Search

This paper presents the results of geochemical and isotope-geochemical (Sm–Nd) studies of the Upper Riphean (?) (in the Stratigraphic Scale of Russia) sedimentary rocks of the Urulungui and Dyrbylkei formations of the Daur Series in the Argun continental massif, as well as U–Th–Pb and Lu–Hf isotope studies of detrital zircons from these formations. Rocks of the Urulungui and Dyrbylkei formations differ significantly in their mineral composition and major-element composition, which is probably related to the degree of chemical weathering of source rocks. Analysis of the trace-element concentrations in the sedimentary rocks of the Urulungui and Dyrbylkei formations enabled us to establish that they were formed in a subduction-related setting. The presence of conglobreccias, conglomerates, gravelstones, fragments of igneous and volcanic rocks, the low level of roundness of clastic material, as well as the abundant Neoproterozoic detrital zircons in the formations, are evidence for tectonic and igneous activities in the region during sedimentation. Sedimentary rocks of the Urulungui and Dyrbylkei formations have two-stage Nd model ages of TNd(DM2) = 2.0–2.1 Ga and 1.6–1.8 Ga, respectively. The abundant Neoproterozoic zircons in sandstones of the Urulungui and Dyrbylkei formations suggest that Neoproterozoic igneous rocks that were widely developed within the western Argun continental massif were the main source of clastic materials. At the same time, the presence of Early Precambrian detrital zircons indicates that Early Precambrian rocks were also present in the source area. However, reliably dated Early Precambrian igneous and metamorphic rock associations have not yet been found within the Argun continental massif. Thus, it is possible that the rocks of the Daur Series form an "exotic block" involved in the Argun massif. [ABSTRACT FROM AUTHOR]

The Transbaikal sector of the Mongol–Okhotsk orogenic belt displays a well-consistent geochemical and metallogenic zoning. The axial zone of the belt, which is controlled by the Mongol–Okhotsk suture zone from the north, is composed of reduced ilmenite-series rocks. Tin, tungsten, and rare-metal deposits and occurrences previously distinguished by S.S. Smirnov as a rare-metal belt are confined to the ilmenite-series rocks. The reduced rocks are framed from the north and south by oxidized magnetite-series rocks with chalcophile mineralization. The distribution zones of the ilmenite-series rocks correspond to anomalies with lower gravity and magnetic fields, whereas the magnetite-series oxidized rocks correspond to anomalies with higher gravity and magnetic fields. [ABSTRACT FROM AUTHOR]

The Transbaikal-Mongolia-North China province of East Asia is ranked as the most productive uranium province. Its area contains hundreds of occurrences, dozens of deposits of different (endo-, exo-, and polygenic) types, and several large uranium-bearing clusters and districts. The largest F-Mo-U clusters were revealed in the Mongol-Argun and Inshan-Liaohe volcanoplutonic zones (VPZ) in the western and southwestern periphery of the Great Xingan belt, respectively. Tectonic data on the Late Mesozoic (J2-K) Tulukuev (Southeastern Transbaikalia), Dornod (East Mongolia), and Guyuan-Duolung (Inshan-Liaohe, China) depressions of the above-mentioned VPZ are analyzed to distinguish their important structural details, which are responsible for the distribution of uranium-bearing fields and deposits in the volcanic rocks. Some of the deposits are associated with paleovolcanic necks, extrusive rocks, subvolcanic bodies, and dikes and have complex morphology. Other deposits are localized in stratified volcanic-pyroclastic rocks or within the basement rocks of the depressions. Recent geochronologic, geophysical, and seismotomographic data indicate not only the synchronous formation of the ore clusters in the Early Cretaceous (Valanginian), but also their localization within a crust of moderate (36-42 km) thickness above the periphery of a stagnant slab in the mantle transition zone. The coincidence of the slab boundary projections and area of their influence with the spatial position of large F-Mo-U ore clusters (Streltsovka, Dornod, and Guyuan-Duolung) separated from each other by hundreds and thousands of kilometers is regarded as evidence for the possible influence of deep geodynamics on the formation of the corresponding mineralization. Some of the aforementioned and similar clusters of the province seem to be insufficiently studied. This refers, in particular, to the exploration of the thick volcanic-pyroclastic fields associated with volcanic paleocenters within the clusters, as well as the basement rocks of the basin. [ABSTRACT FROM AUTHOR]