博茨瓦纳和中国含金刚石金伯利岩的地质特征及对寻找类似岩体的启示

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。     

郯庐断裂带金伯利岩的构造意义

郯庐断裂带形成的时间问题历来争论较大,其断裂深度一直也没有较好的方法确定。笔者试图从新的角度作一尝试,通过金伯利岩提供的构造信息对这两个悬而未决的问题进行探讨。 1.郯庐断裂带与金伯利岩浆活动的关系该断裂带中段和北段广泛分布有受断裂带控制的金伯利岩。而世界上几乎所有的金伯利岩均出现于稳定的克拉通地区,且金伯利岩岩墙具有非常稳定的宽度(1～3.m),只有用裂隙侵入的方式才能解释金伯利岩浆从深处向地表的快速上侵。上地幔橄榄岩固相线温度随压力(深度)的升高而升高,在正常克拉通地区地温情况下,上地幔中不可能达到地幔岩石的熔融温度,然而断裂作用为深部富含不相容元素的C-H-O流体加入到地幔中提供了通道,引起地幔富集作用,使地幔密度和粘度降低,出     

华北克拉通金伯利岩型金刚石矿床含矿性判别

金伯利岩型金刚石原生矿床含矿性评价是了解金刚石成矿规律和指导区域性找矿的重要依据。华北克拉通是我国重要的金刚石成矿远景区,目前已发现了两个重要的含金刚石金伯利岩岩区(山东蒙阴和辽宁瓦房店)。近年来,对岩区内不同品位金刚石典型矿床的研究表明金伯利岩中的金刚石品位取决于以下两个重要的地质过程:金伯利岩岩浆对金刚石的捕获作用和金伯利岩岩浆对金刚石的熔蚀作用。在华北克拉通下方存在一个金刚石稳定区,温度为950～1200℃,压力为4.4～6.1 GPa。山东蒙阴和辽宁瓦房店金伯利岩岩浆在上升过程中穿过该区域并捕获其中的金刚石,成为含矿岩浆;而铁岭和鹤县等金伯利岩岩浆在上升过程中不穿过该区域,成为不含矿岩浆。在穿过金刚石稳定区时,富矿的金伯利岩捕获了更多的金刚石以及金刚石的伴生指示矿物(如镁铝榴石,镁铬铁矿);而贫矿的金伯利岩则相对捕获了少量的金刚石和指示矿物。这些指示矿物往往参加了金刚石的形成过程,有些作为缓冲剂控制了反应(岩浆或熔体)的氧化还原状态,有些则作为反应物或者生成物参与到反应中。部分指示矿物(如镁铝榴石)还进一步地记录了金刚石形成后金伯利岩岩浆对金刚石的交代作用。金伯利岩岩浆温度(T...     

对湖南寻找金刚石原生矿的建议

最近在《中国地质》(1998年第6期)上看到了一篇引人注目的学术论文——“建议在湖南寻找陨击作用有关的金刚石”(以下简称“专家建议”)。作者解放思想,大胆提出找矿的     

我国金伯利岩型金刚石矿床的若干地质特征及其找矿标志

金伯利岩型金刚石矿床是世界上重要的金刚石矿床类型:也是我国迄今所发现的唯一的金刚石原生矿床类型。本文阐述了我国金伯利岩分布的构造部位、时代、产状、与暗色岩类的关系、围岩及其蚀变特征、岩管的岩相特征、岩石类型及岩石化学特征,并指出了该类矿床的找矿标志。     

金伯利岩含金刚石性的矿物学判别标志

金刚石及其紧密伴生的镁铝榴石、铬尖晶石、钛铁矿、单斜辉石及橄揽石,形成于高温,尤其是高压的上地幔条件下,被快速上升的金伯利岩浆携带至地表,这些矿物形成的特殊的地质条件和环境,必然反映到它们的矿物学特征上来。文章通过对我国金伯利岩中金刚石伴生矿物的研究,探讨了金伯利岩含金刚石性的矿物学标志。     

Prospects of search for diamondiferous kimberlites in the northeastern Siberian Platform

