BHP Billiton and Rio Tinto have announced plans to invest $4.5bn (£2.9bn) to expand the world's biggest copper mine.
They are betting that the copper price is in for a prolonged flight and intend to expand the output of the Escondida mine in Chile by about three quarters to 1.3 million tonnes a year by June 2015.
BHP also said it would invest $195m to reopen its Pinto Valley copper mine in Arizona, which was mothballed in 2009.
The price of copper is seen as a barometer for industrial demand, since it is used in everything from power transmission to plumbing.
Although the price has fallen from a record high of more than $10,000 a tonne a year ago, it is still more than double what it was in the thick of the financial crisis. It is 10 per cent up so far this year, at $8,525 a tonne, as economic confidence improves, and is expected to keep rising.
Credit Suisse says the copper price could hit $9,500 a tonne by the end of the year, while the consultant Wood Mackenzie predicts the global market for refined copper will rise from a 718,000-tonne surplus in 2015 to a 316,000-tonne deficit by the end of the decade.