A native nickel-heazlewoodite-ferroan trevorite assemblage from Mount Clifford, Western Australia

CSIRO, Div. Mineral., Wembley, W.A., Australia
West. Min. Corp., Australia

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.

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