Geologic, petrographic and tectonic background

Sapphire-anorthite-phlogopite nodule from the Andranondambo mine

Due to the very special mineralogical compositions of the metasomatized rocks as described above, their chemical compositions are characterized by unusual low concentrations of SiO2 and high concentrations of Al2O3 (Rakotondrazafy et al., 1996).

According to these authors, the chemistry is also characterized by a relatively high concentration of Zirconium (Zr). Zr is interpreted as a typical, mobile element in the course of metasomatism in these skarns. The same holds true for Thorium and Uranium. Baddeleyite alone (without zircon) occurs in hibonite bearing rocks. Incompletely identified metamict Ca-Zr-Ti oxides with variable amounts of Th, U and Nb, with compositions close to that of zirconolite [(Ca,Th,Ce)Zr(Ti,Nb)2O7)] (Bayliss et al., 1989) are present in rocks containing hibonite as well. The coincidental occurrence of hibonite and sapphire in the mines at Andranondambo is confirmed by Kiefert et al. (1996), as well as Schwarz et al. (1996).