Mitsui has already informed the majority owner of Caserones, JXTG, which also controls Japan's biggest copper smelter, Pan Pacific Copper.

The sale of the loss-making copper mine is being “executed as a part of [Mitsui's] reorganization and reconstructing of [its] asset portfolio,” the Japanese company said. 

Mitsui acquired the stake in Caserones in 2010. At the time, had a planned annual capacity of 150,000 tonnes of copper concentrate and 30,000 tonnes of copper cathodes.

Located at the head of a valley in the Atacama region of Chile, Caserones is a low-grade copper deposit exposed to extreme weather in an area with water shortages - making it prone to low-level operations and resulting in high ramp-up costs. 

In 2018, JXTG posted an impairment loss of ¥65 billion ($606.8 million) following a review of its long-term mining plan after a fault in an ore pit. 

Last month, Japanese conglomerate Sumitomo also put its 45% stake in the Sierra Gorda copper-molybdenum mine in northern Chile up for sale. 

Mitsui's  offloading announcement comes despite the red metal price rebounding strongly in recent months. The three-month copper price on the London Metal Exchange was $6,968 per tonne on the morning of November 9, up 31% from six months ago.