—For a multirate approximation, we have determined the dynamics of rock heating by a magmatic-fluid flow in a flat permeable zone cutting the cratonic lithosphere of the Siberian Platform from a magma chamber at a depth of 50 km to the Earth’s surface. This dynamics is compared with the dynamics of infiltration metasomatism in a three-layer lithosphere section: (1) harzburgitic mantle (depth 50–40 km), (2) crystalline basement (39–7 km), whose composition was simulated by the section of rocks hosting the skarn deposits of the Aldan Shield, and (3) platform cover (6–0 km), with its simplified rock compositions specified on the basis of the rock compositions in the southern and northern parts of the trap area of the Siberian Platform. Numerical modeling of the metasomatic transformation of rocks was performed in a multireservoir flow reactor, using the Selektor software. The initial composition of fluids in a magmatic source varied from highly reduced (water–methane) to ordinary (water–acid) (lg pO2 from –13.0 to –12.0). The obtained balances of the interacting phases show no significant change in the mass of aluminosilicate rocks in the mantle and Earth’s crust sections and a significant loss of their mass under replacement of carbonate and sulfate deposits.

Occurrence of alkali chlorides in kimberlites from the Yakutian diamondiferous kimberlite province (Udachnaya-pipe and Mir) are not rare, however there origin is ambiguous. Several papers have reported the primary magmatic origin of alkali evaporitic chlorides and alkali carbonates from Udachnaya (Kamenetsky et al., 2004, 2006; 2007a, b; Maas et al., 2005). Most recent paper suppose evaporitic source of xenoliths (Sharygin et al., 2007). This study provides the first extensive database on inclusion associations in halite from chloride xenoliths in kimberlites through: (1) a very detailed microstructural study of saline melt inclusion, from which some of the mechanisms of entrapment can be deduced, (2) the characterization of inclusion compositions in different textural locations within the xenoliths. We described not only fluid and melt inclusions, but cavities, microveins and mineral inclusions, presuming they are rock-forming minerals, important to understand the nature of former salt melts.

The paper presents systematized and synthesized data on the parameters and evolutionary sequence of metasomatic processes that accompanied interaction between Permian–Triassic trap complex and rocks of the sedimentary cover of the Siberian Platform at the large skarn iron deposits. Relations of the textural–compositional, morphological, and genetic diversity of the skarns and ores with the phases and stages of the origin of ore-bearing volcano-tectonic edifices are demonstrated with reference to the Korshunovskoe and Rudnogorskoe deposits. The genetic reconstructions are based on survey materials and data on the mineralogy of the rocks and ores (obtained by optical and scanning electron microscopy, microprobe analysis, EPR, Raman and IR spectroscopy, and by studying inclusions in minerals). A principally important feature of the volcano-tectonic edifices of the large mineral deposits is their multistage evolution and combinations of fluid-conducting zones, which are related to (1) volcanic apparatuses, (2) shallow-depth magmatic chambers (laccoliths) hosted in carbonate–salt rocks, and (3) multistage fracture structures produced by the collapse of the leached space. The major ore-bearing structures were formed simultaneously with the development of an intermediate magmatic chamber hosted in Cambrian carbonate–salt rocks beneath a seal of terrigenous sedimentary rocks. Magmatic-stage magnesian skarns with disseminated ores in them and in the calciphyres were produced during the prograde stage in the apical parts of the laccoliths, at contacts between the dolerites and dolomites. During the early prograde stage, skarn–ore bodies developed around injection bodies of globulated dolerites, laccoliths, and sills; stockworks and steep bodies of fragmentary magnesian and calcic skarns and ores were formed within the diatremes; and conformable bodies and veins were produced in the splay fracture zones. The later reactivation of faults and fractures and the involvement of connate brines and solutions from the evaporite complex triggered the redeposition of the ore masses, crystallization of the mineral assemblages of hydrated skarns, development of large domains of serpentine–chlorite–epidote–amphibole rocks, calcic skarns, and ores. Data on multiphase fluid inclusions in the forsterite, apatite, and halite indicate that the mineral-forming fluid initially was a highly concentrated solution–melt (total salinity of 60%) with high-density reduced gases. The magnesian skarns were formed during the following stages: (1) forsterite + fassaite + spinel + first-population magnetite (820–740°C); (2) phlogopite + titanite + pargasite + second-population magnetite (600–500°C), and (3) clinochlore + serpentine + tremolite + pyrrhotite + chalcopyrite (≥450°C).

