Massive Volcaniclastic Kimberlite Research Articles

Geology of the Renard 65 kimberlite pipe, Québec, Canada

Renard 65, a diamondiferous pipe in the Neoproterozoic Renard kimberlite cluster (Quebec, Canada), is a steeply-dipping and downward-tapering diatreme comprised of three pipe-filling units: kimb65a, kimb65b, and kimb65d. The pipe is surrounded by a marginal and variably-brecciated country rock aureole and is crosscut by numerous hypabyssal dykes: kimb65c. Extensive petrographic and mineralogical characterization of over 700 m of drill core from four separate drill holes, suggests that Renard 65 is a Group I kimberlite, mineralogically classified as phlogopite kimberlite and serpentine-phlogopite kimberlite. Kimb65a is a massive volcaniclastic kimberlite dominated by lithic clasts, magmaclasts, and discrete olivine macrocrysts, hosted within a fine-grained diopside and serpentine-rich matrix. Kimb65b is massive, macrocrystic, coherent kimberlite with a groundmass assemblage of phlogopite, spinel, perovskite, apatite, calcite, serpentine and rare monticellite. Kimb65c is a massive, macrocrystic, hypabyssal kimberlite with a groundmass assemblage of phlogopite, serpentine, calcite, perovskite, spinel, and apatite. Kimb65d is massive volcaniclastic kimberlite with localized textures that are intermediate between volcaniclastic and coherent, with tightly packed magmaclasts separated by a diopside- and serpentine-rich matrix. Lithic clasts of granite-gneiss in kimb65a are weakly reacted, with partial melting of feldspars and crystallization of richterite and actinolite. Lithic clasts in kimb65b and kimb65d are entirely recrystallized to calcite + serpentine/chlorite + pectolite and display inner coronas of diopside-aegirine and an outer corona of phlogopite. Compositions are reported for all minerals in the groundmass of coherent kimberlites, magmaclasts, interclast matrices, and reacted lithic clasts. The Renard 65 rocks are texturally classified as Kimberley-type pyroclastic kimberlites and display transitional textures. The kimberlite units are interpreted to have formed in three melt batches based on their distinct spinel chemistry: kimb65a, kimb65b and kimb65d. We note a strong correlation between the modal abundances of lithic clasts and the textures of the kimberlites, where increasing modal abundances of granite/gneiss are observed in kimberlites with increasingly fragmental textures.

The geology and emplacement of the volcaniclastic infill at the Voorspoed Group II kimberlite (orangeite) pipe, Kroonstad Cluster, South Africa

