Normative quartz as an indicator of the mass transfer intensity during the postmagmatic alteration of the Botuobinskaya pipe kimberlites (Yakutia)

The intensity of postmagmatic processes in the Botuobinskaya pipe kimberlites was estimated from the calculated content of normative secondary quartz (Q). Several simple algorithms are proposed to calculate the Q content from chemical analyses of kimberlites. Ten groups of altered kimberlites have been recognized from the Q contents. The contents of MgO, some trace elements, and LREE in the groups, the contents of Cr and Ca of crimson garnets, the diamond contents of kimberlites, and the average weight of diamonds decrease as the Q content increases. It is shown that the negative SiO₂–MgO correlation is the most effective indicator of the postmagmatic alteration of kimberlites. As the degree of their secondary alteration increases, the kimberlites transform into an assemblage of quartz and clay minerals enriched in some trace elements and almost completely lacking REE and diamonds.     

Behavior of major and rare-earth elements during the postmagmatic alteration of kimberlites

During serpentinization and subsequent alteration in the absence of brucite, kimberlites accumulate uncompensated silica. Its amount can be calculated from the average compositions of the rock-forming minerals (olivine, calcite, phlogopite) and the chemical compositions of the rocks. The contents of rock-forming oxides and REE were determined in 12 kimberlite pipes of the Yakutian kimberlite province, in 413 samples from secondary-alteration zones and of unaltered kimberlites. Columns of successive kimberlite alterations were constructed for each pipe on the basis of secondary-quartz data; here, the behavior of rock-forming oxides and REE was assessed. All the studied rocks had experienced different degrees of postmagmatic hydrothermal metasomatism at different depths in all the pipes. The changes were reflected in the supply/loss of rock-forming oxides and REE. The supply of REE during the hydrothermal metasomatism enriched the kimberlites with TiO₂, P₂O₅, and CaO. During the removal of REE, most of the rock-forming oxides were partially lost. The maximum REE supply was 67% in the Udachnaya-Vostochnaya pipe and 59% in the Nyurbinskaya pipe as compared with the unaltered kimberlites. The maximum REE loss was 87% in the Aikhal pipe and 81% in the Internatsional’naya pipe as compared with the unaltered kimberlites. The initial REE contents of the postmagmatically altered kimberlites changed considerably in all the studied cases. This conclusion was drawn owing to the use of normative-quartz content as a criterion for secondary alteration.     

Diamond resource potential of kimberlites from the Zimny Bereg field,Arkhangel’sk oblast

Kimberlites with different diamond grades from the Zolotitsa, Verkhotina, and Kepina occurrences of the Zimny Bereg field (Arkangel’sk oblast) have been compared in order to ascertain geochemical criteria of their diamond resource potential. A new collection of 21 core samples taken within a depth interval of 207–940 m from nine boreholes drilled in the central and western portions of the high-grade diamond-bearing Grib kimberlite pipe was subjected to comprehensive petrographic and geochemical examination, including Sr, Nd, and Pb isotopes and trace elements determined with ICP-MS. The compositional variations in kimberlites are controlled by the structural types of rocks. Porphyritic kimberlite (PK) distinctly differs from autolithic kimberlite breccia (AKB). Autoliths (Av) and PK are enriched in Th, U, Nb, Ta, La, Ce, Pr, P, Nd, Sm, Eu, Ti, LREE, and MREE, whereas HREE contents are rather uniform in all types of kimberlites. No lateral zoning was observed in pipes pertaining to the same structural type. The composition of kimberlites in the Zimny Bereg field and their diamond resource potential are variable. In the series of the Zolotitsa, Verkhotina, and Kepina occurrences, the Ti content increases, the La/Yb ratio grows from 18–44 to 70–130, and the diamond grade diminishes in the Kepina occurrence. The variations in kimberlite compositions are considered in terms of the degree of partial melting in the mantle, the role of volatiles, etc. As follows from the variation in the Ce/Y ratio, kimberlites from the Zolotitsa occurrence were formed at a lower degree of partial melting in comparison with the Kepina occurrence. Products of different degrees of partial melting are recognized within the Grib pipe; Av were likely formed at a somewhat higher degree of melting than AKB. An appreciable isotopic heterogeneity of the mantle is recorded in variable Nd and Sr isotopic compositions of kimberlites. The Kepina kimberlites were derived from a source slightly depleted relative to CHUR (?_Nd(t) reaches +4) and are close to kimberlites of group I in South Africa. Kimberlites from the Grib pipe with transitional Nd isotopic composition plotted near the Bulk Silicate Earth (BSE) value in the ?_Nd(t)-?_Sr(t) diagram adjoin the first group. The source of kimberlites of the Zolotitsa occurrence falls in the field of enriched mantle and is considered to be a product of interaction of an asthenospheric plume with the ancient enriched lithospheric mantle. Kimberlites depleted in Ti, Zr, and Th are related to a source formed as a result of a multistage process that included mantle metasomatism with participation of fluids. Devonian kimberlites derived from sources that involve crustal material (a shift of ²⁰⁶Pb/²⁰⁴Pb, minimums of Th, U, Nb, and Ta contents) are diamond-bearing both in the East European Platform (the Zolotitsa and Verkhotina occurrences) and in the Siberian Craton (the Nakyn field).     

