Kimberlite is a type of potassic
volcanic rockVolcanic rock is an igneous rock of volcanic origin.- Texture :Volcanic rocks are usually fine-grained or aphanitic to glassy in texture. They often contain clasts of other rocks and phenocrysts. Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye...
best known for sometimes containing
diamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
s. It is named after the town of
KimberleyKimberley is a city in South Africa, and the capital of the Northern Cape. It is located near the confluence of the Vaal and Orange Rivers. The town has considerable historical significance due its diamond mining past and siege during the Second Boer War...
in
South AfricaThe Republic of South Africa is a country located at the southern tip of Africa, with a coastline on the Atlantic and Indian Oceans. To the north lie Namibia, Botswana and Zimbabwe, to the east are Mozambique and Swaziland, while Lesotho is an independent country surrounded by South Africa.Modern...
, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the
Big HoleThe Big Hole, Open Mine or Kimberley Mine is an open-pit mine in Kimberley, South Africa and claimed to be the largest hole excavated by hand.-History:...
.
Kimberlite occurs in the Earth's
crustIn geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in...
in vertical structures known as kimberlite pipes. Kimberlite pipes are the most important source of mined diamonds today. 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
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
and other
volatileIn planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.
Kimberlite has in many ways attracted more attention than its relative volume might suggest that it deserves. This is largely because it serves as a carrier of diamonds and garnet
peridotiteA peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
xenolithA xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
s to the Earth's surface. Furthermore, its probable derivation from depths greater than any other
igneous rockIgneous rock is one of the three main rock types . Igneous rock is formed by magma being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks...
type, and the extreme
magmaMagma [from Greek μάγμα, paste] is molten rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber inside a volcano...
composition that it reflects in terms of low silica content and high levels of incompatible
trace elementIn analytical chemistry, a trace element is an element in a sample that has an average concentration of less than 100 parts per million measured in atomic count, or less than 100 micrograms per gram....
enrichment, make an understanding of kimberlite
petrogenesisPetrogenesis, also known as petrogeny, is a branch of petrology dealing with the origin of igneous rocks. Petrology is the branch of geology dealing with the origin, occurrence, structure and history of igneous, metamorphic, and sedimentary rocks....
important. In this regard, the study of kimberlite has the potential to provide valuable information on the composition of the deep mantle, and melting processes occurring at or near the interface between the
cratonA craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
ic continental
lithosphereThe lithosphere is the rigid outermost shell of a rocky planet.- Earth's lithosphere :...
and the underlying convecting
asthenosphericThe asthenosphere is the mechanically weak ductily-deforming region of the upper mantle of the Earth...
mantle.
Morphology and volcanology
Kimberlites occur as carrot shaped, vertical intrusions termed 'pipe'. This classic carrot shape is formed due to a complex intrusive process of kimberlitic magma that inherit a large proportion of both CO
2 and H
2O in the system which produces a deep explosive boiling stage that causes a significant amount of vertical flaring (Bergman, 1987). Kimberlite classification is based on the recognition of differing rock facies. These differing facies are associated with a particular style of magmatic activity, namely crater, diatreme and hypabyssal rocks (Clement and Skinner 1985, and Clement, 1982).
The morphology of kimberlite pipes, and the classical carrot shape, is the result of explosive
diatremeVolcanic pipes are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. They are considered to be a type of diatreme. Volcanic pipes are composed of a deep, narrow cone of solidified magma , and are usually largely composed of one of two...
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 is varied but generally includes a sheeted dyke complex of tabular, vertically dipping feeder dykes in the root of the pipe which extends 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
diatremeA diatreme is a breccia-filled volcanic pipe that was formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone, a filled relatively shallow crater known as a maar, or other volcanic pipes...
, which erupts to the 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.
Two
JurassicThe Jurassic is a geologic period and system that extends from about Ma to Ma, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the "Age of Reptiles". The start of the period is marked by...
kimberlite
dikesA dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
exist in
PennsylvaniaThe Commonwealth of Pennsylvania , often colloquially referred to as PA by natives and Northeasterners, is a state located in the Northeastern and Middle Atlantic regions of the United States...
. One, the Gates-Adah Dike, outcrops on the
Monongahela RiverThe Monongahela River is a river on the Allegheny Plateau in North-Central West Virginia and southwestern Pennsylvania in the United States...
on the border of
FayetteFayette County is a county located in the U.S. state of Pennsylvania. As of the2000 census, the population was 148,644. The county is part of the Pittsburgh Metropolitan Statistical Area....
and
GreeneGreene County is a county located in the U.S. state of Pennsylvania and is part of the Pittsburgh DMA. As of 2000, the population was 40,672.Greene County was created on February 9, 1796, from part of Washington County and named for General Nathanael Greene....
Counties. The other, the Dixonville-Tanoma Dike in central
IndianaIndiana County is a county located in the U.S. state of Pennsylvania. In 2000, its population was 89,605. Indiana County was created on March 30, 1803, from parts of Westmoreland and Clearfield Counties, and is probably named for the Indiana Territory. Its county seat is Indiana...
County, does not outcrop at the surface and was discovered by miners.
Petrology
Both the location and origin of kimberlitic magmas are areas of contention. Their extreme enrichment and geochemistry has led to a large amount of speculation about their origin, with models placing their source within the sub-continental lithospheric mantle (SCLM) or even as deep as the transition zone. The mechanism of enrichment has also been the topic of interest with models including partial melting, assimilation of subducted sediment or derivation from a primary magma source.
