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Pyrochlores from the Lueshe carbonatite complex (Democratic Republic of Congo): a geochemical record of different alteration stages

Mohamed Nasraoui 1, 2 Essaïd Bilal 1, 3, 2
3 LMV-ENSMSE - Laboratoire Magmas et Volcans
Mines Saint-Étienne MSE - École des Mines de Saint-Étienne, SPIN, CNRS - Centre National de la Recherche Scientifique : UMR6524, LMV - Laboratoire Magmas et Volcans : UMR 6524
Abstract : Magmatic pyrochlores from the Lueshe syenite-carbonatite complex from the northeastern part of Democratic Republic of Congo (ex-Zaire) are characterized by Ta/Nb ratios in an increasing order from pyroxenite, calcite-carbonatite (sovite), silicate xenoliths (nodules) to syenite. Substitutions invoking Nh, 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, a(Na+), a(Ca2+) and a(HF), and high a(Sr2+) and a(LREE3+). 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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Submitted on : Friday, July 2, 2010 - 3:29:45 PM
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Mohamed Nasraoui, Essaïd Bilal. Pyrochlores from the Lueshe carbonatite complex (Democratic Republic of Congo): a geochemical record of different alteration stages. Journal of Southeast Asian earth sciences, Elsevier, 2000, 18 (2), pp.237-251. ⟨10.1016/S1367-9120(99)00056-5⟩. ⟨emse-00497149⟩



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