Obscuring the sun, a blue-gray cloud sweeps down over the low hills and sifts through the pines, carrying an acrid stench of sulfur.

Ten thousand pounds of sulfur dioxide, mercury, arsenic and other toxic chemicals billow every hour from the smokestack of an iron ore processing plant just outside this small mining town. They ride the prevailing winds in a visible plume to the northern Minnesota wilderness 35 miles away.

American environmentalists have long been concerned that the polluting debris of Atikokan may be damaging the largest U.S.-protected wilderness east of the Rocky Mountains.

So they were shocked to learn recently that the Ontario provincial government had quietly approved construction of a large coal-burning electric power plant just a few miles from the polluting mine operation. The plant will be built with no sulfur pollution controls and could triple the tonnage of airborne chemicals falling on U.S. wilderness.

U.S. scientists fear that as the sulfurous gases from the power plant are changed into sulfuric acid by moisture in the air, the resulting "acid rain" will poison lakes and damage forests fdownwind.

The territory in question is the million acres of northern Minnesota called the Boundary Waters Canoe Area.Ironically, this picture-postcard land of pine forests has been the object of five years of U.S. court battles by groups who differ over how the area should be recreationally developed. But they all agree that it is a unique place for American urbanites to escape the mechanized world.

At the very least, scientists say, pollution from the power plant will render meaningless a federal law - only two weeks old - which prohibits any significant deterioration of the pristine air quality of these regions.

Prodded by Minnesota pollution control officials and national environmental groups, the U.S. State Department pressed reluctant Canadians to agree to discuss the design and location of the 800-megawatt power plant, which is slated to be operating in 1983.

In Ottawa talks, America is asking Canada either to build the plant farther from the border or to install sulfur pollution controls. But most observers say privately the chances are slim that the United States will be able to effect any major change in the Canadian plans.

For the Atikokan power plant is a creature of politics: the politics of a major construction project bestowde on a town faced with economic extinction and the politics of a province where environmental protection is regularly shunted aside in the interests of industrial growth.

Atikokan, a ramshackle town of 6,000, Survives because of its two iron mines; gigantic slashes in the land where earth-moving machines crawl like insects in fierce red wounds. Like most of his fellow townsmen, Jack Pierce, the Atikokan reeve, or mayor, works at a mine.

Four years ago. Pierce says, "We had the situation where both mines announced that they would be out of the region sometime between 1980 and 1984. In order to maintain the community at all, we had to find some new industry."

While the mines were rumbling about closing down, Ontario Hydro was looking for a place to build an electricity generator.

Pierce and a handful of other Atikokan civic leaders extended an enthusiastic invitation to the utility and the more than 200 permanent jobs that would go with the power plant. Remarking Atikokan-s location on the main rail line from the western Canadian coal fields and close to lakes and for cooling, Hydro accepted.

While most of the townspeople embraced the proposal, there was some concern about the 20,000 pounds of sulfur dioxide and other chemicals that Hydro said could be released into the air each hour.

Hydro assured the town that the pollution would not violate Ontario law and said that, under most circumstances, the emitted gases would "remain in a relatively concentrated form and travel long distances, in excess of 30 miles, before reaching ground level."

Those few townspeople who dared oppose Hydro in a series of twon meetings organized by the utility were shouted down, booed, even threatened.

Although lower-echelon Ontario environmental officials criticized various aspects of the Hydro plant, the provincial cabinet approved the project in late June - before Hydro answered the criticisms.

"We still have questions, and I wish to hell I knew what Hydro is going to do," said Michael Barker, - an official in charge of Quetico Provincial Park, a Canadian wilderness area 11 miles from the plant site.

Ontario Hydro is a rare animal - accountable neither to stockholders nor to the voters of Ontario. It is almost a branch of the provincial government, with the Minister of Energy responsible for overseeing it.

Ontario, with its vast western lands and mineral resources, had the fastest economic growth rate among the provinces. Its politicians are anxious to maintain that, and to make sure there is enough energy to supply new industry.

Hydro has never been cited, sued or coerced in any way by the province to clean up pollution from its power plants.

The federal government has the authority to intervene when the actions of a province have international effects. But that authority has never been exercised, and U.S. officials doubt that the administration of Prime Minister Pierre Tudeau is willing to make the political decision to overrule a province.

Pollution controls called "scrubbers" are capable of removing more than half the sulfur from smokestack gases. Scrubbers from the Atikokan plant would costs about $60 million, 7 per cent of the project total.

Hydro maintains that Atikokan's pollution will be within Canadian legal limits and therefore will not require scrubbers. The utility also contends that scrubbers, widely used in the United States, are not a proven technology.

Finally, the people of Atikokan say that, even if scrubbers were installed, the sulfur removed from the gases would have to be disposed of some where - an environmentally difficult job.

"The sulfur's going to be there one way or the other," says Mayor Pierce. "We have to dump the stuff someplace - your back yard or ours."