The Mount Clifford nickel sulfide deposit consists of several stratabound lenses of disseminated sulfides, averaging 1.5 to 2.0 percent Ni, in a serpentinized periodotite. The sulfide assemblage is Ni rich, represented by varying proportions of godlevskite, millerite, heazle-woodite, and Ni-rich pentlandite.In a single 50-cm section of core below the disseminated sulfide lenses, native nickel has been found as a significant component of irregular oxide-sulfide veins, stringers, and blebs in a serpentinized skeletal-textured periodotite. It is present as grains and granular aggregates ranging from 100 mu m to 3 mm in size, and averaging about 500 mu m. The chemical composition (average wt % from 24 electron microprobe analyses) is Ni, 96.41; Fe, 1.41; Co, 0.45; and Cu, 1.51; total, 99.78. Euhedral to subhedral spinels dominate the veins (approximate mode 75%) and are typically zoned from cores of nickeloan magnetite to rims of ferroan trevorite. The spinel both encloses and is included in an intergrowth of native nickel (15%) and heazlewoodite (10%). Minor phases include chalcocite, bornite, and maucherite, together with a rare unnamed mineral, close to Ni 4 S(Sn, Te, Sb) in composition.Secondary sulfides have developed in response to weathering and/or low-temperature hydrothermal alteration processes. Pentlandite is replaced by violarite-polydymite and millerite; godlevskite is replaced by millerite; heazlewoodite is replaced by godlevskite, which in turn is replaced by millerite. Carbonate-bearing solutions have caused alteration of coexisting native nickel and heazlewoodite to assemblages comprising native copper, nickel-iron carbonates, nickel and nickel-iron hydroxycarbonates, and nickel-rich silicates.The native nickel-heazlewoodite-ferroan trevorite assemblage is believed to have crystallized in a hydrothermal environment characterized by high nickel and low sulfur activities, and in which oxygen activity was sufficiently high to make nickel-iron sulfides and nickel-iron alloys unstable relative to nickel-iron oxides, nickel sulfides, and native nickel.