The syenite-associated Beattie deposit (measured and indicated resources of 60.9 Mt grading 1.59 g/t Au, and inferred resources of 29.7 Mt grading 1.51 g/t Au), located in the Abitibi greenstone belt, consists of two styles of gold mineralization: lithology controlled and structure controlled. Lithology-controlled mineralization is hosted by the syenite intrusion and associated with iron carbonate and sericite alteration. Lithology-controlled mineralization is low grade (1–2 g/t Au) and associated with arsenian pyrite and arsenopyrite, with gold being in solid solution within the spongy As-rich cores of pyrite. Structure-controlled mineralization is present within fault zones that are within the syenite intrusion and adjacent to its margins. This type of ore consists of high-grade mineralization (5 g/t Au) in silicified breccia with both hydraulic and tectonic features, cherty veins, and polymetallic veins. In this facies gold is visible as electrum filling microfractures of brecciated pyrite. The deposit has a distinct paragenesis. The initial stage of alteration involved hematite alteration due to deuteric oxidation. This was followed by a shift toward more reducing conditions, triggered by the introduction of CO2-rich hydrothermal fluids that led to sulfide precipitation and gold deposition in As-rich pyrite and arsenopyrite. A later-stage alteration event implies the input of silica-rich fluids that produced sulfide brecciation and their redistribution in the fault zones and gold remobilization in microfractures of brecciated pyrite. Calculated composition for δ18O and measured composition for δD on quartz veins associated with this late event are respectively 6.9 to 10.8‰ and −53 to −83‰, indicating a potential magmatic-metamorphic fluid mixing. This event is associated with enrichment in Te, Hg, Mo, As, Au, Se, Ag, and Sb. In the Beattie deposit, interaction between magmatic and hydrothermal activities, coupled with external fluid ingress, led to a multistage process of sulfide and gold deposition.