GEOCHEMISTRY, ZIRCON U-PB GEOCHRONOLOGY, ND-HF ISOTOPIC CHARACTERISTICS AND TECTONIC IMPLICATIONS OF THE SOUTH MUYA BLOCK METASEDIMENTS (NORTHEASTERN CENTRAL ASIAN OROGENIC BELT)

The Neoproterozoic to Cenozoic collage of the Central Asian Orogenic Belt is well-known to include Precambrian continental blocks and microcontinents traditionally attributed to rifting of Siberia or Gondwana prior to CAOB assembly that significantly contributed into the geochemical and isotopic composition of younger subduction- and accretion-related crustal lithologies via processes of crust-mantle interaction and crustal recycling.The Neoproterozoic to Cenozoic collage of the Central Asian Orogenic Belt is well-known to include Precambrian continental blocks and microcontinents traditionally attributed to rifting of Siberia or Gondwana prior to CAOB assembly that significantly contributed into the geochemical and isotopic composition of younger subduction- and accretion-related crustal lithologies via processes of crust-mantle interaction and crustal recycling

GEOCHRONOLOGY AND SR-ND ISOTOPE GEOCHEMISTRY OF LATE PALEOZOIC COLLISIONAL GRANITOIDS OF UNDINSKY COMPLEX (EASTERN TRANSBAIKAL REGION)

There are several geodynamic models of the Central Asian Orogenic Belt (CAOB) development [Şengör et al., 1993, Zorin, 1999; Parfenov et al., 1999, 2003; Willem et al., 2012; and others]. The Mongol-Okhotsk Orogenic Belt (MOB) represents important part of CAOB. All geodymanic models of Late Riphean to Paleozoic structures of CAOB emphasize significance of subduction processes along Northern Asian craton margin at that time. Collage of CAOB terrains formed as a result of accretion of island arc, accretionary wedge, turbidite, and continental margin terrains to the Siberian paleocontinent.There are several geodynamic models of the Central Asian Orogenic Belt (CAOB) development [Şengör et al., 1993, Zorin, 1999; Parfenov et al., 1999, 2003; Willem et al., 2012; and others]. The Mongol-Okhotsk Orogenic Belt (MOB) represents important part of CAOB. All geodymanic models of Late Riphean to Paleozoic structures of CAOB emphasize significance of subduction processes along Northern Asian craton margin at that time. Collage of CAOB terrains formed as a result of accretion of island arc, accretionary wedge, turbidite, and continental margin terrains to the Siberian paleocontinent

CENTRE OF ISOTOPIC AND GEOCHEMICAL RESEARCH (IGC SB RAS): CURRENT STATE OF MICRO- AND MACROANALYSIS

Centre of Isotopic and Geochemical Research based on the Analytical Department of Vinogradov Institute of Geochemistry SB RAS (Irkutsk) performs a wide range of analytical studies to solve mineralogical and petrological, geochemical, prospecting, ecological, paleoclimatic and applied problems. The studies are supported by the modern equipment for electron microprobe, X-ray diffraction, X-ray fluorescence, atomic emission and mass spectrometric (including isotope) analyses, as well as the necessary international certified reference materials (SRM) and a collection of SRM of the natural and technogenic composition of our production

ЦЕНТР КОЛЛЕКТИВНОГО ПОЛЬЗОВАНИЯ «ИЗОТОПНО-ГЕОХИМИЧЕСКИХ ИССЛЕДОВАНИЙ» ИГХ СО РАН: СОВРЕМЕННОЕ СОСТОЯНИЕ МЕТОДОВ ИЗУЧЕНИЯ ВЕЩЕСТВА НА МИКРО- И МАКРОУРОВНЕ

