all 19 comments

[–]Gargatua13013Regional geology and structural geology 53 points54 points  (5 children)

Your observation hits upon a half-truth: Yes, the RSA is a mineral rich territory. Yet, it is not alone, and its competitors share similar wealth for the same reasons.

This relative richness has to be put in context, and when one does so, it turns out that the RSA is neither richer nor poorer than other similar terranes.

What matters here is the notion of "similar terranes", and those are what us geologists call "Cratons" (well ... we do that ...). The key to the RSAs mineral wealth is the Kaapvaal craton, an assemblage of (mostly) Archean meta-intrusive and meta-volcanic rocks 3.6 to 2.5 billion years old. For a wide variety of reasons, rocks of that age and nature have experienced a number of ore-deposit generating processes, many of which operate differently, or even not all, today. This ties into the concept of metallogenic epochs, where different ore deposits are formed as the Earth, and the processes operating there, change through time. For instance, the appearance of free oxygen in abundance in the atmosphere during the Proterozoic (and hydrosphere) had tremendous consequences for the formation of iron and uranium deposits as it changed the chemistry of surface waters. In the specific case of the Kaapvaal, one may find there:

and others.

However, the Kaapvaal in pretty much on par with other cratons of that age range. Compare it for instance with the Superior province of the Canadian Shield, which is one of the richest areas on the globe in terms of known mineral resources and past production, with similar types of deposits (or, if you can access it, see: Card, K. D., Poulsen, K. H., & Lucas, S. B. (1998). Geology and mineral deposits of the Superior Province of the Canadian Shield. Geology of the Precambrian Superior and Grenville Provinces and Precambrian Fossils in North America, Geology of North America, 1, 13-204.)

The same can be said of several other cratons. I'll grant you however that some of these may have been mined out as they are closer to long populated areas and were suject to a protracted history of production (the Fennoscandian shield for instance). But many of those are the treasure chests on which we currently rely for a vary large part of the mienral resources we currently depend on.

[–]arcosapphire 6 points7 points  (3 children)

Can you explain why the Kaapvaal and Pilbara cratons are said to be the oldest pristine cratons, while the oldest rocks are in the Canadian shield which is considered to be from a later period? I assume the key word is "pristine"--like the Canadian shield got messed around with, but still contains small regions of the oldest untouched rocks.

[–]ShibbyWhoKnew 3 points4 points  (2 children)

I found a bit of info.

"The oldest rock formation is, depending on the latest research, either part of the Isua Greenstone Belt, Narryer Gneiss Terrane, Nuvvuagittuq greenstone belt, or the Acasta Gneiss (on the Slave craton). The difficulty in assigning the title to one particular block of gneiss is that the gneisses are all extremely deformed, and the oldest rock may be represented by only one streak of minerals in a mylonite, representing a layer of sediment or an old dike. This may be difficult to find or map; hence, the oldest dates yet resolved are as much generated by luck in sampling as by understanding the rocks themselves.

It is thus premature to claim that any of these rocks, or indeed that of other formations of Hadean gneisses, is the oldest formations or rocks on Earth; doubtless, new analyses will continue to change our conceptions of the structure and nature of these ancient continental fragments.

Nevertheless, the oldest cratons on Earth include the Kaapvaal craton, the Western Gneiss Terrane of the Yilgarn craton (~2.9 – >3.2 Ga), the Pilbara Craton (~3.4 Ga), and portions of the Canadian Shield (~2.4 – >3.6 Ga). Parts of the poorly studied Dharwar craton in India are greater than 3.0 Ga. The oldest dated rocks of the Baltic Shield are 3.5 Ga old."

I believe that goes pretty in line with what you said.

[–]arcosapphire 3 points4 points  (1 child)

Thanks for your help. It does seem like the gneiss areas being the result of major deformation is the key differentiator. Extrapolating, the two previously-mentioned cratons have, I imagine, not been subject to any significant deformation over time and are thus considered "pristine", and similar in structure to how they were billions of years ago. Which makes them "older" by a larger number of criteria than we could claim about the Canadian gneiss regions, even if the gneiss may be older if considering only a limited amount of criteria.

[–]ShibbyWhoKnew 1 point2 points  (0 children)

I do agree that that is the key with those two. Pristine in this context meaning no major deformations have affected the craton. In the case of the Kaapvaal it was laid over with deposits that themselves may have changed or deformed but the craton remained "pristine".

[–]Chlorophilia 3 points4 points  (0 children)

Well, a lot of the PGEs are from the Bushveld Complex, which is a truly extraordinary layered intrusion. The fact that it was intruded into an already ancient Craton has given it long-term stability (and might have played a role in the intrusion's evolution, although I'm not an expert on this).

Diamonds are also associated with cratons as they tend to form at the roots of Archaean cratons (the Kaapvaal Craton in the case of South Africa) and are later brought up in explosive events.

But there are many cratons all over the world - in terms of why the Kaapvaal Craton has been so particularly productive, I'm honestly not sure.

[–]NormalCriticism 3 points4 points  (3 children)

Many of them are there because the underlying rock is very old and minerals have had time to move around to places where we can mine them. There are plenty of places in South Africa with proterozoic rocks and rocks older than 2.5 billion years... Many of the minerals we care about accumulate in deposits we can mine because water slowly picks it up and redeposits it other places. This is a little like the lime-scale on your water faucets at home but it can happen to gold!

The other one is due to a structure called kimberlites and they are famously rich in diamond. Kimberlites are reasonably rare and South Africa happens to have them.

[–]hmspain 0 points1 point  (2 children)

The other one is due to a structure called kimberlites and they are famously rich in diamond. Kimberlites are reasonably rare and South Africa happens to have them.

Would it be logical to find similar structures in South America?

[–]furtiveglans 5 points6 points  (0 children)

South African here (not a geologist). Africa is an old continent with much of the continental mass still exposed i.e. not buried under a kilometre of ancient seabed. South Africa has been exposed to European interests for a long time so we've had a number of geologists and miners exploring and exploiting. We also have reasonable infrastructure, industry and education so mining is relatively easy compared with, say, Irian Jaya.

TL;DR: there are a lot of minerals here but also the means to find and exploit them

Fun fact: the mineral for Crayola's purple colour was apparently mined on a neighbour of mine's father's farm.