Diamond Geology & Kimberlites
Diamonds form at a depth greater than 93 miles (150 kilometers) beneath the earth's surface. After their formation, diamonds are carried to the surface of the earth by volcanic activity. As this molten mixture of magma (molten rock), minerals, rock fragments, and diamonds approaches the earth's surface it begins to form an underground structure (pipe) that is shaped like a champagne-flute.
These pipes are called "kimberlites" or "kimberlite pipes" (see diagram below). Kimberlite pipes can lie directly underneath shallow lakes formed in the inactive volcanic calderas or craters.
Kimberlite is a diamondiferous igneous-rock matrix composed of carbonate, garnet, olivine, phlogopite, pyroxene, serpentine, and upper mantle rock, with a variety of trace minerals. Kimberlite occurs in the zone of the Earth's crust in vertical structures known as kimberlite pipes (above, right). Kimberlites are found as "dikes" and "volcanic pipes" which underlie and are the source for rare and relatively small volcanoes or "maars" (above, left). Kimberlite pipes are the most significant source of diamonds, yet only about 1 in every 200 kimberlite pipes contain gem-quality diamonds. Many kimberlite pipes also produce alluvial diamond placer deposits.
Diamond bearing kimberlite in some parts of South Africa is black in color (above, right). Most kimberlite is called "blue-ground" kimberlite (above, left) or "yellow-ground" kimberlite and can be found worldwide. The name "Kimberlite" was derived from the South African town of Kimberly where the first diamonds were found in this type of rock conglomeration (see section on "Kimberley - North Cape" below).
Open Pit Mine - Photo: Public Domain
Lamproite pipes produce diamonds to a lesser extent than kimberlite pipes. Lamproite pipes are created in a similar manner to kimberlite pipes, except that boiling water and volatile compounds contained in the magma act corrosively on the overlying rock, resulting in a broader cone of eviscerated rock at the surface. This results in a martini-glass shaped diamondiferous deposit as opposed to kimberlite's champagne flute shape.
The Argyle diamond mine in Western Australia is one of the first commercial open-cast diamond mines that is dug along an olivine lamproite pipe. The Argyle pipe is a diatreme, or breccia-filled volcanic pipe that is formed by gas or volatile explosive magma which has breached the surface to form a "tuff" (consolidated volcanic ash) cone. Breccia is a rock composed of angular fragments of several minerals or rocks in a matrix, or "cementing material," that may be similar, or different in composition to the fragments themselves. There are several types of breccia which are categorized based on their geological origin, including: hydrothermal breccia, igneous breccia, impact breccia, sedimentary breccia, and tectonic breccia.
Alluvial (Placer) Diamond Deposits
The location of alluvial (secondary or placer) diamond deposits is controlled by the surrounding topography. Alluvial diamond deposits are usually located within river terrace gravels that have been transported from their location of origin, usually from kimberlite deposits.
Diamondiferous material tends to concentrate in and around 'oxbow lakes,' which are created by abandoned river meanders. These dried 'lakes' receive river water during seasonal flooding which transports large amounts of sediment held in suspension.
Alluvial Diamonds from Africa - Photo: Public Domain
Diamonds in Kimberlite Matrix - Photo: NASA
The alluvial terrace gravels (below, left) and marine gravels of the south-western coastline of Africa represent the some of the world's largest placer diamond deposits. The world's largest known gem quality alluvial diamond deposits are located along the Namib Desert coastline of southwestern Africa, known as the Sperrgebiet or "forbidden territory," and along the Orange River near Alexander Bay. Namibia's placer diamond deposits are between 40 and 80 million years old, carried from their primary origination point on the Kaapvaal Craton, in central South Africa and Botswana.
Alluvial diamond mining in Angola takes place along a meandering stretch of the Cuango River flood-plain which is also along the south-western coastline of Africa. Some of the largest and highest gem-quality diamonds produced from alluvial placer diamond mining have come from this region, including Angola's two largest diamonds at 105.9k and 101.8k.
Alluvial Terrace Gravels - Photo: Wiki
Diamond-Bearing 'Diamondiferous' Gravel
Many of these alluvial diamond deposits occur in Pleistocene and Holocene successions (1.8 million to 10,000 years ago). The diamonds within these deposits were transported from deeply-eroded diamondiferous kimberlites or, to a lesser extent, from olivine lamproites formed during the Cretaceous or Permo-Triassic period. Westward draining river systems transported these diamonds to Africa's continental coastline for final deposition within on-shore marine terrace gravels. Diamonds that were transported downstream, but were not deposited on land, made their way to the sea bed just offshore. Diamonds in marine areas are typically trapped in bedrock depressions such as gullies, potholes, depressions, channels or other trapsites for diamondiferous deposits.
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