Home – Home – Jornals – Russian Geology and Geophysics 2014 number 11

We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U-Pb dating (SHRIMP II) and Lu-Hf isotope study of detrital zircon from garnet-staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet-staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet-staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite-trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7 Ga. Older source rocks (2.80–3.35 Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet-staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources.

On the northeastern slope of the Kuznetsk Alatau, small differentiated alkaline basic intrusive massifs form an isometric area ~100 km across. They are composed of subalkalic and alkali gabbroids, basic and ultrabasic foidolites, nepheline and alkali syenites, and carbonatites. Results of complex (U-Pb, Sm-Nd, and Rb-Sr) isotope dating suggest that alkaline basic magmatism developed at two stages in the Middle Cambrian-Early Ordovician (~510-480 Ma) and in the Early-Middle Devonian (~410-385 Ma). Finding of accessory zircons (age 1.3-2.0 Ga) in alkaline rocks suggests that the ascent of mantle plume was accompanied by the melting of fragments of Proterozoic mature continental crust composing the basement of the Caledonian orogen of the Kuznetsk Alatau. Probably, parental Cambrian-Ordovician alkaline mafic melts initiated metasomatism and lithosphere erosion. During the next melting of lithosphere substrate in ~100 Myr, this caused the generation of magmas of similar composition with inherited isotope parameters (ε_Nd(T) ≈ +4.8 to +5.7, T_Nd(DM) ≈ 0.8-0.9 Ga) pointing to the similar nature of their matter sources in the moderately depleted mantle.

We present results of isotope-geochemical studies of Late Paleoproterozoic basites from intrusions located in different parts of a dike swarm traceable for more than 200 km within the Baikal marginal salient of the Siberian craton basement (northern Baikal area). The basites of the southern (Khibelen site) and northern (Chaya site) parts of the dike swarm show both similarity and difference in their sources and formation conditions. For example, the Khibelen basites correspond in chemical composition to basalts and trachybasalts, and the Chaya basites, to basalts and andesite-basalts. Based on petrographic and petrochemical data, the basites of both sites can be referred to as medium-alkali (subalkalic) series. All analyzed basites show distinct negative Nb-Ta and Ti anomalies on element spidergrams, negative ε_Nd(T) values, and indicative geochemical ratios Th/Nb_pm, La/Nb_pm, and La/Sm_n > 1. All this points to the formation of basites of both sites from mantle sources contaminated with continental crust. Contamination might have occurred in intermediate magma chambers localized in crust. Differentiated basic varieties of both sites resulted from fractionation of clinopyroxene. For the Khibelen basites, the mantle source (probably, with geochemical parameters close to those of IAB) might have been initially contaminated with middle-crust rocks and then, with lower/upper crust material. The source of the Chaya basites was, probably, produced during the interaction of mantle components similar in composition to IAB and N-MORB with a crustal component. The performed studies testify to the heterogeneous composition of the upper mantle beneath different sites of the Siberian craton basement.

The abundance of various chemical elements (including trace, rare-earth, and radioactive) in the carbon dioxide mineral waters of the Choigan complex has been investigated. Three groups of waters are recognized according to the geochemical conditions and chemical composition: groundwaters of regional-jointing zone with oxidizing conditions; CO₂-enriched groundwaters of regional-jointing zone with oxidizing conditions; and groundwaters of fault zones with reducing conditions. It is shown that water-rock interaction intensified by high temperature and carbon dioxide action is the main process determining the chemical composition of groundwaters.

We present new original data on the geochemistry of scandium in the coals of Asian Russia, Mongolia, and Kazakhstan. In general, the studied coals are enriched in Sc as compared with the average coals worldwide. Coal deposits with abnormally high, up to commercial, Sc contents were detected in different parts of the study area. The factors for the accumulation of Sc in coals have been identified. The Sc contents of the coals depend on the petrologic composition of coal basins (composition of rocks in their framing) and the facies conditions of coal accumulation. We have established the redistribution and partial removal of Sc from a coal seam during coal metamorphism. The distribution of Sc in deposits and coal seams indicates the predominantly hydrogenic mechanism of its anomalous concentration in coals and peats. The accumulation of Sc in the coals and peats is attributed to its leaching out of the coal-bearing rocks and redeposition in a coal (peat) layer with groundwater and undergroundwater enriched in organic acids. The enrichment of coals with Sc requires conditions for the formation of Sc-enriched coal-bearing rocks and conditions for its leaching and transport to the coal seam. Such conditions can be found in the present-day peatland systems of West Siberia and, probably, in ancient basins of peat (coal) accumulation.

