Channel iron deposits of the Hamersley Province, Western Australia
The fluvial goethite‐hematite channel iron deposits (CID) of fhe Pilbara region of Western Australia, formerly known as the ‘Robe Pisolite', represent a major source of iron ore mined in the Hamersley Province. The CID occupy Early Tertiary palaeochannels that are typically less than 1 km but range to several kilometres in width and from 1 to ∼100 m thick. The Robe palaeochannel is the longest, with CID partly preserved over a distance of 150 km, whereas the Marillana palaeochannel contains CID along 80 km of its length. The CID range from goethitic mudstone to fine hematite‐goethite gravel and intraformational conglomerate varying in distribution along the channels in a range of massive, bedded and altered types. The granular type that forms the bulk of the ore is typically ooidal, comprising pelletoids with goethite cortices around hematite nuclei (which often consist of fossilised wood), with typically abundant coarse goethitised wood fragments, a varied proportion of peloids, minor pisoids, and a diverse porous goethitic matrix. The Robe deposits tend to have more spherical pelletoids, with more regularly developed layering, and smaller cores than CID from the Yandi deposits of the Marillana valley. Generally, the presence of ferruginised fossil wood is the principal diagnostic field criterion used to distinguish CID from other comparable goethite‐hematite detrital deposits. Electron microprobe X‐ray maps reveal that lattice Al and Si increase from the hematitic nuclei to the matrix, and then the cortex. In the Robe CID, alumina in the nuclei varies between 0.5 and 2.8%, whereas at Yandi, alumina in the nuclei ranges from 0.2 to 0.7%. The goethite cortex of a granule is usually richer in both alumina and silica than its nucleus, in a cortex : nucleus ratio of ∼2:1 at Robe and 3:1 at Yandi. The nucleus and cortex of individual pelletoids generally exhibit a similar trace‐element distribution, which can often be related to the surrounding country rock and, with the exception of Al, is different to that shown by the matrix. A visual classification (labelled co‐manual) based on the relative amount of cortex (co), matrix (ma) and nucleus (nu) enables an estimate of the deleterious minor elements, mainly aluminium (al), for separating samples into ore, near‐ore and non‐ore.
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