Petrographic study of igneous rocks and breccias from the Mt. Bellview prospect, Elk Mountains, Gunnison County, Colorado

Abstract

The Mt. Bellview prospect is located in the Elk Mountains of Gunnison County, approximately 12 miles north of Crested Butte, Colorado (Figure 1). Geologic mapping by Mutschler (1970) revealed that an igneous complex had intruded and hornfelsed the Mancos Shale in the Mt. Belleview area. The complex is Tertiary in age and consists of granodiorite, granodiorite porphyry, quartz monzonite porphyry, and a large body of intrusive igneous rubble breccia. Petrographic study of the samples revealed prophylitic, argillic and phyllic hydrothermal alteration and/or silicification of the rocks of the Mt. Bellview stock. Geochemical studies revealed anomalous concentrations of Cu, Pb, Zn, Mo, Au and Ag in the rubble breccia pipe, higher than in the intrusive rocks. Diamond drilling perhaps to a depth of 3000-4000 feet would adequately test the prospect as a potential site of porphyry molybdenum mineralization.