The Voorspoed Group II kimberlite pipe is atypical in terms of South African kimberlite pipes. Located in the central region of the Kaapvaal Craton (South Africa), the Voorspoed pipe is one of six pipes comprising the Kroonstad Kimberlite Cluster. Reconstruction of the palaeo-stratigraphy at the time of emplacement, of the Kroonstad kimberlites, indicates that significant post-emplacement erosion has occurred and the pipes are currently exposed at ~1600m depth from the original land surface. The volcaniclastic kimberlite (VK) infilling the pipe is distinctly different from typical tuffisitic kimberlite infilling other South African type kimberlite pipes. Three textural volcaniclastic kimberlite varieties are observed: massive volcaniclastic kimberlite (MVK), bedded volcaniclastic kimberlite (BVK) and fine-grained volcaniclastic kimberlite (fgVK) layers. The BVK and fgVK are volumetrically insignificant and the bulk of the pipe is infilled with MVK. The MVK can be further sub-divided into seven varieties. The two dominant varieties are described in detail, which include: main MVK (mMVK) and the juvenile-rich MVK (jrMVK). The MVKs are typically massive, clastic, poorly sorted and supported by an interclast material consisting of juvenile and xenocrystic crystals in a mud-rich matrix. Magmaclasts within the MVKs are spherical, crystalline with a typical coherent hypabyssal texture and different mineralogical varieties are typically juxtaposed. BVK units are composed of several normally graded beds of volcaniclastic material. fgVK layers are well sorted relative to the MVKs and all constituents are typically <2mm in size. The BVK and fgVK can be deposited only within an open vent in order to account for the sorting of the components. A distinct basalt and sandstone rich unit (bsBreccia), which is essentially devoid of kimberlite material, is also observed. Three volcaniclastic zones are identified based on consistent vertical variation in the internal stratigraphy of the infill: outer east/west zone with a sequence bsBreccia–BVK–mMVK/softMVK; a central zone with a sequence mMVK–BVK–jrMVK–mMVK and an outer north zone with a uniform mMVK sequence.The evidence suggests that the volcaniclastic infill has formed through complex processes involving re-working of extra-crater tephra back into the vent and coeval pyroclastic deposition. It is also likely that there is significant mixing of the resedimented and pyroclastic deposits. Distinction between deposits formed through re-working of material through repeated explosive eruptions within the vent and that formed through varying degrees of re-working extra-crater tephra from the surface (with possible coeval pyroclastic eruption) is probably impossible. For this reason we interpret the mMVK as being deposited by dominantly resedimentation of tephra whereas the jrMVK has been deposited dominantly by pyroclastic processes. The presence of BVK and fgVK deposited deep within the pipe through slumping of the crater margins at the surface suggests that a crater ~2000m in depth was periodically open.The identification of the complex, layered, variable internal structure of the volcaniclastic infill at the Voorspoed has significant implications for diamond ore evaluation in South Africa. Typical South Africa kimberlite pipes are infilled with tuffisitic kimberlite, which are characterised by their homogenous nature over large vertical intervals (>1000m). This is not the case at the Voorspoed pipe where MVK layers are variable in thickness (<10m up to 230m).

Geology of the BK9 kimberlite (Damtshaa, Botswana): implications for the formation of dark volcaniclastic kimberlite

The BK9 kimberlite consists of three overlapping pipes. It contains two dark varieties of massive volcaniclastic kimberlite, informally termed dark volcaniclastic kimberlite (DVK). DVK(ns) is present in the north and south pipes and is interbedded with lenses of basalt breccia at the margins of the pipes. DVK(c) is present within the central pipe where it is overlain by a sequence of basalt breccias with interbedded volcanogenic sediments. The features observed within the DVK units of the BK9 kimberlite provide strong evidence for gas fluidisation of the accumulating pyroclastic material. These include the massive interior of the pipes, marginal epiclastic units, well-dispersed country-rock xenoliths and small-scale heterogeneities in lithic clast abundance. The upper portions of the central pipe provide a record of the transition from pyroclastic eruption and infill to passive epiclastic infilling of the crater, after the eruption has ceased. The wall-rock of the BK9 kimberlite dips inwards and is interpreted as post pipe-fill subsidence of the adjacent country rock. The two DVK units contain interstitial, silt-sized pyroclasts. The DVK(ns) has a higher fraction of former melt and displays evidence of incipient welding, as a result of differences in eruption dynamics. These units demonstrate that whilst DVK is comparable in many respects to MVK and forms part of a spectrum of volcaniclastic rocks formed by fluidisation, it differs in frequently containing silt-sized particles and including agglutinated and welded varieties with a high melt fraction. The DVK varieties, studied here, also have a distinctive hydrothermal assemblage, resulting from the abundance of low-silica accidental lithic clasts. Both the hydrothermal alteration and the abundance of silt-sized particles contribute to the DVKs distinctive dark colour.

The internal geology and emplacement history of the Renard 2 kimberlite, Superior Province, Quebec, Canada

