Search

Orogenic gold deposits associated with metamorphic orogenic fold belts are the major sources of gold currently being mined in the world. The role of metamorphic and magmatic processes and crustal and mantle sources in the gold mineralization genesis are of much debate. The formation of orogenic gold deposit in Precambrian metamorphic belts is usually described by а metamorphic devolatilization model. Some of the orogenic deposits, however, are proved to be much younger than the peak of regional metamorphism. Geological features of these deposits suggest close relation of their gold-forming systems with deeper lithospheric processes. The Baikal-Muya Belt (BMB) of the Central Asian Orogenic Belt is an example of such a region. The BMB comprises ca. 20 orogenic gold deposits and occurrences of Permian age. Gold mineralization at the deposits is usually located in quartz lodes situated in the vicinity of fault zones in association with Late Paleozoic sub-alkaline and alkaline basic and acid intrusions. Using high-precision (0.02–0.03%, 2SD) MC-ICP-MS method, we studied in detail Pb isotopic composition of the Permian gold-bearing mineralization at six BMB deposits, as well as metamorphic and igneous rocks of Tonian to Permian age. The 46 studied sulfide samples representing BMB gold-bearing lodes are highly variable regarding their Pb isotopic composition: 206Pb/204Pb from 16.628 to 18.293, 207Pb/204Pb from 15.325 to 15.614, and 208Pb/204Pb 36.742 to 38.384. Individual Permian orogenic gold deposits and occurrences differ in Pb isotope ratios and degree of their Pb isotopic composition heterogeneity. The Pb isotope features weakly correlate with the geological setting of the deposits. Age-corrected Pb–Pb data for the 41 samples of the BMB metamorphic and igneous rocks are similar to those of the gold mineralization: 206Pb/204Pb from 16.89 to 18.57, 207Pb/204Pb from 15.34 to 15.65, and 208Pb/204Pb 36.67 to 38.4. The high variability of model parameters (µ2 = 9.02–9.78, ω2 = 35.2–42.5, Th/U = 3.84–4.46) and Pb-isotope mixing trends for the ore lead suggest different sources of the metal. One of the sources was the BMB Neoproterozoic metamorphic and magmatic rocks hosting the gold deposits. The obtained data also suggest a common source for these deposits which is identical in its Pb-isotope characteristics to the Permian alkaline and sub-alkaline igneous rocks. We propose that the metasomatized subcontinental lithospheric mantle (SCLM) as well as the SCLM derived alkaline mafic magmas could be the important source of metals (including gold) for the Permian orogenic gold mineralization of the BMB. [ABSTRACT FROM AUTHOR]

Plagioclase varieties from leucogranites of the Raumid pluton (Pamirs) were studied by isotope Rb–Sr method. The studied varieties are: (1) fresh, crystal-clear magmatic oligoclase and (2) altered by deuteric saussuritization, turbid plagioclase. Comparison of data obtained for the coexisting varieties shows that saussuritization of the leucogranites led to Rb and Sr content and 87Rb/86Sr growth, and (87Sr/86Sr)0 distortion. This imply gain of alkaline and alkaline earth elements during saussuritization. The case of the Raumid pluton suggests a possibility of the gain from country rocks. [ABSTRACT FROM AUTHOR]

Geochronological studies have been conducted for ore-hosting rocks (quartz rhyolites, felsitic rhyolites, porphyry granites) of some U deposits of the Yakshi-Yangistau complex within the Balkashino ore center of the Northern Kazakhstan uranium province. According to geological maps of various scales, the rocks were traditionally ascribed to the Early–Middle Devonian (quartz and felsitic rhyolites) and Middle–Late Devonian (porphyry granites). The results of U–Pb, Rb–Sr, and K–Ar dating have shown that, according to the current version of the Geochronological Chart, the age of quartz rhyolites is Early–Late Silurian and that of felsitic rhyolites and porphyry granites is Late Silurian. The features of the chemical composition and different initial Sr isotopic composition of these rocks suggest that quartz rhyolites, felsitic rhyolites, and porphyry granites of the Yakshi-Yangistau complex are hardly comagmatic. They are characterized by different ages and formed from independent magmatic sources. Our data indicate the minimum possible age of the formation of primary U ores within the Balkashino ore center. [ABSTRACT FROM AUTHOR]

