Molybdenum mineralization in the Southwest orebody at the Questa deposit is associated with crystallization of aplite porphyry, explosive brecciation, and exsolution of magmatic hydrothermal fluids. Quartz, K feldspar, fluorophlogopite, and molybdenite precipitated from exsolving aqueous fluids and cemented aplite porphyry and andesite wall-rock clasts forming a magmatic hydrothermal breccia ore.Fluid inclusions in quartz in the breccia matrix trapped aqueous fluids that transported and precipitated molybdenum. Two fluid inclusion populations associated with molybdenite mineralization were identified. The largest population commonly contains liquid, vapor, halite, sylvite, and one or more opaque daughter crystals. These inclusions homogenize between 200 degrees and 500 degrees C and have a prominent mode at 360 degrees to 400 degrees C; salinities vary from 31 to 57 wt percent NaCl equiv. Low first ice-melting temperatures suggest that additional fluid components, possibly calcium or magnesium chlorides, may be present. Approximately 80 percent of these inclusions homogenize by halite dissolution; phase equilibria constraints require that these inclusions trapped fluids in the vapor-absent field. The remaining saline inclusions homogenize to liquid at temperatures above halite dissolution temperatures. Significantly, the saline fluid did not coexist with a low-density aqueous fluid.A second population of inclusions contains fluids with salinities of 2.7 to 25.0 wt percent NaCl equiv and homogenizes by vapor bubble disappearance, vapor bubble expansion, or critical behavior. Textural relationships and phase behavior during microthermometry indicate that these inclusions constitute a single population which formed as pressure and temperature fluctuated at near-critical conditions.Pressure changes related to brecciation followed by crystallization and system resealing produced the observed fluids. Saline fluids were trapped as the system sealed following a major brecciation event. These high-salinity fluids were produced as the melt, under pressure that increased from hydrostatic to lithostatic, partitioned increasing amounts of chlorine into the exsolving aqueous fluid. Renewed fracturing and abrupt pressure decrease to hydrostatic conditions reduced the mass of chlorine partitioning to the exsolving fluid. Lower salinity fluids exsolved at near-critical conditions and were trapped by inclusions that homogenize to liquid, to vapor, or by critical behavior. The lack of cogenetic liquid- and vapor-rich fluid inclusions and phase equilibria constraints indicate that high-salinity fluids (< or = 57 wt % NaCl equiv) were generated directly by the crystallizing magma and were not a product of aqueous fluid immiscibility.