Diamonds are formed deep within the earth’s crust (the diamond stability field). Sudden, deep-seated volcanic eruptions can bring the diamond from depths of 150 to 200 km to the surface. Such deep volcanoes in the cratons are called kimberlites or lamproites, depending on their mineralogical and chemical composition.
Formation of a Kimberlite pipe - Generalised model
The kimberlite magma comes to the surface over a period of a few hours, following deep fractures in the earth, known as fissures. When the magma cools, the kimberlite forms “dykes”, i.e. walls, in the deep fractures. A few kilometres from the surface, the confining pressure of the overlying rocks is no longer sufficient to contain the gas-rich kimberlite magma and a violent eruption follows. This eruption creates a carrot-shaped explosion pipe (diatreme) topped by a crater of pulverized rock pieces that, after being ejected into the air, fell back into the exploded pipe forming layered tuffs.
Kimberlites erupted
- 1,100 million years ago (e.g. Premier, Argyle, Mali, India),
- 520 million years ago (Venetia (South Africa), Russia),
- 250 to 90 million years ago (most African kimberlites),
- 85 to 120 million years ago (Brazil)
- 50 million years ago (Lac de Gras, Tanzania) and
- 20 million years ago (Ellendale in Western Australia).
Not all kimberlites are diamond-bearing though.
On average, about 1% of kimberlites contain diamonds in economic quantities. Diamond-bearing kimberlites with grades better than 0.1 carat/tonne are extremely rare.
Presently about 30 major kimberlites are being mined world-wide.
The erosion of the kimberlites frees the diamonds, along with other minerals (including the so called indicator minerals) formed at great depths and brought to the surface by the kimberlite magma from the kimberlite rock, then disperses them in the rivers.
Apart from sampling for indicator minerals in the rivers, other exploration techniques applied to kimberlites are magnetic surveys. The earth tends to be more iron-rich at greater depths and the deep magma brought to the surface by kimberlites is often richer in magnetic iron-bearing minerals than the rocks at the surface. As a result, the kimberlites often are more magnetic than the surrounding rocks. These magnetic anomalies can be detected by measuring systematically the magnetic field along survey lines from the air (by aeroplane or helicopter), or on the ground.