DATING FLUID FLOW IN ARCHEAN TERRANES: U-PB AGE DEPTH-PROFILES OF ZIRCON FROM THE NORTH CARIBOU GREENSTONE BELT, CANADA

The large-scale movement of heat and fluids during orogenic events is known to play a key role in the distribution of ore deposits. The consequences of these hydrothermal fluids include mineralized and altered bedrock forming a "halo" that defines the footprint of the deposit. In moderate to high metamorphic grade regions it can be very difficult to distinguish the alteration halo from the background metamorphic signature. Zircon, in the presence of high temperature alkaline fluids, can undergo coupled dissolution-reprecipitation. This fluid mediated alteration manifests itself as µm-scale rims on the outer surfaces and within existing imperfections of zircon crystals. We are studying the 3.0-2.7 Ga North Caribou greenstone belt of the Western Superior Province, Canada, which hosts Goldcorp’s >5.8M oz. Musselwhite gold mine. Metasedimentary rocks containing zircon, adjacent to or within fluid-mediating conduits were targeted for sampling using peak metamorphic signatures (And, Crd, Grt), aeromagnetic data (magnetic highs), and deformation structures (high strain). Unpolished zircon crystals ranging from 106-250 µm, commonly characterized by a mottled or microporous outer surface, were mounted in indium and analyzed at UCLA for SIMS U-Pb depth-profile geochronology. This approach revealed sub-micron age resolution of thin <5 µm zircon rims, which are 100 m.y. younger than interior (core) ages. Associated with the younger ages is a relatively low zircon Th/U value of <0.1 (metamorphic derivation) compared to the interior of the crystal, which ranged between 0.4 and 0.7 (igneous derivation). Trace element analyses of these types of altered zircon rims are typically enriched in light REEs relative to their unaltered equivalents. A regional potassic alteration event broadly coeval with Au mineralization at the mine has been resolved from zircon and whole rock geochemical evidence of the surrounding granitoid batholiths, and may be further recorded as the µm-scale zircon rims within the metasedimentary sequences of the greenstone belt. Consequently, a major component of the Au mineralization equation may be present and further endowment remains a possibility distal from the main deposit site provided a trap can be identified.