The Eocene Morrison porphyry Cu-Au-Mo deposit, Babine Lake area, British Columbia

English Abstract:

Morrison is a calc-alkaline porphyry Cu-Au-Mo deposit in the northern Babine Lake area of British Columbia. Mineralization is genetically and spatially related to Eocene plagioclase-hornblende-biotite porphyry intrusions, which intruded into the Upper Jurassic Ashman Formation of the Bowser Group sedimentary rocks. The plagioclase-hornblende-biotite porphyry intrusions yielded a U-Pb age of 52.21 ± 0.37 Ma. The mineralization can be divided into 3 stages: (1) vein-type and disseminated chalcopyrite and minor bornite (associated with potassic alteration); (2) vein-type molybdenite (associated with weak potassic alteration); (3) polymetallic sulfide-carbonate veins (dolomite ± quartz-sphalerite-galena-arsenopyrite-chalcopyrite; related to sericite-carbonate alteration). Re-Os dating of molybdenite from stage 2 yielded ages of 53.55 ± 0.22 and 52.69 ± 0.22 Ma. One of the ages (52.69 ± 0.22 Ma) overlaps within error the age of the plagioclase-hornblende-biotite porphyry and suggests a genetic relationship. Gold is associated with copper mineralization, whereas molybdenite mainly occurs at the edge of copper mineralization zone. Stage 1 vein fluids are higher temperature (Th = 400° to 524°C) and saline. (39.8 to 47.8 wt.% NaCl equiv.). Stage 1 disseminated chalcopyrite-pyrite mixtures have δ³⁴S_CDT compositions of -0.2 and 0.8‰, and calculated δ¹⁸O_fluid values from vein quartz range from 5.1 to 7.9‰. These data suggest that the early Cu-Au mineralizing fluids were likely of magmatic origin. Stage 2 is associated with cooler and slightly less saline fluids (Th = 320° to 408°C; salinities = 37.0 to 43.1 wt.% NaCl equiv.). Molybdenite and pyrite from stage 2 veins have δ³⁴S_CDT values of -2.1 to -1.2‰ respectively, and calculated quartz δ¹⁸O_fluid values range from 0.9 to 4.6‰, possibly indicating increased groundwater involvement. Fluid inclusions in stage 3 veins have homogenization temperatures of 163° to 218°C and salinities of 3.1 to 3.9 wt.% NaCl equiv. δ³⁴S_CDT compositions of sphalerite and pyrite from these veins range from -7.1 to -5.6‰, and the calculated maximum δ¹⁸O_fluid values derived vein quartz range from -16.8 to -6.0‰. These data suggest that stage 3 is associated with a cool and dilute fluid, likely dominantly of meteoric water origin.