STRUCTURAL AND ALTERATION FRAMEWORK OF A BASE METAL MINERALIZED QUARTZ VEIN SYSTEM THAT OVERLIES A CLIMAX-TYPE PORPHYRY MO DEPOSIT AT CRESTED BUTTE, COLORADO, USA

Redwell Basin is located on the flank of Mount Emmons, 6 km NW of Crested Butte, Colorado. Cretaceous to Eocene clastic sedimentary rocks host a 16 to 17 Ma Climax-type granitic porphyry Mo deposit 600 m below surface and vein-hosted base metal (Pb-Zn, ± Cu, ± Ag) mineralization of unknown age near surface. Based on historic drill core, base metals occur in veins close to a rhyolite volcanic pipe that connects with the mineralized porphyry. This study builds a structural and alteration framework to investigate relations between the two deposit types.

Vein-structure groups in the pipe area are differentiated by orientation and vein/fault characteristics. All groups contain steeply dipping to subvertical veins and faults (up to 30 cm wide and up to 4 m of offset). In order of decreasing abundance: (1) NW-striking coarse-grained quartz (anhedral to euhedral crystals) ± fine-grained quartz (microcrystalline to small crystals) ± epidote ± breccia veins; lesser stockwork veins; abundant massive sulfide veins; high fault to vein ratio; (2) NE-striking coarse-grained quartz ± stockwork veins; lesser fine-grained quartz ± epidote veins; local massive sulfide veins; low fault to vein ratio; (3) E-striking coarse-grained quartz veins; lesser fine-grained quartz ± stockwork veins; very local massive sulfide veins; high fault to vein ratio; (4) N-striking coarse-grained quartz veins; lesser fine-grained ± epidote ± breccia veins; abundant massive sulfide veins; moderate fault to vein ratio.

Coarse- and fine-grained quartz veins are widespread but decrease in abundance away from the pipe. Epidote and breccia veins occur in two discrete NW-striking zones within the pipe and an extensive NE-striking corridor N of the pipe. Massive sulfide and stockwork veins are adjacent to the pipe and wrap around its SW to N side. Faults decrease in width and offset size but increase in density in localized zones away from the pipe. Veins and faults trend toward NE-striking orientations away from the pipe.

Disseminated and vein pyrite is most abundant in and around the NNW part of the volcanic pipe in areas of dense veins. Chlorite alteration is most intense at intersecting fault-vein zones SW and N of the pipe and continues with moderate intensity surrounding the pipe and along a NE-striking corridor N of the pipe.