Formation of Diamonds (Text Version of Animated Video)

Description of Kimberlite Pipe Animation

Definition of Craton: Part of the Earth's crust that has been stable for at least one billion years.

The Slave Craton of the Northwest Territories and Nuvanut in northern Canada is a prime geological target for diamond exploration.

A cross-section of the Earth's structure is shown as being composed of a solid inner core, a fluid outer core, the mesosphere (lower mantle), the asthenosphere (a partially molten part of the upper mantle), and the lithosphere (includes the crust and very top of the upper mantle).

Diamonds (crystallized carbon) formed between 47 million and 540 million years ago at high temperatures and pressure (1000 degrees Celsius and 55 kB).

The animation depicts molten rock (magma) from the Earth's mantle rising 250 km in the lithosphere and passing through a diamond stability field at progressively faster speeds (from 20 km/hr initially to 1200 km/hr). The magma erupts at the surface of the Earth after about nine hours.

Over some 60 million years, diamond-bearing kimberlites were constantly weathered by wind and water. Between 20 000 and 13 000 years ago, the kimberlites were further eroded by glaciers. As a result, diamonds can now be found in glacial deposits or spread hundreds of kilometres from where they originated.

Diamond Exploration Process

Indicator Mineral Train

The eruption of the kimberlite ejects rock and ash onto the surface, forming a tuff ring around the volcanic vent. During glaciation, the ice sheet scours the earth's surface and moves the kimberlitic debris from within and around the source pipe in a direction parallel to the direction of ice travel. The entrained debris is deposited as the ice sheet melts, leaving a trail of minerals and rock fragments. Geologists carefully sample the surficial glacial deposits searching for indicator minerals. The location of these minerals map out what are known as indicator mineral trains, which are then followed back to the kimberlite source. The common minerals that can occur in indicator mineral trains are pyrope garnet, chrome-rich diopside, picroilmenite, chromite, olivine and diamond.

Crater Zone

As the kimberlite volcano erupts, the debris is deposited into the crater zone. The crater zone is composed of two types of kimberlite: 1) pyroclastic kimberlite - composed of tuffs deposited on the surface and within the pipe vents; and 2) epiclastic kimberlite - composed of eroded pyroclastic kimberlite that has been redeposited either outside of or within the pipe vent. Sedimentary features such as bedding and sorting are common in the crater zone. There are often xenoliths of surficial cover rock, vegetation (e.g., wood) and terrestrial organisms (e.g., microfossils) that existed at the time of emplacement. Usually in the Slave Craton all of the tuff ring of epiclastic kimberlite and part or all of the pyroclastic kimberlite have been eroded off by glaciation. Diamonds are found in this zone.

Diatreme Zone

Explosive events, originating at deeper levels in the root zone, initiate the formation of the diatreme zone. As the kimberlite ascends towards the surface, variations in the kimberlite composition and the physical properties of the surrounding host rock determine the size and shape of the diatreme zone. Most often, this is the thickest zone in a kimberlite pipe and underlies the crater zone. It is typically greater than 1 km in length, steep-sided, and carrot-shaped. The diatreme zone usually has a fragmented appearance, containing variable amounts of fragments (xenoliths) of host rock, deeper mantle rocks, and kimberlite (autoliths). Diamonds are found in this zone.

Root Zone

Prior to and during the kimberlite eruption, the root zone is formed by the crystallization of magma beneath the diatreme zone. Kimberlite in the root zone, known as hypabyssal kimberlite, commonly exhibits crystalline igneous textures and contains varying amounts of xenoliths as well as diamonds.

Dyke

Kimberlite dykes are typically narrow tabular bodies comprised of hypabyssal kimberlite and are thought to originate from the root zone. These dykes cross-cut surrounding host-rock lithologies and can have a steep or shallow orientation. Kimberlite dykes contain varying amounts of xenoliths and contain diamonds.

Kimberlite

Kimberlite is volatile-rich, potassic, ultrabasic igneous rock that occurs as small volcanic pipes, dykes and sills.