Abstract

Mineral inclusions in gem quality sapphires from the new mining area in southern Malagasy (Andranondambo) include calcite, fluor-apatite, fluor-phlogopite, fluorite, baddeleyite, zirconolite, potassium-feldspar, anorthite, scapolite (meionite), thorianite (thorianite-uraninite solid solutions) and varieties of spinel, such as green Mg-Al-spinel, spinel-hercynite and spinel-hercynite-gahnite solid solutions. These minerals are typically found in various rocks in the mining area belonging to the so-called U-Th skarns (various types of calcsilicate marbles). Therefore, these skarns are considered to be the host rocks for sapphire formation.

Based on the different generations of primary fluid inclusions in the sapphires, various generations of solid inclusions and the absence of hibonite inclusions, it is concluded that the Andranondambo sapphires have formed during several different metasomatic events. The later blue sapphire generations have grown from CO2-rich fluids of complex chemical composition with minor concentrations of fluorine, water and metals such as aluminium, titanium, iron, chromium and zinc as well as other elements (Mg, Si, Ca, Na, K, P, Zr, U, Th).