Abstract
The Merensky Reef's economic base metal sulphide and platinum-group mineralisation is not
confined to the pyroxenite or pegmatoidal reef, but occurs, to varying extents, with the
overlying pyroxenite hanging wall and the underlying leuconoritic to anorthositic footwall.
Thus, it is standard practice for mining companies to extract a mining cut of about 1 m thick
that includes the Merensky Reef's hanging wall, reef, and footwall lithologies. Previous
minerals processing work on the Merensky Reef has not fully addressed the individual host
rock lithologies' impact within the mining cut. It is essential to understand the variable nature
and effect of the host rock lithologies to determine optimum processing conditions. This study
aimed to qualitatively and quantitatively investigate and compare the effect of the different
host rocks' characteristics on the ore mineralogy and mineral processing performance of the
Merensky Reef on the Bushveld Igneous Complex's western limb at Impala Platinum's
Rustenburg Shaft 16. A process mineralogical approach was followed to achieve the study's
aims by conducting the following procedures on the individual host rock lithology samples
collected from drill core or underground channel samples: petrographic examinations; 3D
microtomographic analyses; laboratory-scale grind tests; benchtop flotation tests; and
chemical and mineral liberation analyser analyses of the flotation feeds and concentrates.
Petrographic and 3D microtomographic analyses indicated that fine, interlocking cumulus
plagioclase feldspar laths in the footwall lithologies resulted in small, isolated interstitial spaces
for the sulphide liquid to fill during formation, which led to the mineralisation of finer base metal
sulphides. In contrast, coarser, euhedral cumulus pyroxene grains in the hanging wall and reef
lithologies allowed for larger, interconnected interstitial spaces into which the sulphide liquid
could fill, resulting in coarser base metal sulphide mineralisation. A downer mineralisation
model is further supported by the gravitationally influenced morphology of the base metal
sulphides and platinum-group minerals in 3D space. The 3D microtomographic analyses and
chemical assays revealed that the highest platinum-group mineral or 4E grade (sum of Pt, Pd,
Rh and Au) to base metal sulphides or sulphur content ratios are located in the stratigraphically
lower lithologies (the pegmatoidal reef and footwall, and the lowest ratios are located in the
stratigraphically higher lithologies (the pyroxenite reef and hanging wall). This points to more
than one sulphide immiscibility event during formation whereby platinum-group elements were
picked up by the earlier immiscible sulphide liquid and allowed to trickle down further through
the cumulate pile than the subsequent platinum-group element-depleted sulphide liquids. The laboratory-scale milling tests revealed that the finer lithologies with more plagioclase feldspar (footwall and pyroxenite reef) have shorter milling times than the coarser pyroxenedominated
lithologies (hanging wall and pegmatoidal reef). This can be attributed to the efficient breakage along the grain boundaries of finer grains of the footwall and pyroxenite reef’s samples in comparison to the coarser grains of the pegmatoidal and pyroxenite reef samples. From the single cell, rougher flotation tests, it was observed that higher cumulative base metal sulphide and platinum-group mineral recoveries are achieved in the hanging wall and reef samples, which comprise coarser sulphides. These sulphide minerals are located on
the grain boundaries of equigranular cumulus pyroxene, which potentially improves sulphide
liberation. In contrast, the footwall samples, which have marginally lower cumulative
recoveries, contain fine sulphides, interstitially located with interlocking cumulus plagioclase
feldspar laths, which potentially liberates poorly during comminution.
Mineralogical data from the mineral liberation analyser of the flotation feed and concentrate
samples revealed that, in all lithologies, the most abundant base metal sulphide is pyrrhotite,
followed by pentlandite, chalcopyrite, and pyrite. In order of decreasing floatability, the base
metal sulphides rank chalcopyrite > pentlandite > pyrrhotite. The main platinum-group element
groups in the concentrate samples are, in order of decreasing abundance, sulphides,
bismuthotellurides, alloys, arsenides, gold, tellurides, and antimonides. Coarse platinumgroup
minerals that are liberated and those that are locked within or are associated with base
metal sulphides have high degrees of floatability. The mineralogical results also revealed that
lithologies with higher feed contents of alteration silicates and orthopyroxene, i.e., the hanging
wall and reef samples, have the lowest relative flotation efficiencies due to concentrate gangue
dilution.
This study has shown through a combination of μXCT scans and separate lithology processing
that the main factors that influence the mineral processing performance of the Merensky Reef
host rock lithologies include the liberation of base metal sulphides and platinum-group
minerals determined by its in-situ texture and the cumulate silicate phase; as well as the
dilution of flotation concentrates by gangue minerals, specifically orthopyroxene, which is
determined by its modal abundance in the host rock. In industry practice, the processing plants
could use these results to better prepare for the incoming ore types with mining cuts composed
of different proportions of host rock lithologies. Alternatively, the mining cuts can be adjusted
to improve metallurgical processing efficiency and cost.