Are Monogenetic Basaltic Explosive Volcanoes Good Analogues for Explosive Kimberlite Volcanoes?
There is uncertainty as to the nature of kimberlite volcanoes because the edifices of ancient kimberlite volcanoes have been largely eroded. Monogenetic basaltic explosive volcanoes, such as scoria or cinder cones and maars, are often proposed as good analogues for kimberlite volcanoes. Basaltic scoria or cinder cones form from explosive eruptions driven by exsolving magmatic gases. The sparsity of country rock xenoliths in the deposits of scoria cones indicates the explosive fragmentation surface lay at or above the pre- eruption ground surface in the vent and conduit system, and therefore a sub-surface diatreme or pipe-like conduit cannot be formed. Basaltic scoria cones are therefore not good analogues for most kimberlite pipes and volcanoes. By contrast, the volcaniclastic kimberlite pipe fill of most kimberlite pipes contains a significant proportion of country rock xenolith and xenocryst debris (10 to 50 modal percent or more), indicating that the fragmentation surface lay below the ground surface, and this was responsible for explosively excavating the subsurface diatreme or pipe like conduit. Some authors propose that this is consistent with kimberlite pipes being the remains of maar volcanoes. The deposits of maar volcanoes usually contain substantial country rock debris suggesting excavation of a significant subsurface conduit has occurred. Maar forming explosive eruptions are phreatomagmatic, driven in large part by explosive superheating of ground water as magma passes upwards through a shallow crustal aquifer. The deposits however commonly show two key characteristics, base surge deposits and accretionary lapilli. Although these occur in some kimberlite deposits, attesting to the occurrence of phreatomagmatic explosive activity in those kimberlite volcanoes, in the absence of such features occurring universally in kimberlite pipes, it is difficult to support a maar volcanic model for kimberlite pipes. If the volatile content in a rising magma is high, volatile exsolution will begin well below the ground surface in the magma conduit. Upon approach to the ground surface, initial opening of a vent is likely involves a vulcanian like blast. Opening of the vent would immediately allow the magma to decompress, so increasing exsolution rates and causing the fragmentation surface to fall deeper into the conduit. Magmatic explosive activity deep below ground surface can therefore excavate a pipe or diatreme like conduit. The vents of stratovolcanoes are often cylindrical conduits tens to hundreds of metres in diameter, at least at the mouth of the vent, similar in scale to kimberlite pipes. There is no evidence that kimberlite volcanoes grew to the dimensions of stratovolcanoes; if they did we would see more of their edifices preserved. The high rate of exsolution of magmatic volatiles from a low viscosity, high volatile bearing, relatively small volume batch of magma, would lead to a relatively short explosive event. The explosive intensity would be high, leading to relatively widespread dispersal of pyroclasts, but in turn not producing a significant edifice around the vent. In some cases however, lower intensity explosive eruptions may have built small cones, such as evidenced by the Fort a la Corne tuff cones.
The Influence of Volcanological and Sedimentological Processes on Diamond Grade Distribution: Examples From the Ekati Diamond Mine, NWT, Canada
The study of the diamond distribution within two kimberlite pipes, Fox and Koala, from the Ekati Diamond Mine, NWT, Canada, in conjunction with detailed facies models has shown several distinct relationships of deposit type and grade distribution. In both pipes the lithological facies represent grade units which can be distinguished from each other in terms of relative size and abundance of diamonds. Positive correlation of olivine grain size and abundance with diamond grade is seen, indicating that density sorting of fragmental kimberlites occurs both in pyroclastic and resedimented deposits. Though surface geological processes do not control the diamond potential of the erupting magma, they can be responsible for concentrating diamonds into economically significant proportions. A good understanding of the eruption, transport and depositional processes responsible for the individual lithological units and the diamond distribution within them is important for successful resource estimation and may lead to recognition of areas suitable for selective mining, making a marginal deposit economic.
