Home – Home – Jornals – Russian Geology and Geophysics 2010 number 9

Four stages of the thermochemical plume-lithosphere interaction generating a broad mushroom-like head of plume and especially fourth regressive cooling phase have the important role for metallogeny. The analysis of a thermochemical plume model together with recent geological and geochronological data on magmatic ore systems in the Siberian, Tarim, Emeishan, Central European, and some other large igneous provinces (LIPs) enabled the following characteristics of the metallogeny in large igneous provinces to be revealed: (1) the specific combination of mineralization types, which include magmatic Cu-Ni-Pt and Fe-Pt, hydrothermal Ni-Co-As (±Ag, U, Au), Au-As, Ag-Sb, Au-Hg, Sb-Hg, and stratiform Cu (copper-bearing sandstones and shales enriched in Co, Ni, Ag, Pt); (2) the areal or spot-like pattern of the location of mineralization types (opposed to the linear-belt localization in subduction and rift settings); (3) the zoned distribution of mineralization types relative to LIP centers, with Cu-Ni-Pt, Fe-Pt and stratiform Cu mineralization localized in a LIP center, and hydrothermal mineralization bound to a LIP periphery; (4) the essential contemporaneity of the formation of each mineralization type in LIPs, and the existence of coeval but spatially separated Cu-Ni-Pt, Ni-Co-As, and Au-As deposits; (5) the close linkage between different mineralization types and particular pulses of mafic, alkaline mafic, and felsic magmatism; (6) the unified succession of ore-forming events; (7) the close relationship between the extent of mineralization and LIP igneous volumes, which, in turn, depend on the plume heat power.
The major characteristics of localization of different mineralization types within LIPs, the timing and genetic relationships between mineralization and types of magmatism, as well as specific geological controls on ore formation provide a basis for establishing new geological, magmatic, lithological, and geochemical criteria crucial for predicting and targeting new mineralization within LIPs.

We present seismic images of the mantle beneath East Russia and adjacent regions and discuss geodynamic implications. Our mantle tomography shows that the subducting Pacific slab becomes stagnant in the mantle transition zone under Western Alaska, the Bering Sea, Sea of Okhotsk, Japan Sea, and Northeast Asia. Many intraplate volcanoes exist in these areas, which are located above the low-velocity zones in the upper mantle above the stagnant slab, suggesting that the intraplate volcanoes are related to the dynamic processes in the big mantle wedge above the stagnant slab and the deep slab dehydration. Teleseismic tomography revealed a low-velocity zone extending down to 660 km depth beneath the Baikal rift zone, which may represent a mantle plume. The bottom depths of the Wadati-Benioff deep seismic zone and the Pacific slab itself become shallower toward the north under the Kamchatka Peninsula, and the slab disappears under the northernmost Kamchatka. The slab loss is considered to be caused by the friction between the slab and the surrounding asthenosphere as the Pacific plate rotated clockwise at about 30 Ma ago, and then the slab loss was enlarged by the slab-edge pinch-off by the hot asthenospheric flow and the presence of Meiji seamounts.

