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1.
We report the first data on the contents of main oxides and REE in rocks and the compositions of pyropes and almandines from the Maiskoe kimberlite body recently discovered in the Nakyn field of the Yakutian kimberlite province.The kimberlites are characterized by low contents of Ti, a slight domination of Mg over Ca, and high contents of K2O in some samples. The pyropes have high contents of Cr2O3 (up to 14.5 wt.%); many of them (～16%) are poor in Ca. In petrochemical and mineralogical features the kimberlites of the Maiskoe body are complementary to the highly diamondiferous kimberlites of the nearby Botuobinskaya and Nyurbinskaya pipes. At the same time, they are not the final link in the evolution of kimberlite magmatism in the Nakyn field, which makes the latter still more promising for diamonds.  相似文献   

2.
金刚石及其寄主岩石是人类认识地球深部物质组成和性质、壳幔和核幔物质循环重要研究对象。本文总结了中国不同金刚石类型的分布,着重对比了博茨瓦纳和中国含金刚石金伯利岩的地质特征,取得如下认识:(1)博茨瓦纳含矿原生岩石仅为金伯利岩,而中国含矿岩石成分复杂,金伯利岩主要出露在华北克拉通,展布于郯庐、华北中央和华北北缘金伯利岩带,具有工业价值的蒙阴和瓦房店矿床分布于郯庐金伯利岩带中;钾镁煌斑岩主要出露在华南克拉通,重点分布在江南和华南北缘钾镁煌斑岩带中;(2)钙钛矿原位U-Pb年龄和Sr、Nd同位素显示,86～97 Ma奥拉帕金伯利岩群和456～470 Ma蒙阴和瓦房店金伯利岩均具有低87Sr/86Sr(0.703～0.705)和中等εNd(t)(-0.09～+5)特征,指示金伯利岩浆源自弱亏损地幔或初始地幔源区;(3)博茨瓦纳金伯利岩体绝大多数以岩筒产出,而中国以脉状为主岩筒次之;博茨瓦纳岩筒绝大部分为火山口相,中国均为根部相,岩筒地表面积普遍小于前者;(4)奥拉帕A/K1和朱瓦能金伯利岩体是世界上为数不多的主要产出榴辉岩捕虏体和E型金刚石的岩筒之一,而同位于奥拉帕岩群的莱特拉卡内、丹姆沙和卡罗韦岩体与我国郯庐带的金伯利岩体类似,均主要产出地幔橄榄岩捕虏体以及P型和E型金刚石;(5)寻找含矿金伯利岩重点注意以下几点:克拉通内部和周缘深大断裂带是重要的控岩构造;镁铝榴石、镁钛铁矿、铬透辉石、铬尖晶石和铬金红石等是寻找含金刚石金伯利岩重要的指示矿物;航磁等地球物理测量需与土壤取样找矿方法相结合才能取得更好效果;(6)郯庐金伯利岩带、江南钾镁煌斑岩带和塔里木地块是中国重要含矿岩石的找矿靶区,冲积型金刚石成矿潜力巨大。  相似文献   

3.
Mn-rich ilmenites (up to 7 % MnO) have been identified in dykes cutting the Malanjkhand porphyry copper mining area in Madhya Pradesh. The dykes are hydrothermally altered and are of tholeiitic affinities. Lamprophyres have been reported from nearby areas. The presence of Mn-rich Ilmenites in the Malanjkhand copper mine dykes and the occurrence of lamprophyres and the pervasive potassium metasomatism in the area strongly suggests a possibility of finding diamondiferous rocks in the area. Such high-Mn bearing ilmenites are associated with diamondiferous deposits in other parts of the world, e.g. Juina kimberlites, Brazil. Mn-bearing ilmenite is considered as an indicator mineral for kimberlite/diamond occurrences. The presence of kimberlite pipes in Raipur district and the association of Mn-rich ilmenite with kimberlites is a fortuitous coincidence for venture-some mining companies. A probable explanation for the origin of manganese layers in the context of ‘rift’ tectonic environment is offered. Also a possible link between the dykes, quartz veins in the Malanjkhand granitic rocks and the copper mineralization is proposed.  相似文献   

