US Edition The surprise move comes less than a month after Chile's state copper company, Codelco, said it planned to exercise an option to buy a 49pc stake in AAS. Anglo's south Chilean properties include the flagship Los Bronces mine, in which it has invested $2.8bn.
Chile's state miner had a long-standing right to buy just under half the assets once every three years. Its decision to do so was seen by many as "resource nationalism" and many thought the move significantly undervalued AAS.
Codelco has secured a $6.75bn bridging loan from Japan's Mitsui & Co to fund the deal, suggesting that it valued the AAS mines at just $13.5bn. Anglo's deal to sell some of the assets to Mitsubishi values the Chilean copper business at $22bn.
Anglo, which announced the sale late last night, said the deal with Mitsubishi was compliant with the Codelco agreement. But it is likely that Codelco could now drop its attempt to grab a stake.
"Anglo American has regularly reviewed its available alternatives and, following a thorough assessment, and in the interests of its shareholders, it entered into a process to explore the potential value of the AAS assets through the evaluation of a sale of a minority stake in AAS," the company said in a statement.
The deal comes less than a week after Anglo took full control of De Beers Group, the world's biggest diamond producer, in a deal worth $5.1bn. Its sale of the stake in the Chilean mines will mean it will not need to dip into its cash flow to pay for upping its holdings in the diamond company from 45pc to 85pc.
The shares closed down 62p to £23.53, before the Mitsubishi deal was announced.