Unearthing hidden Gold: The potential of refractory ores of the Vaal and Crystalkop reefs in the Witwatersrand Basin, South Africa

Abstract The Witwatersrand Basin, accounting for one-third of all gold ever mined, hosts both free milling and refractory ores, with the Vaal and Crystalkop reefs at Kopanang Gold Mine serving as examples of the latter. Traditionally, gold ores from the basin are classified as free milling, with cyanidation providing high recovery rates. Achieving higher recovery rates of gold ore is crucial for maximising resource utilisation, reducing waste, and enhancing the profitability and sustainability of gold mining in South Africa amidst declining ore grades. Refractory gold in reefs can be trapped within sulphides or silicates, necessitating additional processing steps. This study investigates the mineralogy of these reefs, emphasising their refractory nature and the challenges they pose to gold recovery. Detailed mineralogical and geochemical analyses were conducted using optical petrography under reflected light, X-ray diffraction (XRD), mineral liberation analysis (MLA), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), and X-ray fluorescence (XRF). The Vaal and Crystalkop reefs predominantly host native gold associated with pyrite and pyrrhotite, with the Vaal Reef exhibiting a notably higher association with sulphides compared to the Crystalkop Reef. Both reefs also contain a relatively high proportion of liberated gold. Leaching tests revealed that, although the final 24-hour gold recoveries were similar at 82.1% for the Vaal Reef and 82.8% for the Crystalkop Reef, the Crystalkop Reef demonstrated a more gradual and stable recovery curve over time. This suggests that gold in the Crystalkop Reef is released more predictably during cyanidation, indicating more favourable leaching kinetics and potentially less variability due to mineralogical locking. The findings demonstrate that most of the inaccessible gold within the Vaal Reef ore is associated with sulphide minerals, while the Crystalkop Reef’s silicate-locked gold presents additional challenges for recovery. Despite the higher gold grade in the profile of the Crystalkop Reef, its moderate recovery rate is attributed to the difficulty of leaching silicate-bound gold. The study confirms the refractory nature of these ores and the need for tailored processing methods to improve recovery to >90%. When comparing the gold recovery rates and grades from the current study to those of a previous study on ores from both reefs, significant regional variability is evident in both parameters. This variability highlights the limitations of generalisations and underscores the need for further research on reefs from the basin. These results highlight the importance of understanding ore-specific mineralogy to optimise metallurgical processes, offering insights into improving recovery rates for refractory gold ores in the Klerksdorp goldfield and the Witwatersrand Basin.