SM-ND DATING OF SCHEELITE AND TOURMALINE - IMPLICATIONS FOR THE GENESIS OF ARCHEAN GOLD DEPOSITS, VAL DOR, CANADA

Sm-Nd analyses of 19 scheelite samples from auriferous quartz-carbonat e-tourmaline-scheelite-gold veins from three mines in the Val d'Or gol d camp in the Abitibi greenstone belt in western Quebec (the Sigma, Pa scalis-North, and Siscoe Extension mines) yield a linear array on an i sochron diagram with a mean square weighted deviation (MSWD) of 3.6 co rresponding to an age of 2596 +/- 33 Ma and epsilon(Ndi) = 2.5. Eight scheelite samples from the Pascalis-North mine alone define an excelle nt linear array with an MSWD = 0.31, an age of 2593 +/- 18 Ma, and eps ilon(Ndi) = 2.7. These ages are interpreted to represent the age of qu artz-carbonate-tourmaline-scheelite vein-hosted gold mineralization in the three mines sampled. Sm-Nd analyses of tourmaline suggest that th is mineral may also be useful for Sm-Nd dating of hydrothermal mineral ization. The Sm Nd scheelite age of ca. 2600 Ma is in good agreement w ith U-Pb ages obtained from mineralization-related rutile and titanite in the Val d'Or area. These results indicate that the quartz-carbonat e-tourmaline-scheelite vein-hosted gold mineralization was emplaced ap proximately 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 hypothese s for deposit formation which involve fluids derived from spatially re lated intrusive rocks or metamorphism, although they do not preclude a n earlier stage of introduction of gold into tile Val d'Or area. The s cheelite Sm-Nd age and depleted mantlelike epsilon(Ndi), and the miner alogy and alteration of the quartz-carbonate-tourmaline veins, are con sistent with the derivation of the mineralizing fluids from a relative ly homogeneous external source, and introduction into the upper crust as part of a single, large hydrothermal system. The formation of the V al d'Or gold deposits also appears to be broadly synchronous with the growth of zircon in high-grade metamorphic rocks of the lower crust un derlying the greenstone belt material, suggesting that these may be re lated processes.