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1.
Prospecting for kimberlites and related rocks in till-covered terrains requires a methodology for recovering a few small grains within tens of kilograms samples, necessitating 1 ppb sensitivity or better. As part of reconnaissance survey for the kimberlite indicator minerals, i.e. pyrope garnet, picroilmenite, chromite and chromian diopside, the Geological Survey of Finland (GTK) developed such a system by significantly modifying and augmenting a 3″ Knelson Concentrator that accomplishes nearly complete recovery of moderately heavy minerals (>0.25 mm) from till samples.Diamondiferous kimberlites occur in the eastern Finland around the Kaavi–Kuopio and Kuhmo areas and much of the rest of the Karelian craton remains prospective based on the empirical evidence necessary for diamond preservation: thick (>200 km) lithospheric mantle, low heat flow and Archaean age rocks. A target area in Lapland, 20×50 km in size, was selected for a pilot study to test extraction of chromite for the (1) discrimination of regionally and locally derived populations, and (2) recognition of possible kimberlitic/lamproitic chromites. Area selection was based on the regional occurrence of a variety of mantle-derived rocks, the recovery of a chromian pyrope grain from till in 1996 and most importantly, the well-established Quaternary stratigraphy in the region. The sample material consisted of sixty-two 80-kg excavator and 40-kg shovel samples. Approximately 1000 chromite grains, almost exclusively 0.25–0.5 mm in diameter, were recovered and analysed by electron microprobe.Tills in the sampling area proved to contain at least two compositional populations of chromite. The first is present in almost every sample and is apparently derived from layered mafic intrusions distal to and up-ice from the study area. The second population consists of chromites with low Ti, high Cr and Mg similar to inclusions in diamond. It is present in approximately one third of the samples, concentrated in a couple of clusters within the target area and is therefore considered to be of more local derivation. Since no high-Ti, high-Cr chromites diagnostic for kimberlites and lamproites were present in the samples, the source for the low-Ti, high-Cr, high-Mg chromite grains remains uncertain, but is probably not kimberlitic. Although this apparently is a negative outcome for diamond exploration in the target area, the main goal of the study was realised by showing the applicability of the system to heavy mineral separation from Quaternary glacial deposits. 相似文献
2.
The Central Mackenzie Valley (CMV) area of Northwest Territories is underlain by Precambrian basement belonging to the North American Craton. The potential of this area to host kimberlitic diamond deposits is relatively high judging from the seismologically-defined lithospheric thickness, age of basement rocks (2.2–1.7 Ga) and presence of kimberlite indicator minerals (KIMs) in Quaternary sediments. This study presents data for a large collection of KIMs recovered from stream sediments and till samples from two study areas in the CMV, the Horn Plateau and Trout Lake. In the processed samples, peridotitic garnets dominate the KIM grain count for both regions (> 25% each) while eclogitic garnet is almost absent in both regions (< 1% each). KIM chemistry for the Horn Plateau indicates significant diamond potential, with a strong similarity to KIM systematics from the Central and Western Slave Craton. The most significant issue to resolve in assessing the local diamond potential is the degree to which KIM chemistry reflects local and/or distal kimberlite bodies. Radiogenic isotope analysis of detrital kimberlite-related CMV ilmenite and rutile grains requires at least two broad age groups for eroded source kimberlites. Statistical analysis of the data suggests that it is probable that some of these KIMs were derived from primary and/or secondary sources within the CMV area, while others may have been transported to the area from the east-northeast by Pleistocene glacial and/or glaciofluvial systems. At this stage, KIM chemistry does not allow the exact location of the kimberlitic source(s) to be constrained. 相似文献
3.
