JOHANNESBURG, AUG. 25 -- Striking black miners reopened negotiations today with the South African Chamber of Mines but were told that the mining companies will not improve their offer on pay, the main issue in the two-week-old strike, but only on benefits.

Cyril Ramaphosa, the general secretary of the National Union of Mineworkers, said that the company's tough, take-it-or-leave-it proposal will be put to the strikers Wednesday for acceptance or rejection. He said that continuation of the strike, the biggest in South African history, will depend on their decision.

Although Ramaphosa would not say what the union leaders will recommend, he did not hide his disappointment with the companies' refusal in four hours of talks today to move beyond their previous pay offer, which the union firmly rejected last month. In reopening the negotiations, he said, the union had believed that wages would be discussed.

"Our members are showing unprecedented determination," he told a news conference. "They are prepared to hold out for as long as it takes. They are saying they have lost too much to go back for this."

The mining companies offered wage increases ranging from 15 to 23 percent and put them into effect unilaterally last month. Since then they have refused to discuss the question. The union had been asking for an across-the-board increase of 30 percent but reduced its demand in the hope that the companies would also compromise.

The decision on whether to accept the companies' offer of improved benefits but no further increase in wages will be difficult for the union, and it could affect negotiating patterns in the mining and other industries for years to come.

The National Union of Mineworkers began the strike Aug. 9. About 11,000 miners have already been fired, and more than 30,000 others are facing dismissal.