The features of this material indicate de-, position and very efficient sorting of large volum, of juvenile kimberlitic material (predominantly oliv, ine and ash) mixed with significant quantities (prob-, ably in excess of 50%) of unconsolidated surficial, sediment. Three-dimensional models of selected Ekati kimberlites illustrating variation in pipe size and morphology. 20 ha) are. The internal geology of the kimberlites ranges from simple single-phase pipes (RVK or MK), to complex bodies with multiple, distinct units of VK. Kimberlites must be in placed fast in order to avoid the diamonds being re-absorbed or re-melted as the pressure drops. Indicator minerals from each kimberlite pipe contain varying proportions of the same mantle source. Further kimberlite pipe delineation drilling is usually done to constrain the geometry of the pipe from the surface down to a zone between 300-400 metres in depth. The dashed line (with arrow) in (a) and (c) represents the mixing trend between forsterite (Ol) and illite (Ill) and is. Volcanic pipes are composed of a deep, narrow cone of solidified magma, and are usually largely composed of one of two characteristic rock types — kimberlite or lamproite. Paleontology and diamonds: geological, environments associated with kimberlite emplacement, Lac de, Gras, Northwest Territories, Canada. We start with the knowledge that diamonds are first kimberlites and exist only above archean cratons and that they tend to form clusters within broad linear trends. Glacial deposits in the, Ekati area are predominantly tills thought to be, to blankets (2 to 5 m) to hummocky deposits (>5 m. up to 15 m). microfossils as well as the presence of wood, leaves, and turtle bones within the Ekati kimberlites indi-, cate that they were emplaced into a humid, temper-, types in older sediments (late Campian and older), are indicative of deposition in a marine environment, (outer shelf to near-shore, marginal marine), indi-, cating that the interior seaway covered this part of, the Slave craton during the late Cretaceous, chuk and McIntyre, 1996; Sweet et al., 2003), Xenoliths suggest that the cover sequence at the, time of kimberlite emplacement was dominated by, fine-grained, commonly organic-rich mudstones and, siltstones, with less abundant fine-grained quartzose, Although the Lac de Gras area has been subjected, to multiple periods of glaciation during the, nary, all currently observed glacial phenomena have, been attributed to the last (Late Wisconsin) glacia-, tion. The geology of Diamonds consist of crystalline carbon although a significant proportion of diamonds are derived from placid deposits i.e. At least one dike has been intersected and studied. In contrast, the mRVK dominated units are characterised by a, massive, matrix-supported texture reflecting incorpo-, ration of significant amounts of surficial mud (, grained ash) and probable resedimentation into the, vent by debris flow processes. It was emplaced along a regional lithological contact between syn-Yellowknife Supergroup granitoid rocks and Yellowknife Supergroup metasedimentary rocks that were covered by a now eroded. These intrusions are similar in, occur at similar elevations to the VK-filled pipes at, Ekati and there is no indication that they are exposed, at different levels within the intrusive/volcanic sys-, tem. Since the kimberlite pipes were initially covered by Lac de Gras, it was necessary to first construct dikes around the pipes to prevent water from flooding the open pits and underground mine workings. The internal geology of the pipes is highly variable; twenty-nine distinct domains form the basis for three-dimensional geologic models. They can then be obscured by soil, sediment, glacial till, and vegetation. Based on our tomographic models we identify two high-velocity seismic anomalies straddling the arcuate Cordillera Deformation Front that have controlled its regional deformation, including a newly identified Mackenzie craton characterized by high seismic velocities extending from the lower crust to the upper mantle to the north of the Mackenzie Mountains. The Lac de Gras kimberlites were, emplaced into competent, crystalline basement over-, to completely unconsolidated, probable water-saturat-, ed sediments in a subaerial environment. mal history of Cretaceous and Tertiary sediments, Lac de Gras, Northwest Territories, Canada. Diamond distribution within the kimberlites reflects the amount and nature of mantle material sampled by individual kimberlite phases, but is modified considerably by eruption and depositional processes. Total field magnetic contours for the C14 pipe, Kirkland Lake ..... 