Geologists have struck gold near an ancient collapsed volcano, some 400 kilometers south of Tokyo. But prospectors better know how to use a submersible, because this deposit is under nearly 1300 meters of water. The finding, described in today's Science, is more likely to be of immediate interest to scientists studying the formation of mineral deposits.
The 7-kilometer-wide volcano, called the Myojin Knoll Caldera, has attracted attention since the late 1980s, when scientists discovered it was spewing hot water. A team of geologists led by Kokichi Iizasa of the Geological Survey of Japan began exploring the caldera with submersible craft in 1991. Five years later, they discovered a major mineral deposit. "They deserve high praise for sticking to it," says Peter Fiske, a volcanologist from the Smithsonian Institution who has made three dives with the team into the pitch black water. "You don't see things unless you are right on them," Fiske says.
Similar deposits are well known on land, and many have been the sites of gold and silver mines. They form when water heated by magma rises through the oceanic crust, dissolving minerals. When the minerals hit the cold water of the ocean, they precipitate as sulfide deposits that are rich in gold, silver, copper, and other metals. Unlike land deposits, the Sunrise deposit, as it is known, is still very active: It is studded with "black smokers," chimneys that release hot mineralized water and continually replenish the deposit.
As big around as the Pentagon building, the Sunrise deposit is larger than many deposits of its kind that have been mined on land, and its gold concentration is 40 times higher. However, many challenges must be overcome before it can be mined: figuring out how deep the mineral layer is, how costly it will be to bring ore to the surface, and what damage may be done to the undersea environment. Fiske is much more confident of the discovery's scientific value. "Many volcanologists never dreamed that [calderas] could exist on the ocean floor," he says. "This is the first one studied in detail, and it opens up a totally new area for exploration."