Sulfide ores of a massive deposit drilled by holes 856H and 856G, Ocean Drilling Project Leg 139 (Juan de Fuca Ridge), are studied by magnetic mineralogy, optical and electron microscopy, Mossbauer spectroscopy, X-ray diffraction, and microprobe analysis. The collection studied consists of 26 samples. The following iron-bearing minerals are recognized monoclinic and hexagonal primary pyrrhotite; sphalerite and chalcopyrite, formed from solutions at temperatures above 350 degrees C; secondary hexagonal pyrrhotite of lambda and intermediate types, pyrite, and marcasite, formed after primary pyrrhotite; and magnetite formed mostly after pyrite at a temperature below 350 degrees C. Both the pyrrhotite and the magnetite exhibit unusual features. Primary monoclinic pyrrhotite is partly disordered and shows high blocking temperatures (340-360 degrees C), whereas intermediate hexagonal pyrrhotite is partly ordered and thereby has remanent magnetization with blocking temperatures of 100-260 degrees C. Magnetite does not contain Mg, Mn, Ni, or other admixtures and is not oxidized; however, it is deficient in Fe and has lower Curie points (520-540 degrees C). This probably reflects specific features of its formation in a water environment. Diagrams are plotted that show the variation of sulfide phase concentrations along the hole 856H section and suggest a nonmonotonic, multistage process of crystallization of sulfide minerals involved in the orebody formation. The results obtained provide a deeper insight into the genesis of such rocks, both recent and ancient.