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Kimberlite
Kimberlite
Kimberlite
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Kimberlite
Igneous rock
Kimberlite from the United States
Composition
Forsteritic olivine and carbonate minerals, with trace amounts of magnesian ilmenite, chromium pyrope, almandine-pyrope, chromium diopside, phlogopite, enstatite and titanium-poor chromite. Sometimes contains diamonds.
False-color scanning electron microscope image of kimberlite from South Africa. Olivine crystals (green) are in a fine-grained matrix made up of clay minerals and carbonates (presented in blue, purple and buff colors).

Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known as the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.70 g) diamond called the Star of South Africa in 1869 spawned a diamond rush and led to the excavation of the open-pit mine called the Big Hole. Previously, the term kimberlite has been applied to olivine lamproites as Kimberlite II, however this has been in error.

Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes, as well as igneous dykes and can also occur as horizontal sills. Kimberlite pipes are the most important source of mined diamonds today. The consensus on kimberlites is that they are formed deep within Earth's mantle. Formation occurs at depths between 150 and 450 kilometres (93 and 280 mi), potentially from anomalously enriched exotic mantle compositions, and they are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation that makes kimberlites prone to hosting diamond xenocrysts.

Despite its relative rarity, kimberlite has attracted attention because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earth's surface. Its probable derivation from depths greater than any other igneous rock type, and the extreme magma composition that it reflects in terms of low silica content and high levels of incompatible trace-element enrichment, make an understanding of kimberlite petrogenesis important. In this regard, the study of kimberlite has the potential to provide information about the composition of the deep mantle and melting processes occurring at or near the interface between the cratonic continental lithosphere and the underlying convecting asthenospheric mantle.

Morphology and volcanology

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Distribution of kimberlites in Africa. Cratons: CA-Central African (Kasai), SA-South African (Kalahari), WA-West African; Kimberlites (shown as red dots): B-Banankoro, Cu-Cuango Valley, Do-Dokolwayo, F-Finsch, G-Gope, J-Kwaneng, Ja-Jagersfontein, k-Koidu, Kb-Kimberley, Ko-Koffiefontein, L-Letlhakane, Le-Letseng, Lu-Lunda, M-Mitzic, Mb-Mbuji-Mayi, Mw-Mwadui, O-Orapa, P-Premier, R-River Ranch, V-Venetia.

Many kimberlite structures are emplaced as carrot-shaped, vertical intrusions termed "pipes". This classic carrot shape is formed due to a complex intrusive process of kimberlitic magma, which inherits a large proportion of CO2 (lower amounts of H2O) in the system, which produces a deep explosive boiling stage that causes a significant amount of vertical flaring.[1] 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.[2][3]

The morphology of kimberlite pipes and their classical carrot shape is 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 eruptions forming these pipes fracture the surrounding rock as it explodes, bringing up unaltered xenoliths of peridotite to surface. These xenoliths provide valuable information to geologists about mantle conditions and composition.[4][5] The morphology of kimberlite pipes is varied, but 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 (4,900–6,600 ft) of the surface, the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to the surface. The surface expression is rarely preserved but is usually similar to a maar volcano. Kimberlite dikes and sills can be thin (1–4 meters), while pipes range in diameter from about 75 meters to 1.5 kilometers.[6]

Petrology

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Both the location and origin of kimberlitic magmas are subjects of contention. Their extreme enrichment and geochemistry have 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 classified into two distinct varieties, termed "basaltic" and "micaceous" based primarily on petrographic observations.[7] This was later revised by C. B. Smith, who renamed these divisions "group I" and "group II" based on the isotopic affinities of these rocks using the Nd, Sr, and Pb systems.[8] Roger Mitchell 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 show closer affinities to lamproites than they do to group I kimberlites. Hence, he reclassified group II kimberlites as orangeites to prevent confusion.[9]

Group I kimberlites

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Group-I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by primary forsteritic olivine and carbonate minerals, with a trace-mineral assemblage of magnesian ilmenite, chromium pyrope, almandine-pyrope, chromium diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group I kimberlites exhibit a distinctive inequigranular texture caused by macrocrystic (0.5–10 mm or 0.020–0.394 in) to megacrystic (10–200 mm or 0.39–7.87 in) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite, and phlogopite, in a fine- to medium-grained groundmass.[10]

The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, is dominated by carbonate and significant amounts of forsteritic olivine, with lesser amounts of pyrope garnet, Cr-diopside, magnesian ilmenite, and spinel.

Olivine lamproites

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Olivine lamproites were previously called group II kimberlite or orangeite in response to the mistaken belief that they only occurred in South Africa. Their occurrence and petrology, however, are identical globally and should not be erroneously referred to as kimberlite.[11] Olivine lamproites are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O). The distinctive characteristic of olivine lamproites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Al-poor phlogopite requiring Fe to enter the tetrahedral site). 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

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Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with chemical compositions that indicate they formed under high pressure and temperature within the mantle. These minerals, such as chromium diopside (a pyroxene), chromium spinels, magnesian ilmenite, and pyrope garnets rich in chromium, are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.

Geochemistry

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Kimberlites exhibit unique geochemical characteristics that distinguish them from other igneous rocks, reflecting their origin deep within the Earth's mantle. These features provide insights into the mantle's composition and the processes involved in the formation and eruption of kimberlite magmas.

Composition

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Kimberlites are classified as ultramafic rocks due to their high magnesium oxide (MgO) content, which typically exceeds 12%, and often surpasses 15%. This high MgO concentration indicates a mantle-derived origin, rich in olivine and other magnesium-dominant minerals. Additionally, kimberlites are ultrapotassic, with a molar ratio of potassium oxide (K2O) to aluminum oxide (Al2O3) greater than 3, suggesting significant alterations or enrichment processes in their mantle source regions.

Elemental abundance

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Characteristic of kimberlites is their abundance in near-primitive elements such as nickel (Ni), chromium (Cr), and cobalt (Co), with concentrations often exceeding 400 ppm for Ni, 1000 ppm for Cr, and 150 ppm for Co. These high levels reflect the primitive nature of their mantle source, having undergone minimal differentiation.

