Vol 13, No 6 (2019)

Based on the data of relief analysis, the gravity and magnetic anomaly axes, earthquake foci sequences, and V_p/V_s ratio values, tectonic fracturing of the lithosphere is studied and tectonic submeridionally and sublatitudinally oriented fracturing zones are distinguished in the Priamurye region. Separate segments of extended submeridional and sublatitudional fracturing zones are evidenced from the regional faults detected using the geological methods. Submeridionally oriented tectonic fracturing zones are mostly revealed, which do not correlate with the regional faults. Fracturing zones of similar orientation which are displayed in lineaments of different type and correlate with the seismoactive zones may be considered as hidden faults.

The Sikhote-Alin Orogen in the southeast of Russia is a collage of geological terranes of various age and tectonic origin, which formed along the East Asian continental margin as a result of the Jurassic–Early Cretaceous subduction of the Pacific oceanic plates. The Jurassic Samarka accretionary complex (AC) and the Early Cretaceous Zhuravlevka turbidite basin in the southern part of the orogen are considered indicators of subduction continental margin and transform plate boundary regimes, respectively. The regime conversion was assumed at the end of the Jurassic, when subduction stopped. Our biostratigraphic study of radiolarians from siliceous and fine-clastic sedimentary rocks has revealed the latest oceanic deposits and the youngest, Early Cretaceous fragment of the Samarka AC in its less studied northeastern part, which is ascribed to the Anuy tectono-stratigraphic element. The well-preserved radiolarians allow accurate dating of cherts, siliceous mudstone, and mudstone. This and other available biostratigraphic data allow the reinterpretation of the stratigraphy of the accreted oceanic sedimentary rocks. The refined stratigraphy is interpreted as a subsequent change in depositional settings on an oceanic plate, which moves to the convergent plate margin. Cherts accumulated in an oceanic pelagic zone from the Middle Triassic to the Late Jurassic (Early Oxfordian). Siliceous mudstone were deposited in a hemipelagic zone in the early Oxfordian–middle Tithonian. Mudstone and siltstones deposited on an external slope of a deep trench in the late Tithonian–Berriasian. The sandy deposits were deposited in an axial part of the trench in the early Valangian, which best corresponds to the timing of accretion. Thus recognized early Valanginian accretion episode shows that subduction underneath the continental margin lasted longer than previously suggested. The onset of the transform continental boundary regime occurred later, probably, in the late Valanginian. This further details the Mesozoic evolutionary scenario, which was previously proposed for the Sikhote-Alin segment of the East Asian continental margin. We also refine the tectonic zonation and evolution of the Samarka AC.

The geodynamic setting of the Agnie–Afanas’evskaya auriferous rotational structure, genetically related to a transtensional left-lateral duplex zone which was formed against the background of the regional roughly north–south lateral contraction (transpression), is discussed. Regional dislocations are accompanied by local dislocations caused by internal stresses, which account for the details of the structural pattern of the gold deposit. This enables us to forecast auriferous stockwork bodies on a new basis.

Numerous phosphorite occurrences are located among the Riphean–Early Cambrian rocks of the Khingan Group (Lessr Khingan, JAO). They are associated spatially and genetically with uranium, iron, and manganese ore occurrences. The phosphorites of the Gremuchy occurrence are enriched in rare-earth elements (REE) and yttrium (up to 813.58 g/t), uranium (up to 52.23 g/t), and gold (up to 17.03 g/t), and can be considered as a complex of phosphorus–uranium–gold–rare-earth ores. REE-rich fluorapatite is the main concentrator of phosphorus in these rocks. REE do not form their own minerals, while gold, silver, and platinum are found as microinclusions. The concentrations of phosphorus, radioactive, and rare-earth elements are positively correlated with the organic carbon content, which supports their coprecipitation from seawater enriched in rare and rare-earth elements supplied by hydrothermal vents on the surface of the slope of seamount volcanoes.