The distribution, petrology, and genesis of nickel ores at the Juan Complex, Kambalda, Western Australia

Abstract

Massive and disseminated Fe-Ni-Cu ores in the Juan complex occur in two belts of contrasting Ni tenor that are interpreted as original sea-floor depressions in basalt flows into which separate sulfidic komatiitic magmas were extruded. Similarities in composition and coincidental areal distribution of massive and disseminated ores indicate that their emplacement was nearly simultaneous. These features, the distribution of metasomatic reaction zones at basalt-ultramafic contacts, and the presence of interpillow massive Fe-Ni sulfides in the underlying basalt, all point to the existence of abundant primary massive sulfides on top of the basalt.A magmatic genetic model is favored for the Juan ores in which massive sulfides were extruded and emplaced slightly in advance of viscous komatiitic magma rich in olivine phenocrysts, as dictated by contrasts in flow rates. The thick komatiitic flow came to rest upon massive sulfides that were partly crystalline, due to rapid marginal chilling. Residual sulfide droplets settled during flow at the base of the flow, thus forming disseminated ore.During ensuing prograde regional metamorphism, massive ore was thickened in some folds and mobilized into some faults and previously barren contacts, but overall redistribution was very limited. Metamorphism did not create new massive ore.During declining metamorphic temperatures, oxidation reactions and attendant sulfur and copper diffusion generated metamorphic magnetite along sulfide-silicate contacts, pyritic layers and lenses in massive ore, and Cu-rich sulfide stringers in wall rocks. On final cooling, sulfide ores were annealed and many tectonic surfaces were overprinted by static recrystallization of silicates and sulfides.