Process mineralogical comparison of the Vaal and Crystalkop Reefs

Abstract

The mining of, and demand for gold has over the years gained traction ever since its discovery in alluvial deposits. As much as the chemical processes of refractory ores have improved since then, there is still some gold that gets lost to tailings. Kopanang Gold Mine is one of the underground Vaal River operations located on the boundary between the North-West and Free State Province. The mine is exploiting gold and associated uranium from two quartz-pebble conglomerates known as the Vaal and the Crystalkop Reefs.

Process mineralogical assessment of the Vaal Reef and the Crystalkop Reef at Kopanang Gold Mine can provide necessary information to assist with improving the gold recovery. This was done using a comparative interdisciplinary approach. In order to investigate this in more detail a series of channel samples were collected from two stope panels (59. DW14.R1. P.2 and 42.BW.26.RSE.P9. E). This study involved the mineralogical, chemical and metallurgical characterisation of the two ores through petrographical studies, automated mineralogical analyses and laboratory based extractive metallurgical test work.

Mineralogical characterisation revealed that both reefs were characterised by very fine gold particles, with about 80% of the gold measuring less than 50 µm. Furthermore, mineralogy showed that there is some percentage of gold that is locked within silicate minerals. Gold particles associated with pyrite are hosted within the matrix of the quartz-pebble conglomerating reefs. Chemical assay revealed that the Crystalkop Reef (156.4 g\t) is characterised by higher grades when compared to the Vaal Reef (77.0 g/t).

Laboratory based leaching investigation conducted on the Composite Reef (with a grade of 85.8 g/t) using AGA plant conditions at 4, 8 16 and 24-hours leach residence time intervals, revealed significant difference between forward and reverse leaching methods. With reverse leaching (76.0 to 84.3%) yielding higher gold recoveries than forward leaching (28.3 to 55.7%). Diagnostic leach test work results demonstrate that 87.0% (74.6 g/t) of gold can be recovered through direct cyanidation and 2.5% (2.2 g/t) of the gold remains occluded within silicate minerals, which is susceptible to loss.

The study concluded that although the two reefs are approximately 270 m apart, they are mineralogically and chemically distinguishable. The study further concluded that mineralogy has broader implications for gold recovery. It was also concluded that the Vaal and Crystalkop Reefs can be reclassified as locally refractory at the Kopanang Gold Mine.