Fragmentation of Kimberlite: Insights into Eruption Style and Energy from Diavik, NWT
One metric of the intensity of a volcanic eruption is the extent of magma fragmentation recorded by pyroclasts. Relative comparisons of volcanic energy are made for historical eruptions by using the combination of grain size distributions and areal extent of pyroclastic deposits. For example, 'F' values which represent the percentage of fragments greater than 1 mm at isopach contour of 0.10max thickness are used for comparative purposes and to deduce volcanic style and energy from grain-size properties of pyroclastic deposits. Kimberlite volcanoes rarely preserve extra-crater deposits that would support this type of analysis. Furthermore, many volcaniclastic deposits of kimberlite are enriched (more than 60 percent) in crystals (e.g., olivine) and, thus, the energetics behind fragmentation of crystal-rich kimberlite magmas may be considerably different from other magmas. Melt-free olivine crystals and juvenile pyroclasts of crystallized kimberlite magma with or without olivine crystals are the dominant pyroclast types in kimberlite. Any estimation of kimberlite eruption intensity from deposits must take into consideration the properties of these components. Here we present data collected from coherent and pyroclastic kimberlite within a kimberlite pipe at Diavik, NWT, to test the following hypotheses: (a) olivine crystals break during kimberlite eruption; (b) kimberlite melt can efficiently separate from crystals during eruption. Based on these data, we discuss possible controls on the modification of pyroclasts during eruption. We suggest that olivine crystals and juvenile pyroclasts of crystallized kimberlite magma can be used to index the relative intensity of kimberlite eruptions. Finally, we present a new fragmentation index for kimberlite eruption using: (1) olivine crystal populations, and (2) ratios of 'free' olivines to juvenile pyroclasts of quenched magma, +/- enclosed olivine crystals.
Kimberlite Wall Rock Fragmentation: Venetia K08 Pipe Development
Volcanic systems impose powerful disrupting forces on the country rock into which they intrude. The nature of the induced brittle deformation or fragmentation can be characteristic of the volcanic processes ongoing within the volcanic system, but are most typically partially removed or obscured by repeated, overprinting volcanic activity in mature pipes. Incompletely evolved pipes may therefore provide important evidence for the types and stages of wall rock fragmentation, and mechanical processes responsible for the fragmentation. Evidence for preserved stages of fragmentation is presented from a detailed study of the K08 pipe within the Cambrian Venetia kimberlite cluster, South Africa. This paper investigates the growth history of the K08 pipe and the mechanics of pipe development based on observations in the pit, drill core and thin sections, from geochemical analyses, particle size distribution analyses, and 3D modeling. Present open pit exposures of the K08 pipe comprise greater than 90% mega-breccia of country rock clasts (gneiss and schist) with <10% intruding, coherent kimberlite. Drill core shows that below about 225 m the CRB includes increasing quantities of kimberlite. The breccia clasts are angular, clast-supported with void or carbonate cement between the clasts. Average clast sizes define sub-horizontal layers tens of metres thick across the pipe. Structural and textural observations indicate the presence of zones of re-fragmentation or zones of brittle shearing. Breccia textural studies and fractal statistics on particle size distributions (PSD) is used to quantify sheared and non- sheared breccia zones. The calculated energy required to form the non-sheared breccia PSD implies an explosive early stage of fragmentation that pre-conditions the rock mass. The pre-conditioning would have been caused by explosions that are either phreatic or phreatomagmatic in nature. The explosions are likely to have been centered on a dyke, or pulses of preceding volatile-fluid phases, which have encountered a local hydrologically active fault. The explosions were inadequate in mechanical energy release (72% of a mine production blast) to eject material from the pipe, and the pipe may not have breached surface. The next stage of fragmentation is interpreted to have been an upward-moving collapse of the pre-conditioned hanging wall of a subterranean volcanic excavation. This would explain the mega-scale layering across the width of the breccia pipe. It must be questioned whether the preserved K08 architecture represents early pipe development in general, or is a special case of a late pipe geometry modification process. Previous literature describes sidewall and hanging wall caving processes elsewhere in the Venetia cluster and other kimberlites world wide. A requirement for emplacement models that include upward pipe growth processes is the availability of space (mass deficit at depth) into which the caving and/or dilating breccia can expand. It is possible that K08 might be connected to adjacent K02 at a depth somewhere below 400m, which would explain the presence of volcaniclastic kimberlite at depth within the K08 pipe. K08 is likely an incomplete ancillary sideward development to K02. The latest stage of brecciation is quantified through an observed evolution in the fractal dimension of the PSD. It is interpreted to be due to complex adjustments in volume in the pipe causing shearing and re-fragmentation of the breccia.
