It said all 19 cases were asymptomatic, 14 had been identified by testing as employees returned to work and of the remaining five, one was a primary contact and four were found through contact tracing.

"Significantly, 17 of the confirmed cases reside locally, suggesting the prevalence of COVID-19 among local communities is far higher than the company's initial estimates had indicated," Implats said.

It confirmed the cases included a local health provider and four mine-employed health providers working at the mine clinic.

It said the facility was immediately closed for sterilisation and testing staff and contacts, and the people identified had been isolated.

Implats said the next shift would not be asked to report for duty next week so the company could complete all health and safety protocols, including verifying test results, sanitising high-risk areas and conducting further testing.

Resources minister Gwede Mantashe welcomed news the employees who tested positive had been screened, tested and quarantined and had not gone underground.

"A disaster at a mining operation will impose an obligation on government to close that operation," he tweeted hours ago.

South Africa has allowed mines to ramp up to a 50% workforce capacity as restrictions in the country continue, and the government is imposing new guidelines on COVID-19 safety measures after a union won a court case seeking binding protections for mineworkers.

Implats CEO Nico Muller said the company was well prepared to deal with employees who had tested positive and would be deploying "all the resources available to us to protect the lives and livelihoods of our people and host communities".

"We wish our infected employees a speedy recovery and will continue to support them and their families through these anxious times," he said.

Implats owns 73% of Marula, which employs 3,988 people and contributes 5.2% to the group's platinum production, according to the company's website.

The company's share price had closed 9.85% higher on Friday to 11,027c to capitalise Implats at ZAR86.3 billion (US$4.6 billion).