Molten Salts: Fluid Inclusion Record and Role in Forming Mineral Deposits

Fluid inclusions provide a wealth of information on the compositions, temperatures, and other properties of fluids that form mineral deposits. Fluid inclusions representative of aqueous-hydrothermal ore-forming fluids have been studied extensively over many years, and our understanding of their properties is comprehensive. But in recent years, evidence has been mounting for widespread occurrence of novel and unexpected types of fluid inclusions. These fluids are composed of molten alkali-calcic chloride, sulfate, and carbonate components, and they contain little to no H 2 O. Their physical and chemical properties are only starting to be explored, but evidence for their capacity to mobilize certain metals, and their participation in crustal ore-forming processes, is growing. The objectives of this review are to showcase these novel solutions and to discuss their origins and roles in forming mineral deposits. ▪ Fluid inclusions in minerals reveal widespread occurrence of natural molten salts. ▪ Evidence for molten salts, composed of chloride, sulfate, and carbonate components, is reported in numerous ore deposits. ▪ Molten salts are low-viscosity fluids, highly chemically reactive, and capable of transporting high concentrations of critical metals. ▪ Molten salts represent a novel and unexpected type of crustal ore-forming fluid.