Distinctive structural features of magmatic complexes of the central type associated with ring faults

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.     

Mineral inclusions in fibrous diamonds: constraints on cratonic mantle refertilization and diamond formation

We analyzed mineral microinclusions in fibrous diamonds from the Wawa metaconglomerate (Superior craton) and Diavik kimberlites (Slave craton) and compared them with published compositions of large mineral inclusions in non-fibrous diamonds from these localities. The comparison, together with similar datasets available for Ekati and Koffiefontein kimberlites, suggest a general pattern of metasomatic alteration imposed on the ambient mantle by formation of fibrous diamond. Calcium and Fe enrichment of peridotitic garnet and pyroxenes and Fe enrichment of olivine associated with fibrous diamond-forming fluids contributes to refertilization of the cratonic mantle. Saline—carbonatitic—silicic fluid trapped by fibrous diamonds may represent one of the elusive agents of mantle refertilization. Calcium enrichment of peridotitic garnet and pyroxenes is expected in local mantle segments during fibrous diamond production, as Ca in the carbonatitic fluids is deposited into the surrounding mantle when oxidized carbon is reduced to diamond. Harzburgitic garnet evolves towards Ca-rich compositions even when it interacts with Ca-poor saline fluids. An unusual trend of Mg enrichment to Fo_95–98 is observed in some olivine inclusions in Wawa fibrous diamonds. The trend may result from the carbonatitic composition of the fluid that promotes crystallization of magnesian olivine and preferentially oxidizes the fayalite component. We propose a generic model of fibrous and non-fibrous diamond formation from carbonatitic fluids that explains enrichment of the mantle in mafic magmaphile and incompatible elements and accounts for locally metasomatized compositions of diamond inclusions.     

New data on kimberlite magmatism in southwestern Angola

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.     

Nano-and micron-sized diamond genesis in nature: An overview

There are four main types of natural diamonds and related formation processes. The first type comprises the interstellar nanodiamond particles. The second group includes crustal nano-and micron-scale diamonds associated with coals, sediments and metamorphic rocks. The third one includes nanodiamonds and microndiamonds associated with secondary alteration and replacing of mafic and ultramafic rocks.The fourth one includes macro-, micron-and nano-sized mantle diamonds which are associated with kimberlites, mantle peridotites and eclogites. Each diamond type has its specific characteristics. Nanosized diamond particles of lowest nanometers in size crystallize from abiotic organic matter at lower pressures and temperatures in space during the stages of protoplanetary disk formation. Nano-sized diamonds are formed from organic matter at P-T exceeding conditions of catagenesis stage of lithogenesis. Micron-sized diamonds are formed from fluids at P-T exceeding supercritical water stability.Macrosized diamonds are formed from metal-carbon and silicate-carbonate melts and fluids at P-T exceeding 1150℃ and 4.5 GPa. Nitrogen and hydrocarbons play an important role in diamond formation.Their role in the formation processes increases from macro-sized to nano-sized diamond particles.Introduction of nitrogen atoms into the diamond structure leads to the stabilization of micron-and nanosized diamonds in the field of graphite stability.     

