The migration of helium from the crystal lattices of sulfides (pyrite, pyrrhotite, chalcopyrite, bornite, and sphalerite) and sulfosalts (tennantite and tetrahedrite) was studied. It was shown that helium occurs in submicrometer inclusions of uranium- and thorium-bearing minerals. The curves of helium thermal desorption from the sulfide and sulfosalts were obtained by the step-heating method and analyzed on the basis of the single-jump migration model. The interpretation of these data led to the conclusion on the possibility of the U–Th–He dating of pyrite. It was shown that the migration parameters of helium in the other sulfides and sulfosalts are not suitable for their potential use as U–Th–He geochronometer. Based on a comparison of data on helium migration in various minerals, it was suggested that high helium retentivity in some sulfides and arsenides (pyrite and sperrylite) is related to the type of their crystal lattice, packing density, and specific electric resistivity.

Based on a study of pyrite from the Uzelga Cu-Zn volcanogenic massive sulfide (VMS) deposit (South Urals) the age of ore mineralization was first determined with the direct age-dating method, based on the fraction of radiogenic helium, incorporated into the pyrite crystal lattice from submicron inclusions of U and Th minerals. Taking into account the measurement errors, the obtained age of 377 ± 8 Ma (MSWD = 1.2) is quite consistent with the independent age dates available for the ore mineralization (Late Eifelian-Early Givetian, 385-390 Ma).

Geochronological (LA–ICP–MS) U–Th–Pb studies of detrital zircon of the Late Cenozoic sand deposits on Ol’khon Island have been performed. It has been shown that their main source is represented by Early Paleozoic igneous and metamorphic complexes of the Ol’khon Terrane of the Central-Asian Mobile belt and Late Carboniferous–Early Permian granitoids of the Angara–Vitim batholite. Sands from the source were transported over a distance of no less than 100 km. It is supposed that it was effectuated by strong air flows, over the ice on the Lake Baikal in particular.

Изучены особенности миграции гелия из кристаллической решетки сульфидов (пирита, пирротина, халькопирита, борнита, сфалерита) и сульфосолей (теннантита и тетраэдрита). Показано, что источником гелия в этих минералах являются субмикронные включения уран- и торийсодержащих минералов. Полученные методом ступенчатого отжига кривые термодесорбции гелия из сульфидов и сульфосолей обработаны, исходя из представлений об односкачковой модели миграции. В результате интерпретации этих данных обоснован вывод о возможности U-Th-He датирования пирита. Показано, что миграционные параметры гелия в других изученных сульфидах и сульфосолях не позволяют рассматривать их как потенциальные U-Th-He минералы-геохронометры. На основании сопоставления данных о миграции гелия в разных минералах высказано предположение, что причина высокой сохранности гелия в некоторых сульфидах и арсенидах (пирите и сперрилите) связана с типом их кристаллической решетки, плотностью упаковки атомов и величиной удельного электрического сопротивления.

Обсуждаются возможности использования U-Th-He метода изотопной геохронологии для датирования самородного золота. Показано, что одной из основных форм нахождения урана и тория и, соответственно, главным источником радиогенного гелия в самородном золоте являются включения уран- и торийсодержащих минералов, в том числе фосфатов редкоземельных элементов. Вследствие субмикронных размеров этих включений образовавшийся в них радиогенный гелий не накапливается, а имплантируется в структуру самородного золота, что позволяет предполагать его высокую сохранность в течение миллиардов лет. В ходе изучения кинетики выделения радиогенного гелия из самородного золота это предположение получило экспериментальное подтверждение. Первые результаты U-Th-He датирования самородного золота из месторождений Педролампи (Центральная Карелия) и Витватерсранд (Южная Африка) находятся в хорошем соответствии с имеющимися независимыми геохронологическими данными. Все это дает основания рассматривать самородное золото в качестве U-Th-He минерала-геохронометра для прямого датирования рудообразующих процессов.

