The heavy air in the tunnel is hot and wet and filled with fine dust and the scream of a drilling machine. No man can stand upright, so the five workers stoop as they claw at the rock with picks and drills. They are black men; the shift boss, who stands a dozen yards away overseeing their work, is white.

Here, more than a mile under the surface of the earth in the gold mines that are the driving wheel of this country's economy, South Africa's racial hierarchy has been this way for generations. Yet changes are coming to the mines -- changes that reflect the turbulence shaking this troubled land and the growing economic power and political restlessness of its black majority.

The most important change is the emergence of a militant black miners' union that only three years after its birth is locked in bitter, high-stakes negotiations for higher wages with six of South Africa's largest and most powerful corporations.

The talks took another sharp turn in Johannesburg yesterday when the union postponed its threatened strike, originally set for next Sunday, until Sept. 1, following new contract offers from five of the companies. The offers vary sizably, and union leaders must now decide whether to settle with some firms and strike at others or maintain solidarity and attempt a risky, industrywide walkout.

Unionization has begun at a time when the companies have improved working conditions in the mines and have significantly narrowed the wage gap between white and black workers.

A company-conducted visit earlier this week to the Blyvooruitzicht gold mine 50 miles west of Johannesburg suggests that the changes have yet to make more than a small dent in the fundamental features of the racial segregation system known as apartheid. In that sense, conditions in the mines mirror those in white-ruled South Africa as a whole.

This mine, an eight-square-mile complex that houses 12,500 workers, is owned by Barlow Rand, generally considered one of the most affluent and progressive of this country's mining giants. Yet housing, dining, school, medical and recreational facilities are strictly segregated. Certain mining jobs are reserved for whites only, and whites earn nearly four times the average salary of blacks.

Even the showers outside the mine entrance are an all-white preserve; black workers must return to their dormitories to wash off the dirt after a day below ground.

Most of the 1,000 whites employed here live with their families in company-supplied houses, while almost all the 11,500 blacks are consigned to the dormitories, where they live 10 or 12 to a room. Most of their families are not allowed to join them, and many go a year or longer without seeing their wives and children.

Some of these restrictions result from South African law, which still reserves certain job classifications for whites in the mines even though this situation has been abolished in all other fields. The law also stipulates that a maximum of 3 percent of black workers can be housed with their families on company premises. Other restrictions are the result of longstanding traditions that are succumbing slowly to the demands of black miners.

"We're not saying it's a perfect system or even that we're happy with conditions as they now exist," said Fourie van Antwerpen, the mine's general manager. "What we are saying is that conditions have improved and that we will continue to improve them."

The company has adopted a nondiscriminatory employment code that officials here say is similar to the principles under which many U.S.-owned firms operate in South Africa. An apprentice training program has been established to train blacks for skilled jobs they have been denied for many decades. However, no more than a few dozen blacks work in skilled classifications, and officials concede it could be decades, if ever, before blacks gain parity and close the wage gap with whites.

"We've got a long way to go, but the Barlow code doesn't say you must do everything overnight," said Fred Ingram, the mine's personnel manager.

South Africa's 720,000-man black mine labor force is a particularly vulnerable group. Nearly half are from the nominally independent black "homelands" created by the South African government, and they have no formal citizenship rights here. Forty percent more are from neighboring countries, mostly from Lesotho, but also from Mozambique, Malawi, Botswana and Swaziland, where they were recruited by some of the nearly 130 labor offices the South African Chamber of Mines operates.

Their salaries often constitute a crucial part of their countries' annual foreign exchange earnings. In Lesotho alone, miners' wages from South Africa account for more than 60 percent of the gross national product.

Most of the men work on year-long contracts, but they can be dismissed on 24 hours' notice and returned to their rural homes. The company can also decline to rehire anyone it deems inefficient or a disciplinary problem, although Barlow officials pride themselves on a 95 percent return rate. There is no pension system, only a form of severance pay that for this mine applies only to those with at least 10 years' seniority.

