Mining

Diamonds are mined in 25 countries on almost every continent. Extensive mining of diamonds shifted from India (17th century), to Brazil (18th century), to the African continent (19th century), and finally Australia and Canada (20th century). The top seven producing countries, which account for 80 per cent of the world’s rough diamond supply, are Australia, Botswana, Zaire, South Africa, Russia, Angola, and Namibia. Russian diamond discoveries were made during the 1950s in Siberia, a most inhospitable mining area where the permafrost is up to 300 metres deep.

The Argyle diamond story started in the early 1970s, when one of the world’s most significant finds was made at Smoke Creek in the remote north of Western Australia, over 2000 kilometres from Perth. these deposits were traced subsequently to the primary pipe known as AK1 in the Kimberley – Lake Argyle region. Even though gem-quality production from the Argyle mine is low, it is the world’s biggest producer of natural diamonds and contributes approximately one-third of the world’s natural supply.

The recovery ratio of diamonds is incredibly low. It takes more than thirty tonnes of diamond-bearing ore to produce a single, polished diamond of one carat – a diamond weighing one-fifth of a gram! These days, diamonds are more likely to be found by sophisticated techniques including satellite imaging than random discovery by a boy kicking at stones on a riverbank.

Diamond Pipes

Millions of years ago, natural carbon was compressed into diamond crystals by great forces of heat and pressure deep within the earth. Later, the molten magma in which the diamonds were formed was forced to the earth’s surface, through fissures such as the throats of ancient volcanoes.

The molten material then cooled into formations called ‘pipes’, composed of an igneous rock called kimberlite or of lamproite. The common term for both kimberlite and lamproite is ‘blue ground’.

Types of Diamond Mining

There are three main types of diamond mining, each depending on the nature of the deposit.

Primary Deposits (at the location of the pipe)
The Open Cast Mine

Initially, all pipes were worked by simple, open-cast (open-pit) methods. Depending on conditions, the maximum depth for opencast mining is generally considered to be 245-305 m.

Today’s open-pit mining is a far cry from South Africa in the 1870s, when thousands of independent claim-holders crowded into the Kimberley Mine and eventually hindered and endangered the entire operation as the depth of the excavation increased. The great risk then was the collapse of surrounding rock into the mine. In modern pits, the reef is gradually cut back in steps as the mine goes deeper – resulting in a wider, deeper, safer hole.

The Underground Mine

Underground mining was not successful in South Africa until 1890, after all the independent claims finally were consolidated into the ownership of De Beers. For this style of mining, vertical shafts are sunk into the ‘country rock’ at a distance from the pipe. There are various methods, but the basic technique is to drive horizontal tunnels leading into the pipe from the vertical shafts, and then to dig out the diamond-bearing ore from below. The hole is made progressively deeper, and the tunnels are driven at successively lower levels, until the entire pipe has been mined out so that only a hole remains.

Secondary Deposits

Erosion of diamond pipes is the source of all the widely distributed alluvial diamonds that have been swept away by streams and rivers to the places where they are now found.

Prospecting for payable alluvial deposits is not an easy task and the ratio of diamonds to waste material is extremely low.

Until the discoveries in South Africa in the latter half of the nineteenth century, diamonds were recovered entirely from alluvial deposits. Methods were simple and labour intensive, relying on a pick and shovel for digging, a hand-held washing pan to separate diamonds from gravel, and hand sorting.

Miners from all parts of the world brought their knowledge to the new fields in South Africa, and they applied gold-mining techniques to the recovery of diamonds from alluvial gravels.

These diggers introduced basic methods of alluvial diamond recovery which have changed only in scale and sophistication in the decades that followed.

Marine Deposits

Marine deposits are a variation on alluvial deposits, resulting from the wave action of the ocean, concentrating diamonds at the base of the surf zone. Waves arriving at an angle to the coast tend to push the diamonds along the coast, causing the diamonds to stream out from where rivers deposited them at the coastline.

There are three types of marine mining operations.

  1. Sand is moved from 10 metres below sea level to as far inland as the sea may have risen, in order to reveal the concentrations on the bedrock.
  2. Divers and boats work in the surf zone to 20 metres of water and use suction pipes to remove gravel and diamonds from the ocean floor.
  3. Deep-sea marine vessels use remote underwater tractors or large excavators to remove overlying sediments and extract the diamond-bearing sand and gravel.

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