An Unusual Example of Coherent Kimberlite From the Muskox Kimberlite (Nunavut, Canada): a Re-evaluation of the Criteria for Recognising Coherent Kimberlite
Although there is no published treatise on the criteria for recognising coherent kimberlite (CK), a review of the kimberlite literature will reveal that there is some consensus on its characteristics. For example, in most publications in the last kimberlite conference (8IKC volume 1) in which CK is discussed, the groundmass is described as 'uniform and well-crystallised' and spinel crystals are almost always present in abundance. Macroscopic features and contact relationships, however, are less commonly reported and do not seem to be important for the distinction of CK from fragmental rocks. Here we report contact relationships, facies distributions, textural features and mineralogy for one coherent and two fragmental facies from the Muskox kimberlite (Nunavut, Canada) to initiate debate on the criteria for distinguishing coherent from fragmental kimberlite, which we also refer to as volcaniclastic kimberlite (VK). Particular focus is placed on the coherent facies, which has unusual groundmass characteristics that have not been previously described in the kimberlite literature. Emplaced within granodiorite country rocks, the Muskox kimberlite is a steeply-tapering, single-vent, kimberlite body, measuring 200 x 220 m, that is infilled with VK and minor amounts of CK. VK is divided into two main facies, a light-coloured, country rock-rich facies (VK1) and a dark-coloured, olivine-rich facies (VK2). VK2 is mostly enclosed by VK1, forming an off-centre nested architecture. The contact between the facies is roughly vertical and gradational over distances up to 10 meters, however, in most drillholes that intersect the contact, the two VK facies alternate several times, thus forming an apparent inter-digitating relationship. Both facies are massive, poorly sorted and range from matrix- to clast-supported and contain olivine crystals, country rock lithic clasts of limestone and granodiorite and rare juvenile pellets. The matrix of VK1 consists mainly of serpentine, calcite, phlogopite, diopside and opaque oxides, whereas the matrix of VK2 consists mostly of serpentine, calcite, phlogopite, opaque oxides and monticellite. Although the matrix of VK2 is similar to the matrix of CK described in the literature (e.g., well crystallised with abundant spinels), the facies is considered fragmental because of the irregular distribution of framework components and because it is in gradational contact with VK1. CK occurs as a late-stage sill-like body found in five drillholes at ~200 m depth below the current surface. Sill thickness ranges from ~0.1 to >13 m thick and all contacts with VK infill are sharp. Most CK is crystal-rich (15-25 percent) and contains evenly distributed, commonly aligned, six-sided crystals (<2 mm) that have been completely replaced by serpentine and carbonate, and are interpreted to represent altered olivine phenocrysts. Groundmass minerals include serpentine, phlogopite, opaque minerals and a mineral (2- 3 microns) that appears to be similar in composition to monticellite. Unusual textures include radiating clusters of minerals that are reminiscent of spherulites or snowflake textures, and arcuate fractures, reminiscent of perlitic fractures. Muskox CK illustrates the importance of combining macroscopic and microscopic features for the recognition of CK and, if the textures are indeed products of devitrification, then this may be the first evidence that kimberlite magmas can quench to glass.
