Multicomponent elemental and isotopic mixing in Ni-Cu-(PGE) ores at Kambalda, Western Australia

Most magmatic Ni-Cu-(PGE) deposits are considered to have formed from sulfide-undersaturated silicate magmas and to contain a significant component of crustal sulfur that was derived via wholesale melting, partial melting, or devolatilization of wall rocks. Under such circumstances, the system may comprise a silicate magma and a sulfide magma, with or without crystalline solids, undissolved wallrock-derived xenoliths, an unmixed silicate xenomelt, or an undissolved xenovolatile phase, each of which may contain distinct chalcophile and lithophile components. Because traditional two-component (silicate: magma - sulfide magma) mass-balance models do not accurately model such systems, we have developed a series of multicomponent elemental and isotopic mass-balance equations to model batch equilibration in magmatic Ni-Cu-(PGE) systems. We have applied them to the type examples of komatiite-associated Ni-Cu-(PGE) deposits at Kambalda, Western Australia. The calculations indicate that the elemental and isotopic compositions of the various components in a multicomponent system will vary considerably as a function of the relative abundances of the components, and that different metals and isotopic systems may decouple from each other, yielding apparently conflicting information regarding the sources of the components. The results suggest that the S isotopic and Zn compositions of the ores are more sensitive indicators of contamination than the Os isotopes, and support a sediment-melting model for Kambalda.