Recent estimates of the thermodynamic behavior of aqueous electrolytes at 2 kb and temperatures to 900 degrees C (McKenzie and Helgeson, 1984) have been combined with thermodynamic data for minerals, gases, and H 2 O (sub (l) ) (Helgeson and Kirkham, 1974; Helgeson et al., 1978; Helgeson, 1982b, 1984) to generate logarithmic activity diagrams for the system CuS-FeS-H 2 S-H 2 SO 4 -HCl-H 2 O at magmatic temperatures and pressures. Equilibrium constraints on the distribution of species in the aqueous phase coexisting with mineral assemblages in this system can be used to characterize the depositional environment of low-grade potassium silicate protores in the central zones of porphyry copper deposits. These protores apparently form at relatively high fugacities of H 2 S. For example, thermodynamic calculations indicate that the sulfide-oxide mineral assemblage in the deep zone of the Butte ore deposit described by Brimhall (1977) is uniquely consistent with a fugacity of H 2 S (sub (g)) (f (sub H 2 ) S) equal to approximately 15 bars, a fugacity of O 2 (f (sub O 2 ) ) equal to approximately 10 (super -16.3) bars, and a ratio of the activity of Fe (super +2) to the square of the activity of Cu (super +) (a (sub Fe (super +2) ) /a 2 (sub Cu (super +) ) ) in the aqueous phase of approximately 10 (super 5.6) at 600 degrees C and 2 kb, which applies to pre-Main Stage mineralization at Butte. With decreasing temperature and f (sub H 2 ) S), a (sub Fe (super +2) ) /a 2 (sub Cu (super +) ) ) increases dramatically. Calculations of this kind make it possible to assess source requirements for ore-forming solutions responsible for mineralization in magma-hydrothermal systems.