FORMATION OF DIAMOND IN KIMBERLITE

#FORMATION OF DIAMOND IN KIMBERLITE

Diamond Mining, Kimberly Town, South Afrika

Formation of Kimberlite

Kimberlites, named after the town of Kimberly, South Africa, where they were first described, are volcanic

rocks that originate in Earth’s mantle.

They are mined exclusively for diamonds. The photo shows the “Big Hole” at Kimberly.

The Hole was mined from 1871 to 1914 and reached a depth of 240 m below the surface. Subsequently,

it filled with water.

Kimberlite eruptions are gas-powered explosive events. The magmas originate at depths of 150 to 450

km, deeper than other igneous rocks.

Most kimberlites are in small vertical columns called kimberlite pipes although some rare sills are

known.

These pipes are the most important source of diamonds today. If kimberlite weathers and erodes, the

diamonds may become concentrated in sedimentary deposits.

Some kimberlites bring mantle xenoliths (pieces of mantle rock) to the surface; petrologists use these

samples to study mantle chemistry and mineralogy.

Kimberlites are ultramafic rocks, having high magnesium and low silicon contents, and are rich in

potassium.

Sample of Kimberlite from Baffin Island, Canada

Mg-rich olivine and carbonate minerals generally dominate, but some kimberlites contain significant

amounts of phlogopite (Mg-rich biotite). Lesser amounts of serpentine, ilmenite, garnet, clinopyroxene,

enstatite and chromite may be present. Most kimberlites are very old, having erupted between about 80

million and 2.5 billion years ago. A few younger ones, 10-20,000 years old, are in Tanzania and the

Democratic Republic of the Congo.

The photo below shows a sample of kimberlite from Baffin Island, Canada. The dark material consists of

pyroxene and olivine. The obvious bright green minerals are chrome diopside. Harder-to-see smaller wine-

red minerals are Mg-rich garnet. The large light-colored fragments are limestone that got caught up in the

kimberlite magma during the eruption.

GEOSPATIAL OUTLOOK