• journal_title：Economic Geology
• Contributor：David J. Porter ; Keith G. Mckay
• Publisher：Society of Economic Geologists
• Date：1981-
• Format：text/html
• Language：en
• Identifier：10.2113/gsecongeo.76.6.1524
• journal_abbrev：Economic Geology
• issn：0361-0128
• volume：76
• issue：6
• firstpage：1524
• section：Articles

The Forrestania group of nickel sulfide occurrences of the Archean Southern Cross province is localized in intrusive dunite bodies at the base of a lower komatiite unit, which is interlayered with tholeiitic mafic rocks, para-amphibolites, quartz-biotite schists, chemical sedimentary rocks (iron-formation and sulfide cherts), and komatiites. Concordant, lenticular, metamor-phosed, sill-like, mineralized dunites have komatiitic affinities and appear to be feeder chambers to overlying thick sequences of dominantly extrusive ultramafic komatiites and rare pyroclastic komatiites. Unmineralized portions of the dunites are extremely rich in magnesium, with MgO values ranging between 45 and 51 percent. The important nickel sulfide occurrences are pentlandite-rich disseminations distributed in the basal portions of metamorphosed intrusive dunites (e.g., Cosmic Boy and Digger Rocks-South Digger Rocks). Similar mineralization also occurs in hanging-wall and internal portions of dunites but is generally of less importance than the basal mineralization because of lower nickel tenor. Massive pyrrhotite-rich sulfides occur within reaction zones between dunite and footwall country rocks and as remobilized veins and stringers in footwall metasediments. This type of mineralization forms important small tonnage-high grade deposits at New Morning and Flying Fox. Supergene alteration of primary sulfides is extensive, and massive violarite-pyrite-magnetite zones are developed in weathering profiles.Polyphase deformation and metamorphism, with peak conditions estimated at 655 degrees + or - 30 degrees C and 4.0 + or - 1.0 kb, resulted in widespread recrystallization and tectonic remobilization of ductile sulfides. Cumulate dunites were metamorphosed to bladed, granular, and granoblastic aggregates of forsterite-talc, forsterite-talc-anthophyllite, and forsterite-talc-enstatite, formed under varying f (sub CO ₂ ) and f (sub H ₂ O) conditions. Retrograde serpentinization and talc-carbonate alteration followed by later supergene alteration has caused textural modification and oxidation of primary sulfides, with development of pyrite-, violarite-, and magnetite-rich ores.Nickel values and Ni/Cu ratios vary for different types of mineralization. The economically important basal disseminations of pentlandite-pyrite-pyrrhotite-chalcopyrite have Ni values in the range 2 to 5 percent and constant Ni/Cu ratios in the range 14:1 to 22:1. Bulk composition of primary sulfides of this type at Cosmic Boy is S rich, with Fe = 39.4 percent, Ni + Co = 18.6 percent, and S = 42.0 percent. Massive remobilized veins of pyrrhotite-pentlandite-pyrite-chalcopyrite have high Ni (3 to 8%) and erratic Ni/Cu ratios, generally in the range from 10:1 to 200:1, with a mean of 45:1. This mineralization is Fe rich, with Fe = 50.4 percent, Ni + Co = 7.4 percent, and S = 42.2 percent. Broad, irregular sparse disseminations (less than 2%) of pentlandite-heazlewood-chalcopyrite form in the internal portions of cumulate and metamorphosed dunites. They have low Ni (0.3 to 0.5%), high Ni/Cu ratio (50:1 to 100:1), and bulk composition close to pentlandite composition, with Fe = 31.8 percent, Ni + Co = 34.2 percent, and S = 34.0 percent. The bulk compositions of sulfides generally plot on the S-rich side of the monosulfide solid solution field at 650 degrees C, and this appears to reflect nickel and sulfur mobility during retrograde metamorphism.