Calculation of the equilibrium C-O-H fluid for ilmenite xenocrysts and estimation of the diamond potential

The composition of the C-O-H fluid is estimated on the basis of the composition of ilmenite xenocrysts. The P-T parameters and oxygen fugacity were assessed for peridotitic ilmenites from the diamond and diamond-free kimberlites of Africa and Yakutia. The composition of the equilibrium C-O-H fluid for these conditions was calculated. The diamond and diamond-free pipes of Angola and Yakutia are characterized by the H₂O-rich and CO₂-rich fluids, respectively. The results indicate that estimation of the composition of the C-O-H fluid in equilibrium with picroilmenite fits the necessary petrological criteria and may be applied for assessment of the diamond potential of mantle objects.     

Kimberlites distribution in Angola and prospective areas for new discoveries

Based on a comprehensive analysis of kimberlite pipes of Angola, including the near surface structural setting, deep lithospheric structure, pipe morphology and emplacement, mineralogical and petrographic features, diamond characteristics and locations of secondary deposits four geographical regions have been outlined within Angola representing four types of diamond bearing potential. These areas include high diamond bearing potential pipes, possible potential, no potential, and unclear potential areas. It was found that the depth of magmatism and diamond potential of kimberlites increases from the Atlantic coast in southwestern Angola into the continent in the north-easterly direction. Areas prospective for the discovery of new primary diamond deposits have been identified.

     

辽宁瓦房店金刚石矿田金伯利岩侵位机制分析

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。     

Zoning of Faults and Secondary Mineralization of Host Rocks of Kimberlites of the Maiskoe Diamond Deposit,Nakyn Field,Yakutia

Study of faults and secondary mineralization of host rocks of diamond-bearing kimberlites yields important data for local prediction of kimberlite bodies. Of special methodological interest are exploration data on deposits where the study of host rocks is based on a dense observation network. Factual material for this paper was collected from cores of all inclined exploration boreholes of the Maiskoe diamond deposit found in the Nakyn field in Yakutia in 2006. The paper shows a nonuniform distribution of tectonic deformations, stringer mineralization, O and C isotopes of calcite, and CO₂ content of Lower Paleozoic host carbonate rocks of kimberlites. Our data agree with different diamond potentials of two areas of the Maiskoe kimberlite body, which can be used to search for and explore deposits.     

Kimberlites and lamproites: Criteria for similarity and differences

Based on original data on the East European and Siberian platforms and materials on the best studied foreign objects, a comparative analysis of kimberlites and lamproites was conducted and the criteria of their differences were formulated. Among most significant differences are the following: (1) the high-Mg potassic rocks (kimberlites and lamproites) show major-component variations, which are significantly wider in lamproites as compared to kimberlites. Kimberlites differ from lamproites not only in the content of SiO₂, but also in alkalis, volatiles, and some trace elements. Kimberlites are characterized by CO₂-dominated regime, whereas formation of lamproites was assisted by essentially H₂O fluid; (2) Kimberlites are localized within ancient cratons, while within-plate lamproites are restricted to adjacent Proterozoic belts. Kimberlites are produced in the low-heat flow regions, whereas lamproites occur in the high-heat flow regions; (3) Kimberlites and lamproites were formed in different time; in particular, most productive kimberlitic magmatism was observed in the EEP and SP in the Devonian; (4) Kimberlite and lamproite bodies have different morphology: lamproites compose small subvolcanic bodies with lava flows, while kimberlites form volcanic pipes with no lavas; (5) Kimberlites contain highly silica-undersaturated minerals, while ultrabasic lamproites—silica-undersaturated ones; priderite and wadeite, the characteristic accessory minerals of lamproites, are not observed in kimberlites; (6) The primary melts of kimberlites and lamproites were derived from different types of mantle. The moderate and low-Ti kimberlites were generated from BSE or EMI type mantle. Precisely these types of kimberlites host diamond deposits, including economic grade objects in EEP. The lamproite sources were localized only in the enriched mantle (EMI and EMII). At the same time, these rocks share some similarities, primarily, with respect to their genesis and classification. Diamonds are common accessory minerals of kimberlites (low-Ti and some other types), but are observed only in only lamproite variety—olivine lamproites.     

