An approximate net slip for the No. 3 Vein fault at Opemiska Copper Mines (Que.) Ltd. property, in northwestern Quebec, has been obtained by comparing ore widths with the potential dilation from several possible fault movements. A reference plane of 096 °strike, 71 °north dip, has been chosen as a compromise between the two common attitudes of the vein. The ore stays close to the footwall of the fault, so that footwall topography has been used to calculate dilation. The zero dilation contours for 10 rectilinear movements and one rotation have been constructed, and their correspondence with the boundary between ore and waste used to choose a net slip of 41 m (135 ft) downward for the footwall, along a line pitching 50° in the reference plane, as the best solution. A close match between ore widths and more detailed dilation contours confirms this choice.