The Renard 2 kimberlite is located in the Otish Mountains region of Quebec, Canada and is one of the largest pipes in the Renard cluster. The cluster consists of nine kimberlite bodies and was discovered in 2001 by Ashton Mining of Canada Inc. and its joint venture partner SOQUEM Inc. Renard 2 was emplaced into Archean meta-greywacke derived migmatite, gneiss and granite of the Opinaca Subprovince of the eastern Superior Province at approximately 640.5 ± 2.8 Ma. An undetermined amount of erosion has occurred since emplacement with the present surface expression of the pipe estimated to be 0.75 ha. This kimberlite is interpreted as a steep-sided diatreme with minor irregularities in the external shape. The dominant infill is a massive volcaniclastic kimberlite (MVK) that is classified as tuffisitic kimberlite breccia (TKB) and is characterized by a high proportion of granitoid country rock xenoliths. A second dominant infill is a texturally complex, less diluted coherent kimberlite (CK) characterized locally by a transitional textures between CK and TKB. Surrounding the diatreme is a significant zone of variable width comprised of extensively brecciated country rock (+/−kimberlite) and referred to as marginal breccia. In addition to the two main rock types infilling the pipe, a number of hypabyssal kimberlite (HK) dykes and irregular shaped intrusions occur throughout the body, along the pipe contacts, within the marginal breccia and in the surrounding country rock. Geological features displayed by Renard 2 are similar to those described from Class 1 kimberlites of the Kimberley area of South Africa, the Gahcho Kué cluster of Canada and the Pimenta Bueno kimberlite field of Brazil. The economic evaluation of Renard 2 is in progress and to date has included extensive diamond and reverse circulation drilling as well as the collection of an underground bulk sample. Results from material sampled from Renard 2, including a 2449 tonne bulk sample, suggest Renard 2 has an estimated diamond content of 83 cpht (carats per hundred tonnes). A three dimension geology model of the pipe has been developed following the investigation of drill cores, subsurface mapping and petrography combined with diamond studies and geophysics. The model produced is being used to guide and direct the evaluation of the kimberlite and unravel the emplacement history of the pipe.

The role of gas-fluidisation in the formation of massive volcaniclastic kimberlite

Massive volcaniclastic kimberlite (MVK) deposits are a volumetrically important constituent in most kimberlite diatremes. MVKs are typically characterised by their uniform texture and homogeneous composition, and previous studies suggest that they exhibit little appreciable internal variation. However, new field results from the Venetia K1 diatreme (South Africa) show that MVK displays significant changes over metre-scales. The textural characteristics and large- and small-scale structure of the deposits are described and interpreted as being due to fluidisation. Laboratory experiments of tapered fluidised beds demonstrate how they are heterogeneous, with fluidisation and mixing limited to the centre of the bed. Marginal wedge-shaped regions remain unfluidised. The unfluidised regions are internally laminated and slip downwards when a high proportion of the bed is fluidised. The observations also demonstrate how fluctuations in gas velocity can produce steep internal boundaries between laminated and well-mixed regions. Data consistent with gas-fluidisation in MVK deposits includes the massive structure, uniform distribution of lithic clasts, presence of steep internal contacts, dominantly sub-vertical crystal fabrics across the vent, occurrence of surface-derived material along the tapered diatreme walls, and the presence of degassing structures. We propose that the overall structure of the Venetia K1 diatreme-fill is consistent with a multi-stage fluidised emplacement, which involved (1) strong initial fluidisation that caused thorough mixing of juvenile and lithic material; (2) a gradual step-wise decrease in gas flux, that produced nested pipe-like sequences; and (3) migration of the narrow fluidised column towards the steep eastern diatreme wall. The occurrence of (4) degassing structures in MVK deposits across the vent is consistent with an overall rapid transition from high to low degrees of fluidisation towards the end of the eruption.

Pyroclastic flow deposits from a kimberlite eruption: The Orapa South Crater, Botswana

The Orapa Diamond Mine (Republic of Botswana) exposes a bi-lobate kimberlite pipe that erupted during the Late-Cretaceous epoch (~ 93 Ma) through Archaean basement and volcano–sedimentary rocks of the Karoo Supergroup. Geological mapping of the crater zone of the South Pipe has revealed a 15–25 m thick in-situ kimberlite pyroclastic flow deposit. The pyroclastic deposit fills in the crater and completely drapes lower units, indicating that the parent flow originated from an adjacent kimberlite pipe. The unit comprises a basal coarse lithic concentration layer exhibiting imbricated clasts, which grades upwards into massive poorly sorted lapilli tuff. The tuff contains abundant sub-vertical degassing structures defined by lithic enrichment and depletion in fine-grained material. Degassing structures commonly emanate from blocks in the basal layer. The presence of degassing structures and a coarse basal layer distinguishes this deposit from pipe-filling massive volcaniclastic kimberlite, which is typically homogeneous in terms of texture and clast size over distances on the order of 100s of metres. Studies of the thermal remanent magnetism in basalt clasts from the deposit, together with serpentine–diopside assemblages, indicate that it was emplaced at elevated temperatures on the order of 200–440 °C, consistent with deposition from a pyroclastic flow. The lithofacies characteristics can be explained by the interaction of the pyroclastic flow with the complex topography of a pre-existing crater.

