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1.
Large diamond placers have been discovered in a Rhaetian basal horizon (Upper Triassic) in the north of the Sakha Republic (Yakutia) in the drainage areas of the Eekit, Nikabyt, Kelimyar, and Bur Rivers. In typomorphic features the found diamonds and indicator minerals of kimberlites are completely similar to those from Carnian basal horizons but, in contrast to them, are well sorted, and pyropes show features of mechanical wear. Analysis of the geologic evolution of the study area, morphology of diamonds and indicator minerals, and composition of the latter showed that the Rhaetian productive sediments resulted from the erosion of Carnian placers.  相似文献   

2.
Many diamond placers in the Siberian craton are heterogeneous consisting of several components that differ in origin and ages of the source. The diamonds are either kimberlite-hosted or are exotic varieties which occur mostly in the northeastern craton periphery and come from primary deposits of unknown types and ages. The two groups of diamond placers in the area represent two evolution trends: those found in Middle Paleozoic kimberlites originated in the Famennian and the exotic diamond groups became involved in sedimentation in the Carnian. The trends have been associated with successive weathering of older diamond hosts and redeposition of the minerals into younger sediments. Having joined since the Late Triassic, the two trends eventually produced a polygenetic and multistage mixture of diamond groups in placers.  相似文献   

3.
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.  相似文献   

4.
The U–Pb (SHRIMP) age was determined for zircons collected from 26 observation and sampling sites of diamonds and index minerals in the northeastern Siberian Platform. This part of the region hosts 15 low-diamondiferous Paleozoic and Mesozoic kimberlite fields, excluding the near economic Triassic Malokuonapskaya pipe in the Kuranakh field. Four epochs of kimberlite formation (Silurian, Late Devonian to Early Carboniferous, Middle to Late Triassic, and Middle to Late Jurassic) of the Siberian Platform, including its northeastern part, are confirmed as a result of our studies. Most observation points, including economic Quaternary diamond placers, contain Middle to Late Triassic zircons, which confirms the abundant Late Triassic volcanism in this region. The positive correlation of diamonds and major index minerals of kimberlites (mostly, garnets) at some observation sites indicates the possible Triassic age of the predictable diamondiferous kimberlites.  相似文献   

5.
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.  相似文献   

6.
Representative sampling of a diamond-bearing basal horizon in the Carnian Stage (Upper Triassic) on the northeastern margin of the Siberian Platform revealed a wide spectrum of indicator minerals, first of all, garnets, whose compositions are the same as in the inclusions in the regional diamonds. Of special interest are garnets of potential eclogite paragenesis with an abnormally high impurity of MnO (0.5–3.2 wt.%), which was earlier detected in more than 20% of garnets present as inclusions in diamonds of northern Quaternary placers and recommended as a new mineralogical criterion for diamond presence. Subcalcic Cr-pyropes of dunite–harzburgite paragenesis were also found in variable amounts, from 0.7 to 3.9 rel.%, in the sample of 973 grains of pyropes of lherzolite and websterite parageneses. Three grains contain 11.9, 12.6, and 16 wt.% Cr2O3, which corresponds to the presence of 30–34% of Mg–Cr-knorringite component. Such pyropes have been revealed for the first time in the study region. Cr-spinels are a mixture of compositions typical of kimberlites and the regional alkali-ultrabasic rocks. All studied samples contain picroilmenites with a variable content of Cr2O3 impurity. Since Mg–Fe–Ca-garnets with Mg# < 35 can be partly hosted in metamorphic rocks of the Anabar Shield, the elevated content of Na2O impurity (> 0.09 wt.%) was also taken into account. The different contents of indicator minerals in the samples might be due to the variable composition of the diamond orebodies. The Carnian placers call for new systematic sampling. Special attention should be given to estimation of the composition of garnets of presumably eclogite paragenesis with elevated contents of TiO2, MnO, CaO, and Na2O and to search for perovskite and Nb-containing rutile. These minerals, together with zircons, are of interest for determining the U–Pb isotopic age of probable diamond orebodies—kimberlites.  相似文献   

7.
Zircon-ilmenite placers in thin sandy clays of the West Siberian lowlands have been assigned ages ranging from Upper Cretaceous to Middle Oligocene. The productive beds were formed under continental conditions close to a Paleozoic sandstone source ridge. Inherited mineral composition, grain size and identical ore minerals are lines of evidence used to relate the placers to their source beds.—W. E. Yasso  相似文献   

8.
Natural diamonds from the Ural alluvial deposits have been studied by FTIR spectroscopy. It is shown that these diamonds are similar in some typomorphic features, such as nitrogen content and aggregation state, to the diamonds of the Coromandel (Brazil) and Verkhnee Molodo (Lena region, Yakutia) placers and to the diamonds from kimberlites of the Arkhangel’sk Region but differ significantly in lower contents of hydrogen and higher contents of platelets. The high contents of hydrogen (5–20 cm–1) determined in some diamonds are due to the specific formation of their internal structure and to the presence of inclusions. The nonuniform distribution of nitrogen A- and B-centers throughout the crystal testifies to the zonal structure of diamonds. The temperature conditions of formation of the Ural diamonds have been estimated.  相似文献   

