Sm-Nd dating of scheelite and tourmaline: Implications for the genesis of Archean gold deposits, Val d'Or, Canada

Sm-Nd analyses of 19 scheelite samples from auriferous quartz-carbonate-tourmaline-scheelite-gold veins from three mines in the Val d'Or gold camp in the Abitibi greenstone belt in western Quebec (the Sigma, Pascalis-North, and Siscoe Extension mines) yield a linear array on an isochron diagram with a mean square weighted deviation (MSWD) of 3.6 corresponding to an age of 2596 +/- 33 Ma and epsilon(Ndi) = 2.5. Eight scheelite samples from the Pascalis-North mine alone define an excellent linear array with an MSWD = 0.31, an age of 2593 +/- 18 Ma, and epsilon(Ndi) = 2.7. These ages are interpreted to represent the age of quartz-carbonate-tourmaline-scheelite vein-hosted gold mineralization in the three mines sampled. Sm-Nd analyses of tourmaline suggest that this mineral may also be useful for Sm-Nd dating of hydrothermal mineralization. The Sm Nd scheelite age of ca. 2600 Ma is in good agreement with U-Pb ages obtained from mineralization-related rutile and titanite in the Val d'Or area. These results indicate that the quartz-carbonate-tourmaline-scheelite vein-hosted gold mineralization was emplaced approximately 70 m.y. later than the youngest observed premineralization magmatic activity and approximately 60 m.v. later than peak regional metamorphism in the Val d'Or area. These ages do not support hypotheses for deposit formation which involve fluids derived from spatially related intrusive rocks or metamorphism, although they do not preclude an earlier stage of introduction of gold into the Val d'Or area. The scheelite Sm-Nd age and depleted mantlelike epsilon(Ndi), and the mineralogy and alteration of the quartz-carbonate-tourmaline veins, are consistent with the derivation of the mineralizing fluids from a relatively homogeneous external source, and introduction into the upper crust as part of a single, large hydrothermal system. The formation of the Val d'Or gold deposits also appears to be broadly synchronous with the growth of zircon in high-grade metamorphic rocks of the lower crust underlying the greenstone belt material, suggesting that these may be related processes.