In this paper, we consider the application of the U-Th-He method of isotope geochronology for native gold dating. It was shown that inclusions of uranium- and thorium-bearing minerals, including rare earth element phosphates, are one of the main form of uranium and thorium occurrence and, consequently, the main source of radiogenic helium in native gold. Since these inclusions are submicrometer-sized, the radiogenic helium formed in them is not accumulated but implanted in the structure of native gold, which suggests its good preservation over billions of years. This suggestion was experimentally supported by the investigation of the kinetics of radiogenic helium release from native gold. The first results of the U-Th-He dating of native gold from the Pedrolampi (central Karelia) and Witwatersrand (South Africa) deposits are in adequate agreement with available independent geochronological data. This allows us to consider native gold as a U-Th-He mineral geochronometer for the direct dating of ore-forming processes.

The Pb-Pb age of phosphorite concretions of the Zigaza-Komarovo Formation, which composes the intermediate horizons of the Riphean stratotype of the South Urals, was determined in fractions resulting from the stepwise dissolution of concretions in 0.1 N, 0.5 N, and 1 N HCl. The determination of the Sr isotopic composition in phosphate fractions was favorable for rejection the fractions polluted with extraneous material. On the 207Pb/204Pb-206Pb/204Pb diagram, the isochron based on 31 points corresponds to 1330 ± 20 Ma (MSWD = 1.12), which is in agreement with the stratigraphic position of the Zigaza-Komarovo Formation. The decreased μ2 value of 9.57 for the phosphorite concretions relative to that of the average earthly lead based on the Stacey-Kramers model (9.74) is related to the rocks with an admixture of mantle lead, which occur in the run-off area of the Zigaza-Komarovo sediments.

Among the Caledonides exposed in the western part of the Kyrgyz Range the Lower Ordovician volcanogenic-sedimentary, plutonic, and tuffaceous-terrigenous complexes were distinguished. Volcanogenic-sedimentary sequences are the Kentash Formation, composed of volcanic rocks, tuffs and subvolcanic bodies of dacitic, andesitic and basaltic composition, sandstones and tuffites with interlayers and lenses of limestone. On the basis of conodonts and U-Pb dating of zircon grains the age of this Formation is in the age interval between Late Tremadocian Stage and Early Darriwilian Stage. Differentiated volcanites are associated with ultramafic-gabbro massifs of the Kokkiya Complex of the Late Darriwilian age (U-Pb zirconology). Features of the chemical composition of rocks of the Kentash Formation and the Kokkiya Complex indicate that they formed in suprasubduction settings within the island arc with a thick heterogeneous basement. Tuffaceous-terrigenous deposits are presented by the olistostrome formation, and coarse-grained deposits of the Taldybulak and Kyzylkainar Formations. The formation of olistostrome formation is associated with the over-thrusting of Cambrian melanocratic complexes on terrigenous-carbonate and shale strata of the Upper Precambrian-Cambrian age. Deposits of the Taldybulak and Kyzylkainar Formations accumulated in the back-arc basin and on the island arc slope, made of rocks of the Kentash Formation.

Предложен новый метод изотопной геохронологии для датирования самородных минералов платины, основанный на -радиоактивности природного изотопа платины 190Pt. Рассмотрение механизма термодесорбции гелия в кристаллической решетке самородных металлов, в том числе и платины, позволяет прогнозировать очень высокую термическую устойчивость (сохранность) радиогенного 4 в самородных минералах платины, вплоть до температуры их плавления. Для подтверждения эффективности предложенного 190Pt-4He метода проведено прямое изотопное датирование изоферроплатины Гальмоэнанского дунит-клинопироксенитого и Кондерского щелочно-ультраосновного массивов. Результаты определения возраста для коренной рудной платины Гальмоэнанского массива (70 ± 5 млн. лет), полученные данным методом, сопоставимы с геологическими наблюдениями и средними значениями Sm-Nd и Rb-Sr изотопных датировок. Установленная для массива Кондер 190Pt-4He датировка (112 ± 7 млн. лет) россыпной изоферроплатины также хорошо согласуется с геологическими данными и близка к значениям возраста косвитов (флогопит-магнетитовым пироксенитам, габбро, нефелиновых сиенитов и метасоматитов по дунитам), оцененным K-Ar и Rb-Sr методами. Экспериментальные данные подтвердили, что 190Pt-4He метод изотопной геохронологии может быть успешно применен для датирования самородных минералов платины.

