Amplats, the world's second-biggest platinum producer, cut its 2020 production guidance for platinum group metals (PGMs) by 900,000 ounces, from between 4.2 million and 4.7 million ounces to between 3.3 million and 3.8 million ounces.

The miner's Johannesburg-listed shares closed down 14.3% and Anglo American, which owns 77% of Amplats, saw its London-listed shares fall 8.8%. Spot platinum prices rose on expectations of tighter supply and were up 2.9% by 1620 GMT.

An explosion on Feb. 10 damaged the Anglo Converter Plant (ACP) phase A plant, part of a chain of processing facilities, at Waterval smelter in Rustenburg, 141 km (88 miles) from Johannesburg.

The phase B unit was set to take over but water was detected in the furnace, which posed the risk of another explosion, so Amplats shut the plant down.

"If we have an explosion then we'll be out for a year with no production from Anglo American Platinum," Amplats CEO Chris Griffith said on a call following the announcement.

The company said repairs to the phase B unit would take about 80 days and it had to declare force majeure - which allows the suspension of contractual obligations because of exceptional circumstances - as it cannot process material during that time.

Repairs on the phase A plant should be completed in early 2021, it said.

Amplats' revised guidance breaks down into between 1.5 million and 1.7 million ounces of platinum, from 2 million to 2.2 million ounces. For palladium, the cut was to between 1.1 million and 1.2 million ounces from 1.4 million to 1.5 million ounces.

"The hit to EBITDA (earnings before interest, tax, depreciation and amortisation) would be more than $1 billion based on current spot prices," Jefferies analyst Christopher LaFemina said.

Amplats' mines will carry on producing and the resulting concentrate can be smelted at one of its four smelter complexes, but the company will not be able to refine its own or third-party production while the plant is under repair.

"The plan is still to continue mining as much as possible so that the metal is not lost to the market, and as quickly as we can catch up we will," Griffith said in response to analysts' questions.

Anglo American Platinum refines platinum group metals from Sibanye-Stillwater, African Rainbow Minerals, Royal Bafokeng Platinum and Siyanda.

In a statement, Sibanye-Stillwater, the world's biggest platinum producer, said it has significant spare processing capacity in South Africa at its Marikana operations and its Brakpan refinery, and will be assessing how best to utilise that.

Sibanye said its Marikana and U.S. PGM operations are unaffected and would benefit from the platinum price increase.

By Helen Reid