Metal transport by magmatic volatile phases in crustal systems

Magmatic volatile phases (MVPs) are multicomponent fluids that are a transport medium for metals being transferred from deep magmatic sources to sites of ore formation. However, the melt-to-fluid exchange of metals remains elusive because existing empirical simulations primarily address metal transport through the fate of one chemical element. We use a comprehensive thermochemical model to simulate the fractional crystallization of a silicate melt that degasses a multicomponent MVP. We show that the major and trace element abundances in MVPs formed from non-enriched magmatic systems are indistinguishable from MVPs found as fluid inclusions in mineralized and non-mineralized systems. We therefore conclude that ore formation is the consequence of repetitive intrusion-fractionation-degassing cycles common to crustal systems without pre-enriched sources, as opposed to scenarios wherein a particular or complex chemical system is required. Instead, the driving force of ore formation is a long-lived system fueled by an H2O- and Cl-bearing melt. Variations in metal signatures of fluids therefore reflect the pressure-temperature path of melt ascent and the changes in major element composition of the melt.