Few people understand what an enormous task it is to bring a diamond mine into production. Few if any of the shoppers on New York’s 5th Avenue or London’s Old Bond Street could possibly appreciate the complexity, cost, or the sheer number of people involved in bringing a diamond mine into existence.
Diamond exploration is an area of the diamond pipeline that is largely detached from the parts of the industry that most of us operate in. It is staffed by experts with dozens of different types of knowledge that understand diamonds at a structural level and how the earth produces them. These experts identify target areas where diamonds are most likely to be found and the best practices for finding them. They don’t always get a lot of credit and their names sometimes remain unknown, however their expertise in identifying prospective mine targets is the starting point that permits the rest of us to employ our own expertise to transform these stones into the beautiful gems that they are.
In my next series of articles, I want to take a look at diamond exploration, discovery and mine development. By examining the many different processes involved before the decision is made to dig, I hope to give my readers a better sense of the scope of work involved in discovering diamonds. I think it will also help to describe just how rare diamonds actually are, which is sometimes a subject of debate in the mainstream media.
Diamonds never seem to be discovered on the outskirts of a major city. Instead, they are typically found in remote areas, often in harsh climates that are difficult and costly to access. But there are very good reasons for this, which have to do with how and where diamonds are brought to the surface of the Earth, and how this event renders the area less hospitable to human settlement. Understanding where diamonds are most likely to be found is perhaps the first step that geology professionals look at in the exploration process.
Diamonds are very old
Scientists have determined that most natural diamonds are very old. There is general agreement in the scientific community that diamonds take approximately 1 billion years to form in nature. Diamonds discovered in Northern Canada are the oldest on Earth and have been dated to be 3.5 billion years old, formed as the continents were forming. This is remarkable since the Earth is 4.5 billion years old, meaning that diamonds have been a part of the planet since its early days.
If nothing else, this knowledge helps geologists to identify which parts of the earth’s crust are even capable of hosting diamonds. Earth is composed of 2 distinct types of crust, continental and oceanic. Continental crust consists of lighter minerals like silicon and aluminum and is underlain by a thick mantle keel that keeps the continents stable above sea level. The oceanic crust has no such mantle keel and is composed of heavier mineral such as iron and calcium, and therefore sits lower which allows for the formation of oceans and seas.
Cratons – Diamond Incubators
Diamonds are found in the oldest parts of the continental crust called Cratons. A craton is basically the oldest and most stable region of a continental plate. Often, the area of a continent around the craton can be wildly active, forming the mountains and valleys of our planet. But a craton is much stronger and stable, often with a sort of root system that imbeds it into the Earth to maintain that stability.
This is important because cratons are where diamonds are found. Although the inside of Earth may have many areas within which diamonds can form, it is only underneath cratons where the conditions exist to allow them to survive. We now know that diamond forms in regions of the upper mantle that yield temperatures in the range of 900 – 1,300 degrees Celsius and pressures of between 45 and 60 kilo bars. If either temperature or pressure were outside of this range, the carbon crystals would revert to graphite instead of diamond.
Cratons essentially act as a shield where the temperature pressure gradient is lower than in other parts of Earth’s crust. The lithospheric mantle beneath oceanic crusts tends to fall back into the mantle as it ages and cools. This does not occur within cratons and instead this material solidifies and forms the thick protective base of the craton. When diamonds are transported to the surface of the planet, this relatively low temperature/pressure gradient allows the diamonds to remain intact through their journey.
Distribution of kimberlites worldwide in relation to Archean cratons
Source: After O.R. Eckstrand et al, 1995, Geology of Canada No. 8, GSC. Image: Geological survey of Denmark and Greenland
Research conducted in the 1960s led to ‘Clifford’s Rule’, which notes that diamonds are found in Archean cratons, from the Archean eon, which is approximately 2.5-4 billion years ago. Clifford concluded that kimberlites from within the Archean age would be capable of hosting diamonds, and newer kimberlites from within the Proterozoic age would not. This relationship holds up extremely well in all diamond deposits discovered so far. In fact, the Argyle mine in Australia is the only known diamond mine to have been formed underneath a craton that is not from the Archean age.
This knowledge has significantly streamlined diamond exploration around the world. Only 7% of the Earth’s surface is covered by Archean cratons and this is where diamonds have been discovered and where exploration activity continues today. Archean cratons exist in northern Canada, northern Brazil and South America, southern, central, and western Africa, India, China, Western Australia and several parts of northern Russia. In all these regions diamonds were discovered.
The understanding of Clifford’s Rule led to the exploration rushes in the Orapa district of Botswana in the 1960s, in Western Australia in the 1980s, and later in Canada in the 1990s. While these regions of earth are still massive in human terms, reducing the size of exploration territory by 93% certainly makes the job of diamond explorers a lot more efficient. In my next article, I will look at how diamonds are transported from beneath these cratons to the surface of the Earth and the implications for discovery.
The views expressed here are solely those of the author in his private capacity. None of the information made available here shall constitute in any manner an offer or invitation or promotion to buy or to sell diamonds. No one should act upon any opinion or information in this website (including with respect to diamonds values) without consulting a professional qualified advise