This paper describes the geology of the Beattie mine, a gold deposit in northwestern Quebec. The mineralization is closely related in space, and possibly genetically, to an intrusion of syenite porphyry situated within an overturned syncline of Keewatin rocks along the eastern extension into Quebec of what is known as the Porcupine Belt.No ore has been found very far from the syenite porphyry or related lath-porphyry dikes, and the orebodies are composite, consisting largely of bleached and brecciated Keewatin tuffs adjacent to altered syenite porphyry, both silicified and mineralized with very fine sulphides.There are three major ore zones at the mine, two of the composite type mentioned above lying alongside pitching syenite lobes, and the third is an oreshoot within the syenite, following a fracture zone.Extensive alteration of the syenite and Keewatin rocks was accompanied by the introduction of potash.There are several ore types, but the most important is "breccia" ore, consisting of fragments of bleached and altered Keewatin rocks cemented by tiny quartz-carbonate stringers. Metallic mineralization is fine-grained pyrite and arsenopyrite. Adjacent to the "breccia" orebodies, and along fracture zones, are bodies of altered "gray massive" porphyry, also mineralized with pyrite and arsenopyrite.The only important sulphides in the ore are gold-bearing pyrite and arsenopyrite, the former averaging 6 1/2 per cent by weight, and the latter 1 1/2 per cent. The gold content averages 0.14 ozs. per ton, and concentration followed by roasting and cyanidation is necessary to recover the values.The structural aspects of the orebodies and their mineralogy suggest that this unusual deposit should be classed as mesothermal or even epithermal.