Ilmenite is one of the common kimberlitic indicator minerals recovered during diamond exploration, and its distinction from non-kimberlitic rock types is important. This is particularly true for regions where these minerals are present in relatively low abundance, and they are the dominant kimberlitic indicator mineral recovered. Difficulty in visually differentiating kimberlitic from non-kimberlitic ilmenite in exploration concentrates is also an issue, and distinguishing kimberlitic ilmenite from those derive from other similar rocks, such as ultramafic lamprophyres, is practically impossible. Ilmenite is also the indicator mineral whose compositional variety has the most potential to resolve provenance issues related to mineral dispersions with contributions from multiple kimberlite sources. Various published data sets from selected kimberlitic (including kimberlites, lamproites, and various ultramafic lamprophyres) and non-kimberlitic rock types have been compiled and evaluated in terms of their major element compositions. Compositional fields and bounding reference lines for ilmenites derived from kimberlites (sensu stricto), ultramafic lamprophyres, and other non-kimberlitic rock types have been defined primarily on MgO–TiO2 graphs as well as MgO–Cr2O3 relationships. 相似文献
4.
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
5.
This paper reviews key characteristics of kimberlites on the Ekati property, NWT, Canada. To date 150 kimberlites have been discovered on the property, five of which are mined for diamonds. The kimberlites intrude Archean basement of the central Slave craton. Numerous Proterozoic diabase dykes intrude the area. The Precambrian rocks are overlain by Quaternary glacial sediments. No Phanerozoic rocks are present. However, mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late-Cretaceous and Tertiary cover (likely <200 m) was present at the time of emplacement. The Ekati kimberlites range in age from 45 to 75 Ma. They are mostly small pipe-like bodies (surface area mostly <3 ha but up to 20 ha) that typically extend to projected depths of 400–600 m below current surface. Pipe morphologies are strongly controlled by joints and faults. The kimberlites consist primarily of variably bedded volcaniclastic kimberlite (VK). This is dominated by juvenile constituents (olivine and lesser kimberlitic ash) and variable amounts of exotic sediment (primarily mud), with minor amounts of xenolithic wall-rock material (generally <5%). Kimberlite types include: mud-rich resedimented VK (mRVK); olivine-rich VK (oVK); sedimentary kimberlite; primary VK (PVK); tuffisitic kimberlite (TK) and magmatic kimberlite (MK). The presence and arrangement of these rock types varies widely. The majority of bodies are dominated by oVK and mRVK, but PVK is prominent in the lower portions of certain kimberlites. TK is rare. MK occurs primarily as precursor dykes but, in a few cases, forms pipe-filling intrusions. The internal geology of the kimberlites ranges from simple single-phase pipes (RVK or MK), to complex bodies with multiple, distinct units of VK. The latter include pipes infilled with steep, irregular VK blocks/wedges and at least one case in which the pipe is occupied by well-defined sub-horizontal VK phases, including a unique, 100-m-thick graded sequence. The whole-rock compositions of VK samples suggest significant loss of kimberlitic fines during eruption followed by variable dilution by surface sediment and concurrent incorporation of kimberlitic ash. Diamond distribution within the kimberlites reflects the amount and nature of mantle material sampled by individual kimberlite phases, but is modified considerably by eruption and depositional processes. The characteristics of the Ekati kimberlites are consistent with a two-stage emplacement process: (1) explosive eruption/s causing vent clearing followed by formation of a significant tephra rim/cone of highly fragmented, olivine-enriched juvenile material with varying amounts of kimberlitic ash and surface sediments (predominantly mud); and (2) infilling of the vent by direct deposition from the eruption column and/or resedimentation of crater rim materials. The presence of less fragmented, juvenile-rich PVK in the lower portions of certain pipes and the intrusion of large volumes of MK to shallow levels in some bodies suggest emplacement of relatively volatile-depleted, less explosive kimberlite in the later stages of pipe formation and/or filling. Explosive devolatilisation of CO 2-rich kimberlite magma is interpreted to have been the dominant eruption mechanism, but phreatomagmatism is thought to have played a role and, in certain cases, may have been dominant. 相似文献
6.
