The National Union of Mineworkers (NUM) has vowed to fight retrenchments at Petra Diamonds' Helam mine in North West, the union said on Tuesday.
"We will fight tooth and nail to make sure that these retrenchments do not happen," chief negotiator Lekhotla Mabea said.
He said the company was planning to retrench 370 employees.
The mine, near in Swartruggens, had 421 permanent employees, of which 253 were NUM members, said Mabea.
He said the company had not complied with the Minerals and Petroleum Resources Development Act (MPRDA).
"The MPRDA states that if the company anticipates to retrench 10 percent or more employees it has to inform the minister of minerals resources."
He said the NUM called on Mineral Resources Minister Ngoako Ramatlhodi to intervene.
The company was not immediately available for comment.