Depositional processes in a kimberlite crater: the Upper Cretaceous Orapa South Pipe (Botswana)

AbstractThe Orapa A/K1 Diamond Mine, Botswana, exposes the crater facies of a bilobate kimberlite pipe of Upper Cretaceous age. The South Crater consists of layered volcaniclastic deposits which unconformably cross‐cut massive volcaniclastic kimberlite of diatreme facies in the North Pipe. Based on the depositional structure, grain‐size, sorting and composition of kimberlite in the South Crater, six units are distinguished in the ∼70 m thick stratiform crater‐fill sequence and talus slope deposits close to the crater wall, which represents a multistage infill of the volcanic crater. Monolithic basalt breccias (Unit 1) near the base of the crater‐fill are interpreted as rock‐fall avalanche deposits, generated by the sector collapse of the crater walls. These deposits are overlain by a basal imbricated lithic breccia and upper massive sub‐unit (Unit 2), interpreted as the deposits of a pyroclastic flow that entered the South Crater from another source. Vertical degassing structures within the massive sub‐unit show evidence for elutriation of fines and probably were formed after emplacement by fluidization due to air entrainment. Units 3 and 5 are thinly stratified deposits, characterized by diffuse bedding, reverse and normal grading, coarse lenticular beds, mudstone beds, small‐scale scour channels and load casts. These units are attributed to rapidly emplaced sheet floods on the crater floor. Units 3 and 5 are directly overlain by poorly sorted volcaniclastic kimberlite (Units 4 and 6) rich in basalt boulders, attributed to debris flows formed by the collapse of crater walls. Unit 7 comprises medium sandstones to cobble conglomerates representing talus fans, which were active throughout the deposition of Units 1 to 6. The study demonstrates that much of the material infilling the South Crater is derived externally after eruption, including primary pyroclastic flow deposits probably from another kimberlite pipe. These findings have important implications for predicting diamond grade. Results may also aid the interpretation of crater sequences of ultra‐basic, basaltic and intermediate volcanoes, together with the deposits of topographic basins in sub‐aerial settings.

Reply to: Discussion by Brown et al. on “In-vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement: An example from the Fox kimberlite, Ekati Diamond Mine, NWT, Canada”

Reply to: Discussion by Brown et al. on “In-vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement: An example from the Fox kimberlite, Ekati Diamond Mine, NWT, Canada”

Problems with an in-vent column collapse model for the emplacement of massive volcaniclastic kimberlite. A discussion of 'In-vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement: An example from the Fox kimberlite, Ekati Diamond Mine, NWT, Canada' by Porritt et al. [J. Volcanol. Geotherm. Res. 174, 90-102

Problems with an in-vent column collapse model for the emplacement of massive volcaniclastic kimberlite. A discussion of 'In-vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement: An example from the Fox kimberlite, Ekati Diamond Mine, NWT, Canada' by Porritt et al. [J. Volcanol. Geotherm. Res. 174, 90-102

Geology of a complex kimberlite pipe (K2 pipe, Venetia Mine, South Africa): insights into conduit processes during explosive ultrabasic eruptions

K2 is a steep-sided kimberlite pipe with a complex internal geology. Geological mapping, logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes, lithic contents and internal structures. The pipe comprises two geologically distinct parts: K2 West is a layered sequence of juvenile- and lithic-rich breccias, which dip 20–45° inwards, and K2 East consists of a steep-sided pipe-like body filled with massive volcaniclastic kimberlite nested within the K2 pipe. The layered sequence in K2 West is present to > 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass-wasting breccias generated by rock fall and sector collapse of the pipe walls: both processes occurred in tandem during the infill of the pipe. Several breccia lobes extend across the pipe and are truncated by the steep contact with K2 East. Dense pyroclastic rocks within the layered sequence are interpreted as welded deposits. K2 East represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption. This phase may have involved fluidisation of trapped pyroclasts, with loss of fine particles and comminution of coarse clasts. We conclude that the K2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement, followed by alternating phases of accumulation and ejection.

