The object of this study is silicate-carbonate rocks located in gabbroid of the Kusinsko-Kopan intrusive complex within the Zelentsovskaya mine in the South Urals. Low Mn and Fe contents and low 87Sr/86Sr ratio in calcite (0.7045–0.7048) indicate that the source of carbonate matter for these rocks was recrystallized limestones of the Lower Riphean Satka Formation. Isotopic-geochemical data (87Sr/86Sr ratio and flat REE distribution pattern) indicate the influence of gabbroid melt on their composition. It is suggested that the silicate-carbonate rocks were formed as a result of the geological process closest to the skarn formation.

—We present results of Pb–Pb geochronology and Sr- and C- chemostratigraphic study of limestones and dolomites of the Ara-Oshei Formation of the Tunka ridge in East Sayan. The δ13C value of the limestones varies from –0.6 to 2.3‰ PDB, the 87Sr/86Sr ratio is within 0.70851–0.70864, and the Pb–Pb age is 521 ± 21 Ma (MSWD = 0.9). The δ13C value of the dolomites varies from –3.9 to 0.5‰ PDB, the 87Sr/86Sr ratio is 0.70844–0.70882, and the Pb–Pb age is 417 ± 8 Ma (MSWD = 2.5). The U–Pb isotope system of the dolomites was disturbed during the geologic evolution of the region, which led to the rejuvenation of their Pb–Pb age in the Early Devonian. The obtained Sr-chemostratigraphic and Pb–Pb data show that the limestones of the Ara-Oshei Formation accumulated in the early Cambrian. This provides a reliable proof that the limestones are part of the Vendian–Cambrian sedimentary cover of the Tuva–Mongolian massif.

Представлены результаты Pb-Pb геохронологического и Sr-, Cхемостратиграфического изучения известняков и доломитов араошейской свиты Тункинских гольцов Восточного Саяна. Значение δ13C в известняках варьирует от -0.6 до 2.3‰ PDB, отношение 87Sr/86Sr лежит в пределах 0.70851-0.70864, Pb-Pb возраст известняков равен 521±21 млн лет (СКВО = 0.9). Значение δ13C в доломитах меняется от -3.9 до 0.5‰ PDB, отношение 87Sr/86Sr 0.70844-0.70882, Pb-Pb возраст доломитов равен 417±8 млн лет (СКВО = 2.5). U-Pb изотопная система доломитов была нарушена в ходе геологической эволюции региона, что привело к омоложению их Pb-Pb возраста в раннем девоне. Полученные Sr-хемостратиграфические и Pb -Pb данные показывают, что известняки араошейской свиты накапливались в раннем кембрии. Это позволяет надежно доказать принадлежность известняков араошейской свиты к венд-кембрийскому осадочному чехлу ТувиноМонгольского массива.

