Shares in some North American silver miners fell sharply on Thursday after Bolivia's leftist government said it might rescind concessions on four mines in the country run by Glencore affiliates and Canada's Pan American Silver.

In afternoon trading on the Nasdaq, Pan American was down almost 10 percent at $37.03. On the New York Stock Exchange, Coeur d'Alene Mines Corp. which also mines silver in Bolivia, dropped 7.4 percent to $31.24.

Earlier, in La Paz, Bolivia's mining minister said mining unions were asking the state to "recover" the silver, zinc, lead and tin mines currently operated by private companies, in line with President Evo Morales' drive since 2006 to regain control over key natural resources.