Coordination of Au and Cu in Peridotite Melts Studied by First Principles Molecular Dynamics Simulations

Chlorine (Cl) and sulfur (S) are two crucial mineralizing agents in silicate melts, and are closely related to the genesis of metallic mineral deposits. Magmatic ore deposits usually form in mafic–ultramafic silicate melts by the separation (liquation) of a cooling, sulfur-rich magma into two immiscible liquids. It is not easy to identify the complexation between gold (Au), cooper (Cu) and Cl, S using the current experiment methods, and the coordination of Au and Cu with Cl and S is still unclear in mafic–ultramafic silicate melts. In this study, by using first-principles molecular dynamics technique, we investigated the structure of Au, Cu, Cl and S in the (a) anhydrous and (b) hydrous peridotite melt to reveal their coordination geochemistry. Our results show that Si4+–Cl−, Cu+–O2−, Au+–O2−, Cu+–Cl−, Au+–Cl−, Au+–S2−, and Cu+–S2− cannot form stable ion pairs in silicate melts; therefore, Au+ and Cu+ cannot form stable complexes with S2−, O2− or Cl− in the melts. But the diffusion coefficients of Au+, Cu+, S2− and Cl−, their RDF values and the bonding time ratio of the silicate melt systems show that, although they cannot form stable complexes, within the range of effective chemical bond lengths, they have a high probability of approaching and interacting with each other, which enables them to form crystal embryos or liquid-phase molecules during magma evolution.