Trace-element, rare-earth element and boron isotopic compositions of tourmaline from a vein-type Pb-Zn-Cu +/- U deposit, NE Turkey

This paper reports trace-element, rare-earth element (REE) and boron isotopic compositions of tourmaline in the Asarck granitoid and quartz veins at Sebinkarahisar. Abundant tourmaline occurs in the vein-type Pb-Zn-Cu +/- U deposit, within the northeastern Pontide Metallogenic Belt, both within the Late Cretaceous Asarck granitoid and in associated quartz-tourmaline veins. Tourmaline in the granitoid forms as rosettes up to 1.5cm in diameter, whereas in quartz-tourmaline veins a few centimetres to a few metres wide, it occurs as black needles and masses that cut the pluton. We present a systematic geochemical study of these tourmalines - all of the schorl-dravite series. Tourmalines from the quartz veins contain high concentrations of ore metals such as Ag, Bi and Zn and have higher Sr, Cr and Sn contents than tourmalines in the Asarck granitoid. Tourmalines from intensively altered parts of the pluton are characterized by higher concentrations of trace elements such as Ba, Rb, V, Sc, Th and Zr compared with occurrences in the quartz veins and in the fresh and less-altered granitoid. Chondrite-normalized patterns of REEs in tourmalines from the Asarck granitoid are generally similar to those of tourmalines in the quartz veins. All vein tourmalines, however, are characterized by heavy REE enrichments, in sharp contrast to those in the granitoid. Boron isotopic compositions of tourmalines range from -14.0 to -2.2 parts per thousand and fall within the 11B range of this mineral in granite-related settings. Tourmaline rosettes from the Asarck granitoid show relatively lighter 11B values (-14.0 to -12.2 parts per thousand) in comparison with those in the quartz veins (-10.4 to -2.2 parts per thousand).