The objects of study are Triassic hypabyssal diamondiferous kimberlites with an age of 220-245 Ma, containing macrocrysts of unaltered olivine. The latter are close in the time of formation to the main stage of intrusion of the Siberian Trap Province (252 Ma), which lasted less than 1 Myr. A comparative high-precision analytical study of the Ti, Ca, Cr, and Al impurity patterns in about 1000 olivine macrocryst samples with a forsterite content Fo = (100Mg/(Mg + Fe)) of 78 to 93 has demonstrated the effect of traps on the lithospheric composition. A comprehensive comparative study of diamonds from northern placers and Triassic kimberlites, including determination of their carbon isotope composition, was performed. Chromatography-mass-spectroscopic analysis of submicron fluid inclusions in diamonds from northern placers and kimberlites has shown predominant hydrocarbons of a wide range of compositions and subordinate contents of N₂, H₂O, and CO₂. These findings, together with the results of previous studies of subcalcic Cr-pyropes and diamonds found in the Lower Carboniferous gritstones of the Kyutyungde graben, lead to the conclusion that the Toluopka kimberlite field is promising for Paleozoic kimberlites. The results of comprehensive studies of diamonds and indicator minerals and U/Pb isotope dating of numerous detrital zircon samples from the basal horizon of the Carnian Stage (Upper Triassic) of the Bulkur site in the lower reaches of the Lena River suggest the presence of diamondiferous kimberlites within the northeastern Siberian Platform. The age of the probable primary diamond sources in the study area can be evaluated by an integrated U/Pb isotope dating of zircons, perovskites, and rutiles from the developed diamond placers and the basal horizon of the Carnian Stage.     

Evaluating the diamondiferous potential of unaltered kimberlites by the population models of their composition

430 chemical analyses of rocks and their diamondiferous potential are used to identify correlations between the diamondiferous potential of rocks and their petrochemical parameters. Samples for this research were collected from selected intervals of core materials, which were also examined for diamond content (a few samples from each interval), from the Nyurbinskaya, Botuobinskaya, Internatsional??naya, Mir, Aikhal, Yubileinaya, Satykanskaya, Udachnaya-West, and Udachnaya-East pipes. Typochemical indications of diamondiferous potential are TiO₂ and K₂O concentrations and the CaO/MgO ratio. System models developed for diamondiferous kimberlites allowed distinguishing two trends of their compositional variability. One of the trends is defined by the negatively correlated TiO₂ and K₂O concentrations of the rocks. This trend is discrete and can be statistically justifiably subdivided into seven segments, each of which represents a population of compositions produced under similar physicochemical conditions. Experimental data confirm that this trend can be closely related to the diamondiferous potential. Diamond richest kimberlites are practically free of TiO₂, whereas diamond poorest ones contain as much as 3% of this oxide. The former and the latter rocks were produced at the greatest and shallowest depths, respectively. The other trend is exhibited in all populations and subdivides them into discrete groups (varieties of the populations) with systematically decreasing CaO/MgO ratio. This parameter is nonlinearly correlated with the diamondiferous potential, and its increase corresponds to a systematic increase in the melting temperature of the source material. Certain kimberlite populations contain anomalously high K₂O concentrations, perhaps, because of mantle metasomatism or the presence of fragments of oceanic crustal material in the magma generation region. In these instances, numerous diamonds could crystallize in the parental melts under high pressures (>100 kbar). The paper presents statistical analysis of pair regressions of the contents of indicative oxides and diamondiferous potential and a graphical multiple-link model for correlations between concentrations of major oxides and diamondiferous potential. Tests of the predictions of diamondiferous potential on the basis of chemical parameters confirm that these predictions are accurate in 85?C90% of the instances.     

Mineralogical zoning of the diamondiferous areas: Application experience of paragenetic analysis of garnets from kimberlites

Paragenetic analysis of pyropes from alluvial deposits of the Muna—Markha interfluve (Sakha–Yakutia Republic) made it possible to distinguish relatively uniform areas that are promising for the discovery of kimberlite bodies.     

A new body of highly diamondiferous kimberlites in the Nakyn field of the Yakutian kimberlite province

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.     

华北克拉通东部含金刚石金伯利岩的侵位年龄和Sr-Nd-Hf同位素地球化学特征

本文首次报道了我国华北克拉通内部金伯利岩中金云母巨晶的Ar-Ar同位素年龄和含金刚石金伯利岩的Hf同位素组成特征,并结合金伯利岩的Sr-Nd同位素组成特征对华北金伯利岩的岩石成因及其构造背景进行了探讨.金伯利岩中金云母巨晶的年代学分析显示华北克拉通内部的蒙阴和复县金伯利岩具有一致的侵位年龄,约465±2 Ma.这与以前发表的金伯利岩中钙钛矿的U-Pb年龄和金云母的Rb-Sr等时线年龄一致.Sr-Nd同位素结果表明蒙阴和复县金伯利岩皆具有很小的Nd同位素变化范围（εNd分别为-0.4～0.2和-3.4～-2.3）.然而,这些金伯利岩却具有极大的Sr同位素组成范围（ISr分布为-0.2～28.1和0～75）.其中复县比蒙阴金伯利岩具有较低的Nd同位素和更宽的Sr同位素组成.Hf同位素分析结果表明蒙阴和复县两岩区内部各金伯利岩岩管具有非常一致的铪同位素组成,类似于Nd同位素组成特征,其中蒙阴金伯利岩的^176Hf/^177Hf初始值为0.282474～0.282416,相应的εHf值为-2.37～-0.30;复县金伯利岩的^176Hf/^177Hf初始值为0.282305～0.282369;相应的εHf值为-6.29～-4.04.这说明蒙阴金伯利岩与复县金伯利岩相比具有较高的Hf同位素组成.结合Sr-Nd同位素组成,暗示蒙阴和复县金伯利岩的Hf同位素组成的不同可能反映金伯利岩岩浆形成时软流圈、岩石圈地幔以及俯冲的洋壳物质参与比例的不同.Sr-Nd-Hf同位素特征说明我国华北古生代金伯利岩的形成同样与华北周边的古大洋岩石圈向华北内部俯冲作用有关.     

