Anglo American has suspended operations at its platinum mines in South Africa to "protect the safety of its employees". All five of its mines in the area are now closed.
The company blamed the "current volatile situation in the Rustenburg area, where employees, who want to go to work ... are being intimidated by the threat of violence".
Shares in Anglo American fell almost 3% in London on Wednesday.
South Africa is home to 80% of known reserves of platinum. The price of the element has risen nearly 20% since the shootings at Marikana, where 34 strikers were killed there by police on 16 August.
Lonmin shares fell another 4% on Wednesday as a wildcat strike at the Marikana mine continued.
The BBC reports the strike at Lonmin has removed almost 60,000 ounces of platinum from the market in a month.