Abstract

Otter Shoot differs from the other shoots at Kambalda in that it contains a significant proportion of millerite-bearing sulfides. The sulfides have been extensively altered by supergene processes. A study was initiated to decide whether the millerite was a product of supergene alteration or was of primary magmatic origin and also how its presence affects weathering of the sulfides.The nickel sulfides in the Otter Shoot occur as a massive sheet at the lower contact of ultramafic rock with metabasalt and as disseminations within the ultramafic. The contact sulfides consist largely of violarite, pyrite, and millerite, but textural evidence indicates that they originally consisted of two different primary assemblages---pentlandite-millerite-pyrite and pyrrhotite-pentlandite-pyrite---that have subsequently been subjected to supergene alteration. The hanging-wall sulfides consist of spherical blebs of millerite, pentlandite, pyrite, and magnetite. Closely associated with the sulfide blebs are blebs of silicate material consisting mainly of talc and chlorite. Both are presumed to have originated as immiscible liquid globules in an ultramafic magma. The ultramafic rock has been altered to an assemblage consisting of talc, serpentine, and carbonates in varying proportions. The millerite-bearing assemblage is preferentially associated with a serpentine-rich zone.Supergene alteration has completely converted the pyrrhotite and pentlandite of the contact ore into violarite and pyrite by means of a coupled reaction in which the nickel released by the replacement of pentlandite by violarite has reacted with the pyrrhotite to produce additional violarite; the remaining pyrrhotite has been converted to pyrite. Where pentlandite is associated with miilerite in the contact ore, the pentlandite has been converted into a nickel-rich violarite, and the iron released by this reaction has participated in the conversion of some of the millerite to violarite. Decomposition of the sulfides at the water table has resulted in the formation of gaspeite and reevesite.

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