Conodonts from carbonate rocks of the Naiman Formation (Chingiz Ridge, East Kazakhstan) have been studied in detail for the first time. The mixed conodont complex is characterized by a high taxonomic diversity and includes species belonging to the stratigraphic interval from the upper part of the Dapingian Stage to the middle of the Darriwilian Stage of the Middle Ordovician. The conodont assemblage includes oceanic/deep-sea species of a wide geographic distribution, in particular, abundant Periodon macrodentatus and P. zgierzensis, which are characteristic of siliceous deposits of Kazakhstan. For the first time, the occurrence of species Scolopodus? mufushanensis, Drepanoistodus latus and Anodontus longus is noted in Kazakhstan. This indicates the existence of the biogeographic zoning by climatic zones typical of pelagic biota. The species Protopanderodus? nogamii indicates that the Kazakhstan conodont fauna belongs to the Australasian biogeographic province of the Eastern Gondwana. The conodont assemblage includes a number of regionally endemic taxa that characterize the Kazakhstan fauna as an independent biogeographic unit and confirm the relative remoteness of water basins of Kazakhstan from the eastern margin of Gondwana and from other paleocontinents in the Ordovician. [ABSTRACT FROM AUTHOR]

Abstract: The paper describes the high-precision U-isotope analysis of rocks. The analysis is based on multicollector inductively coupled plasma mass spectrometry (MC−ICP−MS) and includes, as an essential part, the three-step ion-exchange chromatography separation of uranium from chemically complex solutions. In the analysis, the 235U and 238U mass-bias of the MC−ICP−MS is corrected against the reference ratio of 233U and 236U in a double-spike added. The doublespiking precedes sample chemical digestion. This approach is useful as it corrects isotope data for a possible U-isotope fractionation that could have occurred in the chemical preparation of samples. The double-spike 236U/233U is 1.03183 ± 0.00005. The extraction procedure ensures uranium yield of more than 80%. The accuracy of the analysis was estimated by the study of 238U/235U in an international standard sample IRMM-3184 as well as in monitor samples. The uncertainty was estimated at ±0.008% (2SD), which is comparable with the accuracy of the results previously published by foreign researches. The analysis was elaborated for studying natural 238U/235U ratio variations. It was used by the authors for the high-precision determination of the U-isotope composition in rocks (granites, shales, volcanic rocks) and U-bearing minerals. [ABSTRACT FROM AUTHOR]

This paper reports the results of a detailed isotopic (Sm-Nd, Pb-Pb, and δ34S) and geochemical studies of Neoproterozoic metasedimentary rocks from the Patom and Bodaibo domains of the Baikal-Patom belt (northern Transbaikalia). It was shown that the metasedimentary rocks of these domains are strongly variable in their geochemical and isotope geochemical characteristics. Regular variations in these characteristics were observed, and their correlation with the main stages of the evolution of the sedimentary paleobasin in the Neoproterozoic was established. [ABSTRACT FROM AUTHOR]

The crystallization age of Zhaunkar granites (829 ± 10 Ma) was determined by U-Pb zircon dating. Taking into account the data obtained earlier on the granite age (791 ± 7 Ma) in the Aktas Complex and the syenite age (673 ± 2 Ma) in the Karsakpai Complex, the Ulutau sialic massif is assumed to be composed of three igneous complexes formed during the Tonian-Cryogenian periods of the Neoproterozoic. [ABSTRACT FROM AUTHOR]

First systematic data on the variations of ε(T) in the Neoproterozoic sedimentary sequence of the Baikal-Patom fold belt (Northern Transbaikalia) are reported. The range of obtained εNd(T) is-19.4...-2.0. It was established that the rocks of the Ballaganach and most part of the Dal'nyaya Taiga groups are characterized by ε(T) from-19.4 to-16.3. Upsection, beginning from the Khomolkha Formation, the rocks show a sharp change in the initial Nd isotope composition (ε(T)-8.3...-2.0). The results of Sm-Nd study of metasedimentary rocks of the BPB, as well as the comparison of their Sm-Nd characteristics with those of the inferred source areas suggest that the input of terrigenous material at the early stage of sedimentation in the Baikal-Patom belt was mainly related to the destruction of the Early Proterozoic crust of the Siberian craton. Owing to a change of sedimentation setting from passive continental margin to the 'foreland' basin at the late stage, the terrigenous material of the Siberian craton was diluted by clastics from juvenile Neoproterozoic crust of the Baikal-Muya belt. [ABSTRACT FROM AUTHOR]