Centre of Isotopic and Geochemical Research based on the Analytical Department of Vinogradov Institute of Geochemistry SB RAS (Irkutsk) performs a wide range of analytical studies to solve mineralogical and petrological, geochemical, prospecting, ecological, paleoclimatic and applied problems. The studies are supported by the modern equipment for electron microprobe, X-ray diffraction, X-ray fluorescence, atomic emission and mass spectrometric (including isotope) analyses, as well as the necessary international certified reference materials (SRM) and a collection of SRM of the natural and technogenic composition of our production.Центр коллективного пользования «Изотопно-геохимические исследования», организованный на базе Аналитического отдела Института геохимии им. А.П. Виноградова СО РАН (г. Иркутск), выполняет широкий спектр аналитических исследований для решения минералого-петрологических, геохимических, геолого-поисковых, экологических, палеоклиматических и прикладных задач. Исследования обеспечены современным оборудованием для рентгеноспектрального электронно-зондового микроанализа, рентгеноструктурного и рентгенофлуоресцентного, атомно-эмиссионного и масс-спектрометрического (включая изотопный) анализа, а также необходимыми международными стандартными образцами (СО) и коллекцией СО состава природных и техногенных сред собственного производства

Evolution history of the Neoproterozoic eclogite-bearing complex of the Muya dome (Central Asian Orogenic Belt) : constraints from zircon U−Pb age, Hf and whole-rock Nd isotopes

U−Pb dating and Hf-isotope analysis of zircons and whole-rock Nd-isotope analyses were carried out on country rocks of the eclogite-gneiss complex of the North Muya dome in the Anamakit-Muya zone of the Baikal Muya accretionary fold belt. Zircons from garnet-biotite gneisses (Qtz + Kfsp + Pl + Bt + Grt) and garnet-biotite-muscovite schist were dated using the LA-ICP-MS technique. Based on U−Pb isotope data and CL images zircon grains were divided into three groups: detrital, magmatic and metamorphic zircons. Metamorphic zircons display no zoning or the cloudy zoning. The grains morphology together with the well-developed oscillatory zoning clearly identifies the igneous origin of magmatic zircons. The metamorphic zircons (ages 576-680 Ma) have Th/U ratios varying from 0.271 to 0.004, whereas the ratio in magmatic zircons ranges from 0.779 to 0.11. Magmatic zircons from granite-gneisses of the North Muya dome exhibit a relatively narrow spread in the crystallization age with the major peak at ca 764 Ma. Younger ages are interpreted as due to the partial resetting of U−Pb system during the subsequent metamorphic evolution. Detrital zircons from two-mica schist sample Mu-93-10 give ages of 1.88-2.66 Ga. The oldest detrital zircon from this sample plots near concordia and has a²°⁷Pb/²°⁶Pb age of 3.2 Ga. Zircons from this sample are characterized by the widest scatter of εHf(t) values (from +13.9 to -15.3) and TCDM model ages (0.82-3.86 Ga). Zircons from other samples have a much narrower ranges of εHf(t) (+11.6 to -0.7) and TCDM (0.85-1.52 Ga). The involvement of older crustal material is also evident from the whole-rock Nd isotopic compositions. The gneisses and schists exhibit a range of Nd isotopic compositions with εNd(t) values ranging from -3.5 to +3.6 and tNd(DM) from 1.64 to 1.09 Ga. The integration of the Hf-isotope data with the age spectra provides with the first evidence for the existence of Mesoarchean crust in the Baykal-Muya sector of the Central Asian Orogenic Belt.11 page(s

GEOCHEMISTRY, ZIRCON U-PB GEOCHRONOLOGY, ND-HF ISOTOPIC CHARACTERISTICS AND TECTONIC IMPLICATIONS OF THE SOUTH MUYA BLOCK METASEDIMENTS (NORTHEASTERN CENTRAL ASIAN OROGENIC BELT)

The Neoproterozoic to Cenozoic collage of the Central Asian Orogenic Belt is well-known to include Precambrian continental blocks and microcontinents traditionally attributed to rifting of Siberia or Gondwana prior to CAOB assembly that significantly contributed into the geochemical and isotopic composition of younger subduction- and accretion-related crustal lithologies via processes of crust-mantle interaction and crustal recycling

