Diamond Mining and Prospecting

Diamond is unusual as a target for mining because it is found in igneous/explosively emplaced  rocks, and the diamonds are carried up with these rocks from the Earth's mantle.  Apparently the depths are somewhere in the vicinity of 160 km (100 miles) below the Earth's surface.

The diamonds can then be mined in this primary rock emplaced in the crust.  These are called primary deposits.  Many of these deposits are carrot-like bodies that have cut through older rocks.   The carrot-like bodies are referred to as kimberlite pipes, or diatremes.  Not all pipes contain diamonds and certainly some pipes have much richer diamond contents than others.

The pipes erode as they and the surrounding rocks are attacked by weathering.  Usually the diamond pipes erode somewhat more rapidly than the surrounding rocks and form bowl-shaped depressions.  As the rocks erode, diamonds are released from the kimberlite.  The kimberlite weathers to a yellowish product referred to as yellow ground.  Diamonds are easily extracted from yellow ground, but fresh kimberlite rock, called blue ground, holds the diamonds and must be crushed in order to release them.

The diamonds in the yellow ground can be washed into streams to eventually wind up in alluvial deposits.  Eventually some diamonds are washed off the land/continents into the sea as in Namibia.

Diamonds in the primary rock kimberlite are found by a variety of techniques, but of great importance is the use of sampling for indicator minerals.  Diamond indicator minerals occur in the kimberlite pipe in greater amounts than the diamonds and are released as weathering products as well.  You are more likely to spot these indicator minerals than the diamonds themselves; they include Cr-Pyrope (chromium-rich magnesium garnet), Cr-diopside, magnesium-rich ilmenite (Mg-ilmenite), and chromite (a type of spinel).

Thus geological sampling can find the indicator minerals and they may then be traced in the direction of greater abundance, up a stream, etc.

Once a kimberlite pipe is found, it must be tested.  The concentration of diamonds must be high enough to allow economic extraction.  This means the minimum concentration of diamonds per ton (2,000 lb) that is practical.  In some instances as little as 0.1 carats (0.02 grams) of diamond per ton is economic enough to mine.  A carat concentration of 0.5 carats or more is preferred.  That is a concentration of 0.1 grams per ton.  That means 10 tons per gram of diamond (or 5 carats per ten tons [20,000 lb of rock!]).  A concentration of only 2 to 4 carats (4 carats = 0.8 grams) per ton is very good to excellent.  According to one source, the Argyle diamond mine of Western Australia, has one of the highest diamond concentration known from kimberlite/primary deposits and reaches only a concentration of 6 carats per metric ton (2,200 lbs of rock).  That is one reason that people think of diamonds as being rare!

The kimberlite is extracted in enormous quantities using huge trucks and giant steam shovels; the pipe is mined from the surface as an open pit.  When it gets too deep, underground mining may begin.

Because of the huge quantities of rock involved, the mining is highly mechanized.  One should keep in mind that early mining efforts such as those used to dig the Kimberley “big hole” are now considered primitive.  In the early days, blue ground was dumped in big piles and the miners waited to extract the diamonds until weathering had turned the kimberlite into yellow ground.

The "Big Hole", Kimberley, South Africa.  This is one of the largest holes created by man and much of it was done without the type of modern equipment that we have today. Digging began 1871 and stopped in 1914   This made this a great labor of man and a monument to man's desire.

Today, kimberlite is extracted and crushed to a size determined to give the best yield.  Huge quantities of rock are crushed.

The diamonds and other heavier minerals are concentrated by mechanical means.  First by methods that separate by specific gravity and then by more selective means such as grease tables and X-ray extraction devices that most directly separate the diamonds.  Grease tables take advantage of the fact that diamond is essentially a water-repellent substance and stays dry.  Other rock components of the kimberlite are wet by water.  The crushed mineral concentrate is fed over a table coated with grease (Vaseline) and the “dry” diamonds stick to the grease while the “wet” minerals are washed over the table and are removed.  Occasionally, the table's grease is scraped up and the diamonds are then boiled to remove the grease.

X-ray extraction involves detection of diamond fluorescence due to x-radiation.  The removal of the “glowing” diamonds takes place as a stream of glowing and “non-glowing” minerals passes a photodetector which directs a blast of air from an extractor gun at any glowing object in the stream.

Ultimately, unimaginable amounts of rock are separated to extract diamonds.

Alluvial diamonds can be found using a shovel, sieve, and pan, but extraction ultimately also involves the moving of large amounts of rock.  Gravels hold the diamonds and running water is put to use.  Diamonds are concentrated in sluices when the water levels of the streams are high.  If there is a dry season, the digging is done then to unearth the gravel, and it is separated for diamonds in the wet season.  Sometimes an underwater dredge is used to scoop sand and gravel off the stream bottom or ocean floor.  Namibian diamonds are found in both beach/stream gravels and on the ocean floor where they are sucked up by giant vacuum systems.  After using the sluice, diamonds may be picked by hand from the concentrate.

In many places, the search for alluvial diamonds may be extremely primitive and laborious and the people involved may make very little profit from their hard work.  Diamonds which ultimately bring joy may have been the result of extremely hard labor, but this is the situation with many luxury goods.