Within the Kokchetav massif (Northern Kazakhstan), Mesoproterozoic granites and acid volcanics are widespread: these are the youngest Precambrian igneous rocks forming basement of the region. The Nd isotopic characteristics (ε( t)-4.4 ÷-9.6, t (DM) 2.1-2.6 Ga) obtained for these rocks indicate that the source of their melts was the Early Precambrian continental crust. Thus, the continental crust of the Kokchetav Massif had basically been formed by the beginning of the Mesoproterozoic and during the Late Precambrian: later it became a source for the granitoid melts. [ABSTRACT FROM AUTHOR]

Several generations of Paleozoic granitic rocks are studied with Sm-Nd isotopic methods in the northwestern part of the Aktau-Dzhungar microcontinent of Central Kazakhstan (Atasu-Mointy divide). The initial Nd isotopic composition of the granitic rocks varies in a relatively narrow range from-0.1 to-3.5ε; the Nd model ages are also similar (1.11-1.46 Ga). These results indicate that the crustal source of all the Paleozoic granitic rocks of the region had similar composition and, probably, age. It is shown that the t (DM) values of the Paleozoic granites reflect different proportions between ancient and juvenile material in the crustal source. [ABSTRACT FROM AUTHOR]

This work presents isotope Sm-Nd data obtained for bulk samples of granites of all 8 emplacement phases of the Raumid granite massif, which occurred 35 Ma ago at a hypabyssal depth during the orogenic stage of development of Southern Pamir fold system. The Sm/Nd ratio in studied collection of granite samples ranges between 0.091 and 0.323; the ε( T) value is-4.0. The Sm-Nd isotope study results suggest that all granite varieties distinguished in the Raumid massif are comagmatic formations and contamination and hybridization processes did not play any role in REE distribution in granites. At this, the source of parental magma did not change during granite generation. We assume that the only process, resulted in the trace element evolution in granites, was differentiation of three batches of magma sequentially uplifted from the source. [ABSTRACT FROM AUTHOR]

The paper presents petrological and geochemical data on mantle peridotite, basalt, and metamorphic rocks sampled in Cruise 36 of the R/V Professor Logachev at the MAR axial zone between 17° and 20° N. These data are interesting not only as providing new information on the inner structure of the oceanic crust in the still-poorly known axial MAR segment but also in the context of the fundamental problem of interaction between magmatic and hydrothermal systems in slow-spreading mid-oceanic ridges. The MAR axial zone between 17° and 20° N was determined to host both weakly and strongly depleted residual peridodites, which suggests that the degree of mantle source melting significantly varied along the ridge axis in this segment. The MAR crest zone comprises slabs of serpentinized peridotite brought to the seafloor surface at various time. The most strongly depleted mantle peridotites likely uplifted later than the mildly and weakly depleted rocks in the same areas. A mantle reservoir beneath the MAR axial zone at 20° N is not isotopically related to the mantle source of the parental MORB melts, and high-Mg metabasites exposed at 17°56- N were derived from a crustal source that was modified near the root zone of a high-temperature hydrothermal system. The studied area seems to display traces of an extinct hydrothermal field and likely an ore occurrence related to it. [ABSTRACT FROM AUTHOR]

The basement of the Zheltav sialic massif (Southern Kazakhstan) is composed of different metamorphic rocks united into the Anrakhai Complex. In the southeastern part of the massif, these rocks form a large antiform with the core represented by amphibole and clinopyroxene gneissic granite varieties. By their chemical composition, dominant amphibole (hastingsite) gneissic granites correspond to subalkaline granites, while their petroand geochemical properties make them close to A-type granites. The U-Pb geochronological study of accessory zircons yielded an age of 1841 ± 6 Ma, which corresponds to the crystallization age of melts parental for protoliths of amphibole gneissic granites of the Zheltav Massif. Thus, the structural-geological and geochronological data make it possible to define the Paleoproterozoic (Staterian) stage of anorogenic magmatism in the Precambrian history of the Zheltav Massif. The combined Sm-Nd isotopic-geochronological data and age estimates obtained for detrital zircons indicate the significant role of the Paleoproterozoic tectono-magmatic stage in the formation of the Precambrian continental crust of sialic massifs in Kazakhstan and northern Tien Shan. [ABSTRACT FROM AUTHOR]

