Iron ore, which accounts for most of Rio's profit and is used in making steel, has provided healthy margins for years but the outlook is uncertain as major buyer China is expected increasingly to rely on recycling rather than importing raw material.

Following a commodity price crash in 2015, Rio put a string of assets on the block, mostly in coal, to decrease its debt load. In iron ore, its push to refocus has meant concentrating on Australia's Pilbara region, where it has lower costs and higher grades.Rio Tinto's revenues are more dependent on iron ore than its rivals, in 2017, iron ore making up more than 60 percent of Rio's earnings.

IOC, 59 percent owned by the Anglo-Australian miner, 26 percent by Japan's Mitsubishi Corporation and 15 percent by Labrador Iron Ore Royalty Company, is one of Canada's largest producers of iron ore. It had revenues of $1.9 billion in 2017.

Rio had tried to sell its stake in IOC in 2012 as it already deemed it a non-core asset but it withdrew the process, saying the sale was not essential.

One banking source said that offers from private equity funds and rival miners had not met Rio's target of between $3.5 billion and $4 billion back then.

Rio was now considering an IPO of its stake on the Toronto Stock Exchange (TSX) as part of its ongoing work to hold only its best assets, but the process was at an early stage and it had not yet hired a financial adviser, the banking source added.

Rio Tinto and Mitsubishi declined to comment. Labrador Iron Ore Royalty Company was not immediately available to comment.

The Pilbara region of Western Australia produces high quality iron ore that sells at a premium and Rio invested heavily in the region's assets, installing driverless trains and maximising productivity.

It is also poised to expand there, having approved $146 million of funding to undertake initial work at the Koodaideri iron ore project, ahead of a final investment decision expected by the end of the year.

IOC produces iron ore pellets and concentrate from operations in Newfoundland and Labrador, and runs port facilities in Sept-Iles, Quebec, as well as a 418-kilometre railway that links the mine to the port.

A labour dispute earlier this year that resulted in a two-month suspension of operations was part of the reason for Rio reducing its underlying core profit by $137 million compared with the 2017 first half, Rio Tinto said when it reported half-year results last week.

(By Clara Denina, Barbara Lewis and Yuka Obayashi; Editing by Jane Merriman)