Banded iron formation also shortly known as BIF is a major source of iron. BIF is a rock type made up of substituting silica- and iron-rich bands. BIF is economically among the most significant rock types as our society is largely dependent on iron, which is principally extracted from this rock. Photosynthetic organisms that were producing oxygen, but reacted with the iron dissolved in seawater to create iron oxide minerals on the ocean floor, ended creating banded iron formations.
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Composition of Banded Iron Formation
Banded iron formation contains layers of iron oxides (essentially either hematite or magnetite) isolated by layers of chert (silica-stocked sedimentary rock). Each layer is generally narrow (millimeters to few centimeters). The rock has a characteristically banded appearance due to differently colored darker iron-rich and lighter silica layers. In some cases BIFs may consist of siderite (carbonate iron-carrying mineral) or pyrite (sulfide) instead of iron oxides and in place of chert the rock may consist of carbonaceous (rich in organic matter) shale.
BIF is a chemogenic sedimentary rock (material thought to be chemically catapulted on the seafloor). Since old age BIFs usually have been metamorphosed to a different degree (particularly older types), but the rock has heavily retained its original appearance since its constituent minerals are reasonably stable at higher temperatures and pressures. These rocks can be defined as metasedimentary chemogenic rocks.
Types of Banded Iron Formation
BIFs formed in three episodes i.e. 3500-3000 Ma (millions of years ago), 2500-2000 Ma, and 1000-500 Ma. The BIFs from these three episodes are known as Algoma-, Superior- and Rapitan-types, respectively. In each case there were several simulations that resulted in their formation.
Algoma-type is the oldest (from the Archaean) and appears to be linked with volcanic arcs. They are majorly found in old greenstone belts. Iron-rich minerals are customarily magnetite. Algoma-type iron ore bodies are comparatively small, generally less than 100 meters in thickness and several kilometers in lateral extent. Algoma-type accumulations are mined in the Bjørnevatn (Norway), Abitibi greenstone belt (Ontario, Canada), Kostomuksha (Russian Karelia), etc.
So far it is also one of the significant types of banded iron formations formed during the Paleoproterozoic (Superior-type). They formed on firm continental shelves. Superior-type accumulations are in vast dimensions (greater than 100 meters in thickness and over 100 km in lateral extent). A crucial iron-bearing phase is hematite, but magnetite also occurs. Iron mines where BIFs pertains to Superior-type include Lake Superior (Canada, USA), Labrador (Canada), Hamersley Basin (Australia), Kryvyi Rih (Ukraine), and Transvaal Basin (South Africa), Quadrilatero Ferrifero (Brazil), Singhbhum (India).
The ocean was also a profuse source of silica to form chert layers since the seawater is thought to have been saturated with silica (120 mg/l) during most of the Archaean-Proterozoic.
This type is the least significant with respect to the volume of ore mined. Their genesis appears to be linked with glaciations, global ice age (Snowball Earth) and related environmental changes. Iron-bearing mineral in Rapitan-type accumulations is hematite1.
The world ocean was almost completely overlaid ice and thus separated from the atmosphere. That reintroduced diminishing conditions in the water column same as those that existed before the oxygenation of the atmosphere. This near global anoxia in seawater is usually perceived to be the reason why BIFs reappeared as iron deposited in the water and were later accumulations when the ice age subsided and the ocean was oxygenated again.
Problem With Banding of BIFs
Another key issue is the banding of BIFs. These bands could display seasonal cycles as modern varves do. Or it could be some other major cyclical alteration in ocean water chemistry or biology. It appears possible that there was some form of biological mediation and the alterations in BIF composition display the cyclical changes in the numbers of organisms.
You can spot a 2.1 billion year-old rock with BIF formation at the National Museum of Mineralogy and Geology, Dresden, Germany.
Approximately a 3-billion-year-old BIF from Canada reveals that the atmosphere and ocean once had no oxygen.
Various controversies exist over BIF origination, and many theories have been proposed.
Banded iron formations, although widely mined, remain mysterious in several ways.
Understanding of their genesis is largely obstructed by the fact that there are no modern analogues.
As per a theory, BIF formation has been distinguishably ascribed to volcanic activity; rhythmic accumulation from iron and silica solutions because of oxidation, seasonal variations; and precipitation from solution as an outcome of unique oxidation-reduction conditions.
All these terms (Algoma, Superior, Rapitan) implies localities in Canada, but they are used to classify BIFs worldwide.