A comprehensive study of the Lower Palaeozoic complexes of the Kokchetav Massif and its fringing has been carried out. It has allowed for the first time to discover and investigate in detail the stratified and intrusive complexes of the Cambrian-Early Ordovician. Fossil findings and isotope geochronology permitted the determination of their ages. The tectonic position and internal structures of those complexes have also been defined and their chemical features have been analyzed as well. The obtained data allowed us to put forward a model of the geodynamic evolution of Northern Kazakhstan in the Late Ediacaran-Earliest Ordovician. The accumulation of the oldest Ediacaran to Earliest Cambrian siliciclastics and carbonates confined to the Kokchetav Massif and its fringing occurred in a shallow shelf environment prior to its collision with the Neoproterozoic Daut island arc: complexes of the latter have been found in the northeast of the studied area. The Early Cambrian subduction of the Kokchetav Massif under the Daut island arc, their following collision and exhumation of HP complexes led to the formation of rugged ground topography, promoting deposition of siliceous-clastic and coarse clastic units during the Middle to early Late Cambrian. Those sediments were mainly sourced from eroded metamorphic complexes of the Kokchetav Massif basement. At the end of the Late Cambrian to the Early Ordovician within the boundaries of the massif with the Precambrian crust, volcanogenic and volcano-sedimentary units along with gabbros and granites with intraplate affinities were formed. Simultaneously in the surrounding zones, which represent relics of basins with oceanic crust, N-MORB- and E-MORB-type ophiolites were developed. These complexes originated under extensional settings occurred in the majority of the Caledonides of Kazakhstan and Northern Tian Shan. In the Early Floian Stage (Early Ordovician) older heterogeneous complexes were overlain by relatively monotonous siliceous-clastic units, that were being deposited until the Middle Darrivilian Stage (Middle Ordovician). [ABSTRACT FROM AUTHOR]

According to this paper, the juvenile crust of the Chingiz Range Caledonides (Eastern Kazakhstan) was formed due to suprasubduction magmatism within the Early Paleozoic island arcs developed on the oceanic crust during the Cambrian-Early Ordovician and on the transitional crust during the Middle-Late Ordovician, as well as to the attachment to the arcs of accretionary complexes composed of various oceanic structures. Nd isotopic compositions of the rocks in all island-arc complexes are very similar and primitive (ε(t) from +4.0 to +7.0) and point to a short crustal prehistory. Further increase in the mass and thickness of the crust of the Chingiz Range Caledonides was mainly due to reworking of island-arc complexes in the basement of the Middle and Late Paleozoic volcanoplutonic belts expressed by the emplacement of abundant granitoids. All Middle and Late Paleozoic granitoids have high positive values of ε(t) (at least +4), which are slightly different from Nd isotopic compositions of the rocks in the Lower Paleozoic island-arc complexes. Granitoids are characterized by uniform Nd isotopic compositions (<2-3 ε units for granites with a similar age), and thus we can consider the Chingiz Range as the region of the Caledonian isotope province with an isotopically uniform structure of the continental crust. [ABSTRACT FROM AUTHOR]

The article presents a study which investigates structural position and age justification of neuproterozoic rhyolites of precambrian massif in Central Kazakhstan. Topics discussed include determination of the age of acid volcanic rocks of the Maityubin zone by U-Pb dating method, chemical decomposition of zircon. and separation of uranium (U) and lead (Pb), and isotope study of the accessory zircon.

This work presents the Nd isotopic composition of Middle Cambrian to Late Paleozoic granitic rocks of the central part of the Chingiz Range in eastern Kazakhstan. All the rocks are characterized by a high positive ɛ( t) value (no less than +4), which is similar to that of the country island arc volcanic rocks. These peculiarities allow us to suggest that the juvenile crust of this region was formed at the island arc stage. The continental crust was further thickened due to the transformation of the island arc rocks with an insignificant contribution of the crustal component. On the basis of these data, it is assumed that the deep crustal levels of the Chingiz Range are composed of tectonically juxtaposed, geochemically and isotopically similar, metamorphosed complexes of the Early Paleozoic island arcs. [ABSTRACT FROM AUTHOR]