The Cretaceous Liqhobong kimberlite cluster comprises six known diamondiferous Group 1 kimberlite bodies: the Main Pipe (8.5 ha), the Satellite Pipe (1.6 ha), the Discovery Blow (0.15 ha), the Blow (0.1 ha), the Main Dyke, and the East Dyke. Emplaced along a strike length of about 2.5 km, the kimberlites intruded Jurassic Drakensberg lavas and outcrop at altitude ranging from 2970 to 2670 m above sea level (masl) in the rugged Maluti Mountain terrain of Lesotho. The cluster’s intrusion was structurally controlled and emplacement occurred in at least three pulses. The dykes and the two blows (which are dyke enlargements emplaced ~900 m apart) comprise the earliest event and the Main and Satellite Pipes were emplaced during two separate, subsequent events. Each pipe has steep contacts with the country rock basalt. The two Blows have inward dipping contacts and narrow considerably at depth. Each of the Main and Satellite Pipes comprises multiple phases which range from largely volcaniclastic to marginally coherent kimberlites. The volume of the volcaniclastic kimberlite is always much more (>three times) than that of the coherent kimberlite. The larger Main and Satellite Pipes are diluted by country rock up to 40 vol% whereas the smaller Blows and Dykes typically have less than 10 vol% dilution. The degree of mantle sampling is highest (up to 40 vol%) in the smaller Blows and lower (~25 vol%) in the larger Pipes. 相似文献
7.
Sixteen kimberlite boulders were collected from three sites on the Munro and Misema River Eskers in the Kirkland Lake kimberlite field and one site on the Sharp Lake esker in the Lake Timiskaming kimberlite field. The boulders were processed for heavy-mineral concentrates from which grains of Mg-ilmenite, chromite, garnet, clinopyroxene and olivine were picked, counted and analyzed by electron microprobe. Based on relative abundances and composition of these mineral phases, the boulders could be assigned to six mineralogically different groups, five for the Kirkland Lake area and one for the Lake Timiskaming area. Their indicator mineral composition and abundances are compared to existing data for known kimberlites in both the Kirkland Lake and Lake Timiskaming areas. Six boulders from the Munro Esker form a compositionally homogeneous group (I) in which the Mg-ilmenite population is very similar to that of the A1 kimberlite, located 7–12 km N (up-ice), directly adjacent to the Munro esker in the Kirkland Lake kimberlite field. U–Pb perovskite ages of three of the group I boulders overlap with that of the A1 kimberlite. Three other boulders recovered from the same localities in the Munro Esker also show some broad similarities in Mg-ilmenite composition and age to the A1 kimberlite. However, they are sufficiently different in mineral abundances and composition from each other and from the A1 kimberlite to assign them to different groups (II–IV). Their sources could be different phases of the same kimberlite or—more likely—three different, hitherto unknown kimberlites up-ice of the sample localities along the Munro Esker in the Kirkland Lake kimberlite field. A single boulder from the Misema River esker, Kirkland Lake, has mineral compositions that do not match any of the known kimberlites from the Kirkland Lake field. This suggests another unknown kimberlite exists in the area up-ice of the Larder Lake pit along the Misema River esker. Six boulders from the Sharp Lake esker, within the Lake Timiskaming field, form a homogeneous group with distinct mineral compositions unmatched by any of the known kimberlites in the Lake Timiskaming field. U–Pb perovskite age determinations on two of these boulders support this notion. These boulders are likely derived from an unknown kimberlite source up-ice from the Seed kimberlite, 4 km NW of the Sharp Lake pit, since indicator minerals with identical compositions to those of the Sharp Lake boulders have been found in till samples collected down-ice from Seed. Based on abundance and composition of indicator minerals, most importantly Mg-ilmenite, and supported by U–Pb age dating of perovskite, we conclude that the sources of 10 of the 16 boulders must be several hitherto unknown kimberlite bodies in the Kirkland Lake and Lake Timiskaming kimberlite fields. 相似文献
8.
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
9.
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
10.
