ORIGIN OF THE NICKEL TENOR ALONG STRIKE OF THE EDWARDS LODE NiS ORE BODY, KAMBALDA (WESTERN AUSTRALIA)

*Department. of Earth Sciences, Monash University, Australia, ylahaye@earth.monash.edu.au,

Nickel tenor (Ni in 100% sulphide) within the Edwards lode, Otter-Juan NiS ore shoot, varies from 10%-20% along strike. This magnitude of internal tenor variation has not been previously documented for a komatiite-associated ore body at Kambalda or elsewhere. Three models were evaluated to explain the observed Ni tenor variation: (1) variations in silicate/sulphide mass ratio (R Factor) during volcanism, (2) variations in ƒ(O2) during volcanism, and (3) Ni redistribution by post-volcanic processes, particularly upper greenschist facies metamorphism-metasomatism. An R factor based model is supported by the higher nickel tenor of matrix (40-80% sulphides) ores compared to massive ores (>80% sulphides). However, depleted mantle-normalised PGE abundances are similar in all ore types, which contradicts an R factor-based model (because sulphide/silicate DPGE>>DNi). In regards to an ƒ(O2) based model, contents of Fe increase with Ni and magnetite (Fe3O4) decreases with increased Ni, opposite to trends predicted by such models. Hence, the petrographic and geochemical features indicate the Ni tenor variations within the Edwards lode cannot be explained solely by the volcanic models.

The third model evaluated to explain the tenor variation invokes metamorphic-metasomatic Ni redistribution within the Edwards lode. This model is supported by the positive correlation of increased contents of pentlandite with pyrite (a metasomatic alteration mineral), presence of pentlandite grains in carbonate veins (indicates Ni mobility), and positive correlation of Ni tenor variation with increased degree of metamorphic alteration. Furthermore, in-situ analyses by laser ablation HR-ICP-MS indicate that most of the P-PGE (Pt, Pd, Rh) are hosted by micro-inclusions of platinum-group minerals, probably secondary in origin. In contrast, the I-PGE (Ir, Os, Ru) are in solid-solution within the major sulphide phases and are inferred to have been immobile. The I-PGE show a strong positive correlation with increased distance along strike. Nickel does not correlate with the I-PGE along strike, consistent with a metamorphic control on Ni distribution. In conclusion, the petrographic and geochemical features indicate that a metamorphic-metasomatic redistribution model best explains the Ni tenor variation within the Edwards lode.