The deaths are a setback for the government and the industry as they try to improve safety and reduce fatalities in South Africa’s mines, which are the deepest in the world.

In 2011, for the first time in living memory, the country’s mining sector recorded no “disaster”, defined as a single incident in which four or more workers die.

Fourteen miners were admitted to hospital after Saturday’s incident at the company’s KDC West mine, Gold Fields said in a statement. The company said the entire mine had been closed until further notice.

“We strongly condemn the company for sending workers on overtime night shift when there is no ventilation,” Frans Baleni, the general secretary of the National Union of Mineworkers, said in a statement.

Gold Fields is the world’s fourth largest gold producer.