by Peter Rosewarne

With the lockdown in force it’s given me some time to revisit my passion for igneous rocks and their minerals, being what we used to call a ‘min and pet’ man whilst studying geology at Kingston University back in the early 70s. Of particular interest to me on the local scene are the Bushveld Igneous Complex, the Pilanesburg Alkaline Complex, kimberlites, ultramafics and the Vredefort Dome.  In my quest to find specimens of the ‘type’ rocks from these sites, in addition to what I already have, I contacted a number of people and Bruce Cairncross put me onto someone who had polished spheres and rock specimens from all of the above except for Pilanesburg. I recently bought a few (excellent service from Postnet from J’burg to Sea Point in two days) so I thought I’d share with you some information and insights gained from reading up on these rocks and some photographs of the specimens. Key references for additional information gained are included at the end of this article. Hopefully some of this is as new and interesting to you, as it was to me. The various rock types and spheres with localities are discussed below, in no particular order.

Kimberlites

I did groundwater studies for Murowa Diamond Mine in southern Zimbabwe and Jwaneng Diamond Mine in southern Botswana many years ago, not to mention an obligatory visit to the Big Hole in Kimberley, and so have some first-hand experience of kimberlite occurrences. 


Figure 1

Kimberlite is an ultramafic rock of very unusual composition and one of the few rock types named after a South African locality. It basically comprises a fine-grained medium to dark grey groundmass of an eclectic array of minerals including chlorite, olivine, apatite, monticellite, calcite and serpentine, typically with phenocrysts of olivine, phlogopite and ilmenite, often with reaction rims, and more interestingly from a petrologist’s point of view, xenocrysts and xenoliths of exotic minerals and rocks. The latter include eclogite and peridotite, and the former, diamonds and garnets, all derived from the mantle. A section through a typical kimberlite pipe is shown in Fig. 1 (Viljoen, 1999, with permission), with current erosion levels indicated of some of the main diamond-bearing kimberlite pipes in South Africa and Botswana. Examples of kimberlite from the Rovic (Roberts Victor Mine), near Kimberley (specimen) and Kaalvallei near Welkom (specimen) diamond mines are shown in Figs. 2 and 3.
                             
Figure 2. Rovik Kimberlite                            Figure 3. Kimberlite. Kaalvallei diamond mine

A specimen of the ‘blue ground’ kimberlite from the Big Hole, collected years ago from a pile of rocks in the tourist centre, is shown in Fig. 4. The Rovic example appears to be of the breccia type with chunks of ‘sedimentary-looking’ rock prominent. The Kaalvallei and Big Hole specimens seem to me to be more typical kimberlites with an appearance approximating that described above, with what appears to be olivine phenocrysts being prominent.

Most diamond-bearing kimberlites in South Africa are relatively young geologically speaking and of Jurassic to Cretaceous age, c. 80 to 60 million years (Ma) old, exceptions being the Cullinan and Venetia pipes at 1 200 and 520 Ma, respectively. The former produced nearly 300 gem diamonds each >100 carats in the 70-year period up to 1995 and a quarter of the world’s recently (again up to 1995) gem diamonds >400 carats (Cairncross and Dixon, 1995).



Figure 4. Kimberlite. Big Hole, Kimberley

The latter is located in the Limpopo Mobile Belt and therefore apparently ‘off’ the cratonic core area for diamondiferous kimberlites (see bullet points below) but this belt was apparently part of the Kaapvaal/Zimbabwe cratons in the early part of the Earth’s geological history.

Some interesting facts about rocks and minerals found in and associated with kimberlites include:

  • ·         They are characterised by xenoliths of exotic rocks derived from the mantle including peridotite and eclogite, the former consisting mainly of iron-rich olivine and the latter green omphacitic (calcium-sodium) pyroxene and dark red pyrope garnet, a very attractive rock when fresh;
  • ·         Diamondiferous kimberlites are almost exclusively limited to those associated with old stable cratons, such as the Kaapvaal Craton – Clifford’s Rule (Norman, 2010); areas of early crust formation that haven’t been subjected to processes such as metamorphism and deformation. The reason for this is that the crust is thickened beneath such areas and deep-rooted within the mantle, where temperatures and pressures are optimum for diamond formation and preservation, instead of the carbon being converted into graphite. 
  • ·         Diamonds are not a primary constituent of kimberlite; they were ripped out of the formations that the explosive kimberlite magma travelled through at depths of c.150 to 200 km in the mantle (the diamond pressure and temperature stability field), on its way to the surface. They are therefore xenocrysts;
  • ·         So-called G10 pyrope garnets, which are calcium-deficient and chrome-rich, are markers for diamondiferous kimberlite pipes. Find these in your sediment sampling and you can be pretty sure that there is a diamond-bearing kimberlite pipe somewhere upstream. This ground-breaking geochemical insight was discovered by the late John Gurney (Norman, 2010).

