Geology, geochemistry, and genesis of the Alvand sapphire-bearing syenitic pegmatite, Sanandaj-Sirjan Zone, Hamedan Province, northwest Iran

Abstract Alvand blue sapphire is related to the igneous rocks of Alvand pluton, located in the Sanandaj–Sirjan zone, Hamadan area. The main geological unit is the Alvand batholith including gabbro, granite, and granitoid. Sapphire occurs within the desilicated pegmatite dykes intruding magmatic-metamorphic lithologies, closely to metasomatic zones. Sapphire-bearing rocks are situated in the central zone of aplitic-pegmatitic composite dykes. In addition to sapphire, the rocks contain albite, microcline, and orthoclase as major minerals, the absence of quartz is noticeable. The whole-rock geochemical characteristics show a relatively silica-poor and alumina-rich composition of syenitic sapphire-bearing rocks. According to peraluminous and S-type signatures, aplite and pegmatites indicate crustal magma origin. The slight increase of Al2O3/TiO2 ratio in the sapphire-bearing pegmatites may suggest mobilization of Al, which either by melt mobilization through alkaline hydroxide-complexation or by alkali-bearing high-temperature fluids such as Na-Al-Si-O polymers. Also based on strong enrichment in Th, sapphire may be formed by late-stage fluids. The high Ti, Fe, and Ga parallel to the low Cr and V contents indicate a magmatic-metasomatic origin of the Alvand sapphire.