Diamond & Gem Mining - Extraction Methods
Article Copyright © 2009 AllAboutGemstones.com
Diamonds and other precious and semi-precious gemstones are extracted from the earth using five basic mining techniques. These diamond extraction methods vary depending on how the minerals are deposited within the earth, the stability of the material that surrounds that desired gem or mineral, and the peripheral damage that will be done to the surrounding environment. The principle methods of diamond extraction are:
- Artisanal Mining
- Hard Rock Mining
- Marine Mining
- Open Pit Mining
- Placer Mining
Hard-Rock Diamond Mining
The term "hard-rock mining" (top of page, left) refers to various techniques used to extract gems, minerals, and ore bodies that are in situ, and can only be accessed by tunneling underground and creating underground "rooms" or "stopes" that are supported by timber pillars or standing rock.
Accessing the underground ore is achieved via a horizontal passageway called a "decline," or a by a vertical "shaft." A decline is a spiral (corkscrew) tunnel which circles the ore deposit, while a shaft is vertical tunnel used for ore haulage, running adjacent to the ore. A decline is typically used for mining personnel, machinery, and access to the ore.
Open Pit Mining
Open-pit diamond mining, also known as "open-cast mining" (top of page, center) is a method of extracting rock and minerals from the earth by removal from a machine-dug open pit or burrow. Open pit mines are typically used when mineral deposits are found close to the surface or along defined kimberlite pipes.
Diamonds in Kimberlite Matrix - Photo: USGS
Alluvial Diamond - Smithsonian Museum
Open pit mining is used when the surface material (overburden) covering the deposit is relatively thin and/or the desired minerals are imbedded within structurally unstable earth (cinder, gravel, or sand) that is unsuitable for tunneling. Small "pit lakes" tend to form at the bottom of open-pit mines as a result of groundwater intrusion.
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, and is a derivative form of open-cast mining used to extract minerals from the surface of the earth without the use of tunneling. Excavation is accomplished using water pressure (hydraulic mining), mechanized surface excavating equipment, or hand digging (artisanal mining). Diamonds and most gemstones are hard, highly resistant to weathering, and have a specific gravity that is higher than that of common minerals, rock or sedimentary soil, therefore, they tend to concentrate in alluvial deposits in the same way that gold placers develop. Gems are separated from waste material using various sifting and sorting techniques such as cone screens, box screens, or pans, which concentrate the heavier gems at the bottom, or sort material according to size.
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, at a maximum depth of 500 feet. 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. While on board, the gravels are separated and sorted into three sizes, with the middle size being a candidate for final sorting.
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.
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.
Mining Technology, New Techniques in Mining Technology . SPG Media Group PLC
Mark S. Lesney, Today's Chemist - Precious Provenance . pubs.acs.org
National Geographic, Diamonds: The Real Story . www7.nationalgeographic.com
GIA, Gemological Institute of America . www.gia.edu
AGS, American Gem Society . www.ags.org
Mining Technology, Ekati diamond mine. www.mining-technology.com
BHP Billiton, BHP Billiton - Ekati Diamond Mine . ekati.bhpbilliton.com
Aurias, Diamonds from the Ekati mine . www.aurias.com
Diavik, Diavik diamond mine . www.diavik.ca
Diamond Blog, DiamondBlog.com . www.DiamondBlog.com
Dept of Geology, UofG, Mineral inclusions from African & Brazilian Diamond . www.minsocam.org