Fluid evolution and mineralization in a subvolcanic granite stock; the Mount Pleasant W-Mo-Sn deposits, New Brunswick, Canada

Abstract

In the Fire Tower zone at Mount Pleasant, New Brunswick, porphyry-style, W-Mo (stage I) and stanniferous, polymetallic vein and replacement (stage II) mineralization is hosted principally by a complex hydrothermal breccia pipe which overlies a subvolcanic, fine-grained granite stock. Wolframite and molybdenite of stage I occur in quartz + or - topaz and/or fluorite veinlets and breccia matrices and as disseminations in altered wall rocks. Quartz and topaz, with lesser green biotite, chlorite, and K feldspar, are the main alteration minerals. Sericite occurs as a late alteration product in the main ore zone. Stage II comprises mineralogically complex, Sn-bearing, sulfide vein and replacement bodies that contain abundant fluorite and chlorite. The stage II mineralization has been superimposed on stage I and is spatially related to later dikes of granite porphyry.Quartz, fluorite, and topaz contain three types of fluid inclusions: type 1 are low-salinity (mostly <10 equiv wt % NaCl + CaCl 2 ) liquid-vapor inclusions which homogenize to liquid; type 2 are vapor-rich inclusions that homogenize to vapor; and type 3 are high-salinity (30-60 equiv wt % NaCl) liquid-vapor-solid inclusions, most of which homogenize to the liquid phase by solid (generally halite) disappearance. All three inclusion types are found in minerals from stage I whereas only type 1 is found in minerals from stage II.Crystallization of the fine-grained granite resulted in the generation of a subcritical mixture of orthomagmatic low-salinity vapors and high-salinity liquids. The increase in volume associated with the separation of these fluids caused overpressuring in the apical portions of the fine-grained granite magma, which resulted in failure of the overlying sequence and in the formation of the breccia pipe. The resultant decrease in pressure caused adiabatic cooling of the fluid and explains some of the observed salinity variations. A consideration of phase equilibria in aqueous-salt systems indicates that the high-salinity liquids must have cooled approximately isobarically to produce the observed fluid inclusions. This cooling is thought to have been achieved through mixing with ground waters. The high-salinity fluids were responsible for the precipitation of the bulk of the stage I quartz and associated wolframite and molybdenite, and for silica, topaz, and biotite alteration. The later intrusion of the granite porphyry and/or porphyritic granite provided a second generation of subcritical orthomagmatic fluids which may have been responsible for tin greisen mineralization, located at depth in the porphyritic granite.After each of these orthomagmatic events, low- to moderate-salinity ground waters dominated the system. In both cases these late fluids deposited fluorite and sulfides and caused chloritic alteration. In the latter event they formed the stage II, Sn-bearing, polymetallic zones.