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The authors analyze the geodynamic settings of large fields of spodumene pegmatites hosting Li and complex (Li, Cs, Ta, Be, and Sn) deposits of rare metals within the Central Asian Fold Belt. Most of the studied fields show a considerable time gap (from few tens of Myr to hundreds of Myr) between the spodumene pegmatites and the associated granites, which are usually considered parental. This evidence necessitates recognition of an independent pegmatite stage in the magmatic history of some pegmatite-bearing structures in Central Asia. The Precambrian-Late Mesozoic interval is marked by a close relationship between the large fields of spodumene pegmatites and extension settings of continental lithosphere. They occur either as (1) zones of long-lived deep faults bordering on trough (rift) structures experiencing the tectonic-magmatic activity or as (2) postcollisional zones of shearing and pull-apart dislocations. Thus, large fields of spodumene pegmatites might serve as indicators of continental-lithosphere extension. Important factors favoring the formation of rare-metal pegmatites both in collision zones and continental-rift settings are the presence of thick mature crust dissected by long-lived, deeply penetrating (down to the upper mantle) fault zones. They ease the effect of deep sources of energy and substance on crustal chambers of granite and pegmatite formation.

The mineral and geochemical compositions of noble-metal (first of all, gold) deposits of the Fennoscandian, Siberian, and Northeast Asian orogenic belts are considered. These deposits are of several types: Au (disseminated Au–sulfide and Au–quartz), Au–Bi, Au–Ag, Au–Sb, Ag–Sb, Au–Sb–Hg, and Ag–Hg. They formed in different geodynamic settings as a result of the active motion of crustal tectonic blocks of different nature. Subduction processes (both at the front and at the rear of continent-marginal and island-arc magmatic arcs) resulted in Au–Ag, Ag–Sb, Ag–Hg, Au–Sb–Hg, and Au-Bi deposits. Collision events gave rise to Au and Au–Bi deposits. Intraplate continental rifting and formation of orogenic belts along the boundaries of block (plate) sliding led to the origin of Au and Au–Bi ores in association with Au–Ag, Au–Sb–Hg, and complex ores. In all cases, the formation of noble-metal mineralization was accompanied by magmatism of different types and metamorphism. Because of this diversity of ores, there is no single concept of the genesis of noble-metal mineralization. Several competing models of genesis exist: hydrothermal-metamorphic, pluton-metamorphic, plutonic, activity of mantle fluid flows, and multistage concentration with the leading role of sedimentary complexes.

Data are presented on chromitites from the northern and southern sheets of the Il’chir ophiolite complex (Ospa–Kitoi and Khara–Nur areas). The new and published data are used to consider similarities and differences between ore chrome-spinel from the chromitites of the northern and southern ophiolite sheets as well as the species diversity of PGE minerals and the evolution of Pt mineralization. Previously unknown PGE minerals have been found in the studied chromitites. Ore chrome-spinel in the chromitites from the northern sheet occurs in medium– and low–alumina forms, whereas the chromitites from the southern sheet contain only medium-alumina chrome-spinel. The PGE minerals in the chromitites from the southern sheet are Os–Ir–Ru solid solutions as well as sulfides and sulfoarsenides of these metals. The chromitites from the northern sheet contain the same PGE minerals and diverse Rh–Pt–Pd mineralization: Pt–Ir–Ru–Os and isoferroplatinum with Ir and Os–Ir–Ru lamellae. Areas of altered chromitites contain a wide variety of low-temperature secondary PGE minerals: Pt–Cu, Pt–Pd–Cu, PdHg, Rh₂SnCu, RhNiAs, PtAs₂, and PtSb₂. The speciation of the PGE minerals and multiphase intergrowths is described. The relations of Os–Ir–Ru solid solutions with laurite and irarsite are considered along with the microstructure of irarsite-osarsite-ruarsite solid solutions. Zoned Os–Ir–Ru crystals have been found. Zone Os_82-99 in these crystals contains Ni₃S₂ inclusions, which mark off crystal growth zones. Different sources of Pt mineralization are presumed for the chromitites from the northern and southern sheets. The stages of PGE mineralization have been defined for the chromitites from the Il’chir ophiolite belt. The Pt–Ir–Ru–Os and (Os, Ru)S₂ inclusions in Os–Ir–Ru solid solutions might be relics of primitive–mantle PGE minerals. During the partial melting of the upper mantle, Os–Ir–Ru and Pt–Fe solid solutions formed syngenetically with the chromitites. During the late-magmatic stage, Os–Ir–Ru solid solutions were replaced by sulfides and sulfarsenides of these metals. Mantle metasomatism under the effect of reduced mantle fluids was accompanied by PGE remobilization and redeposition with the formation of the following assemblage: garutiite (Ni,Fe,Ir), zaccariniite (RhNiAs), (Ir,Ni,Cu)S₃, Pt–Cu, Pt–Cu–Fe–Ni, Cu–Pt–Pd, and Rh–Cu–Sn–Sb. The zoned Os–Ir–Ru crystals in the chromitites from the northern sheet suggest dissolution and redeposition of Os-Ir-Ru primary mantle solid solutions by bisulfide complexes. Most likely, the PGE remobilization took place during early serpentinization at 450–600ºC and 13–16 kbar. During the crustal metamorphic stage, tectonic movements (obduction) and a change from reducing to oxidizing conditions were accompanied by the successive transformation of chrome–spinel into ferrichromite–chrome–magnetite with the active participation of a metamorphic fluid enriched in crustal components. The orcelite–maucherite–ferrichromite–sperrylite assemblage formed in epidote–amphibolitic facies settings during this stage. The PGE mineral assemblage reflects different stages in the formation of the chromitites and dunite–harzburgite host rocks and their transformation from primitive mantle to crustal metamorphic processes.

