Last week I wrote about the places where diamonds are found around the world. The reality is that our planet might host enormous quantities of diamonds below its rocky surface.

However, it is only the diamonds that are found at the surface of the earth that are commercially interesting for us. The rarity of diamonds derives mostly from a combination of the unique conditions beneath cratons that allow diamonds to survive on their journey to the surface, and also the violent means by which they travel.

 

Diamonds arrive at the surface of the earth through volcanic eruptions that bring high-pressure magma from the lower layers of the earth to the surface. It is only when these eruptions pass through the diamond stability field, which enjoys the relative protection of a shielding craton that we find quantities of diamonds that can be mined economically. Human beings have never witnessed such an eruption and the last one is believed to have taken place more than 25 million years ago.

We will now take a closer look at these rare and unusual events.

 

All the way to the top

The vast majority of minable diamonds come to the surface in a kimberlite – a type of intrusive igneous rock originating in the asthenosphere, a region from about 80-200 km below the earth’s surface. Kimberlites originate at depths of about 200 km, much lower than that at which most volcanic activity takes place. The asthenosphere is much hotter and much more volatile than the lithosphere above it. Temperatures within the asthenosphere range from 1300-1500 degrees Celsius. At these temperatures, rock is liquefied and flows like lava under the crust of the earth. The rolling asthenosphere forms the “surface” on which the tectonic plates of the earth shift. The convection currents within this layer are what cause the volcanic activity on the surface.

 

Kimberlite eruptions are caused by a differential in pressure that forces hot magma up through weaker areas of rock. The heat of the magma can often combine with carbon dioxide that reverts into a gaseous state and forces the kimberlite up with even more force. As the hot magma approaches the surface, the pressure exerted on it by surrounding rocks gets weaker and weaker. This often causes the kimberlite to expand, leading to the recognizable carrot-shaped profile of most diamond mines, which lends itself well to modern open-pit mining. The kimberlites often encounter ground water close to the surface. This water is quickly vaporized and the resulting gases add to the power of the kimberlites and cause them to violently explode upon reaching the surface.

 

Kimberlites were only documented thanks to the discovery of diamonds within them. Actually the name “kimberlite” comes from the Kimberley area of South Africa where they were first discovered in the diamond rush of the late 1800s. Kimberley diamonds were originally found in weathered kimberlite, colored yellow by a mineral called limonite, which led to it being called yellow ground. Later digging uncovered serpentinized kimberlite high in olivine, which gives the rock its noticeable blue/green color, which miners aptly named blue ground.

 

A complex rock

Diamonds within kimberlites are just a tiny fraction of their total mineral composition. Some mines can economically produce diamonds at a rate of just 20 carats per hundred metric tonnes of rock. This equates to just four grams of diamond per 100 million grams of rock. The largest capacity dump trucks used in construction typically carry around 350 tons of rock. Imagine a massive dump truck filled with rocks yielding just 15 grams, or about one cubic centimeter of diamonds, of which only a fraction are gem quality.

Although research has shown that diamonds take approximately one billion years to form in nature with some dating back as far as 3.5 billion years, kimberlites themselves are a relatively recent phenomenon in geological terms. Most kimberlite eruptions are believed to have taken place during the Mesozoic era, between 70 and 150 million years ago. Some kimberlites in South Africa are believed to be as old as 1.6 billion years, though such an old kimberlite is extremely rare. It is possible that older kimberlite eruptions have occurred but that hundreds of millions of years of weathering and erosion have hidden them from sight beneath the earth.

Approximately 6,400 kimberlites have been discovered to date. Of those, approximately 900 have been found to be diamond bearing, and less than 40 have shown diamond quantities sufficient for mining. New kimberlites continue to be discovered as a result of diamond exploration, primarily in Botswana, Canada and Russia. 

Not just kimberlites

Although most diamond mines form out of kimberlites, this is not the only way for it to happen. Lamproites are a similar type of rock formation which develop from volcanic explosions. Lamproites were long thought to be kimberlites, however research in the 1940s identified important differences from their kimberlite cousins. Diamond bearing lamproites have been discovered in Western Australia, the US, and India. The Argyle Mine is the first economic diamond deposit known to occur in a lamproite, with the nearby Ellendale Mine (now closed) the world’s only other known example.

 

Both kimberlites and lamproites explode violently when they reach the surface of the earth. Some of the minerals within, including diamonds, can be found several kilometers away from the site of the explosion, although most tend to settle back within the crater that usually forms. In some cases, the kimberlite will fissure when it gets close to the surface and this can lead to long kimberlite dyke systems.

However, over millions of years, natural weathering and erosion can cover the surface of the kimberlite with soil and sand, as well as transport kimberlite far away from the source. Alluvial and marine deposits are a perfect example of such a phenomenon, where the course of ancient rivers and tidal basins can transport diamonds across continents. Some diamondiferous deposits, such as those in Saskatchewan, Canada remain un-mined to this day because they are covered in more than 100 m of over-burden that must first be stripped away at great expense.

In northern climates like Canada and Russia, the glacial action of the last Ice Age had a profound effect on surface kimberlites. The massive movement of ice scraped across the top of kimberlite craters, which spread their minerals across a wide expanse of land. In addition, as ice melted and receded, it left behind water, which formed lakes on top of some kimberlites. For example, the very high-grade Diavik diamond mine was discovered at the bottom of a lake covered in 56 m of water. Knowledge of this glacial action has been successfully used to pinpoint the source of some deposits, a subject I will discuss in further detail next week.

The views expressed here are solely those of the author in his private capacity. No one should act upon any opinion or information in this website without consulting a professional qualified adviser.