François-Xavier Bonin, MSc

Most of the gold mined in Canada today comes from quartz veins in orogenic-type deposits. In the Abitibi greenstone belt, the gold-bearing quartz veins are commonly described as late orogenic with regard to the N-S D₂ shortening event (~ 2669 - 2643 Ma). However, not only are the quartz-carbonate veins reported over a large time period overlapping with D₂ (2665 - 2640 Ma) but several younger post-D₂ ages have also been identified in those veins (~ 2705 - 2360 Ma). These dates thus need better constraints to isolate the different events responsible for gold mineralization.

The Bonnefond gold deposit is located within the Val-d’Or vein field. Mineralization is associated with quartz-tourmaline-carbonate (QTC) veins. Gold is hosted mostly in pyrite as Au ± Te inclusions or in pyrite micro-fractures and at grain margins. Near-infrared imaging and LA-ICPMS trace elements mapping of pyrite show textural and chemical zoning. An inclusion-rich core domain is commonly overgrown by a hydrothermal oscillatory-zoned gold-hosting pyrite domain. A dissolution-reprecipitation front delineates the border with the next generation, showing as a barren, silicate inclusion-rich pyrite. This dissolution-reprecipitation texture supports a two-stage mineralization process. In situ LA-ICPMS U-Pb xenotime geochronology provides us with texturally-controlled temporal constraints on gold mineralization. These analyses yield a syn-D₂ hydrothermal event associated with QTC veins at ca. 2662 Ma and a second post-D₂ hydrothermal event at ca. 2607 Ma. Both hydrothermal episodes must therefore have contributed to local high gold grades at the Bonnefond deposit via its remobilization. Our work not only highlights the early- to syn-D₂ timing of gold introduction in the system within QTC veins locally but is also consistent with a documented district-scale post-D₂ gold remobilization event.