A member of the mining community walks past the Doornkop Gold Mine, about 30 km (19 miles) west of Johannesburg in this file photo.

Johannesburg - South African bullion producer Harmony Gold said on Tuesday an investigation into a fire at its Doornkop mine last month that killed nine miners would take at least another four weeks.

Harmony also said in a statement that operations at Doornkop had resumed on Monday but it will be some time before they return to normal levels as the build-up will be gradual and follow a “detailed start-up plan.”

Operations were halted after the outbreak of the fire, which followed a seismic event and rockfall.

“Meanwhile, an external safety audit team has been appointed voluntarily by Harmony to assess and report on safety at all of the company's South African operations,” the company statement said.

The disaster was a blow to efforts to bring down the death toll in South Africa's gold mines, the world's deepest and among the most dangerous.

Despite safety improvements since the end of apartheid two decades ago, 112 people were still killed at South African mining operations in 2012, the last full year for which records are available. - Reuters