Electron microprobe and scanning electron microscopy. The platinum-group minerals found in the chromitites are laurite, porous Ru-rich alloys, Os-Ir- and Pt-rich alloys with highly variable compositions, an Ir-Cu-rich sulfide, a Pt arsenide, and three relatively rare platinum-group sulfides. The platinum-group minerals are 1 to 30 mu m in diameter and occur as inclusions in chromite grains; some of these inclusions also contain silicates and millerite. Base metal alloys (largely Ni-Fe) occur in cracks in the chromitites, along with magnetite, ferritchromite, chromian chlorite, and serpentine. The primary platinum-group mineral assemblage is interpreted to have formed before or during crystallization of the chromite grains and not as a result of exsolution from the chromite.--Modified journal abstract.