Extremely Ni-rich Fe-Ni sulfide assemblages in komatiitic dunite at Betheno, Western Australia: results from synchrotron X-ray fluorescence mapping

Fresh unserpentinised komatiitic dunite at Betheno (Western Australia) contains a distinctive sulfide assemblage of pentlandite, pyrite and millerite. The Ni tenor (i.e. Ni concentration in the original sulfide liquid) of this assemblage is in excess of 30 wt%, and the bulk sulfide composition falls within the compositional range of monosulfide solution (MSS) above 800 degrees C. Such assemblages have conventionally been interpreted as the result of hydrothermal upgrading of normal lower Ni-rich magmatic assemblages, but this explanation is not applicable at Betheno. Subtle zonation of Ni concentration in the host olivine, revealed by high-resolution X-ray fluorescence mapping using the Maia detector on the Australian Synchrotron, suggests that coupled subsolidus re-equilibration of Ni and Fe between olivine and sulfide is not a plausible explanation, and the olivine appears to have gained Ni from sulfides rather than the other way around. This leaves a primary magmatic origin as the favoured interpretation, and supports the existence of a stable pyrite-millerite tie-line in the Fe-Ni-S system at low temperatures. Further evidence for this comes from the existence of similar assemblages in fresh dunites from the nearby Perseverance nickel deposit. Hydrothermal alteration is evidently not necessary to form unusually Ni-rich sulfide assemblages. The exceptionally high Ni tenors are attributed to open-system equilibration of sulfide liquid with typical Ni-undepleted olivine, under conditions where sulfide compositions are essentially buffered by the olivine composition, and to the known positive correlation between the Fe/Ni distribution coefficient between olivine and sulfide and the Ni tenor. Other Ni-rich, millerite-bearing assemblages, such as those from the Black Swan nickel deposit, may also have primary origins.