The Quebec Antimony deposit contains a rare type of mineral association in which gudmundite (FeSbS) is the main antimony ore mineral and native antimony is common. Gudmundite is intimately associated with ferroan dolomite and pyrrhotite as early impregnations in the host rocks at the margin of veins and in silicified fault zones, whereas native antimony occupies the cores of these structures and is practically devoid of associated iron-bearing phases. Hypogene stibnite, kermesite (Sb 2 S 2 O), valentinite (orthorhombic Sb 2 O 3 ), and senarmontite (cubic Sb 2 O 3 ) formed at the expense of the earlier antimony phases during a late replacement stage.The deposit is hosted by altered quartzofeldspathic rocks of the Caldwell Group at their contact with serpentinites of the Lac Nicolet Ophiolitic Complex, southeastern Quebec. The antimony mineralization may have formed at a late stage of the Acadian orogeny during a brittle, south-southeast--north-northwest-directed compressive event.Unusual physicochemical conditions must account for ore deposition in this type of deposit. The paragenesis and presence of methane-rich fluid inclusions indicate that conditions were strongly reducing during the main stage of the ore-forming process. We propose that elevated CH 4 and H 2 fugacities resulted from fluid-rock interaction and/or from mixing with fluids derived from the nearby serpentinite bodies. In contrast, fluids responsible for late stibnite, kermesite, and Sb oxides were oxidizing and relatively iron poor.