Rare Earth and lithophile elements

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Kimberlites show enrichment in rare earth elements (REEs),[12] which are pivotal for understanding their genesis and evolution. This enrichment in REEs, along with a moderate to high large-ion lithophile element (LILE)[13] enrichment (more than 1,000 ppm) including potassium, barium, and strontium, points to a significant contribution from metasomatized mantle sources, where the rock composition has been altered by fluids.

Volatile content

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A defining feature of kimberlites is their high volatile content, particularly of water (H2O) and carbon dioxide (CO2). The presence of these volatiles influences the explosivity of kimberlite eruptions and facilitates the transport of diamonds from deep within the mantle to the Earth's surface. The high levels of H2O and CO2 are indicative of a deep mantle origin, where these compounds are more abundant.[14]

Exploration techniques

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Kimberlite exploration techniques encompass a multifaceted approach that integrates geological, geochemical, and geophysical methodologies to locate and evaluate potential diamond-bearing deposits.[15]

Indicator minerals sampling

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Exploration techniques for kimberlites primarily hinge on the identification and analysis of indicator minerals associated with the presence of kimberlite pipes and their potential diamond content. Sediment sampling is a fundamental approach, where kimberlite indicator minerals (KIMs) are dispersed across landscapes due to geological processes like uplift, erosion, and glaciations. Loaming and alluvial sampling are utilized in different terrains to recover KIMs from soils and stream deposits, respectively. Understanding paleodrainage patterns and geological cover layers aids in tracing KIMs back to their source kimberlite pipes. In glaciated regions, techniques such as esker sampling, till sampling, and alluvial sampling are employed to recover KIMs buried beneath thick glacial deposits. Once collected, heavy minerals are separated and sorted by hand to identify these indicators. Chemical analysis confirms their identity and categorizes them. Techniques like thermobarometry help understand the conditions under which these minerals formed and where they came from in the Earth's mantle. By analyzing these indicators and geological curves, scientists can estimate the likelihood of finding diamonds in a kimberlite pipe. These methods help prioritize where to drill in the search for valuable diamond deposits.[16][17]

Geophysical methods

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Geophysical methods are particularly useful in areas where direct detection of kimberlites is challenging due to significant overburden or weathering. These methods leverage physical property contrasts between kimberlite bodies and their surrounding host rocks, enabling the detection of subtle anomalies indicative of potential kimberlite deposits. Airborne and ground surveys, including magnetics, electromagnetics, and gravity surveys, are commonly employed to acquire geophysical data over large areas efficiently. Magnetic surveys detect variations in the Earth's magnetic field caused by magnetic minerals within kimberlites, which typically exhibit distinct magnetic signatures compared to surrounding rocks. Electromagnetic surveys measure variations in electrical conductivity, with conductive kimberlite bodies producing anomalous responses. Gravity surveys detect variations in gravitational attraction caused by differences in density between kimberlite and surrounding rocks. By analyzing and interpreting these geophysical anomalies, geologists can delineate potential kimberlite targets for further investigation, such as drilling. However, the interpretation of geophysical data requires careful consideration of geological context and potential masking effects from surrounding geology, highlighting the importance of integrating geophysical results with other exploration techniques for accurate targeting and successful diamond discoveries.[15][18]

3-D modeling

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Three-dimensional (3D) modeling offers a comprehensive framework for understanding the internal structure and distribution of key geological features within potential diamond-bearing deposits. This process begins with the collection and integration of various datasets, including drill-hole data, ground geophysical surveys, and geological mapping information. These datasets are then integrated into a cohesive digital platform, often utilizing specialized software packages tailored for geological modeling. Through advanced visualization techniques, geologists can create detailed 3D representations of the subsurface geology, highlighting the distribution and geometry of kimberlite bodies alongside other significant geological features such as faults, fractures, and lithological boundaries. Within the model, efforts are made to accurately depict the internal phases of kimberlite pipes, incorporating different facies, country rock xenoliths, and mantle xenoliths identified through careful interpretation of drill-core data and geophysical surveys. Once validated, the 3D model serves as a valuable decision-making tool, offering insights into potential diamond-bearing potential, identifying high-priority drilling targets, and guiding exploration strategies to maximize the chances of successful diamond discoveries.[19][20]

Historical significance

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Kimberlites are a valuable source of information about the composition of the Earth's mantle and the dynamic processes that occur within it. The study of kimberlites has contributed to our understanding of the Earth’s deep geochemical cycles and the mechanism of mantle plumes, which are upwellings of abnormally hot rock within the Earth's mantle.[21]

Moreover, kimberlites are unique in their ability to transport material from the Earth's mantle to its surface. This process, known as xenolith transport, provides geologists with samples of the Earth's mantle, which are otherwise inaccessible. Analyzing these samples has led to significant advances in our knowledge of the Earth's deep interior, including its physical conditions, composition, and the evolutionary history of the planet.

The role of kimberlites in diamond exploration cannot be overstated. Diamonds are formed under the high-pressure, high-temperature conditions of the Earth's mantle. Kimberlites act as carriers for these diamonds, transporting them to the Earth's surface. The discovery of diamond-bearing kimberlites in the 1870s in Kimberley sparked a diamond rush, transforming the area into one of the world’s largest diamond-producing regions. Since then, the association between kimberlites and diamonds has been crucial in the search for new diamond deposits around the globe.[22][23]

Kimberlites also serve as a window into the Earth's past, offering clues about the formation of continents and the dynamic processes that shape our planet. Their distribution and age can provide insights into ancient continental movements and the assembly and breakup of supercontinents.[24]

Economic importance

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Kimberlites are the most important source of primary diamonds. Many kimberlite pipes also produce rich alluvial or eluvial diamond placer deposits. As of 2014 about 6,400 kimberlite pipes are known on Earth including about 900 that have been found to contain diamonds, with mining of diamonds occurring at about 30 pipes.[25]

The discovery of diamond-rich kimberlite pipes in northern Canada during the early 1990s serves as a prime example of how challenging these deposits can be to locate, as their surface features are often subtle. In this case, the pipes were hidden beneath ice-covered shallow ponds, which filled depressions formed by the softer kimberlite rock eroding slightly faster than the surrounding harder rock.[26]

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". Yellow ground kimberlite is easy to break apart and was the first source of diamonds to be mined. Blue ground kimberlite needs to be run through rock crushers to extract the diamonds.[27]

Mir mine

See also Mir Mine and Udachnaya pipe, both in the Sakha Republic, Siberia.

