Some 1,400 feet below the old saguaro cactuses and blooming ocotillos of the desert, the thick darkness of a copper mine is broken by the stray beams of the miners' lamps.

They are lonely pinlights in the tunnels that go here, then there. A rare and sad silence - even the drip of the seeping groundwater seems loud - fills the miles of tunnels where the din of drilling, blasting and crushing of rock is usually the rule. Tere should be some 400 men mining copper on the day shift, but there is only a handful underground.

And no copper is being mined.

For when the price of copper fell to 60c a pound Aug. 12, the Helca Mining Co. faced a painful but necessary choice. "At some point," said Helca's Gil Valez, "you quit trying to maximize profits and start trying to minimize losses."

Seventeen days later, the company announced it was shutting down its $200 million Lakeshore copper mine and processing plant on the Papago Indian reservation near her; 1,508 workers would be laid off. A $404,000 weekly payroll would stop. It would last 60 days.

Eight months later, however, the mine has yet to reopen.

Worse, the layoffs here at Casa Grande are only a portion of the estimated 5,000 copper jobs lost in a score of communities in Arizona, which produces 63 percent of the nation's newly mined copper.

The problem is that worldwide copper prices still remain well below the cost of production, pushed there in part by foreign copper mines grinding out the ore - no matter what the losses - in a desperate drive for jobs and dollars to repay loans so crucial to poorer, developing nations.

The Labor Department estimates that as many as 12,000 U.S. copper workers may end up receiving special federal payments designed to ease the blow of the import-related layoffs.

So the impact of cheap copper is felt in the United States, in places like Casa Grande, a rural community between Tucson and Phoenix whose population reached perhaps 16,000 before the Hecla layoffs. Here, Sun Belt optimism has given way to uncertainty, financial hardship for some, bitterness and a broad dependence on federal assistance checks.

"This layoff is costing me five years of retirement," 56-year-old Glen McKechnie said of the savings he is rapidy depleting - $2,800 so far - while unemployed. He once made $1,495 a month as a electrical planner at Hecla and paid $200 every two weeks in taxes. Now he's been getting $208 a week back from the state and federal governments to see him through.

Total unemployment here is 13.2 percent, down from a high of 16 percent apparently only because some of the unemployed just packed up and left. Thus a community that was growing rapidly - the population jumped 50 percent in seven years - is now losing people, many of whom left as abruptly as they came when the Hecla mine opened two years ago. School enrollment is down and some teachers, caught in the vortex of one company's unemployment, face the loss of their jobs.

School budgets have been thrown askew because of reduced state aid, which is based on average attendance.

Retail sales are off, businessmen say, but not so much as they were before special federal payments began shortly after the first of the year. Jerry Grady, who runs a tire and minning supply store, said, "This has had a pretty staggering effect on us."

His retail sales are off 6 percent, in addition to the $3,000 to $10,000 a month business he did supplying Helca.

"They were damn good customers," Jimmie Kerr, a county supervisor and dry cleaner, said of mine employes. "They want to go out on Saturday night and they want to look sharp" - in nicely pressed clothes. So his business is off. Citywide, sales tax revenues are down 16 percent.

There is a virtually certainty of decreased property tax revenues for the county and school districts because of the mine's shutdown - only the amount is unknown and whether there will have to be offsetting cutbacks in services or increases in tax rates.

And no one knows how far into the future 3,500 more copper jobs have been pushed, jobs that were to be created by three new copper facilities nearby that have been indefinitely postponed.

The U.S. International Trade Commission has scheduled a May 22 hearing in Tucson as part of a process that could lead to reduced imports, and thus higher prices, through tariffs or quotas.

The Carter administration has asked Congress to start stockpiling copper to help raise prices, a move apparently designed, in part, to help the president's popularity in the West, where most of the layoffs have occured.

Copper producers say they face unfair competition in world markets because federal regulations, such as polution controls, add 10 to 15 cents a pound to the cost of producing copper in this nation.

Hecla and the copper industry in Arizona have turned to the state legislature for property tax relief that, if enacted by a legislature usually sympathetic to mining interests, would cut Helca's costs by about 1 1/2 cents a pound.

Such a reduction still would not make U.S. copper competitive with that from, say, Africa, but state Rep. Jim Hartdegen calls such minor efforts "honey to catch the fly," an effort that would show federal officials "we've done as much as we can." Hartdegen was a safety inspector with Hecla until he was laid off.

But the efforts to ease the mine's tax burden has brought to the surface hard feelings and difficult questions over the growing dominance of copper mining in a town that once was totally, and still is significantly, agricultural.

"When the going gets tough," said Jim Henness, a farmer and a member of the board of the Casa Grande elementary school district No. 4, "the mines want to say, 'I'll give you (other taxpayers) the problem.' But when the going gets good again, they won't ask to have it back."

Henness says individual taxpayers would have to make up any lost property tax revenue. As things stand now, mine property is assessed at a far higher rate than other types of land. "When they start shifting the burden," said Henness, "I get touchy."

To be sure, the going will get good again. For while the Lake Shore mine, which Helca operates with El Paso Natural Gas Co., lost $23.8 million last year, the Superior Oil Co. is considering purchasing the mine. That parallels a trend of oil companies buying copper operations nationwide and overseas with the belief that, before too many years pass, copper prices will rise substantially - well above the 70 cents a pound or so it now takes to break even at Lake Shore.

Some 40 percent of U.S. copper supplies are now believed to be under the control of oil companies.

But if the ripples of adversity hit more people sooner in a small town like Casa Grande, they also have the effect of drawing the local's wagons closer together. The banks here ran newspaper ads urging those affected by the Helca layoffs to come in an make adjustments in their loan payments. Other merchants, like Grady, are simply carrying bad debts until better times come. "You can't blame them," said Grady. "It's not their fault."

One side-effect that concerns public leaders in this community, though, is the loss of white-collar professionals from Helca who have taken new jobs elsewhere - the sort of people who tended to be active in community affairs. For example, six of the city's seven soccer coaches have gone, and an environmental engineer who served on the planning commision has departed. A local drive to establish a YMCA and YWCA program collasped after the organizers moved.

Meantime, there are the individual hardships for the people laid off, who face an uncertain future over the prospect of returning to work.

"I don't know what we're going to do," said Bette Sallen as she sat in the sparsely furnished living room of her double-wide mobile home here. Her husband is working at a mine 100 miles away. He commutes home on weekends, but the expense of maintaining two homes has cut severely into the family budget.

The family includes an 18-year-old daughter finishing high school this June and a 10-year-old daughter. "You just don't move in your senior year," Sallen said of the family's decision to stay put until her 18-year-old graduates.

What then?

"We haven't thought that far ahead," she said, "it's too hard to handle."

The family has tapped its savings account for $1,200 and has borrowed from relatives. "So far, we've hung in there," said Sallen, "but I don't know if we're going to make it to June or not.

"We take it one day at a time."