The firm's shares rose 6 percent after the diversified miner said the strike, called by the Association of Mineworkers and Construction Union (AMCU), was no longer legal and all employees should report for work on Saturday Dec. 15.
The company said in a statement that the extension means all AMCU members and employees not represented by a union are bound by the agreement.
Gold producers in Africa's most industrialised economy have argued that above inflation wage hikes have added to the cost burden of the bullion industry, which has been hit by depressed prices and labour unrest.
On their part, the unions have demanded what they describe as a living wage for its members.
Sibanye signed a three-year wage agreement with three other unions in November. Talks with the AMCU, which represents around 43 percent of the firm's employees, broke down, triggering the strike.
In a statement, the union described the extension of the agreement as "unscrupulous" and said it would pursue all legal avenues to protect the rights of its members.
It called on other unions to not sign any agreements that do not reflect the 12,500 rand ($887) living wage.
Independent Securities trader Ryan Woods said the news was positive for Sibanye.
"The market doesn't like any uncertainty so this essentially puts them back in the driving seat as getting back things to normal," he said.
The share traded up 6.3 percent at 10.29 rand by 0949 GMT.
($1 = 14.1000 rand)
(Reporting by Emma Rumney and Nqobile Dludla; writing by Emma Rumney; Editing by James Macharia)