Rhyolite-granite volcanoplutonic association was identified among the Precambrian basement complexes of the Aktau-Mointy Massif, Central Kazakhstan. This association comprises rhyodacites, rhyolites, subalkaline rhyolites, tuffs, and felsic volcanogenic-sedimentary rocks of the Altyn Syngan and Urkendeu formations, as well as granitoids of the Uzunzhal Complex. U-Pb (ID-TIMS) dating of accessory zircons from the volcanic rocks and granites showed that the association was formed in the Neoproterozoic (Tonian, 925-917 Ma). The Neoproterozoic volcanic rocks and granites are the youngest Precambrian magmatic complexes and mark the final stage in the formation of the Precambrian crust of the Aktau-Mointy Massif. In terms of major and trace element composition, the volcanic rocks and granites resemble A-type granites, thus indicating the within-plate settings of their formation. It was established that their primary magma could be derived by melting of metatonalitic or metagraywacke protolith at T ≥ 940°C and P ∼ 8-10 kbar in response to mantle magma underplating. Sm-Nd isotope data on the volcanic rocks and granites (T(DM) = 1.9−1.7 Ma, ɛ(T) from −1.9 to −3.5) testify the Paleoproterozoic age of their crustal protolith. Available data have revealed strong similarity between the Neoproterozoic tectonomagmatic evolution of the Aktau-Mointy Massif and the Congo-São Francisco paleocontinent, which, with other cratons, composed the southern Rodinia supercontinent. This suggests that the formation of the Tonian anorogenic volcanoplutonic association of the Aktau-Mointy sialic massif was related to the global-scale divergent processes in the southern Rodinia supercontinent (Congo-São Francisco paleocontinent). [ABSTRACT FROM AUTHOR]

Petrogenetic conditions are evaluated for a poorly studied segment of the MAR axial zone. Mantle peridotites from MAR at 19°42′-19°59′ N are determined to show low to moderately low degrees of melting. The geochemistry of the peridotites is close to that of typical abyssal peridotites hosting active hydrothermal fields in the Atlantic (such as the Ashadze-Semenov-Logachev), except that plagioclase lherzolite occurs among the peridotites. Some of our samples collected in the area represent magnesian metabasites which could have been produced by metasomatic transformation (rodingitization) of harzburgite at contact with gabbroids. Cr and Ni concentrations in serpentine were proved to be indicative of the precursor mineral of the serpentine. The distribution of the isotopic composition data points of the MAR abyssal peridotites in the Sr/Sr-Nd/Nd diagram suggests that outcrops of mantle peridotites and related plutonic rocks in the crest zone of the ridge expose blocks of deep-sitting rocks that were not simultaneously brought to the surface. The MAR axial segment between 19°42′ and 19°59′ N can be viewed as prospective for searches for hydrothermal ore mineralization. Compositional parameters of basalt in our rock collection correspond to those of the most depleted MORB varieties in the Atlantic. [ABSTRACT FROM AUTHOR]

The Rb-Sr age of metasomatic rocks from four gold deposits and occurrences localized in Archean granite-greenstone belts of the western, central, and southern Karelian Craton of the Baltic Shield has been determined. At the Pedrolampi deposit in central Karelia, the dated Au-bearing beresite and quartz-carbonate veins are located in the shear zone and replace Mesoarchean (∼2.9 Ga) mafic and felsic metavolcanic rocks of the Koikar-Kobozero greenstone belt. At the Taloveis ore occurrence in the Kostomuksha greenstone belt of western Karelia, the dated beresite replaces Neoarchean (∼2.7 Ga) granitoids and is conjugated with quartz veins in the shear zone. At the Faddeinkelja occurrence of southern Karelia, Aubearing beresite in the large tectonic zone, which transects Archean granite and Paleoproterozoic mafic dikes, has been studied. At the Hatunoja occurrence in the Jalonvaara greenstone belt of southwestern Karelia, the studied quartz veins and related gold mineralization are localized in Archean granitoids. The Rb-Sr isochrons based on whole-rock samples and minerals from ore-bearing and metasomatic wall rocks and veins yielded ∼1.7 Ga for all studied objects. This age is interpreted as the time of development of ore-bearing tectonic zones and ore-forming hydrothermal metasomatic alteration. New isotopic data in combination with the results obtained by our precursors allow us to recognize the Paleoproterozoic stage of gold mineralization in the Karelian Craton. This stage was unrelated to the Archean crust formation in the Karelian Block and is a repercussion of the Paleoproterozoic (2.0-1.7 Ga) crust-forming tectonic cycle, which gave rise to the formation of the Svecofennian and Lapland-Kola foldbelts in the framework of the Karelain Craton. The oreforming capability of Paleoproterozoic tectonics in the Archean complexes of the Karelian Craton was probably not great, and its main role consisted in reworking of the Archean gold mineralization of various genetic types, including the inferred orogenic mesothermal gold concentrations. [ABSTRACT FROM AUTHOR]

