Geology

The Pilbara region is located in the northwest of Western Australia, approximately 1,100km north of Perth. The region contains the 80,000km2 Hamersley Iron Province. The geology of the Province is characterised by a 2,500 million years old group of late Archaean and early Proterozoic rock formations known as the 'Hamersley Group'.

The Hamersley Group was formed by chemical sedimentation of minerals in a marine (ocean) environment.  The processes which formed the Hamersley Group occurred after volcanic activity introduced basalt rocks in the area. Sedimentary rocks like sandstones also occur in the group due to weathering and transportation of rocks which occurred during the period when the Hamersley Group was formed.

During the period of formation, layers of rock with different chemical composition were deposited.  Iron-rich layers alternate with silica-rich layers in the rock formation.  Occasionally these layers are interspersed with layers of sedimentary rock.  This type of rock formation is called a Banded Iron Formation (BIF).

The Hamersley Group is approximately 2.5km thick. It contains several large units of BIF: rock with bands of iron minerals (magnetite and hematite) and gangue minerals (mostly carbonates, silicates and chert).  Typically, unenriched BIF contains about 30 per cent iron by weight.

Geologists have identified and named distinct units within the BIF of the Hamersley Group. Commercially, the most important BIF units are the Brockman Iron Formation (620m thick) and the Marra Mamba Iron Formation (230m thick). These layers host many of the large iron ore deposits in the Province.  The Marra Mamba Iron Formation is older than the Brockman Iron Formation and is situated below the Brockman unit in the original rock sequence.

The BIF layers have been deformed by geological processes, and eroded by weathering resulting in the geological formations that we see today.

Bedded Iron Deposits

Where Banded Iron Formation (BIF) has been enriched by natural processes, these zones can become Bedded Iron Deposits.  Many of the commercially important iron ore deposits in the Pilbara were formed by natural enrichment of BIF. These enriched deposits include the commercially viable ores in the Brockman and Marra Mamba Iron Formations.

The BIF in the Brockman and Marra Mamba Iron Formations was enriched to a high degree forming ore grade ore with more than 60 per cent iron. The natural processes that accomplished this (hypogene and supergene enrichment) involved circulating ground waters. Non-iron minerals in the BIF were largely replaced by hydrous iron oxides (notably goethite) and partly dissolved out. At the same time, magnetite in the BIF oxidised to hematite. Favourable climate and geological structures (folds and faults) stimulated the process. Sedimentary rocks that were interspersed with the BIF became shales.

In the Pilbara, other types of ore deposit exist, notably channel iron deposits, but these are derived from the original bedded iron formations.

Brockman Iron Deposits

The Brockman Iron Deposits have four parts: the lower most Dales Gorge Member, the Whaleback Shale and Joffre Members, and the uppermost Yandicoogina Shale Member.  Brockman ore is mostly in the Dales Gorge and Joffre Members within the Brockman Iron Deposits.

Brockman Iron Deposits typically have hematite as the dominant iron mineral. Brockman deposits also have goethite in varying amounts and have varying phosphorus content and physical characteristics.

The variation exhibited by Brockman deposits is a result of different degrees of dehydration of goethite to microplaty haematite which also affects the amount of residual phosphorus content.

Brockman deposits range from blue grey in colour for deposits with the greatest degree of dehydration to grey-yellow-brown for deposit with higher amounts of goethite and less dehydration. 

Marra Mamba Iron Deposits

There are numerous high grade Marra Mamba Iron Deposits. Marra Mamba deposits all have goethite hematite mineralogy, with a greater proportion of goethite compared to Brockman ores.  There is also a range of physical properties exhibited within Marra Mamba deposits.

The iron content of most high grade Marra Mamba ores is about 62 per cent but can vary significantly.  Key characteristics of Marra Mamba ores include a lower phosphorus content compared to most Brockman ores and a higher loss on ignition which reflects the different goethite mineralogy exhibited in Marra Mamba deposits compared to Brockman ores.  Phosphorus is usually less than 0.07 per cent. Silica and alumina percentages are moderately low. Marra Mamba ores are typically grey-yellow-brown.

Channel Iron Deposits

The Channel Iron Deposits (CIDs) were formed in ancient meandering river channels. As bedded iron deposits were eroded by weathering, iron particles were concentrated in these river channels.  Over time these particles were rimmed with goethite deposited by percolating iron-enriched ground water approximately 15-30 million years ago, which also fused the particles together.

Channel Iron Deposits appear as low flat-topped hills called mesas and have also been located concealed under the cover of more recent rocks.  These deposits range in thickness between 5m and 40m thick.  This type of deposit is believed to be unique to Western Australia.

CIDs are quite different from bedded ores. Their chief characteristic is their pisolitic 'texture': rounded hematitic 'pea-stones', 0.1mm to 5mm in diameter, rimmed and cemented by a goethitic matrix. The ore is brown-yellow in colour.  They typically contain minor amounts of clay in discrete lenses.

Detrital Iron Deposits

Detrital Iron Deposits (DIDs) are found where weathering has eroded bedded iron deposits and deposited ore fragments in natural traps formed by topography, usually drainage channels or valleys. Some Detrital Iron Deposits are loose gravels while others are naturally cemented (hematite conglomerate). Both types are often found in the same deposit.

The quality of the iron ore in these deposits is dependant on the bedded iron ore deposit which was the source of the ore particles.  Typically these deposits are valued for the high proportion of high quality lump contained within them, as lump sized particles have a greater tendency to be captured in the trap site.