The ore bodies consist of nodules of hematite, limonite-goethite with some clay, sand and silt matrix. These nodules (pisolites) are concentrated into alluvial and elluvial washes leading from the source regions, and typically fill palaeochannels.
Layers of limonite-goethite and hematite are deposited on fragments of hematite gravel at surface in a laterite profile as part of the weathering process of the source banded iron formation. The process of lateritisation forms round pisolite nodules which are washed into alluvial channels and stranded.
The major source in the Pilbara is the Marra Mamba Formation, which can be up to 500m thick. In the Murchison region, the Jack Hills banded iron formations and several prominent Archaean BIF formations in the Western Gneiss Terrane of the Yilgarn Craton are the source of the iron pisolites.
The key economic criteria for channel iron deposits are, firstly tonnage and location relative to infrastructure similar to other bulk commodities. Thereafter, the nature of the cement is important, particularly in the cases of carbonate cements containing magnesite, as magnesium is a problem. Rare channel iron deposits are rendered uneconomic because of a silica cement proving too durable for easy mining and crushing. The water content of channel iron deposits (quoted as Loss on Ignition) is from 7% to 12%, which is the highest of all iron ore types, generally due to the presence of goethite-limonite. Phosphorus, aluminium and sulfur levels are another concern, typically being above normal levels in-situ although if the phosphorus and aluminium are hosted in a weak cement, they can often be washed out during beneficiation. Most channel irons are upgraded via washing of the pisolite gravels to remove the cements and matrix.
Channel iron deposits are rarer outside of the West Australian landmass, due to the relative youth of the regolith in the rest of the continental land masses, although there are smaller examples from Brazil and in Africa.
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