Abstract

Serpentinization of barren and weakly mineralized Archean dunites in the Yilgarn Block, Western Australia, occurred without any apparent loss of major components, including the important ore elements Ni and Fe. Sulfur addition during initial serpentinization resulted in about 30 percent of original silicate Ni being redistributed to newly formed sulfide by reactions similar to: olivine + water = lizardite + brucite and lizardite + brucite + S = lizardite + brucite + magnetite + heazlewoodite + water + H 2 . Serpentinization at a higher temperature or metamorphism of the lizardite-bearing assemblage formed antigorite serpentinite in which up to 60 percent of bulk rock Ni is incorporated in sulfide form: lizardite + brucite + magnetite + heazlewoodite + CO 2 + S = antigorite + magnesite + magnetite + Ni sulfide + water + H 2 .Complete CO 2 metasomatism resulted in talc-carbonate rocks with still more Ni redistributed to sulfides, and the formation of some Fe sulfide: antigorite + magnesite + magnetite + Ni sulfide + CO 2 + S = talc + magnesite + pentlanditc + Fe sulfide + or - magnetite + water + O 2 . In areas where low S activity precluded sulfide formation, Ni was strongly enriched in magnetite in serpentinites or talc in talc-carbonate rocks.Preexisting disseminated Ni sulfides of magmatic origin were significantly upgraded by serpentinization. Up to 30 percent of sulfide Ni in ores averaging about 0.7 percent Ni is of metamorphic origin, and this proportion may be higher in talc-carbonate ores. Fe sulfides of metamorphic origin form up to 20 percent of the total sulfide fraction in talc-carbonate-altered disseminated Ni ores, so that about half the sulfide content of these ores may be metamorphically derived by redistribution of elements originally held in olivine.

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