The U.S. Geological Survey said the tremor was centred in Orkney, a town around 120 km (70 miles) southwest of Johannesburg, an area with a high concentration of deep gold mines.
"One of the buildings collapsed on a man believed to be in his 30s. By the time paramedics arrived there was nothing they could do for him," Werner Vermaak, spokesman for emergency service provider ER24, told local television.
"I have since been alerted to mines in the local area where they have received reports of various miners trapped," Vermaak added, without giving any figures.
Officials at AngloGold Ashanti, Harmony Gold, Gold Fields and Sibanye Gold said they had felt the tremors in their headquarters but had so far received no reports of anything untoward in their mines.
The area around Johannesburg is not prone to seismic activity but it is home to some of the deepest gold mines in the world. The quake is the largest in the southern Africa region since a 7.0 tremor in Zimbabwe in 2006.