Abstract This paper deals with the earliest stages of formation of porphyry Cu deposits, starting with the production of partial melts in the mantle and ending where upper crustal magmas reach their solidus and potentially have exsolved a metal-bearing hydrothermal fluid. During all these stages magmatic sulfides exert a major control on the budget of ore-forming metals in the magma. High metal concentrations in mafic arc magmas are favored by low degrees of partial melting in the mantle source region, and by limited removal (or effective redissolution) of magmatic sulfides in the lower crust. Ascending magmas accumulate in large, compositionally stratified magma chambers in the upper crust (5- to 15-km depth), which represent the exsolution source of the mineralizing fluids for the shallower porphyry Cu deposits. Interaction between mafic and felsic magmas in these magma chambers leads to partial mingling/mixing, volatile release, and the formation of magmatic sulfides that incorporate large amounts of Cu and Au, but only little Mo. For porphyry Cu mineralization, it is essential that these magmatic sulfides are subsequently destroyed and thereby release their contained metals to the mineralizing fluids. Evidence from experimental phase equilibria studies and melt inclusions hosted in phenocrysts from plutonic and volcanic rocks in arc environments, combined with fluid inclusion evidence from porphyry Cu deposits, suggest that silicate melts that ultimately give rise to porphyry Cu deposits are likely saturated first with a CO 2 -rich fluid and later give way to single-phase, low-salinity (typically 5–10 wt % NaCl equiv) aqueous fluids. At the typical f O2 conditions of porphyry Cu-forming magmas (ΔFMQ + 1 to ΔFMQ + 3), sulfur occurs mostly as SO 2 in the fluid. Efficient Cu removal from the magma into the overlying porphyry environment is favored by the exsolution of an S-bearing volatile phase that has a low HCl/alkali chloride ratio. The ability of the ore fluid to scavenge and transport Cu increases with increasing f O2 and the concentration of K in the aqueous fluid, and may be maximized at high ratios of SO 2 /H 2 S of the fluid. Once formed, efficient focusing of the ore fluid into the upper portions of the magma chamber may be favored by the development of permeable melt channels that act as conduits for the ascent of ore fluid in a pressure gradient through the crystallizing magma. These conduits likely facilitate the contribution of S, Cu, and other metals from mafic silicate melt that ponds at deeper levels of the magma system.

Abstract The term “fluid” is used in different ways in the geologic literature. Sometimes “fluid” is used to denote any kind of mobile phase, including silicate melts. In this chapter, we will, for purely pragmatic reasons, define a fluid as a mobile phase which is not a silicate or carbonate melt. Sometimes the term is defined even more narrowly as a mobile phase in a regime of pressure and temperature where no distinction between “vapour” and “liquid” is possible anymore. We will not follow this use, i.e. a “fluid” in the sense as it will be used in this chapter can have either “vapour-like” or “liquid-like” or transitional properties, unless otherwise stated. Evidence for the composition of fluids in the Earth’s interior comes essentially from three sources of evidence: (i) the analysis of volcanic gases, (ii) the investigation of fluid inclusions and (iii) considerations of phase equilibria. Gases from volcanoes with a non-explosive eruption style can sometimes be directly sampled, while direct sampling is impossible during major explosions. Naturally, this introduces some bias in the data on volcanic gas compositions, since gas analyses can be much more easily acquired from basaltic magmas than from the often highly explosive andesitic and rhyolitic ones. However, in recent years remote sensing of gas compositions by infrared spectroscopy has become possible and it is to be expected that the further development of these methods will ultimately allow a more representative sampling of volcanic gases from a variety of magma sources and tectonic environments. In any case, although there are quite significant variations, virtually all available analyses show that the predominant constituents