Origin of variations in nickel tenor along the strike of the Edwards lode nickel sulfide orebody, Kambalda, Western Australia

Nickel tenor (Ni concentration in 100% sulfides) varies from 12.5 to 14.5 w t% on the eastern flank, and from 16 to 18 wt% on the western flank of the Edwards lode orebody, Kambalda, Western Australia. The center of the orebod y exhibits a large internal variation in tenor, from 9.6 to 18.7 wt% Ni. Te nor variation of this magnitude within a single orebody has not previously been documented at Kambalda. Three models were evaluated for the observed v ariation. (1) Variation in R factor. This model is supported by a higher ni ckel tenor within the matrix ores than the massive ores. Pyrolite-mantle-no rmalized abundances of the platinum-group elements (PGE) are consistent amo ng massive, matrix and disseminated ores. This does not support an R-factor model for tenor variation, because sulfide liquid / silicate melt partitio n coefficients for the PGE are higher than for Ni (D-PGE much greater than D-Ni) (2) Variations in oxygen fugacity f(O-2). The f(O-2) control on Ni te nor is not supported by geochemical or petrographic observations. The propo rtion of magnetite is relatively consistent within the orebody, demonstrati ng no correlation with Ni tenor. (3) Partial redistribution and sulfidation of the nickel ores during upper-greenschist- to lower-amphibolite-facies m etamorphism. Evidence includes increased pentlandite abundance correlating with an increase in the abundance of secondary metasomatic pyrite. Some pen tlandite grains demonstrate a genetic relationship with carbonate veining, indicating nickel mobility. The large variation in Ni tenor observed within the central domain of the Edwards lode orebody correlates with the ore sur face that exhibits the greatest degree of metamorphic heterogeneity. A lase r-ablation ICP-MS study of the PGE distribution within sulfides shows that the Ir-group PGE (IPGE) are homogeneously distributed, in contrast to the P t-group PGE (PPGE). The distribution of Pd reflects the distribution of pen tlandite, and the other PGE (Rh, Ru, Pt) and Au seem controlled by PGM and Au-bearing phases. The IPGE show a strong primary magmatic control with dis tance along the strike of the orebody. Nickel does not demonstrate a correl ation with the IPGE, indicating that post-volcanism processes of remobiliza tion partly control the distribution of Ni along strike.