Lithology and Mineral Resources

The initial normative mineral composition of some metasedimentary rocks (high-alumina crystalline schists, aluminous gneisses, and eulysites) from the Archean Bol’shoi Cheremshan Group is considered. Based on some data (distribution of data points available for these rocks in diagnostic diagrams, values of several indicator ratios, and initial rock composition), it is assumed that examined rocks were partly formed after highly mature sediments (high-alumina crystalline schists and gneisses), while biotite-garnet-andesine gneisses represent products of graywacke sediments. Elevated concentrations of some indicator elements (Fe, Cr, Ni, V, Zn, Pb, and others) imply that material in the sedimentation paleobasin was likely derived from provenances composed of variable lithologies (ultramafic, mafic, intermediate, and acid rocks). It seems that conditions in this paleobasin were favorable for the development of organic life. It is shown that correlation between some elements (Ti, V vs. Al; Ba, Pb vs. K; Sr vs. Ca; La vs. Na; and others) typical of Phanerozoic sedimentary rocks is also observed in the rocks. The possible contents of organic matter (Corg) and U in primary sediments of metasedimentary rocks of the Bol’shoi Cheremshan Group in the East European Platform basement are reconstructed.

Issue of the influence of eolian processes on the formation of gold placers in the Tuva region is considered. It has been revealed that the formation of placers is related not only to hydrodynamic processes but also to eolian processes, which are reflected in the formation of relief, redistribution of metal, and reworking of the useful component. Study of typomorphic features of the eolian gold and regularities of its redistribution revealed the possibility of discovery of a new type of placer gold (eolian and heterogeneous placers).

New data on geochemical features of the Lower Paleozoic terrigenous rocks in the Mamyn terrane (eastern Central Asian Fold Belt) and U–Pb geochronological studies of the detrital zircon from these rocks are presented. The obtained results suggest the following conclusions. 1. At present, the Kosmataya sequence includes different age Lower Cambrian terrigenous–carbonate and Lower Ordovician terrigenous rocks or represents Lower Ordovician olistostromes including limestone blocks with the Lower Cambrian fauna. Lower Ordovician terrigenous rocks were formed in an island arc or active continental margin, mainly, owing to the erosion of Cambrian–Early Ordovician plutons and volcanics that are widespread in structures of the Mamyn terrane and weakly reworked by the chemical weathering. 2. The Silurian Mamyn Formation was developed at a passive continental margin. The main sources of clastic material for this formation were the same Cambrian–Early Ordovician igneous rocks as for the Cambrian sequence, with the participation of Early Silurian and Vendian igneous complexes. The obtained data significantly refine concepts about the geological structure of the Mamyn terrane, which is a member of the Argun Superterrane, one of the largest tectonic structures in the eastern Central Asian Fold Belt.

The results of study of geochemistry of terrigenous rocks from the contrast (in structure) Cretaceous-Paleogene complexes of Sikhote Alin and Kamchatka are summarized. The data obtained were interpreted based on comparison with the geochemical composition of recent and ancient sediments accumulated in the well-known geodynamic settings. It is shown that the chemical composition of terrigenous rocks and some petrochemical ratios can serve as reliable indicators of various island-arc settings. These indicators make it possible to discriminate sufficiently reliably these settings in paleobasins of orogenic zones.

The origin of friable sediments blanketing the Barents Sea shelf is considered. It is shown that their characteristic seismoacoustic record patterns reflect low degree of diagenetic transformations and indicates continuous sedimentation. According to traditional views, this single sedimentary complex also includes diamicton, and the section is interpreted as a three-unit structure: diamicton, which is considered a till; the overlying friable sediments accumulated under different conditions of deglaciation in glaciomarine settings; and the postglacial marine sediments. It is demonstrated that such views are inconsistent with geomorphologic features (datings by physical methods included) indicating a prolonged hiatus that separates epochs of the diamicton accumulation and formation of friable sediments. The analysis revealed that the composition, vertical succession, and lateral distribution of different lithological types of friable sediments are related to the regular spatiotemporal replacements of different facies settings in the transgressing Arctic sea rather than by the glacial process. This inference is confirmed by the composition of foraminiferal assemblages.

The composition of Aptian–Albian rocks in the western Ciscaucasus is considered. Their lithofacies features and formation conditions are established. The data obtained can be used to determine the mineralogical potential of terrigenous complexes of the western Ciscaucasus.

Cyclic-facies analysis of stratified volcanic sequences in the ore-controlling depressions makes it possible to recognize the recurrent and genetically related sets of rock layers that make up micro-, meso-, and mega-scale eruptive cycles (elementary cycle, mesocycle, and megacycle). Massive sulfide ores occupy a specific position in geological sections. They are confined to the upper portions of elementary eruptive cycles and hosted in volcanosedimentary units, indicating their formation during the periods of waning volcanic activity. The elementary cycles are not all accompanied by ore mineralization. The mineralization is most complete in the upper elementary cycles of each eruptive meso- and megacycles.

Nummulite facies are known to be a good indicator of Eocene shallow-water paleoenvironments. Nummulitic limestones are widespread in the Lower Eocene of Armenia—the Tethys/northern Peri-Tethys transition zone, but they are studied insufficiently so far. The objective of this work was an integrated study of Lower Eocene deposits in the typical Urtsadzor section (Ararat region, Armenia): detailization of the stratigraphic subdivision based on larger benthic foraminifers (LBF); restoration of sedimentation settings based on the facies analysis and tectonic conditions of the basin origin, and evolution in the Early–Middle Eocene by studying the rock fracture system and formational analysis. For the first time, the Ypresian shallow benthic zones SBZ 9-10 and SBZ 10-11 of the Tethyan Chronostratigraphic Scale were identified in Kotutz and Sevan formations—an important contribution to the development of a modern stratigraphic chart of Armenia. Restoration of depositional settings based on the microfacies analysis was accomplished for Lower Eocene carbonate rocks of Armenia for the first time. Nine microfacies were determined and interpreted in the Ypresian part of the Urtsadzor section. Evolution of paleobiocoenoses represented mainly by nummulites, orthophragmines, and red algae was traced through the section. A transgressive succession of microfacies types was revealed from the littoral to lower–middle parts of the middle ramp. Comparison of the section with the adjacent Shagap and Landzhar sections allowed us to estimate the facies variability of rocks and the main direction of transgressional expansion. Ypresian LBF in paleobiocoenoses of southern Armenia are similar to the Peri-Tethyan varieties at the generic and specific level, and distinguished from the Tethyan biocoenoses by the absence of porcelaneous foraminifers (alveolinids and soritids). The South Armenian Ypresian paleobiota is distinguished from the assemblages in northern Armenia and northern Peri-Tethys by the abundance of red algae remnants. Wide Late Ypresian transgression resulted in the proliferation of LBF in the shallow oligotrophic basins, and their hydrological differences provoked the diversification of biocoenoses. The study of jointing systems in nummulitic limestones healed with aluminous products of karstification demonstrated that the jointing was formed at the Early/Middle Eocene transition due to the northeastern to sublatitudinal tension during uplift of the southern rift shoulder of the emerging Middle Eocene Shirak–Sevan–Megri rift. Rifting was preceded by the Early Eocene attenuation of tectonic activity and expansion of a wide carbonate sedimentation basin with development features typical of pre-rift basins.