Kimberlite is a type of igneous rock that is ultramafic and ultrapotassic. Its mineral content usually includes olivine, phlogopite, pyroxene, and garnet, accompanied by a variety of trace minerals. Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. These pipes are the most important source of mined diamonds today. In addition, the scientific study of kimberlite helps reveal some of the geological processes that take place within the Earth's crust and mantle.
The rock kimberlite was named after Kimberley, South Africa, where it was first recognized. Kimberley diamonds were originally found in weathered kimberlite, which was colored yellow by limonite, and was therefore called yellow ground. Deeper workings produced less altered rock, serpentinized kimberlite, which miners call blue ground.
The general consensus is that kimberlites are formed deep within the mantle, at depths between 150 and 450 kilometers, from anomalously enriched exotic mantle compositions. They are erupted rapidly and violently, often with the release of considerable amounts of carbon dioxide (CO2) and volatile components. The violent explosions produce vertical columns of rock—volcanic pipes or kimberlite pipes—that rise from the magma reservoirs. The depth of melting and the process of generation makes kimberlites prone to hosting diamond xenocrysts.
The morphology of kimberlite pipes is varied, but it generally includes a sheeted dike complex of vertically dipping feeder dikes in the root of the pipe, extending down to the mantle. Within 1.5-2 kilometers (km) of the surface, as the magma explodes upward, it expands to form a conical to cylindrical zone called the diatreme, which erupts to the surface.
The surface expression is rarely preserved, but it 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, but in Southern Africa, there are several that formed between 60 to 1,600 million years ago (Mitchell, 1995, p. 16).
Based on their mineralogy, kimberlites are divided into Group I (basaltic) and Group II (micaceous) kimberlites.
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 consist of carbon dioxide (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 titanium-poor chromite.
Group-II kimberlites (or orangeites) are ultrapotassic, peralkaline rocks rich in volatiles (mainly water). The distinctive characteristic of orangeites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously iron-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.
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 garnets rich in pyrope plus chromium—are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.
The indicator minerals are generally sought in stream sediments in modern alluvial material. Their presence, when found, may be indicative of the presence of kimberlite within the erosional watershed that produced the alluvium.
The geochemistry of kimberlites is defined by the following parameters:
Kimberlites are the most important source of primary diamonds. Many kimberlite pipes also produce rich alluvial diamond placer deposits. However, only about one in 200 kimberlite pipes contain gem-quality diamonds.
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