About 20,000 black miners were fired today from three of South Africa's largest platinum mines, and 10,000 more were threatened with dismissal today if they continue a strike begun a week ago.

Impala Platinum Holdings Ltd., the West's second-largest platinum producer, said that it had come to a virtual halt over the weekend at its four mines and its processing plant in Bophuthatswana, one of South Africa's nominally independent tribal homelands. Only a few hundred miners continued to work, Impala said.

The firings constitute the largest mass dismissal of black workers here in recent years and represent a direct challenge to the emerging political ambitions of the National Union of Mineworkers and other black labor unions.

Gary Maude, Impala's acting chief executive officer, told reporters that the company had decided to fire the strikers because it found their demands for pay increases "far too high" and their calls for improved working conditions and stricter safety standards "expensive and unreasonable in current circumstances."

But Maude and other Impala officials did not hide the company's intention of preventing the unionization of the platinum miners and dealing a major blow to the National Union of Mineworkers, whose Bophuthatswana affiliate claims to represent most of the 30,000 miners involved. The company has refused to recognize the union, although the miners' support of the strike is beyond dispute.

Impala's parent company, General Mining Union Corp., prides itself on its toughness in labor negotiations and has boasted about its ability to keep wage increases to a minimum. Four months ago, it fired about 1,200 striking gold miners, also members of the National Union of Mineworkers, but was forced to reinstate them when the union accused the company in court of unfair labor practices and won.

The National Union of Mineworkers, which claims more than 160,000 dues-paying members, said later today that it is planning a strategy to get the fired workers reinstated at the Impala mines and to reopen wage talks with Gencor, as the parent company is known.

Maude said that new workers would be hired immediately to replace all the dismissed miners, and he predicted that Impala would be back in production within two weeks. Company officials added that they will try to ensure that none of the fired workers would be hired by other mines. With unemployment at record levels, even in mining, Gencor appeared likely to succeed in both efforts.

Impala miners currently earn about $100 to $120 a month at top scale, plus room and board, according to union officials.

In South Africa's continuing civil unrest, a black man was killed, the apparent victim of black militants who suspected him of cooperating with the minority white government. Police in Cape Town reported that they found the man's body, burned beyond recognition, in a garbage dump in Guguletu township. Militant youths and more conservative older blacks have been feuding there for the past week.