The lithologic unit known as the “porphyry” at Geraldton, Ontario is host to widespread gold mineralization, and yet its origin is poorly understood. The porphyry occurs within the Beardmore-Geraldton greenstone belt in the Superior Province of the Canadian Shield. Samples from the porphyry contain approximately 1mm porphyroclasts of plagioclase within a groundmass composed of approximately 0.1mm quartz, plagioclase, and muscovite grains. The plagioclase porphyroclasts commonly have asymmetrical tails, and muscovite crystals tend to bend around the porphyroclasts. Undulose extinction and subgrain boundaries indicate dislocation creep in quartz. Plagioclase commonly contains deformation twins. The porphyry also has a well-developed foliation defined by parallel alignment of muscovite. The microstructures are typical of a mylonite. The porphyroclasts give a minimum original grainsize of approximately 1mm for the protolith. Based on microstructural analysis, it appears that the most likely protolith for the porphyry is a felsic plutonic rock. Approximately 30km east of Geraldton, near Longlac, Ontario, is a 150 km2 elliptical, granitic intrusion, the Croll Lake stock. This intrusion is the nearest felsic and plutonic rock to the porphyry at Geraldton. The Croll Lake stock is also deformed. Deformational features in quartz include undulose extinction, subgrains and serrated grain boundaries. Plagioclase commonly contains deformation twins. Evidence for deformation of the stock increases towards the west where it resembles the porphyry. Microstructural analysis of the porphyry and the Croll Lake stock suggest that the “porphyry” is a mylonitized fragment of the Croll Lake stock.