Renard 65, a diamondiferous pipe in the Neoproterozoic Renard kimberlite cluster (Québec, 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.
相似文献The Victor South pipe has a simple bowl-like shape that flares from just below the basal sandstone of the sediments that overlie the basement. The sandstone is a known aquifer, suggesting that the crater excavation process was possibly phreatomagmatic. In contrast, the pipe shape and internal geology of Victor North are more complex. The northwestern part of the pipe is dominated by dark competent rocks, which resemble fresh hypabyssal kimberlite, but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias±volcaniclastic kimberlite. Current evidence suggests that the hypabyssal-like kimberlite is, in fact, not intrusive and that the northwestern part of Victor North represents an early-formed crater infilled with contrasting extrusive kimberlites and associated breccias. The remaining, main part of Victor North consists of two macroscopically similar, but petrographically distinct, pyroclastic kimberlites that have contrasting macrodiamond sample grades. The juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically. The nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different, indicating that they derive from different magma pulses, or phases of kimberlite, and thus represent separate eruptions. The initial excavation of a crater cross-cutting the earlier northwestern crater was followed by emplacement of phase (i), a low-grade olivine phenocryst-rich pyroclastic kimberlite, and the subsequent eruption of phase (ii), a high-grade olivine phenocryst-poor pyroclastic kimberlite, as two separate vents nested within the original phase (i) crater. The second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade. Thus, the final pyroclastic pipe infill of the main part of the Victor North pipe appears to consist of at least three geological/macrodiamond grade zones.
In conclusion, the Victor kimberlite was formed by several eruptive events resulting in adjacent and cross-cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal-like kimberlite, which is now interpreted to be of probable extrusive origin. Within the pyroclastic kimberlites of Victor North, there are two nested vents, a feature seldom documented in kimberlites elsewhere. This study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism. 相似文献
Distinctly different petrographic features characterize crater-facies kimberlite in each of the three pipe classes. In crater-facies kimberlites of Class 1 pipes, small pelletal magmaclasts and abundant microlitic diopside are characteristic. These features appear to reflect the derivation of the crater-facies material from the underlying diatreme zone. Most Class 2 pipes have shallow craters and the crater-facies rocks are predominantly pyroclastic kimberlites with diagnostic amoeboid lapilli, which are sometimes welded and have vesicles as well as glass. Possible kimberlite lava also occurs at two Class 2 pipes in N Angola. The possible presence of lava as well as the features of the pyroclastic kimberlite is indicative of hot kimberlite magma being able to rise to levels close to the surface to form Class 2 pipes. Most Class 3 kimberlites have very steep craters and crater-facies rocks are predominantly resedimented volcaniclastic kimberlites, in some cases characterized by the presence of abundant angular magmaclasts, which are petrographically very similar to typical hypabyssal-facies kimberlite found in Class 1 pipes. The differences in crater-facies kimberlite of the three classes of pipe reflect different formation and depositional processes as well as differences in kimberlite composition, specifically volatile composition. Kimberlite forming pipe Classes 1 and 3 is thought to be relatively water-rich and is emplaced by processes involving magmatic exsolution of volatiles. The kimberlite magma forming Class 2 pipes is CO2-rich, can rise to shallow levels, and can initiate phreatomagmatic emplacement processes. 相似文献
The Renard 2 pipe is currently the deepest-drilled and most extensively studied kimberlite body in the Renard cluster, central Québec, Canada, forming the major component of the Mineral Resource of Stornoway Diamond Corporation’s Renard Mine. Renard 2 is infilled with two distinct kimberlite units that exhibit Kimberley-type pyroclastic kimberlite and related textures. Hypabyssal kimberlite also occurs as smaller cross-cutting sheets and irregular intrusions. The units are distinguished by their rock textures, groundmass mineral assemblages, olivine macrocryst size distributions and replacement products, mantle and country rock xenolith contents, whole rock geochemical signatures, bulk densities and diamond grades. These differences are interpreted to reflect different mantle ascent and near-surface emplacement processes and are here demonstrated to be vertically continuous from present surface to over 1000 m depth. The distinctive petrological features together with sharp, steep and cross-cutting internal contact relationships, show that each unit was formed from a separate batch of mantle-derived kimberlite magma, and was completely solidified before subsequent emplacement of the later unit. The mineralogy and textures of the ultra-fine-grained interclast matrix are consistent with those described at numerous Kimberley-type pyroclastic kimberlite localities around the world and are interpreted to reflect rapid primary crystallization during emplacement of separate kimberlite magmatic systems. The units of fractured and brecciated country rock surrounding the main kimberlite pipe contain kimberlite-derived material including carbonate providing evidence of subsurface brecciation. Together these data show that Renard 2 represents the deeper parts of a Kimberley-type pyroclastic kimberlite pipe system and demonstrates that their diagnostic features result from magmatic crystallisation during subsurface volcanic emplacement processes.
