Home – Home – Jornals – Russian Geology and Geophysics 2015 number 3

Sr, Nd, and Pb isotope data for basaltic rocks of different ages from Kunashir Island (southern Kurile island arc) provide clues to investigate the subduction magmatic history. Signatures of a high-temperature slab component (melt and/or supercritical liquid produced by melting of slab sediments) involved in Early Miocene-Pleistocene back-arc basaltic magmatism indicate a relatively hot (>800 ºC) slab surface. Depleted isotope characteristics of Holocene basaltic lavas in both volcanic front and back arc indicate their origin with the participation of a cold aqueous fluid produced by dehydration of altered oceanic crust of the Pacific MORB type. The difference in geological, geochemical, and isotope patterns in the Pleistocene and the Holocene lavas may be a response to stress change from extension to compression in the Kurile back-arc basin and the Kurile arc.

Early Paleozoic alkaline basic magmatism in the Kuznetsk Alatau is manifested in the Upper Petropavlovka pluton of gabbro, feldspathic rocks (theralites, mafic foidolites, and nepheline syenites), and Ca-carbonatites. According to Sm-Nd and Rb-Sr isotope data, the pluton formed in the Middle Cambrian (509 ± 10 Ma). The silicate igneous rocks correspond in the contents of silica, alumina, and alkalies to derivates of a K-Na alkaline basic association. The Ca-carbonatites are characterized by a high-temperature (600-900 ºC) paragenesis of apatite, clinopyroxene, ferromonticellite, phlogopite, and magnetite. They are enriched in P ₂O ₅ (up to 6.4 wt.%), Sr (up to 3000-4500 ppm; Sr/Ba ~ 5-7), and REE + Y (up to 800 ppm) and show evidence for liquation genesis. The predominant magmatic source (ε _Nd( T ) = 5-7) was moderately depleted PREMA, possibly combined with E-MORB and EM. According to the isotopic data (( ⁸⁷Sr/ ⁸⁶Sr) _T ~ 0.7024-0.7065; δ ¹⁸O ~ 6.3-15.5‰; δ ¹³C ~ -3.5…-2.0‰), the fractionation of the melts was accompanied by their crustal contamination. The trace-element composition of the mafic rocks testifies to the participation of a substance similar to the substrata of the parental magmas of MORB, IAB, and OIB in the magma generation. This suggests intrusion in the geodynamic setting of interaction between the active continental margin and an ascending mantle diapir. Most likely, the intrusion led to the mixing of material from different sources, including the components of PREMA, enriched suprasubduction lithospheric mantle (EM), and continental crust. The assumption is made that the complexes of highly alkaline rocks and carbonatites in the western Central Asian Fold Belt are of plume origin and belong to an Early Paleozoic large igneous province.

The largest nephrite-bearing province of Russia is located on the southern folded periphery of the Siberian craton. Deposits of two formation types were established here: apoultrabasic (East Sayan and Dzhida areas, Parama massif) and apocarbonate (Vitim area). Nephrites compose schlieren and lenticular bodies usually in the zones of contact of serpentinous (lizardite-antogorite) dunite-harzburgite rocks and dolomitic marbles with aluminosilicate rocks of different compositions. Significant difference in composition has been established for nephrites of different formations. Apocarbonate nephrites are more magnesian and fluoric and less ferroan. The contents of Li, Be, Rb, and Cs in them are two orders of magnitude higher and the contents of Sc, Ti, and Mn are lower than those in apoultrabasic nephrites. The isotope data evidence that the fluid phase of apoultrabasic nephrites was released from serpentinites during metamorphism, whereas the fluid phase of apocarbonate nephrites is only meteoric water. Oxygen in minerals of metamorphosed carbonate rocks was borrowed from the matrix subjected to replacement, sometimes with the participation of formation waters.

