Birdwing-shaped REE profiles and/or isovalent Nb/Ta, Zr/Hf ratios in the Bondy gneiss complex, Grenville Province, Québec: sensitive geochemical markers of a fossil hydrothermal system in mineral exploration

CIM Montreal 2003
Louise Corriveau, Marc Richer LaFlèche, Olivier Blein,
Abstract Chondrite-normalized rare earth elements (REEs) profiles and high field strength elements (HFSEs) in rocks and minerals represent important tracers of geological processes. Of interest to mineral exploration research is the birdwing-shaped REE profile with negatively-sloped light REE ((La/Sm)N >1), positively-sloped heavy REE ((Tb/Yb)N <1), and strong negative Eu anomalies. This unusual profile is mostly found in rocks associated with large-scale hydrothermal systems. Another key indicator is the severe deviation of Nb/Ta, Hf/Sm ratios from chondritic values as these ratios generally remain stable and chondritic through normal magmatic and metamorphic processes as a result of the nearly identical ionic radii and charge but may significantly fractionate in hydrothermal systems.

In this study, we present a geological setting where birdwing-shaped REE profiles, together with suprachondritic Nb/Ta, Hf/Sm ratios, are systematically developed in a series of garnetite, quartzofeldspathic gneiss, Fe- or Mg-rich aluminous gneiss, and sulfides and magnetite. These units and associated Cu-Au mineralizations form a VHMS-style hydrothermal system in the granulite-facies 1.4 Ga -1.35 Ga volcano-plutonic, arc-related Bondy Gneiss Complex (BGC), western Grenville Province, Canada. Outside the recognized hydrothermal system of the BGC, such REE profiles are not observed. Instead the surrounding felsic and mafic gneisses display primary trace-element characteristics of island-arc magmas, suggesting that granulite-facies metamorphism did not modify significantly the trace-element signatures of the protoliths. Although magmatic fractionation of accessory minerals can produce birdwing-shaped profiles, it cannot explain the extreme fractionation of isovalent trace elements (Nb-Ta) and the high Zr content of the hydrothermal rocks. Neither is it compatible with the lack of significant Nd depletion that would normally incur during this process. Following our study, we demonstrate that F-rich fluid and mineral phases enriched in LREEs and HREEs respectively must have been involved in the main hydrothermal alteration and REE fractionation process in the BGC. Preferential removal of REEs from the interstitial glass of the volcanic protoliths together with dissolution of plagioclase and subsequent external influx of HREEs and HFSEs can produce birdwing-shaped REE profiles. In turn, such profiles and/or unusual Nb/Ta, Hf/Sm ratios can be used as a sensitive marker of hydrothermally altered rocks in targeting mineral exploration.
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