Pb Isotopic Ages Research Articles

Late Cenozoic high and low temperature magma generation from primordial and age-modified mantle materials beneath Dariganga in Southeast Mongolia: Factors of mantle degassing and adiabatic upwelling

Representative sampling of the Dariganga volcanic field was conducted to decipher its inner structure in terms of its deep magma sources. Rocks with a high La/Yb ratio (40–54) and a high MgO content (11–15.8 wt%) are identified among the predominantly moderate La/Yb ratio (7–40) and a moderate MgO content (5–11 wt%) rocks. These rock markers, traced along linear volcanic zones, are considered as indicators of high and low temperature magma generation processes. A general agreement exists that partial melting predominated in the transitional asthenosphere–lithosphere region; however, between 10 and 5 Ma, these processes were complicated by melts that either adiabatically ascended from a residual slab source in the deep mantle at a high potential temperature (Тр = 1489°С) or were generated due to mantle fluid degassing at a low temperature or both these processes, simultaneously. Magmas were subsequently adiabatically upraised from an OIB-like source in the deep mantle with potential temperatures (Tp) of up to 1423°C and were also generated by mantle fluids at low temperature. The rock markers yield Pb-isotope age estimates of the proto-mantle (at 4.47 and 4.45 Ga) and the age-modified mantle (at 3.11 and 2.74 Ga) beneath Dariganga. In the last 16 Ma, encompassing the late geodynamic epoch of Earth's mantle, similar high and low temperature magmas have erupted across vast sections of the Japan-Baikal Geodynamic Corridor; these magmas are not reported from the adjacent Abaga and Dalinuoer areas, with the exception of the final (Holocene) fluid-derived compositions.

Insight into Oligocene–Early Miocene palaeogeography of the Carpathians in Poland: first cycle and recycled detrital zircon provenance in the Menilite and Krosno formations

A set of 6368 U–Pb isotope ages of detrital zircons from 53 samples of the Oligocene to Early Miocene Menilite and Krosno formations (Outer Western Carpathians) was obtained and interpreted in terms of their sources. In both units the most frequent are mid-Variscan and Cadomian zircons that were derived from the Variscan Internides underlying the studied basin, and from the Brunovistulia Terrane, respectively. The coincidence of detrital zircon ages and clast ages indicates that zircons of that age can be at least partly of first cycle. Sveconorwegian and Calymmian–Statherian recycled zircons of Baltica origin, highly frequent in the Menilite Formation, are most probably sourced from Lower Devonian sediments of the Małopolska Massif. The Oligocene basin of the Menilite Formation was supplied in detrital material largely from the north where the Meta-Carpathian Swell was elevated in the area of the Małopolska Massif and Brunovistulia Terrane. During sedimentation of the Krosno Formation this source expired. A considerable input to the Menilite Formation of Baltica derived zircons and a limited number of zircons from sources located SE of the study area suggests that they were deposited in a relatively shallow and isolated basin. Supplementary material : A list of samples collected, lithological features of sampled rocks and lithostratigraphic sub-units, methods of zircon separation and U–Pb dating, and detailed results of the dating of detrital zircons, with concordia plots and probability density diagrams are available at https://doi.org/10.6084/m9.figshare.c.7100071

New Data on the U–Pb (LA-ICP-MS) Isotopic Age of Zircon from Intrusive Rocks of the Kuru-Tegerek Skarn Au–Cu–Mo Deposit, Middle Tien Shan, Kyrgyzstan

New Data on the U–Pb (LA-ICP-MS) Isotopic Age of Zircon from Intrusive Rocks of the Kuru-Tegerek Skarn Au–Cu–Mo Deposit, Middle Tien Shan, Kyrgyzstan

Disparate Pb-isotopic ages of silicate and phosphate minerals in the diabasic angrite Northwest Africa 12320