The gold distribution in 32 pyrite samples and some samples of other ore minerals is studied using the method of statistical samplings of analytical data for single crystals. The samples were recovered from deposits of different genetic types within the largest gold provinces of Russia and Uzbekistan. The contents of uniformly distributed gold and the ratios of its structurally to superficially bound forms have been determined. According to the Au-As diagram for the chemical states of gold, uniformly distributed gold in pyrite is chemically bound in the overwhelming majority of cases. The previous experimental data suggest that it is partly incorporated into pyrite and partly into the structures of nanosized nonautonomous phases on the surface of the pyrite crystals. Micro– and nanoparticles of native gold might appear during postgrowth transformations of these phases. Data on the other ore minerals suggest that the dependence of the content of uniformly distributed gold on the size or specific surface area of the crystal and the superficial position of its considerable part are common to the ore minerals. It is shown for pyrite that the observed features are commonly found at deposits of different genetic types, only with differences in the slope and determination coefficients of the dependences. The size dependences of the contents of gold and other elements in pyrite are genetically significant, because they give an insight into the ore–forming processes. The data on structurally bound gold permit comparative evaluation of gold concentrations in ore fluids forming gold deposits of different genetic types.

When prospecting ore deposits in the Trans-Baikal region, the endogenous geochemical fields (EGF) are taken as the main search element, as was proposed by L.V. Tauson. Such fields are classified into: geochemical fields of dispersion (GFD), concentration (GFC), and removal (GFR). With regard to their formation conditions, they are subdivided into magmatic (associated with magma chambers), intratelluric (associated with activity of intratelluric emanations), hydrothermal-metamorphic (vadose-thermal solutions), metamorphogenic, and sedimentary-metamorphogenic. Magmatic EGF are divided into three groups: magmatic, pneumatolytic, and hydrothermal stages. This study identified their polygenetic origin and association with ore-magmatic systems. The geochemical fields of ore zones, fields, and deposits result from the late and postmagmatic processes; they also include the EGF of host rocks and those which altered at the pre-ore stage of the natural system development. In ore deposits, the EGF are responsible for the supply and redistribution of elements through the ore formation process. The fields were divided into EGF of poor concentration (contrast coefficient CC normalized after background up to 10), mean (CC > 10–100), and intense (CC >> 100). The EGF intensity progressively increases at the hierarchy stage: “host rock-pre-ore metasomatite-syn-ore hydrothermalite-orebody-ore pillar”. To summarize, the fields, ore districts, zones, and deposits are characterized by diverse patterns of dispersion, concentration, and removal. The specific features of composition, structure, and zonal distribution of elements in geochemical fields are exemplified by some gold-bearing zones of the Trans-Baikal region. The paper reports new approaches to investigating these natural formations. The authors promote transition from the generally accepted evaluation of a halo separation to the volumetric survey of endogenous geochemical fields (GFD, GFC, and GFR included) of ore deposits and ore-magmatic systems, in general. The acquired evidence supports the assumption that endogenous geochemical fields should be regarded as a complete system differentiated in space and time, preserving specifics and pattern of the internal structure.

