The Trout Lake deposit is associated with a granodiorite-tonalite stock that intruded greenschist facies calcareous and pelitic metasedimentary rocks. It is typical of the fluorine-deficient or granodiorite class of molybdenum deposits. The stock is surrounded by a contact metamorphic aureole in which the mineral assemblages record evidence of fluid infiltration. At the present erosional surface, temperatures were below 400 degrees C and X (sub CO 2 ) in the calcareous rocks was <0.01. The confining pressure is estimated to have been 2 kbars. Contact metamorphism was followed successively by skarn and potassic alteration. The latter developed as biotite halos around quartz-albite veins in the metasedimentary rocks and as K feldspar halos around quartz veins in the intrusive rocks. Fluid inclusions in veins formed during potassic alteration are dominantly aqueous and have low to moderate salinities. The ensuing quartz-feldspar-muscovite and muscovite-ankerite alteration events were relatively pervasive. Aqueous-carbonic fluid inclusions are commonly observed in quartz-feldspar-muscovite-altered samples and are the dominant fluid inclusion type present in veins associated with muscovite-ankerite alteration. The aqueous fluid inclusions from all stages of alteration homogenize at temperatures between 170 degrees and 310 degrees C, with a peak at 250 degrees C. The aqueous-carbonic fluid inclusions homogenize to the H 2 O phase between 240 degrees and 340 degrees C. Oxygen isotope compositions for muscovite-quartz pairs yield temperatures of 370 degrees to 400 degrees C for quartz-feldspar-muscovite and muscovite-ankerite alteration. This indicates that the fluids were trapped at approximately 1,400 to 1,700 bars, i.e., above the two-phase region in the H 2 O-CO 2 -NaCl system. Calculated delta 18 O values for the fluid range from 8.0 to 8.4 per mil and, taken in conjunction with pressure and temperature estimates, suggest a magmatic and/or modified ground-water source. Phase relations of alteration minerals suggest that pH decreased during alteration. Mineralization is spatially and temporally associated with quartz-feldspar-muscovite alteration; molybdenite is typically intergrown with muscovite that replaced alkali feldspar. This textural relationship is interpreted to indicate that molybdenite deposited in response to decreases of temperature and/or pH. The common occurrence of these intergrowths in other fluorine-deficient and -enriched deposits suggests that the controls of mineralization for these deposits were similar to those at Trout Lake.