Gold in the mantle: A global assessment of abundance and redistribution processes

Abstract

We have compiled whole-rock analyses in mantle rocks to investigate the abundance of gold in the lithospheric mantle, and how this distribution can be modified. We show that while gold is heterogeneously distributed in peridotites from the upper mantle, the concentrations in most geographical regions cluster around the global median value of 1.2 ppb. This means Au concentration in the modern lithospheric mantle is similar to estimates for the crust and primitive upper mantle, indicating that Au has not been substantially lost from the mantle into the crust, or that it has been effectively recycled between the crust and the mantle, over Earth's history. The only region that shows substantial deviation from the typical lithospheric mantle values is the Eastern Block of the North China Craton, which shows elevated Au contents (median 3.5 ppb) across multiple studies from seven different sampling sites.Au shows no systematic relationship with the LILE, and does not appear to be enriched by hydrous fluids in the mantle, which strongly contrasts with its behaviour in the crust. There is likewise no evidence that Au is enriched by carbonatitic metasomatism in the mantle. A weak correlation with Ta and Hf, along with significantly elevated Ta concentrations in samples with Au concentrations >5 ppb is observed, which possibly indicates the transport of Au by silicate melts in the mantle.We also show that the Au content of pyroxenites is commonly elevated relative to that of peridotites in the lithospheric mantle (median Aupyroxenite = 2.0 ppb; median Auperidotite = 1.2 ppb). The data indicate that the melts from which these pyroxenites have crystallised may be effective metasomatic agents within the lithospheric mantle. Furthermore, these gold-rich pyroxenites may play an important role in mineralisation processes.