South African gold production fell to its lowest level for 20 years in 1980, according to figures released by the Chamber of Mines. But the giant mining companies are far from worried, because income from gold almost doubled, experts say. Earnings from gold leaped from $4.3 billion in 1979 to $7.68 billion in 1980 on an average price of $615 an ounce. Meanwhile, the price of gold plunged below $500 an ounce Thursday, its lowest for nine months. Gold has lost more than $ 70 in trading in the past week.