Channel-iron deposits

Channel iron deposits, also known as pisolitic iron ore are a major source of cheap, high grade iron ore exploited primarily in the Pilbara and Murchison regions of Western Australia.

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.

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Source

The source of pisolites and iron are erosional surfaces developed upon major banded iron formations, which provide the iron oxides to form the pisolites. The pisolites are hypothesised to form via concretion of hematite within arid climates, which has washed from the source banded iron formations into a river bed.

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 Fomation, 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.

Transport and trap

The pisolites are transported downstream and accumulate as gravels. The gravels are cemented via a variety of agents, usually a mixture of limonite, clays, carbonate minerals (magnesite, calcite and sometimes siderite), and occasionally silica. This process may form an in-situ concretion of pisolites which may be very resistant to erosion - some mesas in the Pilbara, and Yilgarn are in fact old cemented ferruginous pisolite river gravels.

Economic importance

Channel iron deposits are an important source of iron ore. Although channel iron deposits are typically low-grade at 53% to 57% Fe in-situ, their relative lack of consolidation in most cases renders them liable to bulk mining with little or no need for drilling and blasting. This then is a significant cost saving to miners, who can offset a lower revenue from Fe percentages in the ore via the ease of extraction. Also, in most cases, beneficiation can increase the in-situ iron grade several percent by washing out the majority of clay, carbonate and hydrous limonite cements.

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.

Type examples

The type deposits are those at Pannawonnica and Robe River, in the Pilbara of Western Australia, which are currently mined by Rio Tinto Iron Ore.

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.

See also

• Iron ore
• Yilgarn Craton
• Pilbara Craton
• Port Hedland
• Banded iron formation
• Palaeochannel