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1.
At present, 48 Late Cretaceous (ca. 70–88 Ma) kimberlitic pipes have been discovered in three separate areas of the northern Alberta: the Mountain Lake cluster, the Buffalo Head Hills field and the Birch Mountains field. The regions can be distinguished from one another by their non-archetypal kimberlite signature (Mountain Lake) or, in the case of kimberlite fields, primitive (Buffalo Head Hills) to evolved (Birch Mountains) magmatic signatures. The dominant process of magmatic differentiation is crystal fractionation and accumulation of olivine, which acts as the main criteria to distinguish between primitive and evolved Group I-type kimberlite fields in the northern Alberta. This is important from the viewpoint of diamond exploration because the majority (about 80%) of the more primitive Buffalo Head Hills kimberlites are diamondiferous, whereas the more evolved Birch Mountains pipes are barren of diamonds for the most part. Petrographically, the Buffalo Head Hills samples are distinct from the Birch Mountains samples in that they contain less carbonate, have a smaller modal abundance of late-stage minerals such as phlogopite and ilmenite, and have a higher amount of fresh, coarse macrocrystal (>0.5 mm) olivine. Consequently, samples from the Buffalo Head Hills have the highest values of MgO, Cr and Ni, and have chemistries similar to those of primitive hypabyssal kimberlite in the Northwest Territories. Based on whole-rock isotopic data, the Buffalo Head Hills K6 kimberlite has 87Sr/86Sr and Nd values similar to those of South African Group I kimberlites, whereas the Birch Mountains Legend and Phoenix kimberlites have similar Nd values (between 0 and +1.9), but distinctly higher 87Sr/86Sr values (0.7051–0.7063). The lack of whole-rock geochemical overlap between kimberlite and the freshest, least contaminated Mountain Lake South pipe rocks reflects significant mineralogical differences and Mountain Lake is similar geochemically to olivine alkali basalt and/or basanite. Intra-field geochemical variations are also evident. The K4 pipe (Buffalo Head Hills), and Xena and Kendu pipes (Birch Mountains) are characterized by anomalous concentrations of incompatible elements relative to other northern Alberta kimberlite pipes, including chondrite-normalized rare-earth element distribution patterns that are less fractionated than the other kimberlite samples from the Buffalo Head Hills and Birch Mountains. The Xena pipe has similar major element chemical signatures and high-Al clinopyroxene similar to, or trending towards, the Mountain Lake pipes. In addition, K4 and Kendu have higher 87Sr/86Sr and lower Nd than Bulk Earth and plot in the bottom right quadrant of the Nd–Sr diagram. We suggest, therefore, that the K4 and Kendu pipes contain a contribution from old, LREE-enriched (low Sm/Nd) lithosphere that is absent from the other kimberlites, are affected by crustal contamination, or both. Based on xenocryst populations, the northern Alberta kimberlite province mantle is dominated by carbonate-saturated lherzolitic mantle. Higher levels of melt depletion characterize the Buffalo Head Hills mantle sample. Despite high diamondiferous to barren pipe ratios in the Buffalo Head Hills pipes, mineral indicators of high diamond potential, such as G10 garnet, diamond inclusion composition chrome spinels and high-sodium eclogitic garnet, are rare. 相似文献
2.
This paper reports new petrographic and mineralogical data on the Manchary kimberlite pipe, which was discovered south of Yakutsk (Central Yakutia) in 2007–2008, 100 km. The pipe breaks through the Upper Cambrian carbonate deposits and is overlain by Jurassic terrigenous rock masses about 100 m thick. It is composed of greenish-gray kimberlite breccia with a serpentine-micaceous cement of massive structure. The porphyry texture of kimberlite is due to the presence of olivine, phlogopite, and picroilmenite phenocrysts. The SiO 2 and Al 2O 3 contents of the groundmass are indicative of typical noncontaminated kimberlites. The groundmass has a significant content of ore minerals: Fe- and Cr-spinels, perovskite, magnetite, and, less commonly, magnesian Cr-magnetite. Pyropes occur in kimberlites as sharp-edged fragments and show uneven distribution. Chemically, they belong to lherzolite, wehrlite, or nondiamondiferous dunite–harzburgite parageneses. Garnets corresponding to lherzolites of anomalous composition make up 8%; this is close to the garnet content of Middle Paleozoic kimberlites from the Yakutian kimberlite province. The pyropes from the new pipe are compositionally similar to those from diamond-poor Middle Paleozoic kimberlites in the north of the Yakutian diamondiferous province. Chemically, pyropes from the Manchary pipe and those from the modern alluvium of the Kengkeme and Chakyya Rivers differ substantially. Consequently, the rocks of the pipe could not be a source of pyropes for this alluvium. They probably occured from other sources. This fact along with numerous “pipelike” geophysical anomalies, suggest the existence of a new kimberlite field in Central Yakutia. 相似文献
3.