The jobs performed by the men are dirty and dangerous. More than 8,500 miners died in accidents between 1974 and 1983, according to government figures. Company officials contend they have done much in recent years to reduce the rate of serious injuries, but the toll continues to rise.

Twenty-nine miners died earlier this month in a coal mine explosion. On Aug. 7, the Chamber of Mines released a statement praising its companies for lowering the gold-mine fatality rate this year. Two weeks later, eight gold miners were killed in another blast.

Along with their vulnerability, the mine workers also have a hold on South Africa's largest and most essential industry. Gold accounts for nearly half this country's foreign exchange earnings and 10 percent of its tax revenues. South Africa supplies 70 percent of the noncommunist world's gold and produces twice the annual output of the world's second leading supplier, the Soviet Union.

Economists here have projected that the mining industry, which also includes coal, diamonds, uranium and a dozen other major minerals, accounts directly and indirectly for nearly 30 percent of the country's gross national product.

The mines cannot operate without workers and the companies and their laborers are locked in a symbiotic relationship. This was best illustrated last month when President Pieter W. Botha issued a warning that he might be forced to return foreign workers to their home countries if the West ever imposes significant economic sanctions on this country.

Hypothetical as it was, the warning sounded alarm bells throughout this industry. Cyril Ramaphosa, the charismatic secretary general of the National Union of Mineworkers, said he will call an industrywide strike if Botha ever makes good his threat. At the same time, mining officials quickly issued statements reassuring their workers and suggesting that the president is playing politics with a vital part of South Africa's economy.

"We've assumed he was just making a political gesture," said Chamber of Mines spokesman John Imrie. "It obviously would not be very desirable for us to have to replace all those workers."

The mining companies themselves, led by Barlow Rand and Anglo-American Corp., the two biggest firms, opened the door to unionization three years ago when they announced they were prepared to negotiate with any union that could demonstrate "significant representation."

It was a case of "enlightened self-interest," according to Neville Huxham, an Anglo-American spokesman, who said the company decided long ago that it would be "smart down the road to be able to talk with responsible, recognized leaders of our employes." About 40 percent of Anglo's black labor in nine gold mines are dues-paying union members.

Ramaphosa's union quickly emerged as the most effective of the new crop of miners' unions. It claims the loyalties of nearly 240,000 workers, although paid-up membership is only about 83,000. Until now it has avoided a showdown with the companies, preferring to build its strength quietly.

Last year Ramaphosa, a tough-talking, 32-year-old labor lawyer, took negotiations with the chamber to the last day before settling. The agreement came too late to alert all of his locals, and 10 workers died in the violence that followed.

This time the union is seeking a 22 percent across-the-board wage increase in the $200 monthly salary that the average unskilled black worker earns plus a three-hour shortening of the work week. It also wants abolition of the white job-reservation clause, a proposal that puts it in direct conflict with 12,000 white miners who see the clause as their last protection against being replaced by lower-paid blacks. The government has said it will move to repeal the measure next year if it is not eliminated before then by negotiation.

The gap in economic issues between the union and the chamber does not appear great. But the bargaining has become a test of wills between an emerging labor movement and a conservative industry that does not believe the union commands enough support among mine workers to conduct an effective walkout.

"We're not that far apart," said Manoko Nchwe, a union spokesman and member of its bargaining committee. "The problem is the chamber thinks it can teach us a lesson, and we are ready to teach them a lesson, too. We think they're going to be in for a rude shock."

Both sides contend the black unrest that has plagued South Africa for the past year has had no direct impact on the mines. They concede that the increasing sense of black frustration is a factor pushing some workers toward a strike.

"We're not striking against apartheid," Nchwe said. "But we cannot see this isolated from the wider aspirations of our people."

For now, however, much of the power appears to remain with the companies, which can rely on police and Army support in a showdown. That was made clear in April when Anglo-American summarily dismissed more than 14,000 workers at Vaal Reefs, the world's largest gold mine, following a six-week dispute. Most were rehired later -- but not before the point was made that the companies, like the government, still have the last word.