Welding of pyroclastic deposits generally involves the sintering of hot glassy vesicular particles and requires the presence of a load and/or high temperatures. Welding can occur on various scales as observed in large welded pyroclastic flows, in small-volume agglutinated spatter rims, or as in coalesced clastogenic lava flows. In all these examples welding occurs mainly by reduction or elimination of porosity within the vesicular clasts and/or inter-clast pore space. The end result of welding in pyroclastic deposits is to produce dense, massive, coherent deposits. Here, we present a possible new end-member of the welding process: welding of non- vesicular pyroclasts in intra-crater kimberlite deposits. Kimberlite melt is a low-viscosity liquid carrying abundant crystals. Because of this, kimberlite eruptions generally produce non-vesicular pyroclasts. During welding, these pyroclast cannot deform by volume reduction to form typical fiamme. As a result, welding and compaction in kimberlites proceeds via the reduction of inter-clast pore space alone. The lack of porous pyroclasts limits the maximum amount of volumetric strain within pyroclastic kimberlite deposits to about 30%. This value is substantially lower than the limiting values for welding of more common felsic pyroclastic flows. The lower limit for volumetric strain in welded kimberlite deposits severely restricts the development of a fabric. In addition, pyroclastic kimberlite deposits commonly feature equant-shaped pyroclasts, and equant-shaped crystals. This, in turn, limits the visibility of the results of compaction and pore space reduction, as there are few deformable markers and elongate rigid markers that are able to record the strain during compaction. These features, together with the low viscosity of kimberlite magma and the stratigraphic position of these kimberlite deposits within the upper reaches of the volcanic conduit, call for careful interpretation of coherent-looking rocks in these settings. In this contribution we explore the possible welded origin for dark and competent kimberlite facies from the Victor Northwest pipe (Northern Ontario, Canada). This volumetrically extensive facies superficially resembles a coherent rock. The following observations on the dark and competent facies are suggestive of a pyroclastic, rather than intrusive or extrusive coherent origin: The facies is completely enveloped by pyroclastic facies; has gradational contacts with adjacent pyroclastic facies above and below; contains faint outlines of primary pyroclasts; shows diffuse grain size variations and rare bedding; shows systematic changes in components from the underlying pyroclastic facies to the dark and competent facies to the overlying pyroclastic facies implying a lack of a depositional break in this succession; and shows a faint, generally subhorizontal fabric despite the presence of an equant grain shape population. In addition, we present evidence that the original inter-clast porosity has been reduced or eliminated by syn-depositional welding rather than by precipitation of secondary minerals in the inter-clast pore spaces. We feel that the latter process (i.e., alteration) is highly unlikely because: The kimberlite package contains intervals with well crystallized groundmass similar to coherent kimberlite, this texture simply cannot be produced by alteration; the kimberlite is in fact the freshest rock within the pipe, containing mostly fresh olivines; and the dark and competent kimberlite does not show a patchy or vein-related heterogeneity typical of alteration. In summary, these deposits likely represent a variably welded succession of proximal spatter/fire fountaining kimberlite deposits.
