A Canadian firm is planning to invest more than a quarter of a billion dollars in the largest strategic metals mine in the United States on 8,000 acres of U.S. Forest Service land near this isolated community 360 miles north of San Francisco.

California Nickel Corp. says the mine, to be developed on a scrub-covered 2,620-foot laterite dirt ridge, could produce enough cobalt to supply 75 percent of the U.S. defense needs over at least the next 18 years. It also is expected to produce vast quantities of nickel and chromium, all of which are in short supply in defense stockpiles, and many of which are purchased from Soviet-bloc or southern African nations.

But development of the mine has kicked off a classic dispute between environmentalists, who say the operation will ruin the near-wilderness coastal area, and economic forces, who say Crescent City and the surrounding Del Norte County can't survive without it.

Currently, 25.9 percent of the employable residents of this little seacoast town (pop. 2,500) are collecting unemployment insurance. It is estimated, however, that as much as 40 percent of the work force is unemployed because the timber and fishing industries--the employment mainstays--have been in such a prolonged slump that employment benefit funds have run out for many.

Cal Nickel says it intends to provide 450 jobs a year over the next 18 years in a county where the workforce numbers only 7,000. County Assessor Gerald D. Cochran estimates the multiplier effect means as many as 1,500 will go to work when the mine gets under way. As much as $1.5 billion will be added to the county tax rolls, he says.

"People are waiting for these jobs with open teeth," said Gerald Oxford, a 16-year lumber worker idled two years ago. "They're so hungry."

But there are tradeoffs. The proposed mine site is the middle of the watershed of the Smith River, a tumbling whitewater stream so clear it glints blue in the midwinter sun. California State Resources Agency Secretary Huey Johnson considers the Smith California's most precious wild river.

It is the only river in the state's wild and scenic river system to be completely undammed anywhere on any of its tributaries. And for the chemical plant the company intends to build on Gasquet (pronounced GAS-key) Mountain, the company wants to build a dam as high as a 15-story building on Hardscrabble Creek, a tributary of the Smith. Having won exemptions from both the federal and state wild and scenic rivers acts, the company must now get 17 permits from 58 state, local and federal agencies.

If the plant is ever built, Cal Nickel also plans a 375-foot-tall smokestack to disperse the refuse from burning hundreds of tons of coal per day in the chemical processing plant. An estimated 99.2 percent of all of the sulfur dioxide created by the burning coal will be captured and used in a sulfuric acid leach process to extract the minerals, according to Lee Hescock, Cal Nickel's Crescent City operations manager. The rest will go up the stack, and that means four to five tons a day of the primary ingredient for acid rain, environmentalists are warning.

Also, according to the mine's critics, a draft environmental impact report produced last spring by the company said enough nitrous oxide to equal the effect of 65,000 cars in a downtown city will be produced daily by the plant. The report was later withdrawn because the firm switched plans to a different metal extraction process, but the new process isn't expected to change particulate levels significantly.

Those problems, according to Del Norte County Planning Director Ernest W. Perry, are dwarfed by the problem of revegetating the areas that Cal Nickel intends to dig up. The company says it can extract about 50 pounds of metals from each ton of fine laterite it digs up. The remaining tailings, pulverized nearly to powder, will be trucked back to the ore pits and put back in the ground.

But Gasquet Mountain receives as much as 140 inches of rainfall annually. At least three times a year, Perry says, the ridge gets storms of the magnitude that devastated areas around the San Francisco Bay earlier this year--up to a foot in a single day. Perry says that if the soil is not revegetated immediately, the red tailings could wash into the Smith River.

The company has done extensive experimental revegetation work. Hescock says botanists have been successful in laboratory conditions in Colorado, but no attempts have yet been made to grow plants at the site, where the soil is so poor it takes 110 years to grow a 10-inch-thick tree.

That has caused a certain amount of disquiet in Crescent City. "You can take my money, or beat up my kids or steal my wife, but don't pollute my river," says Jan Seils, deputy U.S. Forest Service supervisor in Eureka, 80 miles north.

In general, however, the brunt of defending the environment has been borne by newcomers who were drawn to the area by its beauty and isolation. One county official estimated that 22 percent of the residents of Del Norte County are retired persons who came there from somewhere else. In an area where work is physical and punishing, subsistence is hard, and, in the words of one environmentalist, "Philosophical differences tend to be settled with fists," nobody likes the outsiders much.

"Some of these people say they are locals, and you ask them how long they've been here. They'll say 'five or eight years,' " said Lillian Griffin, a Crescent City merchant. "It takes longer than that to get to be a local up here."

Accordingly, when environmentalists have attempted to push ballot measures, they have been soundly drubbed. Over the past several years, voters have convincingly voted down referendums to limit the spraying of herbicides to control unwanted forest vegetation and have refused to stop the aerial application of 2,4,5-T, the primary ingredient in Agent Orange, leading one local politician to say that "these people are so mad at the environmentalists that they voted to continue pouring poison on their own heads."

Whether the mine is developed appears to hinge not on a decision by the community but by the Senate Banking Committee in Washington, which is considering price subsidies on strategic metals. Cobalt slumped from a war-panic high of $40 in 1977 to its current $14. Richard A. Haft, an attorney for Cal Nickel, says price supports of about $20 per pound would be desirable.

"Then it is a matter of financing. Financing is the key," Haft said. "The environmental permit we can get. The question is whether we can get the financing." If the company indeed obtains the financing, Cal Nickel says, construction will begin in the spring of 1983.