The blue and yellow ground were both prolific producers of diamonds. After the yellow ground had been exhausted, miners in the late 19th century accidentally cut into the blue ground and found gem-quality diamonds in quantity. The economic situation at the time was such that, with a flood of diamonds being found, the miners undercut each other's prices and eventually decreased the diamonds' value down to cost in a short time.[28]

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References

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Further reading

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Kimberlite

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Kimberlite is a rare, ultramafic, volatile-rich formed from deep mantle-derived , characterized by its low silica content, high (>25 wt%), and abundance of phenocrysts in a fine-grained matrix often altered by serpentinization and carbonatization. It typically occurs as vertical pipes, dikes, or sheets emplaced in ancient cratonic regions, resulting from explosive volcanic eruptions that transport mantle xenoliths, xenocrysts, and diamonds to the surface. These rocks originate from small-degree partial melting of peridotite in the asthenospheric mantle at depths exceeding 150–200 km, often triggered by mantle plumes or tectonothermal events, with magma ascending rapidly (up to 400 m/s) due to its low and high volatile content (CO₂, H₂O, and ). Kimberlites are silica-undersaturated and ultrabasic, lacking or , and commonly contain ≥35% alongside , serpentine, , and accessory minerals like and . Kimberlite magmatism has been episodic, occurring from the Archean to as young as approximately 30 million years ago in the early Cenozoic (primarily during the Mesozoic to early Cenozoic), with over 1,000 documented occurrences worldwide, predominantly in stable continental interiors such as those in , , and . Kimberlite volcanism is not currently active, there is no evidence of ongoing or imminent kimberlite volcanism, and there are no indications that new kimberlite pipes will form in the foreseeable geological future. The primary geological significance of kimberlite lies in its role as the principal host for , which form as xenocrysts at depths of 150–700 km under high-pressure conditions and are delivered to the surface via these violent, low-volume eruptions without significant alteration. Beyond diamond , kimberlites serve as critical windows into deep processes, revealing insights into mantle composition, evolution, and ancient dynamics through their geochemical signatures and entrained mantle fragments. Named after the Kimberley region in where diamond-bearing pipes were first identified in the , kimberlites remain enigmatic due to their episodic emplacement and association with cratonic stability.

Definition and Overview

General Characteristics

Kimberlite is a rare, potassic, ultramafic derived from mantle , distinguished by its role in transporting from depths exceeding 150 km within the to the surface. This rock type is characterized by an inequigranular texture, featuring macrocrysts—large, often rounded crystals of and —embedded in a fine-grained groundmass primarily composed of , , and . The term "kimberlite" was coined in 1888, derived from the town of Kimberley in , where diamond-bearing examples were first systematically described in the late 1870s following the 1871 discovery of the Kimberley Mine. Kimberlites typically occur as vertical volcanic pipes, subvertical dikes, or horizontal sills, forming small intrusive bodies that represent the conduits for rapid mantle-derived eruptions. These structures are found worldwide, with emplacement ages spanning from the eon (over 2.5 billion years ago) to the era, though the majority are or younger. For instance, the Fort à la Corne kimberlite field in , , hosts bodies dated to approximately 94–101 million years ago, exemplifying Cretaceous-age occurrences preserved under sedimentary cover. Diamonds in kimberlite appear as xenocrysts, entrained from the mantle source during ascent, underscoring the rock's significance in diamond exploration.

Physical and Textural Properties

Kimberlite exhibits a variable density typically ranging from 2.5 to 3.3 g/cm³, with fresh hypabyssal varieties approaching the higher end and altered volcaniclastic forms falling toward the lower end due to processes like serpentinization, which replaces denser olivine with lower-density serpentine, and carbonatization, which can introduce lighter carbonate phases. This density variation influences geophysical detection methods, such as gravity surveys, where altered kimberlites may show reduced contrast with surrounding country rocks. In hand specimen, kimberlite displays color variations from green to in fresh or minimally , attributed to the presence of and in the groundmass, while weathered exposures often appear yellow-brown due to oxidation and formation. These colors can grade laterally or vertically in pipe structures, with deeper "blue ground" contrasting surficial "yellow ground." Texturally, kimberlite is characteristically or macrocrystic, featuring phenocrysts and xenocrysts up to 1-2 cm in size—often rounded or embayed—set within a fine-grained, to groundmass that may appear glassy or peloidal. Common structural features include breccias with angular country-rock fragments, tuffisitic textures involving fine-grained matrices infilling irregular voids, and concentric zoning in pipe-filling deposits reflecting episodic emplacement phases. Kimberlite is relatively soft, with a Mohs of 2-4, owing to its altered mineral assemblage, making it prone to mechanical breakdown during weathering and mining. This softness contributes to rapid surface alteration, where competent fresh rock weathers into friable yellow soil known as yellow ground in South African contexts, facilitating dispersal but complicating extraction. Hypersolidus textures preserve primary magmatic features like euhedral phenocrysts of and , whereas subsolidus textures reflect post-emplacement alteration, including serpentinization rims and veining. These textural distinctions aid in distinguishing primary igneous fabrics from secondary modifications, with the rapid ascent implied by hypersolidus preservation helping maintain integrity during emplacement.

