corundum, ruby, sapphire, Madagascar.12, Placer in alkali-basalt, Gem corundum deposits, Chemical composition, Oxygen isotopes, Genetic model, gems

Madagascar is a major producer of gem corundum, primarily recovered from continental basaltic fields. Key mining areas include sapphire-bearing palaeoplacer deposits such as Ambondromifehy and Nosy Be in Antsiranana Province (northern Madagascar), Soamiakatra–Mandrosohasina in Antananarivo Province (central Madagascar), and the Vatomandry district in Toamasina Province (eastern Madagascar). In Antananarivo Province, Soamiakatra is a primary deposit where rubies occur in metagabbro and pyroxenite xenoliths transported to the surface by the Ankaratra volcanic activity. Petrographic evidence indicates two distinct ruby-forming conditions: near the eclogite domain (T ≈ 1100 °C, P ≈ 20 kb) and in granulite facies (T ≈ 1100 °C, P < 15 kb). In contrast, sapphires from placer and paleoplacer deposits mainly originate from two sources: (i) magmatic sapphires (90%) derived from alkaline magma differentiation in lower crust–mantle boundary chambers, associated with syenite and anorthoclasite xenoliths in alkali-basalts. These are linked to asthenospheric upwelling and lithospheric thinning during Oligocene–Quaternary times. Zircons from Mandrosohasina and Ambatomainty sapphire deposits yield U–Pb ages of 7 Ma. These sapphires have low δ¹⁸O values (4.1 ± 0.4‰, n = 8), matching those from syenitic sources. Inclusions such as columbite-(Fe), tantalite-(Mn), pyrochlore group minerals, samarskite group minerals, uraninite, and anorthoclase further confirm syenitic origins. (ii) The remaining 10% are metamorphic xenocrysts brought up by the same magmatic event. Rubies from placer deposits show δ¹⁸O values of 3.1 ± 1.1‰ (n = 6), consistent with ruby in mafic–ultramafic rocks and metasomatic ‘plumasite’ zones (δ¹⁸O = 1.25–7.5‰, n = 35). In Toamasina Province, ~85% of the sapphires from Vatomandry are metamorphic, with δ¹⁸O = 4.1 ± 0.4‰ (n = 9), overlapping the range for metasomatic sapphires from ‘plumasites’ and biotite schists in shear zones. The remaining ~15% are magmatic, with low δ¹⁸O values consistent with sapphire-bearing syenites. In Antsiranana Province, rubies are absent; sapphires are ~40% magmatic and ~60% metamorphic, with δ¹⁸O = 4.5 ± 0.5‰ (n = 11), comparable to the other provinces. Zircons yield contrasting U–Pb ages: 40.6 Ma (Ambondromifehy) and 0.7 Ma (Nosy Be). These data—corundum characteristics, isotopic signatures, and radiometric ages—demonstrate two distinct corundum sources in Madagascar’s alkali-basalts. One comprises rubies from metagabbro and garnet–pyroxenite complexes of eclogite facies, retrograded to granulite facies during the Pan-African orogeny. The other includes sapphires either magmatic, coeval with volcanic events involving differentiated alkaline magma, or metamorphic, extracted from Precambrian basement during magma ascent.