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Banded Ironstone (帶狀鐵岩)

Banded ironstone (or banded iron) is formed by repeated stacking of thin layers of iron oxide minerals, such as magnetites and hematites, with occasional inclusions of shales or chert rocks. The resulting rock formations are called banded ironstone formations (BIFs). These rocks are ancient sedimentary rocks formed in the Archean and early Proterozoic Era.

The formations of these banded iron layers can be dated back to the dawn of Earth’s formation, when there were no oxygen on the surface and the atmosphere is full of carbon dioxide and methane. The primordial ocean contains large amounts of dissolved reduced iron (Fe2+) that came from the magma. About 3 billion years ago, the emergence of cyanobacteria and blue-green algae brought about massive changes to the Earth’s ocean and the atmosphere components.

These cyanobacteria and blue-green algae began photosynthesis, converting carbon dioxides into oxygen, and slowly the Earth’s oceans and atmosphere is filled with oxygen. The reduced iron in the oceans are oxidized into Fe3+ and slowly precipitated out of water into magnetites and hematites. The process of stacking these reddish-brown iron ores on the ocean floors began 3 billion years ago, and only stop until 2 billion years ago when most the oxidized iron in the oceans are exhausted. Today these banded iron deposits, some measure hundreds of meters thick, are widely distributed in the world, often spanning thousands of kilometers, and form the majority of the world’s iron ore deposits.

Iron compositions
Mainly composed of magnetite and hematite.

Major deposits

The lateral extensions of the banded ironstones are extensive, reaching to every continents of the world. Some of the famous iron mines in the world are: Hamersley deposits in Australia (estimated at 20,000,000 tonnes of iron ore); Lake Superior areas of the United States; South Africa’s Transvaal regions.

Main purpose
Banded ironstones are major sources of iron ores in the world.


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