Geochemistry of Quartz- Tourmaline Vein in Bagho Granitoid Body, Damghan, NE Iran

Bagho granitoid is located in the southeast of Damghan and has been cut by many quartz-tourmaline veins with about 1 mm to 30 cm thickness. Based on petrography and electron microprobe analyses, these tourmalines show schorl- dravite- foitite composition with a tendency toward schorl end member, and located in alkali and vacancy groups. Compared with the ideal composition of schorl- dravite, many of tourmaline samples have high Al contents and alkali - site vacancies. The increase in octahedral aluminum reflects a combination of substitutions in tourmaline involving deprotonation (O-OH exchange) and vacancies in the alkali-site and then they have magmatic origin. In contrast, the presence of zoning, its occurrence as vein form, having high Mg compared with Fe in some samples and tendency away from alkali- deficient and proton- deficient tourmaline vectors, show that these tourmalines have hydrothermal origin. Then, based on these results, it appears that tourmaline veins form by interaction of boron-rich magmatic-hydrothermal fluids of granitic-dacitic provenance with various quartz-tourmaline and metapelitic-metapsammitic host rocks.