Banded iron formations (also known as banded ironstone formations or BIFs) are sedimentary rocks that are composed of alternating layers of iron oxides and silica-rich chert. They are some of the oldest rocks on Earth, with the oldest known BIFs dating back to the Archean Eon, over 3.8 billion years ago.
Most of the major iron deposits worldwide occur in rocks called banded iron formations (or BIFs for short), which are finely layered sedimentary rocks composed of alternating chert (a form of quartz) and iron oxide bands.
Banded iron formations (BIFs) are chemically precipitated sedimentary rocks . They are composed of alternating thin (millimeter to centimeter scale) red, yellow, or cream colored layers of chert or jasper and black to dark gray iron oxides (predominantly magnetite and hematite), and/or iron carbonate (siderite) layers. Banded iron formations have greater than 15% sedimentary iron content. Banded iron formations are of economic interest as they host the world's largest iron ore deposits and many gold deposits.
|Jaspilite banded iron formation (Soudan Iron-Formation, Soudan, Minnesota, USA
Image Credits: James St. John
Algoma-type banded iron formations were deposited as chemical sediments along with other sedimentary rocks (such as greywacke and shale) and volcanics in and adjacent to volcanic arcs and spreading centers. Iron and silica were derived from hydrothermal sources associated with volcanic centres. Algoma-type iron formations are common in Archean green-stone belts, but may also occur in younger rocks.
Formation of Banded Iron Formations
The precise mechanisms underlying BIF formation remain an active area of research, but the prevailing theory suggests their genesis in oxygenated marine environments. As photosynthetic cyanobacteria flourished in the early oceans, their oxygen production triggered the oxidation of dissolved iron, leading to the precipitation of iron oxides from the water column. These iron oxides settled to the seafloor, intermingling with silica-rich deposits to form the characteristic banded structure.
Several key factors are thought to have contributed to the formation of BIFs:
Abundant Dissolved Iron: Early Earth's oceans contained significantly higher levels of dissolved iron compared to today's oceans. This abundant iron supply was a crucial ingredient for BIF formation.
Photosynthetic Cyanobacteria: The emergence of photosynthetic cyanobacteria marked a turning point in Earth's history. These early organisms, through the process of photosynthesis, began to produce oxygen, gradually changing the composition of the atmosphere.
Shallow, Oxygenated Marine Environments: BIF formation is thought to have occurred in shallow marine environments where oxygen levels were sufficiently high to trigger the oxidation of dissolved iron. These environments were often associated with volcanic activity, which provided a source of iron and silica.
Sedimentary Processes: The alternating layers of iron oxides and silica in BIFs reflect cyclical changes in the environment, such as variations in oxygen levels or seasonal fluctuations in productivity. These changes influenced the precipitation of iron oxides and silica, leading to the formation of distinct layers.
Composition of BIFs
BIFs are composed primarily of iron oxides and silica, with minor amounts of other minerals such as manganese, magnesium, and aluminum. The iron oxides in BIFs are typically hematite (Fe₂O₃) and magnetite ( Fe3O4.). The silica in BIFs is typically chert, which is a microcrystalline form of quartz.
The ratio of iron oxide to silica in BIFs can vary widely, from less than 20% to more than 80%. The iron content of BIFs also varies widely, from less than 15% to more than 30%.
|Banded iron formation, Karijini National Park, Western Australia
Depositional Environment of BIFsBIFs are thought to have formed in shallow marine environments where there was a lot of dissolved iron and oxygen. These conditions are likely to have existed in the early oceans, before the rise of land plants. They are often found in association with other sedimentary rocks, such as shales, carbonates, and sandstones.
BIF Global Distribution and Economic Importance.
|Banded Iron Formations Global Distribution Map
BIFs are found worldwide, with notable examples occurring in Australia, Brazil, Canada, India, the United States, and elsewhere. These formations span a wide range of ages, with the oldest known BIFs dating back to the Archean Eon, over 3.8 billion years ago.
The economic importance of Banded Iron Formations
Iron Ore Reserves: They account for over 60% of global iron ore reserves and contribute significantly to the global economy.
Steel Production: The production of steel, which heavily relies on iron ore from BIFs, drives countless industries and supports global infrastructure development.
Banded Iron Formations Implications
BIFs hold profound implications for several reasons:
Iron Ore Deposits: BIFs are the world's primary source of iron ore, an essential component of steel production. Steel, the backbone of modern infrastructure and machinery, would not exist without the iron derived from BIFs.
Evidence of Early Oxygenation: The presence of BIFs provides compelling evidence of oxygenation in Earth's atmosphere as early as 3.8 billion years ago. This event, known as the Great Oxidation Event (GOE), significantly altered the composition of the atmosphere, paving the way for the evolution of multicellular life.
Paleoenvironmental Record: BIFs serve as a valuable record of Earth's early geological history. The composition and layering of BIFs can provide insights into past environmental conditions, including oxygen levels, temperature, and nutrient availability.
|Banded Iron Formation of Hamersley Range, Australia.
Photo: Masha Pastuhov, Dronesandstones
Scientists Discover Earth's Youngest Banded Iron Formation