Diamonds | Diamond Mines & Mining

Diamond Mining - Extraction Methods

Diamonds and other precious and semi-precious gemstones are extracted from the earth via three main types on mining. These diamond extraction methods vary depending on how the minerals are deposited within the earth, the stability of the material surrounding the desired mineral, and the peripheral damage done to the surrounding enviroment. The principle methods of extraction for diamonds are:

• Artisanal Mining
• Hard Rock Mining
• Marine Mining
• Open Pit Mining
• Placer Mining

Hard-Rock Mining

The term "Hard Rock Diamond Mining" (top of page, left) refers to various techniques used to mine gems, minerals, and ore bodies by tunneling underground and creating underground "rooms" or "stopes" supported by timber pillars of standing rock. Accessing the underground ore is achieved via a "decline" or a "shaft". A "decline" is a spiral tunnel which circles the flank of the ore deposit or circles around the deposit. A "shaft" is vertical tunnel used for ore haulage and runs adjacent to the ore. A "decline" is generally used for personnel and machinery access to the ore.

Open Pit Mining

Open-Pit diamond mining or "Open-Cast Mining" (top of page, middle) is a method of extracting rock or minerals from the earth by removal from an open pit or burrow. Open pit mines are used when deposits of minerals are found near the surface or along kimberlite pipes. Open pit mining is used when the "overburden," or surface material covering the deposit, is relatively thin and/or the minerals are imbedded in structurally unstable earth (cinder, sand, or gravel) that is unsuitable for tunneling. "Pit lakes" tend to form at the bottom of open-pit mines as a result of groundwater intrusion.

Diamonds in Kimberlite Matrix

Placer Mining

Placer Diamond Mining, also known as "sand bank mining" (top of page, right) is used for extracting diamonds and minerals from alluvial, eluvial, and or colluvial secondary deposits. Placer Mining is a form of open-pit or open-cast mining used to extract minerals from the surface of the earth without the use of tunneling. Excavation is accomplished using water pressure (a.k.a. hydraulic mining), mechanized surface excavating equipment, or digging by hand (artisanal Mining).

Alluvial Diamonds

Marine Mining

Marine mining technology only became commercially viable in the early 1990s. Marine diamond mining employs both "vertical" and "horizontal" techniques to extract diamonds from offshore placer deposits. Vertical marine mining uses a 6 to 7 meter diameter drill head to cut into the seabed and suck up the diamond bearing material from the sea bed. Horizontal mining employs the use of Seabed Crawlers (remotely controlled, CAT-tracked underwater mining vehicles) move across the sea floor pumping gravel up to an offshore vessel.

Diamond Geology

Kimberlite Pipes

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. A mixture of magma (molten rock), minerals, rock fragments, and occasionally diamonds form pipes shaped like champagne flute glasses as they approach the earth's surface. These pipes are called kimberlites (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.

Photo (left) Attribution - Unknown/Public Domain

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).

Photo (left) Attribution - Unknown/Public Domain

Lamproite Pipes

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.

Alluvial (Placer) Diamonds

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.

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.

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.

Artisanal Mining

Artisanal diamond mining (aka "small-scale mining") involves nothing more that digging and sifting through mud or gravel river-bank alluvial deposits (above, right) with bare hands, shovels, or large conical sieves. Laborers who work in artisanal diamond mining are called "diamond diggers" (below left). Artisanal diamond mining is a form of "subsistence based" non-mechanized mining that is used in poorer countries throughout the world.

Photo Attribution - Unknown/Public Domain

Artisanal diamond mining is used throughout west Africa, in conflict zones where mechanized mining is impractical and unsafe. Artisanal diamond mining accounts for 90% of Sierra Leone's diamond exports and is the country's second largest employer after subsistence farming. It is also used extensivly in Angola, the Congo (DROC), and Liberia.

Bibliography & Suggestions for Further Study on Diamond Mining

1. Mining Technology, New Techniques in Mining Technology . SPG Media Group PLC

1. Mark S. Lesney, Today's Chemist - Precious Provenance . pubs.acs.org

1. National Geographic, Diamonds: The Real Story . www7.nationalgeographic.com

1. GIA, Gemological Institute of America . www.gia.edu

1. AGS, American Gem Society . www.ags.org

1. Mining Technology, Ekati diamond mine. www.mining-technology.com

1. BHP Billiton, BHP Billiton - Ekati Diamond Mine . ekati.bhpbilliton.com

1. Aurias, Diamonds from the Ekati mine . www.aurias.com

1. Diavik, Diavik diamond mine . www.diavik.ca

1. Diamond Blog, DiamondBlog.com . www.DiamondBlog.com