Gold in the mantle: The role of pyroxenites

Abstract

Mantle pyroxenites are the crystallised products of mafic silicate melts, which are commonly invoked as metasomatic agents in the upper mantle. This study has analysed the trace elements of sulfides, with a specific focus on gold, hosted in a suite of mantle pyroxenite xenoliths from Qilin in the Cathaysia Block, southeast China. These are compared with sulfides hosted in peridotite xenoliths from the same locality to assess the difference in the abundances of Au, and a suite of siderophile and chalcophile elements between the sulfides hosted in mobile melts in the upper mantle and their host “wall” rocks.Both the peridotite- and pyroxenite-hosted sulfides show a wide spectrum of trace element contents. The pyroxenite-hosted sulfides typically have PGE and Au concentrations that are an order of magnitude or more below those measured in the peridotite-hosted sulfides (lherzolite-hosted sulfides: total PGE=95±118 ppm, Au=1.4±2.6 ppm; pyroxenite-hosted sulfides: total PGE=0.25±0.70 ppm, Au=0.14±0.39 ppm). Furthermore, the Ir group PGE (Ir, Os and Ru) are present in lower concentrations than the Pd-group PGE (Pd, Pt and Rh). This may lead to a distinct signature if the melts from which these sulfides crystallise interact with lherzolitic sulfides. The overall low abundances of these elements within the pyroxenites suggests that the parent melts are an inefficient medium for enriching any of these elements in the upper mantle.