Two metamorphic complexes of the Yenisei Ridge with contrasting composition are analyzed to unravel their tectonothermal evolution and geodynamic processes during the Riphean geologic history of the area. The structural, mineralogical, petrological, geochemical and geochronological data are used to distinguish two stages of the evolution with different ages, thermodynamic regimes, and metamorphic field gradients. Reaction textures, chemical zoning in minerals, shapes of the P–T paths, and isotope dates provide convincing evidence for a polymetamorphic history of the region. The first stage is marked by the formation of the ~ 970 Ma low-pressure zoned And–Sil rocks (P = 3.9–5.1 kbar, T = 510–640 ºC) of the Teya aureole and a high metamorphic field gradient with dT/dH = 25–35 ºC/km typical of many orogenic belts. At the second stage, these rocks experienced Late Riphean (853–849 Ma) collisional medium-pressure metamorphism of the kyanite-sillimanite type (P = 5.7–7.2 kbar, T = 660–700 ºC) and a low metamorphic field gradient with dT/dH < 12 ºC/km. This metamorphic event was almost coeval with the Late Riphean (862 Ma) contact metamorphism in the vicinity of the granitic plutons, which was accompanied by a high metamorphic field gradient with dT/dH > 100 ºC/km. At the first stage, the deepest blocks of the Garevka complex in the vicinity of the Yenisei regional shear zone underwent high-pressure amphibolite-facies metamorphism within a narrow range of P = 7.1–8.7 kbar and T = 580–630 ºC, suggesting the burial of rocks to mid-crustal depths at a metamorphic field gradient with dT/dH ~ 20–25 ºC/km. At the second stage, these rocks experienced the Late Riphean (900–850 Ma) syn-exhumation dynamometamorphism under epidote-amphibolte facies conditions (P = 3.9–4.9 kbar, T = 460–550 ºC) and a low gradient with dT/dH < 10 ºC/km accompanied by the formation of blastomylonitic complexes in shear zones. All these deformation and metamorphic events identified on the western margin of the Siberian craton are correlated with the final episodes of the Late Grenville orogeny and provide supporting evidence for a close spatial connection between Siberia and Laurentia during early Neoproterozoic time, which is in good agreement with recent paleomagnetic reconstuctions.

The geologic position, development stages, age, and geochemical features of metasomatic and felsic igneous rocks along the southern edge of the Siberian craton are compared. The comparison shows that all the studied metasomatic rocks are confined to the faults feathering the main suture zone of the craton. From Biryusa zone in the southwest and farther northeast, from Primor’e zone to Davan shear zone and Katugino–Ayan zone in the Aldan area, the metasomatic rocks are of similar composition but show higher mineralization. The process begins with blastocataclasis (barren stage). During the second stage, ore-bearing (Nb, Zr, Hf, and REE) potassic solutions circulate along the blastocataclastic zones. They form metasomatic potassic rocks of the early alkaline stage, expressed subalkalic granitization. The next (acid) stage is marked by the formation of greisens with Sn, Be, Th, U, and W mineralization. The igneous stage might precede or follow the metasomatism. At the time of ongoing tectonic movements, it produces rapakivi–like granites rich in the same elements. Also, a huge volcanoplutonic belt develops along the craton edge during this time. The geochemical features of its felsic volcanics are close to those of the metasomatic rocks and granites. The age of all these rocks is within 2.1–1.6 Ga.

The age of the major igneous complexes in the western part of the Selenga–Stanovoy superterrane has been estimated by ⁴⁰Ar/³⁹Ar dating: trachyandesite-basalts of the Kuitun (Chichatka) complex — 259.4 ± 6.2 Ma; gabbro of the Tukuringra complex — 156.3 ± 4.8 Ma; granites and pegmatites of the Tukuringra complex — 153.1 ± 3.8, 154.0 ± 4.4, 156.8 ± 4.0, and 151.2 ± 3.2 Ma; granodiorites, granites, and leucogranites of the Amudzhikan complex — 131.7 ± 2.4, 134.5 ± 2.8, and 131.6 ± 4.2 Ma; and lamprophyre dikes — 125.2 ± 2.4 and 125.2 ± 3.4 Ma. Two stages of hydrothermal ore formation process have been recognized: 132–131 and ~125 Ma. It is shown that the deposit formation and superposed processes follow the general regularities of the Late Mesozoic evolution of the Pacific Asia margin.

Data on the Cu content in native gold and silver and the Ag and Au contents in native copper are summarized. The standard thermodynamic functions of solid solutions the Au–Cu and Ag–Cu binary systems and the Ag–Au–Cu ternary system have been estimated. The corresponding calculation module is prepared for the Selektor software.

The Late Neoproterozoic sediments of the Yenisei Ridge formed in several isolated basins. These sediments are correlated, and the composite section of this age in the region is described. Two age limits are of extreme importance: pre-Chapa (650 Ma) and pre–Vendian (600 Ma). The former, observed in a much larger area, predominates. The great importance of the pre–Chapa transformation becomes evident owing to its coevality with the Marino–Nantuo global glaciation and the preceding tectonic events. This glaciation was immediately followed by a significant biotic event that gave rise to the Doushantuo–Pertatataka microfossil assemblage and, afterward, Ediacaran fauna. The Chapa Group is proposed as a prototype of the Angarian–a unit of the General Late Precambrian scale, which is of the same rank as the Vendian. According to these data, the main Late Neoproterozoic units are the Baikalian, Angarian, and Vendian.