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Petrostructural Signature of the Monazite of Andoharano Ambatofinanadrahana, Madagascar



Author(s)

Dieudonné Razafimahatratra¹ and Alfred Andriamamonjy²

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DOI:10.17265/2161-6213/2018.5-6.004

1. Ecole Normale Supérieure, University of Antananarivo, BP 881, Madagascar
2. Sciences of Earth and Environment, Faculty of Sciences, University of Antananarivo, BP 906, Madagascar

The monazite deposit at Andoharano is 7 km NE of Ambatofinandrahana. It is in the form of filons of hydrothermal chalcedony origin both with monazite and barite, which intersects the small syenitic post-tectonic massif, aged 550 to 510 Ma. The syenite is part of the Ambalavao-Kiangara-Maevaranomagmatic suite located in the Itremo sub-domain. The mineralized filon’s length is approximately 100 meters with an average thickness of 2 to 5 meters, direction N75 and a subvertical dip. It is characterized by the mineralogical association of monazite, chalcedony, barite, quartz and magnetite. Two deformation phases affect the emplacement of the deposit. The first phase corresponds to the crystallization of the automorphic monazites, while the second phase affects the filon which leads to the fracturing of the first through silicification and which leads to the formation of the small xenomorphic monazite crystals. The monazite appears as brownish spots scattered in the rocks. It is rich in ceric earth (32% to 33%) and low in thorium. It constitutes ceric earth ore in the region.

Monazite, syenite, ceric earths, chalcedony, Andoharano.