— Clay subfractions (SFs) with particles size of 2–5, 0.6–2.0, 0.3–0.6, 0.2–0.3, and 0.1–0.2 μm of two shale samples (Upper Riphean Inzer Formation, South Urals) are studied using transmission electron microscopy, X-ray diffractometry (XRD), and U–Pb, Sm–Nd, Rb–Sr, and K–Ar methods. The SFs are composed of low-temperature 1M d illite; quartz, chlorite, and 2M 1 illite are observed only in some coarse SFs. Irrespective of the size, the clay particles are equant. The standardized illite crystallinity indices (CIS) of all SFs are typical of the dia(cata)genesis zone. The CIS value increases, the I 002 /I 001 ratio of XRD patterns decreases, and the K content and K/Rb ratio increase with decreasing particle size of the SFs from 2–5 to 0.1–0.2 μm. Leaching with 1N HCl and 1N NH 4 OAc and U–Pb, Sm–Nd, and Rb–Sr analysis of untreated SFs, leachate, and residue allowed us to study the isotope systematic of mixing in mobile and silicate material of shales. The 208 U/ 204 Pb and 87 Rb/ 86 Sr ratios of leachates are lower and the 147 Sm/ 144 Nd ratio is higher than those of residues. The leachates are also characterized by less radiogenic Pb and Sr and more radiogenic Nd relative to residues. With decreasing size of SF particles, the U, Pb, Sm, Nd, and Sr contents of leachates gently decrease and the Rb content increases. The 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios of leachates of fine SFs are significantly higher, whereas their 238 U/ 204 Pb ratio is lower in comparison with coarse SFs. What is more, the data points of residues of various SFs occur along the mixing lines in the 87 Rb/ 86 Sr‒ 87 Sr/ 86 Sr and 1/ 86 Sr‒ 87 Sr/ 86 Sr plots. The data points of corresponding leachates also form linear trends in 238 U/ 204 Pb‒ 206 Pb/ 204 Pb, 206 Pb/ 204 Pb‒ 207 Pb/ 204 Pb, 147 Sm/ 144 Nd‒ 143 Nd/ 144 Nd, and 87 Rb/ 86 Sr‒ 87 Sr/ 86 Sr coordinates. Apparent Rb–Sr ages calculated from slopes of “inner isochrons” (“leachochrons”) as well as K-Ar ages gradually decrease from 835–836 and 721–773 Ma, respectively, for SFs of 2–5 μm and to 572‒580 and 555‒580 Ma, respectively, for SFs of 0.1–0.2 μm. Thus, the XRD and isotopic data indicate that both clay and mobile shale constituents are mixtures at least of two components, and the silicate phase contains authigenic illites of various ages. First-generation illite abundant in the coarse SFs of 2–5 μm and 0.6–2.0 μm was formed immediately after the deposition of the Inzer sediments and its age of 803–836 Ma is consistent with stratigraphic age of the Inzer Formation. The formation of this illite was facilitated either by lithostatic burial or intense horizontal fluid flow caused by tectonic inversion in the eastern regions of the Uralian paleobasin. The age of the second-generation illite is 572–580 Ma; it was formed as a result of vertical movements or renovation of the composition of the pore fluids during deformations and metamorphism of the South Urals related to the evolution of the Beloretsk metamorphic complex.

A mineralogical-geochemical study of globular phyllosilicates (GPS) of the glauconite-illite series of the Dolgokta Formation from the stratigraphic well Chunkinskaya Well 282 was conducted, and their Rb–Sr age was determined. The suitability of the mineral for geochronological studies was estimated based on the Mössbauer spectroscopy data and the model of cation distribution in the octahedral sheet of GPS. The mineralogical and crystallochemical characteristics indicate epigenetic defects in the glauconite crystal structure; therefore, its Rb–Sr isochronous age (1300 ± 7 Ma) is “rejuvenated” and reflects a partial recrystallization event. The 87Sr/86Sr ratio in dolomites of the Dolgokta and Kuyumba formations varies from 0.70602 to 0.72230 and thereby confirms the epigenetic recrystallization of rocks. The model Rb–Sr age of glauconite, which was estimated taking the enrichment in radiogenic 87Sr into account, is within 1340–1400 Ma. The age estimate defines the upper limit of the stratigraphic distribution of the Mesoproterozoic fossil Tappania that was found in the Yurubchen and Dolgokta formations of the Baikit Anteclise.