MINERALOGICAL AND GEOCHEMICAL EVIDENCE FOR MULTI-STAGE FORMATION OF THE CHERTOVO KORYTO DEPOSIT

Introduction. The Lena gold province is one of the largest known gold resources in the world. The history of its exploration is long, but the genesis of gold mineralization hosted in black shales in the Bodaibo synclinorium still remains unclear. The studies face the challenge of discovering sources for the useful component and mechanisms of its redistribution and concentration. This study aims to clarify the time sequence of the ore mineralization in the Chertovo Koryto deposit on the basis of detailed mineralogical and geochemical characteristics of the ore, wallrock metasomatites and the Early Proterozoic host black shales, and to assess the applicability of the Sukhoi Log model for clarifying the Chertovo Koryto origin.Geological setting. The Lena gold province is located in the junction area of the Siberian platform and the Baikal mountain region (Fig. 1). The main element of its geological structure is the Chuya-Tonoda-Nechera anticline. Its axial segment is marked by horsts composed of the Early Proterozoic rocks with abundant granitoid massifs. The Chertovo Koryto deposit is located within the Kevakta ore complex at the Tonoda uplift, the largest tectonically disturbed block between the Kevakta and Amandrak granitoids massifs. The 150 m thick and 1.5 km long ore zone of the Chertovo Koryto deposit is confined to the hanging wall of the fold-fault zone feathering the Amandrak deep fault (Fig. 2).Composition. In the ore zone, rocks of the Mikhailovsk Formation include carbonaceous shales of the feldspar-chlorite-sericite-quartz composition with nest-shaped ore accumulations of the pyrite-quartz composition and quartz veinlets. In our study, we distinguish five mineral associations resulting from heterochronous processes that sequentially replaced each other:- The earliest association related with the quartz-muscovite-sericite metasomatism and the removal of REE and other elements from the rocks and their partial redeposition;- Metamorphic sulphidization presented by scattered impregnations of pyrrhotite, as evidenced by small lenses of pyrrhotite, which are considerably elongated (axes up to 0.7 cm long) along the foliation planes (Figs 3, a, b);- Ore mineralization represented by a superimposed hydrothermal gold association with arsenopyrite (Fig. 3, d);- Late chalcophilic mineralization formed at the final stage of hydrothermal-metasomatic process (Figs 3, e, f);- Post-ore silification.Geochemical characteristics. The geochemical study of rocks and ores from the Chertovo Koryto deposit show that the rocks of the Mikhailovsk Formation are characterized by higher contents of rock-forming elements, such as of Al2O3, Fe2O3total, MgO, K2O, and P2O5, in comparison to the PAAS standards [Condie, 1993] and the black shale standard composition (SChS-1) [Petrov et al., 2004]. A characteristic feature of the ore zone is that the contents of practically all the oxides, except SiO2, tend to decrease (Table 1). The distribution of rare elements repeats the pattern established for major elements. The least metamorphosed rocks of the Mikhailovsk Formation have higher contents (up to three times) of Cu, Mo, Ba, W, As, Pb relative to the values in the PAAS and SChS-1 standards. In the ore zone, the contents of almost all rare elements are considerably reduced (Table 2). The contents of elements in the siderophile group (Co, Ni) are clearly correlated with the ore processes and increased more than twice in the area of metamorphic changes. Samples with gold-ore grade contents show the highest concentrations of Co and Ni.Conclusion. In our opinion, the Chertovo Koryto deposit was formed in five stages, the first two of which were pre-ore, with ore preparation, and probably considerably distant in time from the main ore-generating event. The staged formation of the Chertovo Koryto deposit correlates with the basic stages in the tectono-metamorphic history of the study region and is consistent with the model showing the formation of Sukhoi Log-type deposits [Nemerov, 1989; Buryak, Khmelevskaya, 1997; Large et al., 2007]

GEOCHRONOLOGY AND SR-ND ISOTOPE GEOCHEMISTRY OF LATE PALEOZOIC COLLISIONAL GRANITOIDS OF UNDINSKY COMPLEX (EASTERN TRANSBAIKAL REGION)

There are several geodynamic models of the Central Asian Orogenic Belt (CAOB) development [Şengör et al., 1993, Zorin, 1999; Parfenov et al., 1999, 2003; Willem et al., 2012; and others]. The Mongol-Okhotsk Orogenic Belt (MOB) represents important part of CAOB. All geodymanic models of Late Riphean to Paleozoic structures of CAOB emphasize significance of subduction processes along Northern Asian craton margin at that time. Collage of CAOB terrains formed as a result of accretion of island arc, accretionary wedge, turbidite, and continental margin terrains to the Siberian paleocontinent