相似文献The pipe shapes and infills of the Gahcho Kué kimberlites are similar to those of the classic South African pipes, particularly those of the Kimberley area. Similar intrusive magmatic emplacement processes are proposed in which the diatreme-zone results from the degassing, after breakthrough, of the intruding magma column. The transition zones represent ‘frozen’ degassing fronts. The style of emplacement of the Gahcho Kué kimberlites is very different from that of many other pipes in Canada such as at Lac de Gras, Fort à la Corne or Attawapiskat. 相似文献
The early Cambrian to late Neoproterozoic Kelvin kimberlite pipe is located in the southeast of the Archean Slave Craton in northern Canada, eight km northeast of the Gahcho Kué diamond mine. Kelvin was first discovered in 2000 by De Beers Canada. Subsequent exploration undertaken by Kennady Diamonds Inc. between 2012 and 2016 resulted in the discovery of significant thicknesses of volcaniclastic kimberlite that had not previously been observed. Through extensive delineation drilling Kelvin has been shown to present an atypical, steep-sided inclined L-shaped pipe-like morphology with an overall dip of 15 to 20°. With a surface expression of only 0.08 ha Kelvin dips towards the northwest before turning north. The body (which remains open at depth) has been constrained to a current overall strike length of 700 m with varying vertical thickness (70 to 200 m) and width (30 to 70 m). Detailed core logging, petrography and microdiamond analysis have shown that the pipe infill comprises several phases of sub-horizontally oriented kimberlite (KIMB1, KIMB2, KIMB3, KIMB4, KIMB7 and KIMB8) resulting from multiple emplacement events. The pipe infill is dominated by Kimberley-type pyroclastic kimberlite or “KPK”, historically referred to as tuffisitic kimberlite breccia or “TKB”, with less common hypabyssal kimberlite (HK) and minor units with textures transitional between these end-members. An extensive HK sheet complex surrounds the pipe. The emplacement of Kelvin is believed to have been initiated by intrusion of this early sheet system. The main pipe-forming event and formation of the dominant KPK pipe infill, KIMB3, was followed by late stage emplacement of additional minor KPK and a hypabyssal to transitional-textured phase along the upper contact of the pipe, cross-cutting the underlying KIMB3. Rb-Sr age dating of phlogopite from a late stage phase has established model ages of 531 ± 8 Ma and 546 ± 8 Ma. Texturally and mineralogically, the Kelvin kimberlite is similar to other KPK systems such as the Gahcho Kué kimberlites and many southern African kimberlites; however, the external morphology, specifically the sub-horizontal inclination of the pipe, is unique. The morphology of Kelvin and the other kimberlites in the Kelvin-Faraday cluster defines a new type of exploration target, one that is likely not unique to the Kennady North Project area. Extensive evaluation work by Kennady Diamonds Inc. has resulted in definition of a maiden Indicated Mineral Resource for Kelvin of 8.5 million tonnes (Mt) of kimberlite at an average grade of 1.6 carats per tonne (cpt) with an average diamond value of US$ 63 per carat (ct).
相似文献The Nxau Nxau kimberlites in northwest Botswana belong to the Xaudum kimberlite province that also includes the Sikereti, Kaudom and Gura kimberlite clusters in north-east Namibia. The Nxau Nxau kimberlites lie on the southernmost extension of the Congo Craton, which incorporates part of the Damara Orogenic Belt on its margin. The Xaudum kimberlite province is geographically isolated from other known clusters but occurs within the limits of the NW-SE oriented, Karoo-aged Okavango Dyke Swarm and near NE-SW faults interpreted as the early stages of the East African Rift System. Petrographic, geochronological and isotopic studies were undertaken to characterise the nature of these kimberlites and the timing of their emplacement. The Nxau Nxau kimberlites exhibit groundmass textures, mineral phases and Sr-isotope compositions (87Sr/86Sri of 0.7036 ± 0.0002; 2σ) that are characteristic of archetypal (Group I) kimberlites. U-Pb perovskite, 40Ar/39Ar phlogopite and Rb-Sr phlogopite ages indicate that the kimberlites were emplaced in the Cretaceous, with perovskite from four samples yielding a preferred weighted average U-Pb age of 84 ± 4 Ma (2σ). This age is typical of many kimberlites in southern Africa, indicating that the Xaudum occurrences form part of this widespread Late Cretaceous kimberlite magmatic province. This time marks a significant period of tectonic stress reorganisation that could have provided the trigger for kimberlite magmatism. In this regard, the Nxau Nxau kimberlites may form part of a NE-SW oriented trend such as the Lucapa corridor, with implications for further undiscovered kimberlites along this corridor.