The spatial distribution of recent (under 2 Ma) volcanism has been studied in relation to mantle hotspots and the evolution of the present-day supercontinent which we named Northern Pangea. Recent volcanism is observed in Eurasia, North and South America, Africa, Greenland, the Arctic, and the Atlantic, Indian, and Pacific Oceans. Several types of volcanism are distinguished: mid-ocean ridge (MOR) volcanism; subduction volcanism of island arcs and active continental margins (IA + ACM); continental collision (CC) volcanism; intraplate (IP) volcanism related to mantle hotspots, continental rifts, and transcontinental belts. Continental volcanism is obviously related to the evolution of Northern Pangea, which comprises Eurasia, North and South America, India, Australia, and Africa. The supercontinent is large, with predominant continental crust. The geodynamic setting and recent volcanism of Northern Pangea are determined by two opposite processes. On one hand, subduction from the Pacific Ocean, India, the Arabian Peninsula, and Africa consolidates the supercontinent. On the other hand, the spreading of oceanic plates from the Atlantic splits Northern Pangea, changes its shape as compared with Wegener's Pangea, and causes the Atlantic geodynamics to spread to the Arctic. The long-lasting steady subduction beneath Eurasia and North America favored intense IA + ACM volcanism. Also, it caused cold lithosphere to accumulate in the deep mantle in northern Northern Pangea and replace the hot deep mantle, which was pressed to the supercontinental margins. Later on, this mantle rose as plumes (IP mafic magma sources), which were the ascending currents of global mantle convection and minor convection systems at convergent plate boundaries. Wegener's Pangea broke up because of the African superplume, which occupied consecutively the Central Atlantic, the South Atlantic, and the Indian Ocean and expanded toward the Arctic. Intraplate plume magmatism in Eurasia and North America was accompanied by surface collisional or subduction magmatism. In the Atlantic, Arctic, Indian, and Pacific Oceans, deep-level plume magmatism (high-alkali mafic rocks) was accompanied by surface spreading magmatism (tholeiitic basalts).

The Phanerozoic history of mafic magmatism in the southern Siberian craton included three major events. The earliest event (~500 Ma) recorded in dolerite dikes occurred during accretion and collision at the early stage of the Central Asian orogen. Injection of mafic melts into the upper crust was possible in zones of diffuse extension within the southern Siberian craton, which acted as an indenter. The Late Paleozoic event (~275 Ma) produced dikes that intruded in a setting of subduction-related extension at the back of the active continental margin of Siberia during the closure of the Mongolia-Okhotsk ocean, as well as slightly older volcanics (290 Ma) in the Transbaikalian segment of the Central Asian orogen. Early Mesozoic magmatism in the southern Siberian craton resulted in numerous 240-250 Ma mafic intrusions in the Angara-Taseeva basin. The intrusions (Siberian traps) appeared as the subducting slab of the Mongolia-Okhotsk ocean interacted with a lower mantle plume. The post-Late Paleozoic ages of flood basalts (290-275 Ma) correspond to progressive northwestward (in present coordinates) motion of the slab beneath the southern craton margin, which likely ceased after the slab had reached the zone of the Siberian superplume. Since its consolidation after the Early Mesozoic activity, the crust in the area has no longer experienced extension favorable for the intrusion of basaltic magma.

Series of continental and oceanic alkaline associations have been compared. Comparison confirms that alkaline plumes originated from the Earth's liquid core under the continents and, less often, under the oceans. The spatial distribution of alkaline complexes has been analyzed in terms of the plume magmatism theory. Analysis suggests that the zoning and lateral migration of alkaline magmatic centers in alkaline provinces were determined by the migration of an alkaline plume (multiplume) and its alkaline basaltic, alkaline ultramafic, carbonatitic, kimberlitic, and other derivates.
Two components are well pronounced in the chemical history of alkaline plume magmatism. The first is the foidaphile component, which persists in all igneous and metasomatic rocks of various alkaline complexes. It includes elements associated with Na and K: rare alkali metals, alkaline earth metals, radioactive elements, rare earths, and others. They make up the important part of the plume that might have separated from the liquid core. The second component is rock-forming mantle-lithospheric, which formed in the asthenosphere during the mixing of mantle and lithospheric sources while the plume ascended to the Earth's surface.

