Solidarity said on Wednesday it would go to court to challenge a move by Sibanye-Stillwater to lock out its members as a strike over wages begins at the company's gold operations.
The National Union of Mineworkers (NUM) and the Association of Mineworkers and Construction Union (AMCU) called a strike beginning on Wednesday evening over an unresolved pay dispute, but Solidarity broke ranks last week and accepted Sibanye's final offer of a 5% annual pay increase.
Solidarity said its members had on Wednesday been informed they would be locked out of the workplace, despite having accepted Sibanye's wage offer.
Sibanye did not immediately respond to a request for comment.
"Solidarity will approach the Labour Court on an urgent basis to obtain an interdict against the lock-out that will be applied to its members at Sibanye-Stillwater," the union said in a statement.
"This comes after Solidarity members today received a notice informing them that they would be locked out from the workplace despite Solidarity's acceptance of Sibanye-Stillwater's wage offer last week."
Solidarity deputy general secretary Riaan Visser said Sibanye's lock-out order meant the union's members could go without pay.
On Tuesday, the company said it would not increase its wage offer, but would take steps to protect employees and company assets in the event of a strike.
The strike is likely to disrupt operations at Sibanye's gold mines at a time when the metal's price is surging.