Origin and Formation

Volcanological Processes

Kimberlite eruptions are characterized by highly explosive styles, often phreatomagmatic, resulting from the interaction of ascending with or water-saturated sediments in the subsurface. This interaction generates thermohydraulic explosions that fragment the and surrounding , producing fine-grained pyroclastic material and well-mixed volcaniclastic deposits. The eruption sequence progresses through distinct : the at the surface, consisting of pyroclastic tuffs and epiclastic sediments; the underlying diatreme facies, a cone-shaped body filled with unbedded breccias; and the deeper hypabyssal , where coherent intrusive rocks form in the root zone. The emplacement of kimberlite involves multistage , beginning with the generation of volatile-rich, ultrabasic melts at depths of 150–200 km in . These melts ascend rapidly through the via narrow dykes, achieving velocities of several meters per second due to exsolution of CO₂ and other volatiles, which enhance and fragmentation. Recent simulations indicate that a minimum CO₂ content of at least 8.2 wt% is necessary for such eruptions to occur, as seen in the kimberlite, ensuring the volatile-driven explosivity required for transport. The process typically spans hours to days, allowing for the incorporation of deep-seated material during transit. This rapid ascent culminates in explosive decompression near the surface, excavating and infilling the pipe structure. Kimberlite pipes exhibit a distinctive carrot-shaped morphology, with steeply dipping walls forming narrow, vertical intrusions that widen slightly upward. These structures range from 0.1 to 2 km in at the surface and extend up to 2 km deep, primarily filled with volcaniclastic comprising fragmented , crystals, and magmatic components in a fine matrix. The breccias result from repeated explosive events that recycle and deposit material within the diatreme. Post-2000 models emphasize the role of fluidization driven by CO₂-rich volatiles in the final stages of eruption, which sustains turbulent mixing of pyroclasts and prevents segregation, thereby enabling the survival and preservation of diamonds during transport. In pipes like those at Lac de Gras, Canada, this fluidization produces high-porosity (>50%), poorly sorted massive volcaniclastic kimberlite, with elutriation of fines enhancing diamond concentration. These processes also facilitate the transport of mantle xenoliths to the surface.

Mantle Source and Emplacement

Kimberlite magmas originate from depths of 150–250 km within the or subcontinental lithospheric mantle, where low-degree (typically <1%) of a carbonated peridotite source produces volatile-rich, carbonate-dominated melts. This process involves the interaction of CO₂ and H₂O with peridotitic mantle, generating primary melts enriched in incompatible elements and volatiles, which are fundamental to kimberlite petrogenesis. The low melting degree ensures that the resulting magma retains primitive mantle signatures while incorporating diamond-stable conditions from these profound depths. Magma generation is often associated with mantle plume activity or lithospheric thinning beneath cratons, which destabilizes the deep mantle and triggers melting. Plumes provide the thermal anomaly necessary for low-degree melting, while thinning reduces the pressure threshold for volatile release, facilitating magma initiation. Initial ascent occurs through hydraulic fracturing of the lithosphere, driven by the high pressure of exsolved volatiles (primarily CO₂ and H₂O) that propagate dikes ahead of the magma body, enabling rapid upward migration with minimal interaction time. This volatile-driven flow maintains low magma viscosity and high buoyancy, allowing the melt to traverse hundreds of kilometers without significant cooling or crystallization. Recent 2025 molecular dynamics simulations of kimberlite melts under varying depths confirm these low-viscosity conditions, tracking atomic movements to model ascent dynamics. Emplacement from mantle source to crustal levels proceeds in distinct phases, typically spanning hours to days, which is critical for preserving mantle-derived xenocrysts such as diamonds and peridotite fragments. The rapid transit minimizes diffusive re-equilibration, retaining sharp chemical zonation in xenocrysts as evidence of minimal residence time. Evidence of pre-emplacement metasomatism is preserved in veined peridotite xenoliths, where kimberlite-like melts infiltrate and alter the host mantle, introducing phlogopite, amphibole, and carbonate veins that reflect fluid-melt interactions prior to ascent. These veins indicate localized enrichment in volatiles and incompatible elements, linking the source region to the final magma composition. Recent seismic tomography studies from the 2020s reveal connections between kimberlite emplacement and deep mantle plumes beneath cratons like the , showing low-velocity anomalies extending from the core-mantle boundary to the lithosphere. High-resolution models, such as AF2019 and AFRP20, image plume-induced lithospheric erosion under southern Africa, correlating with kimberlite clusters and suggesting that plume upwelling thins cratonic roots, promoting magma generation over extended periods. These insights highlight how recurrent plume activity sculpts the mantle architecture, influencing kimberlite distribution across terranes. As of 2025, isotopic studies of primordial neon in kimberlites further support origins in the deep convecting mantle, potentially triggered by plumes interacting with ancient reservoirs, resolving debates on source depth. Kimberlite magmatism is episodic, with distinct pulses rather than continuous activity, and is not currently active. The youngest known kimberlite pipes are tens of millions of years old, primarily from the Mesozoic to early Cenozoic, with some as young as approximately 30 million years. There are no indications that new kimberlite pipes will form in the foreseeable geological future, and there is no evidence of ongoing or imminent kimberlite volcanism.

