Exploration and Mining of Diamonds
Diamonds form deep inside the earth´s crust under high pressure and high temperature. The necessary pressure of about 60.000 bars and the temperature of 2000 °C is found in depths of 150 km and more below the surface. Vulcanic rocks are the means of transportation that bring diamonds all the way upwards. During their journey to the surface there is a high chance for diamonds to burn and turn to graphite or carbon dioxide. Together with great amounts of magma they catapulted to the surface and out of the ground. The funnel-shaped remains of rock are called kimberlit (or lamproit in Australia). These vulcanic funnels reach 1-2 km deep into the earth and were first discovered close the the South-African city of Kimberley.
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Kimberlite funnels consist of clayey sand at the surface which is also called "yellow ground" due to its yellow color tone. Its the weathered product of the kimberlit rock formation which are also called "blue ground". Nowadays often foreign materials and vegetation cover the former vulcanic craters.
The beginning of the diamond exploitation in and aroudn Kimberley are interesting as well: The famous Kimberley mine was operated without machinery from 1871 to 1914. The result is the biggest hole made solely by human hand measuring 460 meters in diameter and about 1 km in depth. In the end not enough diamonds were found in the underground mining and the mine was closed due to cost-effectiveness. However, in total about 3 tons of diamonds were mined which equals to about 14,5 million carats.
Formation and exploitation of kimberlite funnel:
From left to right:
1. Vulkanic eruption and transportation of the diamonds to the surface from depths of 150-200 km
2. Erosion of the vulcano
3. The remains of the funnel vanish
4. Site development of the funnel for surface and underground mining
Primary and secondary places of discovery
Geologists differentiate between primary and secondary finding spots: Kimberlite and lamproit funnels are primary sources while sedimentary rocks in rivers and at the coast are considered secondary sources. Erosion and weathering wash the diamonds out of the kimberlite rocks and they find their way through ditches and streams to placer deposits. However, to be precise one may consider all places of discovery secondary because diamonds are formed inside the earth.
Diamonds are mined four different waysDepending on how the diamonds are mined there are different ways of exploitation:
Surface pit mining
Diamond mining in kimberlite funnels is the most important way of exploitation and accounts for about 80% of world production. The production is still cost effective by gaining 0.2 carats per ton of rock. However, the exploitation-ratios vary greatly depending on mine, development site, and mining method. As a rule of thumb one can expect 0.5 - 1 carat per ton of rock. In comparison: The Australian Argyle mine makes 6-7 carats per ton. At the same time only 5% of the precious stones produced are suitable for the use in jewelry (usually about 20-25%). Then again, at the same time the mine accounts for up to 90% of the world´s production of pink diamonds.
It is much more expensive to extract diamonds of the blue ground. By using vast machinery in surface pit and underground mining the rocks need to be broken, washed and sieved. Modern techniques of filtering the rocks make use of a diamond´s special properties: Especially a diamonds adhesiveness to fat - which later is rather contraproductive when it comes to cleaning and caring of jewelry- comes in handy. Fluorescence and x-rays and optical devices help to filter the precious stones. However, despite all the technological advancements the final sorting process is still manual labor.
The easiest way is the exploitation in placer deposits in rivers and creeks where diamonds accumulate after being dissolved from the weathered kimberlite rocks. By sieving the yellow ground sediment diamonds are filtered and remain. Even though these placer deposits account only for 10-15% of global production, the diamonds found are of above-average size and quality.
Before discovering the kimberlite funnels diamonds were only produced in sediment deposits. In coastal areas diamonds are mined using ships that filter sand sediments like giant vacuum cleaners.