By Sue Thomas

JOHANNESBURG, March 18 (Reuters) - South Africa's vital gold and platinum mines were saved from power cuts on Tuesday after Eskom fixed two power generating units, but the state-owned electricity firm said the situation remained serious.

Millions of South Africans faced returning from work to darkened, cold homes as rolling blackouts continued across the country due to wet weather that raised electricity demand with nine generators tripped and another nine closed for maintenance.

"Things are serious but we're not declaring force majeure, but it's one of the possibilities if we continue to see generators fail," Eskom spokesman Andrew Etzinger said. Force majeure would mean the firm could not meet contract obligations.

Eskom had said on Tuesday morning that power could be cut to the mines if two more of Eskom's 160 generators failed.

Etzinger said two units came back into operation during the day, and two more would be back in service early on Tuesday evening. "That has really made the difference, it's a strong improvement to where we were."

Global platinum prices rose because of the possibility of new cuts in the world's top producer. South Africa is also the second-biggest miner of gold after China.

The electricity grid supplying Africa's biggest economy came close to collapse in January, forcing gold and platinum mines to shut down for five days. Since then mines have been operating below full power, driving up precious metal prices and raising fears of possible job losses and slowed growth.

A gold analyst said investors were hoping that January's cuts would not be repeated.

"It's extremely negative. It is not just the loss of production, it is investors' perception of South Africa as a whole, not just the mining industry," the analyst said.

"Ultimately it raises risk and makes capital more expensive and the companies will suffer low ratings as well as lost revenue as a result of lost production."

South Africa's power crisis follows years of underspending by Eskom on generation capacity. On Tuesday, it asked the energy regulator to revise the annual electricity price rises to 53 percent from 14.2 percent it was granted in December.

The regulator said it would give the request urgent attention because of the power cuts.

Eskom restarted a programme of rolling black-outs, known as load-shedding, and many Johannesburg suburbs -- including the Sandton financial district -- were without power on Tuesday, causing heavy congestion and delays as traffic lights failed.

MINING SHARES TUMBLE

Gold and other mining shares on the Johannesburg bourse tumbled as investors worried that output would suffer, and despite higher gold and platinum prices.

"People are worrying power might be cut off to the mines and if we haven't got power, that is not good," said one Johannesburg trader.

The JSE Securities Exchange's gold sector ended 3.4 percent lower.

Spot platinum turned positive following the Eskom comments after falling over 2 percent to a 1-week low. The metal was last quoted at $1,990/2,000 an ounce.

The powerful National Union of Mineworkers (NUM) said it would be disappointed it Eskom cut power days after the utility had said it was moving to restore 95 percent power to mines.

"All of a sudden they are faced with force majeure, and it seems to us they are not sure about how their system operates," said NUM spokesman Lesiba Seshoka.

A power industry source said Eskom's Duvha power plant was running at 50 percent generation, Majuba and Lethabo had two units down, Kriel had three down and Kendal was at 75 percent.

Eskom's Etzinger said the power problem was compounded by persistent rain in north-eastern Mpumalanga province where coal mines and the utility's coal-fired power stations are concentrated.

Anglo American said three open cast coal mines supplying Eskom had closed because of rain. South Africa generates most of its electricity from coal.

BHP Billiton said the rains had slightly cut its domestic and export coal supply.