Based on the isotopic-geochemical analyses of zircons from granites of the Belokurikhinsky massif in the Gorny Altai using the U-Pb method, the ages of three intrusion phases have been determined for the first time: the age of the first phase refers to the time interval of 255-250 Ma, the second and the third phases have similar ages of about 250 Ma. The formation time of the Belokurikhinsky massif is estimated as not exceeding 5-8 Ma. The δ18O values for zircons from granites of the second and the third intrusion phases average around 11.5-12.0 ‰, indicating a significant contribution of a crustal component in the formation of the parent melts for granites of these phases. The crystallization temperature values of the zircons by the Ti-in-zircon thermometer for three phases range from 820 to 800 °C. The P-T crystallization parameters of titanite from the first phase, determined using a titanite thermobarometer, average around 770 °C and 2.7 kbar. The zircons from the first phase mostly exhibits geochemical characteristics of typical magmatic zircons. The zircons from the second and the third intrusion phases either may be unaltered magmatic zircons or enriched in incompatible elements (LREE, Th, U, Ti, Ca, etc.) due to fluid influence, resembling hydrothermal-metasomatic type zircons in terms of their geochemical characteristics. A number of zircon grains from the second and the third phases of granites demonstrate anomalous geochemical characteristics – the REE distribution spectra atypical for zircons (including “bird's wing” type spectra with oppositely tilted of light and heavy REE distribution profiles), as well as significantly higher contents of certain trace elements compared to other varieties. Such an enriched zircon composition and wide variations in the incompatible element content are due to non-equilibrium conditions of zircon crystallization and evolution of the fluid-saturated melt composition during the final stages of the massif formation.
A study of the trace element composition of beryl varieties (469 SIMS analyses) was carried out. Red beryls are distinguished by a higher content of Ni, Sc, Mn, Fe, Ti, Cs, Rb, K, and B and lower content of Na and water. Pink beryls are characterized by a higher content of Cs, Rb, Na, Li, Cl, and water with lower content of Mg and Fe. Green beryls are defined by the increased content of Cr, V, Mg, Na, and water with reduced Cs. A feature of yellow beryls is the reduced content of Mg, Cs, Rb, K, Na, Li, and Cl. Beryls of various shades of blue and dark blue (aquamarines) are characterized by higher Fe content and lower Cs and Rb content. For white beryls, increased content of Na and Li has been established. Principal Component Analysis (PCA) for the CLR-transformed dataset showed that the first component separates green beryls from other varieties. The second component divides pink and red beryls. The stochastic neighborhood embedding method with t-distribution (t-SNE) with CLR-transformed data demonstrated the contrasting compositions of green beryls relative to other varieties. Red and pink beryls form the most compact clusters.
The special geological issue of the Journal of the Mining Institute includes articles discussing the problems of domestic geological science and reproduction of mineral resources of the country.
The article presents original data of chemical composition of tholeiitic basaltoids and andesites, dredged from the Shaka Ridge (South Atlantic) in the course of field research in spring 2016 on the scientific expedition vessel “Akademik Fedorov”. The analytical part of the work on estimating the contents of petrogenic, trace and rare-earth elements was carried out using the classical method (“wet chemistry”), X-ray fluorescence analysis (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). The studied samples demonstrate elevated concentrations of large-ion lithophile elements, or LILE, (Ba, Rb, Pb) and light rare earth elements, or LREE, (La, Ce, Nd, Sm) relative to high field strength elements, or HFSE, (Nb, Ta) and heavy rare earth elements, or HREE, (Dy, Yb, Lu). The specifics of trace element geochemistry suggest a significant contribution of crustal or subduction components to the magmas of the Shaka Ridge. Discrimination diagrams of basaltoids and allied rocks with fields of different geodynamic settings indicate that they were formed in the setting of the mid-ocean ridge basalt (MORB). The reason behind the appearance of subduction and crustal marks in the rocks is possibly associated with assimilation of crustal matter by magmas or lies in their inheritance from the mantle source.