Pb-isotopic dating of the recently discovered diabasic angrite Northwest Africa (NWA) 12320 yielded ages of 4564.17 ± 0.50 Ma and 4562.82 ± 0.37 Ma for acid-insoluble and soluble minerals respectively. These ages are calculated using measured 238U/235U ratios of soluble and insoluble minerals, thereby excluding the possibility that the apparent age difference is caused by internal U-isotopic variations. The age of the insoluble minerals is indistinguishable from published Pb isotopic ages of other medium-grained angrites SAH 99555 and D'Orbigny. The age of acid-soluble minerals is distinctly younger, with a 1.35 ± 0.62 Ma age gap between the PbPb ages of the two mineral fractions. The mineral hosts of U, Th and radiogenic Pb in the meteorite were identified through in-situ SIMS measurements of these elements in all minerals. The principal carriers of U, Th and radiogenic Pb are acid insoluble Al-Ti-rich augite and soluble Si-rich phosphate. Closure temperatures for diffusion of radiogenic Pb in these minerals are calculated to be respectively 820 ± 90 °C and 450 ± 80 °C using the observed range of crystal dimensions and the published experimental data for Pb diffusion in augite and apatite. The age difference between the pyroxene and phosphates has two potential explanations: (1) a late reheating event >450 °C which may have re-set the UPb systematics of the phosphates but not the pyroxenes or (2) slow cooling of the rock from ∼450–820 °C. If slow cooling is the reason for the age difference, we can estimate a model cooling rate of 270 ± 150 °C/Ma over the estimated closure temperature range between pyroxenes and phosphates. This rate is ∼11 orders of magnitude slower than petrologic cooling rates of quenched angrites at temperatures above 1000 °C determined experimentally using the composition of Asuka 881371 groundmass (Keil, 2012), which is similar to NWA 12320 groundmass. If there was no late reheating event, the discrepancy between fast cooling rates at high temperatures near the solidus, followed by much slower cooling at lower temperatures, might have been caused by impact melting. It could also be attributed to the initial eruption of the source magma of NWA 12320 near the surface of the parent body, followed by burial due to subsequent eruptions.

Корневые структуры Даригангского и Витимского вулканических полей Японско-Байкальского геодинамического коридора: Соотношения источников позднекайнозойских магматических расплавов и современных локальных низкоскоростных аномалий в верхней мантии

In the Dariganga volcanic field of southeastern Mongolia, mainly basalt and tephrite melts with a MgO content of 5–11 wt. % and La/Yb ratio 7–40, derived from the mantle region of transition from the asthenosphere to the lithosphere, erupted in the last 14 million years. Basanite melts with a high MgO content (11–15.8 wt. %) and phonotephrite melts with a high La/Yb ratio (40–54) played a special role. Some basanites erupted after melting of the material of a residual slab source with a high potential temperature (Tp = 1489 °C) in the time interval of 10–5 million years ago, others due to melting of an OIB-like source at Tp up to 1423 °C about 4–3 million years ago. Phonotephrites resulted from low-temperature degassing of mantle fluids during the Quaternary. For the sources of basanites and phonotephrites, Pb-isotope age estimates yield age estimates for the protomantle (4.47 and 4.45 Ga, respectively) and age-modified mantle (3.11 and 2.74 Ga, respectively). We suggest that Quaternary degassing of mantle fluids with phonotephrite eruptions in the Dariganga volcanic field is reflected in the generation of the local East Mongolian anomaly of low S-wave velocities in the upper mantle on the SSW flank of the Japan-Baikal geodynamic corridor and that similar mechanism of Quaternary degassing of mantle fluids is responsible for the formation of the local North Baikal low-velocity root structure of the Vitim volcanic field at the axial part of the geodynamic corridor. High-Mg rocks (basanites and picrobasalts) of the latter volcanic field has been shifted relative to the root structure by 300 km in the last 16 million years.

Гарганский тип континентальной тектоносферы: Pb-изотопные оценки возраста событий ранней, средней и поздней геодинамических эпох Земли в слоях внешней оболочки, коромантийного перехода и литосферной мантии

From Pb-isotope age estimates of protoliths in sources of Cenozoic volcanic rocks and older dikes and sills, as well as Pb age estimates of galena from ore deposits, we define events of the early, middle, and late geodynamic epochs of the Earth's evolution in the Gargan zone of Eastern Sayans. We connect the tectonosphere development in the early geodynamic epoch with impact events of the Earth in the Hadean and Archean, in the middle one – with the Great Oxidation event in the Early Paleoproterozoic, and in the late one – with tectonic integration of the Gargan zone into the basement of the Tuva-Mongolian microcontinent in the Neoproterozoic and early Paleozoic. The conclusions obtained serve as a basis for identifying the Gargan-type continental tectonosphere.

