Abstract

The volcanic peridotite-associated nickel deposits at Kambalda, Western Australia, consist of a number of orebodies or shoots located on the flanks of a broad domal structure within an Archean greenstone belt in the Yilgarn Block. Chemical and mineralogical analyses of carted ore samples demonstrate that there is a systematic variation in sulfide bulk composition and mineralogy between shoots. This is shown by positive correlations between the Ni tenor of the shoots and their Co, Cu, pyrite, and spinel contents. These variations permit division of the shoots into three broad groups occupying distinct geographic positions in relation to the Kambalda dome. The variation in Ni tenor between shoots is suggested to be caused by oxidation occurring prior to or during extrusion, resulting in disequilibrium in f (sub o 2 ) between the sulfide and silicate melts. As equilibrium is reapproached, oxidation of the sulfide proceeds, resulting in a change in the partitioning behavior of Fe, and consequently the Ni content of the sulfide melt increases. Tapping of the silicate-sulfide melts at different degrees of departure from equilibrium could account for the observed compositional variation between the shoots.Emplacement of the sulfides is interpreted to have taken place as a single eruptive event. The coincidence of spinel-rich zones with high pyrite and Cr contents at most major sulfide-rich-sulfide-poor interfaces (e.g., between massive and disseminated sulfides) within the ore zone suggests that post extrusive but premetamorphic oxidation of the sulfides occurred at these positions. This oxidation is due to disequilibrium in f (sub o 2 ) between the silicate and sulfide during cooling and crystallization of the magma.

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