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

摘要

This paper reports trace-element, rare-earth element (REE) and boron isotopic compositions of tourmaline in the Asarcik 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 Asarcik granitoid and in associated quartz–tourmaline veins. Tourmaline in the granitoid forms as rosettes up to 1.5 cm 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 Asarcik 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. Chondritenormalized patterns of REEs in tourmalines from the Asarcik 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‰and fall within the δ~(11)B range of this mineral in granite-related settings. Tourmaline rosettes from the Asarcik granitoid show relatively lighter δ~(11)B values (-14.0 to-12.2‰) in comparison with those in the quartz veins (-10.4 to -2.2‰).