Geology of Thabaphaswa Mountain Sanctuary

Geology of Thabaphaswa Mountain Sanctuary



Figure 1: The Thabaphaswa granite dome dominating the sanctuary.

Regional Geology

The Mokopane area has a long history of geological change over 3 billion of years.

Archean greenstones, which can be gold-bearing, form the basement of the area with the Petersburg Group, aged at over 2.9 billion years old, outcropping just south of Thabaphaswa. The greenstones were later intruded by granites on multiple occasions spanning millions of years. In general, Archaean cratons are the oldest remaining fragments of the original continents of the earth, predominately comprising greenstones, and are commonly intruded by the crystalline granitic and gneissic rocks. The earlier granites which intruded the Thabaphaswa area were metamorphosed into gneisses (2.7-2.8 billion years) while the younger granites (2.4 – 2.7 billion years) have largely kept their original texture and composition. These cycles reflect changing stages in the formation and genetic evolution of the early crust in this region.

The Transvaal Supergroup sediments (2.5 billion years) were deposited after the extensive erosion and weathering. Some sequences of the Transvaal sediments originated from eroded granites. A large igneous, platinum bearing intrusion occurred (2.05 billion years) in the form of the Bushveld Igneous Complex and an outcrop can be seen before entering the sanctuary in the form of the dark pyroxenite hill near the sanctuary turn off the Percy Fyfe road. The mines surrounding the sanctuary are platinum mines exploiting the pyroxenites of the Bushveld Igneous Complex.

Local Geology



Figure 2: Rock types in the Thabaphaswa Mountain Sanctuary are dominated by granite and gneiss.

The Thabaphaswa Mountain Sanctuary is situated on a massive, unfoliated (no repetitive layering) granite batholith or pluton (large intrusion) which intruded the pre-existing Petersburg Greenstone Belt and Hout Rivier gneiss (a metamorphosed granite).

During Early Neoarchaean times magmatic activity caused largely post-tectonic granitoid emplacements (intrusions) in the Pietersburg and Murchison areas. The area is directly underlain by the Hout Rivier gneiss which was later intruded by the younger potassium-rich granite plutons (Uitloop and Lunsklip) which forms part of the Mashashane Granite Suite (a collection of related granite intrusions in the region).

The gneiss shows some banding and a different grain texture to the younger granites. The Mashashane Suite represents a large sheet-like intrusion (of multiple phases) with a shallow south westerly dip. These cycles reflect possible stages in the formation and genetic evolution of the early crust in these regions. The intruding granites contain xenoliths of the initial fine-grained, dark grey, tonalitic granite or greenstones ripped from the basement and entrained with the magma. The rounded domes of the mountains are the remnants of the discordant intrusions into the surrounding greenstone rocks. Neoarchaean granites generally form prominent topographic features in the area (due to their resistance to weathering) relative to the older granitoid gneisses.

The ages of these intrusions range from 2800 million years and 2650 million years. The sites of the intrusions were controlled either by active tectonic processes or by structural features in the crust (large fractures and weak zones spanning large distances).

Chemical and Mineral Properties

Granite is a felsic rock (consisting of light coloured silicate rich minerals) which originated when remelted or partially melted crustal rocks intruded into pre-existing rocks.

Silica rich minerals melt at a lower temperature and thus are nominate mineral groups in the cooler, crustal melting environments. The potassium content in the granites gives them the characteristic pink colour. The potassium is housed in K-feldspar minerals such as orthoclase. These minerals appear pink and orange. The quartz normally appears smoky white but can appear light blue. Plagioclase feldspar is not easily identified due to its low proportion in the rock but it is generally white and grey but is not visible as it occurs

Primary minerals groups:

K-feldspar minerals - KAlSi3O8 - Pink and orange

Quartz - SiO2 - White and light blue

Plagioclase feldspar - Al2Si2O8 - White and grey

Accessory minerals:

Biotite - K(Mg,Fe)3(Al,Fe)Si3O10(OH)2 - Purple and black sheets

Hornblende - (Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2 - Green

Garnet - Fe3Al2(SiO4)3 - Purple crystals



Figure 3: Dark purple garnet and green hornblende in a medium grained granite (S 24°03.925', E 029°02.248')

Textural Properties

The main texture of the rocks can be described as medium to coarse grained with a cumulus (assemblage of visually identifiable well-fitting crystals) texture.

In some areas, phorphoritic texture scan be seen where larger minerals (of K-feldspar and quartz) are surrounded by a fine-grained matrix. In the core of the granite body, a coarse-grained pinkish to pinkish-grey rock occurs (Figure 4) called the Lunsklip Granite.

Around the central phase is a younger, grey to pinkish grey, medium- to coarse-grained leucocratic granite of the Uitloop Granite composed of orthoclase and quartz with minor plagioclase, biotite and hornblende (Figure 5).

The Uitloop granite forms the prominent topographic features in the sanctuary. Clusters of garnets and radiating hornblende occur in this medium-grained granite (Figure 3). Contacts between the two granite types are hard to spot and can be sharp contacts (Figure 4) or gradational as the older granite would be slightly remelted when the younger granite intruded.



Figure 4: A sharp contact between fine grained and course grained granite.

Xenoliths

Xenoliths are older fragments of rock which are broken off a pre-existing rock when moving magma erodes a rock surface. Xenoliths of dark grey, mafic rich rock from the basement greenstones can be seen at the sanctuary. Keep a look out for these small fragments.



Figure 5: A rounded mafic xenolith (foreign rock) within a coarse grained granite. Probably a piece of greenstone ripped off the original rock in the area when the granite intruded.

Dolerite Dykes and Sills

Dykes and sills of Karoo Supergroup age (180 million years) are present to the north of Thabaphaswa in the Witvinger reserve. Keep your eyes open for this distinct rock type called dolerite which is a very common dyke around the country. Dykes make sharp contacts with the surrounding rocks as they intruded once the surrounding rocks had already been emplaced and crystallised. Their dark appearance is due to Fe-rich pyroxene crystals. There may be some in the sanctuary!



Figure 6: Dolerite intrusion showing dark mafic (pyroxene – iron-rich minerals) and plagioclase clasts (white minerals).

Veins

Joints (cracks) originate due to regional pressure and rock contraction when cooling. Silica rich liquid enters the spaces and solidifies creating quartz veins. Quartz crystal can grow if given enough space.



Figure 7: Quartz veins in granite which fill parallel joint spaces. (S 24°02.279', E 029°02.300')



Figure 8: Aplite veins (S 24°02.786', E 029°02.576')

Soils

The geology of the area controls the type and proportion of minerals present in the soil. The soil in turn controls the type and abundance of plant and animal species.

Since granite is a coarse, crystalline rock it breaks down fairly slowly. The quartz-rich material tends to produce nutrient poor, acidic soils. Rounded grains of quartz can be found in the river beds.

Conclusion

By understanding the local geology, we have a deeper perception how the puzzle pieces of the natural world fit together.

We can then understand how certain plant and animal species have adapted to differing areas of the country.

There are still undiscovered secrets in these mountains!

Jason Germiquet – Geologist

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