MELBOURNE, Australia--A strategy by the world's biggest mining companies to keep feeding the iron-ore market even as prices have tumbled has created another victim.

Australia's Arrium Ltd. said Friday it has been forced to close one of its two mines, slash 580 jobs and will book a 1.34 billion Australian dollar (US$1.07 billion) writedown, as the price of the steelmaking commodity languishes at more-than-five-year lows.

It is the latest upset in an industry where producers including BHP Billiton Ltd. and Rio Tinto PLC--which can more easily make profits when prices are low--are continuing to ramp up output, even as demand from China for iron ore and other commodities cools.

Arrium said it was closing its Southern Iron operation in the state of South Australia to focus on its Middleback Ranges business, a move it expects would bring production costs back below the weakened price for iron ore, which plunged by almost 50% last year.

The company said it would also rein in capital spending, adding that the restructuring would cost it about A$70 million in the current fiscal year, as well as contributing to the hefty impairment charge.

Arrium, a steel producer spun off from BHP Billiton in 2000, has in recent years turned to iron ore to diversify and reduce its reliance on slower demand for its steel products. It changed its name from OneSteel Ltd. in 2012 to reflect the shift.

Arrium said it now aims to produce about 9 million metric tons of iron ore a year, sharply lower than the 13 million it had been targeting. The spot-market price for iron ore has fallen below US$70 a metric ton, its lowest since mid-2009, as big mining companies added to a building glut of the commodity, and amid signs that China's steel mills are struggling to absorb supplies as the country's economic growth slows.

Analysts have been warning that some smaller iron-ore producers around the world might be washed out of the market, and that some projects might have to be shelved because of falling prices and as big mining companies stick with their aggressive output plans.

"The longer we see prices stay down, the more we're going to see higher-cost producers curtail production," said Tim Schroeders, a Melbourne-based fund manager at Pengana Capital.

Rio Tinto, the world's second-largest iron-ore producer after Brazil's Vale SA, this week said it would produce about 330 million tons from its mines in Australia's western Pilbara region this year after digging up 280.6 million tons in 2014--itself a record and up 12% from the year prior.

BHP and another of its rivals, Fortescue Metals Group, have also been maximizing output from their Pilbara operations. Coupled with big new mines such as billionaire Gina Rinehart's Roy Hill venture, Australian exports of the commodity might grow by as much as 100 million tons this year, following a record year for shipments in 2014.

Midsize U.S.-based mining company Cliffs Natural Resources Inc. has already faced writedowns, laid off workers, and has said it could offload its Bloom Lake iron-ore mine in Canada. Insolvency specialists were last year called in to Australia's Western Desert Resources Ltd. Meanwhile, Chinese conglomerate Citic Ltd. this week said it would record an impairment loss of up to US$1.8 billion for its Sino Iron project in Western Australia to reflect the fall in prices.

Arrium's chief executive, Andrew Roberts, said that despite efforts to cut costs and spending, the company's mining unit had been absorbing cash.

The company said it planned to reduced capital spending by about A$200 million, or 30%, over the next four fiscal years, adding that an impairment charge of A$1.17 billion would be booked for the fall in iron-ore prices and the closure of Southern Iron in the fiscal first half through December.

Another A$130 million would be written off the value of its steel and recycling operations, it said, while about 200 full-time jobs would be lost and a further 380 contract positions cut.

Write to Robb M. Stewart at robb.stewart@wsj.com

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