Volume 69, Nº 9 (2024)

The results of long-term geochronological and isotope-geochemical studies of mesothermal gold deposits of orogenic type in the world’s largest Lena metallogenic province are summarized, and the evaluation of existing conceptions of their genesis is given. On the basis of geochronological data the presence of three ore-forming events manifested in the region in the Paleozoic time is proved. The early Late Ordovician-Early Silurian (450-430 Ma) event is associated with large-scale vein-embedded gold-sulfide mineralization in Neoproterozoic carbonaceous terrigenous-carbonate rocks of the Baikal-Patom fold belt (BPB). In terms of time, ore formation coincides with the development of metamorphic processes in the region. The reactivation of hydrothermal action in the BPP, which led to the formation of vein gold-quartz mineralization, occurred in the Middle Carboniferous (340-330 Ma) and was produced by post-collisional granitoid magmatism. The most recent ore-forming event occurred in the Early Permian (290-280 Ma) and was manifested exclusively in the Precambrian structures of the Baikal-Muya fold belt (BMP). It was synchronous with the development of intraplate alkaline and subalkaline magmatism in the region.

Isotopic-geochemical data (⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, Pb-Pb, and δ³⁴S) indicate that the mesothermal ore-forming systems of the Northern Transbaikalia were heterogeneous in their isotopic characteristics, which distinguishes them from ore-magmatic (intrusion related type) systems. Comparison of Sr, Nd and Pb isotopic composition of ores and rocks of the region, including magmatic rocks of the same age as gold mineralization, proves the leading role of the Precambrian continental crust in the supply of mineral-forming components to the hydrothermal systems of gold deposits. The contribution of the magmatic source of matter in the genesis of orogenic gold mineralization was limited and is established only for the Early Permian ore-forming systems of the BMP, for which the input of metals from alkaline melts of basite composition is assumed.

An original database compiled by the authors on volatile components of mineral-hosted fluid inclusions currently includes 12 470 analyses from 480 publications and was used to calculate the average gas phase composition of fluids that formed hydrothermal deposits throughout the Earth’s geological evolution, from the Archean to Cenozoic. The paper reviews the methods used in the study, their potential errors, and limitations. Characteristics of the gas composition of fluids are traced for more than 300 ore deposits of Au, Sn, W, Cu, Cu, Pb, Zn, Sb, Mo, and U. The dominant volatile component of natural mineralizing fluids in the Earth’s crust is carbon dioxide, regardless of the geologic age. The fluids contain subordinate amounts of reduced carbon species (methane) and nitrogen, as well as minor amounts of hydrogen sulfide and some other gases. The Cenozoic fluids commonly contain more nitrogen than methane. These relations are occasionally also found in the Precambrian fluids. The CO₂/CH₄ ratio as an indicator of the redox state of the system notably increased over the Earth’s geological history.