Stellenbosch University PhD graduate Dr Steve Chingwaru has uncovered what is potentially the world’s largest invisible gold resource.

The geometallurgist’s findings could help unlock gold to the value of R450-billion “hiding in plain sight” in mine dumps around Johannesburg.

Historical mine waste from the Witwatersrand, called tailings, contains over six-billion tons of material with significant gold content, Chingwaru explains.

The research for his master's degree, which was upgraded to a PhD, aimed to calculate and characterise these gold reserves. He also explored ways to extract the gold efficiently while addressing environmental concerns related to the tailings, such as the release of acid mine drainage (AMD) owing to pyrite oxidation.

Invisible gold refers to minuscule particles locked inside other minerals, and Chingwaru is the first scholar to calculate that the six-billion tons of tailings around Johannesburg's mines contain up to 460 t of gold.

“Historically, the low concentration of gold inside tailings was considered too low-grade to be of value. But now that extensive mining has depleted most of the high-grade concentration of gold, it's becoming unfeasible to mine – some shafts are already reaching 4 km underground. Looking for gold in low-concentration sources is becoming more viable," Chingwaru notes.

Some big mining companies have started to process the tailings to extract the leftover gold, but the traditional way of extraction through cyanide is not very effective and also damaging to the environment, Chingwaru points out.

“Typically, they manage to extract just 30% of the gold through this process. So, in my PhD research, I asked where the remaining 70% is and how it can be safely removed from the pyrite,” he says.

“When sulphides become oxidised, they produce sulphuric acid, and when that goes into the groundwater, it increases the mobility of several toxic elements. It's a big problem in some parts of Johannesburg where they're scared that their groundwater is becoming polluted by tailings-related AMD. That's why I'm passionate about highlighting the economic potential, as well as the environmental benefits of reprocessing tailings dumps efficiently.

“If you process the pyrite, you are taking out the key cause of AMD, plus you're getting economic value from it. The process has the potential to recover additional valuable by-products such as copper, cobalt and nickel, and reduce or even eliminate the heavy metal pollution and AMD associated with tailings dumping," Chingwaru adds.