Five compositional-textural types of ilmenite can be distinguished in nine kimberlites from the Eastern Dharwar craton of southern India. These ilmenite generations record different processes in kimberlite history, from mantle to surface. A first generation of Mg-rich ilmenite (type 1) was produced by metasomatic processes in the mantle before the emplacement of the kimberlite. It is found as xenolithic polycrystalline ilmenite aggregates as well as megacrysts and macrocrysts. All of these ilmenite forms may disaggregate within the kimberlite. Due to the interaction with low-viscosity kimberlitic magma replacement of pre-existing type 1 ilmenite by a succeeding generation of geikielite (type 2) along grain boundaries and cracks occurs. Another generation of Mg-rich ilmenite maybe produced by exsolution processes (type 3 ilmenite). Although the identity of the host mineral is unclear due to extensive alteration and possibility includes enstatite. Type 4 Mn-rich ilmenite is produced before the crystallization of groundmass perovskite and ulvöspinel. It usually mantles ilmenite and other Ti-rich minerals. Type 5 Mn-rich ilmenite is produced after the crystallization of the groundmass minerals and replaces them. The contents of Cr and Nb in type 2, 4 and 5 ilmenites are highly dependent on the composition of the replaced minerals, they may not be a good argument in exploration. The highest Mg contents are recorded in metasomatic ilmenite that is produced during kimberlite emplacement, and cannot be associated with diamond formation. The higher Mn contents are linked to magmatic processes and also late processes clearly produced after the crystallization of the kimberlite groundmass, and therefore ilmenite with high Mn contents cannot be considered as a reliable diamond indicator mineral (DIM) and kimberlite indicator mineral (KIM). 相似文献
11.
This study examines the major element composition of mantle-derived garnets recovered from heavy mineral concentrates of several Proterozoic kimberlites of the diamondiferous Wajrakarur Kimberlite Field (WKF) and the almost barren Narayanpet Kimberlite Field (NKF) in the Eastern Dharwar Craton of southern India. Concentrate garnets are abundant in the WKF kimberlites, and notably rare in the NKF kimberlites. Chemical characteristics of the pyropes indicate that the lithology of the sub-continental lithospheric mantle (SCLM) beneath both the kimberlite fields was mainly lherzolitic at the time of kimberlite eruption. A subset of green pyropes from the WKF is marked by high CaO and Cr 2O 3 contents, which imply contribution from a wehrlitic source. The lithological information on SCLM, when studied alongside geobarometry of lherzolite and harzburgite xenoliths, indicates that there are thin layers of harzburgite within a dominantly lherzolitic mantle in the depth interval of 115–190 km beneath the WKF. In addition, wehrlite and olivine clinopyroxenite occur locally in the depth range of 120–130 km. Mantle geotherm derived from xenoliths constrains the depth of graphite–diamond transition to 155 km beneath the kimberlite fields. Diamond in the WKF thus could have been derived from both lherzolitic and harzburgitic lithologies below this depth. The rarity of diamond and garnet xenocrysts in the NKF strongly suggest sampling of shallower (<155 km depth) mantle, and possibly a shallower source of kimberlite magma than at the WKF. 相似文献
12.
Exploration for diamond-bearing kimberlites in the Chidliak project area by Peregrine Diamonds has generated a grid-like till sampling pattern across four discrete areas of interest totalling 402 km2 that is densely populated with research-grade compositional data for 10,743 mantle-derived Cr-pyrope garnets. The available dataset is well suited to statistical analysis, in part due to the relatively unbiased spatial coverage. Previous workers showed empirically that the TiO2 and Mn thermometry (Ti-TMn) attributes of Cr-pyrope populations at the Chidliak project may serve as source-specific “fingerprints”. In this work, we employ a simplified version of the multivariate Mahalanobis distance technique to formally examine the variability of, and differences between, Ti-TMn attributes of Cr-pyrope subpopulations recovered from a Laurentide-age glaciated terrain that also contains 30 known kimberlites within the four areas of interest. We show the simplified Mahalanobis distance approach enables accurate discrimination of Cr-pyrope subpopulations with subtly to distinctly different Ti-TMn attributes, and permits proper demarcation of their respective kimberlite source(s), specifically in areas with straightforward glacial histories. Redistribution and blending of Cr-pyrope subpopulations from known kimberlite sources is also observed, and typifies areas at Chidliak with complex late-glacial histories. Our results support <1 km horizontal scale subtle to obvious variability in the proportions of TiO2-rich and high-temperature (> 1100 °C) Cr-pyropes between closely spaced kimberlite source(s) and also between physically adjacent magma batches within single kimberlite pipes. The local scale variability is attributed to protokimberlite fluid or melt interacting with, and metasomatizing discrete conduits within, the ambient diamond-facies peridotitic mantle at times closely preceding eruption of kimberlite magma batches at Chidliak. 相似文献
13.
