JOHANNESBURG, South Africa (AP) _ A suspected methane gas explosion in a coal mine 430 feet underground killed 34 men and injured 16, the owners said today. It was South Africa's second underground coal mine explosion this week.

General Union Mining Corp. officials said they did not know what caused the 6:30 p.m. explosion Thursday at the Ermelo mine, 130 miles east of Johannesburg in eastern Transvaal province.

''The accident is suspected to have involved methane gas, but the actual cause is still under investigaiton,'' the company said in a statement.

Methane is a colorless, odorless gas formed by the decomposition of vegetable matter and is found naturally underground.

''The dead miners were apparently overcome by carbon monoxide fumes,'' the company said in a statement issued more than 10 hours after the blast.

The remaining 700 night shift workers returned safely to the surface. The company said the area was being cleared of noxious fumes and that normal operations at the mine, which employes 2,500, had not been fully resumed.

Gencor said 10 injured miners were being treated at the local Ermelo Hospital and were in satisfactory condition. The others had been transferred to Rand Mutual Hospital in Johannesburg and their condition was not immediately known, the company said. The more serious cases are usually transferred.

The company said the dead included 31 blacks and three whites.

The National Union of Mineworkers, which represents more than 300,000 black miners nationwide and about 800 at Ermelo, assailed the company's safety standards and accused Gencor of failing to implement adequate safeguards following a methane gas explosion at the Ermelo mine in November 1982 that killed 11 men.

Cyril Ramaphosa, the union's general secretary, also noted that Gencor owned the Kinross gold mine, where 177 miners were killed in an underground fire last September.

''Gencor is emerging as the butcher of the mining industry,'' Ramaphosa said at a news conference. ''It is bent on destroying as many lives as possible.''

Gencor's first report of the accident said 30 had been killed, and nine hospitalized. Four more bodies were recovered today, said the company, one of South Africa's biggest mining conglomerates.

The company indentified the dead whites as an electrician, Petrus-Pieterse; a fitter, Abel Erasmus; and miner Andre Veldsman.

The identities of the dead black miners, many of whom are migrant laborers from distant rural areas and other countries, will be released after their relatives are notified, the company said.

Ramaphosa said government mine safety officials had frequently refused to hold inquiries into methane gas explosions, despite seven fatal blasts over the past 36 years which he said had killed more 170 miners.

''We should learn from our mistakes,'' he said. ''In other countries, inquiries are held immediately and the reports are made public.''

Ramaphosa said union safety and legal experts were at the mine, waiting to join company officials on an inspection once the area of the explosion was judged safe.

South Africa's worst mining disaster occurred Jan. 21, 1960, when 437 miners were buried alive at the Coalbrook North Colliery, near Sasolburg south of Johannesburg.