Geological, fluid inclusion, H–O isotopic, and U–Pb geochronological constraints on the genesis of the Xiahuolong gold deposit, southern Jilin Province, NE China

The Xiahuolong gold deposit (3.05 t Au, average grade: 6.7 g/t), located in the Ji'an district of southern Jilin Province in the northeastern part of the North China Craton, contains orebodies in a Proterozoic metamorphosed sedimentary sequence. The distribution of the orebodies is controlled by NW- and NE-trending shear zones and faults, and the orebodies consist of auriferous quartz veins and auriferous altered rocks. The mineralization can be divided into four stages, namely (I) quartz-sericite, (II) quartz-pyrite-copper-gold, (III) quartz-pyrite-sphalerite-galena, and (IV) quartz-carbonate, with gold being introduced mainly during the second stage. Systematic analysis of the fluid inclusions suggests that two-phase vapor-rich (WV-type) and liquid-rich (WL-type) are the dominant types in the quartz veins. The early (stage I), main (stages II and III), and late (stage IV) fluid inclusions homogenize at temperatures of 273–367 °C, 201–346 °C, and 120–196 °C, corresponding to salinities of 0.68–0.79, 0.69–0.88, and 0.90–0.96 wt% NaCl equiv., respectively. The ore-forming fluid system evolved from CO2-H2O-NaCl ± CH4 to H2O-NaCl during ore formation, and fluid immiscibility and mixing caused the precipitation of sulfides and gold in the main stage. The H–O isotopes indicate that the ore-forming fluids were of magmatic origin, with a continuous input of meteoric water during ore formation. Two types of diorite porphyry have been identified, one cutting orebodies and the other filled with auriferous quartz veins. The diorite porphyry associated with the auriferous quartz vein yielded a zircon U–Pb isotopic age of 124.1 ± 1.9 Ma, indicating that Xiahuolong mineralization occurred in the Early Cretaceous, and that the high water content and low oxygen fugacity (ΔFMQ = -2.53 to +0.23; logƒO2 = -17.56 to -20.61) of the dioritic magma provided an important basis for mineralization. The results, combined with the deposit geology, fluid geology, H-O isotope geochemistry, timing of mineralization, and tectonic setting, suggest that Xiahuolong is a mesothermal gold deposit formed in an extensional setting characterized by lithospheric thinning related to the rollback of the subducting Paleo-Pacific Plate.

THE OLDEST TRACE FOSSILS IN ASSOCIATION WITH EDIACARA-TYPE BIOTA IN THE UPPER VENDIAN OF THE SOUTH URALS

Trace fossils and palaeopascichnids have been studied from the Basa Formation of the Vendian Asha Group of the South Urals (southwestern periphery of the Suleiman Anticline, Shubino quarry in the Ust’-Katav city, Chelyabinsk Region. The general morphology and preservation of ichnofossils was shown to be similar to the Late Vendian-Early Cambrian subhorizontal bilobate burrows of Didymaulichnus. The paleontological remains were found to be confined to extremely shallow depositional settings and located stratigraphically below the volcanic tuff layer, the zircon from which has a U–Pb isotopic age of 578 ± 7 Ma. Until now, the first occurrence of trace fossils and the associated “Agronomic Revolution” were thought to be timed to the interval of ~560–550 Ma. Our results show that mobile benthos and biological mixing of sediments may have occurred 20–30 Ma earlier and support the hypothesis that the onset of the first “Agronomic Revolution” and the “Cambrian Explosion” were not significantly coincident in time.

The Oldest Trace Fossils in Association with an Ediacara-Type Biota in the Upper Vendian of the South Urals

Trace fossils and palaeopascichnids have been studied in the Basa Formation of the Vendian Asha Group of the South Urals (southwestern periphery of the Suleiman Anticline, Shubino quarry in Ust’-Katav, Chelyabinsk Region of Russia). The general morphology and preservation of ichnofossils have been shown to be similar to the Late Vendian–Early Cambrian subhorizontal bilobate burrows of Didymaulichnus. The paleontological remains have been found to be confined to extremely shallow deposition settings and located stratigraphically below the volcanic tuff layer with a zircon U–Pb isotopic age of 578 ± 7 Ma. Until now, the first occurrence of ichnofossils and the associated “Agronomic Revolution” were dated at ~560–550 Ma. According to our study results, mobile benthos and biological mixing of sediments could have occurred 20–30 Ma earlier. The hypothesis that the onset of the “First Agronomic Revolution” and the “Cambrian Explosion” were far from being close in time was confirmed.