Tuffisitic Kimberlite (TK): A Canadian perspective on a distinctive textural variety of kimberlite

Tuffisitic Kimberlite (TK) is a textural variety of kimberlite first described from a number of southern African kimberlite pipes. This particular variety of kimberlite is a specific type of Massive Volcaniclastic Kimberlite (MVK) and bodies infilled with TK are classified as Class 1 kimberlites. TKs are fragmental rocks which dominate the steep sided diatreme zones or conduits of kimberlite pipes and are less commonly encountered within more complex root zones and blind intrusions. This textural variety of kimberlite is characterised by a unique set of macroscopic and microscopic features which are distinctive compared to those present in fragmental rocks or MVKs found within Class 2 and Class 3 pipes. The distribution of olivine macrocrysts and phenocrysts within TKs is most similar to texturally unmodified kimberlites classified as hypabyssal kimberlites (HK). In addition, they are dominated by a restricted range of distinctive juvenile clasts. In contrast, MVKs from Class 2 and Class 3 pipes are often characterised by an olivine distribution and morphology that is unlike that encountered within texturally unmodified kimberlites or HKs. They are also characterised by more complex and highly variable juvenile clasts, consistent with higher degrees of textural modification. A number of the kimberlites discovered in Canada over the last two decades display geological features similar to those described from the Kimberley area of South Africa. The general geology of pipes infilled with TK from Canadian occurrences is summarized in this contribution. It is also presented as a schematic diagram which can be used as a guide for kimberlite evaluation as well as for emplacement models for Class 1 kimberlites. The geology represented in this generalized model summarises observations and interpretations of drill cores, thin sections, diamond distribution data, geophysics and a variety of other data sets from five different kimberlite clusters across Canada. All the pipes investigated have undergone evaluation work in order to determine their economic potential.

Geochemical trends in kimberlites of the Ekati property, Northwest Territories, Canada: Insights on volcanic and resedimentation processes

The Ekati property, Northwest Territories, Canada, hosts in excess of 150 volcanic kimberlite pipes occupied by a wide variety of rock types including coherent, magmatic material and a range of fragmental, volcaniclastic deposits of both pyroclastic and resedimented origin. Geochemical analysis of a suite of samples from several of these bodies provides valuable insight into the nature of their components and the processes by which they form. Observed variations in the bulk composition of coherent kimberlites correlate with their mode of emplacement and petrographic characteristics. High-volume bodies of coherent kimberlite, occurring within and in several cases dominating volcanic pipes (pipe-fill CK), are depleted in CO 2 and key incompatible elements (e.g. Ti and Nb) but enriched in Ni and SiO 2 relative to samples from narrow kimberlite dyke intrusions. The composition and certain textural features of pipe-fill CK can be interpreted to reflect formation by pyroclastic processes that involved a loss of volatiles and fines, and a concentration of olivine crystals relative to intrusive magmatic kimberlite. If this is the case, then the apparent coherent character of these rocks suggests that they represent coalescence of hot, fluidal pyroclasts (i.e. spatter) generated by fire-fountain style eruptions. Samples of massive pyroclastic kimberlite (PK) are geochemically similar to pipe-fill CK but generally show a greater degree of depletion of incompatible elements, suggestive of more explosive eruption processes and a greater degree of physical fractionation. The geochemical data support petrographic observations that Ekati PK contains very little xenolithic material. Resedimented volcaniclastic kimberlite (RVK) shows variable degrees of enrichment in Al 2O 3 relative to PK. The extent of Al-enrichment correlates with the proportion of dark, fine-grained clastic matrix material in the RVK samples and their composition can be explained as a mixture of a very olivine-rich, fines-depleted, juvenile component with varying amounts of argillaceous sediment likely derived from surface deposits present at the time of kimberlite eruption. However, the increased Al 2O 3 content in RVK is also positively correlated with Nb concentration indicating that the matrix consistently includes a small amount of kimberlitic ash. Geochemical trends in massive volcaniclastic kimberlite (MVK) and fine-grained volcaniclastic kimberlite (VK) from the Fox pipe are very different to those of other VK samples from Ekati and appear to reflect mixing of juvenile kimberlitic material with granodiorite. The data suggest very little if any incorporation of exotic fine-grained sediments into these rocks. Geochemical modelling suggests that the eruption(s) forming Fox MVK involved a significant loss of fines, similar to that interpreted for PK in other Ekati kimberlites. However, the large quantity of fragmented granodiorite (~ 30 to 45%) within Fox MVK indicates that, in contrast to other localities, the Fox eruption was unable to completely clear its vent and disperse the excavated granodiorite prior to infilling the pipe. The composition of Fox VK provides no evidence for a loss of fines and suggests that these were either retained during eruption or reintroduced by mixing with ash deposits from previous eruptions.

