Evidence of multi-stage orogenic gold mineralization at the Bonnefond deposit, Val-d’Or, Québec

A significant part of gold production in Canada is associated with orogenic-style quartz veins. However, some critical parameters – the timing of mineralization, the source and transport of gold, and the gold precipitating mechanisms – remain enigmatic in several districts. The Bonnefond deposit, located in the southeastern Abitibi Subprovince, Québec, is part of the world-class Val-d’Or vein field (VVF). Gold mineralization is associated with pyrite in quartz-tourmaline-carbonate (QTC) veins that cut a subvertical tonalitic plug. Near-infrared imaging and trace element mapping of this Au-bearing pyrite show complex chemical zoning. An inclusion-richer core (Py1) is overgrown by a euhedral, oscillatory-zoned domain (Py2). A sharp front delineates a final pyrite generation (Py3), barren of gold. Highest trace element contents are recorded in Py1 (Co ~ 2000 ppm, Ni ~ 1500 ppm, As < 60 ppm) whereas Py2 displays lower contents (Co < 1750 ppm, Ni < 1000 ppm, As < 50 ppm). The Py1 shows δ34S = -7.7‰ to -2.2‰ and Δ33S = -0.04‰ to 0.04‰; Py2 displays δ34S = -4.7‰ to 4.0‰ and Δ33S = -0.15‰ to 0.08‰; and Py3 shows δ34S = -1.6‰ to 4.7‰ and Δ33S = -0.06‰ to 0.03‰. Gold is found as Au ± Te inclusions in Py2 (Au1), as trapped inclusions at Py2-Py3 border (Au2), and at pyrite margins and in microfractures (Au3). Trace element contents and multiple S isotopes suggest that fluid-rock interactions drove a coupled fO2, fS2, and fTe2 decrease in the auriferous fluid which precipitated Au1. The dissolution-reprecipitation (DR) textures and the multiple S isotopes suggest that pyrite DR triggered gold remobilization (Au2). In situ U-Pb xenotime geochronology yields a QTC mineralization age of ca. 2663 Ma whereas a ca. 2608 Ma age indicates gold remobilization. The proposed multi-stage mineralizing process supports recent studies in the VVF, suggesting that fluid-rock interactions and gold remobilization via DR are key mechanisms to orogenic gold mineralization.