9.
Fluorine contents have been determined in about forty samples of amphibole, mica and apatite in alkali basalt and kimberlite and their incorporated xenoliths. They show a wide variation ranging from 15,000 to 100 ppm, corresponding to about 40 to 0.2 per cent substitution of F for OH in hydroxyl site of hydrous minerals. Fluorine abundances in these minerals reflect those of their host magmas or rocks; Itinome-gata xenoliths are the lowest and South African kimberlites and their xenoliths are the highest. F/OH and also. D/H (Kuroda et al. 1975) ratios in coexisting phlogopite-potassic richterite from peridotite and mica nodules are thoughts to have formed under no simple equlibrium conditions.  相似文献   

10.
The comprehensive mineralogical analysis of the Taman nearshore Ti-Zr placers and their provenances in the adjoining late Pliocene sedimentation basin is presented. Taking into account paleogeographic reconstructions and contemporary mineral potential of sediments in the Sea of Azov, a contribution of specific feeding sources of terrigenous minerals to the formation of the late Pliocene placers have been estimated. These sources are crystalline and sedimentary rocks of the Caucasus, the southern Russian Plate, and the southeastern Ukrainian Shield (listed in the order of their contribution). The Miocene placers of the Stavropol Arch and Adygea Prominence, as well as the Cretaceous and younger placers of the Ukrainian Shield and the Voronezh Anteclise are suggested to be transitional reservoirs. Economic deposits are forecasted in the zone of paleostraits.  相似文献   

11.
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.  相似文献   

12.
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   

13.
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.  相似文献   

14.
Early Proterozoic kimberlites of Karelia are among the most ancient diamond-bearing primary source rocks in the world. They compose the large (2.0 × 0.8 km) Kimozero body localized in the predicted Zaonezhskoe kimberlite field. The established and assumed occurrences of kimberlite magmatism are located within the Karelian craton, which was stabilized during the Early Archean. They are confined to the central part of a large geophysical anomaly detected by gravity, magnetic, seismic, and heat-flow studies and mark a deep-seated magma chamber. Kimberlite bodies occur within structural blocks bounded by zones of plicative-rupture dislocations.The Kimozero kimberlites form an extensive but thin saucer-like body cut by narrow quasi-cylindrical feeders and dikes. It consists of metamorphosed kimberlites, their breccias and tuffs with widely varying amounts of mica. The body includes fragmentary fine-layered crater formations. The rocks contain olivine and phlogopite phenocrysts in an extremely altered groundmass of serpentine, chlorite, calcite, mica, and ore minerals as well as indicator minerals of kimberlites, such as Cr-spinel, manganiferous ilmenite, Cr-diopside, and rare pyrope. About 100 diamonds were extracted from 12 samples (total weight 815 kg). The crystals are colorless resorbed octahedra and, more seldom, combined octahedra-dodecahedra and spinel twins with abundant green spots caused by natural irradiation, which often make the whole crystal surface green. The diamonds contain inclusions of Mg-rich orthopyroxene and pentlandite suggestive of peridotitic lithospheric mantle derivation and dating of the sulfide inclusion implies a late Archean mantle source. By petrochemistry, the rocks are classified as kimberlites.The Kimozero kimberlites differ from classical Phanerozoic ones in having higher Fe contents, low contents of alkalies and P2O5, and intense superimposed carbonate, magnetite, and amphibole mineralization. The saucer-like bodies with narrow feeders without developed diatremes have no analogs in Russia but are similar to the saucer-like kimberlite bodies in Canada (Fort a la Corne), India (Tokapal), and Central Africa (Bakwanga) and the West Kimberley lamproites in Australia. By analogy with these bodies and on the basis of some common petrographic features (presence of pyroclastics and specific amoeba-like autoliths, scarcity of fragments of the enclosing rocks, local reworking of the deposited matter), the Kimozero kimberlites are considered to be the products of subaerial volcanic central-type eruptions.  相似文献   

15.
Typomorphic features of native gold and its contents in complex Ti–Zr placers in the southern West Siberian Plain are reported. Three of the placers are of littoral-marine genesis, and two formed under conditions of an alluvial piedmont plain. Native gold from the studied Ti–Zr placers occurs mainly as flattened thin (?0.1 mm) particles which underwent mechanical action. It is marked by wide fineness variation and the abundance of a very fine (990–1000‰) variety. Most likely, this is chemically transformed clastogenic metal. The gold content of the productive bed (5–30 mg/m3 native gold and 8–140 ppb bulk gold) is consistent with the dispersion of heavy ore and accessory minerals during mechanical migration in water flows simultaneously with their concentration on geochemical barriers. The native-gold content of complex Ti–Zr placers shows a higher negative correlation with the primary source–placer distance than those of Ti and Zr minerals and a positive correlation with the degree of hydrodynamic reworking (gravity concentration) of transit terrigenous material. On the southern framing of the West Siberian Plain, some regions of northern Kazakhstan are promising for gold of complex Ti–Zr placers as well as fine- and thin-gold placers, gold-bearing weathering crusts, and primary gold deposits.  相似文献   