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
14.
西村岩管是在苏北地区发现的第一个金伯利岩管,颠覆了苏北地区无金伯利岩的历史。从岩石学、地球化学和伴生矿物等方面分析了西村岩管的地质特征,并进一步探讨其金刚石找矿意义。从区域背景和金刚石形成条件看,西村地区具备了金伯利岩侵位和金刚石矿形成的基本地质条件,而西村岩管为金刚石矿就位提供了母岩条件;西村金伯利岩与山东、辽宁金伯利岩具有相似的地球化学特征,是幔源岩浆低程度部分熔融的产物,且在岩浆上升过程中普遍遭受了壳源物质的混染,后期碳酸盐化现象普遍发育;其相容元素含量与山东金伯利角砾岩相似,均为典型的金伯利岩型配分模式,稀土元素表现为轻、重稀土元素强烈分馏的特征;伴生指示矿物主要为榴辉岩型含铬镁铝榴石、富铬透辉石和富镁铬尖晶石,其特征均表现出含矿金伯利岩的特点。 相似文献
15.
The paper discusses the petrophysical properties of kimberlites from the Komsomolsky Pipe and statistical analysis of their
relationships with the data of petrographic and ore microscopic studies. Comparison of the obtained results with data of analogous
studies in other Yakutian kimberlite pipes showed that these data can be applied for prognostic evaluation of kimberlite contents
in kimberlite bodies of this region. 相似文献
16.
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
17.
Our study of olivines from Canadian kimberlites shows that the application of FTIR spectroscopy significantly improves the reliability of olivine as a kimberlite indicator mineral (KIM). We have developed an algorithm that yields the water concentration and the normalized intensity of the OH IR absorption band at 3572 cm −1 from unpolished olivine grains of unknown thickness. For 80% of kimberlitic olivines these two parameters are significantly higher than those for olivines from non-kimberlitic magmas and consequently, olivines with water concentrations >60 ppm and a strong absorption band at 3572 cm −1 can be reliably classified as being kimberlitic.We have identified two major spectral features in the OH absorption bands of kimberlitic olivines that allow for a more detailed classification: (a) the presence of three types of high-requency OH absorption bands (Group 1A, 1B and 1C) and (b) the proportion of low-frequency OH absorption bands (Group 2) relative to high-frequency bands (Group 1). Comparison of our results with experimental studies suggests that differences within Group 1 OH absorption bands are due to different pressures of crystallization or hydrogenation. The three identified types of Group 1 OH absorption bands approximately correspond to high ( P > 2 GPa, Group 1A), moderate (2-1 GPa, Group 1B), and low (<1 GPa, Group 1C) pressures of hydrogenation. Group 2 OH IR absorption bands in olivines with NiO > 3500 ppm are interpreted to reflect olivine-orthopyroxene equilibria and hence are indicative of xenocrystic olivine derived from lherzolitic or harzburgitic mantle sources. Interaction of xenocrystic olivine with hydrous kimberlitic melts with low silica activity likely will cause a gradual increase in Group 1 absorption bands. Therefore, FTIR spectra of olivine can be used to obtain qualitative estimates of the duration of interaction between mantle material and a kimberlitic melt.In addition to applications in kimberlite and diamond exploration, FTIR spectra of olivine phenocrysts, combined with mineral chemical data, may also provide insights into kimberlite evolution. Our data suggest that in some instances the ascent of kimberlitic magmas could have been interrupted at or near the Moho, followed by olivine crystallization and exsolution of aqueous fluids. 相似文献
18.
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
19.
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
20.
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
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