In Venezuela, kimberlites have so far only been found in the Guaniamo region, where they occur as high diamond grade sheets in massive to steeply foliated Paleoproterozoic granitoid rocks. The emplacement age of the Guaniamo kimberlites is 712±6 Ma, i.e., Neoproterozoic. The Guaniamo kimberlites contain a high abundance of mantle minerals, with greater than 30% olivine macrocrysts. The principal kimberlite indicator minerals found are pyrope garnet and chromian spinel, with the overwhelming majority of the garnets being of the peridotite association. Chrome-diopside is rare, and picroilmenite is uncommon. Chemically, the Guaniamo kimberlites are characterized by high MgO contents, with low Al 2O 3 and TiO 2 contents and higher than average FeO and K 2O contents. These rocks have above average Ni, Cr, Co, Th, Nb, Ta, Sr and LREE concentrations and very low P, Y and, particularly, Zr and Hf contents. The Nb/Zr ratio is very distinctive and is similar to that of the Aries, Australia kimberlite. The Guaniamo kimberlites are similar in petrography, mineralogy and mantle mineral content to ilmenite-free Group 2 mica kimberlites of South Africa. The Nd-Sr isotopic characteristics of Guaniamo kimberlites are distinct from both kimberlite Group 1 and Group 2, being more similar to transitional type kimberlites, and in particular to diamondiferous kimberlites of the Arkhangelsk Diamond Province, Russia. The Guaniamo kimberlites form part of a compositional spectrum between other standard kimberlite reference groups. They formed from metasomatised subcontinental lithospheric mantle and it is likely that subduction of oceanic crust was the source of this metasomatised material, and also of the eclogitic component, which is dominant in Guaniamo diamonds. 相似文献
4.
Manganoan ilmenite was identified in Juina, Brazil kimberlitic rocks among other megacrysts. It forms oval, elongated, rimless grains comprising 8–30 wt.% of the heavy fraction. Internally the grains are homogeneous. The chemical composition of Mn-ilmenite is almost stoichiometric for ilmenite except for an unusually high manganese content, with MnO = 0.63–2.49 wt.% (up to 11 wt.% in inclusions in diamond) and an elevated vanadium admixture (V 2O 3 = 0.21–0.43 wt.%). By the composition, Mn-ilmenite megacrysts and inclusions in diamond are almost identical. The concentrations of trace elements in Mn-ilmenite, compared to picroilmenite, are much greater and their variations are very wide. Chondrite-normalized distribution of trace elements in Mn-ilmenite megacrysts is similar to the distribution in Mn-ilmenites included in diamond. This confirms that Mn-ilmenite in kimberlites is genetically related to diamond. The finds of Mn-ilmenite known before in kimberlitic and related rocks are late- or postmagmatic, metasomatic phases. They either form reaction rims on grains of picroilmenite or other ore minerals, or compose laths in groundmass. In contrast to those finds, Mn-ilmenite megacrysts in Juina kimberlites are a primary mineral phase with a homogeneous internal structure obtained under stable conditions of growth within lower mantle and/or transition zone. In addition to pyrope garnet, chromian spinel, picroilmenite, chrome-diopside, and magnesian olivine, manganoan ilmenite may be considered as another kimberlite/diamond indicator mineral. 相似文献
5.
Some garnet and omphacite megacrysts which are compositionally similar to phases in the eclogite xenoliths from the Jagersfontein kimberlite are ascribed to fragmentation of coarse-grained eclogites. They differ compositionally from other megacrysts from Jagersfontein and those found at other localities that are believed to be high-pressure phenocrysts precipitating from kimberlite. The eclogite suite differs from those in other southern African kimberlites, mainly in lacking eclogites containing Ca-rich garnets. 相似文献
6.
This report considers experimental studies of the gravitation fractionating of xenocrysts (diamond, garnet, and olivine) in
kimberlite magma at 4.0 GPa and 1400°C. The values obtained (0.6–0.7 m/h for 0.3-mm diamond crystals, 0.36 m/h for garnet
grains, and 0.6–0.29 m/h for olivine grains) point to a high rate of xenocryst sinking in the ultralow-viscous kimberlite
magma (as high as 1 m/h and more, depending on the densities and grain sizes of minerals). A high rate of xenocryst sinking
in kimberlite magma results in the impossibility of preservation of heterogeneity in these melts for a sufficiently long time. 相似文献
7.
The majority of the diamond mines in Botswana were discovered as a direct consequence of soil sampling for indicator minerals such as garnet and picroilmenite. Over the past 60 years the application of soil sampling for indicator minerals as a primary exploration tool has declined while aeromagnetic surveys have increased in popularity. The rate of kimberlite discovery in Botswana has declined significantly. The obvious magnetic kimberlites have been discovered. The future of new kimberlite discoveries is once again dependent on soil sampling for kimberlite indicator minerals. It is essential to have an in depth understanding of the transport mechanism of kimberlite indicator minerals from the kimberlite to the modern day surface of the Kalahari Formation, which is solely via termite bioturbation. Field observations indicate that the concentration of indicator minerals at surface is directly dependent on the physical characteristics and capabilities as well as behavioural patterns of the particular termite species dominant in the exploration area. The discovery of future diamond mines in Botswana will be closely associated with an in depth understanding of the relationship between size and concentration of kimberlite indicator minerals in surface soils and the seasonal behaviour, depth penetration capabilities, earthmoving efficiencies and mandible size of the dominant termite species within the exploration area. Large areas in Botswana, where kimberlite indicator minerals recovered from soil samples have been described as distal from source or background, will require re-evaluation. Without detailed termite studies the rate of discovery will continue to decline. 相似文献
8.
