Workers at Harmony Gold mines in Free State, South Africa, have started a strike after talks on job cuts failed.
About 21,000 workers failed to turn up for the night shift
About 21,000 miners - or 90% of the workforce at its Harmony and Freegold sites - failed to turn up for the Wednesday night shift.
The two sites make up about 40% of Harmony's overall output but it said the action would have little initial impact because of the Easter holidays.
The firm plans to restructure its operations, cutting up to 5,000 jobs.
Industry observers fear the strike could be a protracted one.
The company obtained a court order banning workers at its recently-purchased Target mine in Free State from taking part in the strikes.
Harmony earlier announced plans to restructure the loss-making mines in Free State.
The world's sixth-largest gold producer has been hit hard by the rise in the value of the rand as many of its sites are based in South Africa.
National Union of Mineworkers (NUM) officials said the strike took place after last-ditch talks broke down.
"Strikes have a way of focusing people, so perhaps we can use this opportunity to find common ground to resolve the issues the NUM have put before us," said Harmony chief executive Bernard Swanepoel.
Stephen Roelofse, fund manager at Sanlam Investment, told Reuters that the high number of jobs at stake would strengthen union resolve.
"They've been bleeding in that area," he added. "I don't think there was any option for Harmony but to take a tough stance on this."
Harmony is currently involved in a hostile takeover for Gold Fields, a merger which would create South Africa's largest gold producer.