Category：Mineral

Year：Proterozoic (about 1.8 billion years ago)

Introduction：Academically, Banded Iron Formation (BIF) is defined following the description of James (1954); that is, strata composed of thin bedded or laminated layers with bulk iron contents greater than 15%. In such strata, the thin layer units range from several millimeters to several centimeters thick. They were so-named for the occurrence of the intercalations of high iron layers and chert layers. The high iron layers are characterized by gray-black colors and mainly consist of magnetite and hematite occasionally with siderite. They usually contain > 50% iron. The chert layers grade from whitish or light grayish to brownish with increasing iron content.

Banded Iron Formations (BIF) were formed via chemical precipitation mostly during 2.1–2.4 Ga. In this period, fungi and algae prevailed in continental shelves. They (e.g., cyanobacteria) released oxygen through photosynthesis. The oxygen then reacted with marine iron ions to precipitate iron oxides (magnetite and hematite). The iron ions were derived from weathering of continental rocks or from magmatic activities at mid-ocean ridges. Alternatively, some researchers postulated iron was mainly supplied by mantle plume magmatism. The supply of iron was episodic. When iron concentration was high in seawater, iron oxide precipitated. In case that the seawater was dominated by silica ion, instead precipitated was white amorphous SiO2.

Banded Iron Formations (BIF) are of very large scale. Six large BIFs in Hamersley Province, western Australia, sum up to 100,000 Gt (Bekker et al., 2010). BIFs of similar scale also occur in south Africa (Transvaal), east of South America (Cauê), east of North America (Sokoman) and (Kursk). Large BIFs produce over 200 Mt iron per year (Johnson, 2018). They are the main sources of iron, responsible for ~80% of iron production.

Accession Number:0011713