The falls came as Rio Tinto reminded the market of just how much more low-cost iron ore is coming to oversupply the market and declared there was a lot more still in the ground.

On Friday night, the benchmark, The Steel Index iron ore price, slipped $US1.10, or 2 per cent, to $US58.20 a tonne, extending a $US2.80 drop the previous night that saw it drop below $US60 for the first time since 2009.

Rio, the nation’s biggest and lowest-cost iron ore miner, reported a small drop in iron ore reserves on Friday night in its annual report.

But it put out a separate announcement declaring it had added an extra 1.68 billion tonnes to its booked iron ore resources (a measure of ore in the ground proved up to a lower level of confidence than reserves) in Western Australia’s Pilbara region.

As well as flagging the extra potential long-term supply, the annual report gave a stark reminder of how oversupplied the global market will be for the rest of the decade, even in the face of demand growth.

“The demand outlook for iron ore remains sound with expected contestable iron ore demand growth in excess of 100 million tonnes a year by 2020,” Rio said.

But “supply growth is expected to outpace demand growth, with approximately 300 million tonnes of additional seaborne capacity currently under construction or in ramp up”.

This would come on over the remainder of the decade, pushing out higher cost Chinese and seaborne iron ore suppliers, Rio said.

Rio Tinto, BHP Billiton and Fortescue Metals have all been ramping up output in recent years and, despite the price falls, Brazil’s Vale and Gina Rinehart’s Hancock Prospecting have plans to keep output growing.

The low cost ore has been forcing some closures but lower oil prices and cost cutting have meant a lot of supply is sticking around longer than thought.

Iron ore has lost ground in nine of the past 10 sessions and has fallen five straight days since news of a rate cut from the People’s Bank of China last weekend.

Rio’s reserves slipped by 109 million tonnes in 2014 to 3.91 billion tonnes as some reserves were mined, some mine plans were changed and the Guinea government took a 7.5 per cent stake in the Simandou iron ore mine.

But Rio trumpeted a resource increase, something it has not previously done much of, saying it had added the extra 1.68 billion tonnes of resources mainly through discovery of the Yandicoogina Braid deposit.

It is a lot of ore, but with Rio sitting on about 24 billion tonnes of resources, it does not move the dial. It does, however, move Rio’s iron ore resources a little higher than BHP’s latest estimate of 23 billion tonnes made in October.