The Dovyren intrusive complex includes the ore-bearing (Cu-Ni-PGE) Yoko-Dovyren layered pluton (728 Ma, up to 3.4 km in thickness), underlying ultramafic sills, and comagmatic leuconorite and gabbro-diabase dikes. Studies of Sr-Nd-Pb isotope systems were carried out for 24 intrusive rocks and five associated low- and high-Ti basalts. The high-Ti basalts show 0.7028 ≤ ( ⁸⁷Sr/ ⁸⁶Sr) _T ≤ 0.7048 and 4.6 ≤ ε _Nd( T ) ≤ 5.8, similar to the values in MORB. The intrusive basic and ultrabasic rocks are geochemically similar to the low-Ti formation, making a compact cluster of compositions with extremely high ratios of radiogenic Sr and Pb isotopes and low ε _Nd values. The maximum enrichment in radiogenic Sr is shown by the rocks near the pluton bottom (( ⁸⁷Sr/ ⁸⁶Sr) _T = 0.71387 ± 0.00010 (2σ), ε _Nd( T ) = -16.09 ± 0.06), which are the products of crystallization of the most primitive high-Mg magmas. The above-located dunites, troctolites, and gabbro show lower enrichment, probably because of the contamination of the host rocks during the filling of the magma chamber and/or because of the slight heterogeneity of the source. Calculations of the proportions of mixing of the parental melt with carbonate terrigenous material have shown that the variations in the Sr and Nd isotope ratios are due to the incredibly high contamination of the sediments, up to 40-50%. This contradicts the succession of the main rock types in the Yoko-Dovyren pluton in accordance with the crystallization of picrite-basaltic magma. The contribution of 5-10% high-Ti component seems more likely and suggests interaction between two isotopically contrasting magmas in this province in the Late Riphean. In general, the minor variations in ε _Nd( T ) of the intrusive rocks and metavolcanics (-14.3 ± 1.1) testify to the isotopically anomalous source of the low-Ti magmas. The time variation trend of ε _Nd( T ) and geochemical features of the Dovyren rocks indicate that the products of melting of 2.7-2.8 Ga suprasubduction mantle might have been the massif protolith. Thus, the Dovyren parental magmas formed from a much older (sub)lithosperic source in the Late Riphean. The source was initially enriched in a mafic component with a low Sm/Nd ratio and was isolated from the convecting mantle and mantle melting processes for ~2 Gyr. The existence of such a long-lived and at least twice reactivated lithospheric substratum is confirmed by the fact that the Nd isotope evolution trend of the initially nonanomalous mantle protolith includes not only the Dovyren rocks but also the Paleoproterozoic gabbro of the Chinei pluton and the Archean enderbites of the Baikal region.

The spatial coexistence and synchronous formation of magmatogene porphyry Cu-Mo mineralization and epithermal gold mineralization are due to the genetic relationship between their formation processes. This relationship might be due to the generation of metal-bearing fluids of different geochemical compositions by the porphyry ore-magmatic system, which then participate in the formation of magmatogene porphyry Cu-Mo(Au) and associated epithermal gold deposits. Synthesis of fluid inclusions in quartz was performed for experimental study of the behavior of Cu, Mo, W, Sn, Au, As, Sb, Te, Ag, and Bi in heterophase fluids similar in composition and aggregate state to natural ore-forming fluids of porphyry Cu-Mo(Au) deposits. We have established that at 700 ºC, a pressure decrease from 117 to 106 MPa leads to a significant enrichment of the gas phase of heterophase chloride fluid with Au, As, Sb, and Bi. The heterophase state of carbonate-chloride-sulfate fluids is observed at 600 ºC and 100-90 MPa. It characterizes the highly concentrated liquid carbonate-sulfide phase-liquid chloride phase-low-density gas phase equilibrium. A decrease in the pressure of heterophase carbonate-chloride-sulfate fluid leads to a noticeable enrichment of its chloride phase with Cu, Mo, Fe, W, Ag, Sn, Sb, and Zn relative to the carbonate-sulfate phase. The processes of redistribution of ore elements between the phases of heterophase fluids can be considered a model of generation of metal-bearing chloride fluids, which occurs in nature during the formation of porphyry Cu-Mo(Au) deposits, as well as a model of generation of gas fluids supplying Au, Te, As, and other ore elements to the place of formation of epithermal Au-Cu and Au-Ag mineralization.

Two intrusive complexes are recognized in the Zhireken deposit: Amanan and ore-bearing porphyry. According to the ages obtained by U-Pb zircon dating (Amanan complex - 162.6 ± 1.4 Ma, granites and monzonite-porphyry of the ore-bearing complex - 159.0 ± 1.6 and 157.5 ± 2.9 Ma), the Amanan complex formed at the end of collision, and the ore-bearing porphyry complex, during the change of the geodynamic regime by the postcollisional (rift) one. The rocks of two complexes have high contents of LILE and LREE and low contents of HFSE and HREE. The (87Sr/86Sr)0 ratio in the gabbro and granites of the Amanan complex is 0.70501 and 0.70534, respectively, and that in the rocks of the porphyry complex is within 0.70451-0.70633. The Amanan gabbro, gabbro-diorites, and granites are characterized by εNd(T) = -1.4, -1.8, and -10.3, respectively, and the rocks of the ore-bearing complex, by εNd(T) = -3.7 to +1.0. The model TNd(DM) age of the Amanan granites is 1.5 Ga, and that of the granites and porphyry of the ore-bearing complex is 1.0-0.8 Ga. The Pb isotope ratios in the rocks of the Amanan and porphyry complexes are: 206Pb/204Pb = 18.086-18.136 and 18.199-18.442, 207Pb/204Pb = 15.487-15.499 and 15.506-15.545, and 208Pb/204Pb = 38.046-38.256 and 38.230-38.456. The results of geological, geochemical, and isotope studies admit that magmas were generated from juvenile and ancient crusts. Melts probably ascended from a depth of no less than 55 km during the melting of crust thickened as a result of tectonic deformations (in the upper horizons) and during the basic-magma supply (in the lower horizons). Juvenile mafic crust is considered to be the major source of fluid components and metals. Favorable conditions for the ore generation in the magmatic system during the formation of the porphyry complex arose at the previous stage, during the formation of the Amanan complex, which we regard as a preparatory stage in the evolution of the long-living ore-magmatic system.

