Chemical composition of tourmaline from the Asarcik Pb-Zn-Cu +/- U deposit, Sebinkarahisar, Turkey
Résumé
Abundant tourmaline, commonly as rosettes up to 1.5 cm in diameter, is present in the Upper Cretaceous Asarcik granitoid (Sebinkarahisar), Northeastern Turkey. The tourmalines also occur in quartz-tourmaline veins up to 4 m thick that cut the Asarcik granitoid. This granitoid was emplaced in the eastern Pontide Metallogenic Belt, where accompanying tourmalines are found in close association with a vein-type, uranium-bearing Pb-Zn-Cu deposit. Tourmaline crystals are generally optically zoned from core to rim with a bluish green core. Electron-microprobe studies show that tourmalines from the Asarcik granitoid and quartz-tourmaline veins display mainly dravite-schorl solid solutions with a tendency to schorl compositions. The tourmaline from the Asarcik granitoid has Fe/(Fe+Mg) ratios from 0.28 to 0.69 (mean=0.52) and Na/(Na+Ca) ratios from 0.57 to 0.93 (mean=0.76). In contrast, tourmaline in the quartz-tourmaline veins has Fe/(Fe+Mg) ratio from 0.14 to 0.92 (mean=0.59) and Na/(Na+Ca) ratio from 0.78 to 1.00 (mean=0.96). Tourmaline in the veins has a more alkali-deficient trend than that in the granitoid. Substitution mechanisms for the chemical evolution of tourmalines at Asarcik are the MgFe-1, (Fe2+Fe3+)(MgAl)(-1), square Al(NaR)(-1), AlOR2+ (-1)(OH)(-1), square Al2ONa-1R2+ (-2)(OH)(-1), Ca-0.5 square Na-0.5(-1), CaMg2 square Al--1(-2), CaMgO square Al--1(-1)(OH)(-1), CaMg3OH square-1Al-3O-1, CaMgNa-1Al-1, CaONa(OH)(-1), CaMg2OHNa-1Al-2O-1, CaMg2 square Al--1(-2), and CaMg3OH square-1Al-3O-1 exchange vectors. Chemical zoning in the tourmalines is consistent with these substitutions. Stoichiometric calculations of electron-microprobe data in the Asarcik granitoid suggest a relatively oxidizing environment during tourmaline crystallization.