This paper reports on isotope-geochronologic, petrologic, and isotope-geochemical (Sr-Nd) studies of Quaternary magmatism in the Geghama neovolcanic area, Armenia, Lesser Caucasus. According to these studies, the period of youngest volcanic activity in the region lasted about 700 000 years, from 700 ka to a few tens of thousands of years ago. We found the time limits of five discrete phases of Quaternary volcanism: I (about 700 ka), II (550−480 ka), III (190−150 ka), IV (110−70 ka), and V (later than 50 ka). These phases seem to have lasted a few tens of thousands of years and were separated by quiescent periods of comparable durations. The petrologic and isotope-geochemical characteristics of Geghama effusive rocks show that they belong to the bimodal association rhyolite-trachyandesite and basaltic trachyandesite; this association was largely generated by fractional crystallization of primary basite melts, with the assimilation of crustal material by deep magmas being much less important. The isotopic parameters of the volcanic rocks studied here (0.70410-0.70437 for Sr/Sr and +3.3 to +4.0 for Є) are practically identical for intermediate to basic and acid varieties in this association and are similar to those for young basites in the other areas of the Lesser Caucasus; this circumstance indicates a common origin for all Quaternary magmatic formations in the region. The petrogenesis of these varieties probably involved a single mantle source of the OIB type with certain regional peculiarities in the composition of the melts it generated. An analysis of the locations of Quaternary volcanoes in central Armenia (Geghama and Aragats areas) and in the Kars plateau, which is adjacent to it in the west, provided evidence of an eastward lateral migration of youngest magmatic activity in the region over time. The latest episodes of this migration took place in the eastern Geghama area, which must be the first to produce eruptions in the future. [ABSTRACT FROM AUTHOR]

The article explores the late Cambrian ophiolites found in the southern periphery of the Kokchetav Massif in Northern Kazakhstan. It discusses the structural position, composition and geochronology of early Paleozoic ophiolites which have been widespread in the central and eastern parts of the country. It notes that ophiolites are associated with island-arc and flysch complexes which confined to narrow zones with an intricate fold-thrust structure. It indicates the determination of the ages of crustal parts of ophiolitic sections in many areas including in the Boshchekul zone, Dzhalair-Naiman zone and Maikain-Kyzyltas. It mentions that the western part of Kazakhstan paleozoides are occupied by sialic massifs that are represented by terrigenous and siliceous.

The paper describes a new high-precision method for the simultaneous precise determination of Sr/Sr and Sr/Sr ratios in a single portion of a geological sample by multicollector inductively coupled mass spectrometry (MC-ICP-MS). The isotope analysis is carried out with mass bias effect correction by a combination of internal normalization to the standard Zr-isotope ratio and bracketing standard method (external normalization). Our results for geochemical IAPSO and BCR-1 standard samples are in a good agreement with the published data. The reproducibility of the Sr/Sr ratio varies from ±0.015 to ±0.05‰ (depending on the sample features) and, together with the analysis accuracy, is superior to the previously reported methods of MC-ICP-MS analysis. Still ahead is only double spike thermal ionization mass spectrometry with its ±0.02‰ reproducibility. However, the new method allows the simultaneous determination of Sr/Sr and Sr/Sr ratios and its productivity is higher by 5 to 6 times. On the other hand, in sample preparation, it is necessary to strive for at least 95% Sr yield from the chromatographic column; otherwise the sorption-desorption process may lead to a 0.6‰ Sr/Sr ratio bias relative to the true value. [ABSTRACT FROM AUTHOR]

The article offers information on a study on the Chelkar Peridotite-Gabbronorite Pluton in Kazakhstan. It informs that Chelkar pluton is composed of peridotites, gabbro, and norites which is intruded by large bodies of Late Ordovician granites. It was found in the study that the Chelkar pluton having a complex layered intrusive body was formed in the late Middle-early Late Ordovician and cannot be a part of the ophiolitic section. It informs that it was marked by the intense magmatic activity of the Stepnyak-North Tien Shan ensialic island arc having the Kokchetav massif in its back part.

