The boron isotope composition of tourmaline from the Val-d'Or orogenic gold deposits, Quebec, Canada

Tourmaline from the Val-d'Or orogenic gold vein field is dravitic, plotting in the alkali field with a sodic, low calcium, composition. The boron isotope composition of tourmaline ranges from -16.7 to -8.3 parts per thousand. Higher delta B-11 in tourmaline is related to higher Ti and Mg/(Mg+Fe). Tourmaline fibers display a circa 2 parts per thousand increase in delta B-11 over a growth length of less than 1 cm. The range in delta B-11 values cannot be explained by boiling in veins or by a range of equilibrium temperatures. The variance in tourmaline delta B-11 values (-16.4 parts per thousand to -10.7 parts per thousand) in one large deposit (Sigma) is almost as large as that for the vein field. The boron isotope composition in the Val-d'Or vein field tourmaline is determined by various mixtures of locally derived boron from metamorphic devolatilization of the crustal sequence. This contrasts with tourmaline delta O-18 values that are homogeneous at the deposit scale in the Val-d'Or orogenic gold vein field (Beaudoin and Pitre 2005).