In-vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement: An example from the Fox kimberlite, Ekati Diamond Mine, NWT, Canada

The origins of massive, poorly sorted fragmental kimberlite and kimberlite breccias in the diatremes of kimberlite volcanic pipes is currently poorly understood. Studies of the textural features of the major infill of the Fox kimberlite, Ekati Diamond Mine, NWT, Canada, show that it may have formed as a result of the collapse of the explosive eruption column above the vent into the deep open pipe during the climactic stage of the eruption. As the eruption intensity increased the column became critically overloaded with dense particles and rapidly collapsed onto itself. Unlike deposits typically preserved from column collapse events, such as ignimbrites, some portion of the collapsing mass was trapped in the deep pipe, forming a distinct type of pyroclastic deposit, “in-vent/vent-fill column collapse lapilli-tuff”. This deposit has previously been tentatively identified as tuffisitic kimberlite (TK) or massive volcaniclastic kimberlite (MVK), based on broadly similar textural characteristics with these lithologies. Though every individual deposit will have formed under slightly different circumstances, mass emplacement from column collapse is proposed as a viable model for massive volcaniclastic kimberlite emplacement, based on extrapolation of modern volcanological processes.

Progressive infilling of a kimberlite pipe at Diavik, Northwest Territories, Canada: Insights from volcanic facies architecture, textures, and granulometry

Processes responsible for the formation of crater-facies kimberlite remain poorly understood because there are few occurrences of well-preserved deposits. New open-pit exposures of the A154N kimberlite pipe at Diavik in the Lac de Gras region of the Northwest Territories, Canada reveal well-preserved crater-facies deposits. Here, we describe the geometries, structures, and componentry of deposits from the uppermost 130 m of the A154N pipe. We then interpret the processes responsible for their emplacement on the basis of these characteristics. The base of the section features a poorly-sorted, primary massive volcaniclastic kimberlite deposit (MVK 1) grading upward into a better-sorted and weakly-stratified volcaniclastic kimberlite deposit (MVK 2); which is, itself, overlain by ~ 45 m of stratified volcaniclastic kimberlite (SVK). The MVK–SVK contact is sharp and marks an abrupt transition from sustained eruption of pyroclastic kimberlite to episodic sedimentation from collapse of an unstable volcanic edifice into the open crater. The SVK deposit is overlain dis-conformably by a poorly to well-sorted, diffusely-bedded and graded pyroclastic kimberlite (PK 1 and PK 2) that erupted from another, younger kimberlite volcano. The PK deposits show a variety of textures supporting a pyroclastic origin, including vesiculated juvenile pyroclasts, a grain and clast-supported fabric, and the absence of mud-size particles in the matrix. Vertical clast-size grading over 40–50 m and the absence of bedding in the PK 2 suggest hydraulic sorting during en-masse deposition of pyroclastic kimberlite into a deep water column. Granulometric analysis shows the PK 2 deposit was originally transported as a pyroclastic density current that was sourced from one of the many adjacent pipes in the Diavik area and inundated the A154N pipe. These observations have implications for kimberlite volcanic processes and grade distribution.