16.
Large deposits of diamonds are associated mainly with kimberlites (and related rocks) of the cratons, but they are also known in the folded belts surrounding them. As an example is the Baltica craton and the surrounding its the Ural‐Timan (UT) folded belt. With the first object are associated diamonds of the Arkhangelsk (kimberlites and placers) provinces, and with the second one ‐ mostly placer deposits of the UT province, the probable source of which are also kimberlites. The structural position, composition and age of the potentially diamond‐bearing complexes of the Urals and Timan make it possible to propose a new petrological‐geodynamic interpretation of their formation. According to this model, during the Vendian‐Cambrian subduction of the Pechora Ocean crust, several different depth complexes have been formed, being changed in the western direction. At a shallow depth level the oceanic crust subduction is accompanied only by fluid processing, without the magmatism participation. As a result, this process leads to the formation of fluidizate‐explosive rocks of the Sertynya complex, which marks the outlet of the ancient subduction zone into the surface. At a moderately deep (up to 100–150 km) level melts are being produced, the derivatives of which are not diamond‐bearing depleted kimberlites of the Khartes (V‐Cm) complex. Apparently by the beginning of the Ordovician the active subduction of the Pechora Ocean stops. It occurs an opening of a new Ural paleoocean, and the earlier submerged the oceanic slab continues moving under the Baltica craton. At a deep (above 150 km) level the slab interaction with the mantle produces typical kimberlite magmas (from the Ordovician to the Middle Devonian) transporting diamonds to the surface of the Ural‐Timan province proper.  相似文献   

17.
Eclogitic xenoliths consisting of tschermakitic augite and pyrope garnet, together with variable amounts of kaersutitic hornblende, are common in a volcanic breccia of Lower Oligocene age at Kakanui, New Zealand. The breccia also contains xenocrysts of these minerals, and xenoliths of peridotite. Modal analyses are given of a number of the eclogitic xenoliths, and chemical analyses of two of them and their component minerals. They are compared with similar xenoliths from Hoggar (Algeria), Salt Lake Crater (Hawaii), and Delegate (Australia), with eclogite xenoliths from kimberlites, and with garnet peridotites. These three types of igneous eclogites can be characterized by the nature of their clinopyroxene: tschermakitic in the xenoliths from basaltic rocks, jadeitic in the xenoliths from kimberlites, and chrome diopside in the garnet peridotites. The eclogitic xenoliths in basaltic rocks probably crystallized in the mantle at depths of about 60 km, but their rarity in contrast to the numerous occurrences of peridotite xenoliths poses some significant problems.  相似文献   

18.
We present the first results of studying the major- and trace-element composition of microinclusions in the coats of type IV diamonds from the Sytykanskaya pipe. These microinclusions are of silicate–carbonate composition. Similar compositions are reported for diamonds from the placers of the northeastern Siberian Platform and cuboids from the Internatsional'naya pipe. The microinclusions studied are close to kimberlites and carbonatites in trace-element composition but depleted in HFSE, Mg, and transition metals and enriched in K and LILE. The distribution of incompatible elements in the microinclusions studied is similar to the “table” pattern, which was observed for high-density hydrous-silicic fluids.  相似文献   

19.
Semi-arid climatic conditions were responsible for the evolution of the vast savanna that stretches across central Africa and for the evolution of placers in this area that carry gemstones, rare earth element minerals and zircon- and titanium-bearing minerals. In combination with a polystage peneplanation, chemical weathering contributed to the emplacement of four different types of placer deposits during the Cenozoic in Malawi. Eight landform types have been defined by their geomorphological form, geometry, grain size parameters, stratification, fabric and rock contacts. Each type describes the landscape of the savanna and can be related to the emplacement of placer types within this morphoclimatic zone. Residual to eluvial placers, represented geomorphologically by landform type V, came into existence under strong chemical weathering during a period of alternating peneplanation and valley incision into soil and saprolite. Gentle valley-side slopes or elongated interfluves have a low gradient favorable for the formation and preservation of eluvial to colluvial placers (landform type Ic). Fluvial placers become economic only on outwash plains of tributaries near the confluence with their trunk rivers, beyond the point of inflection (landform types III, II). In the lower reaches of fluvial drainage systems around shallow lakes, fluvio-deltaic placers evolved during periods of rapid uplift and regression of the lake (landform type IV). Other landform types established throughout this study are cast in the role of ‘ore guides’ or, alternatively, have a negative impact on the likelihood of placer formation. The key associations of landforms discussed in this paper may be used as a geomorphological-sedimentological tool in search of modern continental placer deposits in areas where semi-arid climatic conditions occur(red) during the Cenozoic.  相似文献   

20.
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