JOHANNESBURG -- The steady fall in the price of gold, which has dropped from more than $400 an ounce in March to less than $350, is putting a severe squeeze on the world's leading producer, South Africa.

Noting that more than half of South Africa's gold is now being mined at a loss, Johan Liebenberg, senior general manager of South Africa's Chamber of Mines, describes the country's mining industry as being in a "critical situation."

According to the chamber, 22 of South Africa's 41 longstanding mines are in the red, and as the big mining houses prepare to shut down uneconomic shafts, tens of thousands of mostly black miners are being let go.

Typical of many is the situation facing Anglo American Corp.'s Freegold, the world's biggest mining complex, which employs 110,000 workers. The company estimates it is losing 100 rand (about $37.75) on each ounce of gold it produces.

Freegold announced this week it would start talks with the mining unions soon on cutting 7,800 jobs.

The effect of the gold slump is hitting the national economy as well. Gold is by far the most important element in the South African economy, and it is estimated that every $10 fall in the gold price costs the country 500 million rand (about $188 million) a year in foreign exchange earnings.

Roughly annualized, that means the decline since March has already cost South Africa about $1 billion for 1990.

This will affect the country's ability to repay its foreign debt and it will keep the exchange rate of South Africa's currency, the rand, depressed, adding to an inflation rate that is already running at about 16 percent.

The combined effects of rising unemployment and increased inflation also pose a threat of further social unrest in a country already in a state of tension as President Frederik W. de Klerk begins negotiations with the African National Congress and other black movements for a new national constitution.

The South African gold industry has been declining for some years. As new, lower-cost mines have opened in the United States and Australia, South Africa's share of overall Western gold production has shrunk from 70 percent to 37 percent.

Employment in the gold mines has declined from 534,255 to 479,874 since 1986 -- a layoff of 54,381 workers.

Now the sharp price drop of the past few months has put more mines into the marginal category. Tom Dale, gold specialist in a Johannesburg stockbroking firm, calculates that at current prices South African production will shrink from its present rate of 608 tons a year to between 550 and 575 tons by the end of 1990, and 450 tons by the end of 1991.

Projections like that prompted South Africa's leading financial newspaper, Business Day, to warn recently that "an historic watershed" has been reached.

"South Africa's golden past is receding, and ahead lies the difficult transition to modernity," the newspaper said in an editorial, indicating that the country would have to enter a tough new phase of industrial diversification that would require stricter discipline on the part of both capital and labor.

One of the problems facing the shrinking gold industry here is what to do with uneconomic mines.

Putting a deep-level mine in mothballs is a costly business, since extensive maintenance work has to be continued to keep it safe for possible future use.

If it is simply closed down, it will flood with underground water, rotting the supports and making it unsafe to reopen if gold prices go up again.

Some economists are suggesting, however, that if some of the uneconomic mines are closed, South Africa's reduced production will send up the price of gold, making other mines more profitable and preventing overall foreign exchange earnings from the industry from being significantly reduced.