4.
中国的原生金刚石矿床首先发现于山东省和辽宁省,它是规模不大而稀少的矿床类型。中国原生金刚石矿床只产于地台区,沿着郯庐断裂分布,主要矿田呈北北东—南南西向有规律的排列。金伯利岩是一种复式的混杂岩,它侵入的地区伴随有地幔的隆起或穹隆构造。侵入作用是多次的,来自深部喷发作用的金伯利岩是极少。金刚石是从岩浆中结晶沉淀的。“兰地”,“黄地”等风化产物,地球化学、磁异常、重矿物源等有助于指导找矿。  相似文献   

5.
初论金刚石原生矿床成矿系列   总被引：1，自引：0，他引：1       下载免费PDF全文
银剑钊 《地球科学》2000,25(4):380-383
目前所知产金刚石的岩石类型包括金伯利岩、钾镁煌斑岩、榴辉岩、蛇绿岩套、碱性超基性杂岩、碱性超基性煌斑岩和橄榄岩类(方辉橄榄岩、纯橄榄岩等) 等偏碱性超镁铁质岩石, 而有经济价值的金刚石原生矿床仅见于金伯利岩和钾镁煌斑岩中, 除此之外的其他岩石类型中仅见有少量微粒金刚石.金伯利岩和钾镁煌斑岩都起源于地幔深部, 就此意义上讲, 二者是同源的, 但其岩石化学成分、主要矿物组成、产出大地构造背景以及同位素资料等, 却存在着比较明显的差异.由此构成了金刚石原生矿床的两个成矿系列: 金伯利岩成矿系列和钾镁煌斑岩成矿系列.金伯利岩成矿系列又可以根据其化学成分划分为3个亚系列, 即: 高Cr, Ti, Mg成矿亚系列, 低Cr, Ti, Mg成矿亚系列和介于二者之间的一种具有复杂化学成分的成矿亚系列.钾镁煌斑岩成矿系列则可以根据其主要矿物组成, 划分出橄榄石钾镁煌斑岩成矿亚系列、白榴石钾镁煌斑岩成矿亚系列以及介于两者之间的白榴石-橄榄石钾镁煌斑岩成矿亚系列共3种次级成矿系列.与此同时, 无论是金伯利岩成矿系列, 还是钾镁煌斑岩成矿系列, 又都可以根据其野外地质产状, 划分为以下3个成矿亚系列(形成时间从早到晚) : (1) 火山沉积凝灰岩成矿亚系列; (2) 火山凝灰角砾岩成矿亚系列; (3) 火山-次火山侵入相成矿亚系列.   相似文献   

6.
This paper outlines the development of a multi-disciplinary strategy to focus exploration for economic kimberlites on the Ekati property. High-resolution aeromagnetic data provide an over-arching spatial and magnetostratigraphic framework for exploration and kimberlite discovery at Ekati, and hence also for this investigation. The temporal, geomagnetic, spatial and related attributes of kimberlites with variable diamond content have been constrained by judiciously augmenting the information gathered during routine exploration with detailed, laboratory-based or field-based investigations. The natural remanent magnetisation of 36 Ekati kimberlites has been correlated with their age as determined by isotopic dating techniques, and placed in the context of a well-constrained geomagnetic polarity timescale. Kimberlite magmatism occurred over the period 75 to 45 Ma, in at least five temporally discrete intrusive episodes. Based on current evidence, the older kimberlites (75 to 59 Ma) have low diamond contents and are distributed throughout the property. Younger kimberlites (56 to 45 Ma) have moderate to high diamond contents and occur in three distinct intrusive corridors with NNE to NE orientations. Economic kimberlite pipes erupted at 55.4±0.4 Ma along the A154-Lynx intrusive corridor, which is 7 km wide and oriented at 015°, and at 53.2±0.3 Ma along the Panda intrusive corridor, which is 1 km wide and oriented at 038°. The intrusion ages straddle a paleopole reversal at Chron C24n, consistent with the observation that the older economic kimberlites present as aeromagnetic “low” anomalies while the younger economic pipes are characterised as aeromagnetic “highs”. The aeromagnetic responses for these kimberlites are generally muted because they contain volcaniclastic rock types with low magnetic susceptibility. Kimberlites throughout the Ekati property carry a primary natural magnetic remanence (NRM) vector in Ti-bearing groundmass magnetite, and it dominates over vectors related to induced magnetisation. Magnetostratigraphic correlation of Ekati kimberlites may therefore present a powerful adjunct to existing exploration techniques, mainly because the diamond content of Ekati kimberlites apparently is related more to the age of eruption than to any other parameter investigated in this work.  相似文献   

