The South African economy, reeling from the effects of racial violence and threatened international sanctions, faces the prospect of further disruption as the black National Union of Mineworkers announced tonight it would strike seven major gold mines Sunday.

The union accepted wage offers from two companies controlling 22 other gold and coal mines, but warned that it would extend the strike to include them if three other companies -- controlling the mines where its workers will be called out -- take strike-breaking action against them.

Labor specialists said tonight there was a strong possibility of this happening, given the stance of some mining companies. These observers said the dispute then could broaden into a near general strike of black workers, with other major unions calling out their members in support of the miners.

Twelve of the biggest unions are due to form a 400,000-member alliance Nov. 30 in which the National Union of Mineworkers will be the leading partner.

"If the NUM gets involved in an industry-wide strike, I think the other unions involved in the alliance will feel obliged to support it," said Tom Lodge, a specialist in black politics at Johannesburg's Witwatersrand University.

Lodge said this could result in the first general strike in South Africa's history, "which in the context of the current state of emergency could be explosive."

The prospect is worrying the South African business community, already troubled by the sharp decline in the economy and the crash of the currency, the rand, which plunged to 36 U.S. cents yesterday, causing the government to suspend trading on the Johannesburg foreign exchange market and stock exchange.

Spokesmen for the commercial banks welcomed that move, saying it was inevitable because of pressure being exerted by foreign banks demanding the repayment of short-term loans. The national commercial association, known as Assocom, expressed fears that it might be the prelude to a reimposition of exchange controls relaxed two years ago.

For the second time in two weeks, the country's only financial daily, Business Day, called for the resignation of President Pieter W. Botha. It first did so after Botha's speech in Durban Aug. 15, when he failed to announce expected changes in the apartheid system of white-minority rule.

Most economists attribute the rand's slide to a loss of confidence caused by the conflict and fears of international sanctions, resulting in foreign banks demanding the repayment of short-term loans.

Tony Andrew, a senior executive of a leading merchant bank, said he believed the government had suspended foreign exchange and stock-market trading until Monday to give itself time to seek a rescheduling of those loans through the Bank for International Settlements.

An effective strike in the gold mines, even if it did not intensify the racial unrest, could seriously affect an economy that depends on gold sales for 45 percent of its export earnings.

South Africa produces nearly three quarters of the noncommunist world's gold, and at the current price of $340 an ounce its annual production is worth $7.14 billion. A month's stoppage could mean a loss of nearly $600 million to the mining companies.

The decline in the local currency means the mining companies are earning more rands for their gold sales, and the mining union has used this to press its claim for a 22 percent across-the-board wage increase. After weeks of negotiation, two of the companies seem ready toagree. But the other three have offered substantially less. One, a predominantly Afrikaner company called Gencor that has close ties with the government, has offered nothing.

The mining companies normally bargain as a unit through an organization called the Chamber of Mines, but for the first time in their 100-year history they have broken ranks to make separate offers to the black union. This posed a problem for the union, which was formed three years ago and still has only 150,000 signed-up members among the estimated 550,000 miners working for the chamber-affiliated companies. It is strongest in the biggest of the companies, Anglo-American Corp., where it has 80 percent of its membership, and weakest at Gencor and the other two companies that are taking a hard line against it.

The union's response to the staggered offer reflects a concern that by striking only in the mines where its membership is weakest, its members there could face mass dismissal or other tough countermeasures.

Observers familiar with labor practices here point out that mass dismissal is commonly used by the mining companies when they are faced with industrial disputes. Earlier this year, Anglo-American, considered the most enlightened of the mining houses, fired 14,000 workers at one mine.