Classification and Petrology

Group I and Group II Kimberlites

Kimberlites are primarily classified into two genetic groups based on distinct petrological, mineralogical, and isotopic characteristics, a system originally proposed by Smith (1983) using Pb, Sr, and Nd isotopic data from southern African occurrences. Group I kimberlites represent the archetypal variety, characterized by hypabyssal intrusions with a primary mineral assemblage dominated by forsteritic olivine, phlogopite, pyrope garnet, and chromite, derived from volatile-rich, low-silica melts originating from deep mantle sources. These rocks typically exhibit inequigranular textures with macrocrysts of olivine and other mantle-derived phases embedded in a fine-grained groundmass of serpentine, carbonate, and secondary alteration products. Representative examples include the Cretaceous pipes of Kimberley in South Africa and the Triassic to Cretaceous bodies in the Canadian Shield, such as those in the Slave Province. In contrast, Group II kimberlites, later termed orangeites by Mitchell (1995) to highlight their distinct petrogenesis and avoid confusion with Group I, are marked by higher abundances of titanium-enriched minerals, including Ti-phlogopite, Ti-rich pyrope, and rutile or Ti-magnetite, alongside phlogopite and lesser olivine. These rocks show macrocrystic textures with abundant phlogopite macrocrysts and a groundmass featuring zoned diopside, perovskite, apatite, and calcite, often reflecting a more evolved composition transitional toward lamproites. They are predominantly found in the of South Africa, with ages ranging from approximately 90 to 140 million years, such as the Orange River occurrences. Petrological criteria for distinguishing the groups include modal mineralogy, with Group II displaying elevated phlogopite (up to 35 vol.%) and reduced olivine compared to Group I. Evolutionary models posit that Group I kimberlites arise from primitive, asthenospheric sources through low-degree partial melting of carbonated peridotite at depths exceeding 150 km, facilitating the transport of deep-seated xenoliths. Group II orangeites, however, are interpreted to derive from shallower lithospheric mantle via a two-stage process involving metasomatism by CO₂- and H₂O-rich fluids followed by partial melting of recycled crustal components, leading to their Ti-enriched signatures. Indicator minerals such as pyrope garnet and chromite serve as discriminators between groups, with Group II variants showing higher Ti contents. Lamproites are ultrapotassic, silica-poor volcanic rocks characterized by the presence of distinctive minerals such as priderite and wadeite, which are rare in other ultramafic lithologies. These rocks are typically diamondiferous, though less commonly exploited than kimberlites, with the Argyle mine in Western Australia representing one of the world's largest and highest-grade lamproite diamond deposits. Petrologically, lamproites differ from kimberlites through elevated TiO₂ contents and generally lower Al₂O₃, reflecting derivation from metasomatized subcontinental lithospheric mantle sources. Orangeites, previously classified as Group II kimberlites, represent a transitional rock type within the broader ultramafic spectrum, featuring macrocrystic assemblages dominated by olivine, ilmenite, and phlogopite. These rocks are highly micaceous and ultrapotassic, with a volatile-rich composition that facilitates rapid ascent, and they are predominantly associated with the Kaapvaal Craton in southern Africa, often linked to episodes of continental rifting. Key petrological distinctions among these rocks include source depth and metasomatism styles: lamproites originate from higher-pressure mantle environments exceeding 200 km, involving intense K-rich metasomatism, whereas orangeites and kimberlites derive from somewhat shallower lithospheric levels with varying degrees of carbonatitic influence. All share a common mechanism of volatile-driven (CO₂- and H₂O-rich) emplacement, enabling explosive diatreme formation, but differ in the extent and type of mantle metasomatism that shapes their mineralogy and bulk compositions. Post-2010 classifications have increasingly adopted the "kimberlite clan" terminology to encompass kimberlites, orangeites, and certain lamproite variants (such as leucite-bearing types), emphasizing shared mantle-derived, potassic-ultramafic affinities while maintaining petrographic boundaries for differentiation. This broader grouping aids in understanding their collective role in diamond exploration, where distinguishing these rocks poses similar challenges due to overlapping indicator mineral suites.

Mineralogy

Primary Mineral Assemblage

The primary mineral assemblage of kimberlite consists predominantly of olivine, , , and , which together define its ultramafic, volatile-rich character and contribute to the rock's distinctive inequigranular texture. Olivine is the most abundant phase, forming rounded to subhedral macrocrysts and microcrysts with forsterite contents ranging from Fo88 to Fo92, though it is commonly altered to pseudomorphs due to interaction with hydrothermal fluids. occurs as euhedral to subhedral plates and flakes, typically Ti-poor in Group I kimberlites, and plays a key role in the rock's foliated or radiating textures. forms interstitial patches and veins, derived from a primary carbonate-rich melt component that facilitated the magma's low during emplacement. crystals exhibit compositional zoning, evolving from cores to magnesiochromite rims, reflecting progressive under changing oxygen fugacity conditions. Accessory minerals such as ilmenite, perovskite, and apatite are ubiquitous but subordinate, appearing as discrete grains or inclusions that mark early magmatic stages. In rare fresh samples, the groundmass includes monticellite and melilite, which form microlites and contribute to a hypabyssal texture before widespread alteration replaces them with secondary phases. Alteration products dominate most kimberlites, with serpentine forming mesh-like pseudomorphs after olivine and clay minerals (such as smectite) infilling fractures, leading to a zoned distribution from relatively fresh cores to highly altered rims in kimberlite pipes. Macrocrysts and phenocrysts, primarily olivine and phlogopite, comprise 30–50% of the rock volume, with the remainder being fine-grained groundmass, though truly fresh kimberlite is exceptionally rare owing to pervasive devolatilization and fluid-mediated alteration. This assemblage may include minor mantle-derived xenocrysts incorporated during ascent.

Indicator Minerals

Indicator minerals in kimberlite are primarily mantle-derived xenocrysts that serve as diagnostic tracers for potential diamond-bearing pipes due to their specific chemical compositions and textural features acquired during transport from depth. These minerals, including Cr-rich pyrope garnet (particularly the G10 suite), chrome diopside, chromite, and ilmenite, originate from the upper mantle and are sampled during kimberlite eruption, providing evidence of the rock's deep-seated origin. Diamond itself acts as the ultimate but exceedingly rare indicator, occurring in concentrations typically below 1.4 ppm in kimberlite. Cr-rich pyrope garnets of the G10 suite are subcalcic (low CaO) and derive from or sources, characterized by high Cr₂O₃ contents (up to 9.9 wt%) and often featuring inclusions or sinusoidal zoning patterns reflective of metasomatic processes in the mantle. These garnets, typically 0.1–1.0 cm in size, exhibit resorption textures such as rounded edges and kelyphitic rims formed during rapid ascent through the . Chrome diopside, a clinopyroxene, is distinguished by its emerald-green color and elevated Cr₂O₃ (>1 wt%), forming prismatic crystals 1–5 mm long that also display resorption due to the explosive ascent. Chromite shows high Cr₂O₃ (>61 wt%) and MgO (10–16 wt%), with octahedral habits and resorption pits indicating disequilibrium during transport. Ilmenite, often magnesian (MgO >4 wt%), appears as black, paramagnetic grains and similarly bears resorption textures from the kimberlite's volatile-rich environment. These indicator minerals equilibrated at depths of 80–150 km in , where they formed in peridotitic or eclogitic assemblages before being entrained by kimberlite . The rapid ascent, at rates of several to tens of meters per second, preserves their diagnostic features while imparting characteristic resorption, enabling their use in to delineate kimberlite targets. In recent advancements from the 2020s, and have emerged as indicators for even deeper mantle sources (>200 km), with mantle-equilibrated zircons identified through filters that distinguish them from crustal varieties and link them to sub-lithospheric processes. These minerals expand the geochemical toolkit for tracing ultra-deep sampling in kimberlite .

