Home – Home – Jornals – Russian Geology and Geophysics 2022 number 9

For the first time, exposures of ultramafic rocks of subvolcanic origin have been investigated at the foot of Mount Khanlauta (Kola Peninsula) located at a distance of ~2 km from the southwestern margin of the Pados-Tundra layered complex of dunite-harzburgite-orthopyroxenite composition, which hosts zones of chromitite and unconventional PGE mineralization. The ultramafic body is composed of micro- to fine-grained harzburgite and subordinate orthopyroxenite and has a cryptically zoned structure. The body is of E-W strike and has a small size with an apparent thickness of ~0.1 km; the exposed outcrops exhibit a blocky surface as a result of extensive cracking caused by degassing and rapid cooling of a parental komatiitic melt. Elements of columnar parting are recognized, which have a hexagonal shape in cross section and are consistent with the inferred subvolcanic origin. Two zones are identified. Zone I is formed by a more magnesian olivine with Mg# = 86.0-87.9 (0.15-0.21 wt. % MnO). In Zone II, olivine grains are notably less magnesian (Mg# = 81.8-84.1) and invariably have higher contents of manganese (0.19-0.30 wt. % MnO). The presence of this zoning is corroborated by the lateral distribution of accessory chromian spinel grains with maximum Mg# values (>20) in Zone I. The Khanlauta rocks also contain subordinate orthopyroxene (Mg# = 86.3-87.2), amphiboles of the tremolite-actinolite series, and anthophyllite of deuteric (autometasomatic) origin along with accessory minerals: ilmenite, hematite (~15 mol.% escolaite, Cr₂O₃, in solid solution), and mono- and diphase grains of sulfides in the form of intergrowths of Co-bearing pentlandite (Ni/Fe = 0.9-1.3; 1.00-16.74 wt. % Co; up to 1.7-6.8 wt. % Cu) and heazlewoodite (locally in intergrowth with hematite). The inferred front of crystallization moved in the western direction, causing the formation of Zone II from a more fractionated melt with a notably lower Mg# value. A sharp increase in oxygen fugacity locally caused the formation of anomalous parageneses of chromite and ilmenite, in which the observed Mg# values of ilmenite are considerably greater than those of the coexisting chromite. The geochemical whole-rock characteristics based on major, minor, and trace elements (including LILE, REE, and HFSE), as well as the compositions and trends of chromian spinels, are similar to those in differentiated (zoned) sills of the Chapesvara complex, which are closely associated with the Pados-Tundra layered complex. The obtained data indicate comagmatic relationships among the Khanlauta massif, zoned sills of the Chapesvara complex, and the Pados-Tundra layered complex. All of them crystallized from a primitive highly magnesian Fe- and Cr-enriched komatiitic magma (Al-undepleted). Thus, they belong to a single subvolcanoplutonic association being part of the Serpentinite belt-Tulppio belt (SB-TB) megastructure of presumably Paleoproterozoic age.

Mantle xenoliths of fresh sheared and granular peridotites from the Udachnaya kimberlite pipe (Yakutian kimberlite province, Siberia, Russia) are studied in terms of mineralogy, chemistry, and behavior of platinum-group elements (PGE), with a focus on difference between the two types of peridotites and on the role of PGEs in their origin. The analyzed samples of sheared and granular peridotites differ in mineral-liquid equilibrium temperatures and pressures: 1230-1350 °C, 56-67 kbar inferred for the former and 750-1300 °C, 30-67 kbar for the latter. The contents of major oxides, trace elements, and PGEs differ both between sheared and granular peridotites and within each group. Some sheared and all granular peridotites are enriched in incompatible elements, have low percentages of clinopyroxene and contain garnet rich in middle rare-earth elements (MREE). These features result from interaction of the peridotites with melts and related fluids of presumably carbonate compositions containing incompatible elements in high abundances. Sheared peridotites were more strongly enriched and gained more Ca, Al, and Ti than the granular variety. On the other hand, granular peridotites have higher light REE (LREE) and lower heavy REE (HREE) enrichment, which may record compositional difference of metasomatic agents. Metasomatism acted in two stages: first silicate agents caused LREE enrichment and HREE depletion of melts and then the fractionated melts percolated through the mantle and affected the compositions of granular peridotites and, partly, sheared peridotites. The two types of peridotites differ also in PGE patterns, with largely variable Os-Pt concentrations in granular peridotite. This difference may be due to uneven distribution of the Os-Ir and Pt-Os-Ir intermetallic compounds, which formed as sulfide melting residue in the lithospheric mantle. Sheared peridotites show almost uniform Os-Pt distribution and are enriched in Pd, Re, and partly Pt. The enrichment possibly occurred during metasomatism when the percentages of garnet and clinopyroxene increased while sulfide phases precipitated into interstitials.

