New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent drilling near Webster Pass, Colorado, 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 2670 meters of core for study. This core was selectively sampled, and 60 thin sections and polished ore mounts were produced.

Preliminary 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 varies from propylitic to intermediate argillic, and alteration zones are not concentric due to local structural control. Hydrothermal alteration assemblages include chlorite plus biotite with fresh orthoclase in propylitic-altered rocks; quartz, white phyllosilicates, including muscovite, and pyrite in phyllic-altered rocks; and illite, smectite, and kaolinite in intermediate argillic-altered rocks.

The most important economic minerals observed are molybdenite and chalcopyrite. These occur as molybdenite veinlets, chalcopyrite veinlets, and minor sphalerite and galena in pyrite. Paragenesis of ore minerals appears to follow the pattern: pyrite → chalcopyrite ± sphalerite ± galena ± pyrite ± trace of bornite → molybdenite.