June 18 (Reuters) - Yamana Gold Inc said it would buy Extorre Gold Mines Ltd for C$412.9 million ($403.44 million) in a cash-and-stock deal, to build its portfolio of high-grade gold and silver deposits in Argentina.

Yamana's offer of C$4.26 per share is 68 percent higher than Extorre's Friday close. Extorre stockholders will receive C$3.50 in cash and 0.0467 of a Yamana share for each share they own.

"It is a relatively small transaction in that it represents only 3 percent of Yamana's market capitalization yet it could ultimately deliver more than 10 percent of our total gold equivalent production," Yamana's CEO Peter Marrone said.

Yamana, which has a market value of C$12.20 billion, plans to increase gold equivalent production by 60 percent to about 1.75 million ounces over the next two years, the company said in its 2011 annual report.

Extorre's most advanced project is the Cerro Moro property in Santa Cruz, Argentina, which has indicated resources of 1.36 million ounces of gold equivalent mineral resources.

Extorre's shares have slumped nearly 70 percent this year, as investors dumped their holdings in junior miners and Argentina's government tightened controls on imports of equipment and supplies by mining companies.

Yamana will have the first right to match a superior bid and Extorre will pay C$15 million in case the deal is terminated. The deal is expected to close in August 2012.

Separately, Yamana raised its quarterly dividend by 18 percent to 6.5 cents a share.