相似文献The Renard 2 kimberlite pipe is one of nine diamondiferous kimberlite pipes that form a cluster in the south-eastern portion of the Superior Province, Québec, Canada and is presently being extracted at the Renard Mine. It is interpreted as a diatreme-zone kimberlite consisting of two Kimberley-type pyroclastic units and related country rock breccias, all cross-cut by coherent kimberlite dykes and irregular intrusives. Renard 2 has been the subject of numerous diamond drilling campaigns since its discovery in 2001. The first two geological models modelled kimberlite and country rock breccia units separately. A change in modelling philosophy in 2009, which incorporated the emplacement envelope and history, modelled the entire intrusive event and projected the pipe shape to depth allowing for more targeted deep drilling where kimberlite had not yet been discovered. This targeted 2009 drilling resulted in a > 400% increase in the volume of the Indicated Resource. Modelling only the kimberlite units resulted in a significant underestimation of the pipe shape. Current open pit and underground mapping of the pipe shape corresponds well to the final 2015 geological model and contact changes observed are within the expected level of confidence for an Indicated Resource. This study demonstrates that a sound understanding of the geological emplacement is key to developing a reliable 3D geological and resource model that can be used for targeted delineation drilling, feasibility studies and during the initial stages of mining.
相似文献The Letšeng Diamond Mine comprises two ~91 Ma kimberlite pipes. An update of the geology is presented based on the 2012–2017 detailed investigation of open pit exposures and all available drillcores which included mapping, logging and petrography. Each of the steep-sided volcanic pipes comprises a number of phases of kimberlite with contrasting diamond contents which were formed by the emplacement of at least four batches of mantle-derived magma. The resulting range of textures includes resedimented volcaniclastic kimberlite (RVK), Kimberley-type pyroclastic kimberlite (KPK), coherent kimberlite (CK) and minor amounts of hypabyssal kimberlite (HK). The pipes are compared with KPK occurrences from southern Africa and worldwide. Many features of the Letšeng pipes are similar to KPK infilled pipes particularly those of the widespread Cretaceous kimberlite province of southern Africa. The differences displayed at Letšeng compared to other large KPK pipe infills described from around the world are attributed to the marginal or melnoitic nature of the magma and the upper diatreme to crater setting of the Letšeng pipes, where processes become extrusive. It is concluded that the pipes comprise a variant of Kimberley-type pyroclastic kimberlite emplacement. The classification of many of the Letšeng rocks as KPK is important for developing the internal geology of the pipes as well as for predicting the distribution of diamonds within the bodies.
相似文献The HK is an archetypal kimberlite composed of macrocrysts of olivine, spinel, mica, rare eclogitic garnet and clinopyroxene with microphenocrysts of olivine and groundmass spinel, phlogopite, apatite and perovskite in a serpentine–calcite–phlogopite matrix. The Ba enrichment of phlogopite, the compositional trends of both primary spinel and phlogopite, as well as the composition of the mantle-derived xenocrysts, are also characteristic of kimberlite. The present-day country rocks are granitoids; however, the incorporation of sedimentary xenoliths in the HK shows that the Archean granitoid basement terrain, at least locally, was capped by younger Proterozoic sediments at the time of emplacement. The sediments have since been removed by erosion. HK is confined to the deeper eastern parts of the Anuri pipe. It is suggested that the HK was emplaced prior to the dominant VK as a separate phase of kimberlite. The HK must have ascended to high stratigraphic levels to allow incorporation of Proterozoic sediments as xenoliths.
