THE ORIGIN OF THE 'METACHERT' IN THE DUBOIS GREENSTONE, WILDCAT GULCH REGION, GUNNISON COUNTY, COLORADO: A MINERALOGICAL AND PETROLOGICAL STUDY

Intercalated within the fine to coarsely crystalline, phyllitic to gneissic amphibolite (calcic amphibole + plagioclase + quartz ± epidote ± magnetite) that represents the mafic lithodeme of the Dubois Greenstone in the Wildcat Gulch area atop Nine Mile Hill are numerous blue-gray to blue-black, fine to coarsely crystalline quartzite lenses and pods. These discontinuous lenses are erosionally resistant and are oriented with the regional east to west strike and subvertical foliation. The variable amounts of magnetite and iron sulphides in the quartzites, as well as the association with the Cu-Zn-Au-Ag exhalative mineralization of this area have led investigators to propose that the quartzites represent chert deposited as chemical sediments in the volcanic pile that were caught up in the regional metamorphism of the Dubois Greenstone around 1775 to 1700 Ma.

However, recent re-evaluation of the tectonic framework of the Colorado Mineral Belt in this region, as well as their absence in the adjacent felsic lithodeme of the Dubois Greenstone, the presence of three different quartzite textures, and other mineralogical and petrographical observations suggest that there is more than one possible mechanism for the presence of these quartzites. This study evaluates several plausible hypotheses for the origin of these quartzites and places them into the structural and tectonic framework of the region.