Home – Home – Jornals – Russian Geology and Geophysics 2012 number 2

We present for the first time the mineralogical and isotope-geochemical pecularities of zircon and baddeleyite from various rocks of the economic ore-bearing Noril'sk-1 intrusion located in the northwestern part of the Siberian Platform. The ultramafic-mafic Noril'sk-1 intrusion hosts one of the world's major economic platinum-group-element(PGE)-Cu-Ni sulphide deposits.
A detailed study of crystal morphology and internal structure identify four zircon populations characterized by different U-Pb (SHRIMP-II) ages. The U-Pb ages of baddeleyite and the defined zircon populations cover a significant time span (from 290 ± 2.8 to 226.7 ± 0.9 Ma). The established U-Pb ages imply that crystallization of baddeleyite and zircon populations corresponds to several stages of protracted evolution of the ore-forming magmatic system (290 ± 2.8, 261.3 ± 1.6, 245.7 ± 1.1, 236.5 ± 1.8, and 226.7 ± 0.9 Ma, respectively) that served as a favorable factor for the accumulation of magmas and ores of unique scales and concentrations.

Along with eclogitized gabbro and gabbronorite bodies, boudinaged metaultramafites such as garnet-pyroxene rocks and orthopyroxenites were revealed in the Archean plagiogneiss strata of the Gridino complex. The garnet-pyroxene rock crope out as a boudin on Vysokii Island. The early stage of the rock evolution is documented by inclusions of diabantite (Fe-Si chlorite), a mineral that occurs in metasomatized peridotites. Diabantite was found in all rock-forming minerals in paragenesis with mineral phases enriched in REE (Ce, Nd, La, etc.), U, and Th. The confinement of ore phases to the inclusion rims and the development of two systems of cracks, radial and concentric, around the inclusions in pyroxenes point to the transformation of the inclusions after their trapping. Thermobarometric studies of the crystal cores revealed that the anhydrous paragenesis garnet + orthopyroxene + clinopyroxene, which replaced the chlorite-bearing rock, formed at ~690 ?C and ~17 kbar. The rims of the rock-forming minerals reflect isothermal decompression to ~12 kbar, which was followed by decompression cooling to ~650 ?C and ~9 kbar with the formation of regressive amphibole-garnet-pyroxene paragenesis. The giant-grained orthopyroxenites compose chains of boudinaged bodies on Izbnaya Luda Island. The orthopyroxene crystals host abundant amphibole, quartz, biotite, and pyrite inclusions pointing to amphibolite metamorphism at the early stage of the rock evolution. There are two types of amphibole: magnesian hornblende and anthophyllite. The hornblende is a primary mineral, whereas the low-temperature anthophyllite forming rims around the quartz inclusions was produced at the regressive stage of metamorphism. There are no indicators of the PT -conditions of the peak metamorphism in the orthopyroxenite. The reaction enstatite + quartz + H₂O = anthophyllite allows deciphering water activity of anthophyllite formation, a ≤ 0.5.

The Primorskii complex in western Cisbaikalia, formed in the Early Proterozoic at the postcollisional stage of the Siberian craton evolution, comprises rapakivi granites, equigranular biotitic and leucocratic granites, and alaskites. It is a K-rich granitoid assemblage with medium and elevated alkalinity, whose F, Ba, Pb, REE, Zr, Th, and Zn contents exceed the clarkes. The complex consists of three plutons: Bugul'deika-Anga, Ulan-Khan, and Trekhgolovyi, which formed in two intrusive stages. The evolution of the major-stage composition was marked by an increase in silica content, with a similtaneous increase in agpaite and Fe contents and a decrease in Na₂O/K₂O. The Bugul'deika-Anga and Trekhgolovyi plutons are the most contrasting in composition and crystallization conditions. The former originated from a weakly differentiated water-undersaturated melt, which crystallized at medium depths (P _tot = 3-4 kbar). The crystallization was not accompanied by considerable accumulation of granitophile elements (the concentration index (CI) of granitophile elements is ~3) in the leucogranites, except the alaskites, which crystallized in the upper part of the magma chamber (CI = 5). The Trekhgolovyi pluton originated from a leucogranitic melt enriched in Cs, Li, Rb, and Sn, which crystallized at a low P _tot (~2 kbar). The average contents of some elements in the leucogranites are higher than their clarkes in Ca-poor granites: by a factor of 4 for Sn, 3.8 for Th, 2.7 for Rb, 2.5 for Cs, and 2 for F (CI ? 9). The final-stage granites in the Trekhgolovyi pluton are associated with quartz-muscovitic (±topaz, fluorite) greisens, which contain cassiterite, columbite, ilmenorutile, wolframite, bismuthinite, and other minerals. The data suggest that the Trekhgolovyi pluton has a Sn potential.

