"Because of these cost pressures, Lonmin has decided to place a number of its marginal shafts under care and maintenance and that is the reason why the 1,139 workers will be affected," Solidarity, which represents mainly skilled workers, said.

London-listed Lonmin, whose mines are all in South Africa, is also battling a strengthening South African currency and numerous operational issues.

(Reporting by Zandi Shabalala; Editing by Susan Fenton)