The company that controls mining rights to 32,000 acres on the doorstep of the Boundary Waters Canoe Area said Tuesday that the land contains 19 percent more copper, gold and palladium than it previously estimated, making Minnesota home to one of the world's largest deposits of precious metals.

"These are not minor mineral holdings," said Bob McFarlin, vice president for government and public affairs for Twin Metals, a partner in the planned project, which released the results of the new geological analysis. "The state is sitting on an absolute economic juggernaut for generations to come."

Twin Metals, a joint venture between Duluth Metals, a Canadian company, and Antofagasta PLC of Chile, one of the largest international mine operators, is proposing an underground mine and processing plant that would be one of the world's largest.

Located along Hwy. 1 near the Kawishiwi River, it is expected to cost $3 billion and eventually could employ hundreds, Twin Metals said.

The value of the mineral wealth in the area was pegged last summer at more than $100 billion. The new estimate released Tuesday assigned high confidence to the presence of 13.7 billion pounds of copper, 4.4 billion pounds of nickel and 21.2 million ounces of palladium, platinum and gold. The platinum and palladium deposits would be one of the largest outside of South Africa.

The project, along with a proposed mine run by PolyMet Mining Corp., has raised alarms among environmentalists, who worry about the potential impact on northern forests and lake country. PolyMet, which plans a $600 million open-pit project near Hoyt Lakes, is now preparing for the environmental approval process, which it expects to start next year.

Unlike taconite, the precious metals would come from sulfide-bearing rock. When exposed to air and water, the waste rock -- which could amount to millions of tons -- produces sulfuric acid that leaches heavy metals that pollute water. Environmental groups say that northeastern Minnesota, with its wetlands and lakes, is especially vulnerable to such pollution.

Those questioning the projects say that sulfide mines have always ended up with problems, often severe.

"This announcement does not change the main question they need to answer," said Paul Austin, executive director of Conservation Minnesota. "The main concern is that there is no sulfide mine that can demonstrate it can be operated and closed without polluting."

Environmental data

Both PolyMet and Twin Metals said they expect to meet all state and federal environmental standards. PolyMet said this summer that it had installed a reverse osmosis water treatment system to remove all contaminants from water.

Twin Metals hopes to complete a pre-feasibility study for the mine, which will include baseline environmental data, sometime in 2014.

As it completes increasingly extensive engineering and geological surveys, Twin Metals has consistently raised its estimates of the deposits within its holdings. But this latest increase, an average of 19 percent more copper, gold, nickel and palladium, was significant. McFarlin said it reflected both the breadth of the deposit and the concentration of ores within the rock.

Josephine Marcotty • 612-673-7394