GEM CORUNDUM DEPOSITS IN MADAGASCAR

Gem-quality corundum is widespread in Madagascar. Three deposit types occur: 1. Rubies and sapphires associated with pyroxenite xenoliths in Cenozoic alkali basalts (e.g. Antanifotsy). 2.Rubies and sapphires related to fluid movement in high grade metamorphic rocks (e.g. Andranondambo). 3. Sapphires related to prograde metamorphism (e.g. Iankaroka). We discuss 1. and 2.

Rubies in Antanifotsy pyroxenite xenoliths often have pink-red zonation, with inclusions of rutile, zircon, plagioclase, or mica. They have low Cr₂O₃ (0.25-0.56 wt%), high FeO (0.30-0.66%wt), and ∂18O = 4-5 ‰. They contain primary high density (d≈1.125) CO₂-dominated fluid inclusions (X_CO2>0.8). Enclosing xenoliths have coronitic structure, with diopsidic pyroxene surrounding corundum. Outside the pyroxene are garnet (Pyr 0.58-0.60, Alm 0.17-0.18, Gr:0.21-0.23) with plagioclase (An 0.84-0.88) and weathered scapolite. The equilibrium assemblage Cor + Gt + Px gives T≈ 1100°C , P≈20Kb, agreeing with data from fluid inclusions in ruby (T≈1100°C, P≈15 Kb).

Andranondambo sapphires occur in metasomatic pegmatities cross-cutting granulite facies (T≈ 850°C; P ≈ 5 kbar) calcareous metasedimentary rocks (Tranomaro Grp: metamorphic age 565-570 Ma). So-called "pyroxenites" consist of Al-diopside, scapolite ± spinel with phlogopite and pargasite crystallized at a later stage. Pegmatite is dominated by K-feldspar with minor apatite, phlogopite, corundum (sapphire) and calcite. K-spar crystallized at the expense of calcite and diopside, implying high mobility of Al in addition to K and Si. Corundum consists of clear hexagonal bipyramidal sapphire (∂18O=1-14). The deposits have carbonic (X_CO2≈ 0.8) fluid inclusions (in sapphire, apatite and feldspar) giving T ≈ 500°C, P ≈ 2kb. These P-T-X conditions agree with stability of the assemblages Cor + K-spar, and Cor + Cc + H-4SiO4(aq). Sapphire has wt% Fe₂O₃ 0.12-0.6, TiO₂2 0.001-0.10, Ga₂O₃ 0.01-0.04; and Cr₂O₃,Vf₂O₅, and MnO all <0.01. Associated phlogopite has high F (XF = 0.37-0.5). For T = 500°C at the calculated HF concentration, solubility of corundum in H₂O-HF is about 10 times higher than in pure H₂O. Calculations indicate that A1(OH,F) is the main aqueous species. Deposition of corundum was probably due to decreasing F activity resulting from co-precipitation of F-rich phlogopite and F-apatite.