Iron formations are divided into Granular Iron Formations (GIF) and Banded Iron Formation (BIF). They are basically chemical sediments (i.e. precipitated from sea water) that are composed of iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3) and cherty silica (SiO2). Iron carbonate (siderite FeCO3) is also a common ingredient and pyrite (FeS2) is ofen present in small amounts. Goethite [FeO(OH)] is common in weathered parts of iron formation deposits. BIFs are typically thinly banded and are believed to have been deposited in deep water, and GIFs are believed to have been deposited in shallow water, where wave action rolled the soft sediment into granules.
Iron formations only occur in Precambrian rocks, and are most abundant in Archean and Paleoproterozoic rocks older than 1800 my. Iron formations are much less common, and also much smaller in the later Precambrian, and are totally absent in Phanerozoic (i.e Cambrian and later) rocks. It is generally assumed that iron formations were formed when the earth's atmosphere contained no oxygen, which allowed iron (presumed to have been derived from volcanic vents and the weathering of recently erupted volcanic rocks) to stay in solution in seawater until precipitated. It is the precipitation mechanism that is not clearly understood. The absence of modern-day analogues only makes the process more enigmatic.
A typical BIF or GIF contains between 25% and 35% Fe (35% to 50% iron oxide) and 50% to 65% silica. These rocks are referred to as "taconite". In contrast to taconite, enriched iron formation was the preferred source for iron before the 1960s. The enrichment process is even more poorly understood than the origin of iron formation. It appears to have been a combination of surface weathering and subsurface solution activity, and has occurred in almost all the major iron-ore districts of the world. Enrichment has either leached silica out of the original BIF or GIF or has dissolved iron in one part of a deposit and replaced silica with iron oxide in another part. Enriched iron ores tend to be very hematitic, while their protoliths were usually composed of magnetite. They often contain a mineral called martite, which is hematite that preserves the crystal shape of the magnetite from which it was derived. Enriched iron ore is often referred to as "direct shipping ore" (i.e. does not need beneficiation in a mill, hence mining operations have a lower capital requirement and a lower operating cost), and this is often abbreviated to "DSO". Enriched ores have grades in the range of 50% to 65% Fe (pure hematite contains 70.0% Fe and pure magnetite contains 71.7% Fe).
While taconite deposits tend to have resources in the hundreds of millions of tons up to many billions of tons, DSO tends to occur in smaller bodies. Mineable DSO deposits are typically measured in tens of millions of tons to hundreds of millions of tons. The very largest DSO deposit was at Mount Hammersley in Australia with initial reserves of 1.8 billion tons at 64% Fe. In Brazil, the Carajás mining district contains multiple DSO deposits that initially aggregated 17.5 billion tons with grades in excess of 64% Fe.