Pyrochlores from the Lueshe carbonatite complex (Democratic Republic of Congo): a geochemical record of different alteration stages |
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| Institution: | 1. Sobolev Institute of Geology and Mineralogy of Siberian Branch Russian Academy of Sciences (IGM SB RAS), Russia;2. Novosibirsk State University, Russia;3. Tuvinian Institute for Exploration of Natural Resources of Siberian Branch Russian Academy of Sciences (TuvIKOPR SB RAS), Russia;4. Geological Institute of Siberian Branch Russian Academy of Sciences, (GIN SB RAS), Russia;5. Institute of Geology and Geophysics, Chinese Academy of Sciences, China;6. A.P. Karpinsky Russian Geological Research Institute (VSEGEI), Russia;1. Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada;2. Department of Mineralogy, St. Petersburg State University, St. Petersburg, Russia;3. Department of Earth Sciences, The Natural History Museum, London, UK;4. Manitoba Geological Survey, Winnipeg, Manitoba, Canada;5. Laboratory of Orogenic Belts and Crustal Evolution, Peking University, Beijing, China;6. Department of Geology and Pedology, Mendel University, Brno, Czech Republic;7. Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic;8. Department of Geology, Brandon University, Brandon, Manitoba, Canada;1. V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Akademika Koptyuga Str., 3, 630090, Russia;2. Geological Institute SB RAS, Ulan-Ude, Sakhyanovoy Str., 6a, 670047, Russia;3. GFZ German Research Centre for Geoscience, Potsdam, Telegrafenberg Str, D-14473, Germany;4. Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany |
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| Abstract: | Magmatic pyrochlores from the Lueshe syenite–carbonatite complex from the northeastern part of Democratic Republic of Congo (ex-Za??re) are characterized by Ta/Nb ratios in an increasing order from pyroxenite, calcite-carbonatite (sövite), silicate xenoliths (nodules) to syenite. Substitutions involving Nb, Ta, Ti and REE have been precisely described. Hydrothermal alteration of Lueshe pyrochlore involves the substitution of Na++F?=VA+VY and Ca+O=VA+VY (VA=A-site vacancy and VY=Y-site vacancy). In calcite carbonatite, hydrothermal alteration of pyrochlore took place during and after the precipitation of ancylite-(Ce), strontianite, celestite, baryte and fayalite according to a fluid composition of relatively low pH, aNa+, aCa2+ and aHF, and high aSr2+ and aLREE3+. The supergene alteration is characterized by complete leaching of Na, Ca and F and partial incorporation of K, Ba, Sr and Ce resulting in the formation of kali-, bario-, strontio- and ceriopyrochlore respectively. The Na-poor pyrochlore may be an intermediate variety corresponding to an alteration stage between the hydrothermal and weathered pyrochlores. The IR spectroscopic study has indicated that the weathered pyrochlore is a hydrated variety containing two bands of OH vibration modes at 3413 and 1630 cm?1. During hydrothermal and supergene alterations, the cations at B-site remain relatively constant. The variable chemical compositions of the pyrochlores from the Lueshe complex represent geochemical memories of the different alteration conditions including the variation in the oxidation–reduction environment. |
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