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GEOLOGY OF THE PLATREEF ON TOWNLANDS
The farm Townlands hosts the town of Mokopane, formerly Potgietersrus (Fig. 4.1). A drillcore intersecting part of the Platreef has been investigated by Manyeruke (2003) and Manyeruke et al. (2005). Since the data generated during these studies will be compared to the data from Nonnenwerth, and since I have generated additional trace element data from Townlands during the present investigation, a short revision of the stratigraphy of the Platreef on Townlands is necessary. The examined drillcore comes from a borehole collared some 2 km to the NE of Mokopane. A simplified stratigraphic column of the Platreef and its floor rocks in the borehole is given in Fig. 4.2. At this locality, the floor rocks of the Platreef consist of hornfels, quartzite and calc-silicates probably belonging to the early Proterozoic Silverton Formation of the Pretoria Group, Transvaal Supergroup. The hornfels possibly formed by heating of the sedimentary floor rocks by hot Platreef magma. The sedimentary rocks are locally layered on a millimeter to centimeter scale with the layering defined by thin (1-2 mm), dark bands. They are intruded by numerous sill-like bodies of pyroxenites which show internal variation in grain size, from fine-grained margins to medium-grained central portions. The widths of the sills range from a few centimeters to several meters. The contacts between the pyroxenite sills and the sedimentary rocks are sharp and may be defined by thin (< 2 mm) reaction rims. Notably, no sills were observed within the Platreef. This may be a coincidence or could indicate that the sills are older than the Platreef. More information from other borehole intersections is necessary to constrain this question. In general terms, the Platreef on Townlands consists of medium-grained gabbronorite and olivine melagabbronorite. Manyeruke (2003) distinguished three distinct units, a Lower, Middle and Upper Platreef (Fig. 4.2). The individual units are separated by thick (several 10s of meters) intervals consisting of hornfels and fine grained pyroxenite sills.
The Lower Platreef is noritic to gabbronoritic in composition, with minor pyroxenitic domains. The contact between the Lower Platreef and the quartzitic floor rocks is formed by a hybrid zone which consists of highly altered metasedimentary rocks apparently injected by medium grained, non-mineralized pyroxenite. Alternatively, the hybrid rock may represent pyroxenite containing a dense load of sedimentary xenoliths. A clear distinction between the two possibilities is not possible in the
borehole core.
PETROGRAPHY
This work follows the international standard nomenclature (IUGS) in naming the different Platreef rock types. It should be noted that new lithologies not described in chapter 3 (Geology of the Platreef on Nonnenwerth), e.g. recrystallized gabbronorite will be discussed in this chapter. It was not possible to pick up such lithologies on hand specimen as these rocks are relatively thin and occur at the interface between two different lithologies probably as a secondary feature i.e. recrystallization.
Gabbronorite
Gabbronorites are the most abundant rocks at Nonnenwerth and constitute about 80 – 85 % of the Platreef. The gabbronorites are mostly medium grained, but finer grained varieties may occur in places. Plagioclase (48 – 65 modal %) occurs as subhedral, equant to prismatic, poorly twinned crystals (up to 4 mm) and as an interstitial phase. The crystals show evidence of minor deformation in the form of bent lamellae (Fig. 5.1a). Slight deuteric alteration to sericite (particularly along cracks and fractures) and to patchy, fine grained, brown clays is common (Figs. 5.1b, c and d). Orthopyroxene (32 – 48 modal %) forms subhedral granular crystals and anhedral poikilitic crystals (up to 1.5 mm) enclosing anhedral plagioclase crystals that are mostly < 0.3 mm in size (Fig. 5.1e). Orthopyroxene typically contains irregular exsolution lamellae and blebs of clinopyroxene. Minor to moderate alteration of orthopyroxene to uralite along cleavage planes (Figs. 5.1b and c) and along grain margins to amphibole, biotite and oxides is common. Clinopyroxene (5 – 10 modal %) forms anhedral and subhedral grains, reaching up to 7 mm in size that may form simple twins. Orthopyroxene exsolution lamellae are developed parallel to prismatic cleavages. They may coalesce to form blebs that are mostly altered to amphibole and sericite, with iron oxides forming along the cleavage planes. Clinopyroxene is commonly replaced by hydrothermal veinlets filled with
uralite. Inverted pigeonite (15 – 20 modal %) is developed in places. This forms poikilitic, anhedral or subhedral crystals that enclose corroded plagioclase and, occasionally, clinopyroxene and orthopyroxene. Inverted pigeonite is characterised by thick, bleblike lamellae of exsolved augite. Occasionally, two sets of augite exsolution lamellae are developed with the second set at about 740 to (100) in the zone of (010). Minor phases are biotite, magnetite and sulphides. Biotite occurs around altered orthopyroxene where the latter is in contact with plagioclase or as flakes in orthopyroxene metamorphosed to amphibolite facies grade. The sulphides (< 1 modal %) consist of pyrrhotite, pentlandite, chalcopyrite and pyrite. Chalcopyrite and pyrrhotite occur intergrown in small interstitial blebs, or as fine disseminations in orthopyroxene altered to amphibole and chlorite. Pyrite forms veinlets, predominantly in plagioclase. Magnetite is interstitial and tends to be associated with the sulphides. In general, the rocks are poorly mineralized.
1. INTRODUCTION
1.1. Statement of the problem
1.2. Aims of the study
1.3. Previous Work
1.4. Methodology
2. OVERVIEW OF THE BUSHVELD COMPLEX
2.1. General
2.2 General geology of the Platreef
3. GEOLOGY OF THE PLATREEF ON NONNENWERTH
3.1. General
3.2. Borehole 2121
3.3. Borehole 2199
4. GEOLOGY OF THE PLATREEF ON TOWNLANDS
5. PETROGRAPHY
5.1. Platreef
5.2. Main Zone
6. WHOLE ROCK CHEMISTRY
6.1. CIPW Norms
6.2. Lithophile geochemistry
6.3. Concentrations of sulphur and chalcophile elements
6.4. Summary
7. COMPOSITION OF THE SILICATE MINERALS AT NONNENWERTH
7.1. Plagioclase
7.2. Orthopyroxene
7.3. Clinopyroxene
7.4 Summary
8. OCCURRENCE, DESCRIPTION AND CHEMICAL COMPOSITION OF THE OPAQUE MINERALS
8.1. Sulphide and oxide minerals on Nonnenwerth
8.2. Sulphide minerals and oxides on Townlands
8.3. Compositions of major base metal sulphides and spinel
8.4. Summary and discussion
9. Platinum-group minerals (PGM), tellurides and trace minerals
9.1. Nonnenwerth
9.2. Townlands
9.3. Summary and discussion
10. S-ISOTOPE GEOCHEMISTRY
10.1. Summary
11. O-ISOTOPE GEOCHEMISTRY
11.1. Summary
12. DISCUSSION AND CONCLUSIONS
12.1. Compositional and lithological variation of the Platreef in the northern lobe
12.2. Magmatic Lineage of the Platreef
12.3. Origin of the mineralization
ACKNOWLEDGEMENTS
REFERENCES
