The grey monazite of Britanny, determined in 1963, makes up small millimetric nodules in ordovician, dinantian and perhaps also precambrian shales, located in the central armorican trough. In the Ordovician, the mineralized layers are mainly localized in the Llanvirnian. These grey-coloured nodules, with a 4.65 specific weight, are in the shape of flat ellipsoids. Their structure is most often either grained polycrystalline, or as radiated mosaics, or as a crown (central monocrystal surrounded by small crystals). Inclusions involve phyllites, quartz, rutile, graphite, iron oxides, averaging to about 10°/0 of the nodule. The chemical composition is characterized by its high grade in Eu 20 3 (0.5%) and in thorium. The monazite shales, poorly metamorphic, include some chloritoid and a little graphite. The monazite nodules are scattered (content ranging from 50 to 200 g/t), arranged across the schistosity and surrounded by pressure shadows with phyllitous infilling. Within the ordovician stratigraphy, the llanvirnian monazite shales overlie iron ore and sandstons rich in detrital rutile-zircon-monazite, deposits in which diagenesis phenomena occur. A noteworthy sedimentary differentiation characterizes the epicontinental deposits of the ordovician sea invading the weathered and flattened brioverian relieves. The early diagenesis of the deposits is of prime importance in the genetic pattern proposed for monazite: 1) gel of rare earth phosphate in marine silts; 2) transformation of this gel into fibro-radiated globules of rhabdophanite; 3) evolution of the rhabdophanite into monazite; 4) occurrence of the syntectonic metamorphism, partial rotation of the nodules. Further occurrences of grey monazite are known from Siberia, Africa (Morocco, Gaboon, Congo) and Madagascar. At present, the placers only display some economical interest, as it is the case in Brittany.La monazite grise de Bretagne, d&erminfie en 1963, forme des petits nodules millim&riques dans des schistes ordoviciens, dinantiens et peut-&re aussi pr6cambriens, situ~s dans la fosse centrale armoricaine. Dans l'Ordovicien, les niveaux mindralis6es se localisent surtout dans le Llanvirnien. Les nodules, de couleur grise et de densitfi 4,65, ont la forme d'ellipsoides aplatis. La structure est le plus souvent soit polycristalline engren6e, soit en mosaique radifie, soit encore en couronne (monocristal central entour6 de petits cristaux). Les inclusions comprennent phyllites, quartz, rutile, graphite, oxydes de fer, reprfisentant en moyenne environ 10% du nodule. La composition chimique se caract~rise par la richesse en EuzO a (0,5O/o) et la pauvretfi en thorium. Les schistes ~i monazite, faiblement m6tamorphiques, renferment du chloritoide et un peu de graphite. Les nodules de monazite s'y trouvent dispers6s (teneurs de 50 ~ 200 g/t), dispos6s en travers de la schistosit6, et <
No abstract
Des veinules quartzeuses à paragenèse cuprifère, hessite (Ag₂Te) et or natif recoupent les schistes verts du complexe volcano-sédimentaire sous-marin briovérien épimétamorphique de Porz-Mellec en Plestin (Massif armoricain).
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