Finely banded, siliceous, hematite deposits found in Precambrian rocks, forming stratiform units often several hundred metres thick and persistent over 150 km or more. They probably formed by chemical–organic processes during sedimentation in stable, shallow basins with little detritus, and so are syngenetic deposits. In their enriched form (40–60% iron) BIFs are mined open-cast. They include the world’s most important sources of iron ore, e.g. at Hammersley, Western Australia; Lake Superior, USA; Labrador, Canada; Ukraine; and Brazil. Compare granular iron formation.