported.
Particular types of structures are present in widely separated districts, indicating a wide geographic distribution. Certain distinctive forms occur in several Proterozoic iron-formations in North America and in an Archean iron-formation in Australia. Other forms occur in iron-formations from the Mesabi Range, Minnesota, and Belcher Islands and Lake Albanel, Canada.