Geochemistry

Major and Trace Element Composition

Kimberlites exhibit an ultramafic-potassic composition dominated by low silica and high magnesia contents, reflecting their derivation from mantle sources. Typical major oxide abundances include SiO₂ ranging from 20 to 45 wt% (median ~31 wt%), MgO from 25 to 40 wt% (median ~27 wt%), and CaO from 2 to 25 wt%. These rocks are also characterized by low Al₂O₃ (<5 wt%), with values often between 1.9 and 4.0 wt%. The alkali content underscores their potassic nature, with K₂O typically 0.5 to 2 wt% (median ~0.8 wt%) and Na₂O remaining low at <1 wt% (median ~0.1 wt%). The following table summarizes representative ranges for key major oxides based on global datasets and regional studies:
OxideTypical Range (wt%)Notes
SiO₂20–45Median 30.9; lower values in uncontaminated samples
MgO25–40Median 27.3; reflects high olivine content
CaO2–25Variable due to carbonate phases
Al₂O₃<5Often 1.9–4.0; low due to minimal crustal input
K₂O0.5–2Median 0.78; potassic signature
Na₂O<1Median 0.12; subdued sodic character
TiO₂0.3–5Variable; Group I typically <3 wt%, Group II higher (3–6 wt%)
Trace element profiles highlight enrichment in compatible elements inherited from , with Ni abundances of 500–2000 ppm (e.g., 1053–2182 ppm in fresh samples) and Cr of 1000–3000 ppm (e.g., 1135–1868 ppm). These high levels correlate with and abundances, though post-emplacement fractionation can deplete compatible elements in evolved melts. Compositional variations exist between kimberlite groups, notably in TiO₂: Group I kimberlites typically have lower values (<3 wt%), while Group II kimberlites exhibit higher TiO₂ contents (3–6 wt%). Wall-rock during emplacement further modifies , elevating SiO₂, Al₂O₃, and Na₂O while potentially diluting incompatible trace elements; contamination indices >1.5 indicate significant crustal influence. Whole-rock geochemical analyses of kimberlites commonly employ (XRF) spectrometry for major elements and (ICP-MS) for trace elements, enabling precise quantification of abundances down to ppm levels.

Isotopic and Volatile Signatures

Kimberlites exhibit Sr-Nd isotopic signatures indicative of derivation from a time-integrated depleted to slightly enriched mantle source. Initial 87^{87}Sr/86^{86}Sr ratios typically range from 0.703 to 0.705, reflecting minimal long-term Rb enrichment in the source relative to bulk composition. Similarly, εNd values fall between +2 and +6, consistent with a moderately depleted mantle reservoir that has undergone limited . These compositions align with EM1-like enriched mantle components in some models of kimberlite petrogenesis, where low-velocity zones in the contribute to the isotopic heterogeneity. Lead isotopic systematics in kimberlites reveal variable influences, with non-micaceous varieties showing radiogenic 206^{206}Pb/204^{204}Pb ratios suggestive of elevated U/Pb in the source, while micaceous types display lower U/Pb and potential crustal contamination signatures in Pb evolution trends. Rare earth element (REE) patterns further support a deep mantle origin involving -bearing residues, characterized by strong light REE (LREE) enrichment relative to heavy REE (HREE), with (La/Yb)N_N ratios exceeding 10 and steeply negative slopes in the LREE portion. The flat HREE segment reflects retention of HREE in residual during at depths greater than 150 km. Volatile components are abundant in kimberlite magmas, with CO2_2 contents ranging from 5 to 30 wt% in primitive melts, facilitating low-viscosity ascent and emplacement. H2_2O concentrations vary from 3 to 12 wt%, often coexisting with CO2_2 in molar ratios that promote saturation at mantle pressures. and are elevated, reaching up to 3 wt% Cl in fresh samples, while occurs primarily as sulfides, contributing to the state of the magma. These volatiles likely result from fluxing by subducted carbon in the mantle source, enhancing melting in thermochemical upwellings. Recent helium isotopic analyses (post-2020) indicate contributions from primordial mantle plumes, with 3^3He/4^4He ratios elevated above mid-ocean ridge basalt values in some kimberlites, despite overprinting by lithospheric components. This supports models where deep mantle domains, including , supply volatiles and drive kimberlite generation.

Exploration Methods

Indicator Mineral Sampling

Indicator mineral sampling serves as a primary exploration technique for identifying kimberlite pipes through the collection of surface and subsurface materials, including , sediments, and , which may contain diagnostic derived from kimberlite sources. Samples are typically 10-20 kg for sandy or materials and 30-50 kg for clay-rich to ensure sufficient heavy for . In glaciated terrains, sampling targets the C-horizon below the solum to avoid modern influences, with systematic grids spaced from 50 m to several kilometers depending on the exploration scale. Heavy mineral concentrates (HMC) are extracted by wet sieving to isolate the 0.25-2.0 mm fraction—often focusing on 0.25-0.50 mm for optimal recovery—and employing acid dissolution to disintegrate carbonates and clay matrices that could obscure grains. Processing of HMC involves density sorting using heavy liquids at specific gravities of 3.1-3.2 to recover dense minerals like (SG 3.3-3.6), followed by to isolate ferromagnetic phases such as . The resulting concentrates are then examined for indicator minerals, including Cr-diopside, Cr-pyrope , and Mg-ilmenite, via electron microprobe analysis (SEM-EDS or WDS) to assess chemical compositions. Diagnostic thresholds include >0.5 wt.% Cr₂O₃ for Cr-diopside identification, with anomalous concentrations in HMC exceeding 1-5 ppm signaling potential kimberlite proximity, as grain abundances decrease with distance from the source. In glacial settings, vertical dispersion models evaluate mineral distribution through —often forming ribbon-shaped plumes 3 m thick—to reconstruct flow directions and estimate source locations, with short dispersal trains (<3 km) indicating nearby pipes and longer ones (10-15 km) reflecting broader transport. Post-2010 advancements in automated mineralogy, such as QEMSCAN and MLA systems, have transformed processing by enabling high-throughput SEM-based scanning of 10,000-20,000 particles per sample in 1-2 hours, bypassing labor-intensive hand-picking and minimizing operator bias. These tools mount polished HMC epoxy pucks for quantitative mapping of mineral assemblages, textures, and chemistries at resolutions down to 10 μm, facilitating analysis of finer fractions and detection of subtle kimberlite signatures like altered inclusions. Such methods enhance vectoring toward pipes in complex glacial terrains by integrating mineral data with geochemical profiles for more precise exploration targeting.

