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Russian Geology and Geophysics

2014 year, number 2

1.
MANTLE PLUMES IN NORTHEASTERN ASIA AND THEIR ROLE IN THE FORMATION OF ENDOGENOUS DEPOSITS

M.I. Kuzmin1, V.V. Yarmolyuk2
1A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
2Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 109017, Russia
Keywords: Rift, hot fields, absolute reconstruction, large igneous province

Abstract >>
The Phanerozoic within-plate magmatism and the related deposits of Siberia are reviewed. The formation of post-perovskite at about 2.5 Ga in the Earth’s interior and the isotope characteristics of within-plate igneous rocks have shown that plate tectonics and deep geodynamics started to operate at about 2–2.5 Ga. The assembly and breakup of supercontinents under the effect of the superplumes formed in layer D″is considered. Thus, the supercontinent-superplume cycles spanning about 700 Ma are recognized in the Earths history. The manifestations of the within-plate magmatic activity are found throughout the whole Phanerozoic. It was demonstrated earlier that between 570 and 160 Ma, the Siberian continent drifted within the African hot mantle field or large low shear velocity province (LLSVP). At least four plumes, excluding the superplume leading to the breakup of Rodinia at 750 Ma, interacted with the Siberian continent. The superplume leading to the breakup of Rodinia was also responsible for the origin of ultramafic intrusions with carbonatites hosting rare-metal (Nb, Ta, REE) mineralization as well as ultramafic-mafic intrusions with CuNiPt mineralization localized along the rift zones. The plumes originated in other Phanerozoic cycles formed most likely at the lower-upper mantle boundary, where most of the stagnant slabs are accumulated. Those plumes were responsible for the origin of within-plate igneous rocks. The granitic batholiths formed in the centers of zonal area surrounded by rift zones containing abundant rare-metal intrusions with rare-metal mineralization. Gold, tin, base metal, and porphyry copper deposits are also related to these zonal area. The studies have shown that the formation of folded zones and related deposits which surround these zones as well as the structures of cratons and their metallogenic specialization should be considered in terms of both plate tectonics and plume tectonics.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.002



2.
MIDDLE PALEOZOIC BASALTIC AND KIMBERLITIC MAGMATISM IN THE NORTHWESTERN SHOULDER OF THE VILYUI RIFT, SIBERIA: RELATIONS IN SPACE AND TIME

A.I. Kiselev1, V.V. Yarmolyuk2, A.V. Ivanov1, K.N. Egorov1
1Institute of Earths Crust, Siberian Branch of the Russian Academy of Science, ul. Lermontova 128, Irkutsk, 664033, Russia
2Institute of Mineral Geology, Petrography, Mineralology, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 190017, Russia
Keywords: Kimberlite basalt, dike swarm, Devonian plume, 40Ar/39Ar ages, Siberian craton, Vilyui rift

Abstract >>
A Middle Paleozoic tectonothermal event in the eastern Siberian craton was especially active in the area of the Vilyui rift, where it produced a system of rift basins filled with Devonian-Early Carboniferous volcanics and sediments, as well as long swarms of mafic dikes on the rift shoulders. Basalts occur mostly among Middle Devonian sediments and are much less spread in Early Carboniferous formations. The dolerite dikes of the Vilyui- Markha swarm in the northwestern rift border coexist with the Mirnyi and Nakyn fields of diamond-bearing kimberlites. The voluminous dikes and sills intruded before the emplacement of kimberlites. The Mir kimberlite crosscuts a dolerite sill and a dike in the Mirnyi field, while a complex dolerite dike (monzonite porphyry) cuts through the Nyurba kimberlite in the Nakyn field. Thus, the kimberlites correspond to a longer span of Middle Paleozoic basaltic magmatism. The basalts in Middle Paleozoic sediments have faunal age constraints, but the age of dolerite dikes remains uncertain. The monzonite porphyry dike in the Nyurba kimberlite has been dated by the 40Ar/39Ar method, and the obtained age must be the upper bound of the dike emplacement. The space and time relations between basaltic and kimberlitic magmatism were controlled by Devonian plume-lithosphere interaction.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.003



3.
LATE PALEOZOIC GRANITOIDS IN WESTERN TRANSBAIKALIA: SEQUENCE OF FORMATION, SOURCES OF MAGMAS, AND GEODYNAMICS

A.A. Tsygankov
Geological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Sakhyanovoi 6a, Ulan-Ude, 670047, Russia
Keywords: Granitoid magmatism, sources of magmas, isotopic composition, geodynamics, western Transbaikalia