Complex petrological, geochemical, and isotope studies of igneous rocks sampled from the core of parametric Maizasskaya BH-1 showed a predominance of dolerite sills, which formed earlier (~263±4 Ma) than most of basalts in the basement of the West Siberian sedimentary basin and in the Siberian Platform traps (248-251 Ma). Their formation took place during the crystallization of basaltic melt in intrusive chambers existing between layers of Silurian sedimentary rocks. The petrochemical, geochemical, mineralogical, and thermobarogeochemical data show that the sills resulted from the activity of complex magmatic systems different from typical oceanic and plateau-basalt melts and related, most likely, to the formation of rift structures under the influence of mantle plume. Study of melt inclusions provided data on the conditions of generation of primary melts from mantle substratum (≤1570 °C, depths to 105-120 km) and crystallization parameters of dolerites - 1130-1155 °C, 1.5-2 kbar. The results obtained show that the studied basalt complexes in West Siberia are genetically related to the mantle plume activity, which led to the breakup of ancient crust and rifting. Formation of oceanic crust took place in the largest rifts; the ascending magma penetrated into the enclosing ancient strata to form sills.

Recently it has been suggested that the major influence on the environment from Siberian Traps magmatism was due to the interaction of magma and organic-rich shale and petroleum-bearing evaporites, with the subsequent creation and outburst of toxic gases (Siberian gas venting: SGV model). In part this idea was supported by a U-Pb age of 252.0±0.4 Ma for one of the dolerite sills in the southeastern Siberian Traps: The age corresponds to the Permo-Triassic boundary and its known mass extinctions of biota. In this study two other dolerite sills were dated using zircons by the U-Pb SHRIMP method at 254.2±2.3 Ma and 249.6±1.5 Ma. The former age is in agreement within error with the age previously published for the dolerite sills, whereas the latter age is in agreement with U-Pb ages published for lava and intrusions from the northern Siberian Traps. The new ages correspond to the Cahngshingian/Wuchiapingian or Permian/Triassic and Spathian/Smithian boundaries, respectively. Review of ⁴⁰Ar/³⁹Ar and U-Pb SHRIMP ages previously published for the southeastern Siberian Traps shows that three other pulses of magmatism probably took place at Anisian/Spathian, Late/Middle Anisian, and Landian/Anisian boundaries, respectively. Thus, it is possible that the SVG model can be applied also to lesser biotic extinctions and recoveries in proximity and aftermath to the main Permo-Triassic extinction.

The Kuznetsk Basin is located in the northern part of the Altai-Sayan Folded Area (ASFA), southwestern Siberia. Its Late Permian-Middle Triassic section includes basaltic stratum-like bodies, sills, formed at 250-248 Ma. The basalts are medium- and high-Ti tholeiites enriched in Nb and La. Compositionally they are close to the Early Triassic basalts of the Syverma Formation in the Siberian flood basalt large igneous province, basalts of the Urengoi Rift in the West Siberian Basin, and the Triassic basalts of the North Mongolian rift system. The basalts probably formed in relation to mantle plume activity: They are enriched in light rare-earth elements (LREE; La_n = 90-115, (La/Sm) _n = 2.4-2.6) but relatively depleted in Nb (Nb/La) _n = 0.34-0.48). Low to medium differentiation of heavy rare-earth elements (HREE; (Gd/Yb) _n = 1.4-1.7) suggests a spinel facies mantle source for basaltic melts. Our obtained data on the composition and age of the Kuznetsk basalts support the previous idea of their genetic and structural links with the Permian-Triassic continental flood basalts of the Siberian Platform (Siberian Traps) possibly related to the action of the Siberian superplume peaked at 252-248 Ma. The abruptly changing thickness of the Kuznetsk Late Permian-Middle Triassic units suggests their formation within an extensional structure similar to the exposed rifts of southern Ural and northern Mongolia and buried rifts of the West Siberian Basin.

