DESCRIPTIVE MODEL OF DUNITIC Ni-Cu

MODEL 6b

By Norman J Page

DESCRIPTION Disseminated sulfide mineralization in intrusive dunites.

GENERAL REFERENCE Marston and others (1981); Ross and Travis (1981).

GEOLOGICAL ENVIRONMENT

Rock Types Dunite, olivine peridotite in subconcordant lenses 500-1,000 m long, 50-100 m thick.

Textures Dunite; coarse-grained (2-20 mm) subequant olivine (Fo87-95) interlocked to give polygonal to mosaic texture; olivine peridotite; ovate olivine with intercumulus pyroxene, sulfide and oxide minerals.

Age Range Precambrian.

Depositional Environment Intruded into contacts between clastic sedimentary and felsic volcanic rocks and mafic to ultramafic volcanic rocks.

Tectonic Setting(s) Greenstone belts.

Associated Deposit Types Komatiitic Ni, synorogenic-synvolcanic-Ni talc-carbonate Ni-Au, layered sedimentary Ni.

DEPOSIT DESCRIPTION

Mineralogy High grade (1-9 percent Ni): pyrrhotite + pentlandite + magnetite + pyrite + chalcopyrite + chromite. Low grade (0.4-1 percent Ni): the same minerals ± millerite ± heazlewoodite ± godlevskite ± polydymite ± vaesite ± awaruite ± bravoite ± cobaltite ± nickeliferous linnaeite ± cubanite ± Fe-Ni arsenides.

Texture/Structure Lenticular shoots of massive, matrix, and breccia ores are fine to medium grained, also occurs as interstitial films. Olivine is commonly rounded when sulfide is present.

Alteration Prograde and retrograde serpentinization after deposition; usually metamorphosed.

Ore Controls Dunitic lenses close to major strike-slip faults and at high stratigraphic position in volcanic pile; most Ni-rich ores concentrated at one margin, perhaps at base of intrusion.

Weathering Lateritic zones may be enriched in PGE.

Geochemical Signature Ni, Cu, PGE, Cr, Co, Mg. Ni/Cu = 19-70+, Ni/Co=30-70. Massive sulfide ores 6-9 percent Ni, disseminated ores up to 3 percent Ni.

EXAMPLES

Agnew (Perseverance), AUWA (Martin and Allchurch, 1975)

Mt. Keith, AUWA (Burt and Sheppy, 1975)