Most of the Anuri kimberlite is infilled with VK which is composed of variable proportions of juvenile lapilli, discrete olivine macrocrysts, country rock xenoliths and mantle-derived xenocrysts. It is proposed that the explosive breakthrough of a second batch of kimberlite magma formed the western lobe resulting in the excavation of the main pipe. Much of the resulting fragmented country rock material was deposited in extra crater deposits. Pyroclastic eruption(s) of kimberlite must have occurred to form the common juvenile lapilli present in the VKs. The VK is variable in nature and can be subdivided into four types: volcaniclastic kimberlite breccia, magmaclast-rich volcaniclastic kimberlite breccia, finer grained volcaniclastic kimberlite breccia and lithic-rich volcaniclastic kimberlite breccia. The variations between these subtypes reflect different depositional processes. These processes are difficult to determine but could include primary pyroclastic deposition and/or resedimentation.
There is some similarity between Anuri and the Lac de Gras kimberlites, with variable types of VK forming the dominant infill of small, steep-sided pipes excavated into crystalline Archean basement and sedimentary cover. 相似文献
Here we present new data from a systematic Sr, Nd, O, C isotope and geochemical study of kimberlites of Devonian age Mirny field that are located in the southernmost part of the Siberian diamondiferous province. Major and trace element compositions of the Mirny field kimberlites show a significant compositional variability both between pipes and within one diatreme. They are enriched in incompatible trace elements with La/Yb ratios in the range of (65–300). Initial Nd isotope ratios calculated back to the time of the Mirny field kimberlite emplacement (t = 360 ma) are depleted relative to the chondritic uniform reservoir (CHUR) model being 4 up to 6 ɛNd(t) units, suggesting an asthenospheric source for incompatible elements in kimberlites. Initial Sr isotope ratios are significantly variable, being in the range 0.70387–0.70845, indicating a complex source history and a strong influence of post-magmatic alteration. Four samples have almost identical initial Nd and Sr isotope compositions that are similar to the prevalent mantle (PREMA) reservoir. We propose that the source of the proto-kimberlite melt of the Mirny field kimberlites is the same as that for the majority of ocean island basalts (OIB). The source of the Mirny field kimberlites must possess three main features: It should be enriched with incompatible elements, be depleted in the major elements (Si, Al, Fe and Ti) and heavy rare earth elements (REE) and it should retain the asthenospheric Nd isotope composition. A two-stage model of kimberlite melt formation can fulfil those requirements. The intrusion of small bodies of this proto-kimberlite melt into lithospheric mantle forms a veined heterogeneously enriched source through fractional crystallization and metasomatism of adjacent peridotites. Re-melting of this source shortly after it was metasomatically enriched produced the kimberlite melt. The chemistry, mineralogy and diamond grade of each particular kimberlite are strongly dependent on the character of the heterogeneous source part from which they melted and ascended.
相似文献Distinctly different groundmass mineralogy characterise the hypabyssal facies, Mesoproterozoic diamondiferous P3 and P4 intrusions from the Wajrakarur Kimberlite Field, southern India. P3 is an archetypal kimberlite with macrocrysts of olivine and phlogopite set in a groundmass dominated by phlogopite and monticellite with subordinate amounts of serpentine, spinel, perovskite, apatite, calcite and rare baddeleyite. P4 contains mega- and macrocrysts of olivine set in a groundmass dominated by clinopyroxene and phlogopite with subordinate amounts of serpentine, spinel, perovskite, apatite, and occasional gittinsite, and is mineralogically interpreted as an olivine lamproite. Three distinct populations of olivine, phlogopite and clinopyroxene are recognized based on their microtextural and compositional characteristics. The first population includes glimmerite and phlogopite–clinopyroxene nodules, and Mg-rich olivine macrocrysts (Fo 90–93) which are interpreted to be derived from disaggregated mantle xenoliths. The second population comprises macrocrysts of phlogopite and Fe-rich olivine (Fo 81–89) from P3, megacrysts and macrocrysts of Fe-rich olivine (Fo 85–87) from P4 and a rare olivine–clinopyroxene nodule from P4 which are suggested to have a genetic link with the precursor melt of the respective intrusions. The third population represents clearly magmatic minerals such as euhedral phenocrysts of Fe-rich olivine (Fo 85–90) crystallised at mantle depths, and olivine overgrowth rims formed contemporaneously with groundmass minerals at crustal levels. Close spatial association and contemporaneous emplacement of P3 kimberlite and P4 lamproite is explained by a unifying petrogenetic model which involves the interaction of a silica-poor carbonatite melt with differently metasomatised wall rocks in the lithospheric mantle. It is proposed that the metasomatised wall rock for lamproite contained abundant MARID-type and phlogopite-rich metasomatic veins, while that for kimberlite was relatively refractory in nature.
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