Banded Iron Formations

Banded Iron Formations

References - a www. scirus.com search will provide a host of sources and links. Google image will give a range of field images

This distinctive lithology was recognised by Ronov (1968) as uniquely P 111c213b recambrian. (Ronov referred to them as Jaspilites)

Typical BIF - Finely laminated (mm scale or less) sediments composed of iron rich (Haematite and Magnetite) and silica rich layers.

These enigmatic sediments and their metamorphosed equivalents are THE major source of high grade iron ores globally.

BIF's are found in the Archaean and Proterozoic, but the major development of BIF's spans the Archaean/Proterozoic transition.

BIF ages:

Isua 3.8 Ga

Hamersley   2.63-2.44 Ga

Transvaal 2.47-2.39 Ga

Superior  2.20-1.85 Ga

Cloud was particularly interested in BIF's because of the oxidised state (Fe³⁺) of the iron in these sediments.

His model envisaged a biogenic origin involving O₂ liberated by photosynthesis of cyanobacteria (=blue-green algae).

Cloud viewed BIF's as part of the evidence for the transition from an anoxic to an oxic atmosphere.

He reasoned that in an oxygen free world the oceans would become enriched in dissolved iron (Fe²⁺) from continental weathering.

Blooms of O₂-liberating cyanobacteria would oxidise the dissolved (ferrous - Fe²⁺) iron to the insoluble ferric (Fe³⁺) form which would be precipitated and preserved.

The silica-rich laminae are alleged to represent the background sedimentation.

Originally all BIF's were viewed as the same and a common origin was sought.

Increasingly distinctions are being drawn between Archaean examples and the typical Proterozoic forms.

The principal difference is one of scale - Proterozoic examples are bigger, in terms of thickness, complexity and areal extent.

The Hamersley BIF's exhibit an astounding laterally persistent fine scale lithostratigraphy

The literature of BIF's is full of 'armchair' geology - papers based on literature rather than field data. A large proportion of these papers lack data, rigour and any empathy with the rocks and their field relations.

There is an emerging consensus about a number of features of Proterozoic BIF's:

• the source of much of the iron is magmatic not continental

• a deep shelf/slope environment is likely, with references to turbidite inputs

• stratified oceanic waters and 'upwelling' feature strongly in many models

• transitions into shallower facies are marked by different iron mineralogies

• a variety of biological controls are debated

• the tectonic context is highly debated

• a number of authors use an oxygenic atmosphere with a stratified ocean - chemocline

These sediments provide evidence for a unique set of atmospheric, hydrospheric, biospheric and tectonic conditions that occurred at the Archaean/Proterozoic transition.

At present those conditions are only poorly understood. BIF's represent a kind of geological 'Rosetta stone' that has not yet been deciphered.

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