The role of fluidisation in the formation of volcaniclastic kimberlite: Grain size observations and experimental investigation

Deep, steep-sided kimberlite pipes in southern Africa are normally partially filled with multiple units of a characteristically massive, structureless volcaniclastic rock, here termed massive volcaniclastic kimberlite (MVK). These units can be arranged in a pseudo-concentric, nested ‘pipes-within-pipes’ structure. Several key observations on typical MVK from the Venetia kimberlite pipes, South Africa, indicate that fluidisation may have been an important process during its emplacement. These include: (1) the thorough mixing together of juvenile components and of lithic components derived from all stratigraphic levels of the country rock and; (2) a matrix composed of void-filling minerals related to hydrothermal metamorphism (mainly diopside and serpentine), which indicates that the primary deposit had a high porosity (> 50%); and (3) particle size distributions poor in fine ash (< 0.0625 mm) and coarse lithic fragments (> 10 cm). Analogue experiments utilising a sand bed laced with marker horizons were conducted in order to investigate the role of fluidisation in the formation of MVK. High-pressure gas, fed through the bed from a point source, resulted in the formation of an upwardly diverging pipe structure of thoroughly mixed fluidised material bounded by undisturbed sand. Strong upward gas flow along the centre of the pipe is characteristically bubbly and results in internal circulation with downward moving regions at the pipe margins. Analysis of the experimental results suggests that the gas velocity decreases linearly with height and has a Gaussian distribution of vertical velocity in the cross-flow direction. A decrease in the gas flow rate resulted in the development of nested pipes reminiscent of the ‘pipes-within-pipes’ structures in kimberlite pipes. The results suggest that fluidisation could be a major control on the structure and sorting of deposits in kimberlite pipes. We propose that the fluidisation of particles in a kimberlite pipe occurs during the final stages of the eruption when erupting pyroclasts and incorporated fragmented lithic material can no longer be ejected out of the deep, wide conduit. This bed is subject to quasi-sustained fluidisation by the erupting gas-particle flow, which imparts on it the distinctive characteristics of a massive well-mixed, poorly sorted deposit.

Dynamical constraints on kimberlite volcanism

Kimberlite volcanism involves the ascent of low viscosity (0.1 to 1 Pa s) and volatile-rich (CO 2 and H 2O) ultrabasic magmas from depths of 150 km or greater. Theoretical models and empirical evidence suggest ascent along narrow (∼1 m) dykes at speeds in the range > 4 to 20 m/s. With typical dyke breadths of 1 to 10 km, magma supply rates are estimated in the range 10 2 to 10 5 m 3/s with eruption durations of many hours to months. Based on observations, theory and experiments we propose a four-stage model for kimberlite eruptions to explain the main geological relationships of kimberlites. In stage I magma reaches the Earth's surface along fissures and erupts explosively due to their high volatile content. The early flow exit conditions are overpressured with choked flow conditions; an exit velocity of ∼200 m/s is estimated as representative. Explosive expansion and near surface overpressures initiate crater and pipe formation from the top downwards. In stage II under-pressures (the difference between the lithostatic pressure and pressure of the erupting mixture) develop within the evolving pipe causing rock bursting at depth, undermining overlying rocks and causing down-faulting and crater rim slumping. Rocks falling into the pipe interior are ejected by the strong explosive flows. Stage II is the erosive stage of pipe formation. As the pipe widens and deepens larger under-pressures develop enhancing pipe wall instability. A critical threshold is reached when the exit pressure falls to one atmosphere. As the pipe widens and deepens further the gas exit velocity declines and ejecta becomes trapped within the pipe, initiating stage III. A fluidised bed of pyroclasts develops within the pipe as the eruption wanes to form typical massive volcaniclastic kimberlite. Marginal breccias represent the transition between stages II and III. After the eruption stage IV is a period of hydrothermal metamorphism (principally serpentinisation) and alteration as the pipe cools and meteoric waters infiltrate the hot pipe fill. Following an eruption an open crater can be filled by kimberlite- and country-rock derived sediments, forming the crater-facies.