Abstract

The Morrison porphyry Cu–Au–Mo deposit is genetically and spatially related to Eocene plagioclase–hornblende–biotite porphyry intrusions. One porphyry intrusion yielded a U–Pb age of 52.54 ± 1.05 Ma. Mineralization occurs in three stages: (1) vein-type and disseminated chalcopyrite and minor bornite (associated with potassic alteration and gold mineralization); (2) vein-type molybdenite (associated with weak phyllic alteration); and (3) polymetallic sulfide–carbonate veins (dolomite ± quartz–sphalerite–galena–arsenopyrite–chalcopyrite, associated with weak sericite–carbonate alteration). Re–Os dating of molybdenite yielded ages of 52.54 ± 0.22 and 53.06 ± 0.22 Ma, similar to the age of the host porphyry intrusion. Stage 1 vein fluids were predominantly of magmatic origin: Th = 400–526 °C; salinity = 39.8–47.8 wt.% NaCl equiv.; δ18Ofluid = 3.7‰–6.3‰; disseminated chalcopyrite–pyrite δ34SCDT = 0.2‰ and −0.8‰ (CDT, Canyon Diablo Troilite). Stage 2 fluids were a mixture of magmatic and meteoric water: Th = 320–421 °C; salinity = 37.0–43.1 wt.% NaCl equiv.; δ18Ofluid values range from 0.3‰ to 3.4‰; molybdenite and pyrite δ34SCDT = −2.1‰ and −1.2‰. Stage 3 fluids were predominantly of meteoric water origin: Th = 163–218 °C; salinity = 3.1–3.9 wt.% NaCl equiv.; δ18Ofluid = −2.3‰ to 3.9‰ for early vein quartz, and 1.1‰ to 6.1‰ for late vein dolomite; sphalerite and pyrite δ34SCDT = −7.1‰ to −5.6‰. Morrison is interpreted to be a typical porphyry Cu–Au–Mo deposit related to a calc-alkaline to a high-K calc-alkaline diorite to granodiorite intrusive suite, generated in a continental arc in response to early Eocene subduction of the Kula–Farallon plate beneath North America.

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