Abstract

The nickel sulfide deposits of Western Australia consist of many intrusive dunite-associated and volcanic peridotite-associated deposits of late Archean age, a few small gabbroid-associated deposits of Archean or Proterozoic age, and some other rare types. Some 96 percent of the nickel metal resource is contained in deposits concentrated in the curvilinear, folded and strike-faulted supracrustal belts of the eastern Yilgarn Block. Very magnesian ultramafic rocks and sulfidic metasediments abound in these belts, which have commonly been metamorphosed to greenschist or lower amphibolite facies conditions. Major strike faults probably influenced the nature and distribution of volcanism and sedimentation, later tectonometamorphic styles, and nickel mineralization.Intrusive dunite-associated deposits occur in semiconcordant lenses of peridotite to olivinite composition and of komatiitic affinities, which are restricted to curvilinear zones 100 km or more long. Nickel sulfides occur centrally or marginally in the thickest part of the lens. Low-grade (< 1% Ni) sulfides are abundant and enclose smaller bodies of higher grade disseminated and massive or breccia ores, which may be in part tectonically displaced. The Ni content of the sulfide fraction is about 10 percent (although low-grade sulfides may be Ni rich), with ranges in Ni/Cu of 19 to 70 and Ni/Co of (30 to 70. The lenses may have been emplaced (1) passively as subvolcanic sills which fractionated olivine and sulfides and gave rise to associated komatiitic lavas, and/or (2) dynamically as dikelike bodies of sulfidic olivine-rich crystal mush.Volcanic peridotite-associated deposits are best developed at or near the base of volcanic komatiitic ultramafics occurring between metabasalts, and at low stratigraphic levels in the succession. There is little or no geographical coincidence between these deposits and the intrusive dunite-associated deposits. The ultramafics are mainly lavas ranging from picrite to olivine peridotite in composition, with thicker and more magnesian flows dominating the lower, mineralized part of the piles, where thin interflow sulfidic metasediments are also common. About 90 percent of the mineralization is at the base of the lowermost olivine peridotite flows, which appear to be tongue shaped and to occupy original depressions in basalt surfaces devoid of sulfidic sediment. Thin, discontinuous massive sulfides resting on metabasalt are overlain by thicker, continuous, and more extensive matrix to disseminated sulfides. Some massive and breccia ores are tectonically displaced. The Ni content of the sulfide fraction varies from 5 to 23 percent, with an accompanying increase in Ni/Cu from 10 to 16 and Ni/Co from 40 to 65. On eruption of sulfidic komatiitic magma, sulfide segregation under differential flow and gravity would probably result in the observed cross sections through the ore but with subsequent physical modification of the ore during metamorphism.Gabbroid-associated deposits occur in layered or composite intrusions of gabbronorite with lesser pyroxenite, peridotite, and anorthosite, which crystallized from basaltic magma of uncertain affinity. The rocks are incompletely hydrated. Low-grade disseminated or blebby sulfides form layers or irregular bodies associated with lenses or veins of matrix, massive, or breccia sulfides. The Ni content of the sulfide fraction is up to about 6 percent, with a range of Ni/Cu ratios of 1 to 7 and an Ni/Co ratio average of about 25. The bulk chemistry of the intrusions and the sulfides and sulfide-silicate textures are consistent with the existence of immiscible sulfide liquids at the magmatic stage. Layered sedimentary-associated and vein-type arsenical deposits are rare but indicate the possible involvement of nickel in volcanic-exhalative and metamorphic-hydrothermal processes, respectively.A complex history of weathering and supergene alteration has affected the deposits and presents major difficulties in exploration. The recognition of siliceous limonitic cappings, some with pseudomorphic textures, led to the discovery of most deposits.Many problems remain before a more complete understanding of the genesis of the parental sulfidic magmas and their mode of ascent and emplacement into the crust is possible. These are discussed with particular reference to the intrusive dunite- and volcanic peridotite-associated deposits, which seem to be restricted to ca. 2.7-b.y.-old Archean terrains worldwide.

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