The basement of the south Ulutau sialitic massif, which is located in the western part of Central Kazakhstan, comprises metamorphized volcanogenic-sedimentary and plutonic complexes of Proterozoic age. The upper boundary of the metamorphism age corresponds to the age of nonmetamorphized syenites from the Karsakpai massif (673 ± 2 Ma, Late Riphean). U-Pb geochronological studies of accessory zircon were made, and a Late Riphean age of biotite alkali granite from the Aktas massif (the youngst metamorphic Precambrian igneous units in South Ulutau) was found. The obtained age estimate of 791 ± 7 Ma can be considered as the lower age limit of metamorphism. Thus, the last stage of regional metamorphism in South Ulutau took place in the second half of the Late Riphean, in the time interval of 790-670 Ma. [ABSTRACT FROM AUTHOR]

The article focuses on the study on which aims to determine the Riphean age of zircons from alkaline syenites of the Karsakpai massif of South Ulutau in Central Kazakhstan with the uranium-lead (U-Pb) method. It mentions that the granitoids varying in chemical composition associated with Precambrian metamorphic strata were considered as coarse-grained gneissic granites (Zhaunkarskii complex), porphyroblastic granite-gneisses, and sub-alkaline potassic granites (Aktas complex). It states that the characteristics features of syenites of the Karsakpai granites and massif of the Aktas complex are massive structures and an absence of metamorphic transformations. Furthermore, it says that the age of intrusion of the massif allows to consider more ancient metamorphic complexes as Pre-Riphean.

The article offers information on the structural position and age substantiation of Middle Riphean gneiss granites of the Kokchetav Precambrian sialic massif in northern Kazakhstan. It says that the gneiss granites studied in the central areas of Kokchetav Massif such as Kokchetav, Chaikino Settlement, and Lake Chelkar have the development of small blocks and boundinized amphibolite, eclogite, and garnet amphibolite. It states that gneiss granites had structural-metamorphic alterations which transformed into blastomilonites with augen structure and well-developed linearity. Moreover, the age estimate of gneiss granites using uranium-lead (U-Pb) geochronological measurements corresponds to the age value of 1148 ± 8 Ma as deterrmined by discordia for zircon isotope composition.

The volcanogenic Kuuspek Formation is a well-defined part of the succession of the Pre-Vendian complexes of the Kokchetav massif (Northern Kazakhstan). The formation is built up of mildly metamorphosed acid lavas, tuffs, and tuffaceous sandstones. At the reference site to the west of the Kokchetav Mountains, the rocks of the Kuuspek Formation compose hinges of small anticlinal folds with sericite-quartz schists of the Late Riphean Sharyk Formation forming the limbs. The Kuuspek Formation lavas are high-alumina rhyolites of high-potassium calc-alkaline series. The U-Pb zircon age of the rhyolites is 1136 ± 4 Ma, thus referring to the Middle Riphean. The Kuuspek rhyolites form the basal part of the Precambrian sedimentary cover of the Kokchetav massif. The cover also comprises schists, limestones, and dolomites of the Sharyk Formation, and quartzites and quartzitic schists of the Late Riphean Kokchetav Formation. [ABSTRACT FROM AUTHOR]

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the 87Sr/86Sr ratio between the basalts and their chilled glasses and less significant difference in ɛNd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear. [ABSTRACT FROM AUTHOR]

