Porphyry-type ore deposits gain their metal budget from fluids released by magmas in response to decompression and crystallization. Therefore, understanding the controls on the efficiency of metal transfer from the silicate melt into the exsolving fluid is critical for ore genetic models. The conventional view placed the bulk of fluid exsolution into shallow crustal magma reservoirs; however, an increasing number of studies propose fluid exsolution and metal extraction taking place at mid to lower crustal levels. Therefore, we aimed to develop fluid/melt partitioning models for halogens and ore metals applicable over broad P, T and compositional (X) range.

To facilitate this, we conducted experiments (n=145) at P=150–830 MPa, T=750–1000 oC with rhyolitic to basaltic-andesitic melt compositions to constrain the fluid/melt partition coefficients of halogens (Cl, Br, I) and ore metals (Cu, Au, Ag, Mo). Experiments were conducted in rapid-quench externally heated pressure vessels and a piston cylinder apparatus with the phase assemblage contained in Au or Au-Cu-Ag alloy capsules. Run product glasses were characterized by a variety of microbeam techniques, and the equilibrium fluid composition was constrained by mass balance for halogens, and mostly by using synthetic fluid inclusions for the ore metals.

The data and the implementation of the resulting fluid/melt partitioning models led to several key conclusions such as: 1) all the studied metals can be effectively mobilized by magmatic fluids as chloride complexes in the absence of reduced sulfur species; 2) At upper crustal depths, efficient halogen and metal extraction will occur only from rhyodacitic to rhyolitic residual melts; 3) The partitioning of halogens and thus ore metals into the fluid is most effective at mid-crustal depths, facilitating metal extraction earlier during progressing magma differentiation; 4) Br/Cl and I/Cl ratios are sensitive indicators of the degassing and fluid fluxing of crustal magma reservoirs.