Evidence for Fe-sulfide and Fe-Ti oxide liquid immiscibility in the Itsindro gabbro Complex, Madagascar

Abstract : The petrology and mineralogy of the Itsindro complex, SW Madagascar has been investigated through samples coming from a borehole drilled, in the early 70’s, in a geophysical and Ni-,Cu-geochemical anomaly. The host rocks are represented by a pyroxenite unit with interbedded gabbro layers and a gabbro unit. A decrease of the Mg number of CPX, independent of the lithology, is observed from the deepest samples (316 m) up to 163 m suggesting a regular fractional crystallization process. The evolution of this parameter is more erratic in the upper part of the bore hole. The mineralisation consists in Fe-Ti-P oxides and Fe-(Cu-Ni) sulfides. The ilmenite-magnetite±apatite and pyrrhotite±chalcopyrite ±pent-landite assemblages are always observed in interstitial position. The contemporaneous character of the oxide and sulfide assemblages strongly suggests a two stages evolution of the tholeitic magma, a first immiscibility process between a Si-rich and a Fe-Ti-P liquids followed by a sulfide contamination of the latter giving way to massive Fe-rich over Cu- Ni sulphides.