The 2–5, 0.6–2, 0.3–0.6, 0.2–0.3 and 0.1–0.2 µm clay subfractions (SFs) separated from two shale samples of the Upper Riphean Inzer Formation, the southern Urals, were studied by the TEM, XRD, and U–Pb, Sm–Nd, Rb–Sr and K–Ar isotopic methods. All the SFs consist of the low-temperature 1Md illite; admixtures of quartz, chlorite and 2M1 illite occur only in the coarsest SFs. The clay particles are isometric, regardless of their size. The CIS (Crystallinity Index Standard) illite values for the all SFs are typical for the dia(kata)genetic zone. As the size of particles in the SF decreases from 2–5 to 0.1–0.2 µm, the CIS rises, the I002/I001 ratio on the XRD diagrams decreases, and the K content and the K/Rb ratio increase. Leaching with 1N HCl and 1N ammonium acetate (NH4OAc) and subsequent U–Pb, Sm–Nd and Rb–Sr analyses of the untreated SF, acid (acetate) leachate and residue made possible to study the mixing systematics in mobile and silicate materials of the shales. The 238U/204Pb and 87Rb/86Sr ratios in the acid and acetate leachates are below, and the 147Sm/144Nd ratio is above those in the residues. Less radiogenic Pb and Sr and more radiogenic Nd are also common for the leachates compared to the residues. As the size of particles in the SFs decreases, the U, Pb, Sm, Nd and Sr contents in the residues are smoothly reduced, whereas the Rb content shows an increase. The 87Rb/86Sr and 87Sr/86Sr values in the residues for fine-grained SFs are well above, and the 238U/204Pb value is well below those for coarse-grained SFs. What is more, in the 87Rb/86Sr–87Sr/86Sr and 1/86Sr–87Sr/86Sr diagrams, data points for the residues of variable size are arranged on the mixing lines. The data points of respective acid and acetate leachates also form linear trends in the 238U/204Pb–206Pb/204Pb, 206Pb/204Pb–207Pb/204Pb, 147Sm/144Nd–143Nd/144Nd, and 87Rb/86Sr–87Sr/86Sr coordinates. The apparent Rb–Sr age values, calculated from the slopes of “inner isochrons” (“leachochrons”), along with the K–Ar ages are smoothly lowered from 835–836 and 721–773 m.y. for the 2–5 µm SF to 572–580 and 555–580 m.y. for the 0.1–0.2 µm SF. Hence the XRD and isotopic data testify that the clay as well as the mobile material of the shale represent the mixtures of at least two components, the silicate phase containing authigenic illites of different ages. The first illite generation enriched in coarse-grained 2–5 and 0.6–2 µm SFs was formed shortly after deposition of the Inzer sediments, and its age of 803–836 m.y. is in agreement with the stratigraphic age of the formation. Simple lithostatic burial or intensive lateral fluid flow induced by tectonic inversion in the eastern regions of the Urals paleobasin may be considered as the geological processes responsible for the forming of this illite. The second illite generation was formed 572–580 m.y. ago. As the starting points for its formation, alternatively, may be concerned either vertical tectonics or renewal of pore fluid compositions during deformations and metamorphism on the southern Urals area related to evolution of the Beloretsk metamorphic complex.

A mineralogical and geochemical study was conducted and the Rb-Sr age of globular layered silicates (GPS) of the glauconite-illite series of the Dolgokta Formation from a parametric Chunkinskaya-282 drillcore was determined. The assessment of suitability of the mineral for geochronological purposes took into account the data of Mössbauer spectroscopy and modeling of the cation distribution in the GPS octahedral net. Mineralogical and crystalochemical characteristics indicate an epigenetic alteration of glauconite structure; therefore, its Rb-Sr isochronous age (1300 ± 7 Ma) is “rejuvenated” and reflects the stage of partial recrystallization. The 87Sr/86Sr ratio in dolomites of the Dolgokta and Kuyumba formations varies from 0.70602 to 0.72230, which confirms the epigenetic recrystallization. The model Rb-Sr age of glauconite, calculated taking into account the enrichment of radiogenic 87Sr, is within 1340-1400 Ma. The age estimate determines the upper limit of distribution of the Mesoproterozoic fossil Tappania, found in the Yurubchen and Dolgokta formations of the Baikit anteclise.

The Sr isotope composition in glendonites from two sections of the Middle Jurassic of Northern Siberia has been determined for the first time. Glendonite was formed by the replacement of icite by calcite during diagenesis. The 87 Sr/ 86 Sr in most of the samples of glendonites (0.70687–0.70715) is insignificantly lower than this ratio in the Late Bathonian–Early Bajocian ocean. This may indicate a contribution from the Callovian seawater in the formation of individual calcite generations or the influence of methanogenic fluid of deep origin.

