The following has been adapted from information provided in Wikipedia (searched in 2012) and the Geology of the Bungalbin 1:100 000 Sheet (Chen and Wyche, 2003).

Banded ironstone formation (BIF) (or banded iron formation) has been described as iron rich chert.  The chert being predominantly silica. The banded colours occur due to different amounts of oxidised iron contained in the minerals. Minerals that contain iron are: haematite, magnetite, grunerite, limonite, siderite and pyrite.  The bands also include the shale and chert which are very low in iron content (iron poor).  BIF is considered to have formed during the Precambrian, 2,000 to 3,000 million years ago or more.  BIF is a sedimentary rock, formed from sediments under the sea. Hence the layering or bands in the BIF rock.  One concept of the sedimentation process that formed BIF is that dissolved iron in the sea combined with oxygen (produced by phyto-plankton) to form insoluble iron oxides that sedimented out (precipitated out) on to the sea floor. This occurred at a time when there was a great increase in the amount of oxygen available following a time of very low oxygen availability. 

The sedimentary BIF rock was altered via pressure and or heat to become a metasedimentary rock.  The BIF ranges were formed when a great uplifting occurred at the earth's surface.   The uplifting resulted in the BIF jutting upwards at angles of 45º or more.  This was followed by weathering processes, where the lateritic duricrust that had formed above the BIF was eroded away.  The BIF is erosion resistant and as other material drops away it is left forming craggy ironstone profiles to hills and ridges.

Banded ironstone formation rocks and outcropping, exposed via the uplifting and weathering, can consist of blue, steel grey or black bands of colour, rich in magnetite – haematite as well as red jaspilite bands and white quartz.  Sometimes the jaspilite bands dominate and, at the other end of the scale, some rocks appear to have no jaspilite bands and are entirely steel grey or grey-black.

The BIF of the Helena and Aurora Ranges mostly dips to the north and north-west (was uplifted from the south).  

The BIF ranges on the Yilgarn Craton (including those in the Great Western Woodlands) are thought by ecologists to have acted like islands.  As the climate became drier the BIF ranges are likely to have provided more sheltered habitat with more reliable water availability, eventually supporting groups of refugia species that do not occur elsewhere.

 


References

Chen SF and Wyche S (2003)  Geology of the Bungalbin 1:100 000 Sheet. Western Australia Geological Survey, 1:100 000 Geological Series Explanatory Notes.  Department of Industries and Resources