Long Evolution of a Magmatic–Ore System of the Muruntau Gold Deposit, Western Uzbekistan, Tien Shan: Evidence from the LA–ICP–MS U–Pb Isotopic Age of Zircon from Granitoids of the Sardara (Sarykty) Pluton

The LA–ICP–MS U–Pb age was determined for the first time for zircon from granodiorites–granites of the Sardara (Sarykty) intrusive in area of the giant Muruntau gold deposit in Western Tien Shan (Uzbekistan). Three crystal groups were identified in a combined sample: their concordant U–Pb age is 322.0 ± 3.7 (four grains), 301.6 ± 2.1 (11 grains), and 289.5 ± 4.9 Ma (two grains) (MSWD = 3.1, 0.17, and 0.98, respectively). The dispersion of the isotopic age can be explained using a model of a successive crystallization of different zircon generations in magmatic chambers at various depths and their further entrapment upon differentiation/crystallization of new magma portions. The similarity of two (youngest) age intervals with concordant U–Pb ages previously published for zircons of granitoid rocks directly at the Muruntau deposit is noteworthy. The latter ages could thus represent dike branching at the corresponding stages of evolution (progressive differentiation) of deeper sources of granitoid magma, the larger intrusions of which include Sardara (Sarykty) and other plutons exposed at some distance from the deposit. The older U–Pb isotopic ages of zircons from granitoids of the Sardara (Sarykty) pluton (~322 and 302 Ma) generally correspond to the subduction stage, whereas the youngest zircons (~289.5 Ma) correspond to the postcollision stage of the region. The origination and initial evolution of the magmatic sources in area of the Muruntau deposit could thus occur under subduction conditions; however, the final differentiation and crystallization of granitoid magma has finished at the postcollision stage.

Jizerka Gemstone Placer—Possible Links to the Timing of Cenozoic Alkali Basalt Volcanism in Jizera Mountains, Czech Republic

The Jizerka Quaternary alluvial placer in the Czech Republic has been a well-known source of gemstones since the 16th century, and the only one in Europe that has yielded a significant amount of jewel-quality sapphire. Besides Mg-rich ilmenite (“iserine”), which is the most common heavy mineral at the locality, some other minerals have been mined for jewellery purposes. These are corundum (sapphire and ruby varieties), zircon (“hyacinth” gemstone variety) and spinel. Here, we present a detailed petrological and geochronological investigation of the enigmatic relationship between the sapphires and their supposed host rocks, supporting their xenogenetic link. Our hypothesis is based on thermal resetting of the U–Pb isotopic age of the zircon inclusion found inside Jizerka blue sapphire to the estimated time of the anticipated host alkaline basalt intrusion. The host rocks of the gemstones (sapphire and zircon) and Mg-rich ilmenite are not yet known, but could be related to the Cenozoic volcanism located near the Jizerka gem placer (Bukovec diatreme volcano, Pytlácká jáma Pit diatreme and Hruškovy skály basalt pipe). The transport of sapphire, zircon and Mg-rich ilmenite to the surface was connected with serial volcanic events, likely the fast ascent of alkali basalts and formation of multi-explosive diatreme maar structures with later deposition of volcanoclastic material in eluvial and alluvial sediments in nearby areas. All mineral xenocrysts usually show traces of magmatic corrosion textures, indicating disequilibrium with the transporting alkali basalt magma. In order to constrain the provenance and age of the Jizerka placer heavy mineral assemblage, zircon inclusion and associated phases (niobian rutile, baddeleyite and silicate melts) in the blue sapphire have been studied using LA–ICP–MS (laser ablation–inductively coupled plasma–mass spectrometry) geochemistry and U–Pb in situ dating. Modification of the zircon inclusion into baddeleyite by exposure to temperature above 1400 °C in a basaltic melt is accompanied by zircon U–Pb age resetting. A zircon inclusion in a Jizerka sapphire was dated at 31.2 ± 0.4 Ma, and its baddeleyite rim at 31 ± 16 Ma. The composition of the melt inclusions in sapphire and incorporated niobian rutile suggests that the parental rock of the sapphire was alkali syenite. The Eocene to late Miocene (Messinian) ages of Jizerka zircon are new findings within the Eger Graben structure, as well as among the other sapphire–zircon occurrences within the European Variscides. Jizerka blue sapphire mineral inclusions indicate a provenience of this gemstone mineral assemblage from different parental rocks of unknown age and unknown levels of the upper crust or lithospheric mantle.