Вулканогенно-осадочная заонежская свита людиковийского надгоризонта (около 2.0 млрд лет) содержит в средней части крупные карбонатные конкреции и линзы, расположенные в шунгитовых слоях. Карбонатные линзы и конкреции имеют преимущественно удлиненную и уплощенную форму, а их толщина варьирует от десятков сантиметров до первых метров. Некоторые линзы сохраняют реликты тонкой слоистости. Конкреции сложены кальцитом или доломитом и содержат много рассеянного органического вещества, а также слюду, тальк, хлорит, кварц и кристаллы пирита. В кальцитовых конкрециях присутствует незначительная примесь доломита (отношение Mg/Ca 0.011–0.045), и они отличаются от осадочных известняков низким отношением Fe/Mn (0.3–2.1). Содержание Sr в большинстве образцов колеблется в интервале 385–505 мкг/г, и лишь в одном образце это содержание меньше (86 мкг/г). Rb–Sr систематика карбонатных конкреций изучалась с применением методики селективного растворения, которая включала обработку раствором ацетата аммония (АА-фракция) для частичного удаления вторичного карбонатного материала и последующее растворение остатка в уксусной кислоте (УК-фракция). В индивидуальных образцах кальцитов различие между измеренным отношением 87Sr/86Sr в АА- и УК-фракциях кальцитов варьирует в пределах 0.0008–0.0033. Первичное отношение 87Sr/86Sr в УК-фракциях исследованных образцов колеблется от 0.7053 до 0.7162. Оно положительно коррелировано с отношением Mg/Ca и долей силикокластической примеси и отрицательно – с содержанием Mn. Конкреции формировались в обстановке погружения осадка, вероятно, при переходе от зоны с “мягкими” восстановительными условиями к зонам активной сульфатредукции и метаногенеза. В зоне сульфатредукции, где возникло большинство пирит-содержащих конкреций, осадок уже потерял геохимическую связь с поверхностным водным слоем и превратился в замкнутую или полузамкнутую систему. Здесь в ходе диагенеза радиогенный 87Sr освобождался из ассоциированных силикокластических минералов, что приводило к повышению первичного отношения 87Sr/86Sr в конкрециях до 0.7108–0.7162. Единичные же кальцитовые конкреции, не содержащие пирита (или содержащие минимальное его количество), вероятно, формировались в тонком поверхностном слое осадка выше зоны сульфатредукции и, таким образом, осаждали Sr, который находился в изотопном равновесии со Sr придонной воды. При этом крупные конкреции и карбонатные линзы с небольшим количеством силикокластической примеси могли, по-видимому, сохранять информацию о среде раннего диагенеза или даже седиментации. Первичное отношение 87Sr/86Sr в одном из таких образцов, в котором доля силикокластической примеси составляет 6.2%, позволяет дать максимальную оценку этого отношения (0.7053) в людиковийском палеобассейне.

The middle part of the volcanosedimentary Zaonega Formation of the Ludikovian Suprahorizon (approximately 2.0 Ga) includes large carbonates concretions and lenses in shungite layers. Carbonate lenses and concretions are primarily elongated and flattened, and their thickness varies from tens of centimeters to a few meters. Some lenses retain relicts of lamination. Concretions are composed of calcite or dolomite. They contain abundant organic matter, as well as mica, talc, chlorite, quartz, and pyrite crystals. The calcite concretions contain some dolomite admixture (Mg/Ca = 0.011−0.045) and differ from sedimentary limestones by a low Fe/Mn value (0.3–2.1). The Sr content is as much as 385–505 μg/g in most samples and is low (86 μg/g) only in one sample. The Rb-Sr systematics of carbonate concretions was studied with the stepwise dissolution procedure, which included processing with the ammonium acetate solution (AMA fraction) to partially remove the secondary carbonate material, with dissolution of the residue in acetic acid (ACA fraction). In individual calcite samples, discrepancy between the measured 87 Sr/ 86 Sr values in the AMA and ACA calcite fractions shows a variation range of 0.0008–0.0033. The initial 87 Sr/ 86 Sr ratio in the ACA fractions of the studied samples varies from 0.7053 to 0.7162. The ratio shows a positive correlation with Mg/Ca and the proportion of siliciclastic admixture and negative correlation with the Mn content. The concretions were formed when the sediments subsided, probably, during the transition from a zone with “mild” reductive conditions to zones with active sulfate reduction and methanogenesis. In the sulfate reduction zone, where most pyrite-bearing concretions were formed, the sediment was not geochemically exchaged with the bottom water and was evolved into a closed or semiclosed system. Processes of diagenesis in this zone promoted the release of the radiogenic 87 Sr from the associated siliciclastic minerals, resulting in growth of the initial 87 Sr/ 86 Sr in concretions up to 0.7108–0.7162. Some calcite concretions, which lacked pyrite (or contained its minimal amount) were likely formed in a thin surficial sediment layer located above the sulfate reduction zone. Therefore, they precipitated Sr in isotope equilibrium with Sr of the bottom water. However, large concretions and carbonate lenses with an insignificant siliciclastic admixture could retain the signature of early diagenesis or even sedimentation. The initial 87 Sr/ 86 Sr ratio in one of such samples with the siliciclastic admixture of 6.2% makes it possible to estimate the maximal value of this ratio (0.7053) in the Ludikovian paleobasin.

