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Russian Geology and Geophysics

2005 year, number 6

1.
PARAMETERS OF HOT SPOTS AND THERMOCHEMICAL PLUMES

N.L. Dobretsov, A.G. Kirdyashkin, and A.A. Kirdyashkin
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Hot spot, thermochemical plume, heat and mass transfer, melt flow rate, thermal power, Lewis number, temperature gradient
Pages: 575-588

Abstract >>
To estimate conditions of heat and mass transfer through plume channel, three kinds of plumes have been recognized: (a) plumes located in oceanic and continental regions far from spreading zones; (b) plumes at the axis of a mid-oceanic ridge, and (c) plumes situated near the ridge axis.
Given geologic and geophysical data, the flow rates of the effused magma have been estimated. The flow rate values permitted us to estimate the thermal power of the source of the plumes formed at the core-mantle boundary. The source powers for the Bouvet, Hawaiian, and Iceland plumes are 1.57 · 107, 3.02 · 108, and 3.78 · 108 kW, respectively.
We have analyzed the heat and mass transfer of a thermochemical plume rising from the core-mantle boundary. The most probable values of melt viscosity in the plume channel and Lewis numbers for a chemical additive which lowers the melting point at the core-mantle boundary have been obtained for plumes with N = (3.0-3.8) · 108 kW, rising for 1-5 Myr. For these Lewis numbers, typical temperature drops ΔTII between the melting point of the massif and the temperature of plume sole have been found. For the given ΔTII at Le = 100-1000, such parameters as additive concentration, temperature drop, power, and time and velocity of rise are calculated for the plume source diameters at the core-mantle boundary, ds, of 70 and 100 km.



2.
RARE-EARTH ELEMENTS IN METASOMATITES AND ORES OF THE DUKAT GOLD-SILVER DEPOSIT (northeastern Russia)

R.G. Kravtsova, M.N. Zakharov, and O.I. Korkina†
Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Geochemistry, rare-earth elements, igneous rocks, metasomatites, ores
Pages: 589-603

Abstract >>
The distribution of rare-earth elements (REE) in metasomatites and ores of the Dukat epithermal gold-silver deposit in northeastern Russia has been studied for the first time. The deposit occurs in the sequence of Lower Cretaceous acid effusive rocks intruded by a granite massif. Granites, acid effusive rocks, rhyodacitic ignimbrites, and felsites form a volcanoplutonic association; in leucogranites, ΣREE = 288 ppm. Rhyodacitic ignimbrites contain more REE (191-203 ppm) than felsitic lavas and dikes (105-127 ppm). The ignimbrites and leucogranites have similar proportions of major lanthanide groups - 78Ce 16Y6Sc and 77 Ce 16 Y 17 Sc , respectively. High REE contents have been found in hornfels (up to 254 ppm) and greisens (up to 410 ppm) developed after acid effusive rocks at their contact with the granite intrusion. Processes of pre-ore areal sericitization and hydromicatization in acid volcanics contribute to the REE accumulation in hydrothermally altered rocks of the roof of the Dukat intrusion, particularly in hydromicatized volcanics (up to 404 ppm). Near-ore propylites developed after andesites on the deposit flanks are enriched in REE (up to 174 ppm). Altered argillizites of the central part of the deposit, which host gold-silver mineralization, are depleted in REE (61 ppm). Despite the difference in ΣREE, all wallrocks, irrespective of their composition and spatial occurrence, are enriched in light lanthanides. The Ce/Yb values vary from 37 to 46 in the altered propylites and argillizites and do not exceed 21 in the other wallrocks. The proportion of lanthanide groups in the altered propylites (88 Ce 10 Y 2 Sc ) is nearly the same as in the altered argillizites (88 Ce 11 Y 1 Sc ). In polychronous ores of the Dukat deposit, ΣREE varies over a broad range of values (3.9-37 ppm). High REE contents (up to 37 ppm) have been found in tin-silver-polymetallic ores of deep horizons of the deposit, and low ones, in later formed gold-silver and silver ores of the upper and middle horizons of its central part. Rejuvenated gold-silver ores show a minor variation in ΣREE (5.1-5.7 ppm). Minimum concentrations have been found in the last produced predominantly silver ores (3.9-5.1 ppm). Hydrophile lanthanides of the cerium group prevail in these ores but to a lower degree than in the altered metasomatites. The Ce/Yb values in the ores do not exceed 22.



