Coherent Kimberlite Research Articles

Reconstruction of a kimberlite eruption, using an integrated volcanological, geochemical and numerical approach: A case study of the Fox Kimberlite, NWT, Canada

An integrated approach involving volcanology, geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the Fox kimberlite pipe. The observed deposits within the vent include a basal massive, poorly sorted, matrix supported, lithic fragment rich, eruption column collapse lapilli tuff. Extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments, dense juvenile pyroclasts and olivine crystals, triggering column collapse. > 40% dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature, where enrichment in Al 2O 3 and Na 2O compared to average values for coherent kimberlite is seen. The wide, deep, open vent provided a trap for a significant proportion of the collapsing column material, preventing large scale run-away in the form of pyroclastic flow onto the ground surface, although minor flows probably also occurred. A massive to diffusely bedded, poorly sorted, matrix supported, accretionary-lapilli bearing, lithic fragment rich, lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage, caused by the explosive interaction of external water with residual magma. Correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble-boulder breccia was deposited. Ash- and accretionary lapilli-rich tephra, deposited on the crater rim during the late phreatomagmatic stage, was subsequently resedimented into the vent. Incompatible elements such as Nb are used as indicators of the proportion of the melt fraction, or kimberlite ash, retained or removed by eruptive processes. When compared to average coherent kimberlite the ash-rich deposits exhibit ~ 30% loss of fines whereas the column collapse deposit exhibits ~ 50% loss. This shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption, indicating the existence of a high sustained eruption column. The deposits within Fox record a complex eruption sequence showing a transition from a probable violent sub-plinian style eruption, driven by instantaneous exsolution of magmatic volatiles, to a late phreatomagmatic eruption phase. Mass eruption rate and duration of the sub-plinian phase of the eruption have been determined based on the dimensions of milled country-rock boulders found within the intra-vent deposits. Calculations show a short lived eruption of one to eleven days for the sub-plinian magmatic phase, which is similar in duration to small volume basaltic eruptions. This is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters, such as predictions of magma ascent rates in kimberlite dykes.

Difficulties in distinguishing coherent from fragmental kimberlite: A case study of the Muskox pipe (Northern Slave Province, Nunavut, Canada)

Kimberlite drill core from the Muskox pipe (Northern Slave Province, Nunavut, Canada) highlights the difficulties in distinguishing coherent from fragmental kimberlite and assessing the volcanological implications of the apparent gradational contact between the two facies. Using field log data, petrography, and several methods to quantify crystal and xenolith sizes and abundances, the pipe is divided into two main facies, dark-coloured massive kimberlite (DMK) and light-coloured fragmental kimberlite (LFK). DMK is massive and homogeneous, containing country-rock lithic clasts (~ 10%) and olivine macrocrysts (~ 15%) set in a dark, typically well crystallised, interstitial medium containing abundant microphenocrysts of olivine (~ 15%), opaques and locally monticellite, all of which are enclosed by mostly serpentine. In general, LFK is also massive and structureless, containing ~ 20% country-rock lithic clasts and ~ 12% olivine macrocrysts. These framework components are supported in a matrix of serpentinized olivine microphenocrysts (10%), microlites of clinopyroxene, and phlogopite, all of which are enclosed by serpentine. The contact between DMK and LFK facies is rarely sharp, and more commonly is gradational (from 5 cm to ~ 10 m). The contact divides the pipe roughly in half and is sub-vertical with an irregular shape, locally placing DMK facies both above and below the fragmental rocks. Most features of DMK are consistent with a fragmental origin, particularly the crystal- and xenolith-rich nature (~ 55–65%), but there are some similarities with rocks described as coherent kimberlite in the literature. We discuss possible origins of gradational contacts and consider the significance for understanding the origin of the DMK facies, with an emphasis on the complications of alteration overprinting of primary textures.

Some major problems with existing models and terminology associated with kimberlite pipes from a volcanological perspective, and some suggestions

Five significant problems hinder advances in understanding of the volcanology of kimberlites: (1) kimberlite geology is very model driven; (2) a highly genetic terminology drives deposit or facies interpretation; (3) the effects of alteration on preserved depositional textures have been grossly underestimated; (4) the level of understanding of the physical process significance of preserved textures is limited; and, (5) some inferred processes and deposits are not based on actual, modern volcanological processes. These issues need to be addressed in order to advance understanding of kimberlite volcanological pipe forming processes and deposits. The traditional, steep-sided southern African pipe model (Class I) consists of a steep tapering pipe with a deep root zone, a middle diatreme zone and an upper crater zone (if preserved). Each zone is thought to be dominated by distinctive facies, respectively: hypabyssal kimberlite (HK, descriptively called here massive coherent porphyritic kimberlite), tuffisitic kimberlite breccia (TKB, descriptively here called massive, poorly sorted lapilli tuff) and crater zone facies, which include variably bedded pyroclastic kimberlite and resedimented and reworked volcaniclastic kimberlite (RVK). Porphyritic coherent kimberlite may, however, also be emplaced at different levels in the pipe, as later stage intrusions, as well as dykes in the surrounding country rock. The relationship between HK and TKB is not always clear. Sub-terranean fluidisation as an emplacement process is a largely unsubstantiated hypothesis; modern in-vent volcanological processes should initially be considered to explain observed deposits. Crater zone volcaniclastic deposits can occur within the diatreme zone of some pipes, indicating that the pipe was largely empty at the end of the eruption, and subsequently began to fill-in largely through resedimentation and sourcing of pyroclastic deposits from nearby vents. Classes II and III Canadian kimberlite models have a more factual, descriptive basis, but are still inadequately documented given the recency of their discovery. The diversity amongst kimberlite bodies suggests that a three-model classification is an over-simplification. Every kimberlite is altered to varying degrees, which is an intrinsic consequence of the ultrabasic composition of kimberlite and the in-vent context; few preserve original textures. The effects of syn- to post-emplacement alteration on original textures have not been adequately considered to date, and should be back-stripped to identify original textural elements and configurations. Applying sedimentological textural configurations as a guide to emplacement processes would be useful. The traditional terminology has many connotations about spatial position in pipe and of process. Perhaps the traditional terminology can be retained in the industrial situation as a general lithofacies-mining terminological scheme because it is so entrenched. However, for research purposes a more descriptive lithofacies terminology should be adopted to facilitate detailed understanding of deposit characteristics, important variations in these, and the process origins. For example every deposit of TKB is different in componentry, texture, or depositional structure. However, because so many deposits in many different pipes are called TKB, there is an implication that they are all similar and that similar processes were involved, which is far from clear.

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.