The first geochemical and Sm-Nd isotopic characteristics of Neoproterozoic-Cambrian manganese ores from the south folded framing of the Siberian Craton have been obtained. For manganese ores from the Podikat deposit, Tsagan-Zaba and, in part, for Slyudyanka ore manifestations, an explicit positive Eu anomaly and variable Ce behavior are typical no depending on degree of metamorphism. In the rocks of Itantsa ore manifestation and, in part, in those of Slyudyanka, REEs have distribution patterns similar to the normal sedimentary pattern and are characterized by a gentle slope with a negative Eu anomaly and by the absence of a Ce anomaly. With the geochemical peculiarities, including REE distribution in them, on aggregate, reconstruction of vast hydrothermal fields within the south framing of the Siberian Craton and spatial position of the studied manganese basins relative to the craton has become possible.

Fine-grained clayey subfractions (SF) with particle sizes of 1–2, 0.6–1.0, 0.3–0.6, 0.2–0.3, 0.1–0.2, and <0.1 μm were extracted from shales of the Vendian Staraya Rechka Formation in the Anabar Massif and studied by XRD and Rb-Sr methods. All the clayey subfractions are represented by illite with high crystallinity indices, which are characteristic of the low-temperature diagenesis/catagenesis zone and grow with the decrease of the particle size. The Rb-Sr systematics in clayey subfractions combined with mineralogical data provide grounds for the conclusion that illite from clayey rocks of the Staraya Rechka Formation was forming during two periods: approximately 560 and 391–413 Ma ago. The first illite generation was likely formed in the course of lithostatic subsidence of the Staraya Rechka sediments and the second one, during the Devonian lithogenesis stage. It is assumed that age of the first generation (∼560 Ma) is close to that of the Staraya Rechka Formation. This inference is consistent with biostratigraphic, chemostratigraphic, and geochronological data obtained for both rocks of the Anabar Massif and Vendian sediments from other regions of Siberia.

Rb–Sr systematics has been studied in 13C-rich carbonate rocks of the Paleoproterozoic (2.09±0.07 Ga) Tulomozero Formation in the northern Onega Lake area, the SE Fennoscandian Shield. The formation is divided into eight members (A–F) consisting of greenschist-facies-grade, variegated sandstones, siltstones, mudstones, stromatolitic dolostones and subordinate crystalline limestones. Samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the Tulomozero Formation. Prior to isotope analysis, the rocks powders were treated with 1N ammonium acetate for partial removal of the late epigenetic carbonate phases. Major resetting of the Rb–Sr systems in the Tulomozero carbonate rocks appears to take place during the Svecofennian regional metamorphic event, and it was screened by using Mn/Sr, Fe/Sr, Mg/Ca, and 18O/16O ratios. High Sr content (up to 2080μg/g in limestones, and 530μg/g in dolostones) coupled with low Fe/Mn (<0.40) ratios in the Tulomozero carbonate rocks of Members A, B (the lower part), D, F, and E are consistent with accumulation of original carbonate sediments in evaporitic lacustrine, playa, and sabkha environments. A decrease in the Sr content with concurrent increase in the Fe/Mn ratio (>0.40) in dolostones of the upper part of Member B, and of Members G and H is indicative of seawater influxes (sea transgression) into the Tulomozero basin. The 87Sr/86Sr values in the least altered (Mn/Sr<2.0) marine dolostones are 0.70418–0.70442 and 0.70343–0.70409 for the earlier and late phases of the marine transgression, respectively. The decrease in the 87Sr/86Sr ratio in ca. 2.1 Ga seawater is attributable to an increase in hydrothermal flux Sr into the Palaeoproterozoic ocean.