Abstract >>
The evolution of Late Paleozoic granitoid magmatism in Transbaikalia shows a general tendency for an increase in the alkalinity of successively forming intrusive complexes: from high-K calc-alkaline granites of the Barguzin complex (Angara-Vitim batholith) at the early stage through transitional calc-alkaline-alkaline granites and quartz syenites (Zaza complex) at the intermediate stage to peralkaline granitoids (Early Kunalei complex) at the last stage. This evolution trend is complicated by the synchronous development of granitoid complexes with different sets and geochemical compositions of rocks. The compositional changes were accompanied by the decrease in the scales of granitoid magmatism occurrence with time. Crustal metaterrigenous protoliths, possibly of different compositions and ages, were the source of granitoids of the Angara-Vitim batholith. The isotopic composition of all following granitoid complexes points to their mixed mantle-crustal genesis. The mechanisms of granitoid formation are different. Some granitoids formed through the mixing of mantle and crustal magmas; others resulted from the fractional crystallization of hybrid melts; and the rest originated from the fractional crystallization of mantle products or the melting of metabasic sources with the varying but subordinate contribution of crustal protoliths. Synplutonic basic intrusions, combined dikes, and mafic inclusions, specific for the post-Barguzin granitoids, are direct geologic evidence for the synchronous occurrence of crustal and mantle magmatism. The geodynamic setting of the Late Paleozoic magmatism in the Baikal folded area is still debatable. Three possible models are proposed: (1) mantle plume effect, (2) active continental margin, and (3) postcollisional rifting. The latter model agrees with the absence of mafic rocks from the Angara-Vitim batholith and with the post-Barguzin age of peralkaline rocks of the Vitim province.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.004



4.
GEOCHEMISTRY OF EARLY PALEOZOIC GRANITOIDS OF THE BAIKAL REGION AND THEIR GEODYNAMIC SETTING EXEMPLIFIED BY THE KHAMAR-DABAN RIDGE AND OLKHON ISLAND

V.S. Antipin, N.V. Gorlacheva, V.A. Makrygina
A.P. Vinogradov Institute of Geolochemisnty, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
Keywords: Geochemistry, granitoids, geochemical types, collision, metamorphic sequences

Abstract >>
Comparative study of geological and isotope-geochemical features of the Early Paleozoic granitoids of the Khamar-Daban Ridge and Olkhon Island located in the Baikal region has revealed their close age and composition. Besides, they were referred to as syncollisional S -type formations derived from gneiss-schistose substratum of metamorphic sequences. Granitoids of the Solzan massif in the Khamar-Daban Ridge, as well as the Sharanur complex on Olkhon Island, occur in the autochthonous and allochthonous facies. They primarily consist of migmatites, plagiogranites, gneiss granites, and K-Na-granites. The igneous rocks of the Sharanur complex include subalkaline granosyenites and quartz syenites spatially proximal to K-Na-granites. In the north of the island we investigated alkaline syenites which might be related to the Budun massif of basic rocks. On Olkhon Island in the Tashkiney valley, the surveyors recognized the geochemical type of pegmatoid rare-metal granites bearing beryllium mineralization. As was found, they are distinguished from Be-muscovite and spodumene pegmatites of the Khamar-Daban by high Rb, Cs, Sn, Nb, Ta, and W but low Li concentrations, which is probably due to Li-enrichment in the protolith of the Kornilova Formation relative to the Olkhon sequence. This points to the inheritance of the protolith composition at all stages of syncollisional granite formation. The geochemical study has shown similarity of calc-alkaline and subalkaline granitoids of the Khamar-Daban Ridge and Olkhon Island and their affinity in age and average composition of the regional continental crust. In addition, it has revealed the evidence for the existence of the Olkhon-Khamar-Daban block occurring as a single terrane in the Baikal region.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.005



5.
ORIGIN AND EVOLUTION OF NEOGENE ALKALI–BASALTIC MAGMAS IN THE SOUTHWESTERN FLANK OF THE BAIKAL RIFT ZONE (Heven lava plateau, northern Mongolia)

S.S. Tsypukova1, A.B. Perepelov1, E.I. Demonterova2, L.A. Pavlova1, A.V. Travin3, M.Yu. Puzankov4
1A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
2Institute of the Earths Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia
3V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
4Institute of Volcanology and Seismology, Far Eastern Branch of the Russian Academy of Sciences, bulv. Piipa 9, Petropavlovsk-Kamchatsky, 683006, Russia
Keywords: Rift zone, plume, basaltic magma, crystallization, sources