3.
LATE PALEOZOIC MAGMATIC ARC ON THE NORTHERN PERIPHERY OF THE ARGUN' TERRANE AND ASSOCIATED GOLD MINERALIZATION (upper-Amur region)

I.V. Buchko and A.A. Sorokin
Division of the Regional Geology and Hydrogeology of the Amur Scientific Center,
Far Eastern Branch of the RAS, 2 ul. B. Khmel'nitskogo, Blagoveshchensk, 675000, Russia
Keywords: Argun' terrane, active margin, gold-ore occurrences
Pages: 604-611

Abstract >>
In the Amur region, a series of gold-ore occurrences belonging to gold-polysulfide-quartz and gold-quartz formations (Tomskoe, Chitkan, Vyruchka, Urusha) have been recognized within the northern periphery of the Argun' terrane. These occurrences are associated with the formation of Late Paleozoic magmatic arc on the active continental margin. The latter is a chain of gabbro-diorite-granodiorite-granite intrusions of the Urusha complex with the isotopic age of 274-278 Ma (U-Pb zircon method), which extend along the structural boundary between the terrane and the Mongolo-Okhotsk fold belt. In geochemical and isotope-geochemical characteristics ((87Sr/86Sr)0 = 0.70527-0.70724 at ε Nd (t) = -1...+2.2) the granitoids of this association correspond to type I and are similar to granitoids of active continental margins. The ore occurrences under study are localized within endo- and exocontact zones of Late Paleozoic granitoids or slightly off them and are quartz-sulfide veinlets inheriting tectonic zones of weakness. Gold contents in the veinlets reach 30 ppm (atomic-absorption analysis). The orebodies are composed mainly of quartz, chlorite, sericite, zoisite, arsenopyrite, pyrite, galena, sphalerite, and chalcopyrite. Gold is present both in free (Chitkan) and in sulfide-associated (Tomskoe) forms. In the latter case, its content in sulfides reaches 300 ppm (chemical-spectral analysis). These ore occurrences evidence that the setting of active continental margin that existed along the northern periphery of the Argun' terrane in the Late Paleozoic favored the formation of various ore objects, including gold-bearing ones. The subsided tectonic blocks of this structure seem to be promising for commercial gold deposits.



4.
Rb-Sr AGE AND SEQUENCE OF FORMATION OF GRANITOIDS OF THE KHORINKA VOLCANOPLUTONIC STRUCTURE IN THE MONGOLO-TRANSBAIKALIAN BELT

V.F. Posokhov, M.G. Shadaev, B.A. Litvinovsky*, A.N. Zanvilevich*, and V.B. Khubanov
Geological Institute, Siberian Branch of the RAS, 6a ul. Sakh'yanovoi, Ulan Ude, 670047, Russia
* Ben Gurion University, Beer Sheva, Israel
Keywords: Alkali granites, Rb-Sr age, Transbaikalia
Pages: 612-619

Abstract >>
The Khorinka volcanoplutonic structure lies in the central part of the Mongolo-Transbaikalian alkali-granite belt (MTB) extending for more than 2500 km through Northern Mongolia and Transbaikalia. The MTB formed in two stages: 280-270 and 230-210 Ma. The Khorinka structure is ca. 3000 km2 in area. Syenites and granites prevail at the modern denudation level of its rocks. Only a minor part of volcanic comagmates (bimodal trachybasalt-rhyolite series) has been preserved in this structure. The volcanic stage was preceded by the formation of a bimodal dike belt. Granitoid massifs resulted from the subsequent intrusion of alkali-feldspathic, alkaline, and syenogranitic magmas. Rb-Sr isotope dating has shown that the Khorinka structure was produced at the early stage of the MTB formation (~280 ± 5 Ma). The Rb-Sr ages for alkali granitoids agree with the literature U-Pb zircon ages of these rocks. The (87Sr/86Sr)0 value is 0.7073 in alkali and feldspathic granitoids and 0.7033 in syenogranites, which points to different sources of their parental acid magmas.



5.
THE ERDENETIYN-OVOO PORPHYRY Cu-Mo DEPOSIT, NORTHERN MONGOLIA: 40Ar/39Ar GEOCHRONOLOGYAND FACTORS OF LARGE-SCALE MINERALIZATION

V.I. Sotnikov, V.A. Ponomarchuk, D.O. Shevchenko, and A.P. Berzina
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Porphyry Cu-Mo mineralization, intrusive complexes, 40Ar/39Ar geochronology, 87Sr/86Sr ratios, Erdenetiyn-Ovoo porphyry Cu-Mo deposit, Mongolia
Pages: 620-631

