Towards quantitative understanding of volatile and metal release from magmas

Porphyry-type Cu-Au-Mo deposits form by ore metal sulfide precipitation from magma-derived fluids. For the efficient generation of such deposits, a magmatic fluid phase simultaneously rich in ore metals and SO2 is required. Silicate melt inclusions from volcanic rocks distributed along strike in the Southern Volcanic Zone of the Andes show that the availability of S and Cl rather than ore metal abundances control magmatic ore fertility. In addition to magma degassing, magmatic sulfides are often invoked in models assessing magma fertility. They may limit ore-forming potential by sequestering chalcophile metals in lower-crustal magma reservoirs, or they may promote ore formation if their metal and sulfur budget later get recycled into a magmatic fluid phase.

I used new experimental data for S and Cl in combination with published values to construct empirical models to predict their fluid/melt partition coefficients in P-T-compositional space. These were combined with the best available models for anhydrite and sulfide saturation and metal partitioning between sulfides and silicate melt to assess volatile and metal extraction from magmas during ascent and differentiation in the crust. The results show that sulfide-assisted metal pre-enrichment and transport is a testable hypothesis via silicate melt inclusion studies in arc volcanics because magmatic sulfides will typically break down before fluids rich enough in Cl to pick up their Cu budget can form. On the other hand, magmatic anhydrite saturation is predicted to be common and play an important role in ore genesis by regulating the supply of sulfur to the ore-forming hydrothermal system.