Out of 35 breccias in volcanic and minor intrusive rocks of the Mexican porphyry belt, only five centrally located ones are mineralized separately with molybdenite and, later, chalcopyrite. The mineralized breccias show potassic alteration in a field of propylitic altered rocks; the unmineralized breccias of the 35 are sericitized and tourmalinized. Molybdenite post-dates but is coincident with potassic alteration. Chalcopyrite fills fractures enveloped by sericitization in the central breccias. Fluid-inclusion studies of quartz from the molybdenite deposits suggest deposition by episodic boiling of a probable magmatic fluid at 440o-350oC. Daughter minerals of fluid inclusions are interpreted as evidence for the solutions having carried sufficient metals to form an ore deposit. Model calculations illustrate the role of water/rock ratios in hydrothermal alteration.-G.J.N.