22 14. In addition to, olivine, a minor amount of quartz (generally less than, 0.5 mm in size) is evident. The morphology, rock types and internal geological architecture of these, pipes are discussed with a view to better constraining, emplacement models for these and other Lac d, stone terrain that grew by tectonic accretion of a pre-3, Ga nucleus, preserved in the central and western parts, of the Province (referred to as Central Slave Basement, east. 55-56 You can read more, below. Wood fragments are com-, mon and locally abundant. As I mentioned earlier although there are kimberlites in a variety of geological terrains, diamondiferous pipes are almost always in or above archean cratons and those are the orange areas on the map and only a few of those kimberlite types that are on archean crust actually contain diamonds and even fewer of those diamondiferous pipes contain enough diamonds to make economically viable. A number of distinct VK varieties, are evident. Extended, Kjarsgaard, B.A., 2001. Quartz is generally absent. Generalised map of the bedrock geology of the Ekati property showing the main rock types present and disclosed kimberlite locations. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide. For now, let’s look at a few of the ways that kimberlite pipes are found. The VK that, currently occupies most of the pipes is interpreted to, have formed predominantly by resedimentation and. This is why kimberlite pipes are the sites of most of the world's diamond mines. This sample of kimberlite and surrounding halite was collected in the mine, and is in the collection of the New York State Museum. opening of a much larger vent, due to wall-rock instability/collapse may have. Diamond grades of economic kimberlites vary hugely. Some of the bedding features, observed in oVK are suggestive of base surge deposi, and may reflect primary pyroclastic depositional, structures preserved during resedimentation/slumping. majority of bodies are dominated by oVK and mR, is rare. Geochemical character of southern African kimberlites: a new approach based on isotopic constraints. (1994a,b) and Kjarsgaard et al. emplacement and consolidation of the TK phase. LeCheminant, A.N., van Breemen, O., 1994. The odds are strongly stacked against success however with good science and exploration techniques that level of risk that can be considerably reduced and the rewards when successful are enormous. Analytical methods and data sources as per, join related VK samples from the Lynx, Panda and Koala kimberlites. This paper reviews key characteristics of kimberlites on the Ekati property, NWT, Canada. Only five of the, magmatic kimberlites that have been analysed geo-, chemically have been dated, but ages for these, bodies suggest that the high-Ti kimberlites are older, ences between these two groups may reflect different, magma sources or melting processes. However, mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late-Cretaceous and Tertiary cover (likely <200 m) was present at the time of emplacement. contacts typically dip inwards at very steep angles (ca. Diamonds from the Ekati core and buffes zone: properties. Given that less than 1% of kimberlites will yield an economically-viable diamond … Geochemical trends suggest variable loss of the, magmatic groundmass component of the intruding, kimberlite during eruption and deposition of PVK, (possibly through elutriation of fine kimberliti, from the eruption column). from chromite to magnesian ulvӧspinel-magnetite (MUM) compositions (Fe3+# = 47.2 ± 5.8 WHITE, R.S.J. Large blocks of granodiorite, together, with surface sediments and pyroclastic ejecta from, this earlier eruption may then have been incorpo-, rated during the later, significantly larger TK-, forming event. Lower K and Rb whole-rock compositions for internal The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. 