The development of the MC-ICP-MS method, which was launched about one decade ago and was largely stimulated by the need to solve geological problems, has opened a new avenue in isotope mass spectrometry. One of the advantages of this method is the possibility of applying a newly developed approach to the correction of analytical results for the effect of mass discrimination by normalizing the measured isotope ratios of an element to a reference (standard) isotope ratio of another element. This makes it possible to overcome the main disadvantage of conventional thermal ionization mass spectrometry (TIMS), in which the effect of mass discrimination cannot be fully taken into account during isotope analysis, and thus to implement a highly accurate method for the analysis of Pb-isotope composition. In application to the capability of the NEPTUNE MC-ICP mass spectrometer, we optimized and calibrated a method for high-accuracy Pb isotope analysis in solutions spiked with Tl, with all currently measured Pb-isotope ratios normalized to the standard 205Tl/203Tl ratio (TLN-MC-ICP-MS). The factors affecting the random and systematic analytical errors were examined, and the optimal operating regime and analytical conditions were determined. Much attention was paid to the correlation of the measurement results and the mass discrimination effect determined from the 205Tl/203Tl ratio. The value of the 205Tl/203Tl normalizing ratio was analytically determined through isotope analyses of the NIST SRM 981, and SRM 982 standard samples of Pb-isotope composition. The data obtained for two mixtures Tl + Pb (SRM 982) and Tl + Pb (SRM 981) in ten replicate analyses were 2.38898 ± 12 and 2.38883 ± 20, respectively. These results are in good mutual agreement, and their general mean 205Tl/203Tl = 2.3889 ± 1 coincides (within the error) with the recently published values of 2.3887 ± 7 [Collerson et al., 2002] and 2.3889 ± 1 [Thirlwall, 2002]. The precision of the method (±2SD), which was assayed by the long-term reproducibility of the results of replicate analyses of SRM 981 and seven galena samples (90 analyses) was 0.016–0.018% for the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios and 0.005 and 0.009% for the 207Pb/206Pb and 208Pb/206Pb ratios, respectively. The precision of the isotope analysis of common Pb was significantly improved (by factors of 6–10 for various isotope ratios) compared with the precision of TIMS techniques acceptable in isotope studies during three decades. The described method was applied to examine the Pb-isotope composition of approximately 250 samples of galena, scheelite, and pyrite from a number of well known (including large) gold, sulfied, and base-metal deposits. The precision of the method (0.01–0.02%) makes it possible to study small inter-and intra-phase differences in Pb-isotope ratios in hydrothermal and magmatic rocks, to assay the scale of regional and variations in the isotope composition of ore Pb, and to correlate the Pb-isotope composition of rocks and ores and reveal its evolutionary trends. [ABSTRACT FROM AUTHOR]

The Sm-Nd systematics of sanukitoids with an age of 2715–2740 Ma in the Western, Eastern, and Central domains of the Karelian craton with various crustal evolutionary histories indicates that the mafic and acid rocks of the sanukitoid series were derived from two contrasting sources: enriched lithospheric mantle and lower crust. The basic sanukitoids of the Western domain were derived from the mantle enriched long before its melting [ɛNd(2715) = −0.48 ± 0.22]. The source of the acid magmas was the young juvenile crust of TTG composition [ɛNd(2715) increases to +1.2]. The mantle source of mafic sanukitoids in the Eastern domain was enriched shortly before melting [ɛNd(2740) = +1.58 ± 0.01], whereas the acid melts came from an ancient crustal source [ɛNd(2740) decreases to −3.0]. For sanukitoids in the Central domain, the time span between the enrichment of the mantle source and its melting was the shortest [ɛNd(2725) = +2.05 ± 0.15], and the contribution of the juvenile TTG crust was insignificant [ɛNd(2725) deceases to +1.7]. The variations in the isotope characteristics of the acid members of the sanukitoid series are consistent with the known age heterogeneity of the crust of the domains. The lateral isotopic-geochemical heterogeneity of the lithospheric mantle source of the sanukitoids is thought to have been related to its two-stage reworking (at 3.2 and 2.8–2.9 Ga) under the effect of TTG granitoids, which are regarded as the melting products of the subducted oceanic crust. The sanukitoids provide information on the geochemical structure of the Archean lithosphere, which is reflected in Archean crust-building processes. The Rb-Sr isotope system of the Neoarchean sanukitoids underwent transformations on the mineralogical scale and within small massifs in the course of at least two Paleoproterozoic tectono-thermal events. A trace of the event at ∼2.1 Ga is left in the Rb-Sr system of monomineralic fractions from a weakly deformed syenite of the sanukitoid series in the Central Domain. Later event (∼1.7 Ga) was recorded in the minerals of the Teloveis sanukitoid massif, which hosts a gold mesothermal deposit in the Western domain. Monomineralic fractions of muscovite and biotite from the wall-rock metasomatites and of plagioclase, microcline, and biotite from metasomatites away from the orebodies yield isochron ages of 1719 ± 60 and 1717 ± 27 Ma. This age of the metasomatic alterations of the Neoarchean sanukitoids is able to explain the broad and unsystematic variations in the Rb-Sr isotope-geochemical characteristics of these rocks. Our data on the Paleoproterozoic age of the mesothermal gold ore mineralization at the Teloveis deposit provide additional lines of evidence for the complex tectonic and metallogenic evolution of the Karelian GGT in the Early Precambrian. [ABSTRACT FROM AUTHOR]