Historically, kimberlites have been subdivided into two distinct varieties termed 'basaltic' and 'micaceous' based primarily on petrographic observations (Wagner, 1914). This was later revised by Smith (1983) who re-named these divisions Group I and Group II based on the isotopic affinities of these rocks using the Nd, Sr and Pb systems. Mitchell (1995) later proposed that these group I and II kimberlites display such distinct differences, that they may not be as closely related as once thought. He showed that Group II kimberlites actually show closer affinities to lamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites as orangeites to prevent confusion.
Group I kimberlites
Group I kimberlites are of CO
2-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
olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
,
pyropeThe mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for fire and eye. Despite being less common than most garnets, it is a...
garnet, Cr-
diopsideDiopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the...
, magnesian
ilmeniteIlmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
and
spinelThe spinels are any of a class of minerals of general formulation A
2+B
23+O
42- which crystallise in the cubic crystal system, with the oxide anions arranged in a cubic close-packed lattice and the cations A and B occupying some or all of the...
.
Group II kimberlites
Group-II kimberlites (or
orangeites) are
ultrapotassicUltrapotassic igneous rocks are a class of rare, volumetrically minor generally ultramafic or mafic silica-depleted igneous rocks.Ultrapotassic rocks are defined by molar K2O/Na2O >3 in much of the scientific literature. In other papers written as recently as 2005, they are...
, peralkaline rocks rich in volatiles (dominantly H
2O). The distinctive characteristic of orangeites is
phlogopitePhlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg
3AlSi
3O
102, or...
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
chromiteChromite is iron magnesium chromium oxide: Cr
2O
4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts; also, aluminium and ferric iron commonly substitute for chromium....
to titaniferous
magnetiteMagnetite is a ferrimagnetic mineral with chemical formula Fe
3O
4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name ferrous-ferric oxide...
); Sr- and REE-rich
perovskiteA perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or
XIIA
2+VIIB
4+X
2-3 with the oxygen in a fcc...
; Sr-rich
apatiteApatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH
−, F
−, Cl
− or...
; REE-rich phosphates (
monaziteMonazite is a reddish-brown phosphate mineral containing rare earth metals and is an important source of thorium, lanthanum, and cerium. It occurs usually in small isolated crystals...
, daqingshanite); potassian barian hollandite group minerals; Nb-bearing
rutileRutile is a mineral composed primarily of titanium dioxide, TiO
2.Rutile is the most common natural form of TiO
2. Two rarer polymorphs of TiO
2 are known:...
and Mn-bearing
ilmeniteIlmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
.
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
pyroxeneThe pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
), chromium spinels, magnesian ilmenite, and garnets rich in
pyropeThe mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for fire and eye. Despite being less common than most garnets, it is a...
plus chromium 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 materialAlluvium is soil or sediments deposited by a river or other running water...
. 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 (>400 ppm), Cr (>1000 ppm), Co (>150 ppm)
- REE-enrichment
- Moderate to high LILE enrichment; ΣLILE = >1,000 ppm
- 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 depositIn geology, a placer deposit or placer is an accumulation of valuable minerals formed by deposition of dense mineral phases in a trap site. The name is from the Spanish word placer, meaning "alluvial sand"...
s. However, only about 1 in 200 kimberlite pipes contain gem-quality diamonds.
The deposits occurring at Kimberley,
South AfricaThe Republic of South Africa is a country located at the southern tip of Africa, with a coastline on the Atlantic and Indian Oceans. To the north lie Namibia, Botswana and Zimbabwe, to the east are Mozambique and Swaziland, while Lesotho is an independent country surrounded by South Africa.Modern...
were the first recognized and the source of the name. The Kimberley
diamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
s were originally found in
weatheredWeathering is the decomposition of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice,...
kimberlite which was colored yellow by
limoniteLimonite is an ore consisting in a mixture of hydrated iron oxide-hydroxide of varying composition. The generic formula is frequently written as FeO·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.Together with...
, and so was called
yellow ground. Deeper workings encountered less altered rock,
serpentinizedSerpentinite is a rock composed of one or more serpentine minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle. The alteration is particularly important at the sea floor at tectonic plate boundaries...
kimberlite, which miners call
blue groundIn reconnaissance for diamond mining, blue ground refers to a layer of nonoxidized kimberlite. Kimberlites are a class of igneous rocks most commonly associated with diamond mining...
.
See also
Udachnaya pipeUdachnaya pipe is a diamond deposit in the Daldyn-Alakit kimberlite field in Sakha Republic, Russia. It is an open-pit mine, and is located just outside the Arctic circle at . Udachnaya was discovered on June 15, 1955, just two days after the discovery of the diamond pipe Mir by Soviet...
.
The blue and yellow ground were both prolific producers of diamonds. After the yellow ground had been exhausted, miners in the late 1800's accidentally cut into the blue ground and found gem quality diamonds in quantity. The economic importance of the time is that with flood of diamonds being found, the miners were undercutting each other's price of the diamonds and eventually decreased the diamonds value down to cost in a short time.
Related rock types
- Lamproite
Lamproites are ultrapotassic mantle-derived volcanic rocks. They have low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively high MgO content and extreme enrichment in incompatible elements....
- Lamprophyre
Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions...
- Nepheline syenite
Nephelene syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known...
- Ultrapotassic igneous rocks
Ultrapotassic igneous rocks are a class of rare, volumetrically minor generally ultramafic or mafic silica-depleted igneous rocks.Ultrapotassic rocks are defined by molar K2O/Na2O >3 in much of the scientific literature. In other papers written as recently as 2005, they are...
- Kalsititic rocks