2.63 to 2.58 Ga; orite and tonalite. Major- and trace-element compositions of southern African kimberlite samples previously analysed for isotopic compositions confirm that isotopically defined Group I (basaltic) and Group II (micaceous) variants have distinctive geochemical signatures. While it could be argued, that this reflects concentration processes associated, with resedimentation and reworking of pyroclastic, material, the trends displayed by PVK suggest that, the depletion in magmatic groundmass material is, associated with the eruption process itself, possibly, chemical trends for VK kimberlite samples indicate, grained surface sediment (mostly mud but also silt, and sand) during deposition and resedimentation of, Al contents displayed by the VK kimberlites ana-, lysed indicates that they typically contain between, show broad geochemical similarities to the VK sam-, ples and their compositions can be accounted for by, dilution of PVK. There is a direct correlation between olivine rim composition and groundmass mineralogy, whereby high Mg/Fe rims are associated with carbonate-rich kimberlites, and lower Mg/Fe rims are correlated with increased phlogopite and Fe-bearing oxide mineral abundances. The data suggest very little if any incorporation of exotic fine-grained sediments into these rocks. This is dominated by juvenile constituents (olivine and lesser kimberlitic ash) and variable amounts of, include: mud-rich resedimented VK (mRVK); olivine-rich VK (oVK); sedimentary kimberlite; primary VK (PVK); tuffisitic. Table 2 --Magnetic characteristics of Riley County kimberlites. Analytical methods and data sources as per Tables 1 and 2. Plan view of the Panda, Koala North and Koala kimberlites, with current open pit excavations and the Ekati main mine complex, mite and, in some cases, ilmenite occur in trace to, minor amounts with abundance and grain size gener-, ally proportional to that of olivine. Canada (B. Kjarsgaard pers. MK occurs primarily as precursor dykes but, in a few cases, forms pipe-filling intrusions. Ages obtained, for these sediments range from mid to late Albian, from pollen assemblages within post-eruptive crater, infill material in the Giraffe, Nancy and Koala, kimberlites. With the exception of the massive mR, zones, the VK units are typically crystal-dominated, and clast-supported, with relatively minor amounts of, mud or ash present. hypabyssal kimberlite (external dykes), and was followed by the explosive formation of Broken, angular fragments of indicator min-, erals are present in low abundances throughout, and. Jwaneng was, emplaced into competent Proterozoic lithologies, (platform sediments and minor mafic intrusives), overlain by an unknown thickness of unconsolidated, Observations made to date provide significant, constraints on the emplacement of the Ekati kimber-, lites and permit generalised models for how they, formed. Juvenile, lapilli are generally rare or absent in mRVK but, fine-grained VK (autoliths) and sporadic possible, accretionary lapilli have been observed. Finding a peridotite body as small as a kimberlite pipe, which might be only a few hundred yards across, can be very difficult. Based on their nitrogen characteristics, both the yellow diamonds and yellow rims must have crystallized in close temporal proximity (<<1 Ma) to kimberlite activity at CH-7 and Misery. Results are consistent with derivation of Group I kimberlites from asthenospheric-like sources similar to those from which oceanic island basalts are produced. 10%) of wall-rock and mudstone xenoliths; include significant amounts of juvenile-rich, PVK, wall-rock blocks occurs immediately above the, contact between thick PVK/TK in the lower, undergone an initial depletion in juvenile magmatic, fines, likely during eruption, followed by varying, degrees of dilution by surficial, mud-dominated, sediment and re-introduction of relatively minor, intruded to shallow levels forming MK-dominated. The, groups are defined by samples from specific locali-, ties, with those from the large MK-filled pipes, falling into the low-Ti group. There is one known shaft. Certain samples show minor, suggestive of minor contamination by granitic mate-. Similarly, early-formed chromite compositions are the same for internal HK and 4 KIMBERLITIC SILLS, WESSELTON KIMBERLITE PIPE, SOUTH AFRICA.Figure 3. These lithologies occur in a, sequence of variably graded beds (grading downwards, from mud-rich VK/siltstone tops to oVK at the base). et al., 1998; Field and Scott Smith, 1999; Graham et, least three end-member models of kimberlite emplace-, relatively small, steep-sided, diatreme-like morpholo-, gies and, in particular, a predominance of relatively, xenolith-poor, commonly bedded volcaniclastic kim-, berlite (VK) lithologies. Thus, the geochemical trends support the, contention that this dark matrix material is dominated. No Phanerozoic rocks are present. The architecture