GEOLOGISTS PRIDE: DIAMOND EXPLORATION AND MINING

  Agbor Taku Junior    April 30, 2019    0

Diamond which has its name from the Indian word “ADAMAS” meaning eternal is a high value commodity which price increase with size because big sized diamonds are rare and are worked easily. There exist many gemstones that have similar properties to diamond such as density and colour, hence making diamond difficult to identify properly. However, the hardness of diamond is unmatchable and constitutes its most distinctive property.



Diamond can be properly identified using a diamond tester which measures the thermal conductivity of de stone (diamond has a high thermal conductivity)

Diamonds generally have grossly rounded shapes because they are chemically eroded in the volcanic fluids that transports them from the earth’s interior to the surface and it retain this shape in its natural setting since no other minerals can scrape its sides to make in angular.

Diamond forms at great depths of about 140km in the earth’s interior, hence require high pressure to form.

Parameters for establishing diamond mines

  • Grade (high grade is about 1-2ct/ton)
  • Tonnage
  • Quality of the diamond

 

Diamond originates from a special types of volcanism rock known as kimberlite volcanism (Kimberlite pipes) derived from the name “Kimberly” in South Africa. This type of volcanism of volcanism is ultramafic, made up of large porphyritic phenocrysts of olivine set in a fine olivine matrix and comes from the mantle.

 DiamondOriginates

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Kimberlite Pipe seen from the surface

 

MINERALOGY OF KIMBERLITE PIPES

It’s a mixture of oilive, igneous carbonates resulting from very high CO2 melt, pyrope which is a garnet with pinkish colour and large crystals, phlogopite which is a mica due to high volatile content in the Kimberlite, water vapour and with or without diamond. However, olivine remains its dominant mineral. They also always contain fragments of wall rock (Xenoliths) resulting from the explosive nature of the high volatile content of the melt.

Kimberlite is a rare rock to find and there is presently no known active Kimberlite volcanism. This type of volcanism is strongly associated to pipe shaped vents, hence the name “Kimberlite pipes”

There exist some relationship between Carbonatite and Kimberlite. Carbonate volcanism is also rare. An example is the Oldoinyai Lenga in Tanzania.

 

Geological setting of Diamond

Mineral inclusions sometimes occur in diamonds and studies reveal that the diamonds with inclusions are always older than the host Kimberlite. This implies that diamond itself is just transported as a xenolith within the Kimberlite and are not formed by it.

Diamond does not occur at the Mid Oceanic Ridges because of slow movement of the magma which chemically erodes and dissolves the diamond. In contrast, Kimberlites take just few hours to transport diamonds from depth to the surface.

At depths less than 140km, the temperature and pressure are not high enough to stabilize diamond, hence it is chemically eroded and explains why diamond if found mainly in very thick lithosphere. At depths equal to 140km, rising magma picks up formed diamond and transports it towards the surface. Diamond will stabilizes at depths of about 75km only when the temperature is as low as O°C.

Hence, Diamond is actually not rare in the earths interior, but requires a complicated mix of temperature, pressure and transport time to reach the surface.

 

 

DiamondGeologicalSettings



DIAMOND EXPLORATION

Identification of Kimberlite pipes is very difficult due to the small sizes, however, the tuff facies associated with them have a wider lateral extent on the earth’s surface can be used to identify the pipes.

Kimberlite pipes usually show up as aeromagnetic anomalies because they sometimes contain magnetite and so geophysical mapping can help isolate their unique shape.

The fast rate of weathering of Kimberlite always leave the pipes behind as rounded depressions which can also be an indicator during exploration. Old Archean rocks as the best targets for diamonds exploration in rocks, since they are mostly related to Archean Cratons which are associated to thick lithosphere.

The erosional debris of most Kimberlite pipes forms conglomerates in fluvial or alluvial deposits, hence constituting potential exploration targets.

Kimberlites hosting diamonds tend to have pyrope, which is relatively high in Chromium and low in Calcium. This is a diagnostic feature which can aid exploration.







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