THERE is no guarantee that one of world's largest uranium mines will be expanded, despite the discovery of fresh deposits, the Northern Territory Government says.

Energy Resources of Australia announced last week that it had found a potential further 40,000 tonnes of uranium oxide near the Ranger mine, about 260km east of Darwin.

The discovery could extend by seven years the life of the mine, which is surrounded by Kakadu National Park.

Ranger already produces 11 per cent of the world's uranium.

"The options are an underground operation, and perhaps an option is an extension of the current open cut mine," the company's chief executive Rob Atkinson said.

But NT Mines Minister Kon Vatskalis said there were no guarantees the mine would be expanded.

He said the mine would have to undergo a "very strong assessment" by the NT Mines and Environment departments, and the Federal Government.

"It's not just a fait accompli," Mr Vatskalis said. "If it happens, it has to be assessed very strictly against the current environmental and mining legislation."

David Paterson, general manager of ERA business development, conceded that any expansion was a long way off.

"All those studies need to be done," he said.

Mining at Ranger is due to be completed in 2012 and processing is due to continue until 2020.

Dave Sweeney, from the Australian Conservation Foundation, warned the environmental impact of the mine was already considerable.

"This is company has had some spectacular mistakes," he said.