The U-Pb and Pb-Pb methods were used for determining age of cap limestones from the Neoproterozoic Tsagaan Oloom Formation corresponding to the lower part of the sedimentary cover in the Dzabkahn microcontinent of Central Asia. The weighted average age value appeared to be equal to 632 ± 14 Ma (MSWD = 0.11, probability 0.74). This value allows the following assumptions: (1) the lower boundary of the Tsagaan Oloom Formation corresponds to the beginning of the Ediacaran; (2) Dzabkhan tillites are correlative with glacial sediments of the Marinoan Epoch. The low 238 U/ 204 Pb and 232 Th/ 238 U ratios observed in initial Pb sources of limestones from the Tsagaan Oloom Formation indicate that the Dzabkhan paleobasin received at its early development stages a bulk of material from eroded upper Riphean juvenile rocks. The 87 Sr/ 86 Sr ratio in fractions of Tsagaan Oloom limestones enriched with primary carbonate material and satisfying geochemical criteria of Rb-Sr systems retentivity (Mn/Sr < 0.20 and Fe/Sr < 1) varies from 0.70676 to 0.70691 and reflects this ratio in the World Ocean approximately 630 Ma ago.

A new method of isotope geochronology was proposed for dating native platinum minerals on the basis of the α-decay of the natural isotope 190 Pt. The analysis of the thermal desorption of helium in the crystal lattice of native metals, including platinum, allowed us to predict a very high thermal stability (retentivity) of radiogenic 4 He in native platinum minerals up to their melting temperatures. In order to validate the proposed 190 Pt- 4 He method, direct isotopic dating was performed for isoferroplatinum from the Galmoenan dunite-clinopyroxenite and Kondyor alkaline ultramafic massifs. The results of dating obtained by this method for primary ore platinum from the Galmoenan Massif (70 ± 5 Ma) are consistent with geological observations and mean Sm-Nd and Rb-Sr isotopic age estimates. The 190 Pt- 4 He age obtained for placer isoferroplatinum from the Kondyor Massif (112 ± 7 Ma) also agrees with geological observations and is close to the K-Ar and Rb-Sr ages of koswites (phlogopite-magnetite pyroxenites, gabbros, nepheline syenites, and metasomatic rocks after dunites). Our experimental data demonstrated that the 190 Pt- 4 He method is a promising tool for dating native platinum minerals.

The U-Pb age of the manganotantalite from rare-metal pegmatites of the Vishnyakovskoe deposit (East Sayan Belt) has been assessed at 1838 ± 3 Ma. The acquired data indicate the pegmatites of this deposit and associated granites of the Sayan complex belong to the postcollision South Siberian igneous belt (1.88–1.84 Ga), which stretches along the southwestern frame of the Siberian Craton by more than 2500 km, from the Yenisei Ridge to the Aldan Shield. Formation of this igneous belt is related to joining (starting from about 1.9 Ga BP) of the series of continental microplates and island arcs to the Siberian Craton; this led to final stabilization of the craton at about 1.8 Ga BP.

This paper presents the results of Sm-Nd isotopic-geochemical and U-Pb geochronological studies of metamorphic (Ilikan Sequence) and associated igneous rocks from the Ilikan lithotectonic zone (terrane) located in the Dzhugdzhur-Stanovoi Superterrane from the Central Asian Foldbelt. The Nd model age, T Nd (DM), of metamorphic rocks from the Ilikan Sequence is 2.6–3.2 Ga pointing to the likelihood that the lower boundary of their protolith formation probably does not exceed 2.6 Ga. The age of detrital zircons from metasedimentary rocks of the Ilikan Sequence is 2700–2900 Ma, which absolutely agrees with Sm-Nd isotopic-geochemical results. The U-Pb zircon age of metagabbro that intruded the rocks of the Ilikan Sequence and underwent high-temperature amphibolite metamophism with subsequent structural transformations is 2635 ± 4 Ma. The obtained results allow us to conclude that the age of the Ilikan Sequence is 2630–2700 Ma. All this gives grounds to state that the Dzhugdzhur-Stanovoi Superterrane in the Central Asian Foldbelt was formed due to amalgamation of non-Siberian terranes as is assumed for the Argun, Bureya, and Mamynskii terranes of the Amur Superterrane from the Central Asian Foldbelt.