Highly altered slag samples from the historical O’Donnell Roast Yards, Sudbury, Ontario, Canada were examined to gain an understanding of the partitioning of metal and metalloids into phases during roasting and weathering processes and to support the environmental or resource assessment of the Roast Yards prior to remediation or smelting of the remaining material, respectively. The O’Donnell Roast Yards were the largest of their kind in the Sudbury area. Open-bed roasting of sulfide ores between 1888 and 1923 was used to reduce the amount of S in the ore by ~50–75 wt.%. Experimental studies and grain sizes for pyrrhotite suggest that the ore in the Roast Yard has been heated above 700 °C, at which temperatures the sulfide minerals completely transform into oxides. The presence of pyrrhotite and chalcopyrite (remnants of the ore) as well as of covellite (roasting product) indicates steep temperature gradients in the former roast heaps. The slags contain predominantly the aforementioned sulfide, spinel, and silicate minerals and the alteration products amorphous silica, goethite, jarosite, antlerite, roemerite, and gypsum. Remnants of the gangue material are highly altered quartz diorite and norite with silica-rich coatings containing phases of the jarosite group. Laser ablation inductively coupled plasma-mass spectroscopy measurements indicate that covellite and spinels are the major sinks for Se and Ni, Co and Zn during roasting, respectively. During low-temperature alteration processes, jarosite becomes the major host for Pb and As, Cu-sulfates for Cu and Se, and Fe-hydroxides for As, Se, Ni, Co, Zn, and Cr. X-ray diffraction and bulk chemical analyses of alteration halos around the center of the remnant slag and rock piles indicate strong variations in the chemical and mineralogical composition of the altered slag with lateral distance and depth. The concentration of trace elements in these surficial slag samples is controlled by the proportions of spinel phases and secondary sulfates.