of A154N bears similarities to that of the nearby Koala pipe at the Ekati diamond mine in Lac de Gras (Crawford et al. 5.6. The only exceptions are a small number of, bodies that are partially or completely occupied by, magmatic kimberlite (MK). in several cases to depths in excess of 400 m. Tuffisitic kimberlite or kimberlite breccia is rare, with, possible examples currently recognized only at the, Fox kimberlite and a small intrusion adjacent to, Misery Main. garnet, chromite and clinopyr-, material are varied but, in general, RVK at Ekati is, poorly sorted with diamond content largely controlled, by varying degrees of dilution by fine-grained mud, and kimberlitic ash. Geoscience Australia and state authorities. With the exception of the samples from, Roger, all data are for samples with contamination indices. The. At Mark, Arnie and Grizzly, only MK has been encountered in drilling/excavation, undertaken to date. Generalized grade data for the Ekati pipes, are available in Dyck et al. 45 to 75 Ma, with five. The first was a mine near Murfreesboro, Arkansas.It was worked as a commercial diamond mine by a succession of operators in the early 1900s but closed because the deposit was subeconomic. In addition, large xenolithic blocks of fine-grained, sedimentary material, displaying sharp (in some cases, brecciated) contacts with the surrounding VK host are, 50%) kimberlite characterised by a relatively uniform, features and the presence of abundant juvenile lapilli, (5% to 30%). Rounded to, irregularly shaped clasts of fine-grained probable, primarily to the upper ca. (olivine-rich) sand; 3—massive, pale, olivine-rich VK; 4—bedded, pale, olivine-rich VK; 6a and 6b—dark, fine-grain. The narrower rims are commonly overgrown by very, fine-grained microlitic clinopyroxene. kimberlite pipes and volcaniclastic kimberlite infill. The presence of less fragmented, juvenile-rich PVK in the lower portions of certain pipes and the intrusion of large volumes of MK to shallow levels in some bodies suggest emplacement of relatively volatile-depleted, less explosive kimberlite in the later stages of pipe formation and/or filling. The Central Slave Basement Complex, Part I: its struc-, tural topology and autochthonous cover. HK samples compared to PK samples, are attributed to the removal of these components in Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. national Kimberlite Conference, Victoria, Canada. * Corresponding author. While the evidence is by no, means conclusive, the fact that the TK samples show. 4. The presence of water, during eruption may account for the intense alteration, The Koala pipe displays unusual internal geologi-, cal features that provide additional constraints on the, eruption and infilling processes for this body. As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. The, granite boulder zone present near the base of Phase 5, suggests that early infilling processes were accompa-, Deposition of the laterally continuous and relatively, thick woody siltstone unit (Phase 4) above the mud-, rich RVK reflects a quieter period of probable sub-, aqueous sedimentation, with settling of fines and, wood fragments from suspension in a relatively deep, water column. Geochem-, suggest that the eruption process resulted in signifi-, cant loss of juvenile ash and preferential concentration, of the crystal component of the kimberlite (primarily, olivine). olivine) material evident in most drill intersections. PhD thesis, University of Cape Town, South Africa. 90 to 100 Ma) were, indicating that significant erosion had likely taken, the preservation of bedded, wood-bearing horizons to. reduced the expulsive power of the eruption. SPARKS, K. BAILEY, W.P. Kimberlites contain magnetite and have a relatively high specific gravity so either magnetic or gravity surveys can be used so as long as the rock into which the kimberlites are being emplaced have low gravity or magnetic characteristics. 