The chemistry of brine, vapor, and low-salinity fluid inclusions measured by proton-induced X-ray emission from the Bismark skarn deposit, Mexico, is consistent with an evolving magmatic-hydrothermal system with no evidence for external fluid inputs. The results support a model that invokes early phase separation of magmatic fluids into brine and vapor (two-phase field) at high temperature and lithostatic pressure, followed by the entrapment of a low-salinity magmatic fluid (one-phase field) at lower temperature and hydrostatic pressure. The early brine and vapor inclusions contain high Pb and Zn concentrations and low Cu; however, the vapor contains significantly more Cu than the brine and was likely transported as a sulfur complex. The fluid phase changes observed and behavior of Cu are comparable to those of porphyry Cu systems. The later, low-salinity fluid at Bismark represents a distinct pulse of magmatic fluid with a high K/Ca ratio in which base metals, including Cu, were transported by chloride complexes. Paragenetic relationships and variations in the relative concentrations of Cu, Pb, and Zn suggest that this fluid was primarily responsible for ore deposition. The relative Cu, Pb, and Zn concentrations in fluid inclusions at Bismark are consistent with those measured from base-metal ores in high-temperature carbonate- replacement deposits throughout Mexico, and suggest that the bulk-metal budget of these deposits is primarily controlled by magmatic-hydrothermal fluids deficient in Cu.