Banded iron formation (BIF) is the principal source of iron. BIF is a rock type composed of alternating silica- and iron-rich bands. Banded iron formation is economically among the most important rock types as our society is heavily reliant on iron, which is mostly extracted from this rock.
An Algoma-type BIF sample from Bjørnevatn in northern Norway. Black ore mineral is magnetite. Width of sample 9 cm.
Banded iron formation consists of layers of iron oxides (typically either magnetite or hematite) separated by layers of chert (silica-rich sedimentary rock). Each layer is usually narrow (millimeters to few centimeters). The rock has a distinctively banded appearance because of differently colored lighter silica- and darker iron-rich layers. In some cases BIFs may contain siderite (carbonate iron-bearing mineral) or pyrite (sulfide) in place of iron oxides and instead of chert the rock may contain carbonaceous (rich in organic matter) shale.
Banded iron formation is a chemogenic sedimentary rock (material is believed to be chemically precipitated on the seafloor). Because of old age BIFs generally have been metamorphosed to a various degrees (especially older types), but the rock has largely retained its original appearance because its constituent minerals are fairly stable at higher temperatures and pressures. These rocks can be described as metasedimentary chemogenic rocks.
Banded iron formations, although extensively mined, remain enigmatic in several ways. Our understanding of their genesis is greatly hampered by the fact that there are no modern analogues. BIFs formed in three episodes 3500-3000 Ma (millions of years ago), 2500-2000 Ma, and 1000-500 Ma. The BIFs from these three episodes are referred to as Algoma-, Superior- and Rapitan-types, respectively. In each case there were different triggers that led to their formation.
The width of layers is not nearly uniform which somewhat speaks against the hypothesis that the layers are seasonal. Bjørnevatn, Norway. Width of sample 17 cm.
Average iron content of Bjørnevatn BIF is 31%. See the hand magnet strongly attached to the rock, demonstrating its high magnetite content. Width of sample 36 cm.
Algoma-type is the oldest (from the Archaean) and seems to be associated with volcanic arcs. They are found in old greenstone (metamorphosed mafic volcanics) belts. Iron-rich mineral is almost always magnetite. Algoma-type iron ore bodies are relatively small, usually less than 100 meters in thickness and a few kilometers in lateral extent. Algoma-type deposits are mined in the Abitibi greenstone belt (Ontario, Canada), Bjørnevatn (Norway), Kostomuksha (Russian Karelia), etc.
By far the most important type of banded iron formations formed during the Paleoproterozoic (Superior-type, named after lake Superior). They formed on stable continental shelves. Superior-type deposits are large in dimensions (more than 100 meters in thickness and over 100 km in lateral extent). The main iron-bearing phase is hematite, but magnetite occurs also. Iron mines where BIFs belong to Superior-type include Hamersley Basin (Australia), Kryvyi Rih (Ukraine), Transvaal Basin (South Africa), Labrador (Canada), Lake Superior (Canada, USA), Quadrilatero Ferrifero (Brazil), Singhbhum (India)2. Rapitan-type is the least important in terms of the volume of ore mined. Their genesis seems to be related to glaciations and associated environmental changes. Iron-bearing mineral in Rapitan-type deposits is hematite1. All these terms (Algoma, Superior, Rapitan) refer to localities in Canada, but they are used to classify BIFs worldwide.
The original banding may be severely disturbed by metamorphic processes. This rock is composed of quartz and magnetite and it comes from the Bjørnevatn mine in Norway (Algoma-type BIF). Width of sample 11 cm.
The material BIFs are composed of comes from the ocean. Iron seems to be mostly provided by the black smokers on the mid-ocean ridges and by the dissolution of the oceanic crust. This is supported by the observation that most BIFs are remarkably free of terrestrial material. It is in contrast with modern ironstones which contain various degree of material from the continents, including the iron itself. The input of black smokers was especially strong with Algoma-type deposits and has diminished with time. The Rapitan-type deposits seem to reflect an average ocean water character at the time. Superior-type remains somewhat of a problem because these deposits are so vast, yet they are located far away from the mid-ocean ridges.
It seems likely that iron-rich water was brought to the continental shelves by upwelling which similarly has provided material for phosphorite deposits. Ferrous iron (Fe2+) brought up to the surface waters reacted with oxygen produced there by photosynthetic organisms, especially cyanobacteria. The oxidation of iron may also be a result of ultraviolet radiation inducing the oxidation of ferrous iron to ferric iron.
Superior-type BIF from North America. Dark gray layers are composed of hematite. Red is jasper (hematitic chert). The rock is about three meters wide and weighs 8.5 tons. Photo by André Karwath. Licensed under CC BY-SA 2.5 via Commons.
The ocean was also an adequate source of silica to form chert layers because the seawater is believed to have been saturated with silica (120 mg/l) during most of the Archaean-Proterozoic2. Currently seawater contains only less than 10 mg/l because modern oceans are home to several organisms (diatoms, radiolarians, sponges) that extract silica from the water. This is likely one of the reasons why BIFs can not form in modern conditions. The problem, however, is the precipitation mechanism. It has been suggested that perhaps the evaporation of seawater promoted local silica oversaturation which resulted in silica precipitating as a gel on the seafloor.
Another major problem is the banding of BIFs. We do not have an adequate explanation for that. These bands could represent seasonal cycles as modern varves do. Or it could be some other major cyclical change in ocean water chemistry or biology. It seems likely that there were some form of biological mediation and the changes in BIF composition reflect the cyclical changes in the numbers of respective organisms.
Superior-type hematitic BIF from Kryvyi Rih, Ukraine. Width of sample 7 cm.
Yet another not fully explained aspect is the formation of BIFs in three distinct periods. This is generally believed to be the result of favorable conditions brought about by changes in tectonics, global climate, biological evolution, and ocean water chemistry. So the explanation needs to be fairly complex and is different for each type which is why we have no generally accepted detailed explanation of BIF formation.
The formation of Superior-type BIFs is closely related to the oxygenation of the atmosphere. Prior to that atmosphere contained little free oxygen because all the oxygen produced by marine photosynthetic organisms was consumed by iron dissolved in the seawater which subsequently after the oxidation settled to the seafloor. Free oxygen finally had a chance to start accumulating in the atmosphere after most of the dissolved iron from the ocean surface layer was precipitated. That took very long time, but finally gave us these vast deposits of iron-rich sediments we are so thankful for and even more importantly oxygen-rich atmosphere without which modern life could have never emerged.
The Rapitan-type BIFs seem to be associated with global ice age (Snowball Earth). The world ocean was almost completely covered by ice and therefore isolated from the atmosphere. That reintroduced reducing conditions in the water column similar to those that existed before the oxygenation of the atmosphere. This near global anoxia in seawater is generally believed to be the reason why BIFs reappeared as iron accumulated in the water and were later deposited when the ice age receded and the ocean was oxygenated again.
This sample, although somewhat similar to BIFs likely has a different genesis. The main constituents are magnetite, quartz and garnet. The latter indicates that the material is terrestrial and deposited either inland or near the coast. It seems to be a lithified and metamorphosed heavy mineral sand deposit. Varanger Peninsula, Norway. Width of sample 18 cm.
References
1. Misra, K. (1999). Understanding Mineral Deposits
Springer.
2. Robb, L. (2005). Introduction to Ore-Forming Processes
Blackwell Science Ltd.