This paper presents a complex mineralogical and geochemical characteristic (based on SEM-EDS, ICP-MS analysis) of the fahlband rocks of the Kiv-Guba-Kartesh occurrence within the White Sea mobile belt (WSMB ). The term “fahlband” first appeared in the silver mines of Kongsberg in the 17th century. Now fahlbands are interlayers or lenses with sulfide impregnation, located in the host, usually metamorphic rock. The level of sulfide content in the rock exceed the typical accessory values, but at the same time be insufficient for massive ores . Fahlbands are weathered in a different way than the host rocks, so they are easily distinguished in outcrops due to their rusty-brown color. The studied rocks are amphibolites, differing from each other in garnet content and silicification degree. Ore mineralization is represented mainly by pyrrhotite and pyrite, and pyrrhotite grains are often replaced along the periphery by iron oxides and hydroxides, followed by pyrite overgrowth. At the same time, the rock contains practically unaltered pyrrhotite grains of irregular shape with fine exsolution structures composed of pentlandite, and individual pyrite grains with an increased Ni content (up to 5.4 wt.%). A relatively common mineral is chalcopyrite, which forms small grains, often trapped by pyrrhotite. We have also found single submicron sobolevskite and hedleyite grains. The REE composition of the fahlband rocks suggests that they are related to Archean metabasalts of the Seryakskaya and Loukhsko-Pisemskaya structures of the WSMB, rather than with metagabbroids and metaultrabasites common in the study area.
The article presents the results of studying the rocks of the pyroclastic facies of the Mriya lamproite pipe, located on the Priazovsky block of the Ukrainian shield. In them the rock's mineral composition includes a complex of exotic mineral particles formed under extreme reduction mantle conditions: silicate spherules, particles of native metals and intermetallic alloys, oxygen-free minerals such as diamond, qusongite (WC), and osbornite (TiN). The aim of the research is to establish the genesis of volcaniclastic rocks and to develop ideas of the highly deoxidized mantle mineral association (HRMMA), as well as to conduct an isotopic and geochemical study of zircon. As a result, groups of minerals from different sources are identified in the heavy fraction: HRMMA can be attributed to the juvenile magmatic component of volcaniclastic rocks; a group of minerals and xenoliths that can be interpreted as xenogenic random material associated with mantle nodules destruction (hornblendite, olivinite and dunite xenoliths), intrusive lamproites (tremolite-hornblende) and crystalline basement rocks (zircon, hornblende, epidote, and granitic xenoliths). The studied volcaniclastic rocks can be defined as intrusive pyroclastic facies (tuffisites) formed after the lamproites intrusion. Obviously, the HRMMA components formed under extreme reducing conditions at high temperatures, which are characteristic of the transition core-mantle zone. Thus, we believe that the formation of primary metal-silicate HRMMA melts is associated with the transition zone D".
The article presents the results of a study of the composition of xenoliths of mantle peridotites (seven samples), collected from the Quaternary basalts of the Sverre volcano, the Svalbard archipelago. The presence of two big (more than 15 cm in diameter) xenoliths of spinel lherzolite allowed us to consider a change in their composition in the cen- tral, intermediate, and marginal parts of the samples. It is proposed to distinguish three types of xenoliths by the distribution of trace and rare earth elements. Enrich- ment of mantle peridotites with light rare earth elements, as well as high field strength (HFS) and large-ion lithophile (LIL) elements, is presumably associated with mantle metasomatism.
A study on the U-Pb age of rutile from the Ichetju polymineral occurrence has been done for the first time by LA-ICP-MS and TIMS methods. It was established that rutile originates from various sources with different ages (presumably, ca. 1000, 1660, 1860 and 1980 Ma), but all the rutile types have undergone a common thermal event at ca . 580 Ma. Obtained results are consistent with U-Pb zircon data for the Ichetju occurrence and the Pizhemskoe deposit. According to modern concepts, the closure temperature for the U-Pb system in rutile is higher than 500 ° С, which suggests fairly high-temperature conditions of the rutile hydrothermal transformation during the formation of the deposits in Riphean. Obviously, a placer hypothesis of formation of titanium deposits of the Middle Timan which is supported by a number of researchers does not explain such temperature of rutile alteration.
