B7
Highly enriched iron ore

About 80% of iron resources are obtained from banded iron formations (BIFs). Most BIFs were sediments deposited from a time span ranging from 3.5 to 1.9 billion years ago. BIFs are found only on cratons such as Australia, North America, South Africa, India, and Greenland. BIFs typically consist of repeated layers of red-brown to black iron oxides (magnetite and/or hematite), alternating with iron-poor bands. Iron minerals may include sulfide such as pyrite. Fe2+ is soluble in water, while Fe3+ insoluble. Since early Earth had a more reducing surface environment, iron was abundantly in the form of Fe2+ in sea water. Oxygen released by photosynthesis oxidized Fe2+ to insoluble Fe3+ in sea water, forming BIFs. BIFs are classified into two types: (1) Algoma-type, which is smaller and accompanied by volcanic rocks, formed mainly between 3.5 and 2.7 billion years ago, and (2) Superior-type, which is larger and accompanied by detritus from continents, formed mainly between 2.7 and 1.9 billion years ago. In order to explain the difference between the two types, numerous factors must be considered including the depositional environment (e.g., chemistry, pH, submarine vs. onland, concentration of metals, source and amount of hydrothermal activity). BIFs contain generally 30% (or less) iron, and thus mining involves moving tremendous amounts of ore and waste. Higher-grade iron ore, on the other hand, is highly concentrated in iron through localized hydrothermal and/or groundwater alteration (i.e., leaching from crustal materials) of BIF. The specimen displayed is iron ore from Mount Whaleback, Australia. This deposit is altered Superior-type BIF formed 2.5 billion years ago. The ore is compact massive hematite, highly enriched in iron.　(Kaoru Kiyota)