Pseudo-eutectic intergrowths involving niccolite, chalcopyrite, pyrrhotite, and maucherite in Sudbury [Ontario] ores have been observed to develop chiefly by replacement of gersdorffite without the latter participating essentially as a component of the 2- or 3-phase aggregates. As shown previously, niccolite, by loss of As, may likewise develop niccolite-maucherite intergrowths. New experiments confirm the reactions by which gersdorffite forms niccolite-maucherite intergrowths and the subsequent replacement of maucherite by chalcopyrite to form a niccolite-chalcopyrite aggregate. Niccolite-pyrrhotite and maucherite-pyrrhotite intergrowths may be due simply to breakdown of ferroan gersdorffite, though the former is also developed by normal replacement of pyrrhotite by niccolite. Conditions for their development require elevated temperatures and a lowering of S (and As) vapor pressure, but reactions do not proceed under appreciable pressures of S. The intergrowths seem best explained as late-stage alterations, involving introduction of quartz and carbonates and remobilization of Cu and other constituents, accompanying deep-seated fracturing.