The mine's annual output of copper concentrates has stabilised at more than 100,000 tonnes after a series of ramp-up delays and cost-overruns forced the three companies to book 445.7 billion yen ($4.3 billion) of impairment losses in total.

Mitsui Mining, which holds 25.87% stake in Caserones, said it would book a 20 billion yen ($193 million) loss from the deal in the current financial year while Mitsui, owner of a 22.63% stake in the mine, said it expected a 7 billion yen loss from the deal.

The Caserones project's cost more than doubled to $4.2 billion from an expected $2 billion because of factors including a spike in labour costs and bad weather.

Mitsui Mining has booked an impairment losses of about 80 billion yen in total since 2014 and Mitsui has written down 95.7 billion yen.

"It would help boost our corporate value more if we use resources that are set aside in Caserones in other areas," Mitsui Mining said in a statement.

For trading house Mitsui, this is a part of a restructuring of its asset portfolio, it said, although copper will remain a core part.

JX, which has posted a total of 270 billion yen of impairment losses on the project, declined to comment on the deal's impact on its earnings, saying its parent Eneos would announce its results later this week.

After the deal, JX aims to increase production and extend the life of Caserones by investing in automation using advanced technology, it said.

($1 = 103.5400 yen)

(Reporting by Yuka Obayashi; Editing by Christian Schmollinger and Barbara Lewis)

By Yuka Obayashi