Scientists, diving into the crater of a submerged volcano off Papua New Guinea, have discovered some of the richest concentrations of gold and silver in the world.

But the deposits may be too deep in the sea to exploit soon.Geologist Dr. Brian Embleton, of Australia's national science body, said by telephone from Perth Friday a multinational team using a Soviet submarine found the deposits 7,000 feet below the ocean's surface.

The team, made up of scientists from Australia, the Soviet Union, Canada and Papua New Guinea, found gold gradings of 21 grams per ton and silver gradings of 500 grams per ton.

"These are amazing grades, very unusual concentrations," said Embleton, of the Commonwealth Scientific and Industrial Research Organization's (CSIRO) geological exploration division.

The gradings were found in samples collected in April from three-yard high protusions on the crater floor of the volcano 75 miles east of the Papua New Guinea coast.

Precious metals analyst Robert Widdup of stockbroker J.B. Were and Sons said the gold gradings were unusual.

"It's a fabulous find, very few mines are at this grade," he said. "Underground deposits are economic at grades of five to six grams per ton."

"At the moment we don't know how extensive the deposits might be," said Dr. Ray Binns, leader of the Australian team. "Considering their depth . . . it is unlikely they could be mined economically in the foreseeable future."

CSIRO spokeswoman Judy Thomson said the discovery gives geologists important clues about how volcanoes accrue and deposit precious metals, clues which could be used to find rich deposits on land.

Scientists believe similar deposits on land, which are few and far between, were once underwater volcanoes that have risen out of the sea and been buried over millennia.

Binns left Australia Friday for the Soviet Union where he will report the findings at a geological conference. CSIRO said it plans more dives in the area with Soviet experts.

The Soviet Mir-2 submarine was launched from the Soviet research vessel Akademik Mstislav Keldysh.