The nature of the ore fluid in high-sulfidation epithermal precious metal systems continues to be a topic of debate, in large part, because researchers are forced to base their interpretations on observations made long after the hydrothermal activity that produced the deposits has ceased. Currently, opinion is divided over whether the ore fluid is a liquid or a vapor. The observation that the alteration of the volcanic rocks in these systems is the product of their interaction with an extremely acid fluid has led to a consensus that this fluid is a condensed vapor. However, as deposition of the ore minerals commonly postdates alteration, many researchers have concluded that the ore fluid is a liquid of volcanic or mixed volcanic-meteoric origin that was introduced later. This interpretation is strongly influenced by the widely held view that only a liquid can transport metals in concentrations sufficient to form economic deposits.
  Observations made at Kawah Ijen, an active stratovolcano (mainly andesitic in composition) located in the Ijen Caldera Complex in Java, Indonesia, are used to address these issues. The crater of Kawah Ijen is host to the world’s largest hyperacidic lake (pH ~ 0), an actively degassing solfatara field and a center of high-sulfidation epithermal alteration. The contribution to science made by this thesis is that I created a model explaining the formation of high-sulfidation epithermal alteration and mineralization, e.g. Au, Ag, Cu, in an active stratovolcano. The results of my study provide compelling evidence that high sulfidation epithermal precious metal mineralization can form directly from condensed magmatic gases.