JOHANNESBURG (Reuters) – Gold Fields said on Thursday the biggest workers union at its sole South African mine, South Deep, planned to strike over proposed job cuts, a move the bullion producer said would threaten output and could lead to even more layoffs.

Gold Fields plans to slash about 1,100 jobs, a near third of its workforce, at South Deep mine, where it has been working to mechanise operations in the face of challenging geology 3 kms (2 miles) below the surface.

“We are very concerned about the further impact that industrial action will have on the mine – and on our employees, with potentially more job losses,” Chief Executive Officer Nick Holland said.

The company said it might consider stopping production for a limited time.

The National Union of Mineworkers (NUM), which represents around 80 percent of the employees at the South Deep mine, planned to down tools on Friday, the bullion miner said.

“South Deep employees are not to be retrenched for poor management. We are going to be on strike until we can engage with South Deep management and the Department of Mineral Resources on how best we can avoid retrenchments,” said NUM South Deep branch Chair Kanetso Matabane.

Gold Fields, which employs about 3,600 people, said in August it would restructure operations at South Deep and would cut jobs in a cost-saving attempt.

The company said it aimed to reduce the number of contractors at the mine by approximately 420 people.

The firm had earlier said South Deep had lost 4 billion rand ($284 million) over the past five years.

Reporting by Tanisha Heiberg; Editing by Tiisetso Motsoeneng and Edmund Blair