The Kataevo Island Arc System of the Paleoasian Ocean (Transbaikalia): Composition, Age, Paleomagnetism, and Formation Geodynamic Settings

Abstract—Based on new data on the geology, composition, U–Pb isotopic age, and paleomagnetism of the metavolcanic rocks of the Kataevo Formation, we consider the geodynamic conditions of their formation and alteration. The Kataevo Formation metavolcanic rocks belong to the K–Na-high-alumina andesite–andesibasalt–basalt volcanic series. Results for U–Pb analysis of magmatic zircon (SHRIMP II, 8 spots) from a metaandesibasalt sample of the stratotype section on Ungo River yielded and age of 852 ± 9 Ma. Isotope systems for Sm–Nd yield a positive εNd(852) = +9.29, which indicates a juvenile magmatic source, close to depleted mantle (DM), with a Neoproterozoic protolith TNd(DM) model age. The content of the less mobile HFSE and REE (ppm) is consistently low for Nb (8–15), Ti (7074–12,410), Ta (0.32–0.93), Eu (1.80–2.29), Се (50–79), Y (21–25), Yb (2.1–2.8), Rb (10–24) and elevated for Sr (1000–1500), Zr (170–270), La (25–41), and Ba (600–800). All studied parameters place the metavolcanic rocks close to the contemporary Kurile–Kamchatka type of developed island arcs. Paleomagnetic analysis of the section of metavolcanic rocks shows a complete remagnetization ca. 120 Ma. This is synchronous with manifestations of intraplate basaltoid magmatism in the studied region, the most typical example of which is the Lower Cretaceous Khilok Formation.

Pb−изотопная оценка возраста модельных источников позднекайнозойских базальтов вулканического поля Чанбайшань (Пектусан) и её значение в общей Pb−изотопной систематике Центральной Азии

From the 207Pb/204Pb – 206Pb/204Pb diagram, sources of Late Cenozoic basalts from the Changbaishan field, located on the Northeast China – North Korea border, are characterized by two age intervals: Late Archean (3.0–2.8 Ga) and Early Proterozoic (2.2–1.9 Ga). The latter interval coincides with the age estimates of the component of subducted sediments involved in their genesis and its upper boundary coincides with the Pb isotope age of the source of Pleistocene volcanic rocks in Northeast China.

Paleoproterozoic Layered Intrusions of the Monchegorsk Ore District: Geochemistry and U–Pb, Sm–Nd, Re–Os Isotope Analysis

The paper concerns the geochemical analysis of rocks from the ore-bearing layered intrusions that belong to two age groups of the Monchepluton and the Imandra–Umbarechka Complex (2.50 and 2.44 Ga) and the largest gabbro-anorthosite of the Main Ridge Complex (2.51–2.45 Ga). The intrusion of these complexes happened at different depths when the endogenous and geodynamic settings changed at the beginning of the Paleoproterozoic Era. Five megacycles are distinguished in a generalized cross-section of the two-chamber Monchepluton. The megacycles differ in rock composition, rock geochemical features, and mineralization types, i.e., the chromite, sulfide Cu–Ni–PGE and low-sulfide PGE types. The abrupt changes in isotope indicators (εNd, 87Sr/86Sr) mark their boundaries. At a depth of 2037–2383 m, the M-1 borehole intersects a standalone intrusive body that is essentially a magma feeder channel. The intrusive body’s geochemical characteristics and U–Pb isotope age correlate to the Monchepluton rocks. The gabbro-anorthosite massifs united in the Main Ridge Complex were intruded in the following order: the Monchetundra, Chunatundra, Volchetundra, and Losevo–Medvezhye tundras. The largest Monchetundra massif was formed as a result of multiple intrusions of mafic magmatic melt from the deep reservoirs. The melts intruded in two stages, i.e., 2.51–2.49 Ga and 2.48–2.47 Ga, and their composition changed gradually. The gabbro-pegmatites and coeval harrisite dykes are more recent ones (2.46–2.45 Ga). The summarized results of the U–Pb, Sm–Nd, and Re–Os systems research allowed us to establish genetic relations between the studied geological objects. We proposed a model where there was an uplift of a mantle plume to the lower crust area at the age of 2.5 Ga, the deep mantle reservoirs were formed, and a large-scale interaction happened between the parental magma and granulite–eclogite complex rocks. Local contamination and assimilation processes took place during the uplifting of magmas in areas where the magmatic feeding system contacted the host amphibolite–gneiss Archean complexes.

