Noble metal nanonugget insolubility in geological sulfide liquids

Abstract

Abstract Noble metals (NMs) in Earth’s magmatic systems are thought to be controlled entirely by their strong partitioning to sulfide liquids. This chemical equilibrium is at the root of various models, ranging from NM deposit formation to planetary differentiation. Noble metals commonly occur as sub-micrometer phases known as nanonuggets. However, the assumptions that nanometer-scale thermodynamic equilibrium partitioning is attained and that NM nanonuggets are soluble in sulfide liquids have never been validated. Using novel experimental methods and analytical techniques we show nanometer-scale NM ± Bi phases attached to exterior surfaces of sulfide liquids. Larger phases (≤1 µm) show clear liquid immiscibility textures, in which Fe, Cu, and Ni partition into sulfide liquids whereas NMs partition into bismuthide liquids. Noble metal compositions of sulfides and their associated NM phases vary between adjacent droplets, indicating NM disequilibrium in the system as a whole. We interpret most nanometer-scale NMs contained within sulfides to be insoluble as well, suggesting that previously reported sulfide–silicate partition coefficients are overestimated. Consequently, sulfide liquids likely play a secondary role in the formation of some NM ore deposits.