Abstract >>
40Ar/39Ar geochronology shows that the formation of the Erdenetiyn-Ovoo porphyry Cu-Mo deposit in Northern Mongolia related to the Erdenet ore-bearing porphyry complex was preceded by multiphase gabbro-granitoid magmatism in the region which produced the 258.6 ± 3.3 and 247 ± 3.7 Ma Selenge granitoids and the 244.6 ± 1.0-239.3 ± 1.4 Ma Shivota complex. The ore-bearing porphyry complex underwent a rhythmic evolution. The main mineralization stage was associated with porphyry magmatism of the first (234.6 ± 1.7 Ma) and second (225.3 ± 1.0-220.3 ± 5.8 Ma) pulses. The 40Ar/39Ar age of sericite from metasomatic rocks of the main mineralization stage is 235.8 ± 1.9 Ma. Intrusives of different ages (dated on accessory apatite) and early high-temperature ore formation products (on anhydrite) have low 87Sr/86Sr ratios of 0.70393-0.70437 corresponding to the mantle values. Mantle sources are likewise indicated by δ34S of sulfides which approach the chondrite norm. Repeated magmatism and mineralization in the Erdenet region provided favorable conditions for the formation of a large porphyry Cu-Mo deposit. Especially important was the rhythmic development of the ore-bearing porphyry complex in which each rhythm was accompanied by mineralization and metasomatism.



6.
CHEMICAL CHARACTERISTICS OF NATURAL WATERS: DEPENDENCE ON THE GEOLOGIC STRUCTURE OF THE URONAI ORE UNIT, EASTERN TRANSBAIKALIA

V.A. Balandis, L.V. Zamana*, and A.B. Ptitsyn*
Institute of Geology, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
*Institute of Mineral Resources, Ecology and Cryology, Siberian Branch of the RAS,
26 ul. Butina, Chita, 627090, Russia
Keywords: Hydrogeochemistry, geoinformation technology, molybdenum, tungsten
Pages: 632-639

Abstract >>
A GIS-based technique has been developed to treat georeferenced geologic and hydrogeochemical data for estimating the buried mineralization within the Uronai ore unit. A GIS project is proposed, which includes data on relief, geomorphology, geology, metallogeny, and chemical composition of natural waters. This project turned to be useful in distinguishing several hydrogeochemical anomalies and promising sites with buried ores of molybdenum and tungsten.



7.
PETROLEUM POTENTIAL OF MESOZOIC, PALEOZOIC AND VENDIAN DEPOSITS IN STEPNOI ALTAI AND THE SURROUNDING MOUNTAINS: GEOCHEMICAL EVIDENCE

N.V. Sennikov, A.N. Fomin, V.P. Danilova, E.N. Ivanova, A.E. Kontorovich, and E.A. Kostyreva
Institute of Petroleum Geology, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Petroleum potential, geochemistry, organic matter, organic matter maturity, Mesozoic, Paleozoic, Vendian, bitumens, biomarker, hydrocarbons
Pages: 640-654

Abstract >>
The petroleum potential of Mesozoic, Paleozoic and Vendian strata in Stepnoi Altai was investigated using geochemical criteria. Maturity of organic matter, distribution of organic carbon, and the contents and composition of bitumens were studied, with the main focus on the molecular composition of saturated hydrocarbons. Bitumens in Paleozoic and Vendian rocks are depleted in asphaltenes because of high maturity of organic matter (up to metamorphic grade) and display the full range of biomarker hydrocarbons typical of phytoplanktonic and bacterial lipids. According to the geochemical evidence, Paleozoic formations may store small accumulations of asphaltene-free light aliphatic syngenetic oil and condensate.



8.
HEAT FLOW IN CENOZOIC RIFTS IN EAST CHINA

S.V. Lysak, and R.P. Dorofeeva
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Rifting, heat flow, deep structure, geodynamics, geothermal activity
Pages: 655-667

Abstract >>
Heat flows in continental East China, which includes four Cenozoic rifts that belong to the western or eastern rift systems, vary from 26 to 106 mW/m2, with the mean of 61 ± 2 mW/m2. The rifts are generally hotter than their surroundings (the mean of 68-70 mW/m2 against 49-56 mW/m2). Especially high deep flow over 80-100 mW/m2 is associated with rift basins and active faults. The intensity of rifting and recent geothermal activity in the region correlate with heat flow variations and are consistent with the evolution and deep structure of main tectonic units. Cenozoic rifting in the eastern system and its abnormal heat flows are induced by heating from a rising mantle plume related to the subduction of the Pacific plate beneath eastern Eurasia. Rifting in the western system has been controlled mostly by horizontal strain and rotation of the Ordos block under the joint effect of India and Eurasia, with minor thermal control.