The spread of the coronavirus across the world has focused metals markets on the risks to demand, causing prices to plummet, but so far investors have largely ignored the mounting threats to supply.

A hint of what may be coming was SA’s decision on Monday to impose a 21-day nationwide lockdown to try to contain the epidemic, a move that will affect the nation’s mines.

SA is the world’s largest producer of platinum, the second-largest of palladium and is also a major exporter of thermal coal, iron ore and gold...