THE NATURE, ORIGIN AND PHYSICOCHEMICAL CONTROLS OF HYDROTHERMAL MO-BIMINERALIZATION IN THE CADILLAC DEPOSIT, QUEBEC, CANADA

Mo-Bi mineralization occurs in subvertical and subhorizontal quartz-mu scovite- +/- K-feldspar veins surrounded by early albitic and later K- feldspathic alteration halos in monzogranite of the Archean Preissac p luton, Abitibi region, Quebec, Canada. Molybdenite is intergrown with muscovite in the veins or associated with K-feldspar in the alteration halos. Mineralized veins contain five main types of fluid inclusions: aqueous liquid and liquid-vapor inclusions, aqueous carbonic liquid-l iquid-vapor inclusions, carbonic liquid and vapor inclusions, halite-b earing aqueous liquid and liquid-vapor inclusions, trapped mineral-bea ring aqueous liquid and liquid-vapor inclusions. The carbonic solid in frozen carbonic and aqueous-carbonic inclusions melts in most cases a t -56.7 +/- 0.1 degrees C indicating that the carbonic fluid consists largely of CO2. All aqueous inclusion types and the aqueous phase in c arbonic inclusions have low initial melting temperatures (greater than or equal to 70 degrees C), requiring the presence of salts other than NaCl. Leachate analyses show that the bulk fluid contains variable pr oportions of Na, K, Ca, Cl, and traces of Mg and Li. The following sol ids were identified in the fluid inclusions by SEMEDS analysis: halite , calcite, muscovite, millerite (NiS), barite and antarcticite (CaCl2. 6H(2)O). All are interpreted to be trapped phases except halite which is a daughter mineral, and antarcticite which formed during sample pr eparation (freezing). Aqueous inclusions homogenize to liquid at tempe ratures between 75 degrees C and 400 degrees C; the mode is 375 degree s C. Aqueous-carbonic inclusions homogenize to liquid or vapor between 210 degrees C and 400 degrees C. Halite-bearing aqueous inclusions ho mogenize by halite dissolution at approximately 170 degrees C. Aqueous inclusions containing trapped solids exhibit liquid-vapor homogenizat ion at temperatures similar to those of halite-bearing aqueous inclusi ons. Temperatures of vein formation, based on oxygen isotopic fraction ation between quartz and muscovite, range from 342 degrees C to 584 de grees C. The corresponding oxygen isotope composition of the aqueous f luid in equilibrium with these minerals ranges from 1.2 to 5.5 per mil with a mean of 3.9 per mil, suggesting that the liquid had a signific ant meteoric component. Isochores for aqueous fluid inclusions interse ct the modal isotopic isotherm of 425 degrees C at pressures between 5 90 and 1900 bar. A model is proposed in which molybdenite was deposite d owing to decreasing temperature and/or pressure from CO2-bearing, mo derate to high salinity fluids of mixed magmatic-meteoric origin that were in equilibrium with K-feldspar and muscovite. These fluids result ed from the degassing of a monzogranitic magma and evolved through int eraction with volcanic (komatiitic) and sedimentary country rocks.