Lithosphere (Russia)

Research subject. Zircon of sandstones of the Basu Formation, Asha Series, Vendian, in the reference section along the Kukrauk stream (Southern Urals). Aim. To determine the age of rocks in the provenance based on U-Th-Pb (LA-ICP-MS) dating of zircon clastics. Materials and methods. In terms of color, detrital zircon grains were divided into three groups. Pale pink grains predominate (about 50%), and pink and colorless grains are present in equal proportions (about 20–30%). Zircon is mainly represented by rounded grains and single grains of a prismatic shape. According to the cathodoluminescence data, most grains retain traces of zonation. Results. The U-Th-Pb concordant ages dates of 166 detrital zircon grains are predominantly in the time intervals of 996–1029, 1079–1110, 1152–1191, 1200–1234, 1250–1324, 1331–1370, 1416–1438, 1447–1557, 1573–1666, 1756–1806, 1824–1874, 1889–1979, 1987–2015, 2022–2074, and 2661–2729 Ma. Individual grains have concordant ages of 579, 776, 2120, 2142, 2148, 2190, 2763, 2874, 2804, 2816, 2889, 2957, 3014, and 3203 Ma. Conclusions. The group of pink grains is dominated by a population of zircon of the Early Karelian and Archaean age, in colorless grains – Early Riphean, and in pale pink grains – Early and Middle Riphean. Among the grains of detrital zircon from the sandstones of the Basu Formation, there are zircons from local Ural sources (776, 1350–1800,   2000–3200 Ma). For clastic zircon with ages of 996–1320 Ma, the sources of demolition among the local feeding provinces have not been identified; however, they are known within the Sveko-Norwegian area in the north-west of the East-European Platform. This allows us to consider the igneous rocks of the Grenville (950–1220 million years) Sveko-Norwegian orogen as sources of zircon clusters over this time interval. The source of zircons with a dating of 579 Ma, closeto the age (573–577 Ma) of zircons from tuff layers in the Basu Formation itself, could be ash material from the explosive activity of volcanoes

Research subject. Seawater, basalts, and products of their transformation. Aim. To assess the behavior of chemical elements, mineral assemblages, and mineral formation conditions during low-temperature seawater–basalt interaction, including the additional input of dissolved CH4 and CO2 to the system. Method. Physicochemical modeling of seawater–basalt interaction was conducted using the Selektor software in closed systems based on changes in the ξ = –lg(seawater–basalt – Sw/Bs) parameter. Results. According to the conducted physicochemical modeling of seawater–basaltic glass inter action (closed system), quartz, goethite, celadonite, chabazite, manganite, and gibbsite are precipitated at the fluid-dominated part of the model (ξ> 3) under oxidizing conditions. An increase in the relative amount of reacted basalt (ξ < 3) leads to a decrease in the Eh value and the replacement of goethite by hematite and magnetite in assemblage with pyrite, saponite, chlorite, and zeolites. The addition of CH4 to the system during early diagenesis under slightly alkaline (pH≈ 10) and reducing conditions (Eh < 0) results in the formation of brucite, chlorite, chrysotile, and pyrite at low Fe concentrations in solution and the absence of quartz, goethite, and manganite. During late diagenesis under alkaline conditions (pH > 10), a significant Si and low Fe amount passes to the solution, while pyrite and magnetite dominate in the system in addition to saponite, chlorite, celadonite, chrysotile, and zeolites. The contribution of CO 2 (1 mole/L) to the system significantly changes the model; thus, only chalcedony is precipitated at the early stages (ξ > 5) under acidic (pH < 3) oxidizing (Eh = 1) conditions. At reduced Eh values under acidic conditions (ξ = 2–3), the high Fe and Al content passes to the solution and strongly decreases under neutral and slightly alkaline (pH > 8) reducing conditions of late diagenesis. At the same stage, Mg silicates, magnetite, pyrite, and hematite are dominant; however, the Fe oxides do not form economic concentrations   in solid reaction products. Conclusions. In general, our results correspond to natural diagenetic products of basaltic glass.

