Mixing of ore fluids with metal-rich sulphide magmas can generate significant volumes of valuable ore deposits, according to a paper online this week in Nature Geoscience. Such late-stage mixing was thought to play a role in ore formation, but direct evidence for this process has remained elusive.

Olivier Nadeau and colleagues sampled lavas and volcanic gases emitted immediately after an eruption at Merapi Volcano, Indonesia. The samples were found to contain similar concentrations of metal elements, which suggests that the metals entered the gases, and thus the ore fluids, during mixing in the underground magma chamber.

In an accompanying News and Views article, Bruno Scaillet says: "The work by Nadeau and colleagues sheds light on a decisive step in the formation of ore deposits and the transfer of economically interesting metals from sulphide melts into the magmatic ore fluid."