Kimberlite is a rare and distinctive igneous rock. This ultramafic, volatile-rich rock forms under intense pressure and temperature, providing a unique window into the Earth’s mantle. It is primarily recognized for its association with diamonds, acting as their natural transport mechanism from deep within the Earth to the surface. Kimberlite’s unique composition and formation processes make it a subject of considerable scientific and economic interest.
Geological Environments
Kimberlite originates from deep within the Earth’s mantle, typically at depths ranging from 150 to 450 kilometers. This formation occurs from anomalously enriched mantle compositions, often involving partial melting processes. The high levels of carbon dioxide and water within the magma are characteristic of this deep origin and contribute to its explosive eruption style.
The ascent of kimberlite magma to the surface is a rapid and violent process, often described as one of the fastest volcanic eruptions on Earth. This rapid ascent is crucial because it allows diamonds, formed at great depths, to be brought to the surface without converting back to graphite due to changes in pressure and temperature. Kimberlite magmas exploit deep fractures and zones of weakness within the Earth’s crust, forming vertical, carrot-shaped structures known as kimberlite pipes, as well as dikes and sills. These pipes are the most important source of mined diamonds today.
Kimberlite deposits are almost exclusively found within ancient, stable continental crustal blocks called cratons. These cratons are the oldest and thickest parts of continents, possessing deep lithospheric mantle roots that extend to depths where diamonds can form and remain stable. The interaction of rising kimberlite magma with these thick cratonic keels, possibly influenced by mantle plumes, facilitates its explosive upward journey.
Major Global Occurrences
Kimberlite deposits are globally distributed but are concentrated in regions characterized by ancient continental cratons. Southern Africa is historically the most prominent area for kimberlite occurrences, with the town of Kimberley in South Africa lending its name to the rock due to the significant diamond discoveries there in the late 19th century. Key kimberlite fields in Southern Africa include those in South Africa, Botswana, Lesotho, Swaziland, and Zimbabwe, with some of the world’s richest mines found in this region, such as Jwaneng in Botswana and the Premier (Cullinan) pipe in South Africa. These deposits range widely in age, with some being as old as 1,700 million years.
Canada has emerged as a significant producer of diamonds from kimberlite deposits, particularly in its northern regions. The Slave Craton in the Northwest Territories hosts numerous kimberlite pipes, including those associated with the Ekati and Diavik mines, which began production in the late 1990s and early 2000s. Other Canadian occurrences include the Attawapiskat kimberlite field in Northern Ontario and deposits in Quebec, further demonstrating the widespread presence of kimberlites across the country’s ancient shield areas.
Siberia, located in Russia, is another major region for kimberlite and diamond mining, home to large and productive kimberlite pipes like the Mir and Udachnaya mines. Australia also hosts notable kimberlite occurrences, particularly in the Kimberley region of Western Australia, although some diamond-bearing deposits there are hosted by a similar rock type called lamproite. The Merlin kimberlite field in the Northern Territory and occurrences in South Australia are other examples of kimberlite presence on the continent. India, too, has a history of diamond mining, with kimberlite fields identified in the southern peninsular region, such as the Wajrakarur field in Andhra Pradesh, and the Majhgawan mine in Madhya Pradesh.
The Diamond Connection
Kimberlite serves as the primary host rock and transport mechanism for diamonds. Diamonds form deep within the Earth’s mantle, under conditions of extreme pressure and temperature, typically between 150 to 700 kilometers below the surface. Kimberlite magmas, originating from similar depths, act as natural “elevators,” rapidly carrying these diamonds from their formation environment to the Earth’s surface. Without kimberlite, most natural diamonds would remain inaccessible in the deep mantle.
It is important to recognize that diamonds are not formed within the kimberlite magma itself but are instead xenocrysts—foreign crystals picked up by the magma during its ascent. The kimberlite merely transports these pre-existing diamonds. While kimberlite is the dominant rock type for bringing diamonds to the surface, not all kimberlite occurrences contain diamonds, and even fewer contain them in quantities that are economically viable for mining. Of the thousands of kimberlite pipes known globally, only a small percentage, perhaps around 1%, have been successfully exploited for diamonds.