Alakit and Daldyn kimberlite fields,Siberia,Russia:Two types of mantle sub-terranes beneath central Yakutia?

Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle.Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields.In the Alakit field.Cr-diopsides are alkaline,and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions,while in the Daldyn field harzburgitic pyropes are frequent.The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions,while in the Daldyn field they show varying compositions and often continuous Pressure-Temperature(P-T) ranges with increasing Fe~# with decreasing pressures.In Alakit,Crpargasites to richterites were found in all pipes,while in Daldyn,pargasites are rare Dalnyaya and Zarnitsa pipes.Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements(LREE)and stronger REE-slopes,and enrichment in light Rare Earth Elements(LREE),sometimes Th-U,and small troughs in Nb-Ta-Zr.In the Daldyn field,the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances,while those from sheared and refertilized peridotites have smooth patterns.Garnets from Alakit show HREE minima,but those from Daldyn often have a trough at Yand high U and Pb.PTX/O2 diagrams from both regions show similarities,suggesting similar layering and structures.The degree of metasomatism is often higher for pipes which show dispersion in P-Fe~# trends for garnets.In the mantle beneath Udachnaya and Aykhal,pipes show 6-7 linear arrays of P-Fe~# in the lower part of the mantle section at 7.5-3.0 GPa,probably reflecting primary subduction horizons.Beneath the Sytykanskaya pipe,there are several horizons with opposite inclinations which reflect metasomatic processes.The high dispersion of the P—Fe~# trend indicating widespread metasomatism is associated with decreased diamond grades.Possible explanation of the differences in mineralogy and geochemistry of the mantle sections may relate to their tectonic positions during growth of the lithospheric keel.Enrichment in volatiles and alkalis possibly corresponds to interaction with subduction-related fluids and melts in the craton margins.Incorporation of island arc peridotites from an eroded arc is a possible scenario.     

Petrogenesis of the crater-facies Tokapal kimberlite pipe,Indravati Basin,Central India

New geochemical data of the crater-facies Tokapal kimberlite system sandwiched between the lower and upper stratigraphic horizons of the Mesoproterozoic lndravati Basin a：：e presented. The kimberlite has been subjected to extensive and pervasive low-temperature alteration. Spinel is the only primary phase identifiable, while olivine macrocrysts and juvenile lapilli are largely pseudomorphed （talc-serpentine- carbonate alteration）. However, with the exception of the alkalies, major element oxides display systematic fractionation trends; likewise, HFSE patterns are well correlated and allow petrogenetic interpretation. Various crustal contamination indices such as （SiO2 ＋ AI：：O3 ~ Na20）]（MgO ~ K20） and Si] Mg are close to those of uncontaminated kimberlites. Similar La]Yb （＇79-109） of the Tokapal samples with those from the kimberlites of Wajrakarur （73-145） and Narayanpet （72-156）, Eastern Dharwar craton, southern India implies a similarity in their genesis. In the discriminant plots involving HFSE the Tokapal samples display strong affinities to Group 1I kimberlites from southern Africa and central India as well as to ＇transitional kimberlites＇ from the Eastern Dharwar craton, southern India, and those from the Prieska and Kuruman provinces of southern Africa. There is a striking ~;imilarity in the depleted-mantle （TOM） Nd model ages of the Tokapal kimberlite system, Bastar craton, th~ kimberlites from NKF and WKE Eastern Dharwar craton, and the Majhgawan diatreme, Bundelkhand craton, with the emplacement age of some of the lamproites from within and around the Palaeo~Mesoproterozoic Cuddapah basin, southern India. These similar ages imply a major tectonomagmatic event, possibly related to the break- up of the supercontinent of Columbia, at 1.3-1.5 Ga across the l：hree cratons. The ＇transitional＇ geochemical features displayed by many of the Mesoproterozoic po~：assic-ultrapotassic rocks, across these Indian cratons are inferred to be memories of the metasomatisi     