Emplacement Temperatures of Pyroclastic and Volcaniclastic Deposits in Kimberlite Pipes in Southern Africa: New constraints From Palaeomagnetic Measurements
Palaeomagnetic techniques for estimating the emplacement temperatures of volcanic deposits have been applied for the first time to pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa. Lithic clasts were sampled from a variety of lithofacies, from three pipes for which the internal geology is well constrained (A/K1 pipe, Orapa Mine, Botswana and the K1 and K2 pipes, Venetia Mine, South Africa). The sampled deposits included massive and layered vent-filling breccias with varying abundances of lithic inclusions and layered crater-filling pyroclastic deposits, talus breccias and volcaniclastic breccias. Lithic clasts sampled from layered and massive vent-filling pyroclastic deposits in A/K1 were emplaced at >590° C. Results from K1 and K2 provide a maximum emplacement temperature limit for vent-filling breccias of 420-460° C; and constrain equilibrium deposit temperatures at 300-340° C. Crater-filling volcaniclastic kimberlite breccias and talus deposits from A/K1 were emplaced at ambient temperatures, consistent with infilling of the pipe by post-eruption epiclastic processes. Identified within the epiclastic crater- fill succession is a laterally extensive 15-20 metre thick kimberlite pyroclastic flow deposit emplaced at temperatures of 220-440° C. It overlies the post-eruption epiclastic units and is considered an extraneous pyroclastic kimberlite deposit erupted from another kimberlite vent. The emplacement temperature results are comparable to the estimated emplacement temperatures of other kimberlite deposits and pyroclastic deposits from other volcanic systems, and fall within the proposed stability field for common interstitial matrix mineral assemblages within vent-filling volcaniclastic kimberlites. This is in the range where welding and agglutination of juvenile pyroclasts occurs in other types of pyroclastic deposits. Such high emplacement temperatures for vent-filling pyroclastic deposits are consistent with volatile-driven eruptions, and suggest phreatomagmatism did not play a major role in the generation of the deposits. The study indicates that palaeomagnetic methods can successfully distinguish differences in the emplacement temperatures of different kimberlite facies. It also indicates that the post-eruption infill of the pipe may be disrupted by the volcanic activity of nearby kimberlite pipes. Studies are in progress to determine the emplacement temperature of other vent-fill lithofacies, such as dense, coherent pyroclastic kimberlite that may represent high-grade welded deposits. The thermal effects of the pipes on adjacent country rock will be investigated and the results used to further constrain the eruption dynamics and emplacement mechanisms of the studied pipes.
Growth of Bultfonteinite and Hydrogarnet in Metasomatized Basalt Xenoliths in the BK9 Kimberlite, Orapa, Botswana: Insights into Hydrothermal Metamorphism in Kimberlite Pipes
The BK9 kimberlite consists of pipe-filling volcaniclastic deposits that contain abundant small basalt xenoliths, which are altered to varying degrees. The metamorphic assemblages of these xenoliths are used to provide constraints on the conditions of alteration. Bultfonteinite (Ca2SiO2[OH,F]4) and hydrogarnet form major components of the alteration assemblage. Bultfonteinite together with chlorite replace the original plagioclase-augite assemblage, in reactions driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. The bultfonteinite-chlorite alteration assemblage is subsequently replaced by fine-grained hydrogarnet and serpentine as the deposits cool. Consideration of mineral equilibria constrains the stability of bultfonteinite and hydrogarnet. They require temperatures between 350-250°C and a water-rich fluid containing F1-. This indicates that the deposits were hydrothermally altered by a meteoric fluid rather than as a result of autometamorphism by deuteric fluids. The temperatures of alteration appear inconsistent with evidence that the BK9 deposits have been welded, a process expected to occur at temperatures in excess of 600°C. We show that there is an upper temperature limit on alteration and that the deposits must cool before substantial alteration can occur, explaining the discrepancy in temperatures. Alteration predominantly occurs between 400-250°C as a result of the increased efficiency of mass transfer and chemical reactions below the critical point of water and as a consequence of the volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. Hydrothermal alteration is expected to take the form of a thermal convection system, driven by the heat of the deposits, in which cold meteoric water is drawn in from the surrounding country rocks, is heated and then flows vertically up the centre of the pipe to be expelled as steam through hydrothermal vents at the surface of the pipe. Above 400°C, prior to the initiation of serpentinising reactions, the system is dominated by convective cooling; once temperatures fall below 400°C the thermodynamic properties of water change to favour alteration and serpentinising reactions initiate resulting in the hydrothermal alteration of the kimberlite and included xenoliths. The composition of the xenoliths is also shown to have an affect on the metamorphic assemblages which form within the adjacent kimberlite and highlights the importance of understanding the petrogenesis of pipe filling kimberlites through the framework of hydrothermal metamorphism of hot volcaniclastic deposits.