HYDROTHERMAL ALTERATION AND MINERALIZATION ASSOCIATED WITH A COPPER-MOLYBDENUM PORPHYRY SYSTEM, WEBSTER PASS AREA, PARK AND SUMMIT COUNTIES, COLORADO

Recent drilling at Webster Pass, Colorado, has revealed the presence of a mineralized porphyry system. The prospect is located 8 km east of the Montezuma district, known historically for its silver-base metal deposits. The area lies along the eastern edge of the Colorado mineral belt, known for several large Climax-type porphyry molybdenum deposits. Four diamond core holes at Webster Pass have yielded approximately 2,670 m of core for study. This core was selectively sampled, and 60 thin sections and polished ore mounts were produced. The petrologic study suggests that Precambrian amphibolite, schist, and gneiss of the Idaho Springs Formation host the porphyry system at Webster Pass. The porphyry intrusions vary from monzodiorite to diorite. Two intrusion-related breccias were also observed. Observations suggest that porphyry mineralization is structurally controlled, and is associated with hydrothermal alteration zones. Hydrothermal alteration zones are not concentric due to local structural control. Hydrothermal alteration assemblages include biotitic alteration, chlorite ± epidote alteration; quartz-sericite-pyrite alteration; and illite-smectite ± kaolinite alteration. The most important economic minerals observed are molybdenite and chalcopyrite. They occur as molybdenite-quartz veinlets, and chalcopyrite veinlets. Other ore minerals include minor sphalerite and galena in pyrite. Paragenesis of sulfide minerals appears to follow the pattern: pyrite → chalcopyrite ± sphalerite ± galena ± pyrite → molybdenite.