TORONTO - A massive swath of northern Ontario boreal forest, considered the world's largest carbon storehouse, will be off-limits to forestry and mining activities under a plan that will also guarantee First Nations a share of resource revenues, Premier Dalton McGuinty said Monday.

McGuinty offered few details, but said the government would consult industry, environmentalists, aboriginal communities and other local residents to develop a plan over 10 to 15 years that would protect half of the province's pristine boreal forest from commercial activities.

"It's home to the largest untouched forest in Canada and the third largest wetland in the world," McGuinty said of Ontario's boreal forest.

The area in question, north of the 51st parallel, measures 225,000 square kilometres - about the size of Nova Scotia, New Brunswick and Prince Edward Island combined. Home to just 24,000 people, it comprises a whopping 43 per cent of the province's entire land mass.

"It's twice the size of the British Isles," McGuinty said. "It is, in a word, immense. It's also unique and precious."

The new plan would also require that mining and forestry companies consult early with aboriginal communities before starting any projects in the other half of the boreal forest, and give First Nations a share of revenues from new projects on their traditional lands anywhere in Ontario.

"We'll make a down payment on that this fall and put some money in the bank (for First Nations)," McGuinty said.

"We get to say to our aboriginal communities: if there is some mining exploration here, and you permit that, you get a piece of the action."

Nishnawbe-Aski Nation Grand Chief Stan Beardy said Monday he was encouraged that the boreal forest would be mapped to determine what areas need to be protected and developed, but wanted to ensure resource revenue sharing with aboriginal communities finally takes place.

"We've had a treaty with the Crown for about 100 years and we have not benefited whatsoever from resource revenues," Beardy said in an interview. "It's absolutely essential that legislation, policy and practices change to make sure that we benefit from resource development as well."

Mining generated about $11 billion in Ontario in 2007, and McGuinty said he was confident the consultations on the new protected area of the forest won't cause any damage to the growing sector.

"We don't want to compromise that, but we do want to ensure that our mining efforts in the province of Ontario are respectful of Ontarians, aboriginal and non-aboriginal alike."

He also promised to introduce legislation this fall to overhaul Ontario's Mining Act to deal with complaints that prospectors can stake lands for exploration without consulting the owners.

The opposition parties warned that McGuinty's announcement could discourage new investment in mining and forestry in the far north by creating uncertainty about what new rules could be developed - a concern echoed by industry associations.

"If you were a business right now looking at investing, you see this huge question mark - we won't know what we're dealing with for 13 to 15 years," said Jamie Lim, president of the Ontario Forest Industries Association.

"Wow, that's got to create a great deal of business uncertainty."

The Ontario Mining Association agreed it could take 15 years to map out the area's mineral potential, environmental features and vulnerable species, but also expressed concerns about the lack of details in the announcement.

"I think it's very important that you don't change the rules after people have invested money, and change the rules mid-stream," said mining association president Chris Hodgson. "But there's no detail on it."

Last year, 1,500 scientists from 50 countries called on Canadian governments - federal, provincial and territorial - to protect the 5.6 million square kilometres of boreal forest in Canada, which holds about 186 billion tonnes of carbon.

That's the equivalent of 27 years of global carbon dioxide emissions from burning fossil fuels.

Lim said there's evidence that shows properly managed forests that are harvested, turned into wood products and replanted actually sequester four to five times more carbon than untended forests, and he worried that the government was not taking that into account.

Environmentalists were quick to applaud McGuinty's announcement, calling it "visionary" and crucial to protecting the environment and to combat global warming.

"Ecosystems and species are going to need to respond to a change in climate, and having wilderness areas that are not fragmented with roads and logging is going to be very important in the future," said Gillian McEachern of ForestEthics.