The Rustenburg Layered Suite

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Comparison with chemical assay values

Cr203 assay values were recalculated to mass per cent chromite, assuming an average Cr203 content in chromite of 42.86%.* Generally, there is good agreement between the chromite content measured by image analysis, and the chromite content calculated from Cr203 values, with differences smaller than 10 per cent for most samples (Table 5.1E). Most of the differences can be ascribed to plucking out of chromite grains during polishing, especially in samples containing sintered chromite on which it is difficult to obtain a good polish. Deviations of the actual Cr203 content from the average value would also lead to discrepancies.

Image analysis results

The relative amounts of the different base-metal sUlphides in the samples examined are reported in Table 5.1F and graphically depicted in Figure 26. With the exception of sample B2, samples from areas A and B are characterised by the presence of pentlandite, chalcopyrite, pyrrhotite and, to a lesser extent, pyrite. Qualitative EDS analysis indicated that Fe>Ni in pentlandite. In sample B2, pyrrhotite has been replaced by pyrite and the pentlandite is nickeliferous (Ni>Fe). Sample B2 also contains low but significant amounts of millerite, a mineral rarely encountered in samples from areas A and B. Millerite, pyrite and chalcopyrite are the major base-metal sulphide phases in samples from area C, with pentlandite (Ni>Fe) and pyrrhotite representing minor components.
Examination of polished sections of samples from area C by optical and scanningelectron microscopy also indicated the presence of occasional grains of siegenite.

Chromie textures and gain-size distrubutions

Textural features of chromite in the samples investigated are similar to those described by Hiemstra (1985), Hulbert and Von Gruenewaldt (1985), and Eales and Reynolds (1986), ranging from scattered chromite subhedra in a silicate matrix (Figure 27), to patches where chromite grains appear to have been sintered together, resulting in large polygonal chromite grains with little or no interstitial silicate (Figure 28).
Chromite grain-size measurements (Figure 29 and Table S.U) indicate that the median measured chromite grain diameters for most samples (AI, A2, Bl, B2, C2, C3, C4, CS) range between 164 and 177 /lm. Due to the sintering of chromite grains, median chromite diameters in samples A3, B3, Cl, and to a greater extent A4 and B4, are much larger, ranging between 184 and 221/lm. All of the samples in the second group were associated with iron-rich replacement pegmatoid and/or potholes. Minor fracturing of chromite grains was observed in most of the samples. Sample AS, collected from an intensely faulted area, is the only sample displaying extensive cataclasis (Figure 30). This is reflected in the small median measured chromite diameter of this sample.

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1. INTRODUCTION
1.1 Historical background
1.2 Reasons for undertaking this study
1.3 Previous work
1.4 Objectives
2. GEOLOGY AND REGIONAL SETTING
2.1 General
2.2 The Rustenburg Layered Suite
2.3 The UG2 chromitite layer
3. SAMPLING STRATEGY
3.1 Project outline
3.2 Sample collection
4. METHOD
4.1 Comminution
4.2 Froth flotation
4.3 Chemical analysis procedures
4.4 Comparison of calculated and analysed feed grades
4.5 Mineralogical techniques
4.6 Image-analysis techniques
5. RESULTS
5.1 Chemical composition
5.2 Mineralogy of crushed feed samples
5.3 Characterisation of milled feed samples
5.4 Characterisation of flotation product samples
5.5 Milling behaviour
5.6 Flotation behaviour
6. INTERPRETATION OF RESULTS
6.1 Indentification of mineralogically and chemically different types of 148 UG2 chromitite
6.2 PGE mass balance calculations
6.3 The effect of postmagmatic alteration processes on the UG2 169 chromitite.
6.4 Flotation behaviour of different types ofUG2 chromitite
6.5 Interpretation of milling and flotation results 176
6.6 Prediction ofPGE recovery based on mineralogical and chemical parameters
7. DISCUSSION
7.1 Characterisation ofPGE mineral-bearing particles
7.2 In situ trace-element analysis
7.3 Effect of geological environment on mineralogy
7.4 Relating mineralogical characteristics to recovery
7.5 Concentrate grade
8. SUMMARY AND CONCLUSIONS
8.1 Characterisation ofUG2 ore and mineral processing products
8.2 Relating variations in mineralogy to geological environment
8.3 Factors affecting the flotation response ofPGE mineral-bearing particles
8.4 Relating ore type to variations in flotation response
8.5 Relating the characteristics of PGE mineral-bearing particles to flotation parameters
8.6 Concluding remarks
9. REFERENCES

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