7.
The Buffalo Hills kimberlites define a province of kimberlite magmatism occurring within and adjacent to Proterozoic crystalline basement termed the Buffalo Head Terrane in north-central Alberta, Canada. The kimberlites are distinguished by a diverse xenocryst suite and most contain some quantity of diamond. The xenocryst assemblage in the province is atypical for diamondiferous kimberlite, including an overall paucity of mantle indicator minerals and the near-absence of compositionally subcalcic peridotitic garnet (G10). The most diamond-rich bodies are distinguished by the presence of slightly subcalcic, chromium-rich garnet and the general absence of picroilmenite, with the majority forming a small cluster in the northwestern part of the province. Barren and near-barren pipes tend to occur to the south, with increasing proximity to the basement structure known as the Peace River Arch. Niobian picroilmenite, compositionally restricted low-to moderate-Cr peridotitic garnet, and megacrystal titanian pyrope occur in kimberlites closest to the arch. Major element data for clinopyroxene and trace element data for garnet from diamond-rich and diamond-poor kimberlites suggests that metasomatism of lithospheric peridotite within the diamond stability field may have caused destruction of diamond, and diamond source rocks proximal to the arch were the most affected.  相似文献   

8.
丁毅 《地质学报》2020,94(9):2763-2771
20世纪90年代初,加拿大地质学家Charles E Fipke先生在加拿大高寒的Lac De Gras地区发现的含金刚石的金伯利岩,把加拿大带入了寻找金刚石矿的高潮,相继在Lac De Gras地区发现金伯利岩管群和在加拿大中部的Fort à la Corne发现的金伯利岩群,使得加拿大金刚石产量位居世界第三。本文综述这两个地区金刚石矿的发现和分析含金刚石的金伯利岩的存在的特征,指出太古宙地台环境是含金刚石的金伯利岩存在的基础,厚度大的太古宙地台底部是金刚石晶体缓慢生长的天然高温高压培育仓、沿冰碛或沿水系沉积物追踪金伯利岩指示矿物是溯源金刚石母岩的手段、认识金伯利质火山地貌形态能起到在寻找金伯利质火山位置过程中事半功倍的效果,有助于理解金伯利质火山口相的变化。  相似文献   

9.
First data on the geologic and geochemical compositions of kimberlites from nine kimberlite pipes of southwestern Angola are presented. In the north of the study area, there are the Chikolongo and Chicuatite kimberlite pipes; in the south, a bunch of four Galange pipes (I–IV); and in the central part, the Ochinjau, Palue, and Viniaty pipes. By geochemical parameters, these rocks are referred to as classical kimberlites: They bear mantle inclusions of ultrabasites, eclogites, various barophilic minerals (including ones of diamond facies), and diamonds. The kimberlite pipes are composed of petrographically diverse rocks: tuffstones, tuff breccias, kimberlite breccias, autolithic kimberlite breccias, and massive porphyritic kimberlites. In mineralogical, petrographic, and geochemical compositions the studied kimberlites are most similar to group I kimberlites of South Africa and Fe-Ti-kimberlites of the Arkhangel’sk diamondiferous province. Comparison of the mineralogical compositions of kimberlites from southwestern Angola showed that the portion of mantle (including diamondiferous) material of depth facies in kimberlite pipes regularly increases in the S-N direction. The northern diamond-bearing kimberlite pipes are localized in large destructive zones of NE strike, and the central and southern diamond-free pipes, in faults of N-S strike.  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   

11.
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 SiO2 and Al2O3 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   

