A. E. Budyak

Sources of clastic material of the Neoproterozoic metasedimentary rocks of the Baikal–Patom belt, Northern Transbaikalia: Evidence from Sm-Nd isotope data

First systematic data on the variations of εNd(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 εNd(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 (εNd(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.

Thermodynamic model of the ore stage in the formation of the Sukhoi Log deposit

296 There are two concepts of the formation of ore on the Sukhoi Log gold deposit: metamorphic–metaso matic [1, 2] and magmatic–hydrothermal [3, 4]. In the first case, black shale was the source of ore mate rial; in the second case, ore was controlled by endogenic fluid related to granitoid magmatism. The most important regularities in the formation of the Sukhoi Log type deposits from the Bodaibo trough are described in [5] on the basis of the geological and geochemical data. Three main stages are distin guished. The first stage is controlled by the formation of the passive margin of the Siberian continent at the boundary with the Paleoasian Ocean. It is character ized by deposition of black shale of the Dal’netaigin skaya Series (Khomolkhinskaya, Aunakitskaya For mations, and others) enriched in gold and sidero and chalcophile elements. The concentration of gold in heavy fractions of carbonaceous matter extracted from rocks of the Khomolkhinskaya Formation reaches 1– 2 ppm with variations of the concentrations in host black shale from 0.00n to 0.0n. The second stage is indirectly related to closing of the Paleoasian Ocean and the collisional environment at the continental margin. This stage is characterized by intense folding and metamorphic alteration of sedimentary rocks with partial migration of gold to the zones of low pressures and temperatures. The third stage is controlled by the evolution of physicochemical conditions due to the influence of the mantle plume on black shale, which resulted from the formation of the Angara–Vitim batholith. Most likely this is reflected in intense ther mal and material (in the form of fluids) influence on sedimentary formations of the Bodaibo trough; finally this was the main reason for the formation of the deposits [5].

Geodynamic background for large-scale mineralization in the southern environs of the Siberian Craton in the Proterozoic

The geodynamic position of the productive Kevaktinskii and Dal’netaiginskii stratigraphic units with Au–U mineralization is considered within the Baikal–Patom highland. The Au–U specialization of rocks of the Kevaktinskaya Group results from its formation in the environments of the passive continental margin with introduction of the juvenile material related to riftogenesis and further collision, whereas the same specialization of the Dal’netaiginskaya Group is explained by exhalation activity in the back-arc spreading zone. The formation of most of the Au and U large-scale deposits within the southern environs of the Siberian Craton was controlled by Paleozoic oogenesis upon closure of the Paleoasian Ocean.

Isotopic (Sm–Nd, Pb–Pb, and δ34S) and Geochemical Characteristics of the Metasedimentary Rocks of the Baikal–Patom Belt (Northern Transbaikalia) and Evolution of the Sedimentary Basin in the Neoproterozoic

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.

Occurrence Forms of Carbon, Sulfur, and Noble Metals in Deposits of the Black-Shale Formation by the Example of the Degdekan Gold-Ore Deposit (Northeastern Russia)

Pyrite crystals and ore-bearing shales of the Degdekan deposit were studied by means of XPS, SEM–EDX, EPMA, and AAS. Five peaks of carbon organic forms were identified, conforming to polymer compounds containing either double bonds of carbon or alkyne groups and compounds containing C–OH and C=O bonds, as well as, probably, small amounts of S-containing compounds and those with functional groups of carboxylic acids. Sulfate prevails over sulfite in pyrites; among the surface sulfide forms, disulfide prevails over monosulfide; the presence of polysulfide is registered. The occurrence of various chemical forms of sulfur on the surface might provide for concentrating of microelements including the noble metals (NMs) in their surface-bound forms. The regular behavior of NMs (Au, Pt, Pd, and Ru) depending on the grain sizes (specific surfaces) of pyrite crystals along with the narrow range of the ratios of structural and surface components of the concentrations of different NMs points to NM coprecipitation with pyrite during the same productive stage. No capture of NM-containing carbonaceous phases took place, which should violate the regularity of Au distribution in pyrites of the Sukhoi Log deposit.

Geochemistry of dispersed organic matter in gold-ore deposits of black shale formations

Bitumens from carbonaceous shales of various ages of the Baikal-Patom highlands and the Degdekan deposit (Yana-Kolyma folded system) were considered. It was determined that bitumens of the Bodaibo synclinorium are mainly represented by asphaltenes, asphaltogenic acids, and hydrocarbons; bitumens of the Degdekan field are represented predominantly by hydrocarbons.