Appian, a London-based investment company which advises two affiliated private equity funds which in turn own the Brazilian mines, said the transaction was worth more than $1.2bn and called Sibanye's failure to close on it "unlawful".
Sibanye-Stillwater declined to comment.
Reuters, the news and media division of Thomson Reuters, is the world's largest multimedia news provider, reaching billions of people worldwide every day.
Go to: https://www.reuters.com/