Whether short-lived or prolonged duration of multistage combined magmatic and hydrothermal events in the giant Chalukou porphyry Mo deposit, China

The duration (or time scales) of multistage combined magmatic and hydrothermal events provides key information constraining the metallogenic history of porphyry deposits, but the duration (time scale) of the mineralising hydrothermal process remains uncertain. Here, we report zircon U–Pb isotopic age, fluorite Sm–Nd isochron ages, and Ti-in-quartz diffusion chronometry for the giant Chalukou porphyry Mo deposit with reserves of 2.46 Mt @ 0.087 wt% Mo in NE China. The deposit is genetically related to hydrothermal fluids associated with a Late Jurassic (148–147 Ma) porphyritic magmatic event coincident with our new fluorite Sm–Nd isochron age of 148 Ma. Porphyry Mo mineralisation is characterised by the superposition of multiple generations of crosscutting quartz-bearing veins including: quartz veins with minor magnetite and hematite associated with K-silicate alteration (A-veins), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralisation (B-veins), and late base-metal mineralisation with phyllic alteration (D-veins), as well as post-mineralisation veins with argillic alteration. Hydrothermal veins from the Chalukou porphyry Mo deposit contain multiple generations of quartz that are distinct in their Ti concentrations and scanning electron microscope-cathodoluminescence images. The A-veins preserve subhedral to euhedral characteristics with alternating CL bright and dark growth zoning. The A-veins probably formed at a near-lithostatic pressure of ∼ 1.35 kbar and contain CL-bright quartz assaying 31–71 ppm Ti with Ti-in-quartz temperatures averaging 603 ± 30 °C, and CL-dark quartz assaying 6–25 ppm Ti with Ti-in-quartz temperatures averaging 496 ± 36 °C. The mineralised B-veins also CL-dark quartz, although they display less growth zoning with lower Ti assays of 0.4–27.9 ppm. These veins have near 0.7 kbar pressure and Titanium-in-quartz temperature averaging 390 ± 79 °C. The D-veins are dominated by CL-dark quartz and systematically lower Ti concentrations of 0.4–8 ppm, being lower than the relatively latter vein generations, formed at a temperature of 348 ± 58 °C and pressure of 0.4 kbars. The Ti diffusion in quartz has become a successful method constraining time scales and duration of magmatic and hydrothermal processes. Ti diffusion modelling indicates that the A-veins were deposited and cooled in a period between 2500 and 50,000 years. This shows that the earliest magma-related hydrothermal fluids activities formed within tens of thousands of years.

U–Pb Isotope Age of Zircon (LA–ICP–MS Method) from Magmatic Rocks and Some Aspects of the Genesis of the Tyrnyauz Mo–W Deposit (North Caucasus)

U–Pb Isotope Age of Zircon (LA–ICP–MS Method) from Magmatic Rocks and Some Aspects of the Genesis of the Tyrnyauz Mo–W Deposit (North Caucasus)

Age of the lower Cambrian Vanadium deposit, East Guizhou, South China: Evidences from age of tuff and carbon isotope analysis along the Bagong section

