Abstract

Previous interpretations of spinels in Archean Fe-Ni sulfide ores from Western Australia must be revised in view of widespread metamorphic effects. When these effects are isolated, however, the spinels remain useful genetic indicators.Modified igneous chromites occur in the metamorphosed ultramafic host rocks but show no tendency toward gravity-induced basal accumulation. They may be distinguished from chromites of metasomatic origin in contact reaction zones and from Cr-bearing magnetites produced during serpentinization. Ragged magnetites also attributed to serpentinization reactions occur in disseminated and matrix ores. In addition, the ores contain large subhedral ferrochromites and more euhedral magnetites which appear to be relict crystallization products of sulfide-oxide melts; their presence provides supporting evidence for an initially magmatic origin of the ores. Despite changes in their distribution during deformation, some gravitational concentration of the ferrochromites within ore is apparent; this concentration is incompatible with a single-stage magmatic model.Both the lithophile and chalcophile chromites in mineralized ultramafic rocks are unusually rich in zinc, a feature apparently inherited from the magmatic stage, and one which may be useful in exploration.

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