JOHANNESBURG, AUG. 31 -- An explosion ripped through a gold mine shaft today, as striking miners returned to their jobs in the wake of South Africa's longest and costliest labor walkout. The blast killed at least eight miners and left 51 others missing and presumed dead or trapped 4,600 feet below the surface.

Officials of the St. Helena mine in the Orange Free State town of Welkom, 140 miles southwest of Johannesburg, said most of the missing miners were descending in a shaft elevator at the time of the blast, which sent the elevator crashing to the bottom of a 4,600-foot shaft.

The company did not rule out the possibility of sabotage. It said it had initiated an investigation into the cause of the explosion.

The St. Helena mine, which is owned by the General Mining Union Corp. (Gencor), was one of dozens targeted by black mine workers during a three-week strike that ended yesterday when a quarter of a million striking members of the National Union of Mineworkers accepted a wage increase offer that industry management had imposed two months ago.

The average black mine worker now earns about $285 a month, plus food and lodging in men's hostels.

Danger pay and death benefits resulting from accidents were among the issues in the bitter labor dispute, which left 11 miners dead and more than 500 injured in strike-related violence.

In the past year, Gencor mines have had two accidents resulting in mass fatalities, including an underground fire that killed 177 workers on Sept. 16 and was blamed on the use of highly volatile materials for coating underground conduits.

A Gencor spokesman said that at the time of the explosion, about 6:45 a.m., 434 workers were underground and that most had found safety through lateral tunnels to an adjacent shaft.

The shaft elevator had a capacity of 82 workers, but the actual number of passengers was not known, the company said.

Five workers have been rescued so far. The survivors and the dead were found at an intermediate pump station 2,100 feet underground. The company said it did not know when rescuers would reach the elevator, because the explosion had mangled much of the steelwork in the shaft.

Cyril Ramaphosa, general secretary of the National Union of Mineworkers, said the union also was investigating the cause of the accident. After a methane gas explosion killed 34 workers at the company's Ermelo Mine April 9, Ramaphosa called Gencor "the butcher of the mining industry."

More than 800 miners, most of them black, died in accidents in South Africa's mines last year.

The strike against the country's all-important mining industry resulted in some improved benefits for families of workers killed in accidents, but the miners did not win the danger pay that they had sought.

Mining companies represented by the South African Chamber of Mines said today that early reports indicated a steady return to work of the estimated 250,000 workers whose walkout had crippled about half the country's gold mines and a fifth of its coal mines.

The union claimed that at the strike's peak more than 340,000 of South Africa's 600,000 black miners were out at 44 gold and coal mines.

Anglo American Corp., the giant conglomorate most affected by the walkout, said strikers were returning to all its mines except one in the Orange Free State. It attributed that delay to "communications problems."

Anglo American fired 37,000 striking workers during the dispute, many of whom had returned to their homes in tribal homelands and neighboring black states such as Lesotho. Mines dismissed about 44,000 miners in all, but the mining companies said the dismissed workers would be rehired provided their jobs had not been given to replacement workers.

An Anglo American spokesman said today that "subject to availability of jobs," fired employes who report to work within seven days will be rehired. In cases where their jobs already have been filled, strikers will be offered lower grade jobs when they become available.

Chamber of Mines officials have refused to estimate how much the strike cost the mining industry, although industry analysts estimated the cost at $15 million a day in lost revenues. The chamber estimated the strikers lost at least $5 million a day in wages.