Abstract >>
The Heven lava plateau in the Hövsgöl field of the South Baikal igneous province formed in the Early-Middle Miocene between 20 and 15.5 Ma. It consists of Early Miocene hawaiites and trachybasalts and Middle Miocene basanites erupted, correspondingly, during two major events in its history. The Heven alkali-basaltic lavas are compositionally similar to their counterparts from other volcanic fields in the southern flank of the Baikal rift system and are richer in Ba, K, Pb, and Sr than oceanic island basalts (OIB). The basanitic, hawaiitic, and trachybasaltic magmas were generated at pressures from 25 to 15 kbar and at temperatures in the range from 1434 to 1358 ºC. The magma sources occurred at 74 to 41 km in asthenospheric and lithospheric mantle and were ~200ºC hotter than the ambient lithospheric mantle in the surrounding areas and the continental geotherm. The crystallization history of dark-colored began with liquidus highly magnesian olivine and Cr-spinel, and then several other parageneses formed successively as pressures and temperatures decreased: Ol + Cpx and Ol + Cpx+ + TiMgt ± Pl phenocrysts and subphenocrysts, Cpx + TiMgt + Ilm + Pl microphenocrysts, and finally interstitial Ne + Kfs alkali aluminosilicates. There were two crystallization stages with different mineral chemistry trends. The chemistry of minerals changed as the rising magmas first reached the crust-mantle region and then moved to shallow depths, erupted, and solidified. The generation of the Heven hawaiite-trachybasalt and basanite magmas was controlled by the depth of the reservoirs and the melt fraction in garnet-bearing asthenospheric and lithospheric mantle associated with progressive and regressive dynamics of the lower heterogeneous mantle plume consisting of PREMA and EMI components.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.006



6.
EARLY CRETACEOUS GRANITOIDS OF THE SAMARKA TERRANE (SikhoteAlin): GEOCHEMISTRY AND SOURCES OF MELTS

N.N. Kruk1, V.P. Simanenko2, V.I. Gvozdev2, V.V. Golozubov2, V.P. Kovach3, P.I. Serov4, V.V. Kholodnov5, E.Yu. Moskalenko2, M.L. Kuibida1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Far Eastern Geological Institute, Far Eastern Branch of the Russian Academy of Sciences, pr. 100-letiya Vladivostoka 159, Vladivostok, 690022, Russia
3Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034, Russia
4Geological Institute of the Kola Research Center, ul. Fersmana 14, Apatity, Murmansk Region, 184209, Russia
5Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620151, Russia
Keywords: Early Cretaceous granitoids, geochemistry, isotope composition, petrogenesis, Sikhote-Alin

Abstract >>
We present new data on the geologic position, chemical composition, and isotope characteristics of the Early Cretaceous granitoids of the Samarka terrane, Sikhote-Alin’, formed on a transform continental margin. Geological and geochronological data show that these granitoids were generated at two stages of magmatism: in the first half (Hauterivian-Barremian, 130–123 Ma) and second half (Albian-Cenomanian, 11098 Ma) of the Early Cretaceous. Granitoids of the first stage form an autonomous (free of basic precursors) unimodal melanogranite-granite association and are characterized by normal alkalinity with domination of K over Na, low contents of Ca, and elevated contents of Al2O3. By composition, these are Sgranites with a model Nd age of ~1.3 Ga. Granitoids of the second stage are of more diverse petrogeochemical types. They show wider variations in K/Na and Shend Index are richer in Ca and, sometimes, Sr, and are poorer in P than the granitoids of the first stage. Their compositions form a continuous trend from Sto Igranites, and their model Nd age is ≤1.2 Ga. Comparison of the petrochemical, trace-element, and isotope characteristics of the Early Cretaceous granitoids and upper-crustal rocks (sandstones and siltstones of the turbidite matrix of a Jurassic accretionary prism and basalts from the inclusions in it) of the Samarka terrane and the coeval garrboids has shown that the potassic Sgranitoids formed at the early (Hauterivian-Barremian) stage of magmatism as a result of the anatexis of upper-crustal sedimentary rocks. At the late (Albian-Early Cenomanian) stage, the intrusion of mantle magmas led to a temperature increase in the lower crust, which favored more active anatexis, involvement of high-melting substrates (oceanic basalts) in the granite formation, and interaction of mantle and crustal magmas. This resulted in a great diversity of granitoids (from Sto Itype).

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.007



7.
LARGE FIELDS OF SPODUMENE PEGMATITES IN THE SETTINGS OF RIFTING AND POSTCOLLISIONAL SHEAR–PULLAPART DISLOCATIONS OF CONTINENTAL LITHOSPHERE

V.E. Zagorsky1, A.G. Vladimirov2,3,4, V.M. Makagon1, L.G. Kuznetsova1, S.Z. Smirnov2,3,4, B.A. Dyachkov5,6, I.Yu. Annikova2,4, S.P. Shokalsky7, A.N. Uvarov8
1A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
3Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
4Tomsk State University, pr. Lenina 36, Tomsk, 634050, Russia
5Altai Geological and Ecological Institute, named after K.I. Satpaev, ul. Karla Libknekhta 21, Ust-Kamenogorsk, 692024, Kazakhstan
6East Kazakhstan State Technical University, ul. Serikbaeva 19, Ust-Kamenogorsk, 070010, Kazakhstan
7A.P. Karpinsky Russian Geological Research Institute, Srednii pr. 74, St. Petersburg, 199106, Russia
8Zapsibgeolsemka, ul. Shkolnaya 5, Elan Village, Novokuznetsk district, Kemerovo Region, 654219, Russia
Keywords: Lithium, spodumene, pegmatites, granite-pegmatite systems, geochronology, collision, rifting, Central Asian Fold Belt