12.
Kimberlite-hosted diamond deposits of southern Africa: A review   总被引：4，自引：0，他引：4  
Following the discovery of diamonds in river deposits in central South Africa in the mid nineteenth century, it was at Kimberley where the volcanic origin of diamonds was first recognized. These volcanic rocks, that were named “kimberlite”, were to become the corner stone of the economic and industrial development of southern Africa. Following the discoveries at Kimberley, even more valuable deposits were discovered in South Africa and Botswana in particular, but also in Lesotho, Swaziland and Zimbabwe.A century of study of kimberlites, and the diamonds and other mantle-derived rocks they contain, has furthered the understanding of the processes that occurred within the sub-continental lithosphere and in particular the formation of diamonds. The formation of kimberlite-hosted diamond deposits is a long-lived and complex series of processes that first involved the growth of diamonds in the mantle, and later their removal and transport to the earth's surface by kimberlite magmas. Dating of inclusions in diamonds showed that diamond growth occurred several times over geological time. Many diamonds are of Archaean age and many of these are peridotitic in character, but suites of younger Proterozoic diamonds have also been recognized in various southern African mines. These younger ages correspond with ages of major tectono-thermal events that are recognized in crustal rocks of the sub-continent. Most of these diamonds had eclogitic, websteritic or lherzolitic protoliths.In southern Africa, kimberlite eruptions occurred as discrete events several times during the geological record, including the Early and Middle Proterozoic, the Cambrian, the Permian, the Jurassic and the Cretaceous. Apart from the Early Proterozoic (Kuruman) kimberlites, all of the other events have produced deposits that have been mined. It should however be noted that only about 1% of the kimberlites that have been discovered have been successfully exploited.In this paper, 34 kimberlite mines are reviewed with regard to their geology, mantle xenolith, xenocryst and diamond characteristics and production statistics. These mines vary greatly in size, grade and diamond-value, as well as in the proportions and types of mantle mineral suites that they contain. They include some of the world's richest mines, such as Jwaneng in Botswana, to mines that are both small and marginal, such as the Frank Smith Mine in South Africa. They include large diatremes such as Orapa and small dykes such as those mined at Bellsbank, Swartruggens and near Theunissen. These mines are all located on the Archaean Kalahari Craton, and it is apparent that the craton and its associated sub-continental lithosphere played an important role in providing the right environment for diamond growth and for the formation of the kimberlite magmas that were to transport them to the surface.  相似文献   

13.
贵州镇远马坪"东方一号"岩体为中国最早发现的原生金刚石矿,受当时只有金伯利岩才含金刚石矿及后来西澳阿盖尔钾镁煌斑岩型金刚石原生矿等的影响,先后定名为金伯利岩、钾镁煌斑岩、金云火山岩等。最近专题调查分析研究表明,镇远马坪地区含金刚石母岩,其岩石学矿物学和地球化学特征均更接近澳大利亚典型金伯利岩,白坟地区岩体则类似于澳大利亚典型钾镁煌斑岩,建议将镇远马坪地区"东方一号"等岩类定名为角砾凝灰质金伯利岩,白坟地区岩类定名为钾碱镁闪石-透辉石-金云母钾镁煌斑岩,镇远地区兼有金伯利岩和钾镁煌斑岩的特征,与西澳大利亚极其类似,镇远地区乃至黔东地区具有较大的金刚石原生矿找矿勘查潜力和研究意义。  相似文献   

