One of the firms, Bernstein Liebhard accused the South African miner of misleading its shareholders, just hours after the UK Competition and Markets Authority (CMA) cleared its bid to take over Lonmin.
Sibanye-Stillwater said that it had noted the announcement by the firms and said that it would defend the class action.
“The company notes the publicity and, if any claims are ultimately filed against the company, intends to vigorously defend itself,” Sibanye-Stillwater head of investor relations James Wellsted said. “We will continue to monitor developments closely.”
On Tuesday, Sibanye-Stillwater tumbled on the JSE after Parliament threatened to withdraw its operating licence, following another death in its South Africa operations
The death at its Driefontein operations brought the total number of fatalities at its mines to 21 since February.
The CMA said that it had given the deal a go ahead after conducting its own investigation.
Sibanye-Stillwater, a precious metals miner, made an all-share offer for Lonmin in December last year in a £285million (R5.14billion) deal aimed at creating the world’s number two platinum producer.
Sibanye-Stillwater and Lonmin said the approval took the transaction a step closer to completion.
“We remain excited about the proposed transaction, which we consider to be in the best interest of our stakeholders,” the companies said.
“We look forward to the combination of the businesses creating a leading mine-to-market player, with enhanced scale and resources, able to compete more effectively,” the chief executives said in a joint statement.
The deal would seen Lonmin shareholders hold an 11.3percent stake in Sibanye-Stillwater.
The companies said they would continue to engage constructively with the South African competition authorities to obtain clearance.
Last year Sibanye-Stillwater chief executive Neal Froneman said that the acquisition would position the enlarged the group as a “leading mine-to-market” producer of platinum-group metals in South Africa.
The groups also said that the offer was expected to be completed in the second half of the year and a further announcement will be made in due course.
Sibanye-Stillwater rose 2.3percent on the JSE yesterday to close at R8.