Geophysical and Modeling Techniques

Geophysical techniques play a crucial role in kimberlite exploration by detecting subsurface anomalies associated with these ultramafic intrusions, leveraging contrasts in physical properties such as magnetic susceptibility, density, and electrical conductivity. Magnetic surveys are particularly effective due to the high magnetite content in many kimberlites, which generates strong positive anomalies; high-resolution aeromagnetic methods are commonly employed for initial regional targeting to identify potential pipe-like features. Gravity surveys complement magnetics by exploiting the relatively low density of kimberlite pipes compared to host rocks, producing negative Bouguer anomalies that help delineate pipe margins and depths. Electromagnetic (EM) methods target conductive sulfides within kimberlites, providing additional constraints on geometry in conductive overburden environments, while induced polarization (IP) surveys measure chargeability to infer depth to basement and distinguish kimberlite from surrounding lithologies. Airborne surveys, including aeromagnetics and airborne EM, enable broad-scale reconnaissance over large areas with challenging terrain, efficiently mapping magnetic highs indicative of kimberlite clusters before ground follow-up. Ground-based surveys, such as detailed magnetic traverses and IP arrays, offer higher resolution for delineating pipe outlines and estimating overburden thickness, particularly in areas with glacial cover or weathered surfaces that obscure surface expressions. These methods are often integrated with indicator mineral sampling to prioritize drill targets, where geophysical anomalies guide till sampling locations. Seismic reflection profiling, though less routine, images the crustal structure around kimberlite pipes, revealing dyke-sill geometries and emplacement pathways through high-velocity contrasts. Recent advancements in core logging following drilling include hyperspectral imaging and X-ray fluorescence (XRF) mapping applied to drill cores, as tested by De Beers Canada in 2024, enabling rapid mineral identification and proposing AI-assisted analysis for improved accuracy and objectivity. Computational modeling advances have enhanced interpretation by inverting geophysical data to construct three-dimensional representations of kimberlite bodies. Joint 3D inversion of gravity and magnetic datasets constrains pipe geometry, density contrasts (typically 0.2–0.5 g/cm³ lower than host), and magnetization, as demonstrated in studies over Botswanan kimberlites where models matched borehole validations. These inversions incorporate priors like smoothness constraints to resolve depth extents up to several hundred meters, aiding in volume estimates for resource assessment. In the 2020s, drone-based magnetic surveys have improved accessibility in remote or hazardous areas, achieving resolutions comparable to ground methods with reduced logistical costs, as shown in diamond exploration trials in India. Artificial intelligence techniques, including machine learning algorithms for anomaly detection, have been applied to process vast airborne datasets, identifying subtle kimberlite signatures amid noise, as evidenced by recent analyses uncovering new targets in Botswana.

Significance and Applications

Economic Role in Diamond Mining

Kimberlite serves as the primary host rock for economically viable diamond deposits, with commercial mining operations targeting kimberlite pipes that contain diamonds formed deep within the Earth's mantle. Diamond grades in these pipes vary widely but typically range from 0.1 to 2 carats per tonne, as observed in operations like the Ekati mine in Canada, where bulk sampling has shown averages within this spectrum for multiple kimberlite bodies. Global production from kimberlite sources reached approximately 111.5 million carats of rough diamonds in 2023, increasing to about 118 million carats in 2024, accounting for the vast majority of the world's natural diamond output, as alluvial deposits contribute only a minor fraction. Mining methods depend on pipe depth and geometry, with open-pit techniques employed for shallower deposits, such as at the Venetia mine in South Africa, where surface operations extracted over 143 million carats before transitioning to underground methods in late 2022. Deeper pipes, like those at the Diavik mine in Canada, utilize underground approaches including blast-hole stoping and sub-level caving to access kimberlite at depths exceeding 500 meters. Post-extraction, kimberlite ore undergoes processing involving primary crushing to liberate diamonds, followed by dense media separation and X-ray transmission sorting to recover stones with high efficiency and minimal waste. Key challenges in kimberlite diamond mining include low overall yields, with approximately 80% of recovered rough diamonds being of gem or near-gem quality suitable for jewelry, while the remainder serves industrial purposes. Environmental remediation of kimberlite tailings poses additional hurdles, as these alkaline, fine-grained wastes require soil amelioration and native plant seeding to restore ecosystems and prevent erosion, often incorporating carbon mineralization techniques to mitigate greenhouse gas emissions. The global rough diamond market from kimberlite sources generated approximately $12.7 billion in value in 2023, underscoring its economic significance despite fluctuations. Following 2020, the industry has shifted toward enhanced ethical sourcing, with strengthened protocols emphasizing traceability, conflict-free production, and sustainable practices across kimberlite operations.

