With gold output in South Africa mired in a multiyear decline, the country’s biggest producer is turning to an overlooked and cheaper source of supply: the dump.

About 6.5 million ounces valued at $8.5 billion is buried inside mammoth mounds of discarded mine waste at sites owned by Sibanye Gold Ltd. They’re leftovers from more than five decades of excavation west of Johannesburg. But there’s a catch. The traces are so small that the company will process rubble heavier than the Statue of Liberty just to retrieve an egg’s weight in gold. And the investment could take 18 years to pay off.