The mineral composition and U-Pb and Rb-Sr systematics of phosphorites from the Satka Formation of Lower Riphean carbonates, the Burzyan Group of Southern Urals, are studied. Phosphorites occurring as small lenses between stromatolite layers are composed largely of fluorapatite with admixture of detrital quartz, feldspars, illite, and chlorite. Phosphorite samples have been subjected to stepwise dissolution in 1 N (fraction L-1) and 2 N (fraction L-2) HCl. As is established, the maximum apatite content is characteristic of fraction L-1, while fraction L-2 is enriched in products of dolomite and sulfide dissolution and in elements leached from siliciclastic components. The Sr content in the Satka apatites (280–560 ppm) is substantially lower as compared with that in unaltered marine apatite. The 87 Sr/ 86 Sr “initial ratio in the phosphorites studied (0.71705–0.72484) and host dolomites from the lower part of the Satka Formation is significantly higher than in the Early Riphean seawater that indicates a reset of the Rb-Sr original systems in sediments. The Pb-Pb age of 1340 ± 30 Ma (MSWD = 6.4) estimated based on 7 data points characterizing fractions L-1 and L-2 is younger than the formation time of overlying Burzyan sediments, being consistent, within the error range, with date of the Mashak rifting event recorded at the Early-Middle Riphean boundary. The comparative U-Pb characteristics of two soluble fractions (L-1 and L-2) and silicate residue of phosphorites show that epigenetic redistribution of Pb and U was characteristic of the phosphorite horizon only. The initial Pb isotope composition and μ ( 238 U/ 204 Pb) estimated according to model by Stacey and Kramers for the early diagenetic fluids in carbonate and phosphate sediments of the Satka Formation suggest that they were in isotopic equilibrium with erosion products of the Taratash crystalline complex.

Fine-grained clay subfractions, SFs (particle size <0.1, 0.1–0.4, 0.4–0.6, and 0.6–2.0 μm) separated from a sample of the Lower Cambrian blue clay of the Lontova Formation were studied with XRD and Sm-Nd methods. The relatively coarse-grained SFs include illite with a small admixture of chlorite, while the finegrained SF (<0.1 μm) consists of mixed-layer illite-smectite. The illite crystallinity index (I c ) increases with decreasing particle size. The leaching of SFs with 1N HCl, analysis of Sm-Nd systematics of leachates and residues, as well as XRD data and results of chemical analysis show that the studied rocks contain at least two generations of minerals. The first (detrital) generation is related to the transformation of provenance material, whereas the second (authigenic) generation was formed at the postsedimentary stage of the evolution of the Lontova sediments. The Sm-Nd date of the first generation (790±90 Ma) is considered a minimal age of rocks in the northeastern and southwestern regions of the East European Platform that served as a source of sedimentary material of the Lower Cambrian blue clay. The date of the second generation of minerals reflects the timing of authigenic mineral formation in the course of burial and diagenetic and catagenetic reworking of clay sediments.

Clay subfractions (SFs) of <0.1, 0.1–0.2, 0.2–0.3, 0.3–0.6, 0.6–2 and 2–5 μm separated from Middle Riphean shales of the Debengda Formation are studied using the TEM, XRD, K-Ar and Rb-Sr isotopic methods. The oxygen and hydrogen isotope compositions in the SFs are studied as well. The low-temperature illite-smectite is dominant mineral in all the SFs except for the coarsest ones. The XRD, chemical and isotopic data imply that two generations of authigenic illite-smectite different in age are mixed in the SFs. The illite crystallinity index decreases in parallel with size diminishing of clay particles. As compared to coarser SFs, illite of fine-grained subfractions is enriched in Al relative to Fe and Mg, contains more K, and reveals higher K/Rb and Rb/Sr ratios. The Rb-Sr age calculated by means of the leachochron (“inner isochron”) method declines gradually from 1254-1272 Ma in the coarsest SFs to 1038-1044 Ma in finest ones, while the K-Ar age decreases simultaneously from 1225–1240 to 1080 Ma. The established positive correlation of δ18O and δD values with dimensions of clay particles in the SFs seems to be also consistent with the mixing systematics. The isotopic systematics along with data on mineral composition and morphology lead to the conclusion that mixedlayer illite-smectite was formed in the Debengda shales during two periods 1211–1272 and 1038–1080 Ma ago. The first period is likely close to the deposition time of sediments and corresponds to events of burial catagenesis, whereas the second one is correlative with the regional uplift and changes in hydrological regime during the pre-Khaipakh break in sedimentation.