Based on the analysis of the petrographic and lithogeochemical features of the Middle Jurassic-Lower Cretaceous strata in the lower reaches of the Anabar River, we have studied the regularities of changes in the composition of the upper parts of the Yuryung-Tumus and Sodiemykha Formations and the lower part of the Buolkalakh Formation. It has been established that the silt-sandy rocks of the first and basal beds of the second formation are graywacke arkoses and essentially feldspathic varieties, and most of the Sodiemykha Formation is composed of quartz-feldspathic and scarcer feldspar-quartz graywackes. A chemical classification of the rocks was made; most of them were assigned to normosiallites. The rocks of the marker beds, namely, the Fe-containing deposits of the Sodiemykha Formation, the basal glauconite bed of the Buolkalakh Formation, and the overlying clay bed, were classified as hypohydrolysates. All the studied deposits are of low sedimentary maturity, with essentially petrogenic clastic material. These are predominantly igneous rocks of intermediate and, less, felsic composition. The provenances were characterized by moderate chemical weathering. In the periods of the formation of the marker beds, chemical weathering intensified, and the amount of mafic and, partly, ultramafic rocks increased. The established changes in the composition of the parental strata are observed in the Middle Jurassic-Lower Cretaceous deposits of the entire considered petromineralogical province, which permits them to be used for correlation.

The paper is concerned with a study of the slope deposits being weathering products of rhyolites and their tuffs in the vicinity of Lake Grand (northern Okhotsk area). The samples were divided into grain size fractions of 2500, 250, 140, 100, 63, 40, 20, and 1 µm. Analysis for major and trace elements and mineralogical and petrophysical studies were performed for each fraction. It is shown that a decrease in the fraction size is accompanied by the enrichment of the sediments with Al₂O₃, Fe₂O₃, TiO₂, MgO, Y, Rb, Ni, and paramagnetic minerals and by an increase in LOI. The fine fractions are characterized by low contents of SiO₂ and Na₂O and high values of CIA, PIA, and Rb/Sr. The maximum magnetic susceptibility, saturation magnetization, and contents of CaO, Sr, and Zr are established in the fractions of 40 μm. The values of magnetic susceptibility, J_s , and J_rs decrease in passing from the fractions of 40 μm to the fractions of 1 μm, whereas the values of B_c and B_cr increase. A specific feature of these fractions is high paramagnetic susceptibility and the presence of lepidocrocite. The qualitative composition of ferrimagnetic minerals is the same for the slope deposits and lacustrine sediments. In passing from coarse fractions to fine ones, the contents of quartz and K-feldspar decrease, and the contents of muscovite, biotite, chlorite, and kaolinite increase. The distribution of petromagnetic and geochemical characteristics in the Lake Grand sediments of varying grain size is similar to that in the slope deposits. The complex characteristics of the lacustrine sediments that formed in cold and warm periods are consistent with the distribution of these parameters in various fractions of deluvium and colluvium. Moreover, the fine-grained material acquired specific geochemical, mineralogical, and petrophysical properties when being on the slopes. Comparison of the SiO₂/TiO₂ and Fe₂O₃/TiO₂ ratios of the slope deposits and lacustrine sediments makes it possible to identify the intervals of biogenic and chemogenic sedimentation in water basins.

