The origin of banding in banded iron formations (BIF)

Banded iron formations (BIF) and iron ore are globally important resources of iron that have very distinctive textures.  Well-preserved BIF contain iron-rich bands consisting of iron oxides (e.g., hematite, magnetite), iron silicates (e.g., stilpnomelane), and iron oxyhydroxides (e.g., greenalite), that are inter-layered with siliceous chert bands.  New research by Rasmussen et al. recently published in Geology uses a simple, yet elegant approach to explain the origin of the distinctive banding found in banded iron formations.

The researchers utilized samples from the Dales Gorge Member from the Hammersley Group in Western Australia.  Samples of BIF from the Dales Gorge Member are important because they are exceptionally well-preserved and have much of their primary minerals and textures are intact, therefore allowing one to infer primary processes that lead to banding in BIF.

Utilizing petrography and scanning electron microscopy Rasmussen et al. illustrate the microgranules in the BIF are composed of the iron-silicate mineral stilpnomelane (Figure 1),  which was likely iron silicates like chamosite or nontronite at the time of original BIF formation that have subsequently been diagenetically transformed to stilpnomelane.  They argue that these granules formed via flocculation of iron-silicates in an iron-rich ocean, where the iron was supplied by hydrothermal venting (i.e., black smokers), forming a colloidal suspension of iron microgranules.  These suspensions of iron microgranules were subsequently deposited as distinct laminae via sedimentary processes (i.e., density currents), resulting in the iron-rich bands found in BIF (Figure 2). These periods of iron flocculation and deposition alternated with periods of low hydrothermal activity in which sedimentation was dominated by silica deposition and chert formation (Figure 2).  These alternating periods of hydrothermal venting and quiescence ultimately resulted in alternating layers of iron-rich and chert-rich material so diagnostic of BIF.

Figure 1.  Stilpnomelane (brown) surrounded by quartz and dolomite-ankerite in plane polarized and cross-polarized light.  These textures are typical microgranules from BIF. From Rasmussen et al. (2013).

They also illustrate that preservation of the well developed banding is also dependent on whether or not there has been early (i.e., diagenetic to hydrothermal) silicification of the BIF (Figure 2).  In cases where the silificiation is early the rock has strength and cannot be compacted, hence, the diagnostic banding is preserved (Figure 2B).  In cases where silicification does not take place the BIF can be compacted resulting in iron-rich bands with very high concentrations of iron, with the original microgranular textures and delicate banding all but obliterated (Figure 2C).  If there is formation of chert nodules during diagenesis this microgranular texture can be preserved in the nodules, but may be surrounded by compacted, higher iron concentration layers (Figure 2D).

Figure 2.  Idealized diagram illustrating the development of layering in BIF.  A) Flocculated iron microgranules are deposited as layers.  B) If silicification takes place early there will be alternating layers of chert and iron-rich layers.  C) If silicification does not take place then compaction of sediment can occur and there can be larger iron-rich bands forming without alternating chert layers.  D) If chert nodules form during diagenesis there can be local preservation of BIF layering, but there will also be compaction and formation of iron-rich bands around the nodules.  From Rasmussen et al. (2013).

The paper also has obvious implications for the transformation of BIF to iron ore.  In particular, BIF that have early silicification and preservation of microgranules would be very difficult to compact, and therefore would have lower iron grades than BIF that did not endure early silicification (e.g., Figure 2C and D).

The elegance and beauty of this paper is that it relies on rather basic observations using petrography and scanning electron microscopy, coupled with a lot of intuition and reasoning to solve a fundamental problem.  A great read for those interested in BIF, iron ore, chemical sedimentation, and really good science!

See also – very interesting graphics and information on iron ore formation from Minerals Downunder.

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