Recent exploration in the Duketon greenstone belt, Yilgarn craton, Western Australia, led to the discovery of a new occurrence of high-grade Ni-PGE (platinum group element) sulfide mineralization associated with komatiite; this is referred to as the Rosie Ni Prospect. The mineralization consists predominantly of disseminated and brecciated semimassive to massive base metal sulfide with 0.5 to 5 cm thick sulfarsenide-bearing lenses. This pilot study focuses on the petrology, mineralogy, and trace element mineral chemistry of sulfides and sulfarsenides, and the mineralogy of minor PGE-rich minerals (sperrylite, melonite, and bismuthotel-lurides) in selected samples representing different parts of the orebody, with a particular emphasis on the sulfarsenide-rich lenses. Our mineral chemistry and mineralogical studies indicate that As-rich phases (either as a melt or as primary minerals) played a critical role in collecting and concentrating PGEs from the komatiitic magma. The concentrations of trace elements within the sulfarsenides and sulfides from the different mineralization types reflect the interaction between the silicate and sulfide liquids. The concentration of PGEs in the As-rich minerals is a function of the volume of sulfide melt with which they have interacted. The smaller the proportion of the sulfarsenide relative to sulfide in the rock is, the higher the PGE concentration in the sulfarsenide will be. In situ Se analysis of the base metal sulfides from the different ore types indicates that Se concentrations in pentlandite and pyrrhotite from sulfarsenide-rich lenses are an order of magnitude higher than those of sulfides found in As-poor samples. This correlation between the Se concentrations in the sulfide minerals and the As concentration in the whole rock indicates that the processes which led to As enrichment at Rosie also contributed to Se enrichment. The particular As-Se enrichment is inferred to have been triggered by the erosion and assimilation of sulfidic sediments enriched in organic matter (now observed as shales and/or black shales) by the komatiitic magma flows, leading to the formation of immiscible S-As-rich melt, where PGEs partition preferentially into the As-rich phases.