Kimberlite

Kimberlite is an ultrapotassic, ultramafic, igneous rock composed of olivine, phlogopite, pyroxene and garnet, with a variety of chemically anomalous trace minerals. Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. Kimberlite sometimes contains diamonds and kimberlite pipes are the most important source of mined diamonds today.

Morphology and volcanology

Kimberlites are found as dikes and volcanic pipes which underlie and are the source for rare and relatively small explosive volcanoes (maars). Kimberlites in the Guyana Shield, in Venezuela and French Guyana, form thin, tabular dipping sills.

Kimberlite pipes are the result of explosive diatreme volcanism from very deep mantle derived sources. These volcanic explosions produce vertical columns of rock that rise from deep magma reservoirs. The morphology of kimberlite pipes are varied but generally include a sheeted dyke complex of tabular, vertically dipping feeder dykes in the root of the pipe which extend down to the mantle. Within 1.5-2 km of the surface the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to surface. The surface expression is rarely preserved but is usually similar to a maar volcano. The diameter of a kimberlite pipe at the surface is typically a few hundred meters to a kilometer.

Many kimberlite pipes are believed to have formed about 70 to 150 million years ago.

Petrology

Kimberlites are divided into Group I (basaltic) and Group II (micaceous) kimberlites. This division is made along mineralogical grounds.

The general consensus reached on kimberlites is that they are formed deep within the mantle, at between 150 and 450 kilometres depth, from anomalously enriched exotic mantle compositions, and are erupted rapidly and violently, often with considerable CO₂ and volatile components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.

The mineralogy of Group I kimberlites is considered to represent the products of melting of lherzolite and harzburgite, eclogite and peridotite under lower mantle conditions. The mineralogy of Group II kimberlites may represent a similar melting environment to that of Group I kimberlites, the difference in mineralogy being caused by the preponderance of water versus carbon dioxide.

Group I kimberlites

Group I kimberlites are of CO₂-rich ultramafic potassic igneous rocks dominated by a primary mineral assemblage of forsteritic olivine, magnesian ilmenite, chromian pyrope, almandine-pyrope, chromian diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group I kimberlites exhibit a distinctive inequigranular texture cause by macrocrystic (0.5-10 mm) to megacrystic (10-200 mm) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite and phlogopite in a fine to medium grained groundmass.

The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, contains forsteritic olivine, pyrope garnet, Cr-diopside, magnesian ilmenite and spinel.

Group II kimberlites

Group-II kimberlites (or orangeites) are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H₂O). The distinctive characteristic of orangeites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Fe-rich phlogopite). Resorbed olivine macrocrysts and euhedral primary crystals of groundmass olivine are common but not essential constituents.

Characteristic primary phases in the groundmass include: zoned pyroxenes (cores of diopside rimmed by Ti-aegirine); spinel-group minerals (magnesian chromite to titaniferous magnetite); Sr- and REE-rich perovskite; Sr-rich apatite; REE-rich phosphates (monazite, daqingshanite); potassian barian hollandite group minerals; Nb-bearing rutile and Mn-bearing ilmenite.

Kimberlitic indicator minerals

Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with peculiar chemical compositions. These minerals such as potassic richterite, chromian diopside (a pyroxene), chromium spinels, magnesian ilmenite and high-aluminium garnets are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.

These indicator minerals are generally sought in stream sediments in modern alluvial material. Their presence, when found, may be indicative of the presence of a kimberlite within the erosional watershed which has produced the alluvium.

Geochemistry

The geochemistry of Kimberlites is defined by the following parameters;

Ultramafic; MgO >12% and generally >15%
Ultrapotassic; Molar K₂O/Al₂O₃ >3
Near-primitive Ni (>400ppm), Cr (>1000ppm), Co (>150ppm)
REE-enrichment
Moderate to high LILE enrichment; ΣLILE = >1,000ppm
High H₂O and CO₂

Economic importance

Kimberlites are the most important source of primary diamonds. Many kimberlite pipes also produce rich alluvial or eluvial diamond placer deposits. However, only about 1 in 200 kimberlite pipes contain gem-quality diamonds. The deposits occurring at Kimberley, South Africa were the first recognized and the source of the name. The Kimberley diamonds were originally found in weathered kimberlite which was colored yellow by limonite, and so was called yellow ground. Deeper workings encountered less altered rock, serpentinized kimberlite, which miners call blue ground.

Related rock types

References

Igneous rocks | Economic geology | Diamond

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