Canadian Grenville Province
Published:January 01, 2004
The 1.4 Ga volcanic-plutonic arc-related Bondy gneiss complex in the western Grenville Province of Québec, Canada, comprises a Cu-Au-Fe oxides hydrothermal system characterized by a series of showings among aluminous and magnesian litho-facies, such as tourmalinite, aluminous sillimanite-bearing gneiss, garnetite, and cordierite-orthopyroxene white gneiss. The latter has a mineral assemblage and a major element chemistry comparable to those of cordierite-orthoamphibole gneiss found in alteration pipes associated with volcanogenic massive sulfide (VMS) deposits. Supporting evidence for a hydrothermal origin for the Bondy cordierite-orthopyrox-ene gneiss is given by mass-balance calculations that record a loss in Ca and gain in Mg, K, and Si with respect to the adjacent nonaltered rhyolitic protolith, compatible with feldspar destruction, sericitization, and chloritization. These lithofacies present birdwing-shaped rare earth element (REE) profiles, reflecting a mass change of REE and high field strength elements (HFSE) involving relative leaching of light rare earth elements (LREE) and enrichment in heavy rare earth elements (HREE), with a general calculated mass increase from Gd to Yb. These trace element characteristics differ from those of known, Cl-driven, VMS-associated hydrothermal cordierite-orthopyroxene or orthoamphibole rocks, but share similarities with F-driven hydrothermal signatures. The decoupling between major element and trace element signatures suggest that the type of fluids driving the postvolcanic or premetamorphic hydrothermal alteration and mineralization event in the Bondy gneiss complex evolved through time from seawater related to orthomagmatic.