Vol 11, No 6 (2017)

The Kiselyovka-Manoma accretionary complex, formed at the end of the Early Cretaceous, is a part of the Early Cretaceous Khingan-Okhotsk active continental margin. It is located at the front of the Amur accretionary complex and is composed of Jurassic-Lower Cretaceous oceanic volcanic-siliceous deposits. The structural study of this complex in the Lower Amur region hSas made it possible to clarify its overall structure and to subdivide the folds with different morphologies and orientations into five types. The analyzed folding sequence is compared to the folds of the Amur accretionary complex. The fold kinematics indicates various senses of motions that do not reveal systematic kinematic patterns of stacking of the accreted tectonic slices.

Bodies of cryptovolcanic rocks (fluidolites) play a significant role in the structure of the Poperechnoe ferromanganese deposit, Lesser Khingan, Russia. Fe-Mn ores in association with fluidolites are localized in ancient carbonate rocks. PGM, gold, and silver minerals up to tenths of a millimeter in size are established in the rocks and ores of the deposit. Characteristic admixtures in the platinum are iron (9-11%), with less common copper, iridium, and osmium. An intergrowth of isoferroplatinum polycrystal with forsterite is observed. There are also individual grains of Os-Ir-Ru-Rh intermetallides. The gold grains (up to 93% Au) are characterized by dendritic and clumpy morphology, frequently showing faceting elements. The grains are frequently rounded and sometimes have an almost perfect spherical shape. Native silver is distributed among the dolomites, as well as in the dolomitic fragments from the fluidolites. Ubiquitous admixtures in the silver are copper, iron, and more rarely zinc; single grains contain admixtures of molybdenum, nickel, gold, and palladium. PGM and high-fineness gold are related to the fluidolites, which are the only magmatic rocks in the studied range of the deposit. Silver, its minerals, low-fineness gold, and sulfides are confined to the rocks and ores subjected to the regional dolomitization, low-temperature hydrothermal reworking, and silicification. Samples contain up to 11.3 g/t platinum, 2.35 g/t gold, 296.2 g/t silver, which allows us to consider the studied object as economically promising for noble metals.

A great volume of original information on the formation of the ultrabasic rocks of the Siberian Platform has been accumulated owing to the study of melt inclusions in Cr-spinels. The inclusions show the general tendencies in the behavior of the magmatic systems during the formation of the ultrabasic massifs of the Siberian Platform, tracing the main evolution trend of decreasing Mg number with SiO₂ increase in the melts with subsequent transition from picrites through picrobasalts to basalts. The compositions of the melt inclusions indicate that the crystallization conditions of the rocks of the concentrically zoned massifs (Konder, Inagli, Chad) sharply differ from those of the Guli massif. Numerical modeling using the PETROLOG and PLUTON softwares and data on the composition of inclusions in Cr-spinels yielded maximum crystallization temperatures of the olivines from the dunites of the Konder (1545–1430°C), Inagli (1530–1430°C), Chad (1460–1420°C), and Guli (1520–1420°C) massifs, and those of Cr-spinels from the Konder (1420–1380°C), Inagli (up to 1430°C), Chad (1430–1330°C), and Guli (1410–1370°C) massifs. Modeling of the Guli massif with the PLUTON software using the compositions of the melt inclusions revealed the possible formation of the alkaline rocks at the final reverse stage of the evolution of the picritic magmas (with decrease of SiO₂ and alkali accumulation) after termination of olivine crystallization with temperature decrease from 1240–1230°C to 1200–1090°C. Modeling with the PLUTON software showed that the dunites of the Guli massif coexisted with Fe-rich (with moderate TiO₂ contents) melts, the crystallization of which led (beginning from 1210°C) to the formation of pyroxenes between cumulate olivine. Further temperature decrease (from 1125°C) with decreasing FeO and TiO₂ contents provided the formation of clinopyroxenes of pyroxenites. For the Konder massif, modeling with the PLUTON software indicates the possible formation of kosvites from picrobasaltic magmas beginning from 1350°C and the formation of clinopyroxenites and olivine–diopside rocks from olivine basaltic melts from 1250°C.