A mathematical model is proposed for the two-velocity nonisothermal dynamics of the interaction between the convecting upper mantle and the multilayer lithosphere with local permeable zones. Based on the statistical processing of data on the bulk compositions of fluids from mantle rocks beneath the Siberian Platform (SP) and the Earth's crustal metamorphic rocks of granulite and amphibolite facies, we discuss the problems of specifying the initial and boundary conditions for the description of the dynamics of convective melting in permeable zones above asthenosphere. To determine the nature of the established linear CO₂-H₂O trend (these are the main fluids of inclusions), we consider the 2D dynamics of formation of the T and P fields and the accompanying physicochemical dynamics of heterophase interaction between supra-asthenosphere magmatogene fluids and depleted rocks of the lithospheric mantle. The performed experimental and computational studies of the bulk composition and nature of the fluid phase in rock xenoliths from the SP lithosphere and Earth's crustal metamorphosed strata showed that: (1) the gas phase of lower-crustal metamorphic rocks differs significantly in bulk composition from the gas phase of mantle lithosphere rocks, (2) about 80 % of the gas phase in the minerals of lithospheric mantle ultrabasites are oxidized products of the re-equilibration of supra-asthenosphere magmatogene fluids transformed in regional fault zones; (3) a periodic decompression of lithospheric mantle strata in the SP deep-fault zones is the main factor of this re-equilibration; (4) data on the composition of the gas phase in primary inclusions in minerals of igneous rocks can be used to calculate the virtual composition of asthenospheric fluids.

Plutonogene ore-magmatic systems of the Noril'sk ore field are unique constituents of the P₂-T₁ trap formation in the East Siberian Platform. We consider the formation of ore-bearing intrusions, evolution of Cr-spinels in intrusive magmatites, possible mechanisms of formation of massive, disseminated, and impregnated magmatic sulfide ores, possible reasons for the abundance of sulfide melts, quasi-anhydrite isotopic composition of sulfur of sulfide ores, and products of interaction of sulfide melts with ore-hosting basites. The unique contents of PGE, Ag, and Au in ores (eutectic Iss-PbSss intergrowths, crystallization products of low-temperature Ni-Fe-Cu-Pb-S melts) have been estimated for the first time. We have established that pneumatolytic Ag-Au-Pt-Pd mineralization is intimately related to the fluid aureoles near magmatic sulfide bodies. Pneumatolytic PGM are subdivided into early (tetraferroplatinum with lamellae atokite, paolovite with lamellae of insizwaite-geversite and niggliite, etc.), late (sobolevskite, froodite, hessite, maichenerite, cabriite, minerals of Au-Ag series, etc.), and the latest (sperrylite). The direct, reverse, oscillation, and complex zoning of gold particles is much due to variations in the Te activity in the fluids. Pneumatolytic noble-metal minerals were produced at <490?C in strongly reducing conditions with extremely low S_{2 fugacity} . The Pb isotope composition evidences that all systems of the trap formation in the Noril'sk region had the same mantle source. The Pb isotope compositions of ore-bearing intrusions, magmatic sulfide ores, PbSss, and Pd-Pt intermetallides in the Noril'sk and Talnakh ore clusters differ significantly: Lead in the Talnakh cluster is more radiogenic. This evidences genetic relations between sulfide ores and particular intrusions as well as different intermediate magma chambers in the Noril'sk and Talnakh clusters, and a higher degree of contamination of mantle magmas in the Talnakh cluster, which might be the explanation of its giant area.

The paper discusses the spatiotemporal and genetic relationships of hydrothermal Co mineralization in the Altai-Sayan orogen with mafic, alkaline mafic, and granitoid magmatism on the basis of isotopic, geochemical, and geochronological investigations. Four stages of Co mineralization have been distinguished for this region: Early Devonian (D₁), Late Devonian-Early Carboniferous (D₃-C₁), Permo-Triassic (P₂-T), and Early Cretaceous (K₁). They correspond to periods of large-scale mafic magmatism. Isotopic (Pb, Sr, He) and geochemical studies have shown that Co mineralization is genetically related to mafic and granitoid magmatism. Also, these studies have confirmed that Co deposits formed with the participation of mantle fluids and are related to chambers of mafic and alkaline mafic melts. Besides, it has been found that ore originated both from magmatic sources and host rocks. A pulsed facies endogenic zonation has been established for Co deposits, Co-bearing ore clusters, and zones with high-temperature Co-As and low-temperature Ni-Co-As mineralization. It has been first established that ores at hydrothermal Co deposits are rich in Pt and Pd.