МИНЕРАЛОГО-ГЕОХИМИЧЕСКИЕ СВИДЕТЕЛЬСТВА ПОЛИСТАДИЙНОСТИ ФОРМИРОВАНИЯ МЕСТОРОЖДЕНИЯ ЧЕРТОВО КОРЫТО

Introduction. The Lena gold province is one of the largest known gold resources in the world. The history of its exploration is long, but the genesis of gold mineralization hosted in black shales in the Bodaibo synclinorium still remains unclear. The studies face the challenge of discovering sources for the useful component and mechanisms of its redistribution and concentration. This study aims to clarify the time sequence of the ore mineralization in the Chertovo Koryto deposit on the basis of detailed mineralogical and geochemical characteristics of the ore, wallrock metasomatites and the Early Proterozoic host black shales, and to assess the applicability of the Sukhoi Log model for clarifying the Chertovo Koryto origin.Geological setting. The Lena gold province is located in the junction area of the Siberian platform and the Baikal mountain region (Fig. 1). The main element of its geological structure is the Chuya-Tonoda-Nechera anticline. Its axial segment is marked by horsts composed of the Early Proterozoic rocks with abundant granitoid massifs. The Chertovo Koryto deposit is located within the Kevakta ore complex at the Tonoda uplift, the largest tectonically disturbed block between the Kevakta and Amandrak granitoids massifs. The 150 m thick and 1.5 km long ore zone of the Chertovo Koryto deposit is confined to the hanging wall of the fold-fault zone feathering the Amandrak deep fault (Fig. 2).Composition. In the ore zone, rocks of the Mikhailovsk Formation include carbonaceous shales of the feldspar-chlorite-sericite-quartz composition with nest-shaped ore accumulations of the pyrite-quartz composition and quartz veinlets. In our study, we distinguish five mineral associations resulting from heterochronous processes that sequentially replaced each other:- The earliest association related with the quartz-muscovite-sericite metasomatism and the removal of REE and other elements from the rocks and their partial redeposition;- Metamorphic sulphidization presented by scattered impregnations of pyrrhotite, as evidenced by small lenses of pyrrhotite, which are considerably elongated (axes up to 0.7 cm long) along the foliation planes (Figs 3, a, b);- Ore mineralization represented by a superimposed hydrothermal gold association with arsenopyrite (Fig. 3, d);- Late chalcophilic mineralization formed at the final stage of hydrothermal-metasomatic process (Figs 3, e, f);- Post-ore silification.Geochemical characteristics. The geochemical study of rocks and ores from the Chertovo Koryto deposit show that the rocks of the Mikhailovsk Formation are characterized by higher contents of rock-forming elements, such as of Al2O3, Fe2O3total, MgO, K2O, and P2O5, in comparison to the PAAS standards [Condie, 1993] and the black shale standard composition (SChS-1) [Petrov et al., 2004]. A characteristic feature of the ore zone is that the contents of practically all the oxides, except SiO2, tend to decrease (Table 1). The distribution of rare elements repeats the pattern established for major elements. The least metamorphosed rocks of the Mikhailovsk Formation have higher contents (up to three times) of Cu, Mo, Ba, W, As, Pb relative to the values in the PAAS and SChS-1 standards. In the ore zone, the contents of almost all rare elements are considerably reduced (Table 2). The contents of elements in the siderophile group (Co, Ni) are clearly correlated with the ore processes and increased more than twice in the area of metamorphic changes. Samples with gold-ore grade contents show the highest concentrations of Co and Ni.Conclusion. In our opinion, the Chertovo Koryto deposit was formed in five stages, the first two of which were pre-ore, with ore preparation, and probably considerably distant in time from the main ore-generating event. The staged formation of the Chertovo Koryto deposit correlates with the basic stages in the tectono-metamorphic history of the study region and is consistent with the model showing the formation of Sukhoi Log-type deposits [Nemerov, 1989; Buryak, Khmelevskaya, 1997; Large et al., 2007]. Введение. Ленская золотоносная