Research subject. Primary platinum-group minerals from the gold placer of the Bolshoy Sap River (Middle Urals) in the southern frame of the Pervomaisk ophiolite-type massif. Methods. The chemical composition of minerals was studied by scanning electron microscopy (JEOL-JSM6390LV) and electron microprobe analysis (Cameca SX 100). The sulfur isotopic composition of laurite and erlichmanite grains was determined using a laser femtosecond ablation system (NWR Femtosecond UC with laser Pharos 2mJ-200-PPam and harmonics module HE-4Hi-A) attached to a MAT-253 mass spectrometer (Thermo Fisher Scientific). Results. A wide species composition of primary platinum-group minerals was revealed, represented by native minerals of the Os-Ir-Ru (osmium, iridium, ruthenium, rutheniridosmine) and Pt-Fe (by stoichiometry close to the composition of isoferroplatinum) systems, as well as Ru-Os sulfides (laurite, erlichmanite). Iridium grains contain isoferroplatinum lamellae, which are a product of solid solution decomposition, as well as the inclusions of cuproiridsite, Ru-bearing pentlandite, kashinite, and tolovkite. Inclusions in isoferroplatinum are represented by braggite, rhodium and palladium sulfides (Pd-Rh-S), and Pd-bearing (5.78 wt % Pd) native gold. Variations in the composition of natural hexagonal Os-Ir-Ru alloys reflect the presence of three trends (i.e., ruthenium, osmium-iridium, and osmium-ruthenium). The sulfur isotopic values of laurite and erlichmanite grains ((1.0–2.5) ±0.2‰) are consistent with derivation of sulfur from a sub-chondritic source, reflecting a minor contribution of crustal sulfur during mantle-crustal interaction processes. The prevalence of primary platinum-group minerals in placers from various platinum-bearing zones of the Middle Urals was analyzed. In the western Serov-Nevyansk zone, Os-Ir-Ru alloys of osmium-iridium and ruthenium trends are common, as well as Pt-Fe minerals of the tetraferroplatinum series PtFe – tulameenite PtFe0.5Cu0.5 – ferronickelplatinum PtFe0.5Ni0.5. Os-Ir-Ru alloys of the osmium-ruthenium trend were established only in the eastern Salda-Sysert and Alapaevsk zones. Os-Ir-Ru alloys of ruthenium and osmium-iridium trends, native iridium and isoferroplatinum are widespread. Conclusions. The wide species composition of primary PGMs in the placer is due to the polygenic nature of chromitites, which is typical of ophiolite massifs in the Middle Urals. The high-temperature Os-Ir-Ru alloys of the ruthenium trend, as well as Os-Ru sulfides, are associated with laterally secreted chromites in the dunite-harzburgite complex. Metasomatic and reactive metasomatic chromitites in the dunite-verlite-clinopyroxenite complex serve as sources of natural Os-Ir alloys of the osmium-iridium trend and Pt-Fe alloys. The highest temperature Os-Ir-Ru alloys of the ruthenium trend, as well as Os-Ru sulfides, are associated with lateral secretion chromitites in the dunite-harzburgite complex. Metasomatic and reaction-metasomatic chromitites in the dunite-wehrlite-clinopyroxenite complex serve as bedrock sources of natural Os-Ir alloys of the osmium-iridium trend, and Pt-Fe alloys. The most likely reason for the appearance of the osmium-ruthenium trend in the chemical composition of natural hexagonal Os-Ir-Ru alloys is the recrystallization of primary high-temperature solid solutions during metamorphic transformations at lower temperature conditions and the change of the oxidative regime to a reducing regime.

Research subject. The aposkarn serpentinites of the Klara mine in the Pitkäranta mining district. Aim. Determination of mineral formation environments for serpentinites of the Klara mine. Materials and methods. In total, 45 rock specimens were studied using optical and scanning electron microscopy, electron probe analysis, powder X-ray diffraction, in frared spectroscopy, and differential thermal analysis. Results. Skarn diopside is replaced by antigorite, lizardite, chrysotile and talc, which intergrown in many cases. The forsterite skarn zone is transformed into chrysotile-antigorite serpentinites with humite-group minerals that are replaced by late lizardite. All serpentine is enriched with F; the concentration of this halogen ranges 0.7–1.8 wt % in lizardite from pseudomorphs after diopside and humite-group minerals, 2.1–3.0 wt % in chrysotile-antigorite and antigorite aggregates, and 2.5–4.6 wt % in serpentine filling cracks. Other minerals are represented by magnetite, fluorite, micas of the phlogopite-fluorophlogopite series, annite, chlorites, Mn- and Fe-containing dolomite, fluorapatite, sphalerite, and pyrophanite. Conclusions. Aposkarn serpentinites of the Klara mine were formed during two stages. (1) Predominantly lizardite serpentinites appeared during the late stages of the regressive skarnification process associated with the intrusion of early granites of the Salmi Batholith, as a result of hydration of forsterite and, partly, diopside. (2) The Li-F granite intrusion caused the re-development of the pneumatolyte-hydrothermal process. The influence of ≈300–480°C F-rich fluids led to the replacement of lizardite by antigorite and chrysotile with a high concentration of fluorine. With a subsequent decrease in temperature, late lizardite was formed due to the preserved skarn diopside and minerals of the humite group.