Mineralisation

Iron ore mineralisation in the Hamersley Iron Province is divided into Bedded Iron Deposit (BID), Channel Iron Deposit (CID) and Detrital Iron Deposit (DID).

The Hamersley Group consists of Banded Iron Formation (BIF), dolomite, shale and felsic volcanic rocks, all of which have been intruded by dolerite sills and dykes. The Hamersley Group is divided into seven Formations. Of these, the Marra Mamba Iron Formation (MMIF) and Brockman Iron Formation (BFM) host BID in the Hamersley Iron Province.

BIDs are predominantly developed within the Mt Newman Member and Nammuldi Member of the MMIF and the Dales Gorge Member of the BFM. There is varied thought as to the genesis of these deposits, however supergene enrichment (during two phases) is the generally accepted process, with supergene alteration resulting in the development of goethite-martite (replacement of magnetite by hematite) ores.

DIDs develop as a result from the deposition of eroded BID. They may contain a range of clast types, sometimes with later goethite cementation (Canga). Economic deposits of DID mineralisation are usually restricted to the upper-most Hamersley Surface.

CIDs represent alluvial deposits rich in ferruginous fragments which were eroded from the country rock and deposited in river channels incised into various basement lithologies as explained in the regional geology. The CIDs are collectively referred to in geological literature as Robe Pisolite due to its ‘pea-stone’ fabric.

Robe Pisolite typically contains concretions of goethite-hematite and fossil wood cemented with iron oxide. The concretions, variably described in the literature as ooids, ooliths, pisoids and pisolite, are sub angular to rounded, up to 6 mm in size. The dominant mineral of CID is goethite, with lesser hematite.

Within the CID, goethite occurs as oolite, pisolite and as a matrix. Hematite is identified mainly as cores to the pelletoids. Occasionally hematite forms a rim to goethite nuclei. (Read Figure 4).

Most CIDs contain ooid-pisoid sized inclusions of wood, pseudomorphed by goethite (and occasionally hematite) which are usually very porous. These tend to have non-porous goethite margins.

The matrix is essentially goethite and is often of similar tenor (grade) to the pelletoids. The porosity of the ironstones usually relates to the abundance of matrix goethite. Some CIDs essentially lack matrix being clast cemented and are highly porous due to the rounded nature of the clasts.