Petrogenesis of the Late Cretaceous northern Alberta kimberlite province

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.     

Temporal, geomagnetic and related attributes of kimberlite magmatism at Ekati, Northwest Territories, Canada

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.     

Chromium spinels in North Chinese kimberlites,Huabei platform

Samples from diamondiferous pipes in the Mengyin and Fuxian regions were investigated. The chemical compositions of Cr spinels in kimberlites of China were found to be similar to those in kimberlites of the Arkhangelsk province in Russia. A long and complex evolution that was individual for each pipe was demonstrated. The kimberlites of the Shandong Province proved to be rich in high-Cr chromites. This means that the kimberlites formed at large depths in the field of diamond thermodynamic stability. Variations in the redox conditions were noted. They manifested themselves as a wide range of fluctuations of the chemical composition of microcrystalline spinels, up to formation to Ti-magnetite and magnetite.     

Geochemistry of peat over kimberlites in the Attawapiskat area, James Bay Lowlands, northern Canada

The James Bay Lowlands, which is the SE part of the Hudson Bay Lowlands, Canada, and within the Paleozoic limestone terrane, is covered mostly by peatlands. Peat samples were examined in the Attawapiskat area, a region of discontinuous permafrost, where more than 19 kimberlite pipes have been found beneath a cover of peat (2–4 m thick) and Quaternary sediments (up to 20 m thick) of Tyrell Sea clay beds and glacial tills. Pore water at a depth of 40 cm in the peat has a consistently low pH, <4, and high Eh, 290 mV, in the areas over limestones far from kimberlites. On the other hand, peat pore water close to kimberlites has a high pH, up to 6.7, and low Eh, down to 49 mV; the values of pH and Eh are inversely correlated. The high pH and low Eh close to kimberlites suggest active serpentinization of olivine in the underlying kimberlites. The bulk compositions of peat indicate precipitation of secondary CaCO₃ and Fe–O–OH. The secondary carbonate contains high concentrations of kimberlite pathfinder elements, such as Ni, rare earth elements (REE) and Y. The ratios of metal concentrations extracted by ammonium acetate solution at pH 5 (AA5) to those in a total digestion confirm that a majority of the divalent cations are hosted by the secondary carbonate, whereas tri-, tetra- and penta-valent cations are not. As these charged cations are not leached in Enzyme Leach, they are most likely adsorbed on Fe–O–OH.The compositions of peat show spatial variation with the distribution of kimberlites, suggesting that they are influenced by the underlying rocks even through there are thick layers of tills and sediments between the bedrocks and peat. However, elevated concentrations of pathfinder elements of kimberlites in bulk peat samples and AA5 leach are not necessarily directly above kimberlites. The diffused metal anomalies around kimberlites are attributed to the dissolution–precipitation of secondary phases (carbonates and Fe–O–OH) in acidic and reduced waters in peat, and the movement of waters through peat. This pilot study suggests that peat compositions do reflect the underlying bedrock compositions. For kimberlite exploration, a geochemical survey of peat is useful to discriminate concealed kimberlites from other anomalies defined by geophysical and other techniques; however, such a geochemical survey is not suitable for delineating the shapes of the concealed kimberlites due to broad dispersed anomalies.