Sapphires related to alkali basalts from the Cerova Highlands, Western Carpathians (southern Slovakia): composition and origin

Blue, grey-pink and pink sapphires from the Cerova Highlands, Western Carpathians (southern Slovakia) have been studied using CL, LA-ICP-MS, EMPA, and oxygen isotope methods. The sapphire occurs as (1) elastic heavy mineral in the secondary sandy filling of a Pliocene alkali basaltic maar at Hajnacka, and (2) crystals in a pyroxene-bearing syenite/anorthoclasite xenolith of Pleistocene alkali basalt near Gortva. Critical evaluation of compositional diagrams (Fe, Ti, Cr, Ga, Mg contents, Fe/Ti, Cr/Ga, Ga/Mg ratios) suggests a magmatic origin for elastic blue sapphires with lower Cr and Mg, but higher Fe and Ti concentrations in comparison to the grey-pink and pink varietes, as well as similar compositional trends with blue sapphire from the Gortva magmatic xenolith. Moreover, blue sapphires show similar delta O-18 values: 5.1 parts per thousand in the Gortva xenolith, 3.8 and 5.85 parts per thousand in the Hajnacka placer, closely comparable to mantle to lower crustal magmatic rocks. On the contrary, pink and grey-pink sapphires show higher Cr and Mg, but lower Fe and Ti contents and their composition points to a metamorphic (metasomatic) origin.