Detailed petrographic examination of Mississagi quartzite from the Sudbury basin in southeastern Ontario, Canada, reveals that the conical fractures previously identified as shatter cones are lined with sheet-silicate minerals characterized by oriented biotite and chlorite. These mineralized conical fractures crosscut the main deformation features in the rocks. Deformation features in hand specimens from the sampled localities — south Kelly Lake and Laurentian University campus — include serrated and interlocked quartz grain boundaries, extensive subgrain boundary development within quartz grains, primary recrystallization of quartz, and plagioclase cataclasis; these features were probably formed during the Penokean orogeny. The conical fracture zones are therefore late Penokean in age. The mineralization associated with the conical fractures developed by syntectonic crystallization under conditions of nonhydrostatic compressive stress and low strain rates. This is indicated by, in particular, en echelon sigmoidal biotite and chlorite aggregates, crosscutting grain relationships, and slickenside surfaces with steps and fibrous striations. The orientation of the sigmoidal biotite and chlorite aggregates is consistent with σ1, parallel to the cone axes. Thus, the conical fractures postdate emplacement of the nickel intrusion, and they do not appear to be a crypto-explosion feature.