Chemical and boron isotopic composition of hydrothermal tourmaline from the Panasqueira W-Sn-Cu deposit, Portugal

摘要

Tourmaline is a locally abundant hydrothermal mineral in the wallrocks surrounding the W-Sn-Cu mineralized veins in the Panasqueira deposit (Portugal) and a minor phase within the veins themselves. Tourmaline chemical and boron-isotope compositions have been determined from three settings: (1) pervasive fine-grained tourmalinization zones in wallrocks within 10 cm of the veins, (2) coarser tourmaline in wallrock-hosted fault zones, and (3) needle-shaped tourmaline from late-stage vugs in the quartz veins. All tourmalines from Panasqueira have ferromagnesian compositions with significant octahedral aluminum contents and variable X-site vacancies. Tourmaline compositions show significant chemical variations at the grain and sample scale but are homogeneous on the deposit scale. Tourmaline is typically optically and chemically zoned, showing significant increases in Fe and F and decrease in Mg, Ca and Al from core to rim. Moreover, as a general trend, Al increases and Mg decreases with proximity to the vein contact. The observed chemical variation seems to have been controlled mainly by the vectors: FeMg-1; (x)square AlNa-1(R2+)(-1); (x)square Al(OH)Na-1(R2+)(-1)F-1; R2+(OH,F)Al-1O-1 and (x)square Al2O Na-1(R2+)(-2)(OH,F)(-1).