Magmatic salt melt and vapor: Extreme fluids forming porphyry gold deposits in shallow subvolcanic settings

The recently discovered Biely Vrch deposit in the Western Carpathian magmatic arc is the most extreme example of a porphyry gold deposit, being practically free of copper, molybdenum or any other sulfi de minerals. Microanalytical data on fl uid inclusions in quartz veinlets, including a characteristic type of banded veinlets, show that this deposit formed from nearly anhydrous Fe-K-Na-Cl salt melts containing ~10ppm Au, coexisting with hydrous vapor of very low density. This exceptional fl uid evolution required an Fe-rich dioritic source magma that was emplaced at shallow subvolcanic depth (<3.5km), directly exsolving a hypersaline liquid and magmatic vapor at high temperature (~850 ᵒC). During ascent to the level of the porphyry intrusion (0.5-1km), fl uid expansion at high temperature but low pressure led to halite precipitation and further water loss to the vapor, generating an increasingly Fe-K-rich salt melt that transported high concentrations of Au but negligible Cu into the fractured porphyry stock. The low sulfur fugacity resulting from fl uid expansion suppressed precipitation of sulfi de, explaining the goldonly enrichment in this globally recurring but rare type of gold ore.