One of the partners in the Pebble copper and gold prospect in Southwest Alaska has invested $4 million to obtain additional mining claims adjacent to Pebble's mining claims.

With its investment, Northern Dynasty Minerals has obtained an additional 23 square miles of mining claims on state land to the northwest of the Pebble claims. The claims had previously been held by a much smaller mineral exploration company, Liberty Star Uranium & Metals Corp. of Arizona.

The companies studying and exploring the Pebble project already hold roughly 163 square miles of mining claims on state land. On a portion of that land, they hope to develop one of the world's largest copper and gold mines. But developing Pebble into a mine is controversial due to the immense size of the project and its location in the upper drainages that feed Bristol Bay's major salmon fisheries.

Late last week, Liberty Star announced to its investors and federal regulators that it had sold 23 of its 177 square miles of mining claims in the region to Northern Dynasty to help pay off a multimillion-dollar debt owed to lenders. As part of the agreement, Northern Dynasty paid $1 million outright and loaned Liberty Star $3 million. Also, the two companies agreed that Northern Dynasty will earn a 60 percent interest in Liberty Star's other mining prospects in Southwest Alaska if Northern Dynasty invests $10 million over the next six years to explore them.

Northern Dynasty officials said they aren't interested in exploring the 23 acres that they have acquired from Liberty Star for now.

"The interest is in consolidating the Pebble land package," said Sean Magee, a spokesman for Northern Dynasty, based in Vancouver, British Columbia. His company is the smaller of the two companies that have joined together in the effort to develop Pebble. The other company is Anglo American, a London-based mining conglomerate.

The two mining companies are finishing up their environmental studies on the Pebble project this year. They plan to publish a preliminary development plan and apply for development permits next year after feedback from residents in the Bristol Bay region. Also, this summer, the companies are drilling exploration holes at a number of locations on their mining claims. Most of that drilling is at least eight to 10 miles away from Pebble's main ore bodies.

"It's exploration on our claim block to increase our understanding of the geology of the area," said Pebble spokesman Mike Heatwole.