American team is out to wrest tin from silt off the shores of Cornwall

Since the days of the Phoenicians, 2,500 years ago, tin mining has been an important commercial activity around Cornwall. Today, three American aficionados of mining are starting an unusual venture in this part of England that combines reclaiming metals from the seabed with that ancient industry.

The United States-backed venture will dredge from the sea, off Cornwall's northern coast, layers of silt that rivers in the region have carried out over the centuries. The silt contains deposits of cassiterite, or tin oxide, most of it dumped into streams by the proprietors of 19th-century tin mines on the mainland. With the technology of the day, they were unable to extract the tin and viewed the silt simply as waste.

Marine Mining, the company behind the project, estimates that the mud under the water contains about 10,000 tons of tin that can be extracted with modern techniques. The company is building a processing plant at Gwithian, a tiny village near St. Ives, which will treat the mud.

The plant, which will employ a novel extraction technique called froth flotation, is expected to account for about 10 percent of Cornwall's tin output.

The mud itself will come ashore via a pipeline attached to a floating buoy 700 meters (2,300 feet) from the coastline. Dredging vessels will moor alongside the buoy to pump the material to land.

''We are blazing a new frontier here,'' said Gilbert Kerlin, one of the project backers. In a telephone interview, Mr. Kerlin said he had become interested in mining in Cornwall some 15 years ago. He acknowledged that the project is unusual but is confident the technology will work.

The project's backers are a trio of American mining enthusiasts who have started mining ventures in Canada and Mexico. They are Mr. Kerlin, a senior partner in the New York legal firm Shearman & Sterlin, and two brothers, Francis and Robert Goelet. The latter combines an interest in metals extraction with a passion for the natural environment. He is chairman of the board at the American Museum of natural History in New York.

If all goes according to plan, the project will process its first load of mud in 1985. The Gwithian plant will turn out about 500 tons of tin per year until the year 2005, when the seabead material will be exhausted.

The county of Cornwall now contains just four tin mines, a far cry from the heyday of tin mining in the area last century when 400 mines employed 30,000 people. At that time, Cornwall was one of the world's most important areas for metals extraction. Today the work force has slumped to 1,500 and Cornwall's main industry is catering to the tourists who flock there to admire the spectacular coastal scenery.

Marine Mining's backers are putting $:3.3 million ($5.21 million into the venture. In a link with local tradition, they have hired an experienced British mining engineer, Michael Proudfoot, to run the Gwithian plant. Until a few years ago, Mr. Proudfoot was in charge of Cornwall's biggest tin mine, Wheal Jane, near Falmouth.