Eclogite

This rock is composed largely of omphacitic pyroxene and pyrope garnet, with some unusual accessory minerals sometimes being present including chrome diopside, kyanite, spinel and, exceptionally (and surprisingly), quartz. A sphere of eclogite from the Rovic Diamond Mine is shown in Fig. 5 and a rough sample from a xenolith in Fig. 6. Eclogite of this type is theorised to be formed by the melting of oceanic basaltic crust and mantle as it is subducted at continental plate boundaries (Norman, 2010). It’s one of my favourite rock types although the Rovic example is a bit dull. Watch this space for a future article including a nice eclogite slab with kyanite. 

            
Figure 5. Eclogite. Rovic Mine                                      Figure 6. Eclogite

Komatiite

Komatiite is another rock type named after a South African locality, the Komati River in the Barberton Greenstone Belt (Viljoen, 1999) where it was first recognised. It is a magnesium-rich ultramafic lava with a unique ‘spinifex’ texture of elongated olivine and pyroxene crystals. It is thought to represent recycled oceanic crust from Archean times (>2 500 Ma) and formed when the temperature of the earth’s interior/mantle must have been higher, based on melting experiments, and considerably hotter than the highest temperatures recorded for basaltic lava, which is of the order of 1 200 oC (McCarthy and Rubidge 2005). A sphere is shown in Fig. 7 and a hand specimen in Fig. 8. The ‘spinifex’ texture is caused by the rapid crystallisation of a magnesium-rich magma.

       
Figure 7. Komatiite sphere                                     Figure 8. Komatiite

Hydrogrossular/chromite

Outcrops of pink, green and grey hydrogrossular occur in the Rustenburg area and its visual resemblance to jade and physical properties led it to being called Transvaal Jade. It was formed by the alteration of plagioclase feldspar in anorthosites of the Bushveld Complex, possibly by dyke intrusions, often leaving behind bands and stringers of black chromitite. An example of the green variety with chromite bands is shown in Fig. 9. The green colouration is due to the presence of chromium and the pink colouration to the presence of manganese (Cairncross, 1995).


Figure 9.  Hydrogrossular

Pseudotachylite breccia

This rock type gets its name from its similarity to the basaltic volcanic glass called tachylite or tachilite. In this case the black glassy material is theorised to have been formed by the impact of a c.10 km diameter meteorite about 2 020 Ma, which gave rise to the Vredefort Dome (details on its presumed formation are beyond the scope of this article), a World Heritage site. This is the oldest known and one of the largest impact structures on Earth. Fragments of the surrounding Parys granite/gneiss are embedded in the glass, as shown on a small scale in the sphere in Fig. 10 and also on a larger scale in a face of one of the many granite quarries in the Parys area in Fig. 11. 

              
        Figure 10. Pseudotachylite, Vredefort Dome         Figure 11 Parys quarry face with pseudotachylite 

Anyolite

Any what? Basically, a very attractive ruby-zoisite metamorphic rock with dark amphibolite originating from Tanzania. This particular sphere is still in transit from California to me thanks to the SAPO as it was ordered/purchased about four months ago. Along with my copies of the Mineralogical Record, anything coming from the USA seems to be on hold because their information is that the SAPO is not functioning. This is a beautiful combination of green zoisite (tanzanite is a blue gem variety of zoisite), red ruby and the amphibole pargasite (part of the hornblende series). It looks black but is apparently dark green in this rock. To my mind, this is the second-most attractive rock in hand specimen after eclogite/garnet peridotite and I couldn’t resist it (Fig. 12). The name is apparently derived from the Massai word anyoli meaning “green”.   

Figure 12. Anyolite

‘Troutstone’

I’ve sneaked this one in as it wasn’t part of the “recently acquired” material but it is interesting, I think. I came across it when finding places in my specimen drawers for the new pieces. Troutstone is a term applied to a distinctive type of troctolite, a type of gabbro, comprising of grey plagioclase studded with black olivines, visually resembling the dorsal colouration of a trout. I picked up this piece/pebble north of Swakopmund many years ago and had it cut and polished by a contact at UCT (Fig.13).

Figure 13. Troutstone

That ends this brief tour around some unusual rock types and thanks for making it this far. Back to books and existing specimens until the lockdown rules change and some of these and other interesting sites can be visited again from Cape Town. This statement might be redundant by the time you read this.

References


Cairncross, B. and Dixon, R., 1995. Minerals of South Africa. Johannesburg: Geological Society of South Africa.

Norman, N., 2010. The Extraordinary World of Diamonds. Johannesburg: Jacana.

McCarthy, T. and Rubidge, B., 2005. The Story of Earth and Life. Cape Town: Struik.

Viljoen, M. and R. W., 1999. An Introduction to South Africa's Geological and Mining Heritage. Johannesburg: Mintek.