Sulphide magma as a source of metals in arc-related magmatic hydrothermal ore fluids

Abstract

As magma ascends in subduction zones, decreasing pressure and temperature cause it to separate into two or more phases in a process known as exsolution. Hydrothermal ore fluids and metal-rich sulphide melts may be products of this exsolution. Conventionally, the ore fluids are thought to acquire their metal content directly from the parent magma. However, it is probable that the ore fluids and sulphide melts interact after exsolution, and therefore possible that the sulphide melts are an important source of metals for the ore fluids. Here we use petrographic analyses of erupted sulphide melts and compositional analyses of volcanic gases emitted immediately after an eruption at Merapi Volcano, Indonesia, to show that magmatic hydrothermal fluids can derive their metals from dissolution of sulphide melts. We find that the volcanic gases exhibit metal ratios that are comparable to those in the erupted sulphide melts. Furthermore, we show that gases sampled immediately after the eruption have metal concentrations that are orders of magnitude higher than those of gases emitted during periods of quiescence. We propose that the metal content of ores forming in volcanic arc environments is established by exsolution accompanying injections of primitive magma immediately before explosive eruptions. The metal content of ore deposits formed during subduction-zone volcanism was thought to be established when the ore fluid separates from the parent magma. Analyses of metal concentrations in erupted melts and the volcanic gases emitted after an eruption in Indonesia reveal that metals can be added to the ore fluid later, during mixing with separated melts.