Inclusions of mineral-forming enviroments in apatite-containing ijolites and magnetite-phlogopite-apatite ores in carbonatites were studied to elucidate the genesis of apatite mineralization in the Guli alkaline ultramafic carbonatite massif. Primary inclusions of carbonate-salt and carbonate melts have been discovered and studied. The carbonate-salt melt inclusions are of alkaline high-Ca composition and are enriched in P, Sr, SO3, and F (wt.%): CaO - 30-40, Na2O - 5-12, K2O - 2-4, P2O5 - 1-3, SO3 - 1.5-3, and SrO - 1-3. They also contain minor MgO, FeO, BaO, and SiO2 (tenths and hundredths of percent). The homogenization temperature of these inclusions is 850-970 ºC. The carbonate inclusions contain predominant CaO (54-67 wt.%) and minor MgO, FeO, SrO, Na2O, and P2O5 (tenths of percent). Their homogenization temperature is 840-860 ºC. Similar primary carbonate-salt and carbonate inclusions were found in garnet, and secondary inclusions were detected in silicate minerals (clinopyroxene and nepheline) of ijolites. Clinopyroxenes of ijolites also contain primary inclusions of alkaline ultramafic high-Ca melts similar in composition to melilitite-melanephelinites, highly enriched in P, SO3, and CO2 (wt.%): SiO2 - 41-46, Al2O3 - 8-16, FeO - 2-8, MgO - 3-6, CaO - 12-20, Na2O - 2-9, K2O - 1-6, P2O5 - 0.4-2.1, SO3 - 0.2-2.3, and Cl - 0.02-0.35. According to the obtained data, apatite of the magnetite-phlogopite-apatite ores and ijolites of the Guli pluton crystallized from phosphorus-rich alkaline carbonate-salt melts at 850-970 ºC. The generation of these melts was, most likely, due to the silicate-salt immiscibility in melilitite-melanephelinite melts highly enriched in salts, which occured either at the final stages of clinopyroxene crystallization or during the formation of melilite. The presence of alkalies, S, F, and CO2 in spatially separated carbonate-salt melts contributed to the concentration and preservation of phosphorus in them at low temperatures, which led to the formation of apatite mineralization in ijolites and ore deposit in carbonatites.

Chromite mineralization in metamorphosed dunites from the southern Klyuchevskoy dunite-harzburgite ultramafic complex (Central Urals) has been investigated using geomagnetic surveys along with laboratory studies of ore-forming and accessory spinels of the same genetic type. Magnetization in the study area is carried mainly by accessory Fe-Cr-spinel of a variable Fe2+(Cr2- x Fe x 3+)O4 composition. Metamorphism caused changes in element contents and in both crystal and magnetic structure of the primary nonmagnetic accessory spinel, unlike the almost fresh ore-forming spinel. Thus, orebodies stand against their host rocks, which is a prerequisite for the use of geomagnetic surveys for exploration of podiform chromite deposits in dunite-harzburgite complexes. Ground magnetic surveys at a test site composed of faulted rocks bearing disseminated chromite mineralization in metamorphosed dunites resolved a chromite ore zone and a fault block boundary showing up as geomagnetic anomalies. Laboratory studies using high technologies (thermomagnetic analysis at 4 to 1000 K, as well as magnetic resonance and magnetic force spectroscopy) revealed, for the first time, magnetic clusters (superparamagnetic phases) in primary nonmagnetic accessory spinel, which are responsible for the magnetic properties of the host rocks. Microscale variations in Cr-spinel correlate with the geomagnetic anomalies recorded by field surveys at the test site.

We consider the variant of automatic correlation between well sections based on construction of multidimensional functions of differences between fragments of logs. The solution is obtained either by successive correlation between the boundaries with regard to the projective model of Haites and definition of the boundary position at the minimum of the difference function or by construction of the lines of optimal trajectories on a 2D Zhekovskii’s plot. These algorithms are implemented as a program package for original interpretation of well data and as a plug-in for automatic correlation module of the Schlumberger Petrel software using the Ocean development tools.

Statistical data are given on the thermal conductivity of rocks in the Komsomol’sk ore district (Khabarovsk Territory). The regularities of variations in the thermal-conductivity and thermal-inhomogeneity factors of the studied types of rocks are considered in relation with their mineral composition, texture, structure, and degree of metasomatic alteration. Thermophysical sections of the studied mineralized zones are presented. The conclusion is substantiated that the thermal properties of rocks are highly informative and detailed thermophysical studies of sections might be useful for their lithological subdivision.