LES CORINDONS ASSOCIES AUX ROCHES METAMORPHIQUES DU SUD DE MADAGASCAR : LE GISEMENT DE SAPHIR DE ZAZAFOTSY(CARACTERISATION & TYPOLOGIE)
Faculte Des Sciences - Sciences de la Terre et de l’Environnement - None ()
Resume
The Zazafotsy sapphire deposit belongs to the Amboropotsy-Ikalamavony groups, in the Vohimena serie, made up mostly by garnet-sillimanite-cordierite-bearing leptynites and amphibole-pyroxene-bearing gneisses. The deposit is contained in a lens-like made of feldspathic gneiss intercalated within the leptynites. The mineralization occurs in metasomatic rocks such as biotitised gneisses and biotitites. Those rocks are composed of corundum, K-feldspar, plagioclase, sillimanite, spinel, garnet and biotite. Sapphires are multicoloured: dark bleu, light bleu, grey bleu, fuchsia, orange, pink, violet, mauve and brown. The change in colour is function of the variation in chromium content (300 ppm < Cr2O3 < 1890 ppm) for a more or less constant iron content (2320 ppm< FeO < 3300 ppm). The mineral inclusions in saaphires are zircon, K-feldspar, plagioclase, sillimanite, spinel and biotite. Most of the crystals are euhedral (hexagonal prism and tabular). The geochemical study of the feldspathic gneisses shows that the initial protolith was sedimentary (shales). The oxygen isotopic data of sapphire are homogenous with d18O values between 8.8-9.0 ‰. The formation of the primary deposits of corundum located in the granulitic domain of southern Madagascar (Zazafotsy, Ambatomena, Iankaroka, Vohitany) is mainly controlled by fluid-rock interaction. The circulation of fluids in the shear zones induces the development of important metasomatic processes (dequartzification, biotitisation, feldspathisation) are linked to the formation of sapphires and rubies. These metasomatic deposits associated with an alkaline metasomatism at high temperature are directly linked to the activity of the shear zones during the Panafrican orogenesis.