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The Horn Plateau covers 25,000 square kilometres in the south-central part of the Northwest Territories. ((CBC))

A Northwest Territories aboriginal group is taking the federal government to court for quietly opening a vast area of once-protected northern wilderness to mining claims.

The Dehcho First Nations has asked the Federal Court to overturn Ottawa's order removing an existing ban on subsurface mining in the Horn Plateau, a 25,000-square-kilometre area in the south-central part of the territory.

In its application for a judicial review dated Nov. 29, the Dehcho said the federal government's decision to remove the subsurface mining ban breaches an agreement made through the N.W.T. Protected Areas Strategy.

The news comes as Ottawa announced proposed boundaries this week for a national marine park in Lancaster Sound, located at the eastern entrance to the Northwest Passage.

The Horn Plateau region is the source of the Horn, Willowlake and Rabbitskin rivers, serves as a nesting area for migratory birds, and is a habitat for caribou, wood bison and wolverine species.

The Dehcho Dene consider the area — which they call Edehzhie in the Slavey language — to be a sacred place and an important hunting ground. It is also known to have potentially significant oil and gas deposits.

The Horn Plateau area has been under interim protection since 2002. Shortly before that protection was set to expire in October, the federal Indian and Northern Affairs Department issued an order-in-council extending that protection until 2012.

But that extension retained only a ban on surface activities such as logging. Protection from subsurface activity such as mining and drilling was not included.

Area vulnerable, First Nation argues

"This leaves the Edehzhie subsurface immediately vulnerable to prospecting, staking and recording of mineral claims by third parties," the Dehcho's court application states.

Dehcho leaders said federal officials had assured them Edehzhie would be protected from development during their land-claim negotiations.

In the court application, the Dehcho First Nations said it, the federal government and the N.W.T. government had agreed to make the Horn Plateau a national wildlife area under the Protected Areas Strategy.

As part of the strategy, the area in question would have interim protection from subsurface activity while all parties negotiate the terms of permanent protection.

Ottawa's move to remove the ban on mining claims "was made without any consultation as required by law, and is regarded by the Dehcho First Nations as a complete betrayal of the letter and the spirit of the Protected Areas Strategy," according to the court application.

The Dehcho First Nations is an organization that represents nine Dene communities and two Metis communities in the southwest corner of the Northwest Territories.

It is the only aboriginal group along the proposed Mackenzie Valley pipeline route that has not signed a land claim agreement with the federal government.

With the Horn Plateau matter going to the Federal Court, it is not clear if federal negotiators will halt their land-claim talks with the Dehcho First Nations.

The federal government has not commented to date on why it removed subsurface protection from the Horn Plateau.

With files from The Canadian Press