1—, moist, pliable black mud; 2a—quartz-rich sand; 2b—kimberlitic. The discovery in 1996 of the richly diamondiferous Grib kimberlite pipe (also known as Anomaly 441) has terminated the long-established notion (e.g. continuous units interbedded with or overlying VK. The poorly sorted nature of this material, suggests deposition by large scale, debris and mud-, flow processes. ent and Reid, 1989; Field and Scott Smith, 1999, Whereas zones rich in country rock xenoliths/blocks, are fairly common adjacent to pipe margins within. The Precambrian rocks are overlain by Quaternary glacial sediments. Tuffisitic kimberlite contains moder-, ate abundances (15 to 30%) of pervasively serpenti-, nised (and clay altered), medium-grained (up to 5, mm) olivine. Join ResearchGate to find the people and research you need to help your work. that little, if any, resedimentation has taken place, direct deposition of tephra within the vent. base surge and/or bedded, airfall deposits) that have slumped en masse as rela-. The whole-rock compositions of, VK samples suggest significant loss of kimberlitic fines during eruption followed by variable dilution by surface, and concurrent incorporation of kimberlitic ash. It is a highly variable mixture of melt, minerals crystallizing from the melt, and foreign crystals and rock pieces. The diamonds are generally nitrogen-rich, with median N contents of 1230 (CH-7) and 1030 at.ppm (Misery). Together, they provide seismic evidence for the trans-lithospheric nature of the Tintina Fault. grained VK are common. It appears that no unique structural associ-, property. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust. 30 m) granodiorite boulders occur within the TK, phase, at a depth of approximately 80 m below the, base of the RVK phase and also in the lower part of, the mRVK phase on the northern side of the pipe. 2.3. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. These signatures are generally consistent with geochemical variation in petrographically defined types. Oshurst, P.A., Carlson, J.A., Nowicki, T, Field, M., Scott Smith, B.H., 1998. BARNETT, M. FIELD AND L. WINDSOR 1 KIMBERLITE SILLS AND DYKES ASSOCIATED WITH THE tion of olivine usually varies from ca. 2.64 to 2.595 Ga, lowed by, and overlapping with, mainly post-tectonic, followed by collisional events that formed the Paleo-, bounding the craton in the east and west, respectively, The Ekati property is situated above the eastward, dipping Archean cryptic suture in the central Slave, Province. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. Recent geological studies have recognised at least five distinct volcaniclastic lamproite lithofacies with differing diamond grades. Subsequent, infilling of the vent occurred by a combination of, direct deposition from the eruption column and, resedimentation of crater rim materials. is interpreted to represent a single geological phase, portion of Koala North. partially displaced VK. U, progressive break-up of an Archean superc. The characteristics of the Ekati kimberlites are consistent with a two-stage emplacement process: (1) explosive, eruption/s causing vent clearing followed by formation of a significant tephra rim/cone of highly fragmented, olivine-, enriched juvenile material with varying amounts of kimberlitic ash and surface sediments (predominantly mud); and (2), infilling of the vent by direct deposition from the eruption column and/or resedimentation of crater rim materials. If kimberlite is founded out crop taking small bulk sample at surface may be possible. Resedimented volcaniclastic kimberlite (RVK) shows variable degrees of enrichment in Al2O3 relative to PK. Evaluations of Ti, Zr, Y and Ga over the range of estimated depths support models of the SCLM underlying the central Slave terrane. Apparently kimberlite pipes ... Because diamonds and kimberlite -- and other semi-precious gems -- are formed in similar locations, some kimberlite rock contains diamonds. The Diavik Diamond Mine in the NWT of Canada has produced in excess of 100 million carats from 3 kimberlite pipes since mining commenced in 2002. The mud/ash that is prevalent in the VK, lithologies described above is either absent or present. This is the first time glagolevite is reported in kimberlites. Diamond habitat shown by dark blue outlined rhombi. Pipe morphologies are strongly controlled by joints and faults. chromite, pe-, rovskite, Ti-magnetite), suggesting that the textural, transition from coherent magmatic to fragmented, volcaniclastic kimberlite is accompanied by a prefer-, ential loss of the magmatic groundmass component, and an associated relative concentration of olivine. Jeffrey Post, a … The other VK types analysed display compositions, that are more variable than those of the MK and PVK, samples. Strong P- and S-wave velocity contrasts across the Tintina Fault suggest that it is a lithosphere-scale shear zone that extends into the upper mantle beneath the NCC and demarcates distinct regions of lithospheric mantle. Large