Tourmaline compositions from the Salikvan porphyry Cu-Mo deposit and vicinity, northeastern Turkey

Tourmaline-bearing rocks occur in the Pontide volcanic island arc of northeastern Turkey, which hosts numerous small- to medium-sized Cu-Pb-Zn and Cu-Mo deposits. Weakly developed porphyry-style Cu-Mo mineralization accompanies the calc-alkaline volcano-plutonic rocks of Salikvan, in southwestern Artvin. Various types of tourmaline have been analyzed chemically, of which three main environments of formation are identified: (1) quartz-tourmaline veins, (2) tonalite porphyry and, to a lesser extent, coarse granodiorite, and (3) tourmaline-rich rocks at the contacts of coarse granodiorite and basic volcanic rocks. Electron-microprobe data indicate that the tourmaline is relatively ferrous and calcic, and shows a general trend from dravite to uvite end-members; the tourmaline at Salikvan formed by the reaction of Fe-rich hydrothermal fluids with Ca-rich amphibole and plagioclase in tonalite porphyry and granodiorite. The dominant variability in composition seems controlled by the exchange vector CaMgO[]_-1Al_-1(OH)_-1. Tourmaline in quartz-tourmaline veins has moderate Fe/(Fe + Mg) values (0.37-0.51; mean 0.43), whereas that in tonalite porphyry is rich in iron, with a Fe/(Fe + Mg) of 0.45-0.53 (mean 0.48). Tourmaline in tourmaline-rich rocks is relatively rich in magnesium, with a Fe/(Fe + Mg) of 0.30-0.46 (mean 0.39). Trace-element contents of the tourmalines are low relative to those in tourmaline from massive sulfide deposits. Tourmaline has a tendency to scavenge trace elements during crystallization, yielding correlations among Ag - (Au, Li, Mo, P, Pb, Sb, V, W), Au - (P, Pb, Sb, V, W), Li - (P, Pb, Sb, V, W), Mo - (Pb, Sb, V), P - (Sb, V, W), Sb - (V, W), Co - (Cr, Mn), Pb - (Sb, V), Ba-Sr, Zn-Cu, V-W, and Cr-Mn. Chondrite-normalized patterns of the rare-earth elements point out the possible contribution of hydrothermal processes to tourmaline formation. Boron isotope analyses of tourmaline from quartz-tourmaline veins and tourmaline from tourmaline-rich rocks have δ¹¹B values of -9.4 and -9.0‰, respectively, consistent with late-magmatic hydrothermal fluids.