Nickel mineral systems within an evolving Archean craton

Nickel mineral systems in the Archean Yilgarn Craton of Western Australia are hosted by komatiites which cluster in space and time. At 2.9 Ga, komatiite-hosted nickel systems are concentrated in the southern Youanmi Terrane, whereas at 2.7 Ga deposits are primarily located in the Kalgoorlie Terrane. This study investigated whether lithospheric architecture, constrained using Sm-Nd and Lu-Hf isotopes from granites and felsic volcanics, controlled the localisation of komatiitic magmas. As the komatiites and associated deposits are from different ages and locations, Lu-Hf isotopes were split into 'time-slices' at 2.9 and 2.7 Ga. These isotopes were subsequently mapped, effectively imaging the evolving lithospheric architecture of the craton through space and time. Results show that during peak periods of komatiite volcanism, the hottest and most voluminous flows occurred at the isotopic contrast between an old, reworked terrane and a younger, juvenile terrane, with the komatiites occurring in the juvenile area; an architecture analogous to a craton-margin. This suggests that the locus of komatiite emplacement moved from 2.9 to 2.7 Ga because the 'ideal' lithospheric conditions changed with the evolving geometry of the craton. Subsequently, the re-working of the southern Youanmi Terrane through time moved the craton margin to the east, focusing 2.7 Ga komatiite activity in this area. The results of this study not only explain why major komatiite events and associated nickel systems cluster in space and time, but may also explain the significant spatial variations in the abundance and type of komatiite across the Yilgarn, and possibly other cratons.