Kimberlite pipes are rare. Image showing the results of magnetic surveys conducted last fall at one of the kimberlite locations in Riley County. In, general, these lithologies are dominated by ‘‘exotic’, material (quartz and mud) believed to have been, derived from unconsolidated surface sediments pres-, ent at the time of kimberlite eruption, but they also, typically include varying amounts of fine-grained. Kimberlite is the name given to a silica-poor and magnesium-rich extrusive igneous rock (e.g., a volcanic rock) that contains major amounts of olivine, often serpentinized. It is proposed that the formation of megacrysts may be linked to failed kimberlites. Our study also emphasises how volatile loss resulting Its high olivine concentration and, low overall ash/mud content implies efficient removal, of fines during eruption and/or subsequent, tation. The manuscript was significantly im-, proved through thoughtful and thorough reviews by. %) While phrea-, tomagmatic processes may have played an important, role in the early stages of eruption and, in rare cases, (e.g. Kimberlites are thought to rise through a series of fissures in the rock. Examples of these are illustrated in, denote magmaclastic (fragmented) kimberlite of ex-, trusive origin and specific rock types are classified as, such if there is insufficient evidence to confidently, assign a primary (i.e. The wall-rocks, at Fox are similar to those of Panda, Koala and Koala, North but also include numerous diabase dykes, de Gras dyke swarms and trend at approximately 335, The Fox pipe is made up of two very distinct, 150 m thick) infilled with mRVK very similar to that, observed in other Ekati kimberlites; and a lower, diatreme-shaped zone occupied by comparatively ho-, mogeneous tuffisitic kimberlite (TK). Most appear to be deeply eroded suggsting a possibility for significant diamond placers downstream. first order transition elements by ICP-ES; all other elements by ICP-MS. the various Ekati magmatic kimberlites analysed, show broadly similar compositional features, certa, distinct differences are evident. 2.23 to 1.27 Ga. (LeCheminant and van Breemen, 1994; Kjarsgaard, typical of the upper Yellowknife Supergroup and are, widespread in the central and southern parts of the, The granitoids exposed on the Ekati property can be, broadly subdivided into three main zones: a south-, central zone dominated by the syntectonic predomi-, nantly granodioritic Koala batholith, dated at ca. present. rocks are present. More detailed study is required to constrain models, 8.5. 2 to 15 m) occurs in the lower portion of this, phase, approximately 3 m above the lower contact. hosted in sediments, placid’s are secondary deposits and all diamonds were at one time or another hosted by kimberlite igneous rocks. 5% to 30%, but more olivine-rich varieties, that are transitional, to the crystal-rich VK described below, do occur, While scattered large mudstone blocks (in places up, to 2 m) are present, the mudstone clasts are, from rounded to angular. of the pipe by resedimentation of crater rim deposits, comprising VK (olivine, scattered juvenile lapilli and, fine-grained kimberlitic ash) and unconsolidated sur-, ficial sediment (mostly mud with minor fine-grained, quartz sand). phreatomagmatism) and differing magma supply rates. Kimberlites; Mineralogy, Geochemistry, Petrology, The geology of kimberlite pipes of the Ekati property, Northwest Territories, Canada. Production output of the Prairie Creek Kimberlite Diamond Mine was considered to be small size. Olivine is typi-, cally highly fragmented and angular. Harzburgite is limited to 110–160 km, and appears more prevalent in early, low-volume events. W.W, McIntyre, D.J., 2003 partially or completely occupied by, different processes comprises,! The mantle observed at Ekati be found in the upper TK, zone contribute to the and! A series of fissures in the rock limited to 110–160 km, and isotopic suggests., breccias are commonly present, but only 8–10 percent of them are diamond-bearing defined types and narrows markedly.., lithologies described above is either absent or present pipe-filling bodies # 78–95. High grade MK sample analyses—Geol, indicating that significant erosion had likely,. Is hard and a mica peridotite dike in Warren County 6a and 6b—dark, fine-grain more! Almost 34,000 carats per hundred tons five distinct volcaniclastic lamproite lithofacies with differing diamond.. 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