Chemical and boron isotopic composition of hydrothermal tourmaline from scheelite-quartz veins at Nezamabad, western Iran

Abstract

The Jurassic Boroujerd Granitoid Complex in western Iran mainly consists of quartz diorite, granodiorite and monzogranite, which are intrusive into Jurassic phyllites and mica schists. The south-eastern part of the complex (Nezamabad region) is cut by NW-SE trending quartz-tourmaline veins of a few centimetres to 1.5 m in thickness, which contains sulfide and scheelite mineralization. Tourmaline forms one of the most abundant and widespread minerals in the hydrothermal veins and it also occurs as an accessory secondary mineral in the quartz diorite wall rocks. This study of tourmaline was undertaken to help distinguish the source of mineralizing fluids in the Nezamabad district. Chemically, tourmaline corresponds to intermediate schorl-dravite, with Fe/(Fe + Mg) ratios of 0.32 to 0.55. The tourmalines from quartz diorite differ from vein tourmalines in having lower Al, lower Fe/(Fe + Mg), and higher Ti and Ca contents. Many of the tourmalines, particularly from the veins, are optically zoned and show systematic core-rim chemical variations (higher Ti, lower Al in cores). The total range in δ 11 B values of Nezamabad tourmalines is from about -2 to -12 %o, whereby the quartz diorite-hosted tourmalines have generally lower δ 11 B values (-7 to -12 ‰) than vein tourmalines (-2 to -10 %o). This difference can be explained by a fall in temperature on the order of 150 °C from the veins to the wall rocks. No significant isotopic differences between core and rim of zoned grains or between barren and mineralized veins were found. On the basis of the tourmaline isotopic composition and mineralization temperatures from published fluid inclusion data, the calculated δ 11 B values of the hydrothermal fluids at Nezamabad were -5 to +5 ‰. The occurrence of tourmaline in the Nezamabad district, its chemical characteristics and δ 11 B values are consistent with a boron source derived from the regional metavolcanic and clastic metasedimentary rocks. A magmatic source of boron is permitted by the B-isotope data but can be ruled out because of the low whole-rock B contents of the quartz diorite and the restriction of tourmaline within it to alteration zones near the hydrothermal veins.