Abstract >>
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.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.008



8.
NOBLE–METAL ORE GENESIS AND MANTLECRUST INTERACTION

N.A. Goryachev
Northeastern Complex Research Institute, Far Eastern Branch of the Russian Academy of Sciences, ul. Portovaya 16, Magadan, 685000, Russia
Keywords: Orogenic gold deposits, genesis, orogenic belts, mantle-crust interaction

Abstract >>
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 Auquartz), AuBi, AuAg, AuSb, AgSb, AuSbHg, and AgHg. 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 AuAg, AgSb, AgHg, AuSbHg, and Au-Bi deposits. Collision events gave rise to Au and AuBi deposits. Intraplate continental rifting and formation of orogenic belts along the boundaries of block (plate) sliding led to the origin of Au and AuBi ores in association with AuAg, AuSbHg, 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.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.009



9.
COMPOSITION AND EVOLUTION OF PGE MINERALIZATION IN CHROMITE ORES FROM THE IL’CHIR OPHIOLITE COMPLEX (OspaKitoi and KharaNur areas, East Sayan)

O.N. Kiseleva1, S.M. Zhmodik1, B.B. Damdinov2, L.V. Agafonov1, D.K. Belyanin1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Geological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Sakhyanovoi 6a, Ulan-Ude, 670047, Russia
Keywords: Chromitites, chrome-spinel, PGE minerals, PGE remobilization and redeposition

Abstract >>
Data are presented on chromitites from the northern and southern sheets of the Il’chir ophiolite complex (Ospa–Kitoi and KharaNur 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 mediumand lowalumina 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 OsIrRu solid solutions as well as sulfides and sulfoarsenides of these metals. The chromitites from the northern sheet contain the same PGE minerals and diverse RhPtPd mineralization: PtIrRuOs and isoferroplatinum with Ir and OsIrRu lamellae. Areas of altered chromitites contain a wide variety of low-temperature secondary PGE minerals: PtCu, PtPdCu, PdHg, Rh2SnCu, RhNiAs, PtAs2, and PtSb2. The speciation of the PGE minerals and multiphase intergrowths is described. The relations of OsIrRu solid solutions with laurite and irarsite are considered along with the microstructure of irarsite-osarsite-ruarsite solid solutions. Zoned OsIrRu crystals have been found. Zone Os82-99 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 Ilchir ophiolite belt. The PtIrRuOs and (Os, Ru)S2 inclusions in OsIrRu solid solutions might be relics of primitivemantle PGE minerals. During the partial melting of the upper mantle, OsIrRu and PtFe solid solutions formed syngenetically with the chromitites. During the late-magmatic stage, OsIrRu 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, PtCu, PtCuFeNi, CuPtPd, and RhCuSnSb. The zoned OsIrRu 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 450600ºC and 1316 kbar. During the crustal metamorphic stage, tectonic movements (obduction) and a change from reducing to oxidizing conditions were accompanied by the successive transformation of chromespinel into ferrichromitechromemagnetite with the active participation of a metamorphic fluid enriched in crustal components. The orcelitemaucheriteferrichromitesperrylite assemblage formed in epidoteamphibolitic facies settings during this stage. The PGE mineral assemblage reflects different stages in the formation of the chromitites and duniteharzburgite host rocks and their transformation from primitive mantle to crustal metamorphic processes.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.010



10.
STRUCTURALLY AND SUPERFICIALLY BOUND GOLD IN PYRITE FROM DEPOSITS OF DIFFERENT GENETIC TYPES

V.L. Tauson, R.G. Kravtsova, N.V. Smagunov, A.M. Spiridonov, V.I. Grebenshchikova, A.E. Budyak
A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
Keywords: Gold, pyrite, speciation, distribution, surface, nonautonomous phase, gold deposits

Abstract >>
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 oreforming processes. The data on structurally bound gold permit comparative evaluation of gold concentrations in ore fluids forming gold deposits of different genetic types.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.011



11.
TYPES OF ENDOGENOUS GEOCHEMICAL FIELDS AND THEIR SIGNIFICANCE FOR PROSPECTING

A.M. Spiridonov, L.D. Zorina, V.A. Romanov
A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, 664033, Irkutsk, Russia
Keywords: Endogenous geochemical fields, classification, composition, structure, zonation, geochemical methods of deposit prospecting

Abstract >>
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.

DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.012