South Africa's largest black trade union today announced Aug. 25 as the date for a nationwide mine workers' strike that could set off more racial unrest and further damage the country's already weakened economy.

The strike call could put the National Union of Mine Workers on a collision course with the white-minority government, which has declared a state of emergency in 36 cities and towns because of racial violence. It also could prove a critical test of strength for the country's emerging black labor movement as it seeks to assert influence on economic and political issues in this racially divided country.

The strike is ostensibly over wages, but behind those demands are several political issues. Union leaders said today that their members had resolved to begin a boycott of white-owned businesses in mining towns if the government's two-week-old emergency decree is not lifted by Wednesday.

Union leaders predicted at least half of the country's 550,000 black mine workers would honor the strike call and said it would affect 29 mines -- 70 percent of South Africa's gold mines and 20 percent of its coal mines.

The strike "could virtually bring the industry to a halt," Cyril Ramaphosa, the union's general secretary, said at a press conference here this afternoon. He said the strike was approved by a special congress of 450 voting union delegates.

Seven miners died and 400 were injured last September in clashes with police and company security men during a walkout by 64,000 workers. Another died during a smaller, wildcat strike in April, that collapsed after two mining companies fired 15,000 striking workers.

Ramaphosa warned that such tactics would not work against an industry-wide walkout. "The mining houses would commit suicide by dismissing over 200,000 workers," he said.

The union says it is demanding a 22 percent pay increase and a shorter work week while employers have granted increases ranging from 14 to 19 percent. Industry spokesmen say they calculate the union's wage demand at 32 percent.

In addition to their threat to boycott white-owned businesses, union leaders also strongly denounced President Pieter W. Botha's threat to send black migrant workers back to their home countries if western nations impose economic sanctions against South Africa.

Ramaphosa said the union would strike in retaliation if the threat, which Botha made last week, was carried out. He said about 40 percent of his members could be affected if South Africa deported foreign workers.

Gold is South Africa's most precious commodity, accounting for about half of the country's foreign earnings and 10 percent of tax revenues. The country produces about 70 percent of the gold mined in the noncommunist world, and its annual output is estimated at twice that of the Soviet Union, the world's second leading gold supplier.

While gold prices have held steady since the emergency declaration, South Africa's currency has dropped sharply in value and stock prices have fallen slightly, reflecting investor unease over political conditions. Analysts say a prolonged mine workers' strike could depress gold stocks further.

The mine workers union has been in the vanguard of the labor movement here since the government legalized black trade unions in 1979. Its paid membership has grown from 6,000 two years ago to more than 83,000, and it has claimed at least 70,000 other registered members and the loyalty of thousands of others.

The six companies that constitute the South African Chamber of Mines have questioned the union's strength and challenged the strike vote last month that the union says authorized the walkout. Industry spokesmen have charged that there were "widespread voting irregularities" that invalidate the result.

In recent years the mining industry, pressed by the union, has pushed hard to close the gap between wages for whites and blacks. Average white salaries remain five times higher than those of blacks, according to the industry, whereas in 1971 they were 21 times higher.

Nonetheless, the union wants the ultimate abolition of racially based pay differentials and of the code that reserves certain higher paying jobs for whites.

Government officials had no comment tonight on the strike call. But a spokesman for Anglo-American Corp., the mining conglomerate that is the country's biggest company, said the company was "disappointed" in the union's decision. He said the union lacked "a clear mandate" from workers to call a strike.

Among the few violent incidents reported today was the burning of a woman in the black township of Reini, in eastern Cape Province. A police spokesman said that the attack appeared connected to allegations that she was a police informer. She was hospitalized.

Police also announced 13 more arrests under the emergency powers declaration, bringing to 1,412 the number of since the decree took effect two weeks ago.