We present results of U-Pb (SHRIMP II) geochronological study of the rocks of the Mukhal alkaline massif in the Vitim alkaline province, western Transbaikalia. The available K-Ar and Rb-Sr dates for the alkaline rocks (Saizhen complex) of the Vitim province, including the Mukhal massif, vary over a broad range of values. The obtained age of crystallization of the Mukhal urtites refines the time when the regional alkaline magmatism began. The age of zircons and magmatic processes within the Barguzin area (315-275 Ma) is close to the peak of main events, which occurred between 295 and 275 Ma. These processes took place at the early stage of evolution of the Late Paleozoic rift system in Central Asia, whose activity was associated with the activity of mantle superplume.

We present results of lithofacies, reservoir, geochemical, well logging, and petrophysical studies of the key section of the Vendian-Lower Cambrian Preobrazhenka productive horizon in the Lena-Tunguska province. We have considered the composition, structure, and formation conditions of the deposits as well as the intensity of postsedimentation processes and the rock geochemistry, petrophysics, and reservoir properties.

The resistivity pattern at the site of the "Crystal" underground nuclear explosion (Daldyn-Alakit district of Yakutia) of 1974 which led to an accident has been imaged using TEM data. The local background pattern corresponds to a three- or four-layer earth with a conductor at the base. The uppermost layer, with a resistivity of tens to hundreds of ohm·m, has its bottom at 190-260 m asl and consists of perennially frozen Late Cambrian carbonates. The resistivity structure of shallow subsurface at the blast epicenter remained unperturbed, though being subject to mechanic and thermal effects. The bottom of the second layer is at 20 to 190 m below the sealevel, and its resistivity is 7-10 ohm·m. It is composed of frost-bound and unfrozen cold rocks that belong to a Late Cambrian water-bearing sequence (an aquifer). The third and fourth layers make up the conducting base of the section (0.2-1.4 ohm·m), while the conductor's top matches the table of a Middle Cambrian aquifer. Anomalous transient response at the site prompts the existence of a local conductor possibly produced by highly saline waters in the containment cavity and in deformed rocks around it. However, the resistivity is too low (0.02 ohm·m) to be accounted for by any model available at the present state of knowledge. Another problem is to explain how the brines circulating at large depths might have reached the explosion cavity and the surrounding strained zones. The study has provided the first idea of the background resistivity distribution and its UNE-induced changes.

The structure of the Earth's crust at the junction of the Siberian craton and Sayan-Baikal Fold Belt was studied along the Bayandai Village-Cape Krestovskii profile (85 km long) by a set of geological and geophysical methods: structural survey, interpretation of long-distance photographs, emanation survey, electrical prospecting with self-potential (SP) and direct-current (DC) resistivity profiling, magnetotelluric sounding, magnetic survey, and hydrogeochemical sampling of water objects. Interpretation of the data refined the main features of the tectonic structure of western Cisbaikalia and revealed the disruption pattern and hierarchic zone-block structure of the Earth's crust. The Obruchev fault system (~50 km wide), which is the northwestern shoulder of the Baikal Rift, is the main interblock zone of the studied region. It consists of the Morskoi, Primorskii, and Prikhrebtovyi interblock zones, traced from depths of tens of kilometers and widening near the surface owing to superior structures. The studies gave an insight into the regularities in the occurrence of interblock zones and the criteria for their identification in different geologo-geophysical fields. An efficient complex of methods for mapping the Earth's crust zone-block structure is proposed.

Samples of magnetite ores of different parageneses and mineral compositions from the Goroblagodatskoe skarn-magnetite deposit were studied by physical and mineralogo-petrographic methods. Electrical-resistance curves were obtained for an ore containing 80-90% magnetite for the temperature range 20-800 ?C. Three groups of samples were recognized according to the pattern of temperature curves and electrical resistance: sulfide-free, sulfide-containing (pyrrhotite), and variolitic ores.
The parameters of high-temperature conductivity (activation energy E ₀ and coefficient of electrical resistance lgR₀) have been determined. A linear relationship between E ₀ and lgR₀ (lgR₀ = a - bE ₀) has been established for magnetite ores of all mineral types. The coefficient a varies from 1.92 to 4.80 depending on the type and mineral composition of magnetite ores. The coefficient b is nearly constant for all studied samples, 6.65. The figurative points of the samples in the E ₀ - lgR₀ coordinates lie in the field bounded by the straight lines lgR₀ = 4.80 - 6.65 E ₀ and lgR₀ = 1.92 - 6.65 E ₀.
The samples of magnetite ores of the skarn and hydrosilicate paragenesis with visible pyrrhotite inclusions and samples of variolitic ores form an individual group in the above field. Within the group, the ores are also subdivided into garnet-magnetite (skarn paragenesis), epidote-chlorite-magnetite (hydrosilicate paragenesis), pyroxene-magnetite, and orthoclase-magnetite (variolitic ores).

We consider the parameters of the magnetoacoustic-emission (MAE) effect in magnetites from the ores of the Urals and West Siberia. It has been shown that the differences in signals are related to various types of the domain structures of samples, whose fixity is determined by the formation conditions of magnetite and the effect of superposed physicochemical processes. On the basis of the field parameters, the magnetites have been divided into three types depending on the area of magnetic fields with MAE. These parameters can serve as the typomorphic features of magnetites of different genesis.