The paper presents the results of the development and use of numerical methods for interpreting logging data in the Bazhenov Formation interval (Upper Jurassic). We consider wells located in the central and southeastern regions of West Siberia. Based on the machine learning method (artificial neural networks) and with the results of detailed lithological and geochemical core studies taken into account, a computational algorithm has been developed and tested to evaluate the material composition of the Bazhenov Formation rocks. In the studies we use the classification of the Bazhenov Formation lithotypes, which is based on the modern concept of the distribution of rock-forming mineral and mineraloid components. In the examined well sections, the lithologic composition of the Bazhenov Formation rocks has been determined, along with its lateral change in the central part of the Salym field, in the Surgut arch region, and in the southeast of West Siberia.

New insights in the aeromagnetic data over the Central Iranian Microcontinent (CIM) have revealed interesting results for future studies and exploration. This work presents the interpretation of different magnetic analyses and the calculated 3D inversion model to provide important insights into the distribution of igneous rocks in the area that may be traced under significant cover. By analyzing several hundred magnetic susceptibility data points and aeromagnetic anomalies of known igneous rocks over the area, it was determined that mafic-ultramafic intrusive rocks generally have a high magnetic susceptibility and produce a strong magnetic response. Intermediate-felsic intrusive rocks have a low magnetic susceptibility and show a smooth gradient variation and commonly regular shape. Volcanic rocks show a wide range of magnetic susceptibility; therefore, the aeromagnetic anomalies are often random or show strong amplitude with high frequency signals and are rapidly eliminated when an upward continuation is applied. Based on the results of analysis of different magnetic maps and 3D inversion of data, and combining this information with known outcropped of igneous rocks, we revealed 1215 concealed intrusive rocks and 528 volcanic rocks in the area. We also renewed the boundaries of tens outcropped igneous rocks. The known and new mapped igneous rocks can be identified as 12 regions (or zones) for intrusive rocks and 4 regions for volcanic rocks. The results indicate that the mafic-ultramafic rocks are mainly located in the Sistan suture zone of eastern Iran along the Nehbandan fault zone. They also show that the many parts of the Lut block as the main structure of CIM have been under magmatic events, so that most of concealed igneous rocks are distributed in the middle and southern part of the Lut block. Volcanic rocks are widespread in the southeastern and northern parts of the area such as the Urumieh-Dokhtar Magmatic Arc, North Lut, and Bam region.

The latest results on electromagnetic geothermometry and its use in geothermics and geothermal exploration have been covered. Temperature models for the well-known geothermal areas, Larderello-Travale (Italy), Soultz-sous-Forêts (France) and Hengill (Iceland), constructed from electromagnetic sounding data and temperature well logs have been analyzed. 2D temperature model built along the magnetotelluric profile crossing the Travale (Italy) geothermal system allowed to locate a potential supercritical reservoir at the depth between two isotherms, T_SCF ~ 400 °C and T_BDT ~ 600 °C, coinciding with the seismic reflectors, detected earlier. 3D temperature model of the Hengill (Iceland) geothermal area built up to the depth 20 km, using electromagnetic sounding data, allowed us to locate high-temperature magma pockets at shallow depth (2-5 km); these magma pockets can be regarded as targets for geothermal exploration. Joint analysis of the specific resistivity and temperature models, taking into account gravity anomalies, allowed us to locate the zones of different thermal regimes within the crust; this, in turn, explained the observed seismicity pattern by thermo-mechanical properties, rather than by crust spreading. Deep temperature model, built for the Soultz-sous-Forêts (France) geothermal area from magnetotelluric data and temperature well logs, used to locate deep heat sources and predict heat transfer mechanism at depth up to 8 km.