* Bolivia says might rescind some mining concessions

* U.S., Canadian silver miners' stocks drop sharply (Updates with analyst, Coeur quotes, options activity)

NEW YORK, April 14 (Reuters) - The 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 International Ltd [GLEN.UL] affiliates and Canada's Pan American Silver Corp <PAA.TO><PAAS.O>.

By the close, Pan American was down 9 percent at $37.35 on the Nasdaq and at C$35.89 in Toronto. On the New York Stock Exchange, Coeur d'Alene Mines Corp <CDE.N>, which also mines silver in Bolivia, but was not mentioned by the Bolivians, had dropped 7.9 percent to $31.08.

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.

"The shares are down because there is a fear that the Bolivian government would potentially take possession of more mines," said Joe Cusick, senior market analyst at Chicago-based brokerage firm optionsXpress.

"The concern is that they are going to take possession for public use and then potentially divest the private ownership.

"In Pan American Silver, the risk is that these state mining companies would rescind some of their contracts."

Tony Ebersole, a spokesman for Coeur, said the company's property rights are not the subject of expropriation.

"We operate in a mine where the title already belongs to Bolivia and is backed by a presidential decree. Our surface mining and silver production from San Bartolome are operating as usual," he said in an email to Reuters, stressing Coeur pays millions in royalties and taxes to the Bolivian state.

In options trading, Coeur was under pressure and its put options were actively traded on Thursday. At the close, Coeur's turnover was seven times greater than typical levels, with about 30,000 puts and 28,000 calls traded, according to options analytics firm Trade Alert.

The April $31 and May $30 CDE puts are the most active, as some short-term players appear to be bracing for additional downside in Coeur, Cusick added. (Reporting by Steve James in New York and Doris Frankel in Chicago; editing by Andre Grenon)