Abundance and distribution of gold, palladium and iridium in some spinel and garnet lherzolites: implications for the nature and origin of precious metal-rich intergranular components in the upper mantle

Abstract

The abundance and distribution of Au, Pd, Ir, Cu, Co and Cr has been determined in mantle-derived spinel lherzolite xenoliths in basanites from Mt Porndon (Victoria, Australia) and Kilbourne Hole (New Mexico, U.S.A.) and in garnet lherzolites from the Matsoku and Thaba Putsoa kimberlites (Lesotho). Minerals in the lherzolites concentrate Au, Pd and Ir in the following sequence of increasing platinum group element (PGE) content; garnet, olivine, orthopyroxene, clinopyroxene, spinel. and demonstrate that there exists a real crystallochemical control on the distribution of PGE. Whole rock PGE abundances calculated from the modal mineralogy are less than actually determined and indicate that the bulk of the PGE (60–80%) occur in a sulphide-rich intergranular component. A metasomatic origin for this component is considered to be unlikely and it is proposed that it represents an immiscible sulphide melt which has been retained in the mantle after extraction of a sulphur saturated basic partial melt. This component may in the case of garnet lherzolites have been modified by metasomatic events in the mantle leading to Au depletion and rare earth element addition. Spinel lherzolites are relatively homogeneous at a given locality but differ in their PGE content regionally. The weighted average abundances of PGE in a spinel lherzolite upper mantle are 0.6 ppb. Au, 4.0 ppb Pd. 3.6 ppb Ir. Garnet lherzolites are very heterogeneous and insufficient data is available to allow calculation of geochemically meaningful averages. Spinel lherzolite-basalt based pyrolite contains 0.9 ppb Au, 4.3 ppb Pd, 3.0 ppb Ir, and indicates that the mantle contains an apparent excess of Au over a calculated abundance based upon the siderophilic equilibrium distribution of Au between core and mantle. This excess is considered to be due to failure to consider the chalcophilic nature of Au in the mantle and not to the addition of a meteoritic component to a mantle equilibrated with the core.