TORONTO -- Operations have been halted at the world's deepest operational mine after more than one-quarter of COVID-19 tests given to those who work there produced positive results.

AngloGold Ashanti said Sunday that it had stopped work at the Mponeng gold mine in South Africa after learning of the 650 tests it has administered since May 14, 164 were positive and "a handful" have yet to be processed.

The "vast majority" of those who tested positive had not shown any symptoms of COVID-19, the company said. Contact tracing has been taking place through data from an electronic tracking system normally used to locate miners who go missing.

Operations at the mine had already been running at half-capacity as part of a gradual ramp-up after the government initially ordered all mines closed amid the pandemic.

According to the company, everybody who tested positive will be placed in isolation as per South Africa's COVID-19 protocols, while "the workplace and key infrastructure" will undergo thorough cleaning.

Considered the world's deepest operational mine, Mponeng's current operations stretch as far as 3.4 kilometres below the surface although the mine itself extends beyond four kilometres down in total. AngloGold Ashanti reached a deal to sell it and other parts of its South African portfolio to Harmony Gold in February, a few weeks before the first case of COVID-19 was confirmed in the country.

Harmony reported May 20 that two employees of a contractor at its Kalgold mine, 300 kilometres west of Mponeng, had recently tested positive for the disease.

Many mines in Canada were also shut down as the COVID-19 risk grew, although normal activity has slowly been resuming at most in recent weeks, according to information from the Mining Association of Canada.

There have been more than 22,500 confirmed cases of COVID-19 and 429 related deaths reported in South Africa, according to figures from Johns Hopkins University.