14.
辽宁瓦房店金刚石矿田金伯利岩侵位机制分析   总被引：2，自引：0，他引：2  
付海涛 《地质学报》2020,94(9):2640-2649
辽宁省瓦房店金刚石矿田位于华北陆块辽东新元古代- 古生代坳陷带。区内各时代地层均有出露,其中新元古界出露面积最大。区内断裂构造发育,较大的有北北东向的金州断裂,已发现的金伯利岩体基本上分布在该断裂以西。矿田内金刚石矿均为金伯利岩型,已发现100多个金伯利岩体,划分成4条矿带,已提交4个大型原生金刚石矿床和3个近源小型金刚石砂矿床,资源量占全国的一半以上,是我国重要的金刚石矿集区,其中50号金伯利岩管因其出产的金刚石质量优越而在宝石界享有盛誉。但本区的金伯利岩绝大部分是20世纪70年代、80年代发现的,为了更好地开展金刚石勘查工作,对本区金伯利岩的成矿条件和控矿因素进行了研究,金伯利岩体的平面分布位置表明,瓦房店地区的金伯利岩体成群、成带分布,既有岩管也有岩脉,以岩脉为主,岩管约占20%左右,岩体大小不等、形态各异,钻孔控制的岩管、岩脉大多具有向下延伸突然中断的特征,钻孔中见到的金伯利岩显示,很多金伯利岩管底界平直或具有多个水平标高上出现平移错动的现象,典型岩管、岩脉与等轴或近等轴状构造盆地的关系密切。通过对区内金伯利体岩空间分布特征、岩体形态特征进行分析,并探讨了本区金伯利岩的侵位过程和就位机制后认为,瓦房店地区的金伯利岩在侵位的浅成阶段,由于岩浆携带大量挥发分,在上升通道不顺畅的地段使上覆地层隆起,当挥发分泄漏掉以后隆起的地层塌陷形成浅碟子状的构造盆地,挥发分泄漏的通道就是金伯利岩体的产出位置,也有部分岩浆沿次级断裂运移固结成岩;由于被晚期推覆构造改造,使岩管、岩脉出现水平错动,造成了钻孔中所见的平底岩管或岩脉向下延伸不大的现象,这一认识为合理部署勘查工作提供了新的思路。  相似文献   

15.
综述了预测金刚石原生矿的六准则;有利于原生矿产出的地质环境、合适的地台基底年龄、特殊的遥感影象和物化探异常特征、特定的金伯利岩指示矿物、金伯利岩及其相关岩石的出现和地貌特征。  相似文献   

16.
Pyrope–almandine garnets (Mg# = 28.3–44.9, Ca# = 15.5–21.3) from a heavy mineral concentrate of diamondiferous kimberlites of the largest diamond deposit, the Yubileinaya pipe, along with kimberlite- like rocks and diamondiferous volcano–sediments of the Laptev Sea coast, have been found to contain polymineral, predominantly acicular inclusions, composed of aggregates of shrilankite (Ti2ZrO6), rutile, ilmenite, clinopyroxene, and apatite. The presence of shrilankite as an inclusion in garnets from assumed garnet–pyroxene rocks of the lower crust, lifted up by diamond-bearing kimberlite, allows it to be considered as an indicator mineral of kimberlite, which expands the possibilities when searching for kimberlite in the Arctic.  相似文献   

17.
《International Geology Review》2012,54(10):1142-1152
On the basis of a study of a large quantity of deep-seated xenoliths from the kimberlites of the Malo-Botuobuya, Daldyn-Alakit, Upper Muna, and Lower Olenek regions of Yakutia, we have discussed the distribution of the ultrabasic rocks and eclogites in the kimberlite pipes both on the basis of petrographic composition, and also on depth facies, and a comparison is presented of the mineral composition of the deep-seated inclusions and of the amounts of defined types of xenoliths with the diamond capacity of the kimberlites. The conclusion has been reached that: 1. the amounts of inclusions of deep-seated rocks vary significantly not only in kimberlites from the various diamond fields, but also in the pipes of a single diamond-bearing region; 2. the composition of the ultrabasic rocks and eclogites of the diamond-bearing pipes is distinguished from that of the inclusions of the non-diamond kimberlites in these rocks; and 3. the diamond capacity of the kimberlites has been determined by the depth of occurrence of the magmatic focus and the velocity of uprise (intrusion) of the melt during the formation of the kimberlitic diatremes —Authors.  相似文献   

18.
It was shown that the crystal morphology, content, and size of diamonds depend on the concentrations of silica, magnesium, calcium, and carbon dioxide in the host kimberlites and lamproites. The character of this dependence suggests that the viscosity of the initial melts of these rocks was the main control of the morphology and properties of diamond crystals and indicates a magmatic genesis for this mineral. Two genetic varieties of diamond crystals were distinguished: larger residual grains coeval with the formation of the sources of kimberlite and lamproite magmas during the slow high-pressure fractionation of the near-bottom peridotite layer of the global magma ocean and smaller early magmatic grains, which crystallized during the decompression-friction transformation of kimberlite and lamproite protoliths into magmas.  相似文献   

