Copper concentrate exports from the Grasberg mine in Indonesia will slump 83 per cent this year as output almost halves due to a shift in mining operations from open pit to underground, a government official said.

Exports of concentrate that’s smelted to make copper cathodes is expected to fall to 200,000 metric tons from an estimated 1.2 million tons last year, Yunus Saefulhak, director of minerals at Indonesia’s Energy and Mineral Resources Ministry, told reporters in Jakarta on Wednesday. Production is set to drop to 1.2 million tons from 2.1 million tons and about 1 million tons will be channeled to a smelter at Gresik, he said.

Freeport-McMoRan Inc., the operator of the world’s second largest copper mine, has previously warned that the imminent shift to mining underground would reduce output before ramping up production to normal levels by 2022.

Freeport yielded majority ownership of the Grasberg mine to state-owned PT Indonesia Asahan Aluminium last month in a $3.85 billion deal that also saw Rio Tinto Group exiting its economic interest. Indonesia Asahan, known as Inalum, now owns 51.2 per cent of PT Freeport Indonesia, with Freeport owning the rest.

Freeport Indonesia has yet to seek government permission to export beyond February, Saefulhak said. Riza Pratama, a spokesman for Freeport’s local unit, didn’t immediately respond to calls seeking comments.