Chemical composition of tourmaline from the Asarcik Pb-Zn-Cu±U deposit, Şebinkarahisar, Turkey

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, □Al(NaR)_-1, AlOR²⁺ _-1 (OH)_-1, □Al₂ONa_-1R²⁺ _-2(OH)_-1, Ca_0.5□_0.5 Na_-1, CaMg₂□_-1Al_-2, CaMgO□_-1Al_-1(OH)_-1, CaMg₃ OH□_-1Al_-3O_-1, CaMgNa_-1 Al_-1, CaONa(OH)_-1, CaMg₂ OHNa_-1Al_-2O_-1, CaMg₂ □_-1Al_-2, and CaMg₃ OH□_-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.