Accueil > Documentations scientifiques > Revues récentes > Mada-Géo : journal des sciences de la terre > Archives > Volume 12, septembre 2008, ISSN 2074-4587 > Geochemical control of monazite and bastnaesite deposits in (...)

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  Geochemical control of monazite and bastnaesite deposits in Ambatofinandrahana area, Central part of Madagascar
  Mada-Géo : journal des sciences de la terre, Volume 12, septembre 2008, ISSN 2074-4587 pp:50 - 65

  Auteur(s) : Rasoamalala V., Andriambololona D.R., Razakamanana T., Salvi S., Rasolomanana E., Randrianja R., Guillaume D.

  Auteur correspondant :

  Mots-clés : RARE EARTH/BASTNAESITE/MONAZITE/AMBATOFINANDRAHANA/ SYENITE/GRANITE
  
  

  Résumé de l’article

  [FR] Quartzites, micaschists and dolomitic marbles which intersected by syenitic and granitic rock formed the Itremo group in Madagascar. The results, obtained from one thousand three hundred analysis realised by the LA-ICP-MS technique and twenty thin sections, show that the monazite is linked to the syenitic rocks of Andoharano and of Ifasina-Marovoalavo ; and the bastnaesite to the granite of Andakatany and within veins of microsyenite in Ankazohambo marble deposit. The main minerals of these rocks are perthitic alkali feldspar, plagioclase, aegyrine calcic amphibole and/or sodic amphibole, titanit and apatite. The rocks are a subsolvus type, which indicates the enrichment of water in the magma, hence they are the alkaline rocks. The mineralized microsyenite is formed by microgeode of chalcedony, monazite or bastnaesite, and baryte. The projections on the Harker diagram of the granite and syenite shows a decreasing evolution of the contents of major elements with the magmatic evolution, and the magma evolutes with fractional crystallization from the central part of the massif to the edge. The multielements studies show that these rocks have the same modality of constitution and should have been subjected by the same events. They are late-orogenic and syn-collisional rocks setting. In this paper we show that the monazite and bastnaesite mineralizations of this region should be the result of two events. The first which is a tardimagmatic event, responsible of the formation of the first generation of automorphous monazite in syenite and microsyenites, and bastnaesite in veins of microsyenite. The second one, a hydrotermal quartzo-baryted episode juxtaposed to the first event and rich in rare earth elements should be responsible of silicification of microsyenite with the formation of second generation of xenomorph monazite and chalcedony veins with bastnaesite in granitic rocks.

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