The paper presents the results of the isotope, geochemical and thermobarometric study of plagio-crystalline schist containing in the Upper Anabar series of the Anabar Shield. Granulite complexes of the paleoplatforms are the most important issue in addressing the fundamental problem of the Earth's crust origin and its composition. The early stages of crust formation which correspond to the deeply metamorphosed rocks of the platform basements, available for study within the shields, are of particular interest. The study of the age and metamorphic conditions of granulites by the case of the Upper Ananbar series allows specifying the stages the Anabar Shield's ancient crust formation. Isotope-geochemical (U-Pb geochronology for zircon and Sm-Nd for garnet-amphibole-WR) and thermoba-rometric (Theriak-Domino) studies of plagio-crystalline schist allowed to identify two Paleoproterozoic metamorphism stages within the territory of the Anabar Shield with an age of about 1997 and 1919 million years. The peak conditions of granulite metamorphism are determined as 775±35 С and 7.5±0.7 kbar, the parameters of the regressive stage are 700 C and 7 kbar. The sequence of the rocks metamorphic transformations can be assumed: high-thermal metamorphism of the granulite facies (T ≤ 810 C) and subsequent sub-isobaric (about 7 kbar) cooling to 700 C with a water activity increase and formation of Grt-Amp paragenesis corresponding to the transition from the granulite to amphibolite facies. Data on the REE and other trace elements distribution in zircon and rock-forming minerals obtained by the ion microprobe analysis contribute significantly to the isotope-geochemical data interpretation.
The paper presents the results of a study (LA-ICP-MS method) of spinel from the collection of mantle xenoliths of lherzolites (seven xenoliths) selected in quaternary alkaline basalts of the Sverre volcano, the Spitsbergen archipelago. The study of two large (more than 15 cm in diameter) xenoliths made it possible to study changes in the composition of minerals in the central, intermediate, and marginal parts of the samples of chromium diopside spinel lherzolites. The sinusoidal character of the REE distribution in spinels, which indicates the manifestation of mantle metasomatism, is established. The results obtained for the first time on the trace-element composition for spinels from mantle xenoliths in alkaline basalts of the Spitsbergen archipelago are supplemented by data on the geochemistry of spinels of mantle origin published in the world literature.
Results of isotope-geochemical studies by PbLS step-leaching method of cassiterite from greisens located in Logrosán granite massif (Central Iberian Zone, Spain) and apatite from hydrothermal quartz-apatite vein on its exocontact indicate that in both cases a hydrothermal event is recorded in the interval of 114-126 Ma, which has been accompanied by lead supply. Within the limits of estimation error, the same age around 120 Ma corresponds to crystallization of hydrothermal apatite, formation of sticks and micro-inclusions in cassiterite from greisens and is suggested for Au-As-Sb-Pb ore mineralization, which calls for further confirmation. Xenogenous zircon from quartz-apatite vein does not react to this relatively low-temperature hydrothermal event either with building up new generations (sticks, areas of recrystallization) or with rebalancing of U-Pb isotope system. The age of greisen formation has been confirmed to be around 305 Ma by PbLS method on final phases of cassiterite leaching. Earlier it was estimated with 40 Ar/ 39 Ar method on muscovite.
A detailed isotopic geochemical study (secondary ions mass-spectrometry – SIMS, time-of-flight mass-spectrometry – TOF) of zircon from ore-bearing syenites of the Yastrebetskoe rare metal-rare earth deposit (the Ukrainian Shield) has yielded proofs of magmatic genesis of the deposit: unaltered central parts of zircon grains typically feature characteristic magmatic spectra of REE distribution, their values of δ 18 O staying somewhat higher than the mantle value (6.5 ‰, on the average). During the final stage of forming the deposit the role of fluorine-water-bearing fluids enriched with Y, REE, Nb, Ве and heavy oxygen had increased, as directly reflected in the anomalous isotopic-geochemical characteristics of zircon rims and zones of zircon alteration (the contents of Y reaches 61874 ppm, that of Nb – 7976 ppm, Be – 1350 ppm, δ 18 O reaches 12.42‰, F – 0.7 % mass, H 2 O – 4% mass).