провинция является одной из крупнейших по запасам золота во всем мире. Несмотря на длительную историю работ на данной территории, генезис месторождений Au, приуроченных к черносланцевым отложениям Бодайбинского синклинория, до сих пор остается открытым. Наиболее дискуссионными являются вопросы, связанные с определением источника полезного компонента и выявлением механизма его перераспределения и концентрирования. Цель настоящей работы – выявление этапности формирования месторождения Чертово Корыто на основании детальной минералого-геохимической характеристики руд, околорудных метасоматитов и вмещающих их черносланцевых отложений раннепротерозойского возраста, а также оценка применимости сухоложской модели для формирования месторождения.Геологическая позиция. Ленская золотоносная провинция приурочена к зоне сочленения Сибирской платформы и Байкальской горной области (рис. 1). Одним из главных элементов геологического строения рассматриваемой площади является Чуйско-Тонодско-Нечерский антиклинорий. Положение осевой части антиклинория подчеркивают выступы пород раннепротерозойского возраста, в которых значительные площади занимают массивы гранитоидов. Месторождение Чертово Корыто расположено в пределах Кевактинского рудного узла, приуроченного к Тонодскому поднятию и представляющего собой крупный интенсивно тектонически нарушенный блок, заключенный между Кевактинским и Амандракским массивами гранитоидов. Рудная зона месторождения, мощностью 150 м и протяженностью 1.5 км, приурочена к висячему боку складчато-разломной области, оперяющей Амандракский глубинный разлом (рис. 2).Вещественный состав. В пределах рудной зоны месторождения породы михайловской свиты представлены углеродсодержащими сланцами полевошпат-хлорит-серицит-кварцевого состава с гнездообразными рудными скоплениями пирит-кварцевого состава и прожилками кварца. Установлено пять минеральных ассоциаций, сформированных в результате разновременных процессов, поэтапно сменяющих друг друга:- наиболее ранняя ассоциация связана с кварц-мусковит-серицитовым метасоматозом и выносом РЗЭ и ряда других элементов из пород с их частичным переотложением;- метаморфическая сульфидизация представлена рассеянной вкрапленностью пирротина с образованием мелких, значительно вытянутых (до 0.7 см по длинной оси) вдоль рассланцевания линзочек пирротина (рис. 3, a, b);- рудная минерализация представлена наложенной гидротермальной ассоциацией золота с арсенопиритом (рис. 3, d);- поздняя халькофильная минерализация, образовавшаяся на завершающем этапе гидротермально-метасоматического процесса (рис. 3, e, f);- пострудное окварцевание.Геохимическая характеристика. В результате геохимического изучения пород и руд месторождения Чертово Корыто установлено, что отложения михайловской свиты характеризуются повышенными содержаниями, относительно стандартов PAAS [Condie, 1993] и СЧС-1 [Petrov et al., 2004], таких породообразующих элементов как Al2O3, Fe2O3общ, MgO, K2O, и P2O5. Характерно то, что содержание практически всех оксидов в рудной зоне имеет тенденцию к снижению. Исключением является SiO2 (табл. 1). Распределение редких элементов повторяет закономерность, установленную для петрогенных элементов. Наименее измененные породы михайловской свиты характеризуются повышением содержания (до трех раз) Cu, Mo, Ba, W, As, Pb относительно значений PAAS и СЧС-1, в то время как в рудной зоне содержание практически всех редких элементов заметно снижается (табл. 2). Содержание ряда элементов сидерофильной группы (Co, Ni) имеет четкую корреляцию с рудными процессами, проявляя увеличение в два раза и более в зоне околорудных изменений. Максимальные концентрации Co и Ni отмечены в пробах с рудными содержаниями золота.Выводы. Формирование месторождения Чертово Корыто происходило в пять этапов, первые два из которых являются дорудными, носят рудоподготовительный характер и, вероятно, значительно оторваны по времени от основного рудоформирующего события. Установленная стадийность формирования месторождений Чертово Корыто коррелирует с основными стадиями тектонометаморфической истории региона и согласуется с моделью формирования месторождений сухоложского типа [Nemerov, 1989; Buryak, Khmelevskaya, 1997; Large et al., 2007]