19.
Several episodes of kimberlite magmatism occurred in the East European Province (EEP) during a long (about 1.5 Gyr) time period, from the Late Paleoproterozoic (ca. 1.8 Ga) in the Archean Ukrainian and Baltic shields to the Middle Paleozoic (ca. 0.36 Ga) mainly in the Arkhangelsk, Timan, and adjacent regions. Based on the analysis of data on 16 kimberlite occurrences and four lamproite occurrences within the EEP, five time stages can be distinguished; one of them, the Middle Paleozoic stage (Middle Ordovician and Devonian), is the most productive epoch for diamond in the northern hemisphere (EEP, Siberian Craton, and part of the China Craton). The analysis of petrological and geochemical characteristics of kimberlites (lamproites were studied less thoroughly) revealed variations in rock composition and their correlation with a number of factors, including the spatial confinement to the northern or southern Archean blocks of the craton, time of formation of the source of kimberlite melts, contents of volatiles and autoliths, etc. Three petrogeochemical types of kimberlites were distinguished: high-, medium-, and low-Ti (TiO2 > 3 wt %, 1–3 wt %, and <1 wt %, respectively). There are two time intervals of the formation of kimberlite and lamproite sources in the EEP, corresponding to TNd(DM) values of about 2 Ga (up to 2.9 Ga in the Por’ya Guba occurrence) and 1 Ga. The latter interval includes two groups of occurrences with model source ages of about 1 Ga (low-and medium-Ti kimberlites of the Zolotitsa and Verkhotina occurrences) and about 0.8 Ga (high-Ti kimberlites of the Kepino and a number of other occurrences); i.e., there seems to be an evolutionary trend in the composition of kimberlites. Concentric zoning patterns were recognized. The role of the crust in kimberlite sources is discussed; it is assumed that buried remnants of the oceanic lithosphere (megaliths) may underlie whole continents. A unique feature of the composition of low-Ti kimberlites, for instance, kimberlites of the Zolotitsa occurrence (to a smaller extent, medium-Ti kimberlites of the V. Grib pipe) is the distinct depletion of highly charged elements and pronounced negative anomalies of Ti, Zr, Th, U, Nb, and Ta in trace-element distribution patterns, which indicates a contribution of crustal material to the source of these kimberlites. It was shown that autoliths exert a significant influence on the differentiation of kimberlite material, resulting in the enrichment of rocks in the whole spectrum of incompatible elements. It was argued that geochemical criteria can be used together with traditional criteria (including those based on indicator minerals) for the assessment of diamond potential in EEP occurrences. We hope that such a combined approach will yield important outcomes in the future.  相似文献   

20.
Hydrothermal alteration of kimberlite by convective flows of external water   总被引：1，自引：0，他引：1  
Kimberlite volcanism involves the emplacement of olivine-rich volcaniclastic deposits into volcanic vents or pipes. Kimberlite deposits are typically pervasively serpentinised as a result of the reaction of olivine and water within a temperature range of 130–400 °C or less. We present a model for the influx of ground water into hot kimberlite deposits coupled with progressive cooling and serpentisation. Large-pressure gradients cause influx and heating of water within the pipe with horizontal convergent flow in the host rock and along pipe margins, and upward flow within the pipe centre. Complete serpentisation is predicted for wide ranges of permeability of the host rocks and kimberlite deposits. For typical pipe dimensions, cooling times are centuries to a few millennia. Excess volume of serpentine results in filling of pore spaces, eventually inhibiting fluid flow. Fresh olivine is preserved in lithofacies with initial low porosity, and at the base of the pipe where deeper-level host rocks have low permeability, and the pipe is narrower leading to faster cooling. These predictions are consistent with fresh olivine and serpentine distribution in the Diavik A418 kimberlite pipe, (NWT, Canada) and with features of kimberlites of the Yakutian province in Russia affected by influx of ground water brines. Fast reactions and increases in the volume of solid products compared to the reactants result in self-sealing and low water–rock ratios (estimated at <0.2). Such low water–rock ratios result in only small changes in stable isotope compositions; for example, δO18 is predicted only to change slightly from mantle values. The model supports alteration of kimberlites predominantly by interactions with external non-magmatic fluids.  相似文献   

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