Two regional mining executives have closed a deal on what may be the biggest silver mining venture in Northwest history after a week of 18-hour-a-day negotiations with Canadian resource giant Noranda Inc.

Harry Magnuson of Wallace and Frank Duval of Spokane, Wash., principals in Montana Reserves Co., will join Noranda in developing what they said is the world's largest silver-copper deposit 20 miles south of Libby, Mont.Acquisition and development costs of the property will exceed $250 million, they said, greater than any previous Northwest mining deal.

The underground mineral property will move swiftly to its pre-production phase after the necessary permits are obtained, Noranda Minerals Inc. Copper Group President Alex G. Balogh said.

The northwestern Montana property, optioned by Magnuson and Duval for $94 million in February from U.S. Borax & Chemical Corp. and two Canadian companies, is estimated by Noranda to contain at least 300 million ounces of silver and 2.5 billion pounds of copper.

Borax has performed extensive exploration work on the property over the past five years.

The mine is expected to produce about 15 million ounces of silver per year for at least 20 years, making the mine the world's biggest silver producer, Duval said.

The mine will employ about 400, he said, and have a yearly output greater than the combined production from the Sunshine, Galena, Coeur and Lucky Friday mines in northern Idaho's famed Coeur d'Alene mining district.