Using the Selector PC application, we studied the process of formation of magnesian skarns at the contact of dolerites with carbonate-salt deposits. The physicochemical parameters of metasomatic processes were estimated by studying the localization of skarn ore shoots and mineral assemblages in the deposits of the Angara-llim type. The action of magmatic fluids on the system dolerite-magnesian salt deposits and dolerite carbonate-salt deposits resulted in zonal columns of infiltration magnesian skarns. The computation was carried out using a dynamic multireservoir model of a flow reactor with a constant temperature gradient and a uniform pressure. We have established that changes in the C/H ratio and Cl content in the fluid source affect the composition of the produced mineral assemblages. Depending on the temperature during the formation of skarns, different mineral assemblages are produced: diopside, enstatite, anorthite, quartz, ilmenite, hercynite, and pyrrhotite at 1040-1010 degrees C; monticellite, forsterite, magnetite, geikielite, periclase, spinel, calcite, and graphite at 980-740 degrees C; and calcite, dolomite, phlogopite, halite, and graphite at 710-380 degrees C. Wollastonite is observed in the rear zone of magnesian skarns. We examined the temperature-dependent sequence of formation of different types of silicates, spinels, and Ti-containing minerals in the metasomatic column. The computation results show that during crystallization, the tholeiitic magma releases a fluid phase with C/H = 0.1-1.0, amounting to 1.5-2.0 wt.%. (C) 2017, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Phase compositions and microtextures of ore minerals in intrusive traps of the western part of the Siberian Platform have been studied using scanning electron microscopy. Oxide and sulfide solid solutions crystallize at the magmatic stage; their grain and aggregate shapes are determined by the cooling rate of magmatic bodies. We have revealed a gradual transition of oxides from fine-grained texture in the quenching zone, through skeleton, case, and frame forms, to isometric aggregates of mixed crystals in the holocrystalline silicate matrix. Sulfide spheroids (either associated with oxides or separated from them) are changed by dissemination and nests. The chemical compositions of both oxides and sulfides are correlated with the petrochemical types of rocks. Chrome-spinels or chrome-enriched ulvospinels crystallize first in the most magnesian dolerites. Iron and titanium oxides with Mn, V, Mg, and Al impurities prevail in the rest rock varieties. As temperature decreases, ilmenite, ulvospinel, and titanomagnetite crystallize after chrome-spinels. Exsolution structures are very intricate for titanium and iron oxides and depend on the oxidation conditions and on the assemblage of impurities and their quantities. The first exsolution particles of ilmenite are more magnesian, while the following ones are more manganese. Subsolvus exsolution is accompanied by the release of impurities, grain stripping, and rearrangement and natural enrichment of ore material. Conjugate transformation of silicates and ore minerals results in aggregate pseudomorphs and minerals, such as titanite, zircon, and baddeleyite. Nickel-containing sulfides formed at the magmatic stage prevail in more magnesian rocks. Copper minerals are more diverse. These are polymorphic modifications of chalcopyrite and cubanite in ore solid solutions formed at the magmatic stage, chalcopyrite in paragenesis with monoclinic pyrrhotite in zones of hydrothermal metasomatites, and chalcopyrite in solid solutions with bornite and chalcosine and in assemblage with low-temperature sulfides. The obtained data on mineral structures and assemblages can be used as indicators to classify the genesis and formation types of ores.

Впервые с помощью сканирующей электронной микроскопии изучен фазовый состав и микроструктуры рудных минералов в интрузивных траппах западной части Сибирской платформы.В магматическую стадию кристаллизуются оксидные и сульфидные твердые растворы, размер, форма зерен и агрегатов которых определяются скоростью охлаждения магматических тел.Прослежен постепенный переход оксидов от тонкой вкрапленности в зоне закалки через скелетные, футлярные, каркасные формы к изометричным агрегатам смешанных кристаллов в полнокристаллической силикатной матрице.В этом же направлении сфероиды сульфидов, сопряженных или разобщенных с оксидами, сменяются интерстициальными гнездово-вкрапленными обособлениями.Химический состав оксидов и сульфидов коррелируется с петрохимическими типами пород.В наиболее магнезиальных долеритах первыми кристаллизуются хромшпинелиды или обогащенные хромом ульвошпинели.В остальных породах преобладают оксиды железа и титана с примесью марганца, ванадия, магния и алюминия.При понижении температуры вслед за шпинелидами кристаллизуются ильменит, ульвошпинель и титаномагнетит

We report results of experiments in which samples of ultramafic xenoliths from the Udachnaya-Vostochnaya kimberlite pipe (Siberian craton, Yakutia) were heated, in a flow reactor, by flows of reduced gas at temperatures between 800 °C and 1200 °C. The heating tests were preceded by investigation into the gas composition of main phases. When subjected to heating by the reduced gas flows, the samples of metasomatized lithospheric mantle released reduced fluids and formed partial melts, the composition of the latter being controlled by fusibility of rocks (“readiness” of liquid to segregate). The initial compositions of the partially molten rocks turned out to correlate with those of secondary inclusions in matrix minerals and of most fusible minerals in reaction rims and in metasomatic veins, as inferred from the analysis of vitrified melting products and structure of the heated rocks. The effect of partial melting of matrix minerals was to increase the contents of Al (melting of garnet) and Mg (olivine and orthopyroxene rims) in the melts. The experimental melts are difficult to compare with natural felsic and mafic rocks in average compositions. The bulk compositions of fluids in altered ultramafics disagree with the computed equilibrium compositions according to some correlation relations (CO2 ↔ CO, r+).

Ore mineral assemblages associated with various types of intrusive traps from the western sector of the Siberian Platform are reviewed. It was shown that various types of mineralization (Norilsk magmatic copper-nickel, Angaro-Ilim skarn-magnetite, and hydrothermal with Zn, Pb, Ag, and other metals) appear during the evolution of the fluid-magmatic systems. A suggestion about the high potential perspectives of this area for the revealing of large accumulation of various ore types was postulated.

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 differ 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 model composition of asthenospheric fluids.

Nonisothermal equilibrium physicochemical dynamics has been numerically modeled to estimate the effect of reduced asthenosphere fluids on continental lithosphere profiles beneath the Siberian Platform (SP). When the over-asthenosphere continental mantle is metasomatically changed by reduced magmatic fluids, the following sequence of zones forms: (1) zone where initial rocks are intensively sublimated and depleted by most petrogenic components; the restite in this case becomes carbonated, salinated and graphitized; (2) zone of Si and Fe enrichment and carbon deposition in initial rocks depleted in Na, K, P, Mn; (3) zone of diamond-bearing lherzolites enriched with Na; (4) zone of hydrated rocks enriched with K; (5) zone of hydrated rocks not enriched with petrogenic components. Zone 1 can be responsible for the formation of kimberlite melts, zones 3 and 4 can be substrates of alkaline magma melting, and zone 5 can be the source of mafic tholeiitic magma.