Think of a gemstone from Africa. With the fine Emerald and Amethyst of Zambia, the Ruby and Paraiba Tourmaline of Mozambique, Sapphire, Garnet and Apatite of Madagascar, the Tsavorite Garnet of Kenya, Tanzanite of Tanzania and vibrant Opal of Ethiopia, there’s plenty to choose from! But some of you may have noticed something about all of these stunning gemstones. They all originate from the eastern half of the African continent. Although some gems have been discovered in western Africa, there are precious few.
A BRIEF HISTORY OF TECTONIC THEORY
The idea that the continents slot together like pieces of a jigsaw puzzle first surfaced in the
1600s with the dawn of more accurate mapping. It was noticed that the eastern coastline of South America and the western coastline of Africa had a very similar shape. However, most geologists still believed that the continents were fixed in position.
The idea didn’t gain much scholarly research until 1915, when the German geophysicist
Alfred Wegener (1880-1930) published 'The Origins of Continents and Oceans'. In this book, he set out the basic premise of what we now call Plate Tectonics, but what he called Continental Drift.
Despite plenty of corroborating evidence, including the shapes of continents, the distribution of fossils, rock types and geological features, as well as ancient climate evidence, Wegener’s hypothesis contained a few small errors and omissions. His contemporaries focused on these defects and for the most part, ignored his ground-breaking theory.
Wegener’s theories were eventually proved to be by and large correct. In the period following the Second World War, oceanographers utilised new equipment and techniques and spent two decades exploring the sea floor. Their discovery of a global oceanic ridge system proved that Wegener’s theories were right. Unfortunately, it came too late for Alfred Wegener to live to see it. He sadly froze to death in the Arctic tundra of Greenland, on an expedition to collect ice cores in 1930. He is now remembered as the father of modern Plate Tectonic theory.**
If you think of the world’s supply of coloured gemstones, you might think of Brazil as being the most famous source. The most historically significant gemstone deposits on earth lie in the east of Brazil; the likes of Bahia, Minas Gerais, Espirito Santo and Paraiba, to name just a few, have for centuries, produced some of the finest coloured gemstone discoveries known to the market. Brazil has effectively built and shaped the modern coloured gemstone market as we know it.
But east Africa has, over the past few decades, risen to prominence in the coloured gemstone market. When Manuel d’Souza discovered Tanzanite near Mererani in 1967, there were few roads, resources or infrastructure at his disposal. From getting the first digger to the site and basic amenities being installed, to the multi-million pound investment in transportation, security and technology that can be seen there today is an astonishing change.
Gem explorers have been looking at new areas to explore, to find the next generation of gemstone deposits. So, the question is… where should they be looking? The simple answer can be found in Plate Tectonics.
Africa has four large cratons – called the West Africa, Congo, Tanzania, and Kalahari Cratons. Cratons are the oldest and most stable parts of the crust. These ancient rocks have survived for longer than the surrounding crust.
Some of the most gemmologically rich areas in Africa are in, and just to the southeast of, the Tanzania Craton, in the Mozambique Belt. The gem deposits of Madagascar, Mozambique, Tanzania, Kenya and Ethiopia are all situated along what was once this belt. Not only are cratons often gemmologically rich, but the interaction of cratons and the surrounding newer rocks are also of geological interest, since these are former plate boundaries. Plate boundaries are where most pressure and heat is generated – the key ingredients for gemstone formation.
When you look at the African plate as a whole, the four major cratons came together during the Pan-African orogeny around 600 million years ago. They formed part of Gondwana, the supercontinent which preceded Pangea. This then joined with the supercontinent Laurasia and formed Pangaea around 300 million years ago. In the meantime, the rocks which now make up Brazil were moving closely alongside (and running through), those that make up Africa today. It is no coincidence that these are some of the most gem-rich areas on earth. Many rocks in eastern Africa have similar counterparts in Brazil. But nestled in between these two similar areas of east Africa and Brazil, is western Africa, which has hitherto been mostly overlooked. This is where many prospectors are looking for new deposits today.
WHAT IS PLATE TECTONICS?
One of the main criticisms of Wegener’s Continental Drift theory was that he couldn’t explain the mechanism by which continents are moving. With modern equipment, we know for sure that the continents are moving. In fact, it is reckoned that the US is now 25 metres further away from the UK than it was in Columbus’ day, but how?
The answer lies in the heat buried in the centre of the Earth. The Earth’s structure is built up of many layers of rock, with different temperatures and pressures in each (see Figure 1). Convection currents through the mantle mean that its rocks are continually on the move. On top of the mantle is the crust, which is a light and thin outer skin of solid rock.
The crust (and uppermost mantle) are broken into pieces called Tectonic Plates, which are constantly moving too. Due to this movement, given enough time, all the continents eventually collide and come together as a giant landmass. This happens every 300-500 million years or so. The last supercontinent is known as Pangaea, and began to break up around 180 million years ago (see Figure 2).
Over the last few years, we have seen exceptionally high grade coloured gemstones making their way to market in a copper-bearing Tourmaline pocket has also emerged, which could be significant knowing that the state of Paraiba is in the very northeast of Brazil, one of the states that formerly sat closest to present-day Nigeria and Cameroon.
We have started to build strong relationships in Nigeria and, to date, have been seriously impressed by the parcels we have sourced, including Sapphire, Rubellite and Tourmaline. We also fell in love with Moroccan Amethyst, which hails from the geological belt surrounding the West African Craton.
Africa has some fascinating and diverse geology. With large areas of continent having survived several orogenies (tectonic movements), the number of older rocks means that there is a higher than usual chance of discovering gem-hosting rock.
When we look at South America and Africa closely, we can see how they were once aligned. Two cratons were torn apart when South America detached from Africa. What is known in Africa as the Congo Craton, was attached to Brazil’s São Francisco Craton, and the West Africa Craton has a small remnant left in Brazil, known as the São Luís Craton.
In and around these cratons are belts of younger rock. These belts can contain magnificent gemstones. The Araçuaí Belt in Brazil and the Mozambique Belt in Africa are both gem-rich. Just like cratons, these belts are contiguous from one continent to the next. Following their paths is one way to pinpoint where new gem sources may be found.
Unfortunately these recent parcels have been very small due to the fact that in the west it is still predominantly artisanal miners. There are other likely problems too. At one time, a large part of western Africa was covered by a shallow sea, which may have deposited sediments on gem-bearing rocks. The modern-day Congo River has a huge sedimentary basin, which may have buried Mother Nature’s treasure under a deep layer of silt. Additionally, not only are there difficulties to be found with the infrastructure in many of these countries, but there are likely to be political instabilities to deal with as well.
There are no assurances yet that there will be a fruitful supply of gems found, but the signs so far are extremely promising. As prospectors move their efforts to this new west African frontier, trekking in the wilderness with basic tools and equipment, spurred on by courage and a glimmer of hope, you can’t help but be reminded of Manuel d’Souza, walking barefooted through the Tanzanian outback in 1967, on the cusp of an extraordinary discovery.
Earth Science, Tarbuck, Lutgens, Tasa, 2012.
Foundations of Earth Science, 7th Ed, F. K. Lutgens, E. J. Tarbuck, D. G. Tasa, 2014.
Geology: The Key Ideas, David A. Rothery, 2010.
Introducing Tectonics, Rock Structures and Mountain Belts, Graham Park, 2012.
With thanks to Dr Paul Anderson and Dr Nick Kettridge of the University of Birmingham.