The late Silurian-Early Devonian natural remanent magnetization of kimberlites and traps in the Yakutian diamondiferous province

Doklady Earth Sciences -     

Kimberlitic zircons — A possible aid in prospecting for kimberlites

Kimberlitic zircons from 16 intrusions in Lesotho, the Republic of South Africa, and Tanzania have been investigated. The following characteristics distinguish the kimberlitic zircons from the zircons derived from most other sources: 1. Rounded to subrounded grains predominate; euhedral zircons are very rare. 2. Perfect parting in several directions, mainly (100) and (111). 3. Lattice distortions occur (decreasing molar volume). 4. High abundancy of fluid inclusions, occupying healed fractures. 5. Uranium contents less than 30 ppm U. Low contents of Th, Y, P, REE. 6. Whitish alteration rims composed of monoclinic and tetragonal zirconia in varying proportions are common. — The use of zircons in order to identify and trace kimberlite intrusives is discussed, as there may be a possible connection between some properties of the kimberlitic zircons and the diamond grade of their host rocks.     

Petrology and geochemistry of diamondiferous Mesoproterozoic kimberlites from Wajrakarur kimberlite field,Eastern Dharwar craton,southern India: genesis and constraints on mantle source regions

The petrology and geochemistry of some new occurrences of Mesoproterozoic diamondiferous hypabyssal-facies kimberlites from the Chigicherla, Wajrakarur-Lattavaram and Kalyandurg clusters of the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are reported. The kimberlites contain two generations of olivine, and multiple groundmass phases including phlogopite, spinel, calcite, dolomite, apatite, perovskite, apatite and rare titanite, and xenocrysts of eclogitic garnet and picro-ilmenite. Since many of the silicate minerals in these kimberlites have been subjected to carbonisation and alteration, the compositions of the groundmass oxide minerals play a crucial role in their characterisation and in understanding melt compositions. While there is no evidence for significant crustal contamination in these kimberlites, some limited effects of ilmenite entrainment are evident in samples from the Kalyandurg cluster. Geochemical studies reveal that the WKF kimberlites are less differentiated and more primitive than those from the Narayanpet kimberlite field (NKF), Eastern Dharwar craton. Highly fractionated (La/Yb = 108–145) chondrite-normalised distribution patterns with La abundances of 500–1,000 × chondrite and low heavy rare earth elements (HREE) abundances of 5–10 × chondrite are characteristic of these rocks. Metasomatism by percolating melts from the convecting mantle, rather than by subduction-related processes, is inferred to have occurred in their source regions based on incompatible element signatures. While the majority of the Eastern Dharwar craton kimberlites are similar to the Group I kimberlites of southern Africa in terms of petrology, geochemistry and Sr–Nd isotope systematics, others show the geochemical traits of Group II kimberlites or an overlap between Group I and II kimberlites. Rare earth element (REE)-based semi-quantitative forward modelling of batch melting of southern African Group I and II kimberlite source compositions involving a metasomatised garnet lherzolite and very low degrees of partial melting demonstrate that (1) WKF and NKF kimberlites display a relatively far greater range in the degree of melting than those from the on-craton occurrences from southern Africa and are similar to that of world-wide melilitites, (2) different degrees of partial melting of a common source cannot account for the genesis of all the EDC kimberlites, (3) multiple and highly heterogeneous kimberlite sources involve in the sub-continental lithospheric mantle (SCLM) in the Eastern Dharwar craton and (4) WKF and NKF kimberlites generation is a resultant of complex interplay between the heterogeneous sources and their different degrees of partial melting. These observations are consistent with the recent results obtained from inversion modelling of REE concentrations from EDC kimberlites in that both the forward as wells as inverse melting models necessitate a dominantly lithospheric, and not asthenospheric, mantle source regions. The invading metasomatic (enriching) melts percolating from the convecting (asthenosphere) mantle impart an OIB-like isotopic signature to the final melt products.