A natural oil seep has been revealed in the floodplain of the middle stream of the Amga River, in zone of the exposure of Middle Cambrian sediments. A distinctive feature of saturated hydrocarbons of this oil is the absence of biomarkers of the series 12- and 13-methylalkanes, biomarkers that are present in oils of the Vendian-Cambrian deposits of the Nepa-Botuobiyan petroliferous province. In this feature the studied seep oil is similar to the Middle Cambrian oil from hydrogeologic wells (1-P and 1-T) drilled earlier downstream of the Amga River.

Data on geophysical fields and petrophysical heterogeneity, parageneses and the thermodynamic conditions and age of their formation, and fluid inclusions were used for the genetic reconstruction of petrophysical zoning at the Blagodatnoe gold-sulfide deposit. Petrophysical associations of the pre-ore and ore stages of the deposit formation are clearly reflected in anomalies of the magnetic and natural electric fields and the aureoles of radioactive elements. At the early pre-ore stage (752 Ma), reduced solutions with high activity of K, enriched in U, Th, and, probably, Au, were supplied to intensely foliated tectonic zones. Their interaction with initial metasedimentary rocks gave rise to contiguous zones of quartz-muscovite and chlorite metasomatic rocks. Accompanying graphitization led to a high electrochemical activity of the metasomatic rocks, which generated anomalies of up to -300 mV in the natural electric field; the most intensely carbonized zones became enriched with U (up to 6.5·10^–4%) and, probably, Au. The quartz-muscovite metasomatic rocks accumulated Th and K (up to 29·10^–4% and 4%, respectively), whereas the chlorite metasomatic rocks accumulated rock-forming elements (particularly Fe), which led to the compaction of these rocks and the acceleration due to gravity in local positive anomalies. The nonmagnetic character of the fresh pre-ore metasomatic rocks suggests the predominantly pyritic composition of early sulfides. At the ore stage (698 Ma), the minerals were deposited from H₂O–CO₂–As–S solutions at 560 to 315 ºC. The activity of these solutions caused a redistribution of radioactive elements and a high petrophysical differentiation of the ore-bearing structure. The amplitudes of the anomalies above this structure vary from 500 to 80 nT in the magnetic field and from –130 to +10 mV in the natural electric field. It has been found that the hydrothermal fluid hardly affected the polarization properties of graphitized rocks at the maximum temperatures but caused an intense removal of U and the development of magnetic pyrrhotite after pyrite. The temperature decrease in the mineral-forming system was favorable for the formation of siderite. Carbonaceous schists subjected to carbonatization lost their electrochemical activity. The binding of carbon dioxide in the solid phase influenced the migration capacity of trace elements and their zonal distribution. With this evolution of the solution, Th accumulated at the lower levels of the mineralized zone, whereas the upper levels of the deposit became enriched with U. Productive gold–arsenopyrite–pyrite–pyrrhotite paragenesis with anomalous magnetic susceptibility evolved at the ore stage. The late galena–sphalerite–chalcopyrite paragenesis (365 Ma) was of strictly local occurrence and reduced the magnetic susceptibility of ores.

The effect the metal casing of a vertical borehole may exert on transient electromagnetic (TEM) responses has been studied in a field experiment. Eddy currents in the casing affect transients only slightly at early times, but the casing effect predominates at late times. Therefore, early-time TEM response measured near a borehole can provide information on shallow subsurface. The late-time TEM signals induced by the eddy currents in the casing show exponential behavior b·exp(–t/τ). The time constant τ refers to the rate of eddy current decay in the casing; the amplitude b is M₁₂· M₂₃ · L^–1 · τ^–1, where L is the casing self-inductance, and M₁₂ and M₂₃ are the mutual inductances between the transmitter loop and the borehole and between the borehole and the receiver, respectively. Both M₁₂ and M₂₃ are controllable, while M₂₃ is especially important for survey applications: by reducing it, one can reduce the casing effect on the TEM data.

Much experience has been gained lately in the use of a radial current source, the so-called circular electric dipole (CED), as a transmitter in transient electromagnetic (TEM) surveys. CED is a source of alternating transverse magnetic (TM) polarized field, a surface analog of a vertical electrical line in VES. In the course of two recent decades, the method has been developed theoretically and tested through the field practice. The respective published results are expected to provide an idea of TEM soundings with the optional use of either TE or TM mode. In this paper we report some new theoretical aspects and share our field experience of surveys with an CED system.

We consider data of study of the parameters of magnetoacoustic emission performed on pyrrhotite from magnetite and pyrite ores from the deposits of the Urals. It is shown that the difference in signals is mainly due to different types of domain structure which forms during the crystallization of pyrrhotite as pinacoids or prisms. Five types of pyrrhotite are recognized depending on the parameters of magnetoacoustic emission. This information can be used as typomorphic features of pyrrhotite of different geneses.