Historical and Scientific Importance

Kimberlite was first identified as the host rock for diamonds in 1871 with the discovery of the Kimberley pipe in South Africa, where surface exposures revealed the distinctive blue ground containing gem-quality stones. This event spurred intensive geological investigation, leading to the formal naming of the rock "kimberlite" in the late 19th century based on its occurrence at Kimberley. Early studies recognized kimberlite's ultramafic composition and volcanic origin, distinguishing it from surrounding country rocks and establishing it as a key carrier of deep mantle materials to the surface. In the 1970s, significant advances in kimberlite petrology came from the mineralogical classification proposed by Skinner and Clement, which categorized southern African kimberlites based on primary mineral abundances such as , , and pyrope garnet. This framework highlighted textural and genetic variations, aiding in the differentiation of hypabyssal and volcanic facies. By the 1980s, further refinement introduced the Group I and Group II distinction, with Group I kimberlites showing primitive mantle signatures and Group II exhibiting more enriched, potassic characteristics linked to metasomatized lithosphere. Scientific studies of kimberlites have profoundly advanced mantle geoscience, particularly through xenolith analyses in the 2000s that revealed craton evolution over billions of years. For instance, Re-Os isotope data from peridotite xenoliths in Lesotho kimberlites indicated ancient depletion events in the Kaapvaal craton lithosphere dating back to 2.9 billion years, with subsequent refertilization. These findings illuminated stabilization mechanisms of continental roots. Kimberlites also transport diamonds formed at depths of 150–250 km (lithospheric) or up to 700 km (sublithospheric) under high-pressure conditions of 900–1300°C, providing direct samples of subcratonic mantle conditions otherwise inaccessible. Isotopic signatures, including Sr-Nd-Hf systems, have informed mantle convection models by tracing kimberlite sources to deep, convecting reservoirs rather than solely lithospheric domains. Recent research from 2023–2025 has extended these insights through advanced analysis of fluid inclusions in kimberlitic diamonds and olivines, linking ancient mantle fluids to broader paleoenvironmental reconstructions via trapped volatiles and isotopic proxies. For example, 2025 studies on primordial neon in kimberlite fluids suggest origins in the deep convecting mantle, while analyses of high-density fluids in diamonds from the No. 50 kimberlite pipe in China highlight connections to kimberlite magmatism. Such studies underscore kimberlites' role in probing deep Earth processes over geological timescales.

Major Occurrences

Kimberlite pipes are predominantly clustered within Archean cratons, with over 7,000 known occurrences worldwide as of the 2020s, of which approximately 10% are diamondiferous. These distributions reflect episodic magmatism tied to deep mantle processes, spanning from the to the , though the majority erupted during the . The Kaapvaal Craton in South Africa hosts one of the densest concentrations of kimberlite pipes, with more than 1,000 identified across the region, many dating from 1 to 2.9 billion years ago (Ga), including some of the oldest known examples around 1.6 Ga in the Kuruman Province. These pipes, such as those near Kimberley, have been instrumental in diamond production, underscoring the craton's economic significance. In the of Canada, over 350 pipes are documented as of 2023, with emplacement ages ranging from 45 million years ago (Ma) to 2.5 Ga, though the majority cluster in the Late Cretaceous to Eocene (45–80 Ma) in areas like Lac de Gras. The Siberian Platform in Russia features more than 1,000 pipes, primarily in the Yakutian province, with ages spanning the Devonian (around 360 Ma) to the Jurassic, including major fields like Mirny and Udachnaya. Beyond these primary provinces, kimberlites occur in other regions, such as West Africa, where clusters in Guinea date to the Jurassic (around 155–180 Ma) and intrude Archean basement rocks of the Man Craton. In Australia, the AK1 field (Argyle), though technically a lamproite, represents a Proterozoic example at approximately 1.18 Ga within the Kimberley Craton. Rare Phanerozoic occurrences include the Cretaceous Fort à la Corne field in Canada, with over 70 pipes emplaced around 95–105 Ma near the edge of the craton. Emerging provinces, such as the Alto Paraguai in Brazil, have seen discoveries of around 10 new kimberlite pipes since 2020. Tectonically, kimberlites are strongly associated with stable Archean cratons, where thick lithospheric roots facilitate mantle upwelling, leading to spatial clustering of pipes. Younger fields often link to rifting events, such as those during the breakup of supercontinents, which trigger volatile-rich melts from the asthenosphere. This pattern highlights the role of cratonic stability in preserving deep-sourced magmas while rifting influences post-Archean emplacement.

Comparisons with Similar Ultramafic Rocks

Kimberlite, a volatile-rich ultramafic rock, differs from komatiites in both source conditions and composition. Komatiites represent high-degree partial melts (>30%) of an essentially mantle source at temperatures exceeding 1600°C, resulting in high-Mg basaltic compositions with low and minimal volatiles. In contrast, kimberlites arise from low-degree (<5%) melting of volatile-enriched sources, yielding potassic magmas rich in CO₂ and H₂O, which facilitate their rapid ascent and distinctive mineralogy. These differences highlight kimberlite's role in sampling metasomatized mantle domains, unlike the primitive, high-temperature origins of komatiites. Compared to carbonatites, kimberlites exhibit a hybrid silicate-carbonate , with typically less than 50 vol% primary carbonates and higher MgO contents (20–29 wt%). Carbonatites, by , contain over 50 vol% carbonate minerals and lower MgO/CaO ratios, reflecting derivation from more Ca-rich, oxidized sources at shallower depths (90–150 km) and lower temperatures (1000–1100°C). Both rock types can host , but kimberlites form steep-sided volcanic pipes that entrain deep mantle xenoliths, whereas carbonatites typically occur in ring complexes or dikes with limited deep sampling. Picrites, as olivine cumulate-rich ultramafic rocks, share high MgO levels with kimberlites but lack the elevated volatile contents and that define kimberlitic magmas. Picrites derive from higher-degree melting of mantle sources with lower CO₂ and H₂O, often without the mica-bearing phases prominent in kimberlites. This distinction underscores kimberlite's unique metasomatic imprint, contrasting with the more primitive, cumulate textures of picrites. In broader context, kimberlites, komatiites, carbonatites, and picrites all originate in or settings, but kimberlites stand out for their ability to sample depths greater than 150 km, entraining and eclogitic xenoliths from the transition zone. Lamproites show some overlap with kimberlites in volatile and potassic signatures.

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