贵州镇远是中国金刚石原生矿找矿的重点区域之一。镇远地区马坪D1号岩体是1965年中国首次发现的含原生金刚石金伯利岩。该岩体岩石具典型的金伯利岩结构和组成特征,其中的锆石捕虏晶U–Pb年代学和Hf同位素分析结果表明,该地区存在未暴露的太古宙基底物质残余。基于壳幔耦合性规律,可能对应有古老的岩石圈地幔,这种古老的克拉通属性是金刚石形成的有利因素。但另一方面,马坪金伯利岩普遍含有伴生矿物含铬镁铝榴石,其CaO含量较高,多数属于G9(二辉橄榄岩)类型,不是全球富含金刚石的方辉橄榄岩原岩类型(G10),暗示当时的岩石圈发生了部分改造而可能不利于高品质金刚石的形成。需要注意的是,在金刚石找矿过程中,应该以详细的野外工作与岩石学对比研究为基础,同时依赖于金伯利岩及其相关的岩浆活动所携带的捕虏体/捕虏晶的研究,配合以岩浆成分来反演地幔源区特征,才能较全面地揭示古老大陆岩石圈的形成年龄与演化历史、物质组成与精细结构,以及大陆岩石圈根的厚度、热状态、氧逸度、流体作用等,进而为寻找金刚石提供重要的依据。 相似文献
9.
Petrographic and chemical criteria indicate that the overwhelming majority of olivines in kimberlites are probably cognate phenocrysts. The implied low volume of xenocryst olivines requires that primitive kimberlite magmas are highly ultrabasic liquids. Two chemically distinctive olivine populations are present in all of the kimberlites studied. The dominant olivine population, which includes large rounded olivines and smaller euhedral crystals, is Mg-rich relative to late-stage rim compositions. It is characterized by a range in 100 Mg/(Mg + Fe) and uniform Ni concentration, reflecting Rayleigh-type crystallization during magma evolution. The most Mg-rich of these olivines are considered to be similiar to those in the mantle source rocks. The second compositional population, generally very subordinate, though markedly more abundant in the megacrystrich Monastery kimberlite, is Fe-rich relative to rim compositions. This group of olivines crystallized from evolved liquids in equilibrium with iron-rich megacrysts, both entrained by the kimberlite magma during ascent. Differences between the chemical fields of Fe-rich olivines in Group I and Group II kimberlites point to relatively deeper derivation of the latter suite. Olivine chemistry can be used to characterize kimberlite magma sub-types, and may prove to be a useful tool for evaluating the diamond potential of kimberlites. 相似文献
10.
In the late 1990s, the Fazenda Largo kimberlite cluster was discovered in the Piauí State of Brazil. As with earlier known kimberlites in this area – Redondão, Santa Filomena-Bom Jesus (Gilbues) and Picos – this cluster is located within the Palaeozoic Parnaiba Sedimentary Basin that separates the São Francisco and the Amazonian Precambrian cratons. Locations of kimberlites are controlled by the ‘Transbrasiliano Lineament’. The Fazenda Largo kimberlites are intensely weathered, almost completely altered rocks with a fine-grained clastic structure, and contain variable amounts of terrigene admixture (quartz sand). These rocks represent near-surface volcano-sedimentary deposits of the crater parts of kimberlite pipes. By petrographic, mineralogical and chemical features, the Fazenda Largo kimberlites are similar to average kimberlite. The composition of the deep-seated material in the Fazenda Largo kimberlites is quite diverse: among mantle microxenoliths are amphibolitised pyrope peridotites, garnetised spinel peridotites, ilmenite peridotites, chromian spinel + chromian diopside + pyrope intergrowths, and large xenoliths of pyrope dunite. High-pressure minerals are predominantly of the ultramafic suite, Cr-association minerals (purplish-red and violet pyrope, chromian spinel, chromian diopside, Cr-pargasite and orthopyroxene). The Ti-association minerals of the ultramafic suite (picroilmenite and orange pyrope), as well as rare grains of orange pyrope-almandine of the eclogite association, are subordinate. Kimberlites from all four pipes contain rare grains of G10 pyrope of the diamond association, but chromian spinel of the diamond association was not encountered. By their tectonic position, by geochemical characteristics, and by the composition of kimberlite indicator minerals, the Fazenda Largo kimberlites, like the others of such type, are unlikely to be economic. 相似文献
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