Abstract The early Cambrian is a critical interval of dramatic oceanic and biochemical changes in geological history. The black shale deposits, which are rich in Mo, Ni, V, and platinum group elements (PGE), are a reflection of that interval. Among all known Cambrian black shale deposits in South China, the vanadium deposits are poorly constrained by geochronology. The newly discovered tuff layer in the Sansui Bagong vanadium deposit in Guizhou Province can provide excellent constraints on the age of vanadium deposits. In this study, we obtain a new zircon U–Pb isotopic age, which can constrain the age of the vanadium deposit. This tuff occurs in the middle part of the ore bed, and the age of the tuff layer can reflect the mineralization age of the V deposit. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is used in this study to obtain the zircon U–Pb age of the tuff and indicates that the event of volcanic activity took place at 520.9 ± 1 Ma. This age is close to the boundary between the Cambrian Terreneuvian and Series 2. This result provides a good constraint on the age of the vanadium deposits in South China and makes the vanadium deposits to be comparable with other Mo–Ni–PGE deposits. The alteration of organic carbon isotope (δ 13Corg) values can reflect changes in the marine environment and is widely used in stratigraphic correlation. The analysis of the δ 13Corg values of the ore bed in the present study reveals a positive excursion at the bottom of the deposit and a negative excursion in the V-enriched layer. The δ 13Corg values in the Bagong section are comparable to those in the Xiaotan and Longbizui sections. The ages of the Sansui vanadium deposit constrained by the U–Pb isotopic age of the tuff and the δ 13Corg values are consistent. All of the data lead us to infer that the vanadium deposit formed at approximately 521 Ma.

Post-Ore Processes of Uranium Migration in the Sandstone-Hosted Type Deposits: 234U/238U, 238U/235U and U\u2013Pb Systematics of Ores of the Namaru Deposit, Vitim District, Northern Transbaikalia

To assess the nature of the post-ore behaviour of uranium in the Namaru deposit (Khiagda ore field), U–Pb isotope systems and the isotopic composition of uranium (234U/238U and 238U/235U) were studied. The studied samples represent different ore zones of the deposit and were collected along cross-sections both vertically and horizontally. Wide variations in the isotopic composition of uranium and U–Pb isotopic age have been established. Deviations of the 234U/238U ratio from equilibrium values, which for some samples exceed 50%, along with significant variations in the isotopic age, indicate that permafrost layer, which covered the catchment areas of paleovalleys with meteoric oxygen-containing waters ca. 2.5 Ma ago, did not lead to preserving uranium ores at the deposit. Uranium migration took place during the Quaternary period. The effective combining the U–Pb dating and 234U/238U data in assessing the post-ore redistribution of uranium made it possible to recognize: removal of uranium from some zones of the ore body and its accompanying redeposition in others. Wide variations in the 238U/235U (137.484–137.851) ratios throughout the entire studied cross-sections can be explained by the different locations of samples relatively to the ore deposition front and change in redox conditions as this front advanced. Depletion of the light isotope 235U in the lower zone of the ore body may be associated with the influence of ascending carbonic waters established in the regional basement. The effect of such waters on uranium-bearing rocks causes predominant leaching of light 235U.

First Data on the U–Pb Isotopic Age for Zircon (LA-ICP-MS Method) from Leucocratic Granites of the Tyrnyauz Mo–W Deposit (North Caucasus, Russia)

First Data on the U–Pb Isotopic Age for Zircon (LA-ICP-MS Method) from Leucocratic Granites of the Tyrnyauz Mo–W Deposit (North Caucasus, Russia)

U–Pb isotopic ages and provenance of some far-travelled exotic pebbles from glaciogenic sediments of the Polonez Cove Formation (Oligocene, King George Island)

Zircon grains from nine erratic pebbles of granite, granodiorite–tonalite and quartzite from the Polonez Cove Formation (southern King George Island) were studied for their U–Pb isotope ages and provenance. The calculated concordia ages of the studied pebbles are 108.79 ± 0.89, 119.7 ± 2.2, 178.6 ± 2.8, 180.7 ± 1.9, 207.4 ± 3.1, 231.1 ± 1.9, 1087.5 ± 4 and 1833 ± 4 Ma. The source area of individual pebbles was analysed and defined. Pebbles of crystalline rocks were derived from the Antarctic Peninsula, as well as from the Antarctic mainland. The erratic made of quartzite was eroded from the southernmost part of the Ellsworth Mountains (Linder Peak Member of Howard Nunatak Formation). Our isotope data analytically support the earlier thesis that the Oligocene ice-sheet covered a substantial part of Antarctica and its nucleus was located in its central part. Supplementary material: The U–Pb isotope data (Table S1) are available at https://doi.org/10.6084/m9.figshare.c.5233335