Abstract

Disseminated nickel-iron sulfides in Archean serpentinite at Black Swan consist of the relatively uncommon assemblage millerite-pyrite-magnetite-"violarite", with the previously unrecorded assemblage pyrite-vaesite-"violarite" occurring in some talc-carbonate rocks. Although interstitial sulfide textures and relic pyrrhotite-pentlandite aggregates suggest a primary magmatic origin for at least part of the mineralization, the sulfide assemblages are too sulfur-rich to have crystallized directly from a sulfide liquid. Progressive serpentinization and carbonation of a dunite body during an essentially static, upper greenschist to lower amphibolite facies metamorphism has clearly controlled the composition of sulfide aggregates and has resulted in at least limited mobilization and concentration of nickel and sulfur. Whether larger scale mobilization of nickel and sulfur resulted in significant concentration of sulfides is problematical, although sufficient silicate nickel does appear to have been released during carbonation to provide the observed concentration of nickel in the mineralized serpentinite.During an early-stage serpentinization iron-poor lizardite with clinochrysotile rims and magnetite dust, all pseudomorphous after olivine, coexisted with pyrite-millerite-magnetite in mineralized serpentinite and with disseminated "violarite" in near-barren serpentinite. Progressive release of silicate nickel and extensive textural modification of sulfide aggregates without significant change in their composition accompanied the replacement of lizardite by antigorite, and its subsequent recrystallization in response to increasing temperature. Carbonation of serpentinites was penecontemporaneous with recrystallization of antigorite in areas where carbon dioxide fugacity of the serpentinizing fluid was high. Sulfide aggregates in contact with carbonate were strongly recrystallized and more sulfur-rich assemblages were generated in talc-carbonate rocks, the final products of carbonation. Sulfide assemblages continued to re-equilibrate after final recrystallization of serpentine and carbonate minerals.

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