An "unparalleled" deterioration in safety standards at Sibanye-Stillwater, resulting in the deaths of 20 employees in nine incidents since February, has prompted the gold and platinum miner to appoint a safety expert, academic Kobus de Jager, as it investigates what went wrong at its Driefontein and Kloof gold mines, and how to address its safety approach.

Last week, five mine workers were killed after entering an unmined area in the Ikamva shaft at Kloof near Westonaria, while 12 miners died in what Sibanye says were two "unrelated" fatalities earlier in 2018.

Work at Kloof, which has five producing shafts of which the deepest operates at below 3.3km underground, has been shuttered until a safety audit there is complete, and the company will hold a safety day on Thursday following Wednesday’s memorial. The five mine workers died after entering an area that was not ventilated.

The seismic event at its Driefontein mine on May 3, located in the West Wits Line near Carletonville, caused a rockfall at a working stope, trapping 13 miners, of whom seven died.

Subsequent investigations have found that the event was "primarily technical" with "no preliminary evidence" that its operational controls had failed.

That incident is under investigation by the gold and platinum miner and the Department of Mineral Resources.

Sibanye’s head of investor relations, James Wellsted, said underground tremors were nothing new for Sibanye, which had recorded more than 780 category-one seismic events and more than 80 category-two incidents over the past five years. The company was now trying to ascertain whether there were any links between the seismic events that resulted in such a heavy loss of life.

During the Driefontein and Ikamva accidents, allegations were made that Sibanye employees were made to work in unsafe conditions, but the company said there was "no evidence to substantiate this".

"We are appalled by the loss of our employees’ lives at our mines over the past few months," said CEO Neal Froneman, who has also urged employees to come forward with information on any intimidation by supervisors who may have forced miners to work in unsafe conditions.

Sibanye also announced that it would be funding an independent study by Wits University to develop practical recommendations to improve risk management at its mines.

Planned production from Driefontein is about 50kg of gold a day, with the Masakhane shaft accounting for about a fifth.

In the shutdown following the incident, Sibanye estimates it lost more than 230kg of gold output. In 2017, Sibanye’s South African gold operations produced 43,634kg, or 1.4-million ounces of gold.